Considerations
When tool designs require components to be fully encapsulated within an anodic coating to prevent galvanic corrosion issues can arise with coating thickness, evenness, and total item dimensional tolerance. Typically, most electro-coatings will result in an item with a total tolerance of 0.005” – ten times the typical tolerance of 0.0005”. When a tighter tolerance is required, it is possible to grind, hone, or otherwise finish the item to size. However, such activity should not be assumed to be practical.
Such processing likely requires new abrasives to be used to prevent the contamination of the item with ferrous material from the abrasives. Changing abrasives on machinery like a centerless grinder can easily take more than 1 hour and cost the wheel itself can cost thousands of dollars. (Thus, driving up the costs.)
Additionally, the materials used to encapsulate the component need to be managed in the removal process. Chrome and Chromium plating are heavy metals that can easily build up in the human body and will, at minimum, need to be treated as hazardous waste. Chromium is a PFAS and its’ use requires managed reporting to the EPA. Trivalent Zinc, while least toxic, may still be of concern.
Stainless Steel (CRES) components would appear to be a suitable solution in most applications. However, mating Stainless Steel parts, even those that have been heat treated to 50+ Rc can easily gall.
Best Practices
The following best practices should be kept in mind while designing tools and managing Galvanic Corrosion.
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Choose a coating that is high on the Anodic Index. Such a coating will act sacrificially and protect the underlying substrate material even when some plating is lost. Zinc is a highly recommended choice for pins because the loss of coating, due to wear, may not result in corrosion. The Zinc plating layer is typically far more reactive than the substrate resulting in the Zinc plating acting as a sacrificial layer. Zinc plating, in most cases, will corrode significantly prior to any corrosion occurring on the substrate. This is why all Carr Lane Mfg Steel Ball Lock Pins are Zinc plated.
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To reduce or eliminate the need for grinding, honing, or finishing locating components to size, after plating, use Stainless Steel (CRES) Bushings with Zinc Plated pins. The pins will experience plating loss, but as discussed above, the Zinc coating will offer continued protection. Many bushings, especially common locating bushings such as Slotted Locator Bushings, are offered as standard in Stainless Steels (CRES) shortening lead time and controlling costs. It is also typically much easier to service, maintain, and replace pins rather than bushings.
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Avoid specifying Hexavalent Chromium and heavy metals, such as Chrome, when grinding, honing, or finishing to a tight tolerance is required due to environmental and employee health concerns.
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When specifying Zinc plating always specify Trivalent processing. Zinc Chromium has typically referred to a Zinc plating that has undergone a Hexavalent conversion process to make it harder and more corrosion resistant. Like Hexavalent Chromium coatings, such a conversion processes on Zinc plating is not only bad for the environment, and is generally banned in the EU, but is also very harmful to humans and are is likely carcinogen.
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Avoid Cadmium plating whenever possible for even more toxic impacts to human health and the environment than Hexavalent Chromium coatings. Cadmium is a known carcinogen and should never be ground or worked after plating without extensive Personal Protective Equipment.