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For many assembly applications, fastener corrosion poses an overriding design concern. But the prospect of corrosion need not send chills down a design engineer’s spine. Coatings and plating can provide a barrier between two dissimilar materials, provide a protective barrier against outside elements, and enhance the cosmetic appearance of a product. The fastener application and performance determine which is most suitable. Usually coatings and plating are less expensive than going to an upgrade of material like stainless steel from a basic carbon steel.
Coatings provide multiple barriers to corrosion. Coatings are typically applied using the dip-spin process. A mesh basket filled with fasteners is immersed in the coating slurry. It is then removed and spun to eliminate excess coating. Afterward, the fasteners are put through a baking oven to cure the coating. This process is someArial repeated between two and four Arial to build up the required coating thickness.
Unfortunately, this application process creates a nonuniform coating thickness that can prove troublesome on threaded fasteners. For correct torque and thread fit, a consistent thickness is needed. Teflon can be added to the coating to create uniform and predictable friction characteristics.
Coated fasteners have a very high salt spray performance (between 400 and 1,000 hours). Coatings typically offer better corrosion protection than plating. They also offer consistent torque tension characteristics. However, coatings are not suitable for very small parts.
Typically, plating offers only a single barrier and is applied electrolytically. The fastener is the cathode in the electrolytic bath. The metal to be plated onto the fastener is the anode. As long as current flows, the plating will continue to build on itself. Thus, plating thickness is controlled by voltage applied and the length of time that the voltage is applied. Most plating is 5 to 8 microns thick.
Fastener Finishes to Prevent CorrosionFastener coatings and plating can be categorized into three major groups: phosphate with oil or wax; organic; and metallic (which are electroplated or mechanically applied).
Phosphate and oil finishes are bulk processed in perforated drums. The fasteners are cleaned in an acid solution. The zinc or manganese phosphate acts as a porous sponge to trap oil. Water-soluable oil or wax is applied to the surface. The phosphate keeps the oil or wax on the surface, which prevents rust. This finish is typically used for internal engine, transmission and nonvisible interior fastener applications. Substituting wax for oil reduces residue, making it useful for visible interior applications in cars.
However, phosphated finishes offer limited corrosion protection. The phosphate crystals tend to break down during handling, creating a greasy residue in feeder systems. They do offer consistent frictional properties.
Organic finishes describe a variety of nonmetallic coatings applied to fasteners using the dip-spin process. These coatings are widely used for suspension, external power train and underbody applications. They are not suitable for fasteners with recessed drives because they tend to fill the recesses.
E-coat is another organic finish. It is a paint-based coating that is applied by electrostatic deposition. This helps to create a uniform finish with consistent thickness. It generally comprises a phosphate layer for adhesion, and an oil or wax topcoat for corrosion protection. It offers moderate corrosion protection (150 hours) and is used primarily for interior and underhood applications.
Metallic or electroplated finishes are applied to fasteners electrolytically. Zinc with a chromate topcoat is the most popular metallic finish. Cadmium is also a good finish, but it has been phased out of the automotive industry for environmental reasons. Most metallic finishes use a chromate layer to improve salt-spray performance. High-performance chromate coatings offer corrosion performance equal to many organic coatings. Electroplated finishes have uniform and predictable thickness characteristics.
Zinc finishes have some limitations. In some cases, zinc-finished fasteners must be baked to expel hydrogen, which can compromise chromate performance. Friction is high, causing chatter during tightening and erratic clamp load. However, developments in wax-based coatings, which are also called torque tension fluid, have made zinc as good as organic coatings. But it is difficult to visually determine whether the coating is present.
Hot-dip galvanizing prevents corrosion by coating steel with zinc. The galvanized coating bonds with the underlying steel. This forms a barrier between the steel and the corrosive environment. The galvanized coating also preferentially corrodes to protect the steel. It is also able to protect small areas of steel that may become exposed when damaged.
Generally, product designers will try to conceal attachment hardware. If that can’t be done, the designer may take the opposite approach. The fastener may then become an aesthetic element of the product. In this case, a coating on the fastener’s head may contribute to the product’s appearance. It may also disguise the fastener’s head so that it blends with the mounting surface.
A coating may also be used as a temporary locking patch. It can be applied to a threaded or unthreaded fastener to hold it in place during automated assembly. This lets the assembler install the fasteners in components prior to final assembly.
