When SUS304 (JIS; AISI 304) is used in chloride-rich seawater, which correctly pairs the localized corrosion likely to occur with a countermeasure?

Answer: B. Pitting corrosion; switch to a Mo-bearing grade such as SUS316

Chloride ions locally break down the passive film at weak points and cause pitting corrosion, in which small holes are dug deeper and deeper. Inside the pit, hydrolysis of the dissolved metal ions acidifies the solution, and chloride ions collect to maintain electrical neutrality, so the corrosion becomes self-accelerating. The large surrounding passive surface acts as the cathode and the bottom of the pit as a small anode, so the pit can advance rapidly to perforation. The countermeasure is to switch to a grade richer in Mo, Cr and N, which strengthen the film (such as SUS316, equivalent to AISI 316, with about 2–3% Mo); the pitting resistance equivalent number calculated from these contents is a guide for selection. Because this is not corrosion that thins the whole surface uniformly, increasing the wall thickness is not an effective countermeasure. More carbon makes Cr carbides form more readily at grain boundaries (sensitization) and actually lowers corrosion resistance.