A program to improve and control the quality of a metal or plastic product should start at the desk of the designer. The metal finisher is restricted in what he can do by certain basic principles of mechanical finishing and of electroplating. The engineer should understand the limitations imposed by shape and size of components to facilitate quality finishing at an acceptable cost. The designer can exert as much influence on the quality attainable in finishing a part as can the electroplater himself. ASTM Standard B-507 can provide the designer with helpful information.
A most important term used in specifying metal finishes is "significant surfaces". In most products the same standard of quality is not required over every square inch of surface. Instead, the quality specifications apply and compliance is expected only for the so-called "significant surfaces" defined by mutual agreement between the producer and purchaser as follows:
Significant surfaces are defined as those normally visible (directly or by reflection) which are essential to the appearance or serviceability of the article when assembled in normal position; or which can be the source of corrosion products that deface visible surfaces on the assembled article. When necessary, the significant surfaces shall be the subject of agreement between purchaser and manufacturer and shall be indicated on the drawings of the parts, or by the provision of suitably marked samples.
Design for Mechanical Finishing
Metal products which are to be coated with copper/nickel/chromium or nickel/chromium finishes are generally subjected to abrasive polishing with belts or wheels in preparation for the plating operations. This is done to aid in securing an attractive uniform, mirror-like or satin appearance on the finished part. Mechanical finishing is an expensive operation. To reduce costs and assist the metal finisher in improving the appearance and quality of the product the designer should consider certain rules applicable for parts requiring mechanical finishing.
· Avoid blind holes, recesses and joint crevices which can retain
polishing compounds and metal debris.
· Avoid intricate surface patterns which will be blurred in polishing.
· Significant surfaces should be exterior, reachable by ordinary polishing wheels or belts.
· Avoid sharp edges and protrusions which cause excessive consumption of wheels or belts.
In small parts which are to be barrel processed the above rules apply plus a requirement that the 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 Racking, Draining and Air Entrapment
Most metal or plastic parts weighing more than a few ounces are not plated in bulk in barrels but are mounted on racks for processing in cleaning and electroplating tanks. Design considerations relating to racked parts are described in the following paragraphs.
Products which would occupy a large volume in processing tanks, large in proportion to surface area, should be designed to be plated in sections for assembly after coating.
Consult your plating vendor to make certain that parts can be held securely on a plating rack with good electrical contact without masking a significant surface. Many difficult racking problems can be solved by design modification.
Provide for good drainage of processing solutions from racked parts. Certain shapes tend to trap solution which then causes contamination by carry over, possible corrosion of the part and waste of materials. Carry over aggravates the problem of waste disposal. In design, avoid rolled edges, blind holes and spot-welded joints. Drain holes are especially important in irregular shapes and tubular parts.
Avoid shapes which can trap air on entry into processing tanks if this
air could block access of solution to areas requiring treatment. Wherever
air can be trapped, hydrogen or oxygen gas may also accumulate during a
cleaning or plating step.
Design for Good Distribution of Electrodeposit
Experience and cost accounting show that simple shapes are always finished more uniformly and more economically than complex shapes. This is rule number one for the designer.
One of the most important factors determining the quality of a coating is its thickness on significant surfaces. Fundamental laws of electrochemistry operate to prevent a perfectly uniform deposition of an electrodeposited coating on a cathode of any practical shape and size. Portions of the work which are nearer the anodes tend to receive a heavier deposit. Sharp edges or protrusions at all current densities tend to steal a disproportionate share of the current. The goal of the designer and the plater is to make thickness variations as small as possible. At the same time, uneconomical wastage of metal by excessive build-up on both non-significant and significant areas must be avoided.
It is possible to estimate metal distribution ratios from models or mock-ups, but there are also empirical rules. These can guide the designer to improved uniformity of thickness, hence to improved quality with greater economy. These general principles and various sketches illustrate what has been learned from practical experience:
· Avoid concave or perfectly flat significant surfaces. Convex
or crowned areas receive more uniform coatings. Use a 0.4 mm per 25.4 mm
(0.015 inch per inch) crown-minimum.
· Edges should be rounded to a radius of at least 0.4 mm (1/64 inch) preferably 0.8 mm (1/32 inch).
· Re-entrant angles or corners should be filleted with a generous radius. Make such radii as large as possible.
· Avoid concave recesses, grooves, or slots with width less than one-half the depth.
· Minimize the number of blind holes because these must usually be exempted from minimum thickness requirements. Where necessary, limit their depth to 50% of their width. Avoid diameters less than 6 mm (7/32 inch).
