Potential and Strain-Rate Effects in Slow Strain-Rate Stress Corrosion Cracking of Type 304 Stainless Steel in 35 Percent Magnesium Chloride at 120°C

Abstract

Constant extension rate experiments have been performed on Type 304 stainless steel round tension-test specimens in 35 percent magnesium chloride at 120°C as a function of electrode potential and extension rate. When potentiostatic measurement was applied, an extension rate of 4.4 × 10−6 s−1 was found to be acceptable as an accelerated test. These tests are compared with constant load measurements by variation of the electrode potential. Both methods indicate that stress-corrosion cracking decreases by lowering electrode potential and disappears below −380 mV (saturated calomel electrode). Current density-time plots made during constant straining and fractographic observations show different types of crack formation. The number of cracks increases with increasing potential, and the crack depth decreases with accelerated straining. Transgranular stress-corrosion cracking is observed accompanied by pitting. The selective corrosion in the early stage of cracking was revealed by scanning electron microscope (SEM) micrographs and SEM analysis. Iron and nickel are dissolved and a chromiumrich phase is formed along the crack walls. Although the first attack appears to be intercrystalline, the fracture shows only transgranular features.