Design For Plating
The program to improve and control the quality of a metal product should start at the desk of the designer. Basic principles of mechanical finishing and electroplating impose important restrictions on the size and shape of components. The designer should know enough about these principles so that he can design to minimize costs of quality finishing while planning products which will have a long service life. Metal finishing processes are at least as complicated as metal stamping, casting or forging with which the designer is usually more familiar. Proper selection of finishes and processes offers many opportunities to improve quality, reduce costs, and increase production. ASTM Designation B-507 can provide the designer with helpful information.
A most important term used when specifying metal finishes is "significant surface" because on many products the same standard of quality is not required at all points on the surface. The significant surfaces can be defined as those normally visible directly or by reflection which are essential to the appearance or serviceability of the article or which, if they are a source of corrosion products, can alter the appearance or performance of visible surfaces. Significant surfaces preferably should be agreed upon between purchaser and manufacturer and should be indicated on drawings. Furthermore points at which thickness measurements are to be made should be identified.
Design for Barrel Processing (Electroplate or Mechanical Plate)
Metal parts which are to be zinc or cadmium plated do not ordinarily require polishing with belts or wheels before plating. Those to be cleaned or smoothened are often treated by barrel finishing or tumbling or other vibrating processes. In designing to improve quality, consideration should be given to certain rules applicable to such processing, whether this be surface preparation or barrel plating:
· Avoid blind holes, recesses and joint crevices which can retain
tumbling compounds and metal debris.
· Avoid intricate surface patterns which will be blurred by barrel finishing.
· Parts must be sturdy enough to withstand the multiple impacts of barrel rotation.
· Small flat parts which tend to nest together should be provided with ridges or dimples to prevent this.
· Design for good entry and drainage of solutions during rotation by using simple shapes.
· Significant surfaces must be exterior for barrel work in order to undergo proper mechanical preparation and cleaning or to receive their share of metal deposit. They should be convex, if possible, rather than recessed.
Design for Racking, Draining and Air Entrapment
There are design considerations other than the above for parts which are to be mounted on racks for processing in cleaning and electroplating tanks. Among them are the following:
· Products which would occupy a volume in processing tanks large
in proportion to surface area should be designed to be plated in sections
for assembly after coating.
· Consult the plating department to make certain that parts can be held securely on a plating rack with good electrical contact without masking any significant surface. Many difficult racking problems can be solved by design modification.
· Provide for good drainage of cleaning and other processing solutions from racked parts. Certain shapes tend to trap solution which then cause contamination by carryover, possible corrosion of the part and wastage of materials. Carryover also aggravates the problem of waste disposal. In design avoid rolled edges, blind holes and spot-welded joints. Provide drain holes in recessed areas.
· Avoid shapes which can trap air on entry into a processing tank if this air can prevent access of solutions to areas requiring treatment. Wherever air can be trapped, hydrogen or oxygen may also accumulate during cleaning and plating.
Design for Good Distribution of Electrodeposits
The most important factor determining the quality of a coating on metal part: is its thickness on significant surfaces. Fundamental laws of electrochemistry operate to prevent perfectly uniform deposition of an electrodeposit on a cathode of any useful shape and size. Portions of the work which are nearer to the anodes tend to receive a heavier deposit. Sharp edges or protrusion: tend to steal a larger share of the current and receive a heavier deposit. The goal of the designer and the plater is to minimize thickness variations over significant surfaces. At the same time uneconomical wastage of metal by excessive build-up on non-significant areas must be avoided. Variations in plate distribution may not allow parts to be assembled properly - due to heavy buildup in high current density areas.
Design Features That Influence Electroplatability
The effect of the basic design of a product or component upon the effectiveness or durability of the plating used has been the subject of much study and research and many failures for which the plater has been blamed can be attributed to the original design.
A major contribution to the plating industry was made by the Zinc Institute, Inc. when it sponsored a design study by Battelle Memorial Institute which has resulted in the establishment of basic design principles to be applied to zinc die castings.
It is possible to estimate metal distribution ratios from models or mock-ups, but there are also empirical rules which can guide the designer to improved uniformity of thickness, hence to improved quality with greater economy. The sketches illustrate the influence of design as it has been developed from practical experiences:
· Avoid concave or perfectly flat significant surfaces. Convex
crowned areas receive more uniform coating.
