Summary for stripped zinc coated screws Discussion
Stripped zinc coated screws are the issue, with some valve-pad fasteners losing their zinc finish after a test bath and dryer, but only randomly. Mohammad Ramezani says the first step is to verify that the plater is meeting the required zinc-plating specifications. He notes that screws that look darker in the thread valleys may only be coated on the crests, while a reddish appearance can indicate the parts were not properly plated at all. He also warns that the test-bath chemistry matters: chlorides and similar chemicals can attack zinc quickly. His advice is to review the screw specs, including plating thickness and required salt-spray hours, and confirm the supplier is meeting them. He suggests using a third party to check coating thickness and perform salt-spray testing if needed.
Q. 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 because considering me being new here, this might be an opportunity to prove myself!
Anyway, we got two different test baths and screws are different in appearance 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 tested 4 different possibilities and showed nothing (two baths water, and 2 screws). We mark on the head of the screws with a black texture 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
2007
A. Mohammad,
You should check if the plater 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 “red-ish” 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 thickness, 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