For over sixty-five years Advanced Plating Technologies has been providing engineered solutions in metal finishing for our customers. Regardless of the industry, finish or application the principle topics surrounding the art and science of metal finishing remain constant. Provided below are eight finishing topics that are central to the metal finishing industry. Select on any of the questions to learn more regarding the specific finishing topic. We hope you find this information a useful resource and value any feedback you may have.
All metals, with the exception of the precious metals, will oxidize when exposed to oxygen and an electrolyte (i.e. atmospheric moisture). It is a chemical reaction of the metal surface with the oxygen present in the air that causes some of the metal to corrode (or oxidize) and form the respective metal oxide on the surface. In some metals such as steel, the corrosion products formed are very visible and loose. Everyone has observed the red color of iron oxide (rust) seen on improperly protected steel products. The red rust formed is generally scaly and loose and easily falls away exposing more and more basis material to the environment. However, metals such as stainless steel (steel with added nickel and chromium) oxidize as well. The difference is that the nickel and chromium oxides formed are a more uniform and tenacious oxide layer that protects the underlying material by “sealing” the surface from further oxidation once formed.
In addition to the surface oxidation that occurs on individual metals, any two dissimilar metals placed in contact with one another with an electrolyte (such as atmospheric moisture or water) will form a corrosion cell. This is the very basis of batteries used in everyday products. One of the two metals in contact will corrode in preference to the other and form that metal’s respective oxide. Which metal corrodes is based on what chemists call the electromotive series of metals. The selection of what plated layer to use is an important one since electroplating in its very essence is the process of placing two dissimilar metals in contact with one another. The plated layer (or layers) can either be an anodic coating (coating corrodes in preference to the substrate) or cathodic coating (substrate corrodes in preference to the plated layer). Whether a coating is an anodic or cathodic coating greatly impacts how the finishing system will perform once in service and there are advantages and disadvantages to each.
Figure F.1 shows a cross section of a two-layer (duplex) nickel plated product as an example of a cathodic coating. The corrosion occurs between the two dissimilar nickel layers (bright nickel and semibright nickel) which forces the corrosion to propagate laterally. The use of two layers of dissimilar nickel helps prevent the galvanic attack from occurring between the basis steel and the plated layer.
To stop the oxidation of substrates would seem to be a simple matter. However, several finishing decisions must be made.
Coatings can prevent substrate oxidation by protecting anodically, as in the case of zinc or cadmium over steel. These “anodic” coatings corrode in preference to the substrate. The oxidation of the deposit often leaves a white chalky film on the surface of the part.
Coatings can also prevent substrate oxidation by encapsulating the basis metal and sealing it from the environment. This method is typical of nickel plating on steel and is referred to as a “cathodic” coating. For this method to be successful the deposit must be pore-free. Holes in the deposit become avenues for the entrance of oxygen and water to reach the underlying steel and start the corrosion process.
The effectiveness of either coating type is highly dependent on the surface texture over which it is laid. If the substrate is pitted, torn, cratered or otherwise discontinuous, the coating will have to be much thicker to effectively cover these substrate flaws than for a coating which will be deposited over a smooth surface morphology. Typical examples of problematic surfaces are those with cut threads and those that are stamped or sawn.
“Rule of Thumb”:
Minimum rust protection starts at about 0.0003″. Those deposits with less will rust fairly quickly, especially in moist environments. If you have an especially poor base, a thickness of greater than 0.0005″ may be required. The use of multi-layer systems such a copper underplate prior to a nickel or tin topcoat, will reduce the overall porosity and enhance corrosion performance. In addition, selection of a more corrosion resistant topcoat such as high phosphorus electroless nickel plating services, can improve corrosion performance.
Coatings such as electrolytic nickel, electroless nickel or tin can provide excellent corrosion resistance but this only occurs if the deposit thickness is sufficient to develop a pore-free surface on a steel substrate. This is due to the fact that nickel or tin protect steel by “sealing” the surface from the atmosphere. This method of corrosion protection is due to the fact that the nickel or tin are less reactive (more noble) than the steel basis material – referred to as a cathodic coating. This is the opposite compared to zinc on steel. Zinc is an extremely reactive (less noble) metal when compared to steel – referred to as an anodic coating. As such, the zinc will corrode in preference to the steel even if the surface is not pore free such as in thin coatings of zinc on steel. This is why white rust (zinc oxide) is seen prior to red rust (iron oxide) on zinc plated fasteners. The zinc sacrifices itself to protect the steel until the zinc is consumed near the pore and then red rust begins.
The amount of nickel or tin that is required to achieve a certain level of corrosion protection is very much a function of the surface finish (smoothness, burrs, pits, etc..) of the substrate. A smooth surface will require less nickel or tin to achieve good corrosion protection as compared to a rough, pitted surface with edge burrs. However, a good general guideline is that “good” corrosion protection begins around 0.0005 inches (20um) of nickel or tin and “excellent” corrosion protection occurs around 0.001 inches (50um) on reasonable substrates. This is very different from an anodic coating such as zinc where the surface condition will not play as large of a role in the overall corrosion performance. In addition, thinner coatings of zinc on steel can afford better corrosion resistance than tin or nickel since zinc coatings do not have to seal the surface to provide scarification corrosion protection.
