The ASTM standard G78 describes a procedure for the evaluation of the susceptibility of a metallic material to crevice corrosion in specific environments. Technically, this procedure works like a charm when noble or corrosion resistant materials are tested in relatively corrosive environments. The multiple crevice assemblies (MCAs) used in this test favor the formation of an even more aggressive chemistry at the interface between the grooves machined in the inert material and the piece of metal being tested and on which the MCAs are pressed with known force.
During a recent discussion thread on the NACE International discussion list the question was raised about the possibility of crevice formation of steel components exposed to natural seawater. The flurry of messages that followed the initial question by Dr. Bob Cottis (> 60 messages in approximately one week) clearly indicates that this topic is at the frontier of our knowledge in this apparent simple matter. It should be noted that no clear consensus was reached during that debate.
In order to verify that such corrosion attack did or did not occur on steel exposed to this harsh environment, the Corrosion Doctors decided to perform some experiments using the G78 MCAs and steel washers of three different sizes (1.5, 3.2, 3.8 cm) exposed to two different waters, i.e. one containing 3% sodium chloride in distilled water (to simulate seawater) and the other simple drinkable Kingston (Ontario) tap water or Great Lake treated best to be more precise. Here are two pictures of these specimens after a twenty-five day immersion of the assembled MCAs.
As you can see the salted water (top photo) is more aggressive to the steel washers generously sacrificing their metallic atoms in these experiments than those exposed to tap water (bottom photo). As Dr. Cottis would probably say, the steel is seawater is more active than the steel in tap water!
Original message sent by our friend Dr. Bob Cottis from UMIST, Manchester
I am currently having a ‘discussion’ over the supposed phenomenon of crevice corrosion of carbon steel in seawater.
I am of the opinion (based on largely theoretical arguments) that it does not occur, since crevice corrosion requires a passive external surface coupled to an active surface inside the crevice in order to drive the current between the anode and the cathode. For carbon steel in seawater the external surface is active, so the potential is rather negative. Inside the crevice some corrosion of iron occurs, but the Fe2+ concentration quite quickly reaches a high enough level to bring the iron dissolution reaction close to equilibrium, and the rate of dissolution becomes lower than that outside the crevice (the late Abi Alavi demonstrated that the crevice in an artificial crevice in 3.5% NaCl became a net cathode).
It is quite possible that I have missed something, and it is very common for dire warnings to be made about the risks of crevice corrosion in seawater. Before I press my case that such warnings are based on a misunderstanding of what crevice corrosion is, does anyone know of any hard evidence of crevice corrosion of carbon steel in seawater? Note that I specifically excluded the enhanced corrosion that occurs in such places as bolted fences, folded car seams and other gaps that retain moisture in atmospheric exposure conditions. In this case the moisture retention is the cause of the problem, not the crevice per se.