A detailed study has been made of the influence of various alloying elements in stainless steels on the course of corrosive attack by eight acids. Factors in the environment (acid concentration, corrosion products and oxygen) which determine the course of corrosion also have been investigated. The acids used were oxalic, formic, acetic, hydroxy-acetic, phosphoric, sulfuric, sulfamic and sodium bisulfate and the stainless steels included straight chromium (AISI 400 series), 18 Cr - 8 Ni (AISI 300 series) and some Cr-Mn-Ni steels.
Corrosion of stainless steels in the acids investigated is inhibited to such an extent by the action of soluble ferric salts, in amounts exceeding a certain critical concentration, that the characteristic bright finish of these steels is maintained for long periods in boiling acids. The minimum amount of ferric ions required for inhibition, which may be considered a measure of the passivating characteristics of the steel, is dependent of the alloy content (Cr-Mn-Ni) of the steel.
The corrosion behavior of a 17.6 percent Cr - 5.6 percent Mn - 4.4 percent Ni steel was found to be very similar to that of 18 Cr - 8 Ni stainless steel. 6.2.5