All metals with the exception of the precious group (gold, palladium, platinum, rhodium) are subject to tarnishing. The tarnishing is typically seen as a darkening or discoloration on the deposit. The occurrence of the tarnish is exacerbated by:
- Choice of Packaging Material (sulfur bearing paper products can greatly accelerate tarnishing, especially on silver products)
To minimize tarnishing, many products receive a post plate anti-tarnish conversion coating which will effectively seal off the plated surface from the precursors of oxidation. Typical of these coatings are chromates, triazoles, clear powder coatings and lacquers. Mechanical barriers are also used to minimize exposure to oxygen and moisture. Typical of these are desiccants and various types of protective wraps.
Advanced Plating Technologies can provide many options to minimize the tarnishing of a plated deposit including nitrogen bagging. In nitrogen bagging the oxygen that is responsible for forming metal oxides is removed by filling the bag with pure nitrogen. APT will guarantee protection from tarnish for your product for up to one year if nitrogen bagging is specified.
Passivation of stainless steel on the surface would appear a simple matter as it is essentially an acid pickling process. In reality proper passivation of stainless steel is one of the most complex “simple” finishes in metal finishing. Unfortunately it is also one of the most incorrectly specified finishes by product design engineers and improperly employed finishes by finishing job shops. Often these two factors result in stainless parts which may have been passivated but are not truly passive which can result in field failures.
The most important point to understand in passivation is that not all stainless steel grades can be passivated the same. In addition, simply stating “passivate” on an RFQ or print does not guarantee that the respective job shop will use the correct process for the specific material grade. To make matters worse, common specifications such as ASTM A967 cover all potential methods of passivation which basically means referencing the spec alone without a nitric or citric method callout is meaningless.
To properly passivate stainless steel the passivation method must specified based upon the basis grade of stainless and the heat treatment that the stainless receives. Several specifications such as ASTM A967, QQ-P-35 and ASTM A380 provide passivation guidelines along this manner but they are often just that – guidelines! Some higher quality grades of stainless such as 316SS passivate very readily; other grades of stainless, especially free machining grades such as 303SS or 416SS, can be extremely difficult to passivate – especially if a high surface luster must be maintained such as with centerless ground shafts.
The other key point of passivation is that validation through testing is critical to ensure the process was successful. Advanced Plating Technologies offers a number of test methods including copper sulfate, potassium ferricyanide, high humidity and salt spray testing to test, validate and certify that stainless parts are indeed passive and “stainless” after processing.
Advanced Plating Technologies has devoted a tremendous amount of research and development over our history in the proper methods and applications that are required to make the respective grades of stainless steel passive. We have a full range of room temperature and heated nitric and citric systems available both with or without inhibitors to proper passivate even the most difficult stainless steel grades. We welcome the opportunity to work with companies on problem parts that have failed in the field and in most cases can validate the problem and provide a process that will solve the issue. APT also offers a wide range of proprietary methods as well as exacting ultrasonic cleaning and rinsing systems available to “precision passivate” demanding products such as implantables used within the medical and dental industries.
Plating and Underplate Selection
Electroless nickel (EN) has several distinct advantages over electrolytic nickel deposits. EN is an amorphous alloy of nickel and phosphorous. The addition of phosphorous provides the deposit with more corrosion resistance, less magnetic properties and a low coefficient of friction. The application of post-plate heat treatment causes the formation of nickel phosphides at grain boundaries which hardens the deposit. Because the deposit doesn’t require the application of an external electrical reduction force (DC rectifier or power supply) to create the deposit, but uses a chemical reducing agent within the solution chemistry, the deposit is extremely uniform in thickness. The plate is uniform across diameters, across threads and in dead end holes. This property often eliminates the need for post-plate machining on critical dimensions.
Figure F.3 below shows two cross-sectional photos of two identical gears, one plated in electroless nickel and one plated in electrolytic nickel. The drastic improvement in uniformity of the deposit is very clearly seen. The top photos show the teeth of the gears, the bottom photos show the inner bore of the gear. Note that EN plates completely within the bore.
Figure F.3: Electroless Nickel Verses Electrolytic Nickel
In summary, the advantages of EN are:
1. superior corrosion resistance – especially in high phosphorus varieties
2. uniform deposit thickness (typical standard deviations are +/- 0.000015″)
3. variable magnetic properties
4. hard deposits (approximately 90% of hard chromium)
5. deposit lubricity
Bright tin is deposited from an acidic solution. The brightening system used is typically based on naphthalene compounds. These organic compounds codeposit with the tin. When the deposit is soldered, the temperature exceeds that required to oxidize the organic portion of the deposit and the codeposited brighteners burn. This is seen as a blackening or darkening on the surface of the deposit or solder joint. This charring prevents the formation of a proper solder joint.
To prevent this, the tin deposit must be devoid of codeposited organics. This is done in the plating systems which do not use any organic brightening systems. These processes are generically called “solderable” and plate out as a dull, matte finish, but they are highly solderable.
Nickel plating is an important electro deposition process for preserving steel, brass and other basis metals from corrosion. Plated as a bright deposit often combined with chromium, nickel is the most effective electroplated coating for preserving a decorative appearance for extended periods of time in corrosive environments. Nickel electro deposition is also popular for engineering applications as a non-decorative functional plate.
Bright nickel electroplated for decorative uses differs appreciably from non-decorative nickel deposits. The high sulfur content (>0.05%) of the bright deposit reduces its ductility and corrosion resistance. Bright deposits are typically deposited from the watts formulation (see below) with the addition of organic-sulfur brightening systems. Functional deposits are typically deposited from the watts bath without the addition of brighteners (commonly referred to as watts non-bright) or from sulfamate nickel chemistries (see below).
