Stripped zinc coated screws

+++++++

I'm working in a company producing some kind of gas cylinders and tanks. I started 3 weeks ago; an issue has came up recently which has caused lots of confusing in process; we have noticed that some of the screws which are used to fasten valve pad to the cylinders after going through test bath and dryer subsequently, are stripped of their zinc coat. I really like to solve this out caz considering me being new here, this might be an oppportunity to prove my self!!
Anyway, we got two diffrent test baths and screws are diffrent in appearence as well, although supplier says they go through same process, some of them are bright and paler and fewer are darker and a bit reddish.
I testd 4 diffrent possiblities and showed nothing(two baths water, and 2 screws). we mark on the head of the screws with a black texter before putting them into bath. the pad that is fastened by screws are painted. and there is some sort of chemical in the water of test baths as well.
The thing happens randomly and hardly, but happens and no one knows what causes that.

Mohammad Ramezani
Production improvement engineer - Melbourne, Vic, Australia

+++++++

Mohammad,

You should check if the platter is compiling with the required specs of the plating.

Usually when the thread is darker in the valley it means that the screw is only covered at the crest of the screw and not in the valley. If you mean "redish" by some kind of rust on the surface, then the screw is not plated.

Also you should know what chemical it is being used in the tests, that chemical can affect the zinc plating, e.g. Chlorides have a strong effect over the zinc, attacking it fast.

Review your screw specs (zinc plating thikness, salt spray hours required) and check if your supplier are fulfilling them.You may ask a third party to to check the thickness and to do the salt spray test.

Saludos.

Guillermo Castorena
plating jobshop - San Luis Potosi, Mexico