Chloride-Induced Stress Corrosion Cracking of Powder Metallurgy Duplex Stainless Steels

The chloride-induced stress corrosion cracking (SCC) resistance of nitrogen-alloyed, powder metallurgically (P/M) produced and hot isostatically pressed (HIP) duplex stainless steels (DSS) was investigated and compared to the SCC resistance of two commercial wrought (forged) DSS. Constant-strain (deflection) SCC tests with four-point, loaded-bend specimens were performed in aerated 50 wt% calcium chloride (CaCl₂) solution at 100°C with pH = 6.5 to 7.0. The pitting corrosion resistance index value (PREN) was not a suitable parameter to predict SCC resistance of the investigated DSS. Instead of pitting corrosion, selective corrosion of the austenite or ferrite phases seemed to determine the SCC resistance of each material. Selective corrosion was not the primary cause for failure, but it assisted the initiation and growth of stress corrosion cracks. Selective corrosion was noticed in all of the investigated DSS. The corroding phase, austenite or ferrite, was dependent on the material. High copper content in the specific material slightly lowered the stress limit at which stress corrosion cracks started to grow in the used test solution. The SCC resistance of P/M-HIP DSS was as good as the SCC resistance of forged DSS. The banded microstructure of forged DSS led to a directional selective corrosion attack. The corrosion grooves were ideal sites for initiation of SCC. Because of this phenomenon, the homogeneous microstructure of P/M-HIP DSS showed clear advantages over the banded microstructure of forged DSS.