While sticking is not a major problem with bolts and nuts, aside from the occasional flat-to-flat contact, it is a headache for manufacturers of washers and other flat fastening devices. Few problems and almost no rejects were ever seen when standard zinc and cadmium platings were specified. Today, the need for increased corrosion resistance has lead the industry to the new dip/spin metallic coatings, bringing with them a whole new set of difficulties. Whereas the electroplated parts tumbled and were evenly coated, the metallic paints, like all paints, make excellent glue when baked. Washers come out in huge "chunks," bonded securely in masses best used as boat anchors and doorstops. When they can be broken apart, the presence of large, uncoated "touch" spots render them useless to the customer.
The number of solutions is limited. Mechanical plating has always been one solution. Its use for thin, high hardness parts that nestle and flat parts (washers) makes this solution an acceptable answer if the user can be satisfied with the negatives. See my article in Automotive Finishing, winter 1998, or on AF's website for a discussion of the pros and cons of mechanical platings.
A second solution is to rack plate the parts. In this way the surfaces, sides and edges are covered, except for the hanger spot. The drawback to this idea is that it is very expensive. Labor to rack the parts, processing on almost an individual basis, and hand unloading, add up to unacceptable cost. Automotive fasteners exist in a world of mammoth volumes and marginal cost factors, making bulk processing mandatory. Unless there is overpowering need for the washer to be finished with a spray coating, fully covering the entire part, it is not going to be in an automotive selection chart. Rack coatings have been successfully used in other automotive components, where lower volumes and larger sizes balance the cost effectively against total area and amount of coated surface or where geometry necessitates spraying.
Electroplating still appears to offer the best chances for success. The requirements of higher corrosion than one achieved with the old standard zincs coupled with ease of application, availability of processors and proven technology lead one to look at zinc alloy plating. It answers all the requirements. A drop shelf is used on the bake oven conveyor surface and/or shaker loader to separate the final chromate dipped parts. This has lead many washer users to ask for this coating on flat parts.
The final solution that is taken by many automotive designers is to delete the use of a washer entirely. The use of bolt and washer assemblies prevents the accidental omission of the washer by the plants and is generally a labor and cost savings when the part is brought in that way to begin with. In those remaining cases where the washer is a must, it is used with one of the old standard finishes, even though it means that there will be corrosion starting before the point that the designer requires. "That is the best that we can do!" is the retort given.
Any answers from the readership? Stainless parts do not effectively work out because of galvanic situations, the softness of the metal or even color requirements (black is preferred in many applications). The same for non-ferrous washers.