Letter 2034

Maximum temperature for zinc plating and chromate

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We are using a threaded steel fitting finished with zinc pllating & chromate treated to a bronze color. The fitting is exposed to conducted heat during an aluminum brazing process, but is not directly exposed to the flame heaters.

After brazing, the fitting and tube are water quenched to cool & remove flux. The plating on the fitting has darkened and is flaking off.

My question is, What is the maximum operating temperature of zinc plating with chromate conversion treatment?

Grant Repar
Halla Climate Control - Belleville, Ontario, Canada

The chromate starts degrading at about 140F because it is a very thin gel and is being dehydrated. The high temp of brazing nearby and then quenching is a very severe test of plating. If the plating is on the aluminum, even with an excellent perparation and zincating, I would expect a high failure rate of the plate. The chromate will turn a very dark color, approaching black, in the 600F range. If at all possible, let the part at least partially cool before you quench. My preference would be to totally cool and then remove the flux with hot water, assuming that you are using a soluable flux.

James Watts - Fl

Dear Grant ,

Whilst I agree with everything that Jim Watts says , I think you have another problem about to rear its ugly old head , correct me if I am wrong , but you are braising an aluminum fitting to a steel part that has been Zinc plated , there is a real corrosion cell looking for somewhere to get wet !!

regards

John Tenison - Woods
- Victoria Australia

With regard to the potential for galvanic corrosion, John has a good point. Now, since the subject has come up though: whether you will have a problem depends not just on how wet the weldment gets, and how active the aluminum is in comparison to the zinc, but (a factor that is crucial but seldom considered) the area of the two parts. A tiny bit of more noble material mounted to an expanse of more active metal will usually have small effect, whereas a tiny bit of active metal mounted to an expanse of more noble material can fail catastrophically in the blink of an eye. I suspect that Grant is already aware of this and that the assemblies perform well.

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

GRANT,

CHROMATES WILL BLOW OFF BETWEEN 140 F AND 170 F

ZINC PLATE WILL MELT ABOVE 375 F

ZINC PLATE WILL BLISTER WITH 2 OF THE FOLLOWING:

1 - THE TYPE OF PLATING SOLUTION IE. CYANIDE/CHLORIDE/ALKALINE

• chlorides are high in organic composition as deposited any heat tends to blow them off ie. blisters
• alkalines are prone to "post delayed blistering" caused by stress in the deposit
• cyanides although not prone to blistering can if organic levels get too high

2 - ZINC PLATE THICKNESSES APPROACHING 1.0 MILS ARE PRETTY WELL GUARANTEED TO BLISTER.

COMMENTS:

WE PLATE GM TRUCK TAILGATE HINGES AND WENT THROUGH HELL!!!!

TO GET THE PLATE TO STICK BECAUSE THEY GET WELDED THEN PAINTED FOR FINAL ASSEMBLY. THE BEST PROCESS WE DISCOVERED WAS, AND THIS IS IN A BARREL APPLICATION, IS A CYANIDE SOLUTION WITH NO ORGANICS ie BRIGHTENER - 0.2 TO 0.4 MILS YELLOW CHROMATE

REGARDS
RAY

ray delorey
industrial processing - cambridge, ontario

sir,

could you please provide me some material on "zinc deposition by theoretical and experimental methods"

thank you

yours faithfully,
sandesh

SANDESH PRABHU
- mysore,karnataka,india

Mr. Prabhu,

This site is not about asking somebody else to do your homework for you.

I've just spent as much time answering your question as you spent asking it. If you'd like us to spend more time answering it, spend more time asking it.

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

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