Background: The CS bolts are available with Black Oxide coating. I have read that the coating is applied using hot dipping and cold dipping methods. Also that hot dipped coating is more durable than cold dipped one.

Problem: How to check if a given bolt is hot dipped or cold dipped. I am looking for a crude method that does not involve any lab test, but for on spot check.

  • $\begingroup$ What does "durable " mean ? Black oxide gives very little corrosion protection . Likely limited to a surface condition that will hold oil which will give some protection. $\endgroup$ Nov 20, 2020 at 21:44
  • $\begingroup$ we have been using bolts with black oxide coating for many years and the coating is quite successful in protection against corrosion. $\endgroup$
    – new2Ubuntu
    Nov 22, 2020 at 15:06

1 Answer 1


I imagine you could just cut it in half and look at the boundary under a microscope or loupe. Compare against a known sample that is hot-dipped and one that was cold-dipped. I imagine it should be fairly obvious.

Another idea is to get some hardness testers and test them on some known samples.

If you can't find known samples (i.e. you can find bolts labelled as cold-dipped and hot-dipped black oxide), then then get some bluing compound and apply yourself to some clean steel and compare it against a a good black-oxide drill (which should be hot-dipped). I would hope the black-oxide coating on a drill would be even better than one on a hot-dipped bolt so keep that in mind.

That said, do any manufacturers actually use cold black-oxide on bolts? Because for a mass-produced item where you already have high tooling and setup costs, going with cold-dipped black oxide seems like a weird cost cutting measure that doesn't seem like it would actually cut costs by much. I would be really disappointed to find anyone using cold-dipping on something small and mass-produced like bolts.

  • $\begingroup$ Thank you for taking time to respond. The idea that I am looking for is something that does not use any lab equipment. For example, I scratched the bolt with a steel piece and the coating was easily scratched. I want to know it that is a sign of bad or normal coating. $\endgroup$
    – new2Ubuntu
    Nov 20, 2020 at 7:47
  • 1
    $\begingroup$ @new2Ubuntu You need known samples to compare against. Find good brand name certified hex bolts or drills. You will probably have to go out of your way to order them since they won't be sitting in a hardware store unless they are made in China in which case you won't know what you are getting. $\endgroup$
    – DKNguyen
    Nov 20, 2020 at 15:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.