· Edges should be rounded.
· Re-entrant angles or corners should be filleted with a generous radius. Make such radii as large as possible.
· Blind holes must usually be exempted from minimum thickness requirements.
· Protruding fins, knobs and ridges tend to rob current from surrounding areas, hence should be avoided or reduced in height.
· Assist the plater by clearly marking significant surfaces in part drawings.
· Avoid use of a variety of basis metals in any one part to be plated. The contact of dissimilar metals may interfere by galvanic action with covering power or with adhesion of the deposit. In addition, the cleaning of dissimilar metals is quite difficult, if not impossible.
Design for Good Distribution of Mechanical Plating
There is available a non-electrolytic, mechanical plating process which utilizes mechanical energy to deposit metal coatings in a tumbling barrel. The designer should be aware that the distribution of thickness follows different rules than in electroplating. Mechanical plating tends to form thinner coatings at edges and corners than on adjacent surfaces. Flat surfaces are well covered but blind holes and reentrant angles are not. Some recessed areas such as the root diameter of threads have a tendency to build-up. It is evident that, in spite of these differences from electroplating, the designer should still avoid sharp edges and corners, as well as blind holes, to aid in improving quality.
SELECTING THE FINISH
The designer should approach the problem of selecting a zinc or cadmium finish with a clear understanding of the requirements expected of the coating, the properties of the metal deposit, the properties of any coating on the deposit and the service conditions the article will be expected to withstand. Another factor to consider when choosing zinc or cadmium is that some countries will not allow parts or assemblies to be imported if they contain cadmium.
In most cases pleasing appearance is an important factor regardless of function. A properly selected finish of high quality will have an acceptable apperance not only on the shelf but also throughout the service life of the part.
Requirements of Finish
Iron and steel surfaces rust readily even in mildly corrosive surroundings. The rust not only is unattractive but may interfere with mechanical functioning of a component or discolor materials in contact with it. Nearly all iron an steel structures or parts are therefore treated for rust prevention. Zinc an cadmium deposits afford excellent protection against rusting under most circumstances. The reason for applying zinc or cadmium may be classified a (a) to retard rusting, (b) to provide a pleasing appearance and (c) to serve some functional capacity.
(a) Rust Prevention - The corrosion rates of zinc and cadmium deposits are much lower than those of steel in most atmospheres a well as in contact with water. In addition, the electrochemic relationships between each of these two metals and steel are such that rusting of the latter is suppressed by galvanic action where the coating is damaged or worn through.
(b) Appearance - In some applications zinc or cadmium is used with the sole objective of providing a durable, pleasing appearance for the period of useful life of a steel part. Applications of this class have been greatly increased by the availability of bright zinc and cadmium deposits and the development of chromate conversion coatings and colored finishes.
(c) Functional Service - Zinc or cadmium may be deposited on steel components
to serve functional purposes while retarding rusting. Examples of such
uses are: (1) to improve solderability, to lower electrical contact resistance
and to provide surface conductivity on electronic equipment, radio and
television chassis, (2) to prevent discoloration of fabrics or other materials
with which the plated coating may be in contact, (3) to prevent seizing
of moving parts, bolts, nuts and latches, (4) to reduce or eliminate bi-metallic
corrosion through the use of mixed metal mechanically deposited coatings.
Properties of the Deposited Metal Coatings
Cadmium and zinc, considered similar in their galvanic behavior to iron in most environments, do differ in many of their properties. These differences should be considered in selecting one for a given application.
(a) Rust Retardation - The potential difference between cadmium and iron is usually less than that between zinc and iron. For this reason cadmium does not retard rusting over as large an area of exposed iron as does zinc. "Pinhole" corrosion of steel is occasionally observed to occur through pores in a cadmium coating. This suggests that the potentials of cadmium and iron may even be reversed in some environments.
Exposure tests have shown conclusively that in industrial locations cadmium coatings fail more rapidly than do zinc coatings of equal thickness. In severe marine atmospheres, however, cadmium is more effective than zinc in retarding rusting. These results have also been shown to be independent of the application method. Mechanically deposited coatings have demonstrated equivalence to electro deposited coatings of these metals when equal thicknesses have been compared.