Brightness in nickel deposits is induced with organic-sulfur compounds that decompose at cathode surfaces forming very small particles of nickel sulfide which refine the grain size of the deposit at the cathode surface (plated part) by at least two orders of magnitude. The selection of brightener additions to the plating bath affects ductility, internal stress, electrical conductivity and corrosion resistance of the deposit, all in a negative manner. Nickel coatings stressed in tension reduce the fatigue strength of steel. Nickel deposits lose corrosion resistance, ductility and electrical conductivity as the amount of co-deposited sulfur or other impurities increases from the brightening system. The change of the above characteristics is a rather complex subject which is primarily dependent on the bath chemistry, but also to a lesser extent on the operational parameters of that chemistry. The following charts will provide general trends/properties of nickel as deposited from the three (3) traditional chemical systems: watts non-bright, watts bright (watts with organic brighteners) and sulfamate.
|Type of Bath||Resistivity, microhm-cm|
|Watts, Organic brightener||10.00|
|Type of Bath||Elongation %|
|Watts, Organic brightener||4-5|
|INTERNAL DEPOSIT TENSILE STRESS|
|Type of Bath||Stress, PSI|
|Watts, Organic brightener||30,000||0-60,000|
The corrosion resistance of nickel deposits is very complex and the object of much study. In general, as any impurity is added to the deposit, the nickel plate loses its ability to fight off corrosion. The most common deposit impurities are hydrogen, oxygen, carbon, sulfur, chloride and metallic impurities. Because the watts non-bright and sulfamate formulations do not co-deposit sulfur and carbon from a brightening system, they are more corrosion resistant.
In summary, for engineering applications where ductility and corrosion resistance of the deposit take precedence over a decorative finish, a watts non-bright or sulfamate formulation is the deposit of choice. If low internal stress is also a concern for parts that will be significantly deflected or bent, sulfamate nickel processes are superior. However, bear in mind that these deposits are not bright and as such do not have as high of an aesthetic appeal. The old adage “bright is right” does not always apply for engineered coatings!
Tin plating is provided in two general types of deposits, bright and matte. Both can be obtained from an alkaline or acidic bath. The acidic chemistries are most common today. The advantages of each type are as follows:
- aesthetic appeal
- corrosion protection
- electrical enhancement to substrate
- electrical enhancement to substrate
For applications where cosmetics, lubricity or the appearance of the deposit are critical, bright tin is generally preferred. However the additional of organic brighteners in the deposit can impede solderability or epoxy bonding. For applications where joining is a key design requirements, a matte tin deposit should be selected. Since the solder bond occurs with the substrate and not the actual tin coating, an underplate of a pure nickel underplate such as sulfamate or unbrightened watts deposit serves as an excellent base to a solderable tin deposit for the best possible solderable finish.
Electroplating is often used to produce a clean, pristine metal surface upon which to solder. Many of the electrodeposited metals are capable of being soldered upon. These include gold, silver, nickel, electroless nickel, cadmium, copper and tin. If the soldering is to be performed on the ultimate layer such as nickel, this layer must be kept devoid of oxidation and transient surface contaminants. Careful rinsing of the surface after plating and packaging of the parts to protect them from surface contamination is critical. Sealed nitrogen bagging serves this application very well to protect the plated surface.
Because it is difficult to keep the final deposit in a pristine state, the most common scheme to promote solderability is to deposit the surface upon which to solder as the penultimate layer and then top coat with a metal that will amalgamate into the solder (tin, gold or silver). The best known version of this is to copper plate or nickel plate a substrate and then apply a matte tin as the final coating. The matte tin should be devoid of any codeposited organics from the electrolyte. In addition, careful rinsing of the surface is critical to prevent organic surface contamination that can impede soldering.
In soldering of tin electrodeposits, the tin becomes part of the solder joint and the bond occurs between the solder and the deposit or base material beneath the tin. For gold or silver soldering the same principle applies – the gold or silver forms and amalgam with the solder in the solder joint. Since high additions of silver or gold in a solder joint can cause embrittlement, thin gold or silver topcoats are preferable in solderable applications. As with matte tin, a high purity soft gold (99.9% pure) or matte silver (99.9% pure) are preferred for solderable applications.
The use of precious metals (gold, silver, palladium) on electrical circuits makes use of the superior electrical properties of this group. These electrical and thermal properties, coupled with the innate corrosion resistance of the precious group, provide a combination that is unsurpassed in electrical design and requirements. In addition, since precious metals do not form oxides under normal conditions, the conductivity of the contact interface will remain constant over time from of an insulating oxide barrier. This is especially critical in low voltage and amperage applications of the telecommunication and interconnect industries.
No other deposits will perform as consistently or reliably as gold plating services and silver plating services in electronic circuits and power distribution networks. The deposits also have the added characteristics of inherent lubricity, solderability and thermal conductivity to further enhance the functionality of the contact system.
Plating Thickness and Uniformity
There are five (5) methods most commonly used for defining plating thicknesses. They are average, range, minimum, maximum and customer negotiated. The differences between each are very distinct. The definition of each is provided below:
An Average deposit thickness requirement is given as a single number i.e. “nickel plate 0.0002”.” An Average deposit thickness provides the target that the mean of multiple thickness readings should be within a 75uin window. In the example given this would be 0.0002” ± 0.000075” (200±75uin). However, it is at the plater’s discretion to choose the location where to measure the product unless the measuring location is indicated on the print. This is a very important distinction as the plating thickness on electroplated products can vary considerably from point to point. The thickness distribution is heavily dependent on part geometry. On products that are drawn out such as rods or pins there can be an extremely wide range of thickness. However, the thickness distribution can be quite minimal on spherical products such as ball bearings.
