Surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Currently, the trend is towards surface treatments. Surface engineering techniques are generally used to develop a wide range of functional properties, including physical, chemical, electrical, electronic, magnetic, mechanical, wear-resistant and corrosion-resistant properties at the required substrate surfaces. In general, coatings are desirable, or even necessary, for a variety of reasons including economics, material conservation, unique properties, or the engineering and design flexibility which can be obtained by separating the surface properties from the bulk properties. Surface engineered products thus increase performance, reduce costs, control surface properties independently of the substrate and medium, thus offering an enormous potential in the finishing Industry.
Electro depositing of metals is a very significant industrial process. Electroplating is both an art and science .It entailed adhering a thin metal coating to an object by immersing it into an electrically charged solvent containing the dissolved plating metal. Electroplating served a number of functions, such as protecting from corrosion and wear, decoration, and electrical shielding. Anodizing most closely resembles standard electroplating. Anodizing or anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. Anodizing increases corrosion resistance and wears resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films are most commonly applied to protect aluminium alloys.
The aim of this handbook is to give the reader a perspective on several metal surface treatment techniques which are generally followed in the finishing Industry. This is a unique compilation and it draws together in a single source technical principles of surface science and surface treatments technologies of plastics, elastomers, and metals along with various formulae of bath solutions, current density, deposit thickness, manufacturing processes, various ingredients used in these processes. It is a very useful guide for the readers, engineers, scientists, practitioners of surface treatment, researchers, students, entrepreneurs and others involved in materials adhesion and processing.
an electrochemical process, is the reverse of electroplating. Therefore, metal
is removed than deposited. The article to be electro polished is made the anode
in an electrolyte which, when low voltage is applied, forms a polarized film
over the entire surface. This film is thickest over the micro depressions. Where
the polarized film is thinnest, electrical resistance is the least and therefore
the rate of metallic dissolution is the greatest. electro polishing selectively
removes the microscopic high points much faster than the rate of attack on the
â€œValleysâ€ or microdepressions. Stock is removed as a metallic salt. Stock
removal is controllable and can be held to 0.0001 to 0.0025 in. However, stock
removal can be much higher if desired, and under stringent condition can be held
to a lesser amount. It is believed that the polarized film is responsible, under
proper operating conditions, for brightening and smoothing the metal surface. It
must be pointed out, however, that brightness and smoothness do not always go
together. For instance, a welded or rough ground piece may be bright but not
smooth. Conversely, a lapped surface may be smooth but not particularly bright.
When a metal surface is polished, the light rays are reflected in
parallel lines, thus the surface acts like a mirror. On a rough or unpolished
surface, the light rays are reflected in a random patter; therefore, no image
can be seen.
obtain a smooth, highly reflective surface on various metals, several factors
come into play. The degree of successful electro polishing is determined, to a
large extent, on the surface conditions of the base metal. Poor conditions for electro polishing
include nonmetallic inclusions, over pickling, heat scale, large grain size,
directional roll marks, insufficient cold reduction or excessive cold working.
Several of these conditions may be inherent in the metal as it comes from the
mill. During electro polishing, metal is removed, revealing these flows. When
metal is removed smoothing can take place. Other factors contributing to a
smooth, highly reflective surface are change in bath chemistry and precipitation
of metal salts. When bath chemistry is out of balance, poor polishing or no
polishing can results or an extended time may be necessary to obtain a
When salts build up in the bath, higher voltages are required to maintain
the desired current density. This can result in burned contact points.
The Electro polished Surface
with Mechanically Finished Surface. The absence of scratches, strains, metal
debris, and embedded abrasive characterizes the electro polished surface. It has
the â€œtrueâ€ crystal structure of the metal undistorted by the cold working
that accompanies mechanical finishing methods.
The finishing by abrasives or other cutting or burnishing action,
regardless of how small the amount of work, always distorts the surface. The
important fact is the unavoidable presence of those rough ridges and loose clods
of metal. Those defects are absent after electro polishing. The cold-work
damages penetrates into the metal and the abrasive is embedded in the surface.
It reveals in cross-section what are probably the rough ridges. The depth of
damages is also revealed, which shows the surface without enough electro polishing
to remove all the scratches and rough ride metal.
Much is known about heat-treating metals to relieve them of cold-work
effects. The superficial damage done to a metal surface by mechanical finishing
does not necessarily disappear as a result of heat treatments known to restore
gross properties. Nor will heat treatment (or any other method) remove embedded
and smeared-over abrasive particles.
Burnishing by lapping, buffing, or coloring decreases the micro inch
roughness and improves the image-defining quality of a surface, but never
completely removes the debris and damaged metal. The minimum of cold work and
mechanical damage is done in metallographic polishing, yet the effects are
there. The fine abnormal structure of nickel plate reveals the presence of the
cold-worked surface. The nickel plate clearly reveals the true, undisturbed
metal in the electro polished surface.
The mechanical strength of the surface metal is lowered by cold work
accompanying simple cutting operations. Machining of steel having 100,000 psi
tensile strength can leave a surface skin of worked metal having only 35, 000
psi tensile strength.
Advantage and Limitations
preceding discussion shows some of the positive technical advantages of electro polishing.
It shows why electro polishing produces a surface with good properties for:
receiving electroplates having better smoothness, better appearance, and because
of few voids, better corrosion protection; resisting corrosion when there is no
plate or other coating; greater reflectivity of light and heat; better emissive
in electronic tubes; and wear against other metal surfaces without loose metal
fragments to cause fretting.
The surface damage and cold-work effects are overlooked if only
smoothness is specified as a criterion of quality in a metal finish. As the
preceding figure show, smoothness is not an independent variable in surface
definition. It is only a part of an important subject that can be called
â€œsurface metallurgy.â€ Smoothness specification, according to gages, can be
met by electro polishing (with or without plating as an adjunct) just as well as
by abrasive polishing.
The principal limitations of electro polishing are: the process cannot
smear over and cover up defects such as seams and nonmetallic inclusions in
metals; multiphase alloys in which one phase is relatively resistant to anodic
dissolution are usually not amenable to electro polishing; heavy orange peel,
mold-surface texture, and rough scratches are not removed by a practical amount
of electro polishing and require â€œcutting downâ€ first, just as needed before
buffing and colouring. This last situation can be reversed, and electro polishing
is used as a â€œroughing operationâ€ before colour buffing on wheels.
Seams and nonmetallic inclusions impose a limitation. In showing up such
conditions, electro polishing is a good inspection tool. Although unpassable by
established inspection habits, an electro polished and plated.
Table 7, 8 and 9 smoothness versus polishing, smoothness of steel and
smoothness of brass and aluminium respectively.
shown is screen size of grit on the belt. Same locations as accurately as
possible before and after mechanical and electro polishing. Average of several
reading. All RMS units, where stated in this paper, are micro inches. Hand
polishing. Automatic-machine polishing.
is a term used to designate the electo polishing of an electroplate as distinct
from the electroplishing of the basis metal. Both electro buffing and electro polishing
remove metal. The nickel-plating operation adds metal. The group of plates was
bright nickel; a mirror-like smooth electrodeposits. The group of plates was
dull nickel, appearing smooth, but lacking luster. Surface can be superior in
performance. Chromium plate has filled a hole left by electro polishing out a
nonmetallic inclusion. The depression in the chromium plate at the site of the
filled-in hole would be termed a â€œpitâ€ and the part would be rejected. Yet,
the inclusion would remain in an abrasively polished surface and be â€œbridged
overâ€ by the chromium plate. A point of weakness would exist, since the
chromium would not adhere to the nonmetallic. The plated products would be
passed by inspection.
Clearly, an informed approach to the use of electro polishing can give
improved products, and processing costs can be reduced in many applications. By electro polishing,
surface appearance and quality can be as easily reproduced as can an
electroplate. Electro polishing baths are not so critical as plating baths to
control and are less subject to contamination.
Types of Metal Electro polished
metal can be electro polished successfully but best results are obtained with
those having fine grain boundaries and free of nonmenttallic inclusions and
seams. Also, those comprising a high content of silicon, lead or sulfur are
Stainless steels are the most frequently electro polished alloys and all
can be electro polished. Castings will polish to a bright finish but not to the
same brightness or smoothness as wrought alloys.
Besides stainless steel, other commercially electro polished metals
include high and low carbon steel, copper, brass, beryllium copper, phosphor
bronze and many grades of aluminium. Other metals, which can be electro polished,
are columbium, gold, silver, tantalum, titanium, tungsten and vanadium.
lists various metals and solutions, which are suitable for electro polishing.
There are numerous suppliers of proprietary solution on the market.
Today. Some are complete
solutions while others are concentrates, which are mixed with acids purchased
locally. Before purchasing a solution, each should be evaluated to obtain the
best operating conditions for a particular application. Type of solution,
operating temperature, ventilation requirements and pollution problems are some
of the things to be considered.
spinning, weldments, castings, drawings, forgings and wire goods are all
suitable candidates for electro polishing. Typical items fabricated from
stainless steel and electro polished include hospitals, medical and surgical
equipment; dairy, food and beverage processing and handling equipment; bone and
joint implants; vacuum equipment; paper mill equipment; automotive and truck
parts; electronic and communication parts; tubing, pipe, valves and fittings;
fasteners and all types, sizes and shapes of wire goods. These parts can range
in size from a small nut to tanks with thousands of gallons capacity.
items are electro polished for micro inch improvement, burr removal and to
obtain a better surface for electroplating. For instance, the I.D. of gun
barrels are electro polished prior to hard chrome plating. electro polishing is
used on brass costume jewelry items, electronic parts and as a finish prior to
by removing a small amount of metal from the surfaces, also removes any
contamination on or just under the surface. This enables a much stronger
subsequent weld or braze to be achieved, many times with less heat or brazing
material being used.