· Countersink threaded holes to minimize electroplate thickness at their peripheries and facilitate insertion of fasteners after plating.
· If fins or ribs are required, reduce their height and specify a generous radius, 1.6 mi (1/16 inch) at each base. Round off tips with radii of at least 1.6 mm (1/16 inch) Multiple parallel fins should have spacing between centers equal to four times th width of the fin. Broad hollow ribs are preferred over slender solid ones.
· Adopt recessed in preference to raised letters and insignia, but round off edges an provide gentle contours.
· Integrated studs for fasteners should be shortened as much as possible and inside angles at each base should be rounded generously. Tips should be similarly rounded.
· Studs or bosses with hollow centers should be shortened as much as possible an angled 90 degrees from the major plane of the part. All bosses should face the same direction.
· 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. Cleaning may also be complicated due to unequal reactions with the different metals.
DESIGN FEATURES THAT INFLUENCE ELECTROPLATABlLlTY
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. 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 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. The principles can be applied to other substrates.
SELECTING THE FINISH
The designer should approach the problem of selecting the proper copper/nickel/ chromium or nickel/chromium finish with a clear understanding of the requirements imposed on the plated product, the properties of the individual metals of the coating system and the service conditions to be satisfied. A properly selected decorative finish of high quality should be expected to perform acceptably throughout the service life of the product.
Requirements of the Finish
While appearance standards vary, the usual requirements are characterized by the deposits that have high reflectivity and are free of pits, clouds, and surface roughness. Such finishes are required in a variety of products including automotive parts, appliances, plumbing fixtures, bicycles, and tools. Chromium over suitable copper/nickel or nickel undercoats meets this need handsomely when high quality of the composite system is specified and achieved. Similarly, functional decorative applications e.g. flashlights, floodlights, electrical heaters, and instruments which require a high luster and mirror-like surface can be satisfied more economically by such finishes as compared to precious metal coatings. Satin and brushed surface effects can be produced as a more subdued finish. In contrast with bright or satin coatings, chromium and nickel can also be deposited as a non-reflective black oxide coating and lustrous surfaces for various applications can be obtained.
(b) Corrosion Behavior
To satisfy the prime requirement of maintaining an acceptable appearance, a decorative coating necessarily must effectively resist deterioration of itself and protect the basis metal from corroding. There are a multitude of finishes which can protect a basis metal from rusting or tarnishing in the broad family of paints, lacquers, sprayed metals and plastics as well as plated coatings. However, where a durable bright metallic finish is desired, the choice is limited. Chromium provides a top coating having excellent resistance to tarnish in atmospheric exposure and the copper/nickel or nickel undercoat provides the proper foundation for the lustrous chromium finish and the corrosion protection of the underlying steel, plastic, zinc, stainless steel, or other basis material.
(c) Wear and Abrasion
If the part must resist cleaning and handling or abrasive wear, copper/nickel/chromium or nickel/chromium finishes are usually specified in preference to softer metallic and organic coatings.
Coating Metals Considerations
(a) Rust Retardation
There are two classes of plated coatings which retard rusting or other corrosion of the basis metal. They do so by different mechanisms. The distinction arises from the different electro-chemical relationships between the coating and the basis metal when these are in contact with moisture. This can happen at a pore or other discontinuity in the coating. If the finish is anodic to the basis metal when exposed to a corrosive medium, then corrosion of the basis metal will be inhibited. This is the way in which zinc and cadmium and other anodic coatings retard the rusting of steel. The other class of finishes, represented by copper/nickel/chromium and nickel/chromium coatings are called cathodic coatings. They show an opposite electrochemical behavior, since copper, nickel and chromium are generally cathodic to common basis metals. Such finishes actually tend to promote galvanic corrosion when only a few pores or cracks are present in the coating.
It is obvious that cathodic coatings must resist corrosive attack which could create pores or pits extending to the basis metal. From outdoor exposure tests it has long been known that a prime factor of quality of copper/nickel/chromium and nickel/chromium finishes is thickness. In general, the thicker the coating the longer the basis metal is protected. However, it was found that just increasing the nickel thickness up to two or three mils did not keep pace over the years with increasing severity of some outdoor urban environments. It was also not economical. Then it was discovered that multiple layers of nickel of differing composition give superior protection without increasing the total nickel thickness. These, together with modifications to the chromium plate, provided the means for a spectacular improvement in protective value while maintaining the appearance.