The slower rate of corrosion of zinc deposits in industrial atmospheres has been attributed to the fact that zinc corrosion products formed in such atmospheres are alkaline in character and of low solubility. Cadmium corrosion products in industrial atmospheres are more soluble and tend to wash away. It is not clear why cadmium outperforms zinc in the salt spray and in pure marine atmospheres. The zinc corrosion products formed under these conditions are visible enough but do not seem to be as protective.
When rust retardation is the prime requirement zinc coatings are commonly used. The zinc coating may be untreated or receive an inexpensive clear chromate finish in such applications, as for example, conduit, strip, wire and fence hardware.
(b) Corrosion Behavior - When zinc is stored in a humid, stagnant atmosphere quantities of white, bulky corrosion products develop which are usually objectionable and may interfere with proper functioning of moving parts or cause seizure of threaded components. Cadmium weathers differently and does not generate voluminous adherent corrosion products. Corrosion of this sort usually can be prevented by application of chromate conversion finishes.
(c) Solderability - Cadmium-coated steel can be soldered readily with non-corrosive fluxes, an advantage important in electronic and other electrical equipment.
(d) Toxicity - Cadmium corrosion products are toxic hence cadmium should not be plated on articles which come in contact with foods or beverages. Cadmium also poses a severe waste water treatment problem for the plater because of its toxicity. Zinc salts are far less toxic. Although it is not advisable to use bare zinc deposits in contact with food, zinc-coated steel wire with organic coatings has been used for many years in the manufacture of refrigerator shelves. Again, cadmium vapors and oxide fumes are toxic, hence cadmium-plated parts should not be welded. Zinc-coated parts may be welded safely if the fumes are removed by adequate ventilation.
(e) Electrical Propetlies - Cadmium has a lower contact resistance than zinc. This, together with its ease of soldering, leads to its use on electronic apparatus, such as, radio and television chassis. The lesser tendency of cadmium to form bulky weathering products also is of advantage in electrical equipment, giving better surface conductivity and contact behavior.
(f) Hydrogen Embrittlement - High carbon and high strength steels having a hardness greater than 35 Rockwell C are susceptible to embrittlement caused by absorption of hydrogen in the processing of the steel in pickling, cathodic cleaning or plating operations. Zinc and cadmium plating can cause this embrittlement on high strength steels used to make springs, lock washers, fasteners and the like. The phenomenon of delayed cracking by hydrogen embrittlement has been studied by hundreds of workers but is still not thoroughly understood. There is no general agreement on the processing cycle for cleaning and plating such steels with zinc or cadmium which will reliably avoid the embrittlement. There is agreement that susceptible steel parts should be heated after plating to reduce danger of subsequent cracking. Baking at 191°C (375 _F) for four hours is a widely used precaution recommended in ASTM Designation B 242. High-efficiency chloride plating processes which generate very small amounts of hydrogen during operation are available for zinc and cadmium requirements. The use of these processes enables a bright, ductile deposit of zinc or cadmium to be plated directly on cast iron, malleable iron or steel with greatly reduced possibility of inducing hydrogen embrittlement. Mechanical plating can also be used to eliminate this problem since the process imparts no significant absorption of hydrogen.
(g) Covering Castings - The phenomenon of "covering power involves the minimum current density at which electrodeposition occurs. Cadmium baths are generally superior to zinc baths in this respect in electroplating cast and malleable iron parts or high carbon steel, although improved zinc plating processes having good covering power are now commercially available. Although cadmium has been specified for barrel plating of cast iron and high carbon steel components, there are available chloride zinc processes which can be used for this purpose. A mechanical plating system, which is not limited by a minimum current density requirement, is well suited for plating hard to cover substances.
(h) Formability - Both cadmium and zinc coatings can be deposited in
a ductile form. Steel coated with properly applied cadmium or zinc electrodeposits
can be formed, stamped or drawn without damage, depending upon the depth
of the draw.