A Range deposit thickness requirement is given as a range of numbers i.e. “nickel plate 0.0001-0.0003”.” The range provided is the range within which the mean of the thickness measurements must lie. Similarly to Average thicknesses, it is at the plater’s discretion where to measure the product unless the measuring location is indicated on the print. If the range is ≤ 0.00015” (150uin) it is treated as a single average with the mid point of the range being the target average and the definition of average thickness above applies.
Note: A range specification does not imply that all readings collected on all articles at any location must be within the range. This is only designated by Minimum and Maximum thickness requirements defined below.
A Minimum deposit thickness requirement is identified with a single number with the word minimum i.e. “nickel plate 0.0002” minimum or min.” A Minimum thickness requirement is defined as all readings measured on the significant surfaces must be greater than the thickness indicated. In the example provided the thickness measured on any significant surface would have to be greater than 0.0002” (200uin). Significant surfaces are generally defined as any portion of a part that can be touched by a 0.75” diameter sphere. However, the manufacturer of a product can identify significant surfaces as required on the blueprint of the product.
Note: There is no upper limit of thickness that applies in this definition.
A Maximum deposit thickness requirement is given as a single number with the word maximum i.e. “nickel plate 0.0002” maximum or max.” A Maximum thickness requirement is defined as all readings measured on significant surfaces must be less than the thickness indicated. In the example provided the thickness measured on any significant surface would have to be less than 0.0002” (200uin). If a significant surface is not defined all readings shall be less than 0.0002” (200uin) on any area of the part that can be touched by a 0.75” diameter sphere.
Note: There is no lower limit of thickness that applies in this definition. Any measurable deposit thickness is acceptable.
Customer Negotiated Specifications
When a customer has a defined specification or sites commonly referenced specifications i.e. ASTM, MIL, AMS etc. they shall be followed unless both the specification and thickness requirement are provide on the print. In this case, the thickness otherwise specified on the print supersedes the spec referenced. For example, specification QQ-N-290 requires minimum plating thickness requirements based on the grade of the plating (i.e. in QQ-N-290 Grade G is 0.0002” Minimum). However, if a customer were to indicate on a print, “Nickel Plate per QQ-N-290 0.0001”-0.0003”,” this is understood as a range plating thickness requirement because the customer has otherwise specified the thickness desired without indicating the grade. If a customer desires the thicknesses defined within the spec, the specific grades should be cited, i.e. “Nickel Plate per QQ-N-290, Grade G.”
Customer-unique specifications can always be developed based on the wishes of our customers but they must be known upfront. Unique specifications developed by a customer will be referenced on the appropriate process routing instructions and will be inspected to accordingly. This document will be the ruling document as to significant surfaces, thickness interpretation and target thicknesses desired.
Electroplating is an electrochemical process wherein the metal deposited on a substrate is supplied through the bath chemicals, containing the metal of interest under the application of a direct electric voltage across the anodic and cathodic sides of the plating cell. The generated current through the bath is an ionic current flow, and the current in the external circuit is electronic. Both are direct currents. Direct current always seeks the path of least resistance from the anode (metal source) to the cathode (work piece). The geometric shape of the work piece can shorten the distance to the anode, and thus decrease the electrical resistance (the resistance of the plating solution is directly proportional to the linear distance between anode and cathode). The least resistant path will carry more current and thus deposit more metal. The classic example of this phenomenon is a sharply pointed object (i.e. a rod) with the ends pointing at the anodes. The ends will have dramatically more plating than the center of the piece.
Often the shape of a part will restrict the ionic movement over its surface and thus, metal deposition. Dead end holes limit effective solution exchange within the hole, that is they have very little plating if any within the hole.
Electrodeposits are notorious for their nonuniformity. Awareness of this property helps the design engineer to build his product such that functional surfaces will not receive diminished coating thickness. Reference the Design for Plating Guide available within the white papers section of the Technical Library for additional information on how part design affects electrolytic plating distribution.
Thickness of electrodeposited coatings varies from point to point on the surfaces of a product. The thickness is less in interior corners and holes. Such surfaces are normally exempt from the thickness requirement. If the full thickness is required on these surfaces, the electroplater will have to use special techniques that probably will increase the cost of the process.
Because of deposit thickness variability, it is necessary to identify the key operational area on the coated article where the deposit must function. These locations are defined as the functional significant surfaces. Functional significant surfaces are best defined as a portion of the surface of a coated article at which the coating is required to meet all of the requirements of the coating specification for that article; significant surfaces are usually those that are essential to the serviceability or function of the article, or that can be a source of corrosion products or tarnish films that interfere with the function or desirable appearance of the article; significant surfaces shall be indicated on the drawings of the parts or by the provision of suitably marked samples.
For applications in precious metals, it is critical to define the functional significant surfaces of the component where the application of the deposit is important to the part function/design. Often times, if surfaces that are functionally critical are clearly identified on the part print, the overall gold or silver usage on the product can be reduced. This can often result in significant cost savings vs. applications where no such distinction is made.
