Zinc Plating Adhesion to Leaded Steels

A. Cold forming will flake off the very best chromate conversion coating. Chromate cannot withstand any mechanical, chemical, or thermal attack.

Your zinc is not sticking to the leaded steel because in the presence of chloride, lead forms an insoluble compound that flakes off. And he is plating in a Zinc Chloride solution with free hydrochloric acid. It did no good to go thru the fluoboric and then set it down in the zinc chloride plating solution.

Have the plater get in a hurry after the fluoboric acid dip, make contact outside the plating solution, and go in "hot".

A. Please forgive me for oversimplification. The reason the zinc plate is non adherent to the leaded areas is that the surface preparation suitable for activation of the steel area leaves the lead passive. Lead is not soluble in any of the common acids used for activation (sulfuric, hydrochloric, proprietary acid salts, etc) and therefore must be treated in an acid it is soluble in, namely fluoboric acid (lead fluoborate is formed in the acid solution). I would try using a 20% by volume solution of fluoboric acid at room temperature for about 1 minute. Move concentration and/or time up as necessary.

By the way, if you are cold forming the part and using an acid chloride zinc process, you also need to look at the ductility of the zinc deposit (flaking is notorious when bending chloride zinc plated parts). If this is the case, constant carbon filtration and frequent additions of an oxidizer to reduce organic contamination are very helpful.

A. I cannot help with the zinc chromate as I know nothing about it. However, the lead in the steel may cause a problem because of lead smear. Lead is normally added to help machinability, but it leaves a thin film of lead on the steel. This inhibits adhesion of overlays. To remove the lead, you will need to soak in fluoboric acid (say 20% for 2-5 minutes, depending on the temperature, amount of lead smeared and the type of steel). To check this, ask your supplier for advice.

Q. Thank you for the responses; the lead oxide explains the problem. I should have been more clear on the electroplating process. We are using an alkaline process and a hex chromate.

Also, ductility was mentioned. Does anyone know where I can get mechanical properties for zinc coatings that I can use to evaluate against our forming process?

Thanks again for the response.

A. Mr Sushil Jain

Hello and Greetings..

Please rinse twice after dragout.
Activate in 20 Mls per liter Nitric acid dip. 2 percent solution...
Rinse
Chromate

Replenish the nitric at 5 mls per liter every 4 hours worked..

This will solve peel off of Chromate over alkaline zinc
Regards

? Hi, Aditya.

I'm not familiar with "EN1a". Can you tell us what standard or numbering system this is found in?

Are the black spots present before the chromating, or only after chromating? Do they appear immediately after chromating or do they only develop after several days? You say you are "vat plating" . . . does that imply that you are rack plating, or are you barrel plating. Some photos of the spots might help.

Regards,

Q. It is rack method also called vat method, trivalent passivation; the black spots are visible immediately after passivation.
I even tried doing fluoboric acid no result.
This material is equivalent to SAE 12l14,11smnpb30.
EN1a leaded is european standard.
Will be mailing the pics soon of the defect soon.