Pitting corrosion is a form of localized corrosion, which produces attacks in the form of spots or pits. Pitting corrosion may occur in stainless steels in neutral or acid solutions containing halides*, primarily chlorides (Cl-), such as seawater.
Pitting corrosion attacks most often take place at points where the passive layer might be weakened, for example by slag inclusions, a damaged surface or imperfections in the passive layer. Once the attack has started, the material can be completely penetrated within a short time.
Problems with pitting corrosion attacks depend primarily on the chloride content, the pH value (the acidity), and the temperature. If pitting has taken place and if the environment in such is not too corrosive for the steel grade, a spontaneous repair of the passive layer will occur in the presence of oxygen.
The chloride content is often stated as ppm (parts per million = 0.0001%). Some typical Cl- values are:
Natural seawater is more corrosive than would be expected from its chloride content. The reason is that biological activity will occur. Therefore, chlorination as biocide is often used as to prevent the growth of micro-organisms in seawater-bearing pipe systems.
Under such conditions Sandvik SAF 2707 HD, Sandvik SAF 3207, Sandvik SAF 2507 and Sandvik 254 SMO* have superior properties. If chlorine addition of 0.5–2 ppm is made intermittently (for example. 2 x 30 min/24 hours), the risk of pitting attack can be reduced. Intermittent chlorination is fully sufficient to prevent micro-organism growth. However, the chlorine addition can cause an increased risk of pitting corrosion due to the oxidizing capacity. Use of continuous chlorination increases this risk, which is why intermittent chlorination is preferred.
These alloying elements have the following positive influence on a material's resistance to pitting corrosion:
One way to estimate the pitting resistance of stainless steels is through the pitting resistance equivalent (PRE), which can be calculated from the chemical composition of the steel. The higher the value, the better the resistance.
PRE can be calculated according to the formula:
% of Cr + 3.3 x % of Mo + 16 x % of N
There are several different methods for testing a steel grade's resistance to pitting. Critical pitting temperature (CPT) determination is often used. ASTM G48, practice A and E (6% FeCl3) is one of the toughest pitting corrosion tests for stainless steels due to the acidic solution combined with chlorides and the oxidizing ability. ASMT G48 practice A is a performance test at one temperature while the practice E determines the critical pitting temperature (CPT). For high-alloy grades testing according to ASTM G150 in the neutral solution of 1M NaCl is frequently used.
* Halides = elements forming salts; e.g. Cl, F, Br, I
** 254 SMO is a trademark owned by Outokumpu OY