Summary for white rust over zinc coated bolts Discussion
White rust over zinc coated bolts is the core issue in this thread: Subhro Naskar says automotive body bolts cannot be CED coated because the nut thread will not fit afterward, so zinc coating is being used instead, but the coating still develops white rust after some time. Jeffrey Holmes, CEF, says there is not enough information to give a precise fix because the plating and chromating specification and the achieved salt spray hours are unknown. His advice is to compare the actual salt spray performance with what the specification should deliver to decide whether the problem is poor plating/chromating execution or simply an inadequate specification. If the coating is underperforming, the plating line or plater should be corrected or changed. If it meets expectations, then a more corrosion-resistant alloy zinc plating or greater plating thickness may be needed, though Holmes notes that time to white rust depends mainly on chromating rather than thickness.
Q. We have some bolts used in final fitment of automotive bodies, which we can’t CED
⇦ huh? 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 19, 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,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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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, CEF
Spartanburg, South Carolina