Stress corrosion cracking behaviors of carbon steel in liquid ammonia

As a carbon-free hydrogen-rich energy carrier, ammonia has gained increasing attention and application in the context of carbon peaking and carbon neutrality. This study evaluated the stress corrosion cracking (SCC) sensitivity of four target materials, A516-70, 16MnDR, 15MnNiDR, and Q370DR, in a liquid ammonia environment at 25 degrees C and 1.03 MPa by slow stress rate tests to determine their SCC sensitivity index. The microstructure, grain size, misorientation, hardness, strength, and micro-fracture morphology of these materials were compared to analyze the SCC mechanism. The results showed that 15MnNiDR exhibited significant SCC sensitivity while both 16MnDR and A516-70 demonstrated certain levels of SCC sensitivity in liquid ammonia. However, Q370DR showed no SCC sensitivity under these conditions. The misorientations observed align with the strains experienced by each respective carbon steel in liquid ammonia. An unstable passivation film formed on the surface of 15MnNiDR steel when exposed to liquid ammonia whereas Q370DR developed a stable oxide film which contributed to its weak SCC sensitivity.