Wrong Corrosion Test Method for 420SS Knife Blades?

A. Hi Andy. Your situation is not entirely clear to me. Why did you send the knives to a test lab for salt spray testing if that is not the proper requirement? Are you responsible for determining the requirements, specifying test methods, and acceptance criteria, or is that responsibility held by another company? I agree that Type 420 stainless steel will not have good corrosion resistance in salt spray testing according toASTM B117, and that another test method might be more appropriate, such as ASTM G50 Standard Practice for Conducting Atmospheric Corrosion Tests on Metals.

No matter what, stainless steels must be properly passivated in order to produce maximum corrosion resistance. I would discuss this issue further with your steel vendor, and investigate the test methods available from ASTM International or other standards bodies.

A. 420 stainless steel is difficult to passivate well enough for a 24 hour salt spray test. First, I assume that the blades are fully hardened. You cannot passivate them unless they are hardened. Second, I assume that they are properly passivated with alkaline treatment followed by either a good citric formulation or nitric acid/sodium dichromate solution (Caution- nitric/dichromate is hazardous and carcinogenic). Third, you are much better to use a boiling distilled water test on this grade to determine the acceptability of the passivation. You could also do high humidity test, but I do not recommend a salt spray test.

A. Salt spray test is the technique for outdoors stuff. You could find the right test from CATRA lab.

A. A certification service has been named, but I expect that they verify compliance with a standard -- not invent their own standards, as you seem to think they have done, Andy. Similarly, a test equipment manufacturer has been named -- but I expect that they build equipment to verify compliance with standards, not invent standards.

Sidebar: I appreciate Sean's input, but it is not true that salt spray testing is only for outdoor stuff unless he is speaking only of cutlery.

There are dozens of test labs, Louis, but it seems to me that what is missing from this thread so far is the name of either a U.S. or international cutlery group (educational society or trade association) that might define what the appropriate ASTM or other corrosion resistance tests for cutlery should be. If that association says that 24 hours salt spray is the right test, or if they suggest a different test, then you have something that a certification service or a piece of testing equipment can test for.

A. It is surprising that all correspondents have missed the International Standard ISO 8442-1 in which the correct method for corrosion testing knife blades made from martensitic steels such as the 420 series is detailed, including assessment. It is implied that CATRA made up the test they use . . . wrong: CATRA uses the internationally agreed test in the standard, it is the proponents of salt spray that are making it up!

Thanks Roger ... and good point. However, I did not imply that CATRA makes up their tests, I said that they "verify compliance with standards, not invent standards".

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey