Zinc Plating "Service Condition" (SC), Thicknesses, & Corrosion Resistance

A. Hi, FG. Wheel bolts can be made from aluminum, titanium, electroplated steel, and perhaps other materials). Steel bolts can be zinc plated, zinc alloy plated, chrome plated, etc. If you are replacing them it is obviously important to double check that they were zinc plated if you are seeking the same look.

Zinc plating is covered by ASTM B633, some aspects of which are very rigid, some aspects rather loose. In one way "Service condition" (SC) is a rigid requirement (greater plating thicknesses are required as you go from SC1 (mild) to SC4 (very severe), and meeting those minimum thicknesses is not optional. But in another way, the spec is rather loose (because which SC an application falls into is handled with a few examples rather than an exact SC for every possible application).

But with the bloviating out of the way, automobile wheels are an extremely corrosion-prone application, and doubtless SC4 :-)
Luck & Regards,

A. Hello Fred!

I work in a company which zinc plates and chromates bolts, screws, nuts and other fixation systems for the construction, automobile and electronic appliance industries. We work with these and other specs (these are kind of old, but some of our clients ask for them) and I think I can answer some of your questions.

1) Yes, it should resist more than 6 hours in most cases. If this spec existed, you would NEED the parts to resist at least 6 hours without white corrosion. You could use a cheap conversion coating and comply with all other requirements.

2) It should resist more than 72 hours, could be 96. This spec does not exist, so we can only extrapolate!

3) Without any conversion coating, there is no white corrosion resistance. Zinc will corrode almost immediately.

4) That standard does not include a method to evaluate corrosion resistance measurement for type I, only defines conditions for the usage of the part to be plated. "Severe" conditions are the recommended for an SC3, and "Extreme" for an SC4 (25 microns). Many platers manage with both standards making SC2 III equivalent to Fe/Zn 8C using only the higher spec (120 hours).

5) Generally, yes. Clear passivation with trivalent chromates is superior than hexavalent. This does not apply for yellow! But... you can lower costs and get some (cheap) trivalent products that are the same as hexavalent.

6) Yes, you delay the appearance of white corrosion, and the red corrosion appears later.

7) It depends on the process of chromating. If the process is well performed, with a good product, I think you can achieve 96 hours without red rust with only 5 microns. Less than 4 microns, passivation of zinc is not homogeneous for trivalent (in my experience with bolts and screws).

Regards!

A. Hi Arash. I don't know what kind of steel hangers you are referring to, but ASTM B633 is a standard for zinc plating of steel items. I don't know of any reason that it wouldn't be applicable for the hangers you are thinking of.

I don't think B633 actually "defines" service conditions; but

1). it specifies the minimum plating thickness for them: Very Severe = 25 µ; Severe = 13µ; Moderate = 8µ; Mild = 5 µ; and

2). it gives typical examples. I don't have a copy handy, but the MFSA Quality Metal Finishing Guide (avail. from www.nasf.org) offers these examples.

Regards,