Chromium offers good corrosion resistance and abrasion resistance in the family of commonly plated metals. Thin chromium coatings over suitable undercoats provide excellent decorative systems. Furthermore, special chromium types can be deposited over copper/nickel or nickel undercoats so that the overall corrosion protective value of the system is increased.
Corrosion protection can be improved by the use of special types of chromium coatings. The useful life of the finish can be extended by the use of microporous or microcracked chromium. These invisible pores or cracks may be achieved by employing one of several existing process techniques. For hexavalent chromium processes, two methods are commonly used in North America to form microporous chromium. First method: a nickel strike containing micron sized inert particles are plated over the bright nickel and before the hexavalent chromium deposit. Pores are formed in the subsequent chromium deposit as it covers the inert particles. Second method: a spray of hard material such as sand or aluminum oxide is shot against the hexavalent chromium deposit just hard enough to microcrack the brittle chromium. In neither method is the appearance of the deposit affected. Microcracked chromium is essentially not used in North America. Chromium deposited from a trivalent bath is inherently microporous up to 0.6 microns, then it generally becomes macrocracked. Corrosion resistance is significanty increased by microdiscontinuous chromium. This improvement is particularly beneficial in such applications as automotive and marine hardware.
Chromium may be deposited from baths with the metal in either the hexavalent or trivalent state. The hexavalent or CR+6 uses chromic acid as its source for metal, along with sulfates for control. There are proprietary catalysts used to improve efficiency and covering power. Hexavalent chromium has been identified as a carcinogen and several regulatory agencies have tightened its emission controls.
The function of nickel is to provide a tough, durable, and ductile undercoat. Nickel protects the basis metal from corrosion and in combination with the chromium top coat results in a lasting decorative finish. Nickel levels micro-roughness in the basis metal, providing a smoother, brighter finish. With increased levelling in the nickel, the need for other expensive substrate preparation work is reduced.
In double-layer nickel undercoats for chromium finishes, the metal immediately under the chromium is bright nickel containing small amounts of sulfur (eg. more than 0.04 mass %) while the layer under that is semi-bright nickel essentially free of sulfur. In any galvanic electrolytic cell set up with the chromium, the bright nickel reacts anodically to the purer semi-bright nickel. If microscopic corrosion sets in through pores in the chromium layer and penetrates the bright nickel layer, galvanic action between the two kinds of nickel tends to cause the microscopic pit to spread laterally in the outer bright nickel layer. The net effect is to retard penetration toward the basis metal, hence, to lengthen the useful life of the coating. The negative effect is that as the lateral corrosion increases, the resulting surface pits may decrease the reflectivity of the corroded finish.
The potential difference between the semi and bright nickel layers can be measured by the S.T.E.P. Test (ASTM B-764). S.T.E.P. potentials between 100 and 200 millivolts are typically specified. The higher the S.T.E.P. value, the more lateral corrosion occurs in the bright nickel layer before the semi-bright nickel layer is penetrated. This extends the time before basis metal corrosion but increases the rate of deterioration of the appearance.
The above galvanic corrosion system can be further enhanced by the use of three layers of nickel of different sulfur contents. In this case, a high potential nickel strike layer (high sulfur in the nickel) is deposited between the semi- and bright nickel. This strike acts as a sacrificial deposit to both these layers. This further retards the corrosion penetration toward the basis metal while improving the appearance of the surface by minimizing the corrosion at the bright nickel/chromium interface. In general, when a high potential nickel strike is utilized, the potential of the bright nickel layer can be decreased to improve appearance after corrosion commences without a loss in basis metal corrosion protection.
(e) Copper Undercoat
Nickel/chromium coatings may be deposited over a copper undercoat. ASTM specification B-456 has been established for copper/nickel/chromium coatings where microporous microcracked or conventional chromium is used. Alkaline copper coatings, generally cyanide, are first plated on zinc die castings to protect the active zinc from attack by the subsequent acid nickel processing.
As with nickel deposition, leveling copper processes can be used to upgrade the basis Metal finish, and thus contribute to an overall improvement in the appearance of the final chromium finish. The excellent buffability of copper permits obtaining a high luster finish, and buffed copper coatings are employed when buffing costs can be justified. Ductile copper plate improves the apparent ductility of overlying nickel-chromium coatings. Bright leveling acid copper plate can minimize the undesirable effect of basis metal porosity by filling and bridging pores and also because of its superior micro-throwing power, will plate into sharp angles. Acid copper deposits are currently being used on some zinc die castings following a thin protective copper plate deposited from a cyanide bath.