Many times electro polishing has been used as an inspection tool. For
example, during the fabrication of heavy walled stainless steel elbows, tees,
crosses, etc. which are used in nuclear applications, it has been found that electro polishing
was a superior method of crack detection than was the standard dye check. Cracks
can develop in the surface which are not detectable by visual inspection and are
ever suspect for xyglo inspection. electro polishing by removing the top
surface, reveals material flaws such as cold shots, cracks and inclusions. This
is not only valuable for nuclear applications but also for high-pressure
systems. electro polishing is currently being used to decontaminate equipment
used in nuclear power plants.
vessels, pipes, fittings, valves, heat exchange, tanks, etc. for the food
chemical and pharmaceutical and polymer industries are currently being electro polished
to provide a smooth, clean surface which provides greater anti-stick qualities
coupled with easier cleanability. In many cases an electro polished stainless
steel fabrication provided anti-stick qualities equal to glass lining. Other
applications for electro polishing include decorative purposes and removing heat
tint from spot welds, heli-arc welds and other types of welding. This is of
great value for wire fabrications which have been spot welded.
90% of electro polishing is performed on stainless steel. All electro polishing
is done in the same manner, therefore, our comments will be based primarily on
solutions are used for electro polishing, both acid and alkaline. The
preponderance of solutions used commercially are acid and are based on one of
the grades of phosphoric acid and one or more additional acids. Basically there
are two types of electro polishing solutions- infinite and finite. When a part
is being electro polished, a small amount of metal is removed. This metal in
combination with components of the bath form a metallic salt which drops to the
bottom of the tank and forms a sludge. This bath has an infinite life. A finite
solution is one in which metallic salts remain dissolved in the solution and do
not settle to the bottom of the tank. To obtain optimum results these metallic
salts must be held within limits. When the upper limit is reached, the solution
must be decanted and replaced with new solution to keep the metallic salt
concentration within satisfactory operating limits. This can be expensive
because of the Resource Conservation and Recovery Act regulations for disposing
of this used acid.
Infinite life solution requires that periodically the solution be removed
from the tank and stored. The sludge in the bottom of the tank is then removed
and disposed of according to RCRA regulations. The solutions. The solution is
then pumped back into the tank and fresh solution added to proper operating
electro polishing, the parts are made the anode in the appropriate solution
which will dissolve the oxides of the metal. As these metal ions are removed
from the metal surface more metal atoms are exposed, thus polishing occurs. As
in plating, a DC rectifier is used for the power source. As work is being electro polished
oxygen is liberated at the anode and hydrogen is liberated at the cathode. Thus
no hydrogen can be imparted to the part. There can be times when an imbalance of
solution occurs and the oxygen liberated at the anode and present in air can
cause an explosion of the hydrogen. Such explosions will be loud but not
dangerous. This condition is preceded by a larger than normal foam blanket on
the solution surface and is ignited by a spark from a work holder.
convenience of discussion requires that the subject of part preparation be
separated into distinct areas, it is a mistake to think that these areas are not
interrelated and interdependent. Rather than to think in terms of any single
step, the plater is encouraged to think in terms of a preplating, or part
preparation cycle, every step of which is terms affected by what went before,
and in turn affects that which comes after.
The subject of this preplating cycle is to remove those surface films,
which can be characterized as soils, and replace them with films, which will be
compatible with the solutions being used to apply the final finish. When the
sequence is properly selected and operated, the parts will enter the final
processing solution with a surface in an activated or receptive state for the
finish to be applied. To accomplish this preparation, four basic steps are
cleaning- the removal of heavy soil.
cleaning-the removal of residues from gross cleaning, along with fine
removal-the removal of the thin layer of oxide, which covers every metallic
adjustment-to bring the residual surface film close to the same pH as
the processing solution.
steps constitute the objectives of each stage of the preplating cycle. The
actual processing sequence may be considerably more elaborate. Any stage may
require more than a single processing solution; some soils may require more that
a previous stage be repeated; the rinsing steps must be considered as part of
each stage, not merely incidental; if a multi-component plate is to be applied,
intermediate activating or preparatory steps may be required. The complete
process, therefore, can become extensive.
In addition to preplating cycles, there also may be less demanding cycles
for other manufacturing operations; cleaning prior to applying rust preventives;
cleaning prior to conversion coating; and specialized cleaning operations.
which affect all of these processes include:
of the soil
to be applied
definition of a soil may be compared to the definition of a weed. A weed is a
plant that is out of place. A rose bush in a wheat field is a weed. A wheat
stalk in a rose garden is a weed. Similarly, a soil is matter out of place. Rust proofing
oil on a part in storage is not a soil. Only when the part moves to the
finishing room does it become a soil. The same is true for cutting oils used in
machining; drawing or stamping lubricants; buffing compounds, etc. The air is
filled with particulate matter, oil sprays and various fumes, all of which can
settle out on parts in storage and which are lumped together as shop dirt. A
part cannot be made without contaminating it to some degree with some sort of
Soils not only vary in their basic nature, but the same soil may present
varied cleaning problems, depending on the method of application and its
history. Some soils are particularly susceptible to these effects.
compounds are mixtures of lubricating materials (usually fatty acids), abrasives
(complex silicates, carbides or metal oxides) and materials to control the
melting point (often high-melt parafinnic compounds or waxes). Since the buffing
process is a friction related process, very high temperatures may be generated
at the point of contact, and all the ingredients can react with each other and
the metal surface. These temperatures can vary widely with buffing conditions;
the reactions can vary as well. It is not unusual, therefore, to find that the
same parts buffed with the same buffing compounds which then bind the residues
to the surface, storage or transfer time between buffing and cleaning operations
will also affect the cleaning process. In general, the shorter the delay between
operations, the easier will be the cleaning process. In extreme cases of delay
it is possible for the buffing residues to react so extensively with the
surface, that when they are removed, and etch pattern will remain.
compounds can roughly be placed in three categories:
- Inorganic, water-soluble
compounds for protection between operations, or short-term protected
storage. These normally do not present any cleaning problems.
- Emulsifiable organic
mixtures cut back with water to form the required emulsion. When the
emulsion â€œbreaksâ€ due to a change of temperature or the evaporation of
water, the organic portion is left on the surface as a protective film. The
formulation usually contains one or more volatile constituents, which
evaporate with the water during drying so the protective film is no longer
emulsifiable. Protection is adequate for long-term protected storage, or
interplant transfer. Cleaning problems are similar to the next.
- Solvent cutback organic
mixtures provide a wide degree of protection, depending on composition and
degree of cutback. Protection may be adequate to permit outdoor storage for
reasonably extended periods. They may be formulated with water-displacing
characteristics so parts to be protected may be immersed wet. The organic
protective materials generally contain an oil base, a highly protective
material such as a fatty acid, a metallic soap, or a polar material with an
affinity for the substrate. If they are not fully dry to the touch they
become magnets for shop dirt. Dryness or lack of tack is usually imparted by
incorporating a wax, a drying oil or a film forming resin. Since these
materials are designed to protect by preventing the penetration of moisture
to the metal surface, they are often difficult to clean in aqueous systems.
Solvent or vapour degreasing before aqueous cleaning is often helpful. A
solvent dip to penetrate the film and reduce its viscosity also helps. If
waxes are used for dryness, the temperature of the cleaning solution must be
higher than the melting point of the wax.
Age of the
film can be an important factor. Some of the polar materials may react with the
metal surface. Unsaturated compounds may polymerize to form varnish-like
materials. Evaporation of the solvent used for cutback will alter the viscosity.
Coiled stock is particularly susceptible to these effects. Depending on the
tightness of the coiling, these variations may occur at different rates in
various areas of the coil. Hence, differences in cleaning requirements from
point to point on the coil are not unusual.
Machining and Forming Oils
often, these oils are being fortified with additives providing extreme pressure
lubrication. Since these adhere strongly to the substrate, aggressive,
high-alkalinity cleaners may be required. Machining and forming conditions by
generating locally high temperatures can affect the cleaning process.
Additionally, poorly designed or poorly maintained tooling can introduce surface
conditions that complicate the cleaning process. Double-cleaning cycles may be
required to compensate for these defects. Post-plating defects, including
roughness, plate porosity, and spotting out can result from these causes.
Certain chlorinated or sulfonated oils may be gelled by high alkalinity
cleaners, and low alkalinity materials may be more effective. Once gelled, they
can be very difficult to remove. Where the presence of these materials is
suspected, a sequence of low alkalinity followed by high alkalinity is the
smut is defined as finely divided particulate matter strongly adherent to the
metal surface. It may be conductive or non-conductive. The non conductive smuts
consist of inorganic residues including carbon from acid treatment of
high-carbon steels, or from heat treating operations such as oil quenching or
controlled atmosphere heat treatment; pigments from the use of pigmented drawing
compounds; insoluble constituents of an alloy brought to the surface by previous
chemical treatment; e.g., silicon in aluminum alloys, beryllium in beryllium
copper, etc.; abrasive compounds from buffing or mass finishing operations; mold
residues from casting operations; and certain types of shop dirt.
