ASTM B 633 and Trivalent Chromate

[dgallup]

I currently specify ASTM B633 Fe/Zn 8 SC 2 with type II finish on a number of components. Out suppliers have been using hexavalent chrome for years but with the tightening restrictions on hexavalent chrome they say I must change my spec. I don't see anywhere in B633 where it says that the chrome must be hexavalent. Can trivalent chrome satisfy the ASTM B633 requirement? Is there a different ASTM that I should look at?

 

[TVP]

There are no ASTM standards that specify trivalent chromates over zinc plating. The existing standard was created for specifying hexavalent chromates (colored chromates), but that does not necessarily preclude the use of trivalent chromates. The requirement for Type II is no corrosion products (red or white) after subjecting the coated part(s) to 96 hours of salt spray testing according to ASTM B 117. Most clear, trivalent chromates do not meet the requirement of no white rust when subjected to 96 hours of salt spray, but some of the better ones do. Many plating companies will plate parts with zinc, apply a trivalent chromate, and then apply a seal (silicate, wax, or acrylate) layer in order to meet the 96 hour corrosion test requirement.