Chloride stress corrosion cracking resistance of 6% Mo stainless steel alloy (UNS N08367)

The UNS N08367 (AL-6XN) alloy was designed to be a seawater resistant alloy and has been successfully used in a multitude of marine and offshore applications including process piping systems, heat exchanger equipment, drilling platforms, and desalination systems. Although the N08367 alloy has improved resistance to chloride stress corrosion cracking (CSCC) it is not completely immune to this mode of failure. This investigation exposed U-bend specimens to various chloride bearing solutions to better define cracking threshold of the N08367 alloy. Samples were exposed in an autoclave using static solutions and no effort was made to either aerate or dearate test solutions. Measurements in solutions with chloride concentrations of 0.02 to 2.0 wt% mixed from ASTM D-1141-52 sea salt, readily produced cracking at temperatures of 260°C. The threshold temperature for the onset CSCC in straight NaCl solutions was found to increase as the chloride concentration decreased. In the range of 0.02 to 15.8 wt% chlorides the threshold temperature in °C for the onset of cracking can be defined as follows. TSCC = 190.05 – 47.42log [Cl]. The results of this testing indicate that it is unlikely to stress corrosion crack this alloy in presence of chlorides at temperatures in the range of atmospheric boiling (≈100°C). At temperatures greater than 120°C cracking can be initiated depending on the chloride content of the environment.

From this testing the following conclusions can be drawn.