Adhesion is clearly paramount in both decorative and functional finishes. In situations where the adhesion of a plated deposit fails, the loss of adhesion is often blamed on the deposit itself. However, in most situations the proper adhesion of a plated deposit has little or nothing to do with the actual plated layer. The key to achieving good adhesion on any product is to ensure an active metal surface sufficiently void of oils, die releasing films, oxides, alloying inclusions and heat treat scale. Ensuring an active metal surface is the sole function of the pretreatment within a plating line. The pretreatment system on a plating line is composed of assorted and various alkaline presoaks, alkaline electro cleaners, acid pickles, deoxidizers, chemical descalers, ultrasonic cleaners, and activating strikes based upon the design and function of the plating line.
All of the pretreatment systems listed above have a finite life within a plating line based upon the level and severity of usage as a function of the plating load and condition of plated product. If any one of the critical pretreatment chemistries looses its effective strength due to age on the line, poor adhesion of the end deposit can occur. Shown below are two examples of a plated deposit that lost adhesion due to a pretreatment system that failed to remove two common adhesion killers. Figure F.4 shows a lead inclusion on the surface of a 360 leaded brass part that was not removed in the pretreatment system. The result was a failed deposit that can be seen directly over the inclusion:
Figure F.4: Loss of Adhesion due to Lead Inclusion at Surface
Figure F.5 shows the surface of a heat-treated iron product that was plated over. Evidenced directly below the failed deposit is residual heat treat scale that was not removed by the pretreatment system. Often excessive heat treat scales can only be fully removed from the surface of a part by mechanical methods such as blasting or grinding prior plating.
Figure F.5: Failed Adhesion of a Deposit due to Heat Treat Scale
Although proper maintenance and selection of pretreatment systems is the plater’s responsibility, there are several key areas that buyers can assist the plater to successfully process their product.
a. Provide the Exact Alloy of the Basis Material on all Paperwork including RFQs: The specific alloy of a product can make a world of difference to the plater. For example 260 brass has no lead whereas 360 brass can contain up to 5% lead. Both brasses are very common in usage but each requires a very distinct pretreatment system to ensure proper activation. It is critical to provide the specific alloy UNS to the plater to ensure the parts are pretreated accordingly.
b. Use less tenacious oils wherever applicable: Not all oils are created equal. Organic based oils derived from vegetable and animal sources are generally very easily removed from the surface of materials, whereas waxes and silicon based lubricates can be extremely difficult. The preference of oils used, in order of most preferred to least, is provided below. Whenever functionally and economically feasible, use a lubricant that is more “plater friendly”. Often extremely tenacious oils will require off-line degreasing to remove which can add considerable cost into the price of finishing a product.
- Animal/Vegetable Oils and Fats
- Light Mineral Oils/Water-soluble Coolants
- General Metalworking Lubricants
- Synthetic Oils
- Heavy Grease
- Buffing Compound
- Mold Release Compounds
- Silicon Bases Lubricants
c. Perform heat-treating in an inert environment (bright hardening): Although heat-treating in an inert environment is considerably more expensive that heat-treating in an atmospheric one, the additional cost of preparing the products to be plated can offset the savings. If blasting is required to remove very tenacious heat-treat scales, the cost the plate a product can be up to an entire order of magnitude more than if a scale-free product is received.
d. Use higher quality “plating-grade” materials: Similarly to “c” above, higher-grade materials such as plating-grade sheet product for stamping are by their very nature more expensive. This is because they are manufactured in such a way to ensure a surface condition that is devoid of metallic inclusions and other contaminants that can present extreme adhesion obstacles to platers. Although more dollars may have to be paid up front for quality raw products, the savings in finishing can very rapidly pay for the additional outlay.
The adhesion of an electrodeposited coating to its substrate is as important as its thickness and selection of the plated layer to the overall performance of the finishing system. The ability to properly test for adhesion of plated layers to a product is of utmost importance. Unfortunately, practical adhesion tests are generally qualitative and difficult to relate to the end application of a product. Although quantitative tests exist – i.e., tests that attempt to express the force necessary to separate the coating from the substrate in numerical terms – they are not suitable for routine use are generally reserved for research purposes.
Several common adhesion tests are provide below along with a basic description of how the test is performed. ASTM B571 is generally considered one of the better specifications for defining adhesion testing and many of the tests listed below are cited in ASTM B571. If the size and shape of the item to be tested does not permit the use of one of these tests, a test piece may be used. However, the test piece must be of the same material and preparation as the product and ideally of a similar configuration such that it can be plated along with the subject parts. In addition, if the plated product is very valuable, the use of test piece may be necessary.
Bend Test: Bend the part with the coated surface away, over a mandrel until its two legs are parallel; the diameter of the mandrel should be 4 times the thickness of the sample. Examine the deformed area under a low magnification (4X) for peeling or flaking of the coating from the substrate. If the coating fractures or blisters, a sharp blade may be used to attempt to lift off the coating. Brittle coatings may crack under this test, but cracks are not evidence of poor adhesion unless the coating can be peeled with a sharp instrument.
Burnishing Test: Rub a coated area, about 5 cm2, with a smooth-ended tool for about 15 seconds. The pressure imparted should be sufficient to burnish the coating but not to dig into it. Blisters, lifting, or peeling should not develop as a result of the burnishing. Note: This test is not suitable for thick coatings.
Chisel-knife Test: Use a sharp cold chisel to penetrate the coating, or at a coating-substrate interface exposed by sectioning the specimen. If it is possible to remove the deposit, the adhesion is not satisfactory. Note: This is not applicable to soft or thin coatings.