This type of smut usually responds well to reverse current treatment in
electro cleaners or alkaline descalers.
The conductive smuts usually consist of metallic fines or finely divided
metallic oxides from a previous operation such as polishing, mechanical
finishing, machining or forming. This type of smut does not usually respond well
to electrolytic treatment, since the gas is generated at the surface of the smut
rather than at the metal surface. Much of the lifting action of the gas is
therefore lost. Relatively strong or specialized acid treatment is often the
only effective procedure. It is not uncommon for oil films to be trapped under
these smuts so their removal results in the reappearance of a â€œwater breakâ€.
Double cleaning cycles may be required.
Both types of smut can occasionally be held to the surface by either
electrostatic or magnetic attraction. Ultrasonic or spray cleaning may be
required to overcome these forces.
Base Metal Effects
The nature of the base metal
has a critical bearing on the type of cleaning system selected. Materials must
be selected to provide the required cleaning action without undue or selective
attack on the base metal. Since metals vary greatly in reactivity, allowable
limits of pH, temperature and concentration and the type and concentration of
inhibiting a gents are dictated by the base metal. Cleaners for aluminum or zinc
will generally by quite different from those of brass or steel.
finishes are applied from solutions which either have cleaning and deoxidizing
action by virtue of their composition, or which are very tolerant of marginal
part preparation. Cyanide zinc plating solutions fall into this category, and it
has been common practice to use very condensed, and often marginal preparatory
cycles ahead of these solutions. The wisdom of the approach is highly
questionable, but acceptable, if not high-quality, work can be produced in this
way. Nickel-plating, on the other hand, is highly susceptible to improper part
preparation, so more extensive and effective cycles must be used. Numerous other
instances can be found to illustrate this point.
Tests for Cleaning
literature on various tests for the effectiveness of cleaning procedures is
extensive. Unfortunately, with few exceptions, these remain research tools
rather than production control methods. Among the methods suggested are analysis
and control of numerous characteristics of the cleaning solution, the waterbreak
test, spray pattern test, atomizer test, residual soil measurements, residue
pattern evolution, fluorescent dye evolution, radioactive tracer tests and the
copper sulfate test.
production, the cleaning process is usually controlled by a combination of
solution analysis and the water break test.
increasing complexity of cleaning blends, and the widespread use of
proprietaries, has reduced control by solution analysis to simple test for
solution concentration or total alkalinity, occasionally supplemented by an
analysis for total surfactant concentration. These may be carried out by
laboratory procedures, but most often are monitored with simple test kits
provided by the suppliers. Dilute solutions, such as used in spray washing
equipment, may be monitored by concentration measurements based on simple
conductivity meters, or hand-held refractometers.
More extensive breakdowns involving measurements of pH,
complete titration curves, emulsification characteristics, surface tension,
colloidal suspension properties, etc. are generally reserved for use by the
suppliers laboratory in trouble shooting, cleaner selection or the development
of new materials.
The Water break Test
is based on the ability of a properly cleaned metal surface to retain an
unbroken film of water. The test is subject to possible misinterpretation due to
retained alkali from inadequate rinsing of cleaner residues, or the presence of
hydrophilic smuts with oil trapped under the smut. These difficulties can be
avoided by using a suitable acid treatment before making the observation. In
production, parts are inspected at various stages of the preplating cycle for
any evidence of â€œwaterbreakâ€ or failure to retain a continuous film of
The other tests listed for determining the cleanliness of a surface are
used as research tools for evaluating cleaning mechanisms and developing new
materials. Occasionally one or more of these techniques may be used as a control
procedure in very demanding applications; e.g., sophisticated electronic
manufacture, or space applications.
are mixtures of suitable solvents and sufficient concentrations of surfactants
to cause the solvent to emulsify when added to water. They may be used full
strength followed by a water rinse; or as a prepared emulsion, generally at a
concentration of 5 to 10% by volume in water. A residual film is always left on
the metal surface. They therefore are generally used as precleaners. Cleaning is
by dissolving action of the solvent on oily soils present on the surface,
although the surfactants used to form the emulsion provide additional cleaning
action. The prepared emulsions are frequently used in spray units. Increasingly
stringent restrications on the levels of hexane solubles in effluents are having
a negative influence on the use of these materials since they contribute heavily
to this type of effluent contamination.
Buffing Compound Removers
are essentially highly specialized forms of soak cleaners, designed for the
effective removal of buffing compound residues. They fall into three basic
Neutral detergent-usually liquids; mixture of surfactants; pH close
to neutral with buffing provided by the surfactants used. Concentrations in the
range of 1 to 10% by volume.
detergent-similar to neutral detergent but fortified with organic alkalies which
can react with the fatty acid in the buffing compound to form organic soaps.
Concentrations 2 to 10% by volume.
soak cleaners-similar to soak cleaners (q.v.) but modified to be especially
effective on buffing compounds. Concentrations of 45 to 120 g/1 (6 to 16
Types 1 and 2 often show poor performance on oily soil other than buffing
compounds. Temperature of operation should be above the melting point of the
buffing compound, 60Â° to 80Â°C (140Â° to 180Â°F). Use of ultrasound, or
vigorous agitation will often permit operation at lower temperatures.
cleaners are blends of various inorganic alkaline salts with deflocculants,
inhibitors and surfactants as required to provide the various cleaning
mechanisms and functions discussed below.
chemical action by which a fatty acid, a fatty oil or other reactible soils is
converted to a water-soluble compound such as a soap. Elevated temperature,
concentration and pH promote the speed and completion of the
reaction. The main advantage is that cleaning will proceed in the absence of
surfactants, and that the reaction products may function as additional cleaning
agents to improve the performance of the cleaner. Disadvantages include the fact
that at least initially only reactible soils will be affected; the reaction
products may build up to levels that cause rinsing and drying-on problems;
incomplete rinsing may result in redeposition of the soils in a subsequent acid
treatment; the solubilized soils unless separated will contribute heavily to
hexane solubles in the effluent, and such separation is not always easy to
The chemical process by which surfactants penetrate oily soils and break
them down into globules sufficiently small to allow dispersion and suspension in
the solution. Advantages include the fact that the reaction is often independent
of pH; temperatures and concentrations required can be somewhat lower
than with sponification; all types of oily soils will be removed; and rinsing
will generally be somewhat better than for saponified soils. Disadvantages are
similar to those for saponification except as noted, and with the added
possibility that the surfactant concentration may be depleted at a rate
different from the alkali depletion. The cleaner may therefore drift out of
balance and fail to perform even when concentrations appear to be within limits.
process by which special chemical compounds surround particles of solid soil,
removing them from the surface and dispersing them in solution. The process is
generally improved by mechanical action and/or the development of gas by
electrolysis. Elevated temperatures may also be helpful. Different deflocculants
may be specific to certain solids, so complex soils may require mixtures of
several agents for effective action.
process by which surfactants lift oily soils from the surface of the parts to be
cleaned. A film of surfactant and solution is left on the part surface. The oily
soil floats to the surface of the cleaning bath. Advantages include longer
solution life and the possibility of operating at lower concentrations and
temperatures. The main disadvantage is the need to continually skim the solution
surface to remove the displaced oil. Failure to keep the solution surface
properly cleared may result in the redeposition of the oily soil as the parts
are removed from the solution. When properly operated, hexane solubels in the
effluent are reduced, since the oily soil is constantly separated from the
solutions, which are sprayed on the parts, sometimes under considerable
pressure. Any of the mechanisms previously discussed, including emulsified
solvents may be used. Careful attention must be given to choosing materials with
low-foaming characteristics. The combination of chemical action and the
mechanical action of the spray produces effective cleaning. Spray patterns must
be designed to provide complete coverage of the parts, and the units given
periodic maintenance to insure that nozzles are not plugged. Except for a
foaming requirement, alkaline spray cleaners are similar to soak cleaners.
Concentrations and temperatures, however, are generally much lower, in the range
of 15 to 30/g 1 (2 to 4 oz/gal) and 35Â° to 60Â°C (100Â° to 140Â°F). The newer
low-temperature spray cleaners often operate at 4 to 15 g/1 (1/2 to 2 oz/gal)
and 20Â° to 30Â°C (70Â° to 90Â°F). Liquid forms of the materials are sometimes
available and operate at Â½ to 2% by volume.
use in the cleaning process of ultrasonic energy provided by an ultrasonic
generator which produces the necessary signal, and transducers which convert the
signal to mechanical energy within the solution. The most commonly used
frequency is probably 20 kHz, although higher frequencies (40 to 100 kHz,) are
sometimes used. The ultrasonic energy alternately compresses and expands the
solution and produces several concurrent effects.
alternate pressure effects can literally tear the solution apart to produce
â€œcavitation bubblesâ€. When these â€œbubblesâ€ are collapsed on the
compression portion of the cycle, high-pressure mechanical effects are created,
blasting solid soil away from the surface. Pressures as high as 180,000 psi may
be generated. In extreme cases, the substrate being cleaned may be etched or
otherwise damaged. Cavitation occurrence will be increased, and cavitation
pressures reduced by increasing temperature. It disappears completely at the
Very high voltage may be developed across opposite faces of the
cavitation bubble. These can neutralize electrostatic charges holding particles
to the substrate being cleaned, or even produce oxidizing effects by generating
ozone from oxygen dissolved in the solution.