In recent years, the development of a non-cyanide alkaline copper strike may allow the copper cyanide strike to sometimes be replaced; therefore permitting a copper strike and copper plating system to be used that is free of cyanides.
(f) Other Specialty Finishes
A number of modified finishes based on copper, nickel, or chromium can also be obtained.
• Antique and relieved copper plating
In this process, the copper plate is colored brown, black or green by immersion in sulfur based or other proprietary antiquing baths. The converted surface can then be relieved echanically by light buffing or mass finishing then optionally sealed with a clear organic top coat. Examples of such applications are electrical fixtures, hardware, and clothing accessories.
• Decorative black nickel
In general, nickel is plated and then it is oxidized by either a dip or by electroplating of an alloy of nickel such as nickel-tin, nickel-zinc, and other available combinations. There are also proprietary black pure nickels that can be applied directly and which can be subsequently relieved. Typical applications of this process are luggage hardware, vets, screws, etc.
• Black chromium
This is produced by using proprietary chromium plating formulations. The process and equipment are similar to those for bright chromium plating. Solar collectors, furniture, and some camera components as well as other specialty decorative black finishes are plated by this process.
Although basis metals with bright nickel/chromium finishes have been successfully drawn, stamped and otherwise formed, the bright nickel/chromium layer is relatively brittle. The best way to withstand forming operations is to plate the workpiece with a very ductile Watts or sulfamate nickel layer that is polished and/or buffed to obtain the desired finish and then plated with a thin micro-discontinuous chromium deposit. An alternative method that is less expensive and time consuming includes the use of micro-discontinuous chromium over the bright nickel coating. This tends to relieve the hydrogen embrittlement in the nickel layer, thereby improving formability
The most economical and simplest method is to substitute decorative
nickel-iron for bright nickel (Note the discussion on the properties of
these deposits above). These coatings are more ductile, especially after
chromium plating, which in some cases allows the plated parts to be formed
or bent without having to resort to the use of special procedures or additional
plating baths. This should only be done when the high corrosion protection
offered by pure nickel is not required.
SPECIFYING THE FINISH
Having selected copper/nickel/chromium or nickel/chromium as the coating system for a steel, plastic, stainless steel, brass, zinc or other substrate, the designer must now specify the type, thickness and other characteristics desired in the coating. High quality can be obtained by properly specifying the types and thickness of layers to be applied. The designer must specify the appearance of the layer's finished surface. These will determine the protective value of the final part. The required performance tests such as adhesion and ductility should also be specified. For specific subjects, refer to the Annual Book of ASTM Standards, Volume 02.05, which covers Electrodeposited Coatings.
Type and Thickness
By type is meant the number and sequence of layers of copper, nickel and chromium which constitute the coating. A variety of multiple-layer decorative finishes have been developed by the various suppliers. Most are superior in protective value to a single layer of bright nickel covered by chromium. The large number of deposit types and combinations of layers can economically meet most performance specifications. See Tables land II for recommended types and thicknesses meeting various service requirements.
Explanation of Tables I and II
(1) Service Conditions
Depending upon the customer, there are four or five service conditions which define the environment to which the plated part may be exposed as a function of the substrate.
The classification letters shown in the tables indicate the type of deposit to be provided. The types of nickel are designated:
b - for nickel deposited in the fully bright condition.
p - for dull or semi-bright nickel requiring polishing/buffing to give full brightness, and containing less than 0.005 mass % sulfur (Note 0.005 mass % of sulfur is essentially a sulfur-free deposit).
d - for a double-layer or triple-layer nickel coating ofwhich the bottom layer contains less than 0.005 mass % sulfur and the top layer contains more than 0.04 mass % sulfur. The low sulfur layer should be from 60% to 75% of the total nickel thickness. If there are three layers, the intermediate one shall contain not less than 0.15 mass % sulfur and shall not exceed 10% of the total nickel thickness.
There is no restriction on the type of chromium used, except that one is not permitted to buff the chromium deposit. There are no restrictions on how the microporous and microcracked deposits are produced. The deposits must meet the following classifications:
r - for regular (i.e., conventional) chromium. This deposit is non-microdiscontinuous
hexavalent or trivalent chromium.
mc - for microcracked chromium having more than 300 cracks per linear cmi (750/in.) in any direction over significant surfaces. The cracks shall be invisible to the unaided eye.
mp - for microporous chromium containing a minimum of 10,000 pores per sq. cm. ( 65,000/sq.in.). The pores shall be invisible to the unaided eye.
