Galvanised bolts have yellow shade over half the surface

A. Hi Chem.

Can you expand upon your situation? Are you the galvanizing shop, or the final end user, or are these bolts a component you purchased for a product that you sell? If you are not the galvanizing shop, how do you know for sure that the bolts were hot dip galvanized?

Galvanizing is not a "decorative" finish, so (to my knowledge) you will not find widely-accepted appearance specifications covering this yellow discoloration. But there is nothing that prevents any purchaser from writing their own appearance specification. Please start by telling us who you are. Thanks!

Regards,

A. The yellowish color is not uncommon to see on galvanized bolts. It is not desirable, and should be controlled by the galvanizer. There are a couple of things that come to mind that can cause this. Threaded products are usually run in steel baskets in the galvanizing kettle and then the basket is spun to create enough centrifugal force to sling off the excess zinc from the threads. These steel baskets get "dirty" after a while as they go through the process multiple times, and have to be stripped of the "dirt" (zinc build-up which traps kettle skimmings). If this is not done, then a yellowish, powdery finish can coat part or all of the product. Some of this can be washed off when the product is quenched in water right after galvanizing. This brings up the second way. The quench water needs to be cleaned or replaced to remove particles of zinc, zinc oxide, etc. that is carried over from the galvanized product.

Either way, you can usually clean off the yellow deposit with a wire brush. A good galvanizer will have procedures in place that should stop this before it becomes a problem.

Insightful answer, David. Thanks!

Regards,

A. Perhaps you are using chromic acid (CrO3) rather than sodium dichromate to quench the bolts. Chromic acid as the source of hex chrome quench typically gives bad coloration to the product including yellow and dark green coloration.

Regards,

A. Sir:

Perhaps your aluminum content in the zinc is too low. Under about 0.005% aluminum will give a golden color to the galvanized product.

Regards,