Since any relatively rigid material will transmit and rediafe ultrasonic
energy with only about 5% loss at each interface, ultrasonic cleaning is highly
effective for blind holes, and internal threads or bores. Soft, resilient
materials such as rubber and plastic are energy absorbers and therefore do not
respond well to ultrasonic cleaning.
of the standard cleaning material may be used with proper adjustment for the
ultrasonic effect. Since much of the ultrasonic energy is converted to heat,
solvents with low flash points or low boiling points may require cooling to
avoid the possibility of fire or excessive evaporation. Alkaline cleaners will
generally require better inhibition for sensitive metals, since the ultrasonic
effects will increase the chemical action of the cleaner on the substrate.
Cleaners can be specially formulated for ultrasonic use and /or to control
The energy levels are generally calculated in terms of watt density/in.2
of surface to be cleaned. Common values are 5 to 10 W/in2 since at
higher levels a layer of heavy cavitation may form immediately adjacent to the
transducer and prevent proper transfer of the energy into the solution. Only one
side of the rack or part is used to calculate this â€œcleaning windowâ€.
Transducer area must be adequate to provide the necessary energy without
exceeding the 10 W/in.2 limit mentioned above. Provision should be
made to remove solid materials from the active area by filtration, or by
settling, or avoid attenuation of the ultrasonic energy, or marking of the
substrate by ultrasonically agitated abrasive particles.
The work-horses of the industry, they remove the major portion of the
heavy oily soils, and often some of the solid soils. They are generally used at
60 at 120g/1 (8 to 16 oz/gal) with an average level of 75 to 90 g/l (10 to 12
oz/gal). Temperatures range from 50Â° to 95Â°C (120Â° to 200Â°F); more commonly
60Â° to 70Â°C (140Â°to 160Â°F). The newer, low-concentration, low-temperature
materials will operate at 15 to 30 g/l (2 to 4 oz/gal) with a maximum of 45 g/l
(6 oz/gal) at temperatures of 20Â° to 40Â°C (70Â° to 100Â°F) in the displacement
mode and 30 to 60 g/l (4 to 8 oz/gal) with a maximum of 75 g/l (10 ozgal) at
temperatures of 25Â° to 40Â°C (80Â° to 110Â°F) in the emulsification mode. Soak
cleaners consists of blended alkalies to establish the desired pH range and
reserve alkalinity, surfactants for detergency, and often deflocculatnts for the
removal of solid soil. Inhibitors for specific base metals may be included. The
pH range is usually established by the reactivity of the metal to be cleaned;
strongly alkaline materials often being used for steel; magnesium and copper
alloys, and mild materials for zinc, aluminum, brass and other sensitive alloys.
Generally only relatively mild materials can be considered for all-purpose use.
Soak Cleaning Equipment
steel equipment is usually satisfactory. Tanks should be equipped with a bottom
drain, a dam type overflow for skimming action, a grease trap and a circulating
pump if displacement type cleaners are used, and a heating coil.
Alkaline blends for use with current. Work can be either cathodic or
anodic, although general practice now emphasizes anodic use. Cathodic electro cleaning
has the advantage that twice as much gas is developed to provide scrubbing
action to remove solid soils. Since the gas developed is hydrogen, sensitive
metals may be subject to hydrogen embrittlement. Additionally, permissible
levels of chromate contamination are considerably lower with cathodic cleaning,
and metallic contaminants may be plated out onto the work. Good practice,
therefore, dictates that when cathodic cleaning is used, special precautions are
taken. These include use of sufficient anodic cleaning to remove plated on
contaminants; prevention of hexavalent chromium contamination; and special care
in regard to hydrogen embrittlement possibilities.
Electro cleaners are commonly used at concentrations of 60 to 120 g/l (8
to 16 oz/gal) with an average of 75 to 90 g/l (10 to 12 oz/gal); temperatures
from 50 to 95 C (120 to 200F), more often 60 to 70 C (140 to 160F) and current
densities of 5 to 15 A/dm2 (50 to 150 ASF). The voltage required to
develop the desired current density is dependent on the type of electro cleaner
used, the tank configuration and the temperature. The normal voltage range is 6
to 12 volts. The newer, low-concentration, low-temperature materials will
operate at 45 to 75 g/l (1 to 10 oz/gal) with a maximum of 90 g/l (12 oz/gal);
temperatures of 45 to 75c (80 to 110F); current densities of 3 to 8 A/don2 (30
to 80ASF). F07 sim8ilar equipment configurations, required voltage will be
somewhat higher than for heated materials.
Electro cleaners consist of blended alkalies to establish the desired pH
range and reserve alkalinity (generally at least some free caustic or other
highly conductive slat is present to promote conductivity); inhibitors to
prevent attack of the base metal (frequently silicates of one type or another);
deflocculants and/ or complexes for solid soil and must removal. Since
precleaning has usually been carried out, surfactants are frequently limited to
the amount needed to provide a mist suppressing foam blanket. Compromise
materials with sufficient surfactant to act as both soak and electro cleaner are
available. Foam levels must be watched carefully. The hydrogen and oxygen
resulting from electrolysis will be trapped in the foam, and if foam levels are
excessive, any sparks from poor contacts, vibration of racks, etc. can produce
ear-shattering, although seldom dangerous, explosions.
plating, as in many other manufacturing processes, â€œthe chain is no stronger
than the weakest linkâ€. While the plating may usually be considered the most
complicated stage in an electroplating sequence, it may frequently be surpassed
in importance by the â€œsimpleâ€ step of rinsing.
Although many processing cycles are presented in this chapter, seldom
will a given cycle meet the requirements of a particular case. Therefore, it is
hoped that from the cycles given and the accompanying discussion, the reader
will gain a sufficient understanding of the requirements in order to be able to
plan the electroplating sequence which will be most suitable for each particular
situation. It is of utmost importance for the reader to refer to other chapters
to understand fully the various steps in a sequence. In additional, the cited
references will be of great aid.
The most elementary cycle prior to the electroplating step is a simple
sequence of cleaning and rinsing. In the majority of cases, one or more
additional steps, such as acid dipping, striking, activating, conditioning,
etc., are required. Even in the above elementary sequence the cleaning may
involve several steps.
In order to understand and be able to project an electroplating sequence,
the following fundamental factors and questions should be considered:
work should be free of foreign matter such as oil, grease, dirt, and oxide
before attempting to electroplate. Long transfer times allowing contamination
and oxidation should be avoided.
should be adequate. In many cases, it may be necessary to use distilled,
deionized or A.S.M. type reagent grade water.
cleaning solutions and water should be of a composition so as not to leave
films, which cannot be rinsed off. This means that the composition of the acid
dip and cleaning solution will usually depend on the composition of the basis
the basis metal require a special conditioning treatment prior to plating?
Examples are the activation of stainless steel, immersion or anodic treatment
for aluminum, and silver striking before silver electroplating.
the solution formulas given in this chapter, the concentrations are
in metric units followed by English units in parenthesis, unless otherwise
Percent by volumes, %, as for example, 5% is equivalent to 5 liters in a
total of 100 liters; grams per liter, g/L; (avoirdupois ounces per gallon,
oz/gal; or fluid ounces per gallon, fl oz/gal).
Grams per liter, g/L; (avoirdupois ounces per gallon, oz/gal).
Unless otherwise stated, the acids used in the solution formulas are
common technical or commercial grade concentrated acids that have the following
Commercial 66 Beâ€™ (93% by weight H2SO4).
Sometimes expressed as g/L (oz/gal) of the 66 Beâ€™ acid.
(Muriatic Acid): Commercial 20 Beâ€™ (31% by weight HCl)
Commercial 42 Beâ€™ (67% by weight HNO3).
Phosphoric Acid, Orthophosphoric acid: (75% by weight H3 PO4).
(42% by weight HBF4).
Acetic Acid- As Glacial: (99.5% by weight CH3COOH).
by weight HF).
Many of the chemicals referred to in this chapter are of a potentially
hazardous nature. Before attempting to use them, the safe and proper methods for
storing, handling, and disposing them should be thoroughly investigated.
Pretreatment is the preparation of the article for the actual
electroplating step. For clarity in discussion, pretreatment is divided into two
stages- preliminary and final. The preliminary treatment removes heavy surface
soils, such as grease, buffing compounds, drawing compound, scale, heavy rust
and burnt oil. Although it can be a part of the plating-room cycle, it
frequently precedes it.
The final treatment removes only the last traces of oil and grease, and
conditions the surface for electroplating. Acid dips in this final stage should
not be expected to remove scale or heavy rust. They are only to neutralize the
last traces of alkaline cleaner remaining after rinsing, and to activate the
surface for electroplating.
In any preparatory cycle where the parts have both oil and oxide
contamination, it is better practice to remove the oily material before
attempting to remove rust or scale unless the oxide scale is to be removed by
mechanical means. This will facilitate uniform removal of the latter.
This involves one or both of two basic steps: (1) Removal of heavy
amounts of oil, grease, buffing compound, drawing compound, etc., and (2)
removal of scale, heavy rust, burnt-in oil, etc.
These steps are only followed when required, as determined by the kind
and degree of contamination. The methods for accomplishing step (1) may in most
cases be used for all basis metals. The methods for accomplishing step (2) vary
depending on the metal and type of article.
