Stainless steels 327Figure 4.17 Pitting corrosion in 1~pe 304 stainless steel- The build up of M + ions within the pit causes the C1- ions to migrate to the
pit in order to preserve electrical neutrality. - The soluble metal chloride hydrolyses to form hydroxide and free acid:
M+CI - + H20 --+ MOH + H+CI -Thus acid is produced by the reaction which decreases the pH to a low level
whereas the bulk solution remains neutral.
Traditionally, the susceptibility of stainless steels to pitting corrosion was
evaluated by immersion tests in acidified ferric chloride solutions (ASTM Prac-
tice G48-76 and Practice G46-76). Such tests involve the measurement of pit
density, size and depth. However, laboratory tests on pitting behaviour are now
more generally based on electrochemical techniques.
The resistance to pitting increases with chromium content but major benefit
is obtained from the addition of molybdenum in stainless steels, e.g. Type 316
(18% Cr, 12% Ni, 2.5% Mo). The addition of nitrogen is also beneficial and the
combined effects of nitrogen plus molybdenum will be discussed later in this
chapter. The potential pitting resistance of stainless steels is often expressed in
terms of a pitting index:
Pitting index = Cr% + 3.3Mo% + 16N%In terms of microstructure, MnS inclusions are important sites for pit initiation
but other features such as delta ferrite, alpha prime and sigma phase can also
promote pitting corrosion.