Properties of the Chromate Finish
Even though zinc and cadmium are excellent rust retarders the surface of steel coated with these metals does not remain bright and stain-free in service. To avoid staining and delay the formation of white corrosion products during storage or in service indoors, practically all zinc and cadmium-coated steel manufactured today is given a thin (20 micrometres maximum) protective film using one of the available chromate conversion finishing processes. There are three classes of chromate finishes in general use on cadmium and zinc coatings: blue brite, clear (single dip or leached) iridescent yellow, black and olive drab. The thickness of the finish increases in the same order, the olive drab being relatively much thicker than the others. Which class of finish is selected depends upon the kind of service to be met and the properties desired in the finish.
(a) Corrosion Protection - The protective value of a chromate finish increases with increasing thickness. In a salt spray test bare zinc-coated steel may show white corrosion products in an hour or two. The same parts, given an olive drab chromate finish can withstand more than 100 hours exposure before showing white products. The protection afforded is particularly valuable in retarding white corrosion under highly humid storage conditions in stagnant air. When used as a base for paint or other organic coating, the chromate coating is extremely beneficial in that it forms a barrier to lateral erosion and undercutting of the top coat. Applications benefiting by this protection include auto parts, washing machines, refrigerator parts and military hardware and equipment intended for tropical service.
Trivalent chromium processes for developing Chromium-containing film on zinc and cadmium are available. The film produced is used similarly as a base for organic finishes and gives improved corrosion resistance to the final product when used in this fashion. The corrosion protection afforded by the film alone is generally equal to a single-dip blue-bright hexavalent chromate process.
(b) Decorative Value - The use of chromate finishes permits wide choice in surface appearance. The chrome-like brilliance of bright zinc or cadmium can be preserved for extended periods of time indoors by application of a blue brite, clear or leached chromate finish. Futher protection can be provided by a clear lacquer coating. Tubular furniture and business machine parts are typical examples.
The thicker finishes offer variety in color ranging from yellow through iridescent yellow to bronze, black, and olive drab. Further choice of colors is possible through application of organic dyes. Some of these are readily absorbed by the chromate finishes to give reds, blues, greens, golds and blacks, along with simulated brass tones which are sufficiently color-fast for indoor service. The primary purpose for use of dyes is color coding of similar parts. The chromate finishes are also used to reduce tarnishing and finger-marking and thus improve the shelf life of hardware and appliance parts.
(c) Electrical Properties - The electrical resistance of the chromate finishes varies with the film thickness - i.e., the thinner the film the lower the resistance. In many applications their protective value can be utilized without losing the advantages of low contact resistance and good surface conductivity of cadmium plated steel. A silver bearing black chromate finish is being used both for cosmetic appearance and to improve static grounding of computer housings. Many electronic, electrical and aircraft components are thus treated.
(d) Solderability - Soldering with rosin fluxes is possible on cadmium-plated surfaces treated with clear chromate finishes. However, clear bright coatings on zinc deposits and colored finishes on both zinc and cadmium usually but not always must be removed in the area of a soldering joint. Applications of clear chromate finishes on plated parts for soldering are typified by electronic equipment, business machines, and telephone equipment.
(e) Abrasion Resistance - Most chromate conversion coatings dehydrate
to form a relatively hard film which can withstand a moderate amount of
wear and handling. The coating does retain entrapped water which, to some
extent, enables the coating to "self-heal" or flow back into a scratched
SPECIFYING THE FINISH
Having selected either cadmium or zinc as the coating for a steel or iron component, and having decided whether or not a supplementary chromate finish is required, the designer must now specify the requirements of the system to assure its quality. High quality can be achieved by specifying the appropriate thickness for the application, adhesion, appearance and freedom of the basis metal from embrittlement, together with appropriate tests for assuring that these requirements have been met.
Atmospheric exposure tests in several countries have supported the generalization that the protective value of zinc or cadmium deposits is proportional to their thickness. That is, a steel part coated with one micrometer of zinc will be protected from rusting about twice as long as another part coated with one-half micrometer and exposed at the same time. Thickness is therefore the most important item in a quality specification.
In case of doubt as to the severity of a planned application one should specify the thickness given for the next more severe class. It should be noted that the tables recognize some applications for very severe service; however, at these thicknesses, mechanical galvanizing (plating) can be used as an alternative to hot dipped coating, metal spraying or other finishes since electroplating becomes economically unfeasible.