This may be
accomplished by one or more of the following methods:
cleaning. Alkali or emulsion-type cleaners are used with a nozzle pressure of
0.2 to 0.4 Mpa (30 to 60 psi.) The temperature, alkalinity, and time should be
adjusted to suit the material being cleaned.
degreasing. Trichoroethylene or perchloroethylene is ordinarily used. The parts
may be washed by agitation in a heated or cold liquid, or by vapour degreasing
by suspending the cooled article in the vapour. Since the latter often leaves
solid matter on the article, a multiphase operation is frequently used. This
involves washing in a heated liquid, followed by dipping in a cool condensed
liquid, and finally vapour degreasing. Proper health precautions should be
taken, and because of fire hazard, the use of mineral spirits is not
recommended. Solvent degreasing is usually followed by very thorough washing or
preferably soak cleaning.
cleaning. This is done by immersion, preferably with agitation, in alkali or
emulsion cleaners and the same precautions should be taken as with spray
alkali cleaning. This may be cathodic or anodic depending on the metal being
cleaned and other conditions. Generally, the same conditions for a given metal
are used as are given in the final cleaning cycle.
parts in bulk. Parts in bulk may be cleaned.
cleaning or solvent degreasing, holding the parts in suitable baskets or trays.
Electrolytic cleaning in a high-conductivity cleaner using an insulated barrel
cylinder will effect more complete removal of grease, etc., but may not be a
necessary step in the preliminary cleaning cycle.
parts may be tumble-cleaned without current in a suitable cleaner.
conjunction with cleaning solutions to produce or increase agitation, which in
turn facilitates the cleaning process.
Through rinsing should follow aqueous cleaning. Where there is a time
lapse or storage period after cleaning, an alkaline cleaner film should remain
on the part.
Scale may be removed by the following methods:
This consists of polishing, tumbling, and sand, grit, or vapour blasting.
Frequently this precedes or makes unnecessary any cleaning in the preliminary
treatment cycle. If not complete, the mechanical treatment at least reduces the
alkaline treatment or acid pickling to a minimum. Where the scale is both heavy
and tenacious, as with hot-formed, high-carbon steel parts, the practice is
first to remove most of the scale by pickling. The polishing then removes the
remainder of the scale and smooths the surface, which has been pitted and
roughened by the pickling operation. On softer metals such as lead alloys, zinc
and aluminum castings, and brasses, the thin surface oxide can be removed by
fine mesh abrasive polishing or buffing, followed by cleaning the surface by
passing it over a clean, dry buffing wheel.
descaling may be conducted on ferrous metal parts. A typical bath, which may be
used as a soak or with current contains:
This method minimizes the effect of acid attack on the ferrous metal and
will not produce smut, both of which could be a problem with acid pickling.
For removal of heavy scale, periodic reverse current (note 1) may be
During periodic reverse (PR), the part is made alternately cathodic and anodic
at intervals of a few seconds using DC current. Barrels or racks may be used:
5-80 C (120-150F)
50-65 C (120-150F)
A/dm2 (30-60 ASF)
5. Sulfuric acid
22.5 g/L (3 oz/gal)
22.5 g/L (3 oz/gal)
70C (160 F)
6. Sulfuric acid
45 g/L (6 oz/gal)
37.5 g/L (5 oz/gal)
Proprietary acids salts, many which contain fluorides, may be used along with or
in place of the acid pickles.
steel. Solution (1) is used where the rust or scale is not heavy or tenacious
scale. Solutions (2) and (3) are used for heavy and tenacious scale. Solutions
(4), (5), and (6) may be used for brown, glazed or burnt-in oil surfaces/
casehardened, and low-alloy steels. High-carbon steel is usually hot-formed,
producing heavy scale; low-alloy steels are cold-formed, producing little scale.
These steels are susceptible to hydrogen embrittlement, and any pickling, except
anodic, should be eliminated where possible or held to a minimum, especially for
As with high-carbon steel, acid pickling of cast irons should be kept to a
minimum since cast irons are susceptible to a low-hydrogen over voltage surface
condition that causes low electroplating efficiency, and acid treatment will
frequently intensify this effect. Formation of smudge can also result from
over-pickling; therefore, scale, rust, and sand should be removed wherever
possible by mechanical means. If it is necessary to acid pickle, two solutions
have been recommended:
1. Sulfuric acid
125 ml (16 fl oz)
125 ml (16 fl oz)
1 L (1gal)
2. Sulfuric acid
90 g/L (12 oz/gal)
37.5 g/L (5 oz/gal)
7.5 g/L (1 oz/gal)
steels. As with some other types of steel. Mechanical methods such as blasting
shot penning, tumbling, and wheel abrading will prove economical prior to
pickling. Iron-containing abrasive should not be used. Frequently, pickling of
stainless steels consists of two steps: scale softening and final scale removal.
65-70 C (150160F)
50-60 C (120140F)
Inhibitors should be used and the parts rinsed thoroughly before going
into the scale removal solution.
60-70 C (140-160F)
Solution (2) is used only for the austenitic stainless steels (except for
type 303) to shorten the time required. The high-carbon grades (types 420, 440A,
440B, and 440C) should be mechanically descaled, if possible. Pickling in the
fully hardened condition should be avoided to prevent pickling cracks. Other
solutions for scale removal have been used:
55-60 C (130-140F)
50-70 C (120-160F)
and copper-base alloys. In addition to copper, the following solutions may be
used for brasses, bronzes, and nickel silver, if buffed, descaling is seldom
Room to 80Â° C (176F)
50Â° C (120F)
Solutions (2) and (5) and the so-called fire-off dip (3) can be used for
removing heavy oxide, solution (3) sometimes being used after (1). For
beryllium-copper, solution (2) is used. The bright dip solution (4) may be part
of the preliminary or final pretreatment cycle, and it should be followed by
very thorough rinsing. Solution (5) may be used for lighter oxides, but the
parts should be clean and dry when immersed.
It is assumed that the work going into the final cycle will be reasonably
free of heavy contamination. As previously discussed, however, there is not
clear demarcation between the preliminary and final treatments. Therefore, some
of the cycles which follow will essentially be duplications of each other, the
differences only being to allow for different degrees of cleaning necessary.
tumble in cleaner, no current
â€“ 5% sulfuric acid
Note 3 â€“ The acid dip may depend on the plating solution but is usually
4 to 10% sulfuric acid or 5 to 25% hydrochloric acid. Alkaline derusters are
sometimes used in place of the acid dips. They are usually operated at 40 C
(104F) and 2 to 5 A/dm2 (20 to 50 ASF), with and without periodic
reverse current (see Note 2). They would be followed by a rinse, acid dip, and
For nickel electroplating
Cycle I-1 through 1-5 may be used.
For copper (see also cycles II-8, II-9), zinc cadmium, and tin
electroplating. The above cycles may be used, but very frequently a room
temperature, 30 to 75 g/L (4 to 10 oz/gal) sodium cyanide dip or alkali dip for
alkaline tin in used just prior to plating in the cyanide or alkaline solution.
For silver electroplating. Proceed after the last rinse in the above
cycles as follows:
silver strike (Note 4)
strike (Note 4)
strike or plate
Note 4- A
rinse may be desirable after the silver strike. Its use will prevent a
carry-over of impurities into the silver plating solution but will result in
some loss of silver and cyanide.
For electroplating directly over steel, two silver strikes are necessary,
as in cycle I-6. Cycle I-8 is usually preferred over I-7.
For gold electroplating proceed after the last rinse in Cycles I-1
through I-4 as follows:
g/L (5 oz/gal) KCN, Cathodic at 4.0 A/dm2 (40 ASF)
distilled or pure water
brass, silver, or nickel strike
(distilled or pure water)
(frequently a potassium cyanide dip and another rinse are used prior to gold
plating in the above cycle)
electroplating. Proceed after the last rinse in cycles I-1 through I-4 as
Lead directly over steel usually lacks good covering power, and unless a heavy
deposit is to be used, cycles I-12 and I â€“13 are preferred.
Cycle I â€“
(see Note 6): 2-10% Fluoboric acid, 2-10% acetic acid, or 2-10% hydrochloric
Note 6: The
acid dip may be omitted, sometimes advantageously, if extra rinsing is used.
For chromium electroplating. Chromium directly on steel is usually for
functional purposes and is known as hard chromium plate.
after the last rinse in cycle I-1, I-2, or I-3 as follows:
chromic acid solution
relieve at 175 C (350F) if necessary
chromic acid solution
Bake at 175
C (350F) to remove occluded hydrogen
coatings are specified by the metal finishing engineer for a variety of end uses
and may be formed on the surfaces of iron, steel, galvanized steel, aluminum and
electrodeposited zinc and cadmium. Phosphate coatings are used to promote the
adhesion of organic coatings to metal substrates and to retard the rate of
interfacial corrosion. Phosphate coatings are used to retain and enhance the
performance of corrosion resistant oils and waxes on metallic surfaces.
Phosphate coatings, with supplementary lubricant, are used to assist in cold
deformation processes such as wire drawing, cold heading and impact forming.
