Chloride corrosion can be caused by the combination of insulation containing leachable chlorides with the 300 series austenitic-stainless-steel surfaces, when moisture is present and temperatures are above 140o F. Concentration of the chloride ion usually results from the evaporation of rain water,or of water used to fight fires, or of process water. Stress-corrosion cracking of insulating jackets often results from airborne salts in coastal regions.

The probability of failure and the speed of crack propagation are governed by the temperature of the stainless steel and the chloride concentration at the metal surface. Solutions containing less than 1 ppm are normally considered safe. Below 176°> F, levels of 100 ppm are not particularly dangerous if continuous surface-wetting occurs; but at higher temperatures, lower levels can result in failure.

In practice, it should be assumed that evaporation of the solution will inevitably occur. Because local concentration of chlorides take place at the metal surface, the bulk concentration may be of little importance. Above 390° F, external stress-corrosion cracking is normally not experienced. The stress required to cause cracking of stainless steel may result either from fabrication or operation (or shutdown).

Water entering the insulation and diffusing inward will eventually reach a region of dryout at the hot pipe or equipment wall. Next to this dryout region is a zone in which the pores of the insulation are filled with a saturated salt solution-this includes any chlorides present. When a shutdown or process change occurs and the metal-wall temperature falls, the zone of saturated salt solution moves into the metal wall. Upon reheating, the wall will temporarily be in contact with the saturated solution (e.g., chlorides), and stress-corrosion cracking may begin.