Stress Corrosion Behavior of AISI 304 Stainless Steel in a Boiling 42% MgCl₂ Solution Under Cyclic Slow Strain Rate Technique

Stress corrosion cracking (SCO) behavior of AISI 304 stainless steel (SS) rod and plate specimens in boiling 42% MgCl₂ was investigated using a monotonic and a cyclic slow strain rate technique (SSRT) in the crosshead speed (CHS) range from 6 × 10⁻⁵ to 1.5 mm/min. A maximum stress (σ_max) of 50 to 330 MPa was selected in a cyclic slow strain rate test. A thermal cyclic stress test was also conducted in the solution. Fracture surface observations revealed that crack mode changed from transgranular to intergranular with increasing stress level in the SCC process, and the crack mode was independent of the CHS change. Time to failure in the cyclic slow strain rate test was constant at a given σ_max in all CHS used (except slow CHS of 6 × 10⁻⁵ at σ_max of 50 MPa). Crack propagation rate (da/dt) increased with the stress intensity factor, and it was independent of CHS. The corrosion potential varied with the stress cycle after cracking started; this indicated that SCC proceeded by a dissolution-repassivation mechanism. The cyclic slow strain rate test is recommended as an SCC test that can reproduce the actual service conditions of stress.