Phosphate coatings are able to provide these valuable functions because
of their particular composition and structure. They are formed by the controlled
corrosion of the metal surface being treated. The resulting coating is tightly
bonded to the surface and is mineral in nature. The coating has a definite
chemical composition and crystal structure. When fully developed, the coating
will cover the surface completely and can be shown to exhibit electrical
The selection of the best coating for a particular application can be
facilitated by an understanding of the characteristics of various coatings now
being applied in commercial practice. Of particular importance is the physical
structure. A series of scanning electron micrographs illustrates the difference
in structure. A typical manganese phosphate coating on steel is characterized by
a dense block-like deposit of angular crystals densely packed in a random
orientation. This heavy coating provides a corrosion resistant or lubricating
surface when the proper oil is applied in a supplementary treatment. The
composite system of manganese phosphate and oil is used to reduce wear and
prevent galling of moving parts such as piston rings, shafts, gears, cylinders,
and pistons. In some cases, the etch pattern formed in the metal surface by the
formation of the manganese phosphate coating is also of importance since it
promotes oil retention on the sliding surface after break-in.
Heavy zinc phosphate coatings on steel exhibit a more platelike structure
than manganese phosphate. Two varieties are normally encountered. When maximum
corrosion protection is desired, an iron content is maintained in the
phosphating solution and a crystal structure is produced. The orientation is
principally parallel to the metal surface and the layered structure offers an
excellent surface for the retention of corrosion resistant oils and waxes.
When a heavy zinc phosphate is used to facilitate cold deformation such
as encountered in the drawing of wire and tubing, cold shaping, cold extrusion,
etc., the phosphating solution is operated under iron-free conditions resulting
in a crystal structure showing a high percentage of crystals oriented vertically
to the metal surface. These thin plates may be lubricated by one of several
methods, including lime, borax, specialized soaps and oils. In some cases, the
surfaces of the individual crystals may be converted to a zinc soap. These
lubricated plates then provide a multitude of slipping surfaces that promote the
ease of cold deformation.
When a phosphate coating is used to promote paint bonding on steel or
zinc-coated steel, a number of options are available to the finishing engineer.
A zinc phosphate coating may be desired for superior performance in retarding
the lateral creep of corrosion between the organic coating and the metal
originating at a scratch through the coating to the metal. Typical crystal
structures for a zinc phosphate formed from the same solution on steel and
galvanized steel. The coating on steel is tightly bonded to the surface and the
effective surface area of the phosphate coating is considerably greater than the
underlying metal. The phenomenon assists in increasing the adhesion of the
organic coating which is further promoted by increasing the numbers of sites for
mechanical locking of the coating to the phosphate crystals.
A zinc phosphate coating on a galvanized surface shows a different
structure. In this case, the individual crystals are well defined and are
distributed in a random orientation on the surface. Note that these
photomicrographs are at a higher magnification than that used for the corrosion
resistant or drawing phosphates. A fine-grained phosphate is desirable for a
paint base in order to provide acceptable gloss with minimum organic coating
A less expensive but still effective phosphate treatment to promote paint
bonding utilizes a so-called iron phosphate treatment on steel or zinc rich
surfaces. It consists of an extremely fine-grained deposit of mixed oxides and
phosphates of iron. The crystal structure is so fine that the coating is often
classified as amorphous. When the same solution used to produce this coating on
steel is used to treat a galvanized surface. This coating differs in structure
and composition from that produced on steel and has the characteristics of a
fine-grained zinc phosphate.
Another popular prepaint treatment is calcium modified zinc phosphate.
The coating consists of densely packed granular crystals showing a high degree
of uniformity that create a surface with greatly increased surface area and an
intricate void pattern well-suited to provide a high degree of mechanical lock
of the organic coating to the treated surface.
Amorphous Phosphate Coatings on Aluminum Surfaces
Coatings consisting mainly of aluminum and chromium (trivalent)
phosphates with traces of other elements are formed on aluminum alloy surfaces
by treating them with acid aqueous solutions containing phosphorus (pentavalent),
chromium (hexavalent) and fluorine. Other elements may be present in the
solutions including boron, silicon, zirconium, titanium, etc. These coatings are
characterized by a degree of crystallinity too low to be readily detected by
electron diffraction or X-ray techniques. They are accordingly designated as
The chemistry of formation of these coatings is not entirely understood.
Apparently direct oxidation of the aluminum surface by the hexavalent chromium
and hydrogen ion couple in the presence of the phosphate radical leads to the
formation of trivalent chromium and aluminum phosphate layer. The role of either
the fluoride or complex fluoride seems to be one of depassivation. Suffice it to
say that very little fluorine appears in the coatings produced.
While the coatings described may be considered to be typical of several
of the types available, their structure and performance may vary with the
specific chemical composition and operating conditions of the phosphating
solution as well as a variety of other factors, including the composition of the
metal being treated, its physical form and surface history; its degree of
cleanliness and surface activation, the effectiveness of rinsing between stages,
and the nature of any seal or other post-treatment that may be applied. The
chemical compositions used to clean, activate, phosphate, seal or post-treat are
usually proprietary products and the specific recommendations of the supplier
should be followed. However certain general comments may be beneficial.
Because the metal surface being phosphated participates in the chemical
reaction to form the coating, the ease and uniformity of its ability to corrode
in the reaction plays an important part in proper coating formation. The
chemical and physical nature of the extreme outer surface is much more important
than the same properties for the article as a whole. The presence of stressed
areas due to cold working, the presence of previously applied corrosion
resisting films, the possibility of decarburization, and the existence of
pre-established etching patterns, all have an influence on the proper
development of the phosphate coating.
In contrast to the stringent cleaning requirements for electroplating,
the surface to be phosphated should be cleaned and conditioned by techniques
designed to form a large number of active sites on the surface in order to form
a fine-grained phosphate coating with even coverage. In general, minimal
chemical cleaning is indicated. Mechanical methods for rust removal are
preferred. Grit blasting and tumbling procedures are effective and are preferred
over acid types of pretreatment. Organic contamination may be removed by solvent
degreasing procedures, emulsion cleaning or alkaline cleaning, providing that
the cleaner will not inhibit the surface and interfere with phosphate coating
formation. In the case of iron phosphates used as a pre-paint treatment, the
cleaning and phosphating operations may be performed by the same solution.
When the maximum in grain refinement is required, a conditioning process
is included. The active materials tend to be finely divided to colloidal in
nature and their application increases the number of active sites for the
development of the phosphate coating. Sometimes this procedure may be
incorporated in the cleaning steps but, preferable, it is applied as separate
treatment after cleaning. The effective life of these solutions is limited and
they should be changed frequently for maximum effectiveness.
In the case of manganese and zinc phosphating solutions, the chemical
composition is such that it provides a source of these elements in solution
together with a source of phosphate. The function of these essential ingredients
is often enhanced by the presence of oxidizing materials such as nitrate,
nitrite, or chlorate. These materials accelerate the formation of the phosphate
coating by increasing the aggressiveness of the acid solution. In addition, the
solution may also contain catalytic amounts of other metals, complexing agents,
and depolarizing agents.
Conventional chemical control of these solutions may include the
determination of free acid, total acid and ferrous iron. The free acid is
expressed as the number of milliliters of tenth normal sodium hydroxide, often
referred to as â€œpoints,â€ necessary to titrate a known volume, usually ten
milliliters of phosphate solution to a pH of 4.0. Total acid is the number of
milliliters of tenth normal sodium hydroxide required to titrate the same volume
of sample to a pH of 8.2. Ferrous iron is usually determined by titration with
The free acid maintains the solubility of the zinc or manganese phosphate
and provides a measure of degree of aggressiveness of the phosphating solution
toward the metal to be coated. Usually maximum-coating rates will be obtained
when the free acid is only slightly higher than that necessary to maintain
solubility of the metal phosphates in the working solution.
The total acid may be considered to be a measure of the amount of coating
materials in solution and its ratio to the free acid is more significant than
either individual value. The ratio of total to free acid in a liquid concentrate
is usually lower than that of the operation bath and either partial
neutralization or processing of scrap work may be necessary to establish the
proper balance between the free and total acid. The latter procedure also
establishes an iron level in the solution, which is generally desirable when the
phosphate coating is applied to assist in corrosion protection. When the
phosphate is to be used in a cold deformation process, soluble iron is excluded
from the solution by the use of an oxidizing agent such as sodium nitrite.
Whenever ferrous metals are phosphated, a stock loss is generated and
iron is introduced into the phosphate solution. Ultimately this results in the
formation of ferric phosphate, which has very limited solubility. This material
is the principal ingredient in the sludge, which is generated as an inevitable
by-product of the phosphating process. It may be removed by filtration or
settling and decantation. Both high-temperature and low-temperature and
phosphating solutions will produce sludge.
The production of another insoluble material, scale, is characteristic of
phosphate solutions operated at high temperature. A general characteristic or
zinc and manganese phosphating solutions is that the metal phosphates become
less soluble as the temperature is raised. This leads to selective deposition of
scale on those surfaces that have the highest temperature; i.e., the heating
surfaces used to maintain the operating temperature. When possible, the
development of this type of scale may be limited by the use of a tank within a
tank type of construction wherein the annular space between the tanks contains a
suitable heat transfer fluid, or by the use of a properly designed external heat
Phosphating solutions are sensitive to metallic contamination and
materials such as lead and aluminum should be excluded in those cases where the
solution is not designed to compensate for the introduction. Obviously, the
introduction of cleaner residues and organic contamination will have a
detrimental effect on the proper operation of a phosphating solution.
plating has white shine, good lustre and hard surface, but it is very costly
metal, and its properties are very similar to those of nickel, owing to high
cost factor it is very rarely used. So it is very less used in specialized areas
due to its special magnetic properties.
