SSINA Stainless Steel Corrosion. Definition The combination of tensile stress and a specific corrosive environment can crack stainless steels. This mode of attack is termed stress corrosion cracking SCC. The most common environmental exposure condition responsible for SCC of stainless steels is the presence of chlorides. Although no stainless steel grade is totally immune to chloride SCC, the relative resistance of stainless steels varies substantially. Influence of Alloy Composition The relative resistance to chloride SCC is dependant on the stainless steel family. The austenitic family of stainless steels is the most susceptible. The resistance of austenitic stainless steels to SCC is related to the nickel content of the steel. The most susceptible austenitic grades have nickel contents in the range of 8 to 1. Therefore, standard grades such as 3. L and 3. 163. 16. L are very susceptible to this mode of attack. Austenitic grades with relatively high nickel and molybdenum contents such as alloy 2. L, and the 6 molybdenum super austenitic grades have substantially better chloride SCC resistance. The ferritic family of stainless steels, which includes grades such as type 4. SCC. The duplex stainless steel with their dual austeniteferrite microstructures has a resistance that is in between that of the austenite and ferrite grades. Corrosion Testing. The relative resistance of a stainless steel to chloride SCC is often quantified by the use of standard boiling salt solutions. The following table summarizes the results of testing in boiling salt solutions of 2. Na. Cl sodium chloride, 3. Li. Cl lithium chloride, and 4. Mg. Cl. 2 magnesium chloride. The boiling Li. Cl and Mg. Cl. 2 test solutions are very aggressive relative to practical applications and only austenitic alloys with compositions that approach those of nickel base alloys will routinely resist cracking in these test solutions. Table 1 Relative chloride SCC resistance measured using fully immersed U bend specimens in standard boiling salt solutions. Taken from producer dataAlloy. Mg. Cl. 23. 3 Li. Cl. 26 Na. Cl. Austenitic SSTType 3.
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November 2017
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