Stainless steel comes in several qualities.

  • AISI 303 (1.4305) (Stainless Steel A1) approximately 304 but more machinable due to sulfur.
  • AISI 304 (1.4301) (Stainless Steel A2) comprises of 18% chromium and 8% nickel.
  • AISI 316 (EN 1.4401) (Stainless Steel A4) is 16% chromium, 10% nickel and 2% molybdenum.
  • AISI 316L is the low carbon variant to enhance weldability.

For seawater 316 quality is often advised. I'd like to understand:

  • By what process does molybdenum make the metal more resistant to corrosion in seawater?
  • How do the material loss rates compare between these grades of stainless steel, as well as to ordinary structural steel (S235, S275 or S355)?
  • $\begingroup$ The short answer is, the metal goes under a similar process as carbruzing but with molybdenum. The chlorine in the seawater is not strong enough to break the covalent bonds in the molybdenum lattice on the surface of the metal.... I think $\endgroup$ – Sam Weston Mar 1 '15 at 1:16
  • $\begingroup$ Years ago, I've asked about steel corrosion on chemistry SE and got a helpful answer: chemistry.stackexchange.com/questions/853/… $\endgroup$ – mart Nov 12 '18 at 12:09

First, I'd like to point out that 316/316L may not necessarily be recommended for permanent immersion (intermittent immersion is okay) in seawater [1]. The rule of thumb is that you need at least 12% chromium in order to form a passive layer that protects the base metal from corrosion [2, chapter 1 page 4]. Molybdenum is added to improve pitting resistance. For permanent immersion in seawater, the recommendation is at least 6% Mo [1].

As for material loss rates, it will depend on various factors such as the chemical environment (e.g. pH, chlorides, sulfides), temperature, pressure, etc. Additives can substantially change properties related to corrosion resistance. Ref [2, chapter 3 figure 1] provides an explanation for the role of various additives / alloying elements.

  1. http://www.worldstainless.org/Files/issf/non-image-files/PDF/Marineapplications.pdf
  2. J. R. Davis, Corrosion of Weldments (#04182G). ASM International, Jan 2006. http://www.iranpejohesh.com/wp-content/uploads/2012/05/Corrosion-of-Weldments.pdf
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    $\begingroup$ Correct me if I'm wrong, but I understood corrosion to be a problem with intermittent immersion but not with permanent immersion. The reason being that oxygen is needed for oxidation, so something permanently immersed gets no oxygen and hence does not corrode. Therefore if its ok for intermittent immersion, it's ok for permanent immersion. $\endgroup$ – AndyT Mar 12 '15 at 12:57

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