Electrolyte composition and operational conditions for electroplating
cobalt are described below:
Electrolyte composition for Bright Cobalt Palating
working conditions of cobalt electrolyte corresponds in the main with those used
for nickel deposition, greater current density can be used and the operational
temperature is kept at 50Â°C and pH value should be 4.
solution for cobalt plating is given below:
temperature is maintained at 35Â°C and current density is 150 amps./sq.ft.
Here is should be noted that corrosion behaviour of cobalt deposits is
generally regarded as good. Without chromium plating it is almost better than
nickel, due to its colour and adequate corrosion protection properties. So it
has achieved some importance in the last few years in unchromed state, for
example spectacle frames. In the chromed condition its behaviour is similar to
that of nickel.
Gold (Au= 197.2 atomic weight) is generally found in the metallic state.
It is one of the metals possessing a yellow colour. Precipitated from its
solution with green vitriol (ferrous Sulphate) or oxalic acid, it appears as a
brown powder without luster, which on pressing with the burnisher acquires the
colour and luster of gused god. Pure gold is nearly as soft as lead, but posses
considerable tenacity. In order to increase the hardness when used for articles
of jewellery and for coinage, it is alloyed with silver or copper. The
â€œFineness of Goldâ€ or its proportion in the alloy is usually expressed by
stating the number of carats present in 24 carats of the mixture. Pure gold is
stated to be 24 carats â€œfineâ€ standard gold is 22 carats fine, 18 carat gold
is a mixture of 18 parts of gold and 6 of alloy. Gold is the most malleable and
ductile of the metals. It may be beaten out into leaves not exceeding 1/10,000
of a millimeter in thickness.
beaten out into thin levels and viewed by transmitted light, gold appears green,
when very finely divided it is dark red or black. The specific gravity of fused
gold is 19.35 and that of precipitated gold powder, from 19.8 to 20.2. Pure gold
melts at about 2016Â°F and in fusing exhibits a sea green colour, 23 carat gold
melts at 2012Â°F, 22 carat at 2009Â°F, 20 carat at 2002Â°F, 18 carat at 1995Â°F,
15 carat at1992Â°F, 13 carat at 1990Â°F, 12 carat 1987Â°F, 10 carat at 1982Â°F,
9 carat at 1979Â°F, 8 carat at 1973Â°F, carat at 1960Â°F.
Gold salts contain 40 percent fine gold and have to be dissolved in
water. Anodes of pure gold are preferable, although platinum or carbon anodes
may be used. If gold anodes are not used, the gold is necessarily taken from the
solution more rapidly.
Brass or copper articles to be gold plated are immersed in the gilding
solution with the gold anode held in the right hand (used to agitate the gilding
solution slightly), the articles held in the left hand being wired as negative
or cathode. About half minute is suffice to obtain a good deposit; if a thicker
deposit is required the articles are swilled and scratch brushed, and then given
a further 10 second in the gilding bath to get a bright finish. Articles made of
base metals, or soft soldered, are first plated in a cyanide copper solution.
Articles, which require bright gilding inside, are first burnished, given
a cleaning bath and then put through the potassium cyanide solution. Large
articles are filled with gold solution and connected to the cathode rod, a piece
of gold wrapped in two or three pieces of swanâ€™s down is connected by a wire
to the anode is suspended in the solution inside the article and moved about
briskly for a time, depending upon the thickness of deposit required. If the
edges are to be gilded, the anode is rubbed on these parts. A fasted appearance
is obtained by sand blasting or using a brushing wheel, then gilding and scratch
The electrolytic process is used for stripped gold from plated articles.
The solution is made up of 500 gms. Potassium cyanide, 250 gms. Caustic soda and
one-gallon water. The articles are used as anodes and are hung on the center
rod. Sheet steel cathodes are suspended from the outer rods, which are connected
to the negative pole of the dynamo a resistance board being used to regulate the
Gold plating may be effected in a hot or cold bath, large objects being
generally plated in the latter and smaller objects in the former. The hot bath
has the advantage of requiring less current-strength, besides yielding deposits
of greater density and uniformity and of sadder richer tones. Hot baths work
with a moderate content of gold â€“ 11.5 to 12.5 grains per quaint of bath-while
cold baths should contain not less than 54 grains per quaint.
Baths prepared with potassium Ferro cyanide are preferred by some
authors, while others work with a solution of gold salt and potassium
bicarbonate and others recommend a solution of cyanide of gold in potassium
cyanide. With proper treatment of the bath good results may be obtained with
either. Generally, the use of baths prepared with potassium ferrocyanide can not
be recommended on account of the secondary decompositions which take place
during the operation of plating and because the baths do not dissolve the gold
anodes. Below only approved formulas for the preparation of gold baths will be
for Cold Gilding
Fine gold in the form of fulminating gold 54 grains, 98% potassium
cyanide 0.35 to 0.5 oz (according to the current strength used), water 1 quiant.
Electro-motive force at 10 cm electrode distance and with the use of 0.35
oz. Of potassium cyanide, 1.35 volts, with the use of 0.5 oz of potassium
cyanide, 1.2 volts.
Current-Density, 0.15 Ampere
prepare this bath, dissolve 54 grains of fine gold in aqua regia in a porcelain
dish heated over a gas or alcohol flame and evaporate the solution to dryness.
Continue the heating until the solution is thickly fluid and dark brown and on
cooling congeals to a dard brown mass. Heating too strongly should be avoided,
as this would cause decomposition the auric chloride would be converted into
aurous chloride and eventually into metallic gold and chlorine, which escapes.
The neutral chloride of gold formed in this manner is dissolved in 1 pint of
water and ammonia added to the solution so long as a yellow-brown precipitate is
formed, avoiding, however a considerable excess of ammonia. The precipitates of
fulminating gold is filtered off, washed and dissolved in 1 quart of water
containing 0.5 oz of potassium cyanide in solution. The solution is boiled,
replacing the water lost by evaporation, until the odor of ammonia which is
liberated by dissolving the fulminating gold in potassium cyanide disappears,
when it is filtered. Instead of dissolving the gold and preparing neutral
chloride of gold by evaporating, it is more convenient to use 108 grains of
chemically pure neutral chloride of gold as furnished by chemical works and
precipitate the fulminating gold from its solution.
Too large and excess of potassium cyanide yields good deposits of an
ugly, pale colour when working with a more powerful current the excess of
potassium cyanide need only be slight, with a weaker current it may be larger.
The fulminating gold must not be dried as in this condition it is highly
explosive but should be immediately dissolved while in a moist state.
the cost of bath for cold gilding with such as high content of gold as given in
formula 1 should appear too great, only 27 grains of gold per quart may be used.
Within a suitable electro motive force deposits of a beautiful shade-yellow
colour are thus also obtained. Such a bath is yielded by the following formula:
Dissolve the gold salt from 0.35 oz. Of fine gold or about 0.7 oz of
neutral chloride of gold in Â½ pint of the water and the potassium cyanide in
the other Â½ pint of water and after mixing the solutions boil for half an hour.
The preparation of this bath is more simple than that of formula I, but the
colour of the gold deposit obtained with the latter is warmer and sadder. The
high content of gold in the bath, prepared according to formula. (II) Readily
cause a red-brown gold deposit and hence special attention has to be paid to be
the regulation of the current.
Formula for Cold Gold Gilding
Formulation for Cold Gold Gilding
(II). Fine gold as neutral chloride
98% potassium cyanide
Electro motive force at 10 cm
A bout 1.5 volts,
Formulation for Cold Gold Gliding
Yellow prostate of potash (potassium ferrocyanide)
(as chloride of gold or fulminating gold)
force at 10 cm electrode distance
For those who prefer gold baths prepared with yellow prostate of potash
instead of potassium cyanide the following formula for cold gilding is given.
To prepare the bath heat the solutions of the yellow prostate of potash
and of the carbonate of soda in the water to the boiling-point, add the
gold-salt and boil Â¼ hour, or with use of freshly precipitated fulminating
gold, until the odor of ammonia disappears. After cooling, the solution is mixed
with a quantity of distilled water, corresponding to the water lost by
evaporation and filtered. The bath gives beautiful bright gilding upon all
metals, even upon iron and steel.
The yellow prostate of potash baths are deservedly popular for decorative
gilding, when gold deposits of different colours are to be produced upon an
object. Certain portions have then to be covered with stopping off varnish, the
latter being less attacked by this bath than by one containing an excess of
This bath is especially suitable for the so-called clock gilding. The
articles are first provided with a heavy deposit of copper in the alkaline
copper bath next drawn through the bright-pickling bath, thoroughly rinsed, and
finally gilded in the bath heated to about 122Â°F.
Gold Baths for Hot Gilding
Formulation for Hot Gold Gilding
(as fulminating gold)
force at 10 cm electrode distance
This bath is prepared in the same manner as that according to formula I,
from 15.4 grains of fine gold, which is converted into neutral chloride of gold
by dissolving in aqua regia and evaporating or dissolve directly 29.32 to 30.75
grains of chemically pure neutral chloride of gold in water, precipitate the
gold as fulminating gold with aqua ammonia, wash the precipitate, dissolve in
water containing the potassium cyanide and heat until the odor of ammonia
disappears, replacing the water lost by evaporation. This bath yields a
beautiful shade gilding of great warmth. All that has been said in regard to the
content of potassium cyanide in the bath prepared according to formula I also
applies to this bath. The temperature should be between 158Â° and 176Â°F and the
current strength 2.0 to 2.5 volts following bath is recommended for hot gilding.
this bath is to serve for directly plating steel, only half the quantity of
potassium cyanide is to be used and the objects should be covered with the use
of a some what greater electro-motive force. Increasing the content of neutral
sodium to 0.5 or 0.7 oz also appears advisable.
