Problems encountered whilst de-embrittling zinc plating

Letter 2083

Problems encountered whilst de-embrittling zinc plating

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We experience problems de-embrittling zinc plated high tensile steel parts. This is not a new problem, we have continuously had problems for 50+ yrs., and other British platers apparently do as well. We used to do cadmium plating, and never had any problems with this process. However, items that were previously cadmium plated and are now zinc plated present problems. This indicates that the zinc plating is the source of the problems.

The problem:

Bad blistering, or dulling of the plating after de-embrittlement. This does not occur on every item in a batch and it is difficult to identify why this should be. The problem does not occur on every base material, but those that give rise to problems are always troublesome. We cannot see any pattern in the base materials, except one job that is heat treated (to harden it) just before we receive it for plating is always bad. However, when we ask our customers if other problem jobs are heat-treated this does not always seem to be the case.

Has anyone experienced these problems? And solved them. Please let me have your thoughts.

David Grimes
plating company - Farnham, Surrey, UK

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Dear Dave ,

I have experienced this same problem years ago when Chloride Zincs were first on the market , it seemed at the time that the "Blistering" was not a blister as such more an Exifoliation , the Zinc used to "grow" and just flake off ,but never to the base steel always to a Zinc base , I thought that the problem was caused by high concentrations of the Wetter component of the Brightener system , any way we seemed to be able to overcome the problem by lessening the wetter component from about 3% by volume to about 2.2 --> 2.5 % maximum with a corresponding drop in the brightener component , from memory the add rate of the brightener was down to 70 % of that recommended by the supplier .

I know there have been improvements in the systems in the meantime but it is worth a try anyway , if it fixes your problem let us know

regards

John Tenison - Woods
- Victoria Australia

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DAVID,

THE HARDNESS OF THE BASE METAL PLAYS AN INTEGRAL PART IN WHETHER THE ZINC PLATE STAYS ON OR BLISTERS. ANY STEEL PART WITH A HARDNESS OF 38 ROCKWELL C OR GREATER, SHOULD NOT BE ZINC ELECTROPLATED. HYDROGEN EMBRITTLEMENT IS THE CULPRIT. MECHANICAL PLATING WHICH DOES NOT IMPART HYDROGEN INTO THE BASE METAL IS REALLY THE CHOICE OF PLATING.

IN N.A. FORD, GENERAL MOTORS, AND CHRYSLER ALL HAVE MECHANICAL PLATING SPECIFICATIONS JUST FOR THIS PURPOSE. CHECK WITH TERRY CLARKE OF MACDERMID G.B. FOR MORE INFORMATION

I KNOW OF SOME PLATERS, BECAUSE I USED TO BE ON THE CHEMICAL SUPPLY SIDE, DO GET AWAY WITH THIS BLISTERING BY REWORKING THE PARTS. GUARANTEED - THEY WILL BE FINE THE SECOND TIME THROUGH. MAKE SURE YOU ADJUST YOUR PRICE ACCORDINGLY.

FINALLY, THE TYPE OF SOLUTION ie. CYANIDE/CHLORIDE/ALKALINE AND THE THICKNESS OF THE DEPOSIT WILL CAUSE BLISTERING

CHLORIDES HAVE A HIGH ORGANIC INCLUSION IN THE DEPOSIT WHICH WILL EXPAND UPON HEATING EG.BLISTERS
ALKALINES ARE PRONE TO "POST DELAYED BLISTERING" DUE TO STRESS
CYANIDES ALTHOUGH THEY DO NOT COMMONLY BLISTER CAN IF OVERLOADED WITH ORGANICS
ANY ELECTROPLATED ZINC DEPOSIT, AS IT APPROACHES 1.0 MILS IN THICKNESS, WILL TEND TO BLISTER.

REGARDS
RAY

ray delorey
- CAMBRIDGE, ONTARIO, CANADA

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I have experienced similar problems with Zinc plating blistering prior to painting when the paint was forced cured, or powder coated. I also believe it is a problem with entrained brighteners,or brightener levels. However, interestingly enough, the main prevention step was determined to be controlling the plate thickness. When the thickness was controlled to .2-.3 mils the problem went away.When the plate thickness was above .4 mils it blistered when heated.

Ward Barcafer, CEF
aerospace - Wichita, Kansas

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While probably stating the obvious, the embrittlement of high-strength (i.e. high-carbon) steels is due to entrapment of atomic hydrogen in the steel.

My experience with this problem was when having high-strength carbon steel parts plated with zinc chromate in a batch plating process. The only solution we found was baking of the plated parts at 350-375 deg F for 1-3 hours (depends on thickness of part). This theoretically removed quantities of entrapped hydrogen from the steel and coating. This completely eliminated the problem without changing the plating process itself. These parts were AISI 4140/4150 steels which were heat-treated prior to plating with tensile strength in the 150 ksi range. The baking requirements for your steels might be somewhat different. If we did not bake these products we encountered periodic embrittlement failures in service. The metallurgical condition of the steel to be treated also influences the embrittlement potential.

It is probable that hardened parts require consideration of a different cleaning method to reduce the potential for hydrogen embrittlement. Blast-cleaning or alkaline cleaning pretreatment may reduce the potential for hydrogen embrittlement for high-carbon, high-strength steels when compared to other methods which evolve hydrogen as a by-product. Those more knowledgeable about plating process might know other plating process controls which could achieve reduction in hydrogen entrapment.

Robert P. McCrossen
- Jacksonville, Florida, USA

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