Measuring and Testing FinishesEnsuring correct fastener finishing thickness is critical to the fastener’s performance in the overall assembly. The method used to measure a fastener finish differs among coatings and plating.
A magnetic or ultrasonic probe is often used to measure coating thickness. The probe measures the distance from the coating surface to the substrate. Everything in between is considered coating. Usually, an uncoated fastener is used as a standard.
To measure plating thickness, X-ray fluorescence or a magnetic probe tip can be used. With the X-ray method, the area to be tested is the target of radiation, and the energy emitted from that surface is measured. The X-rays are produced by an X-ray tube. The radiation measured is secondary emissions from the interaction of the X-rays with the plating and substrate. The emissions measured are specific for each metal.
Coating thickness can also be calculated based on thread fit differential. This measurement is based on measuring an uncoated fastener using a functional pitch diameter gauge. Generally, the effect of coating on a thread is to increase the pitch diameter by four Arial the coating thickness.
Plating and coatings are typically tested in a salt-spray chamber, or by using the Kesternich test. Salt-spray testing using sodium chloride was developed by the automotive industry for testing salt corrosion resistance on vehicles subjected to winter road salting. Salt-spray testing is a suitable quality control tool for ensuring that parts, such as fasteners, are coated correctly. A salt-spray chamber’s major weakness is that it offers a different corrosive environment than the real world. A fastener in a real environment will be exposed to hot and cold, daylight, nighttime and condensing humidity. All that will affect the rate of corrosion.
The Kesternich test was developed by the automotive industry in Germany for Volkswagen. This test method is a severe measure of corrosion resistance.
The components are prepared and placed in a cabinet called a Kesternich Test Cabinet. Two liters of distilled water are placed in the bottom of the cabinet. After the cabinet is sealed, sulfur dioxide is injected into the cabinet, and the internal temperature is set to 104 F for the cycle.
Each 24-hour cycle begins with 8 hours of exposure to the acidic bath in the cabinet. The cabinet is purged and opened, and the components are rinsed with distilled water and dried at room temperature for 16 hours. The test specimens are examined for red rust at the end of each cycle.
A fastener affected by hydrogen embrittlement can cause as much trouble as a corroded fastener. During the plating process, the fastener can absorb hydrogen. It gets trapped underneath the plating layer and migrates into the grain boundaries. When the fastener is stressed, those grain boundaries weaken. This can cause sudden catastrophic failure under load.
For this reason, plating is a high-risk surface finish for fasteners, unless they’re not going to be used in areas of high tensile load. Typically, parts are baked after electroplating to expel potential hydrogen.
Trends in Fastener FinishesHexavalent chromium-based compounds, found in all chromate coatings, are among the most efficient and cheapest corrosion inhibitors available. However, hexavalent chromium is a suspected carcinogen.
The automotive industry is one of the largest users of hexavalent chromium and is leading the effort to eliminate the material.
There are numerous hexavalent chromium-free finishes currently available. But the performance of each differs from the hexavalent chromium finishes that are currently in use. Typically, trivalent chromates are first-choice substitutions for hexavalent chromates.
Organic finishes that do not contain hexavalent chromium are available. They do offer corrosion resistance that is equal or superior to hexavalent chromates. But they are generally not suitable for parts smaller than 6 millimeters in diameter or for parts with recesses.
Another trend in the finishing industry has been to reduce emissions of volatile organic compounds (VOCs). Organic solvent-based coatings have been the coating of choice, because they require minimum surface preparation, dry quickly at ambient temperatures, offer easy color change and need little capital equipment for application.
However, these materials have coating-to-total-spray ratios of only 30 percent to 40 percent. This leads to large amounts of VOCs being emitted. There is always the potential for significant solid and hazardous waste, depending upon the coating compound being used.
VOC-reduction choices are often unclear. Such concerns as required finish quality, surface preparation, capital equipment requirements, operating costs, and possible tightening of emission limits are factored into the final decision.
Some possible choices include: eliminating emissions by selecting an alternative fabrication path; using contract coating vendors; capturing and recovering the organic coating for recycling or fuel; incinerating the problem hydrocarbons; changing to a lower emittive coating process; changing to water-based or ultraviolet-cured coatings; and switching to powder coatings.