RECOMMENDED STANDARDS FOR QUALITY
NICKEL/CHROMIUM FINISHES ON STEEL,
IRON AND ZINC PRODUCTS
SC 1 MILD
Toaster bodies, rotisseries, Exposure indoors in normally warm, dry atmospheres; coating subject to minimum wear or abrasion. p r 10 0.1 waffle makers. oven doors and liners, interior auto hardware, trim for major appliances, hair driers, fans, inexpensive utensils, coat & luggage racks, standing ash trays, interior trash receptacles, inexpensive light fixtures.
SC 2 MODERATE
Steel& Iron: stove tops, oven liners, home, office and school
Exposure indoors in furniture, bar stools, golf club shafts. Zinc Alloys: bathroom sation of moisture accessories, cabinet hardware. may occur: example, in kitchens and bathrooms.
SC 3 SEVERE
Patio, porch and lawn furniture, Exposure which is bicycles, scooters, wagons, likely to include hospital furniture, fixtures and occasional or frequent wetting by rain, dew or poss. cleaners and saline solutions.
SC 4 VERY SEVERE
Service conditions Minimum requirements for auto which include likely bumpers, grilles, hub caps and damage from denting, lower body trim. Light housings. scratching, or abrasive wear in addition to corrosive media.
SC 5 EXTENDED - VERY SEVERE
Extended usage in exterior auto-Service conditions automotive and other items as in SC4
which include damage from denting, scratching, or abrasive wear in addition to exposure to corrosive enviornments where long time protection of the substrate is
Note: 5 micrometers (millionth of a meter) copper is required on zinc
and zinc alloys prior to nickel-chromium. 15 micrometers may be used on
steel. 25.4 micrometers = 1.0 mil
Adapted from ASTM Standard B-456
RECOMMENDED STANDARDS FOR QUALITY
NICKEL/CHROMIUM FINISHES ON
COPPER AND COPPER ALLOYS
SC 1 MILD
Exposure indoors in normally warm, dry atmospheres with coating subject to minimum wear or abrasion. Toaster bodies and similar appliances, oven door and liners, interior autro hardware, trim for major appliances, receptacles, and light fixtures.
SC 2 MODERATE
Exposure indoors in places where condensation of moisture may occur: for example, in kitchens and bathrooms. Plumbing fixtures, bathroom accessories, hinges, light fixtures, flashlights, and spot lights.
SC 3 SEVERE
Exposure which is likely to include occasional or frequent wetting by rain or dew or possibly strong cleaner and saline solutions. Patio, porch and lawn furniture and light fixtures, bicycle parts, hospitial and laboratory fixtures.
SC 4 VERY SEVERE
Service conditions which include likely damage from denting, sctatching or abrasive wear in addition to corrosive media. Boat fittings, auto trim, hub caps, lower body trim.
(3) Use of Copper
Copper undercoats are preferred for deposits on plastic, zinc die castings and aluminum, where it protects the underlayers from acid solutions during subsequent acidic plating steps. Copper is sometimes specified for steel to improve the part's appearance and to cover up corrosion enhancing surface defects. Copper thicknesses are not substitutable for any part of the specified nickel thickness.
(4) Typical Application
The lists of a typical applications will help the designer or purchaser select a suitable coating specification if he is not certain of the service conditions to be met. In case of doubt as to the severity of a planned application, a classification should be specified suitable for the next more severe type of service condition.
The problem of specifying performance of decorative coatings involving copper/nickel/chromium and nickel/chromium composites has intrigued the metal finishing industry continually since the start of the last century. The factors controlling corrosion are so numerous that the only valid test is exposure of parts to actual service conditions over the life of the part.
In recent years several accelerated corrosion tests have been developed which have shown some correlation with the performance of copper/nickel/chromium and nickel/ chromium coated parts in moderate and severe outdoor urban and outdoor seacoast service. There is copious data available to support the specification of the number of hours of resistance to these accelerated corrosion tests, as a measure of minimum acceptable quality in coatings intended for a given type of service. The tests themselves are described under the heading "Testing the Finish".
Cyclic corrosion tests involving cycles of heat, humidity, spray, etc. are beginning to be used in an attempt to better simulate natural environmental corrosion. The existing tests have not been standardized at this time, but individual companies do specify them.
