Joe Bender-Zanoni wrote:
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> Chuck,
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> I can't post to the list but I know a bit about this so if you could copy the list.
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> The issue with chroming works like this:
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> 1) The plating thickness (attraction of chromium ions) depends on the strength of the electric field.
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> 2) The electric field varies with the part geometry.
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> 3) To understand the geometry issues- at the extreme, the electric field INSIDE a sphere can be proven to be zero. That means if lightning hits your elatively spherelike car (or aircraft more likely), usually it is not a big problem. In the same way but to a lesser degree, an inside radius like a seat lug gets poor plating because of a weaker field. An even bigger problem on a bike frame is between the BB shell and the brake bridge. Notice the rust on most chromed frames in this nearly enclosed area.
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> 4) Sharp edges have the opposite problem, strong electric fields and excess chrome buildup. The extreme is a metal "whisker" or burr. After plating it can be ten times larger than the initial feature. Invisible sharp edged defects can lead to visible defects after plating. For this reason, industrial parts use electropolishing, an etching process which selectively attacks the high electric field areas to remove material prior to plating.
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> The discussion up to now relates to the chrome thickness. The very same areas are hard to polish. Surfaces to be chromed must be highly polished because, guess what, any defects in the surface cause deviations in electric fields and magnify the defects after plating. So that explains the "frostiness".
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> Joe Bender-Zanoni
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> Former engineer responsible for GAU-8 gun barrels as used on the A-10 aircraft, highest quality and most demanding chrome plating application I know of.