Dissimilar metal unions (galvanic corrosion) and vibration between tightly joined components (fretting) can also cause corrosion. The effects range from catastrophic failure of studs and nuts on compressors to seizure of door-lock components.

Xylan coatings, particularly the formulations made with PTFE, offer a simple solution to the problem. Xylan is an excellent corrosion barrier, even if applied in a thin film. Most formulations form functional films at about 25 microns/0.001 in. However, there may be microscopic pin holes in the coating.

For even better protection, the coatings can be applied in two thin layers so that pin holes in one layer are covered by the second layer.

In ASTM B-117 salt fog tests, Xylan coatings provide a minimum of 100 hours of protection. Use of special primers increases these values to thousands of hours. Xylan coatings form excellent barriers to acids, bases and bromine.

Even if corrosives eventually penetrate the coating and attack the substrate, little or no underburrowing occurs, so the parts may still be easily disassembled for refurbishing.

This is particularly important for process equipment in extremely corrosive atmospheres, such as chemical mixers, pumps and marine equipment. In spite of their appearance, heavily corroded fasteners remain functional if they are coated prior to placing them in service.

When ferrous, aluminum or even galvanized parts are to be exposed to oxidizing fluids or fumes, Xylan coatings can help protect them.

If corrosion is the dominant failure mode, choose a coating that offers the best protection from the specific environment. If the problem is a combination of corrosion and wear, a good choice would be a coating which performs well in the presence of corrosive elements and has a low K value (wear rate).

For example, if corrosion is compounded by fretting (as found on compressor housings or other components subject to cyclic stresses), a hard, wear-resistant coating is the best choice.

Fastener class coatings
One of the greatest contributions made by fluoropolymer coatings is increased resistance to corrosion. Xylan coatings designed for fasteners and other small parts have improved corrosion resistance by a factor of five.

As the petrochemical industry developed, it began to demand better corrosion protection. Then came the automotive industry. But they wanted corrosion protection and low friction.

Specific formulations of Xylan were developed to combat the severe corrosion that affects the massive studs and nuts on oil drilling rigs and petrochemical processing equipment, as well as other items associated with the Chemical Processing Industry (CPI).

These coatings also permit the use of less expensive (and stronger) metals in place of stainless steel and other more exotic and costly materials. They are applied by conventional spray and, when fully cured, resist both corrosion and mechanical damage. Note: If multiple coats are to be used, oversize nuts may be required.

The problem with automotive fasteners was somewhat different. The typical automobile uses about 2,000 small nuts and bolts on trim, accessories, brake components and engine subassemblies. Pressured by more and more consumers complaining that their new cars were showing severe rust, automakers began a search for a better way to protect fasteners.

Previously, small fasteners were plated with cadmium. Corrosion resistance was about 96 hours as measured in a salt fog cabinet (ASTM B-117). Unfortunately, cadmium has serious environmental side effects and has been severely regulated or banned in many countries. Also, because cadmium is applied in a plating bath, the fasteners are exposed to hydrogen and are thus subject to hydrogen embrittlement.

Cadmium's replacement, zinc plating, only provides 40 to 50 hours of salt fog protection. The auto industry's search for better fastener protection led to a new set of standards of coating performance. One of the first was issued by General Motors (GM 6046-M). It calls for a coating that provides at least 336 hours of salt fog protection on self-drilling and self-tapping screws — after the screws have been driven through and removed from sheet metal panels.

In 1982, a new Xylan Series (5000) was introduced to meet this standard. These coatings and their derivatives can be applied economically via the dip/spin method.

Testing this material, automakers and other users of threaded fasteners found that salt fog resistance increased to about 500 hours, more than 5 times the previous "best," with no danger of hydrogen embrittlement.

Another advantage of the 5000 Series is that the torque required to preload coated fasteners is more uniform than that for other fastener finishes. In the idiom of the industry, "torque scatter" is narrowed, meaning that preloads on fastened joints tend to be more uniform and the joints more secure.

Since the coatings permit the fastenings to be tighter, backout, or loosening from vibration, is greatly reduced, if not eliminated.