The conventional salt spray test, ASTM B-117 which was instituted in 1914, has been generally discredited as an accelerated corrosion test for these decorative coatings largely because of lack of reproducibility of results and questionable correlation with service. It is recognized, however, that the test is still used in some segments of the plating industry because it can serve as an inspection tool to reveal gross defects in the coating and bare areas. It is mainly classified as a porosity test.
It can be specified that the coating shall be sufficiently adherent to the basis metal and the separate layers of composite coatings shall be sufficiently adherent to each other, that the finished part will pass the adhesion tests described under the heading "Testing the Finish".
Appearance cannot be readily specified because it involves factors such as brightness roughness and uniformity of color which are not easily assessed objectively. Location and extent of surface defects may influence acceptability. It is suggested that sample be prepared which are acceptable to both manufacturer and purchaser as standards of quality in appearance.
If the product is to be used in such a way that the coating will be formed or deformed in service it may be desirable to specify ductility requirements. If the coating contains semi-bright nickel, then ASTM Specification 456 calls out a minimum 8% elongation for that deposit.
TESTING THE FINISH
There are several methods for measuring the thickness of copper/nickel/chromium and nickel/chromium coatings, some being destructive of the part tested and others not. Most only measure a small area of the part and so thickness measurements are ofte made at several specified set-points or at different current density areas.
(1) Microscopical Method
The part under test is cut on a plane perpendicular to an area being measured and is mounted for metallographic examination. The cross-section is polished and etched to contrast the plated coating with the basis metal, (See Illustration 1. for a good example) The thickness is then measured with an optical microscope at a magnification great enough to permit measurements of thickness with an accuracy of plus or minus one micrometer (0.00004 inch) or one percent of the coating thickness, whichever is greater Thicknesses below one micrometer are too thin for this method.
The method is obviously destructive, is time consuming and a high level of skill and experience is required by the operator. Despite these disadvantages, it is used to some extent for production testing. When there is disagreement in thickness measured by other methods, the microscopical method is often selected as the referee test. This method has been accepted by ASTM as described in ASTM Standard B-487.
2) Magnetic Methods
Since the magnetic properties of the basis metal and the various layers of copper/nickel/chromium finishes differ, it is possible to use these differences to determine the thickness of coatings. To use ASTM Standard B-499, the coating being measured must be non-magnetic and plated over a magnetic surface. Instruments are available that measure the force necessary to detach a small magnet from the surface of the finished part. Other instruments utilize the reluctance of a magnetic flux passing through the coating and basis metal to measure the thickness. Both are easy and rapid to operate. They are calibrated against standards of known thickness. The tests are non-destructive. Results are within 10 percent of the true thickness. These instruments cannot measure the thickness of nickel if there is an undercoat of copper present.
ASTM Method B-530 describes a magnetic test method for coatings on magnetic and nonmagnetic substrates. As described in these standards, only the appropriate method must be used for the deposits and substrate being tested.
(3) Coulometric Method
This method is also known as the electrochemical or anodic solution stripping methods. It depends upon measuring the number of ampere-minutes or coulombs required for a controlled anodic current to dissolve a coating from a small fixed area of the surface. The test is destructive. Instruments are available which record the current flow automatically so that they are simple and fast to operate. Coating thickness of chromium (greater than 0.0785 micrometers), and nickel and copper (between 0.75 and 50 micrometers) can generally be determined by these instruments. ASTM Standard B-504 provides some particulars on this method.
This method is useful to simultaneously determining both the thickness and electrochemical potential of individual nickel layers in multi-layer nickel deposits. The S.T.E.P. (Simultaneous Thickness and Electrochemical Potential) Test method is based upon the Coulometric Method and is destructive. As each nickel layer is stripped under a constant current, it requires a given voltage (electrochemical potential). This potential is determined by comparison to a standard electrode. The precision and accuracy of this method has not been independently determined.
The main function of this test is to monitor these variables on production parts. These measurements can also be used to indicate the corrosion protection offered by the overall system, ASTM Method B-764 describes this test in detail. Commercial equipment is available and requires minimal experience.
(5) X-Ray Spectrometry
By the use of X-Rays, this method measures the mass of the coating per unit area, which can also be expressed in units of linear thickness provided that the density of the coating is known. Equipment is available for this easy to operate, non-destructive method. Depending upon the instrument and the metals being measured, the accuracy decreases as the thickness increases. Known standards are used to calibrate the instrument. ASTM B-568 describes this method.
