Local corrosion such as pitting and crevice corrosion of stainless steel usually occurs in the presence of halide ions, usually chlorides (eg, coastal and deicing chloride salts – sodium chloride, calcium chloride or magnesium chloride; hydrochloric acid; bleach – sodium hypochlorite or Calcium hypochlorite; and other chloride compounds).
Pit corrosion can occur when there is local destruction of the passivation layer on the exposed surface of the stainless steel plate. Once started, the growth rate of these pits will be relatively fast, resulting in deep holes and even penetration. Other metals such as aluminum may also cause pitting corrosion.
Crevice corrosion occurs at sites where oxygen does not circulate freely, such as in tight joints, fastener heads, and other conditions in which metal sheets are in intimate contact. Chloride salts, pollutants, and moisture in the environment can accumulate in the gaps. The environment in the crevice becomes depleted of oxygen, and the chloride-rich acid is easily acidified, thereby promoting the destruction of the passivation film and the anodic dissolution.
An important environmental factor that contributes to local invasion is high chloride content. The higher the temperature, the lower the pH and the higher the corrosion potential.
Figure 1: Effect of temperature on pitting resistance of S44660 superferritic stainless steel in 6% ferric chloride test solution
Crevice corrosion in a 304L stainless steel piping system, which starts in a gap created by the lack of full penetration in the orbital weld.
Crevice corrosion on 316 bolts
Influencing factors of the alloy:
The resistance of stainless steel to local erosion is closely related to its alloy content. The main factors that can improve the pitting and crevice corrosion resistance of austenitic and duplex stainless steels are Cr, Mo, W, and N. Tungsten, although not commonly used, is essentially equivalent to the weight percentage of molybdenum in terms of improved corrosion resistance.
Corrosion resistance equivalents (PREn) have been developed to correlate the composition of stainless steel with its resistance to pitting corrosion. The PRE relationship between austenite and duplex stainless steel is usually as follows.
PRE =%Cr + 3.3%(Mo + 0.5W) + x%N (where x is usually given as 16 or 30)
Although this relationship has developed into a graded corrosion resistance, it also provides a relative ranking of the crevice corrosion resistance of stainless steel.
The PRE number is related to the grade resistance under ideal conditions and does not involve the presence of intermetallic phases, improper heat treatment or poor surface conditions. Due to this restriction, care must be taken when using the PRE number to make material decisions.
The relative resistance to local corrosion of different alloys can be quantified by determining the critical temperature required for the initial attack. Critical pitting temperature (CPT) or critical crevice corrosion temperature (CCCT) is measured using an immersion test method (as outlined in ASTM G48) or an electrochemical method (such as the ASTM G150 test method). When a higher critical temperature is required to cause the stainless steel alloy to erode, it is more resistant to local chloride attack. These critical temperatures are related to the standard laboratory environment and are not easily transferred to the actual operating environment, but they provide an indication of relative performance.
Source: China Stainless Steel Plates Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)