File Test: Saw off a piece of the coated specimen and inspect it for detachment at the deposit-substrate interface. Apply a coarse mill file across the sawed edge from the substrate toward the coating so as to raise it, using an angle of about 45 degrees to the coating surface. Note: This test is not suitable for soft or thin coatings.
Grind-saw Test: Hold the coated article against a rough emery wheel such that the wheel cuts roughly from the substrate toward the deposit. Often an aluminum oxide grinding wheel is used for this test. Note: This test is not suitable for thin or soft coatings.
Heat-quench Test: The article is heated in an oven to a temperature prescribed for the coating-substrate combination, then the specimen is quenched in water at room temperature. After quenching the test sample is inspected for any blistering or flaking. Note that the heating may actually improve adhesion by diffusion alloying, or it may form a brittle alloy layer at the interface. These effects limit the applicability of the test.
Impact Test: This test consists merely of hammering the specimen severely to see whether blistering or exfoliation occurs. The exact details of the impacts can be specified as desired by a customer.
Peel Test: A strip of steel or brass is bonded to the specimen by solder or a suitable adhesive; at an angle of 90 degrees the strip is pulled off the specimen. Failure at the deposit-substrate interface evidences poor adhesion.
Push Test: Drill a blind hole 7.5 mm in diameter from the underside of the specimen until the point of the drill tip comes within about 1.5 mm of the deposit-substrate interface on the opposite side. Support the specimen on a ring about 25 mm in diameter and apply steady pressure over the blind hole, using a hardened steel punch 6 mm in diameter, until a button sample is pushed out. Exfoliation or peeling of the coating in the button or crater area is evidence of poor adhesion. Note: This is not suitable for soft, thin or very ductile deposits.
Scribe-grind Test: Scribe two or more parallel lines or a rectangular grid pattern on the article using a hardened steel tool. The distance between the scribed lines should be about 10 times the nominal thickness of the coating, with a minimum of 0.4 mm. Cut through the coating to the substrate in a single stroke. If any portion of the coating between the lines breaks away from the substrate, adhesion is inadequate.
If not explicitly stated, in all of these tests peeling, flaking, blistering or exfoliation of the coating is evidence of poor adhesion.
Aesthetics and Appearance
The application of a bright finish, especially a bright “flaw-free” finish, has been a major challenge for the metal finishing industry for decades. Successful brilliant, reflective surfaces are the result of proper basis metal selection, preplate process(es), proper application of chemically leveling deposits used under controlled conditions, post plate buffing and continuous careful handling of the product. Figure F.6 below shows the ability of leveling deposits to reduce the surface roughness of a basis material. However, note that the plated layers are still not fully level. Without a very good starting surface, a flaw-free, mirror bright surface can not be achieved.
Figure F.6: Electrolytic Leveling Deposits
Often the purchaser of a bright finish feels that the electroplating bath is the sole agent responsible for the desired appearance. Actually the bath is most often the least responsible. Proper basis metal selection which is free of macro discontinuities such as scratches or machining lines is of primary importance. Substrates that are corroded, full of porosity (often found in castings), tooling marks or handling damage require extensive preplate polishing. Polishing can often remove basis metal defects, but it is expensive. Polishing that provides consistent surface finishes of Ra <1.0 are not found at all facilities.
Even with the best substrate selection, preplate polishing and bright, leveling bath, careless handling or improper packaging can easily damage a product, causing delivery delays and substantial scrap costs. Successful application of flawless bright finishes requires coordination between customer and the metal finishing vendor. Discussion of the customer’s needs and expectations before finishing begins is crucial to providing the correct sequence of finishing steps to meet the finished products requirements.
Baking of hardened steel products is a common step to relieve hydrogen embrittlement (see What is Hydrogen Embrittlement and How is it Prevented? ). A common product requirement for bright nickel plated products to retain their luster after baking. It is a common misconception that the nickel deposit darkens during baking at common hydrogen embrittlement temperatures of 375-400F. Bright nickel deposit can maintain their luster at these temperatures as they generally form a relatively passive oxide with good spectral properties.
However, any nickel deposit that is plated to a common (thin) commercial thickness of 0.0001-0.0003 inches is generally a porous deposit that can be thought of metalugically as a screen on the surface of the steel. During baking the steel itself is susceptible to oxidizing or slightly scaling with a darker oxide film. The ferrous scale has a tendency to propagate through the pores in the nickel deposit resulting in a loss of luster to the overall deposit. In order to mitigate this phenomena it is recommended that the nickel deposit be increased to a minimum thickness of 0.0004 inches in order to reduce the porosity of the deposit. It has been shown that this higher nickel thickness generally works to better “seal” the substrate steel and thus prevent darkening of the deposit during the baking process. Slight variations from this rule of thumb may be needed but as a generally design guideline this increase thickness provide acceptable results.
Many of the finishes offered by Advanced Plating Technologies have a high degree of cosmetics associated with them. Bright gold and lustrous nickel finishes such as bright electrolytic nickel or medium phosphorus electroless nickel are good examples of finishes that will have a higher deposit luster and more cosmetic appearance due to intrinsic depositional characteristics and the specific elements in the deposit.
Other finishes offered by Advanced Plating Technologies are functional finishes intended fur a specific engineering purpose. Examples of these finishes include matte tin, tin/lead, lead or sulfamate nickel finishes as well copper deposits. These finishes are intended for specific engineered purposes including soldering, brazing, babbitting, current carrying capacity or heat treat stop off. Due to the very nature of these deposits they can be susceptible to certain cosmetic shortcomings that are generally noted on the quotation if applicable.
Some examples of cosmetic disclaimers include:
Matte Deposits such as Matte Tin, Tin/Lead or Unbrightened Nickel such as Sulfamate Nickel:
These deposits are fully matte and can have a slightly mottled appearance due to the fact that they are plated free of any organic additives to control the appearance or grain size of the deposit. These deposits can also be very susceptible to water staining and in some cases finger printing due to the structure of the deposit. The very reason why these finishes function well for their intended purpose are why they may have the cosmetic shortcomings listed above. If alterations to the bath or process are made to improve cosmetics, the deposits will not perform as intended for the application.
Reactive Deposits such as Copper or Silver: Elements such as copper and silver can react with elements in the atmosphere including sulfur bearing compounds and oxygen. The reaction of the deposits with these elements is a thermodynamic fact that can not be altered. These reactions can cause issues such as surface staining or discoloration and tarnishing. The environment in which the parts stored can have a large impact on the reactions and timeframe in which the reactions can occur. There are steps that can be taken including tarnish inhibitors and sealed nitrogen packaging which can compensate for the reactivity of the deposit. If the appearance of reactive deposits is a concern, a representative of the technical sales group at APT can advise of inhibits or packaging methods that can help mitigate these effects.
Commercial deposits are very thin and are generally regarded as the most competitive thickness option for a given electro deposit. Typically commercial deposits are the thinnest functional deposit thickness for a given deposit which are greater than a flash deposit but are still not of enough thickness to impart significant corrosion protection for environments other than mild service environments. Common commercial thicknesses are typically in the range of 0.0001 to 0.0003 inches per side for non-precious metal deposits such as tin, nickel or copper.
Ferrous products (e.g. steel, ductile iron, cast iron) components plated with commercial thickness deposits that are cathodic to the iron substrate will have very minimal corrosion protection. Examples of cathodic coatings on steel include electrolytic nickel, electroless nickel, silver, gold, tin and copper. Reference, the Corrosion section of the Plating Topics within the APT Technical Library for additional information on cathodic verses anodic coatings and deposit thicknesses required to prevent corrosion and rusting. Since these coatings are cathodic to the surface, a thin commercial deposit will be insufficient to seal the steel from the atmosphere. This can result in visible rust or corrosion that occurs with even moderate exposure or service applications.
The environment in which commercially plated parts are stored can have a significant impact on if visible rust will occur on the surface. For example commercial deposits on ferrous products which are plated and inventoried in dry environments – such as a Midwest warehouse in January – will have very different results when plated and inventoried in the same warehouse during hot, humid summer months. Since Advanced Plating Technologies can not control the use or application of commercial deposits, a disclaimer is typically added to the quote warning of the corrosion limitations of the commercial deposit.
The technical sales group welcomes discussions regarding the corrosion requirements or service application of a component. Deposit recommendations beyond standard commercial thicknesses can be provided based upon the specific engineering function. Proprietary corrosion inhibitors or rust preventatives can also be offered in lieu of increased deposit thickness. Finally, custom packaging methods can be suggested utilizing volatile corrosion inhibitor (VCI) paper or bags as well as sealed nitrogen bag packaging methods to greatly extend the storage life of commercially plated components.
Gold and Silver Price Fluctuation
Gold and silver are commodities that are actively traded on the world markets. As such, they are subject to the market forces of supply and demand. This causes the price to change over time. Typically, in times of international insecurity (i.e. wars, changes of governments in world powers) the prices of gold and silver will increase as people purchase gold and silver to use as a trading medium in lieu of paper currency. Additionally, the price of gold and silver paid by industry is higher than the published bullion rate. This is because users of the metals must pay a fabrication and distribution charge, which is added to the bullion price.
As precious metals costs change, this is reflected in the price of the plated component that consumes the precious metal. This is typically changed on a monthly or bi-monthly basis (weekly or even daily in times of rapid change), and the base price of the gold and silver is printed on the bottom of the invoice. Advanced Plating Technologies bases gold and silver invoicing on the Engelhard Fabricated indexes which can be found on the Wall Street Journal commodities page.
All precious metal parts quoted by Advanced Plating Technologies are based upon the current Engelhard Fabricated metal pricing on the day of quote (see above link). APT does game precious metals quotes by quoting parts at a market gold price well below the current fair market price.
The specific market gold value is listed at the bottom of the quote for reference. Along with this information the gold or silver adder is provided. The adder is the amount that the price to plate the part will increase or decrease for every $0.25 change in market silver or $5.00 change in market gold.
Example 1: Gold Increases
Part 123 is quoted at $1.00/Ea with gold at $1500.00/toz and a $0.01 adder. If gold increases to $1550.00/toz the cost to process the part will be the quoted price plus 10 adders (there are 10, $5.00 increments or gold adders between $1500 and $1550). Thus the cost to process the part will be $1.00 + 10 * $0.01 = $1.10 at $1550.00 gold.
Example 2: Gold Decreases
Part 123 is quoted at $1.00/Ea with gold at $1500.00/toz and a $0.01 adder. If gold decreases to $1475.00/toz the cost to process the part will be the quoted price minus five adders (there are 5, $5.00 increments or gold adders between $1500 and $1475). Thus the cost to process the part will be $1.00 – 5 * $0.01 = $0.95 at $1475.00 gold.
