Non-stabilized (like AISI 304) Stainless Steels are prone to intergranular corrosion cracking due to precipitation of M23C6 at grain boundaries in heat affected zone (HAZ). This precipitation depleates chromium in matrix there by reduces the corrosion resistance. You can use 'stabilized' Stainless steels like 321, 347 or low carbon grades such as 304L or 316L

with regards,

Harsha

There is not really such a thing as being able to say steel "A" is more corrosion reistant than steel "B". The corrosion resistance of all metals vary based on what the service environment is. Whoever told you that 304 is not truly corrosion resistant, may be right in a few instances, based on certain environments, but in general, it is considered a corrosion resistant steel, as it contains enough chromium to form a protective layer of chromium oxide. The only environment you mentioned is "Coastal industrial environment". If it is exposed to salt spray, usually, 316 will provide better corrosion resistance than 304 due to its higher moly content. However, there are instances where 304 out performs 316 also, so it is all relative to the corrosive media.

I want to agree with Mr. Roberts. You need to see relative corrosion resistance in terms of the corrosion environment. To state that 304 is not corrosion resistant, is incorrect. It is a corrosion resistant steel, but can still corrode under certain circumstances. These are rather too numerous to mention. One typical issue (sensitization) was raised by another respondent. Even in the type of atmospheric corrosion environment you reffered to, there are numerous possible corrosion mechanisms that can lead to failure. This would include the failure of type 316 S/Steel.

If you would like more details, let us know. In particular, under which circumstances do you want to use the type 304?

Regards
Niekie Jooste

As GRoberts correctly points out, 304 SS won't resist a "coastal environment", if by that you mean that it'll be used close to the sea side. The salty mist (mainly sodium chloride) produced by the sea and carried away by the wind will not only corrode the 304 SS but eventually will also make it to collapse. This is because chloride ion produces a special form of corrosion in SS called "stress corrosion". For some reason which is not completely understood, in the presence of chloride ions SS starts corroding when subjected to some kind of stress (like internal pressure in a piping) and eventually the corroded metal will no longer withstand the stress, getting in collapse.
Attention though! The same thing happens also with 316 SS.
You don't say for which application you want to use the SS (piping, chemical tanks?). If we knew that we might possibly help you.
Giovanni S. Crisi
Sao Paulo - Brazil

304 stainless steel will develop a light surface rust and possibly some pitting when exposed to a marine atmosphere (salt spray). 316 would perform better. Stress corrosion cracking (SCC) is possible, but unlikely unless the chloride concentrations are very high or the service temperatures are above 150 °F. Using "L" grades will eliminate the potential for sensitization (formation of M23C6 carbides)of the HAZ, which reduces the risk for SCC in the HAZ.

There is some very good literature available from the Nickel Development Institute (NiDI) on corrosion of stainless steels (www.nidi.org). Type in the keyword "marine" into thieir article search engine. Also, there is some corrosoin data information available from Avesta Polarit and Sandvik on thier websites.

Unless you have some extreme service requirements and conditions, 316 should be just fine.

I would suggest contacting one of the metal suppliers for advice if you have a special service requirement. There are a number of grades of stainless steels that will perform better than 304 / 316 in cases with special requirements.

Regards.

I question the use of the term "bare" in your reference to 304 stainless. If you have made the surface "bare" by some mechanical means such as sandblasting, machining or grinding, then they are correct. You will no longer have "stainless" steel as you have removed the CrOx needed on the surface to be resistant to corrosion and thus called "stainless". The surface will again gain it's "stainless" status as it oxidizes. You can expect the surface to have rust on it when it has been bared and is exposed to moisture. The metal, after all is approx. 70% iron.

You should supply us with more data.