Salt spray test for austenitic stainless steel.

A. Hi Arwin. This seems to be a good example of the damage that can be done by poor application of salt spray specification requirements. Certainly the designer of this object was not intending for WD-40 or parafin or mineral oil to gum up the works :-(

What is this item you are treating? I assume it is actually type 316 SS, not a casting? Castings are sometimes labeled 316 SS but they are not, and they do not exhibit the same corrosion resistance.

It's possible that your overall passivation treatment is not as good as it could be; but if passivation isn't working you could try electropolishing the item.

Please read the rest of this thread for some good insights.

Luck & Regards,

A. QQ-P-35C [from DLA] says that the parts (they mention 303 containing sulfur) should pass 2 hours salt spray. They say to use Type II passivating solution.

A. I have salt fog tested 304, 304L and 316 for 2500 hours and did not see red rust.

I do not have a report on this since I am not employed by that firm any longer.

The test was per ASTM B117, surface was scribed. the test was to assure the paint adhered to the stainless after 2000 hours, not to find red rust. In the same test 409 stainless was tested.

I have routinely tested 304L and 409 stainless at 1500 hours per ANSI C57.12.28, Draft 5.

Hope this helped you.

Ty

A. Hi Palkannan,

That's a broad question and, while it probably seems carefully framed to you, each of those issues involves a dozen "ifs, ands, & buts", so there are no quick answers. You could start getting the answers to those questions with the following AESF illustrated lectures: Corrosion, Testing, Metallurgy, Zinc Plating, Chromium Plating, Anodizing, and Phosphate Conversion Coatings. Good luck. You might also want to sign up for an AESF intensive training course, or a course by Kushner Electroplating School, the Institute of Metal Finishing in Birmingham UK, or the Metal Finishers Association of India. Good luck!

A. If you have used SS 304 for a coolant tank then there should be no rusting and if you find some red rust like formation most likely they are some acid cleaning in that area which might have caused this, you can simply use a buffing wheel and your problem should be solved.
We had a similar problem and with just buffing we were able to get back our SS304 back in action, all the best.

A. Hi Hadi. Can you tell us what you're involved in here? I wouldn't try to predict the results you should expect from this test, and probably you shouldn't be doing it anyway :-)

Good luck but please tell us your situation.

Thanks & Regards,

A. Hi Nick. All stainless steel should be passivated. It's not an optional topcoating; rather it's a process of dissolving and removing any remnants of carbon steel from the surface. Without it, rust from those remnants of the manufacturing process must be expected.

I think it may be possible for 316 stainless steel to "last 1000 hours before rusting" because a number of knowledgeable people on this and other sites have made that claim ... but can you quote a spec? ASTM B117 is only instruction for how to construct & operate a salt spray chamber, not for how to evaluate the results. The processing specs often tell you how to interpret failures. Without a spec telling you how to interpret results, you're instantly into no man's land (for example, do you use the naked eye, a jeweler's loupe, or a microscope to look for the rust? It depends on what the applicable spec says.)

Regards,

  Thanks Ted, appreciate the quick reply. Our requirements are usually spelled out clearly but not on a current project so I'd thought I'd seek out your expertise. "All should be passivated" is helpful.

A. Hi Balachandar. Sorry, but the question is a bit backwards. Salt spray hours for stainless vs. galvanized will not give you even the faintest clue about which is better for the application. Salt spray hours cannot be used to compare the corrosion resistance of different materials, especially galvanizing which can last 75 years and even more in a natural environment when glassy zinc carbonates can form on the surface as intended, but which cannot form in salt spray testing :-(

Properly passivated type 304 stainless steel should be excellent in a non-salty, non-chlorine environment as long as it is not in metallic contact with any other metal. If it actually is in contact with or splashed with salt water, 304 SS is unlikely to be acceptable, you'll probably want 316 SS. Please explain your application. Thanks. Good luck.

Regards,

A. Hi Matheus. I think it's pretty well established that 316 SS with its molybdenum additions or 316Ti is more corrosion resistant than 304, and that 3xx is better than 4xx, and that passivation (or electropolishing) is necessary for rust-free corrosion resistance. But, yes, beyond that there are lots of different environments and situations so it's hard to say how long a given stainless steel will resist corrosion.

Yes, generally, high stress (tension from bending) does make the metal in that area more anodic and prone to corrosion; additionally, some types of bends and shapes might cause salts or moisture to accumulate, leading to faster corrosion. But in mild environments austenitic stainless steel can last a very long time, as witness tens of millions of sets of stainless flatware many decades old.

Regards,

A. Hi again Matheus. There's a good reason why you're unable to find something that you and a lot of people want, and it's that what you want isn't reality-based. Please read on and look at, for example, the simple graph of corrosion of coated materials vs. stainless steel :-)

Regards,

A. Hi Matheus. Hi Gian Carlo. Except for the previously quoted QQ-P-35-C, which was cancelled over 20 years ago, and which requires 2 hours of salt spray resistance, I'm not aware of other specs. Hopefully another reader will enlighten us if there is one :-)

Regards,

A. NASM 1312-1 which is still active but relates to the old MIL-STD-1312-1. It is a 4 hour salt spray test for passivated Stainless and used for fasteners. Good luck, it's a bear to pass on threaded regions without media blasting them prior to passivation.

A. Hi Matevz. Thank you!

Your question and other Q&A's on this page made me uneasy because although I have a basic understanding of salt spray testing, I couldn't understand how people could talk about 2-hour, 4-hour, 400-hour, and 1500-hour salt spray testing requirements for the same stainless steel materials :-(

It motivated me to find one of the best 15-minute technical articles I've read in years, which explains the situation in one quick graphic:

Many people have asked on this page "how many salt spray hours...?" After reading that paper, my answer to them would have to be: the answer is more go/no-go than quantitative; the stainless item will resist corrosion or it won't :-)

Regards,