Hydrogen Embrittlement and Baking
Hydrogen embrittlement results from the simultaneous codeposition of the primary metal and hydrogen on the surface of the work piece (cathode). The hydrogen is available from the water in aqueous plating bath chemistries and is also exposed at the surface of the work piece during the acid pickling steps of the pretreatment process. Because the hydrogen atom is much smaller in size than the metal of the deposit, it is able to migrate into the crystal lattice of the basis metal, and reside interstitially between the individual metal atoms. The interstitial hydrogen can greatly amplify the stress of applied forces within the basis metal which can produce catastrophic fracture at loadings much lower than the typical yield strength of the material. Hardened steels (> 40 Rc) are particularly susceptible to this phenomenon and as such are generally required to be baked after plating to protect against hydrogen embrittlement.
Hydrogen codeposition can occur in the plating process either during the actual electrolytic deposition or during the cleaning and acid pickles preceding the plating bath. As such, hydrogen embrittlement is a concern even in electroless plating processes. It is readily removed from the metal lattice by baking the product immediately after plating. The requirement for baking is a time-at-temperature cycle that is generally specified on the part print or within a plating specification. A typical cycle is to bake at 375ºF for 4 hours within 1 hour after plating.
Environmental Issues and Compliance
The Restriction of Hazardous Substances Directive (RoHS) 2002/95/EC was adopted in February 2003 by the European Union. The RoHS directive took effect on July 1, 2006. The directive restricts the use of six hazardous materials in the manufacture of various types of electronic and electrical equipment.
- Hexavalent Chromium
- PPB (Polybrominated Biphenyls)
- PBDE (Polybrominated Diphenyl Ether)
It is closely linked with the Waste Electrical and Electronic Equipment Directive (WEEE) 2002/96/EC which sets collection, recycling and recovery targets for electrical goods and is part of a legislative initiative to solve the problem of huge amounts of toxic e-waste or waste from any broken or unwanted electrical or electronic appliance. In casual conversation, it is often pronounced “rose”. The directive applies to equipment as defined by a section of the WEEE directive as:
- Large and small household appliances
- IT Equipment
- Telecommunications Equipment
- Consumer Equipment
- Lighting Equipment
- Electrical and Electronic Tools
- Toys, leisure and sports equipment
For a plated finish to meet the RoHS directive, it can not contain any of the six banned materials. Advanced Plating Technologies leads the industry in RoHS compliant solutions for products in nearly any industry. APT has taken the initiative to switch all in-house chemistries to RoHS complaint varieties where possible and can certify any of our compliant finishes to meet the RoHS directive if required.
Advanced Plating Technologies also offers a RoHS complaint alternative to hexavalent chromium plating with a tin/cobalt alloy plating process. Tin-cobalt is generally plated over a bright nickel underplating similarly to traditional chrome plating. Tin-cobalt has a similar “blue-bright” appearance of chrome but unlike hex chrome is fully RoHS compliant.
If you have a product that must be certified to RoHS requirements, Advanced Plating Techologies has a breath of finishing solutions to meet your needs. Our current line card details which finishes can be certified to meet RoHS compliance as well as the MIL and ASTM specifications that the finishes meet.
The European Union’s (“EU”) Regulation on Registration, Evaluation, Authorization and Restriction of Chemical Substances (“REACH” Regulation (EC) 1097/2006 Dec 18 2006) applies to a specific set of chemical substances, when they are a component of a product, or if the substance is intended to be released from an article under normal or reasonably foreseeable conditions of use. In nearly all cases, the actual metal finish on a component falls under the article exemptions of REACH in the that the deposit is not intended to be released from the article under normal use.
What does that mean in simple terms? The applied finish is generally an integral part of a product that is not released or separated from the product during normal use. A simple example of a component that is intended to be released would be ink from a ballpoint pen. It is part of the products normal function to release the ink from the pen itself. Conversely, the nickel on a nickel plated fastener is not intended to be released from the article under normal use and as such falls under the article exemption.
Advanced Plating Technologies can provide additional guidance as it relates to REACH and can help companies fulfill their obligations to their suppliers with this regulation. Contact our Sales Department with any additional questions regarding this recent EU directive.
Chromium metal as an electroplated deposit is a regulated categorical pollutant controlled under the Code of Federal Regulations 413 or 433. This applies to either the trivalent (Cr +3) or hexavalent (Cr +6) valence state. In addition, hexavalent chromium is one of the six hazardous materials under the current RoHS directive. Advanced Plating Technologies currently offers a tin/cobalt chrome alternative that is not regulated under any Federal regulation since neither tin or cobalt components are regulated and can be certified fully RoHS complaint.
Hexavalent chromium(+6) is the more severely controlled of the two valence states. Its introduction into drinking water supplies is regulated in the parts per billion range and as of 1 July 2006 it is prohibited from entry into the EU on coatings of products sold therein at levels greater than 1000 ppm (RoHS Directive). Total chromium either +3, +6 or the sum of both is regulated in End of Life Vehicle (ELV) regulations in the EU and similar regulations are in the draft stages within several US regulatory agencies.
Tin/Cobalt is free from any of the above issues as it is not chromium in either valence state or as the nascent state. Chromium is under growing pressure in the US due to reflective environmental concerns as addressed by RoHS. Its continued use will become increasingly difficult in any form. The application of green processes to goods and services has excellent value in the market place and within government particularly the Department of Defense (i.e. Joint Strike Fighter F-35 which is serving as a test bed for several new and green technologies). The tin/cobalt process avoids all of these existing and potential future manufacturing environmental concerns.