Reliance is not placed upon the various chromate finishes to add significantly to the protective value of the metal coating. Instead, the chromate treatments are employed to improve appearance of the coatings, retard staining and delay formation of voluminous white corrosion products which might interfere with functioning of a component. The thinner chromate finishes are specified when solderability and good electrical characteristics of cadmium-coated electronic equipment must be maintained. Those treatments are also specified for service in which maintenance of appearance and long shelf life are the sole considerations as with thousands of varieties of hardware parts and wire goods.
As indicated earlier, specifying a minimum thickness of a zinc or cadmium deposit is the best way to guarantee protective value of high quality. Some buyers do additionally specify performance in the standard salt spray test as an acceptance requirement. The salt spray gives cadmium coatings a much better rating than zinc coatings of equal thickness. In actual exposure tests cadmium deposits on steel usually show a superiority over zinc in marine atmospheres, while they are definitely inferior to zinc in industrial atmospheres. The best course to follow is not to rely too heavily on salt spray performance as a measure of rust retarding ability or protective value of zinc and cadmium coatings.
Under normal exposure conditions, a clear or blue bright passivating dip is sufficient to prevent the formation of white corrosion products on zinc or cadmium coatings until the coating has eroded away and red rust starts to appear. There is some evidence that the strong passivation of yellow or olive-drab chromates reduce the galvanic cell protection of the plating and red rusting may be more extensive than without these heavy chromates. This is due to the fact that a larger amount of zinc is dissolved during chromating with these heavier types, leading to a corresponding reduction in zinc thickness and overall galvanic protection.
In the case of salt corrosion, such as marine or road salt conditions, the use of heavier chromate coatings will delay the formation of white corrosion products. Salt spray hours may be useful in measuring the chromate protection in these cases.
The coating must remain adherent to the basis metal when subjected to bending, cutting or grinding. There is no accepted standard of adhesion and these tests can only be considered qualitative. In high quality plating the adhesion of metallic coatings is so good that in any destructive test the bond between coating and basis metal does not fail at the interface.
Appearance cannot be readily specified because it involves factors which are not easily assessed, such as brightness, roughness, uniformity of color and freedom from blisters, pits and other surface defects. It is suggested that samples be prepared which are acceptable to both manufacturer and purchaser as standards of quality in appearance.
For chromated finishes the general appearance is automatically specified when the type of finish is selected. However, if the requirement of color is critical, use of standard samples for comparison is recommended.
All high strength steel parts with hardness greater than Rockwell C 35 are susceptible to embrittlement caused by absorption of hydrogen during the processing of the steel or in pickling, cleaning or electroplating operations. Cathodic cleaning should be avoided if possible in favor of anodic cleaning and activating. To avoid possible failure of such components on the shelf or in service, due to delayed cracking, it should be specified that they shall be heated after plating for 3 or 4 hours at 191 to 205C (375_ to 400_F) or as prescribed in ASTM Designation B 242.
If immunity from hydrogen embrittlement is a paramount requirement, coating high strength steel parts by mechanical plating should be considered because no hydrogen is introduced into the steel during the plating process itself. It should be noted that the use of this process for the purpose of controlling hydrogen embrittlement to a minimum must recognize the possible embrittling effects of any cleaning and pretreatment procedures. Proprietary zinc and/or cadmium plating processes which deposit zinc or cadmium with little, if any, hydrogen embrittlement should be considered.
ZINC ALLOY PLATING
Zinc coating of steel components has been thus far considered the most economical and viable industrial finishing process for steel, where sacrificial type corrosion resistance is required. For most applications, zinc finishes afford from 24 hours to "white" rust and up to 240 hours to "red rust" in accelerated neutral salt spray testing, depending on zinc thickness, type of chromate and availability of organic top coat.
Recently, specifications for much improved corrosion resistance, especially in the automotive industry, prompted the industry to develop new finishing processes. Alloy plating offers many practical possibilities in this respect.
