Stress corrosion cracking (SCC) is characterized by cracks propagating either transgranularly or intergranularly (along grain boundaries). There are several types of stress corrosion cracking (SCC), for example, chloride-induced SCC and H2S-induced SCC.
Stress corrosion cracking (SCC) results from the combined action of three factors:
A precursor of stress corrosion cracking in chloride-bearing environments is pitting corrosion, occurring if the stainless steel is not sufficiently resistant to pitting.
The risk of stress corrosion cracking (SCC) can be minimized through plant and equipment design. It is especially important to avoid any mechanical tensile stress concentration, which will occur at sharp edges and notches. In many cases, problems with stress corrosion cracking (SCC) can be solved by selecting a suitable material.
Process fluids in wet and sour service within the oil and gas industry often contain a certain amount of hydrogen sulphide, H2S. When considering the corrosivity of a sour process fluid, the partial pressure of H2S has to be taken into account as well as the pH value. Also the temperature, the oxygen and chloride contents and the presence of any solid particles, such as sand has to be considered.
It has been shown that H2S-induced stress corrosion cracking attack is worst at temperatures around 80°C (176°F), but cracking can occur also at temperatures below 60°C (140°F).
Laboratory testing can be carried out according to NACE TM0177 (5% NaCl and 0.5% acetic acid saturated with H2S) or in relevant simulated service environments at elevated temperature and pressure. Several Sandvik grades, including SAF™ 2205, SAF™ 2507, Sanicro® 28, Alleima® 41 and Alleima® 254 SMO* are covered by the standard MR0175 and should not normally need further testing.
Chloride-induced SCC is tested experimentally in the laboratory using a chloride-containing environment. Testing can be carried out, for example, in boiling 40% CaCl2 or chloride-containing water.
Duplex stainless steels, austenitic stainless steels with a high nickel content (> 25%). Ferritic steels are also resistant to cracking but may corrode.
The following Sandvik grades have very good or excellent resistance to chloride-induced stress corrosion cracking (SCC):
For good resistance to H2S-induced stress corrosion cracking (SCC) an alloy with high nickel content is recommended, and for most sour environments this type of alloy is now used. A Alleima grade with very good resistance to this form of stress corrosion cracking is Sanicro® 28. For less demanding environments, duplex steels can be used.
The following Alleima grades have very good or excellent resistance to H2S-induced stress corrosion cracking (SCC):
Austenitic steels of type ASTM 304 and 316 austenitic steels have limited resistance to stress corrosion cracking (SCC), even at very low chloride contents and temperatures.
* 254 SMO is a trademark owned by Outokumpu OY