(6) Spot Test Method for Chromium
Like other chemical methods, the spot test for chromium thickness is destructive. A circular spot on the surface is exposed to a drop of hydrochloric acid solution which attacks the chromium at a known rate. The time required for penetration through the chromium layer is a measure of the thickness. Precision of up to plus or minus 10 percent can be achieved. Details of the test method are given in ASTM Standard B-556.
(7) Weight Loss on Stripping
It is often possible to strip or dissolve a coating, chemically or anodically, without appreciable dissolution of the basis metal. By weighing the speciman before and after the stripping and estimating its surface area, the average thickness of the coating can be calculated. Sometimes, the entire sample, basis metal and coatings, is dissolved and the solution analyzed for nickel, chromium, copper and the ingredients of the basis metal. Again, the average thickness is what is obtained from the calculations. This method will not indicate significant surfaces with insufficient coating thicknesses. This method is often used in testing the quality of the finish on small barrel-plated parts. It is also used on other parts in which surface roughness, lack of accessibility, and other restricting factors make thickness measurements impractical by other methods.
The accelerated corrosion tests specified in Table III are a means of controlling the continuity and quality of copper/nickel/chromium coatings, but the duration of such tests does not have a good correlation with the service life of the finished article. Generally, the deterioration of the appearance of the deposits' surface is greater after these tests than typically observed in natural corrosive environments. Passing accelerated corrosion tests at the plating plant level helps to assure consistent quality parts. Details of the test methods are found in the corresponding ASTM Standards. The designer and purchasing agent need only know the general methods employed which can be briefly described as follows:
The CASS Test, ASTM Standard B-368, involves exposure to a fog of droplets of five percent sodium chloride solution containing enough acetic acid to maintain a pH in the acidic range of 3.1 to 3.3. The droplets also contain a small amount of cupric chloride to accelerate corrosion.
The CORRODKOTE test, ASTM Standard B-380, is conducted by applying a slurry of corrosive salts and kaolin to the significant surfaces of the specimen, allowing the slurry to dry, and then exposing the slurry-coated part to a highly humid atmosphere for a specified period of time.
After subjecting an article to the corrosion test for the specified time, it is examined for evidence of corrosion of the basis metal or blistering of the coating. It is up to the plater and the purchaser to determine which test to use and what constitutes failure after accelerated corrosion testing. These corrosion tests were developed to give a relative measurement of the protection of the basis metal and are not reliable for predicting surface appearance after use. The extent to which such surface alteration will be tolerated is again subject to agreement between purchaser and manufacturer.
Two other accelerated corrosion tests that are still found in the older literature but which are no longer considered useful are the NEUTRAL SALT SPRAY Test, ASTM B-117 and the ACETIC ACID-SALT SPRAY Test. The latter test had an ASTM number of B-287 but it was discontinued by ASTM in 1988. A general salt spray standard can be found in ASTM G-85.
There is no accepted quantitative standard test for adhesion. Qualitative tests are suggested in ASTM Standard B-571 A bend test involves repeated flexing or deformation of the plated parts until fracture occurs. Any separation of the layers or coating constitutes failure. In the file test a piece is cut out of the part with a saw, then a coarse file is applied to the cut edge of the coating so as to attempt to raise or separate it. There is also a quenching test in which the finished article is heated in an oven to an elevated temperature and then is quenched in cold water. If no test method is specified, pick the one that most closely simulates the possible adhesion failure mode for the part during actual service.
ASTM provides two recommended practices for evaluating ductility. ASTM 13-489 describes a procedure that consists of bending over a mandrel a narrow strip cut from a metal plated article. An elongation measurement is obtained from the smallest diameter mandrel that does not cause the deposit to fracture. When the shape is such that a suitable specimen cannot be cut from the plated part, a test panel may be prepared of appropriate basis metal or plastic, with the same coating system in the same baths.
ASTM B-490 is suitable only for evaluation of deposits having low ductility, such as nickel. It describes a procedure for measuring the ductility of electrodeposited foils obtained from the actual plating solutions. The recommended practice consists of measuring the bend of a foil held between the jaws of a micrometer; these are closed until fractures or cracks appear. This is the typical method used for decorative electroplated nickel deposits.
For enhanced corrosion protection with the same nickel thickness, microporous and microcracked chromium is used instead of standard, conventional, or "non-porous" chromium deposits. Several methods are available producing equivalent results. Microdiscontinuity is invisible to the unaided eye and so must be tested. ASTM B-456, Appendix X4 contains the most established method, the Dubpernell Test. This test enlarges the sites by plating copper only into the pores and cracks so that they can be visible under 100 to 200x magnification. A newer method used by the automotive industry corrodes the part for 22 or 44 hours in CASS, then strips off the chromium and observes, at low magnification, the surface pits created in the nickel layer through the microcracks or micropores of the chromium when exposed.