Electrochemically, alloys have different corrosion potentials from their alloying elements, and can be designed to achieve specific properties. Alloys of zinc, if sufficiently high in zinc content, can still maintain an anodic potential to steel, yet remain less active than pure zinc.
In a corrosive atmosphere, this means that the zinc alloy will still protect the steel sacrificially, but will corrode at a much lower rate than zinc. There are several zinc alloys commercially available:
Zinc-iron - Zinc-cobalt - Zinc-nickel and Tin-zinc. Except for the latter, zinc accounts for 85-99% of the alloy composition. As in zinc plating, chromate conversion coating as a post treatment is important in improving the overall corrosion resistance of zinc alloys.
The process produces zinc alloys containing l5-25% iron. The alloy has good weldability, workability and can be adapted to commercially electroplated strip steel. The alloy composition and process can be varied to enhance weldability or adhesion of electropainting processes. Black chromating is the most suitable for this type of alloy.
Commercially available processes are similar to low ammonia or ammonia free acid chloride zinc baths. Some newer baths operate on the alkaline side. The deposit contains from 1-3% cobalt. The acid type bath has a higher cathode efficiency, and reduced hydrogen embrittlement, but its plating thickness distribution varies substantially between low and high current density areas. Table X - Typical bath composition. Chromate conversion coatings in iridescent, black and yellow are available.
There are two types of zinc-nickel processes currently available commercially. Acid and alkaline non cyanide types. Alloys deposited contain from 5 to 15% nickel. Corrosion resistance improves with nickel content up to 15-18% but the deposit becomes more noble and loses its sacrificial protection property. Chromate film formation was found to be at optimum in the 5-10% nickel content range. Above this range the deposit tends to be passive and chromating becomes very difficult.
Of all the available zinc alloys, zinc-nickel has consistently produced the highest corrosion protection as shown by accelerated corrosion testing, with the exception of the SO2 (Kesternich) test which shows zinc-cobalt to be better and equal to zinc.
One advantage of zinc-nickel alloy processes is their capability of retaining a higher corrosion resistance after forming and after heat treating the plated chromated parts. This is of important significance in industries that require these manufacturing steps after plating. Brake and fuel lines and other under the hood components are typical of such applications.
Another important area of application for zinc-nickel alloys, particularly the alkaline type, is their potential as a substitute for cadmium plating. In view of the recent stringent restrictions imposed by regulatory agencies on cadmium, the search for adequate replacement has been going on for the past several years. The retention of high corrosion resistance after baking and overall performance compared to cadmium plating makes this alloy a favorable candidate for this substitution.
Chromating of Zinc Alloy Finishes
Since the zinc alloys assume new chemical activity properties, conventional chromates are not suitable for their passivation. Special chromating solutions are designed for these coatings and due to the alloying elements, produce characteristically strong iridescent films. Heavier films such as bronze, green-purple iridescent and black give the highest corrosion resistance, and are usually specified for optimum performance.
ZINC ALLOYS AVAILABLE TO REDUCE ACTIVITY WITHOUT REDUCING SACRIFICIAL PROTECTION
Salt Spray Test Results:
Corrosion resistance to red rust @ gmm (.003 in.) thickness
Zinc - 250 hours Zinc-Nickel - 1,000 hours
Zinc-Iron - 350 hours Tin-Zinc - 1,000 Hours
Zinc-Cobalt - 500 hours
This is an alloy of 70-90% tin, 30-10% zinc electrodeposited from either a cyanide type bath or from a newly commercially available neutral non cyanide system as well as acid types.
Tin-zinc is more expensive than other zinc alloy baths, but offers high corrosion resistance to salt water as well as sulfur dioxide, as well as excellent solderability and deposit ductility.
Generally, tin-zinc deposits are satin to semi-bright as plated. A top layer of bright zinc plate can be used where bright deposits are required. Special cast tin-zinc anodes are used for the process.
Chromate passivation on tin-zinc alloys is generally limited to yellow or clear applied electrolytically. A top coat of conventional zinc allows the use of any of the available conventional chromates.
