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topic 60842

White rust over zinc coated bolts


September 19, 2018

Q. We have some bolts used in final fitment of automotive bodies, which we can't CED as it will not fit with nut thread after CED. So we are applying Zinc coating. But that zinc coating is also getting white rust after some time. How can I prevent or extend life of zinc coating from white rust?

Subhro Naskar
shop employee - India


September 2018

A. Hi Subhro, Unfortunately, we have insufficient information yet to suggest a course of action. We don't know the plating and chromating specification, and we don't know how many salt spray hours the bolts survive ... so we don't know if your request for longer life from the current specification is reasonable or not.

The generic answer is to compare the salt spray hours you actually obtain to what should be expected from such a specification in order to determine whether the problem is poorly executed plating/chromating or an inadequate specification for the needs. If the actual plating doesn't match reasonable expectations, fix the plating line or find another plater. If the salt spray resistance does meet expectations for that specification, then you must switch to an alloy zinc plating with greater corrosion resistance or at least increase the plating thickness (but time to white rust is primarily dependent on chromating rather than plating thickness). Good luck!

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


September 19, 2018

A. Are you applying a yellow chromate after zinc? If not, then do so. If that's not good enough, you could add a final dip in a water or solvent based rust preventive.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina



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