ON OTHER SUBSTRATES
Aluminum usually requires special conditioning treatments to remove the natural oxides and alloying surface elements prior to plating. The most critical steps in plating on aluminum alloys are those relating to surface preparation.
A typical sequence consists of oil, grease, and buffing compound removal followed by removal of the surface oxide layer. The oxide layer is heavier and chemically more resistant on heat treated alloys. The choice of acid deoxidizers versus alkaline etchants earlier in the cycle may depend on the extent of this oxide layer. Alkaline etching following degreasing is often used to remove extrusion or machining lines and other surface imperfections. Depending on the alloy composition, smut develops on the surface after etching. The choice of desmutter is based on the type of aluminum alloy processed.
A thin layer of oxide redevelops on the surface soon after desmutting and will affect adhesion when plated. This oxide must be replaced by thin metallic coatings applied by immersion. Single or double zincates and stannates are typical of such coatings.
Recent developments include modified alloy zincates which incorporate small amounts of nickel, iron, or copper in addition to zinc. These modified alloy zincates are more dilute and less viscous than conventional zincates. They produce fine, dense crystalline alloy zincate layers that typically improve adhesion of subsequent coatings.
A bronze strike is applied to a stannate treated surface. Whereas either a copper strike, electroless nickel, or a sulfamate nickel strike is applied following zincate. After striking, normal electroplating procedures may be followed. SAE Standard J-207 specifies types and thicknesses of copper, nickel and chromium coatings on aluminum. ISO 1456 includes specifications for decorative nickel-chromium deposits on aluminum and aluminum alloys.
Plastics and some other non-conductors can be finished by electroplating operations once the article is made conductive. The electroplate type can be selected as the user desires.
Among the plastic substrates which have been successfully electoplated in production are acrylonitrile butadiene styrene (ABS), polyphenylene oxide, polypropylene, polysulfone and nylon. The substrate most widely used today is ABS. The plastics mentioned all require different preparatory techniques.
The "preplate" operations prepare the plastic surface for electroplating. Virtually all the techniques involve the use of either electroless copper or electroless nickel to provide a conductive layer for subsequent electroplating. The techniques used are mostly proprietary and complicated enough to preclude discussion in detail here.
The electroplated layers on metallized plastic substrates are essentially the same as plated on metal substrates. While plastic substrates do not corrode in the same manner as metal, the galvanic relationships of the electroplated layers in the coating is the same whether the substrate is metal or plastic. Therefore, when an article is exposed to corrosive atmospheres, the electroplate structure is selected to withstand corrosion penetration.
When plating plastics there is one layer that must be given special consideration; that is the copper electroplate. Copper plating serves two primary functions:
1) It effectively levels the etched, micro-roughened surface.
2) Its ductility serves as a cushion to absorb the temperature induced movement between the plastic and the coating.
Generally, 12 to 25 micrometers (0.0005 to 0.001 inch) of copper is deposited. The copper electroplating bath must provide a levelled and ductile deposit. ASTM Standard B-554 covers measurements of thickness of metallic coatings on non-metallic substrates.
Decorative applications for copper/nickel/chromium finishes on plastics include: small appliance trim, automotive interior trim, automotive grilles and light bezels, plumbing faucet trim, knobs and shower heads, marine hardware, camera parts, candlestick holders, fashion trim on handbags and shoes and nameplates. ASTM Standard B-604 covers the specification for "Decorative Electroplated Coatings of Copper/Nickel/ Chromium on Plastic" as does ASTM Standard B-727 for "Preparation of Plastic Materials for Electroplating".
Austenitic and ferritic stainless steels can be electroplated with decorative nickel and chromium to provide improved appearance and corrosion protection. Surface oils and buffing compounds are removed by hot alkaline soaks and electrocleaners similar to those used for mild steels. An anodically electrified acid is used to remove surface oxides and in conjunction with a Woods nickel strike provides acceptable adhesion. ASTM Standard B-571 describes typical adhesion tests that must be met.
The subsequent bright nickel layer is of sufficient thickness to provide the desired appearance. Microdiscontinuous chromium is required for severe service conditions to prevent the formation of large pits during corrosion.
Decorative applications for nickel/chromium finishes on stainless steels include: automotive window moldings, bumpers, light bezels, body side moldings a well as marine hardware and hospital supplies.