Letter 8010

Can a topcoat substitute for Zinc iron Plating? 

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A customer of ours wants to use Alkaline Zinc plating instead of Zinc Iron plating with silver free black chromate as they have been advised by an alkaline zinc supplier that alkaline zinc used with silver containing black passivation & a topcoat, passes a salt spray test just as well as regular zinc iron and silver free black can perform a Salt spray test.

If this were true, why would people go to the trouble of developing and mantaining costlier Zinc alloy baths in the first place ? May I say that we are trying to meet a VOLVO spec here.

How differently would the zinc iron alloy plating behave in the real world viv a vis an alkaline Zinc plated part which passed a Salt spray test ?

Khozema Vahanwala Automaxx   Bangalore, Karnataka, India

My experience, which is not all inclusive, indicates that the reason people prefer zinc-iron plating for black chromate is color stability rather than salt spray resistance. Those silver-based black chromates used on plain zinc turn different colors on you whereas my limited experience is that the black chromates on zinc-iron stay black.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

Khozema,

My company currently uses both platings: alkaline zinc and alkaline zinc-iron. Zinc-Iron definitely will last longer in salt spray that plain zinc. I have done side-by-side salt spray tests on these platings and have witnessed a significant difference. Zinc-Iron will also be the easiest plating to use if you need a good black color. However, zinc-iron is more expensive than plain zinc.

You must find out how long the part needs to last in salt spray and/or real life. If the specification for your part is only 150 hours or so, then plain zinc is all you need. You do not want to pay for a better plating if the less expensive one does the job just as good. Locate a copy of this Volvo spec and determine what the customer needs. If plain zinc passes the spec, then they should not be paying for more than what they need.

Tim Neveau
- Rochester Hills, MI

Dear Mr Mooney & Mr Tim, TQ for your inputs.

Yes the Silver free passivation used on Zinc Iron is not UV sensitive, and hence stays black longer than Silver containing passivations, but its the Silver free, which gives the higher corrosion resistance.

The Volvo Spec in question is clearly for Zinc Iron (5732) which calls for 260 hours to white rust. I believe in the integrity of the Zinc Iron deposit itself which reduces the rate of corrosion, by making the Zinc less active and hence the deposit lasts and lasts in real life apart from 1000 hours to red rust in a SST.

My question is again, can a topcoat which does not have these above mentioned properties of Zinc iron, take its place ??

Khozema Vahanwala Automaxx  Bangalore, Karnataka, India

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