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
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.
HOW IS THE ADHESION OF PLATED LAYERS ON PRODUCTS MEASURED?
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.