TESTING THE FINISH
There is no practical method for measuring the thickness of chromate conversion coatings, although electrical impedance measurements are useful in characterizing the coatings. The appearance of such finishes itself is an index of relative thickness because the olive drab films are known to be thicker than the clear finishes, while the iridescent yellow films are of an intermediate thickness. For the metal coatings there are several methods of test for thickness, some being destructive of the part tested, others not:
(a) Microscopical Method - The part under test is cut on a plane perpendicular to a significant surface and is mounted for metallographic examination. The cross-section is polished and etched to contrast the plated coating with the basis metal. The thickness is then measured optically in a microscope at a magnification great enough to permit measurements of thickness with an accuracy of plus or minus one micrometre (0.00004 inch) or one percent of the coating thickness, whichever is greater. The method is obviously destructive. Because of the skill required by the operator and the time involved, it is not ordinarily used for production testing. Where there is disagreement in thickness measured by other methods the microscopic is generally accepted as the referee test. This method has been described as given in ASTM Designation B 487.
(b) Coulometric Methods - Instruments are available which determine the thickness of zinc and cadmium coatings on steel automatically by recording the number of ampere-minutes or coulombs required for a controlled anodic current to dissolve the coating from a small, well defined area on the surface. Although the test destroys the coating it is rapid and simple. This method is described in ASTM Designation B-504.
(c) Magnetic Methods - Since zinc and cadmium are non magnetic while iron and steel are magnetic, it is possible to determine the thickness of these coatings by commercially available instruments which employ the magnetic field principle.
Instruments are available that measure the force necessary to detach a small magnet from the surface of the finished part. Other instruments utilize the relutance of a magnetic flux passing through the coating and basis metal to measure the thickness. Both types of instruments are calibrated against standards of known thickness. The tests are nondestructive and rapid.
Results are well within 10% of the true thickness. ASTM Designation B-499 covers the magnetic methods.
(d) Destructive Methods - Dropping and jet tests destroy the coating but not the part. They determine thickness by measuring the time required for penetration through the coating to the basis metal by the action of a steady stream of drops or a jet of a corrosive solution. Reproducability of 5% may be achieved. The solution is chosen so as to provide a constant rate of attack of the coating. ASTM Designation B 555 specifies use of a chromic acid - sulfuric acid solution for drop testing zinc and cadmium coatings on steel.
(e) Weight Loss on Stripping - A test popularly used to determine average thickness of zinc or cadmium deposits over the entire surface of a part depends upon measuring the loss in weight of the part after subjecting the coating to chemical stripping. From the area of the surface and density of the deposit, thickness can be calculated. The method is destructive, not rapid and does not determine local thickness. It does find use in determining the thickness of small, barrel plated parts.
(a) Salt Spray Test - The salt spray test has been used to assess the protective value of zinc and cadmium coatings however the results do not correlate with any given service exposure. At best the test is only able to distinguish very poor from very good coatings by picking out bare areas or other areas where the coating is very thin. Although ASTM does not recommend the salt spray test for determining the protective value of zinc and cadmium coatings, the procedure is described in ASTM Designation B 117. Most purchasers do use the salt spray test to assess the quality of chromate treatments on zinc and cadmium by specifying the number of hours to appearance of white corrosion products on a significant surface. ASTM Designation B 201, Tentative Recommended Practice for Testing Chromate Coatings on Zinc and Cadmium Surfaces, is applicable and suggested performance is indicated.
(b) Lead Acetate Spot Test - The presence of clear chromate finishes is often hard to detect visually. On zinc or cadmium coatings such films can be detected by spot testing with a drop of solution containing 5 percent lead acetate in water. The time elapsed to the appearance of a colored spot is compared with the time required for a spot to appear on an unchromated coating. There is no valid correlation with the salt spray test.
While there is no accepted standard test for adhesion of zinc and cadmium coatings, qualitative methods are some times agreed upon which involve bending, twisting, filing or otherwise maltreating the deposit. It is agreed that no separation of the coating should occur in such tests.
For chromate finishes a qualitative test is sometimes used which indicates degree of adhesion and at the same time tells something about its abrasion resistance. The chromated surface is rubbed with a gritless soft gum eraser for 2 or 3 seconds by hand (about 10 strokes) using moderate pressure and a two-inch stroke. The chromate film should not be worn through to the zinc or cadmium layer as a result of this treatment.