Dissolve in a porcelain dish, with the aid of moderate heat, the sodium
phosphate and sodium Sulphate and when the solution is cold add the neutral
chloride of gold prepared from 15.43 grains of gold is equal to about 30.86
grins of commercial chloride of gold, and the potassium cyanide. For use, heat
the bath to between 158Â°F and 167Â°F and 167Â°F.
Formulation for Hot Gold Gilding
pure catalyzed sodium phosphate
force at 10cm electrode distance
For the preparation of gold baths for hot and cold gilding, double gold
salts and triple gold salts as well as gold solutions, as brought into commerce
by some manufacturers may also be used.
Many gold-platters prepare their gold baths with the assistance of the
electric current. This is accomplished as follows:
Dissolve 12 ozs of potassium or sodium cyanide (98 to 99%) in a gallon of
distilled water and heat a temperature of 130Â°F. Place in this cyanide solution
a porous cup which is attached to the negative or cathode rod. A carbon cathode
is suspended in the porous cut, which must contain sufficient cyanide solution
to bring it to the same level or a little higher, than the outside solution.
Place an anode of fine gold weighing about 1.5 oz. Troy in the larger solution
of cyanide, attaching it to the positive rod. Allow an electric current from two
Bunsen cells or a dynamo to pass at an electromotive force of from 3 to 4 volts
until 1 odwts of gold have been dissolved. The solution after removing the
porous cup is ready for use.
The only advantage of this mode of preparing the bath is that it excludes
a possible loss of gold, which may occur in dissolving god, evaporating the gold
solution, etc. by breaking the vessel containing the solution. However, by using
commercial chemically pure chloride of gold such loss is avoided and the bath
prepared according to the formulae given yields richer tones than a gold bath
produced by electrolysis. Besides, the preparation of the gold bath with the
assistance of the electric current can only be considered for smaller baths,
since the saturation of a larger volume of potassium cyanide solution requires
considerable time and the potassium cyanide is strongly decomposed by long
Tanks for Gold Baths
Gold baths for cold gilding are kept in tanks of stoneware or enameled
iron, or small baths in glass tanks, which to protect them against breaking are
placed in a wooden box. Baths for hot gilding require enameled iron tanks in
which they can be heated by a direct fire, or better, by placing in hot water
(water bath) or by steam. For small gold baths for hot gliding porcelain dish
resting upon a short-legged iron tripod may be used. Beneath the iron tripod is
a gas burner supplied with gas by means of a flexible India-rubber tube
connected to an ordinary gas burner. Across the porcelain dish are placed two
glass rods, around which the poles wines are wrapped. In heating larger baths in
enameled tanks over a direct fire it may happen that on the places most exposed
to the heat the enamel may blister and peel off it is, however better to heat
the baths in a water or steam bath. For this purpose have made a box of stout
iron or zinc sheet about Â¾ inch wider and longer, and about 4 inches deeper
than the enameled tank containing the gold bath.
Execution of Gold-Plating
Most suitable current density, 0.15 to 0.2 ampere. Like all other
elector-plating operations, it is advisable to effect gold plating with an
external source of current, this is to use a battery or other source of current
separated from the bath.
apparatus required for salt water gold solution is as follows. A copper kettle
having a steam coil of copper pipe at the bottom, a red porous jar and a piece
of sheet zinc Â½ inch thick, which should be formed into the shape of a
cylinder. To the latter is riveted a copper rod, so shaped that it extends over
and above the opposite side of the zinc cylinder. Place the porous jar and zinc
on glass insulators in the bottom of the copper kettle. In the water surrounding
the porous jar dissolve rock salt until the brume hydrometer registers 15
degrees. The gold solution which should be made with yellow prostate of potash
is placed inside the porous jar. The copper kettle, zinc and rock salt generate
a feeble current of electricity, which deposits the gold upon articles suspended
from the copper rod and immensed in the gold solution, the temperature of which
should be maintained at 170Â°F.
The gold deposit seldom needs to be made extravagantly heavy and the
rough surface formed would require more laborious polishing with the burnishers
and on the other, the gold deposits adhere quite well to highly polished
surfaces, provided the current-strength is correctly regulated and the bath
accurately composed according to one of the formulae given.
The current-strength must, under no circumstances be so great that a
decomposition of water and consequent evolution of hydrogen on the objects, take
place, since other wise the gold would not deposit in a and coherent form, but
as a brown powder. By regulating the current-strength so that it just suffices
for the decomposition of the bath and avoiding a considerable surplus a very
dense and uniform deposit is formed and by allowing the object to remain long
enough in the bath, a beautiful, mat gold deposit can be obtained in all the
baths prepared according to the formulae given. It may, however, be mentioned
that this mode of mat gilding is the most expensive, since it requires a very
heavy deposit, and it will therefore, be better to matten the surface previous
For gilding with cold baths, two freshly-filled Bursen cells coupled for
electro-motive force suffice in almost all cases, while for hot baths one cell
is as a rule, sufficient, if the anode surface is not too small. The more
electropositive the metal to be gilded is, the weakens the current can and must
Though gold solutions are good conductors and therefore, the portions of
the articles which do not hang directly opposite the anodes gild well, for solid
plating of larger objects it is recommended to frequently change their
positions, except when they are entirely surrounded by anodes.
The inner surfaces of hollow-ware, such as drinking-cups, milk pitchers,
etc. are best plated after freeing them from grease and pickling, by filling the
vessel with the gold bath and suspending a current-carrying gold anode in the
center of vessel, while the outer surface of the latter is brought in contact
with the negative conducting wire the lips of vessels are plated by placing upon
them a cloth rag saturated with the gold bath and covering the rag with gold
For Gold-Plating in the Cold Bath the Process is as Follows
The objects, thoroughly freed from grease and picked (and if of iron,
zinc, tin, Britannia, etc. Previously coppered) are suspended in the bath by
copper wires, where they remain with a weak current until in about 8 or 10
minutes they appear uniformly plated. At this stage they are taken from the
bath, rinsed in a pot filled with water and the latter, after having been used
for some time, is added to the bath to replace the water lost by evaporation.
The articles are finally brushed with a fine brass scratch-brush and tartar
solution, thoroughly rinsed, again freed from grease by brushing with lime paste
and then returned to the bath, where they remain until they have acquired a
deposit of sufficient thickness.
When an article is to have a very heavy deposit, it is advisable to
scratch-brush it several times with the use of tartar or its solution, or with a
solution of size and water between the intermediate coats of gold. By these
means a very durable and lasting coating of gold will be secured.
For gold plating with the hot bath, the operations are the same, with the
exception that a weaker current is introduced into the bath and the time of the
plating process shortened frequent scratch brushing also increases the solidity
of the deposit and prevents its prematurely turning to a dead brown-black. Since
in hot plating more gold than intended is readily deposited it is especially
advisable to place a rheostat and voltmeter in the circuit, as otherwise the
operator must remain standing along side of the bath ad regulate the effect of
the current by immersing the anodes more or loss.
When taken from the bath, the finished gilded objects should show a deep
yellow tome, which after polishing yields a full gold colour. If the objects
come from the bath with a pale gold tone, the deposit, after polishing shows a
meager, pale gold colour, which is without effect. Gold deposits of a dark or
brown colour also do not yield a shade gold tone.
With a some what considerable excess of potassium cyanide and if the
objects to be plated are not rapidly brought in contact with the
current-carrying object rod, hot gold baths cause the solution of some metal.
Therefore when silver or silver plated objects are constantly plated in them
they yield somewhat greenish gilding in consequence of the absorption of copper,
if copper or coppered articles are constantly plated in them. Hence, for the
production of such green or reddish colour, gold-plating baths, which have thus
become argentiferous or cupriferous, may be advantageously used.
AND THEIR PREVENTIVE MEASURES AND POLLUTION CONTROL CONSIDERATION
As is known, metals have
shaped the development of modern world. This is why metals are:
Worldâ€™s increasing wealth,
Aspiration of the nation and
of our civilization. But metal do deteriorate because of their reaction with
environment, namely (1). Industrial atmosphere, (2) marine atmosphere,
atmosphere, and (4) aqueous solutions (pH 0-14), etc. This results in huge metal
loss incurring 1.5% of GNP.
Corrosion has been defined in many ways. A sub-committee of the Inter
Society Corrosion Committee recommends the definition:
â€œCorrosion is the deterioration of a substance (usually a metal)
because of a reaction with its environment. The corrosion is stated to be an
Corrosion Loss in System
Corrosion leads to Numerous
Damages, which are as below:
of valuable products
of safety and reliability
and operating costs
rusted items leave impression. Here, service life versus dollars is not the
shut down are due to lack of knowledge, neglect and ignorance about
value of the products, pigments, foods, drugs, semi-conductor is related to
its purity and quality. This demands expensive material of construction.
on materials of construction is not desirable, if safety is risked.
- Substantial savings if
close co-operation between the corrosion engineers and process and design
In fact, our economy would be drastically changed if there is no
â€œcorrosionâ€. While corrosion is inevitable, its cost can be reduced
considerably by judicious selection of preventive measures.