CHLORIDE-INDUCED STRESS-CORROSION CRACKING OF POWDER-METALLURGY DUPLEX STAINLESS-STEELS

The chloride-induced stress corrosion cracking (SCC) resistance of nit rogen-alloyed, powder metallurgically (P/M) produced and hot isostatic ally 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 s pecimens were performed in aerated 50 wt% calcium chloride (CaCl2) sol ution at 100 degrees C with pH = 6.5 to 7.0. The pitting corrosion res istance index value (PREN) was not a suitable parameter to predict SCC resistance of the investigated DSS. Instead of pitting corrosion, sel ective corrosion of the austenite or ferrite phases seemed to determin e 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 d ependent on the material. High copper content in the specific material slightly lowered the stress limit at which stress corrosion cracks st arted to grow in the used test solution. The SCC resistance of P/M-HIP DSS was as good as Me SCC resistance of forged DSS. The banded micros tructure of forged DSS led to a directional selective corrosion attack . The corrosion grooves were ideal sites for initiation of sec. Becaus e of this phenomenon, the homogeneous microstructure of P/M-HIP DSS sh owed clear advantages over the banded microstructure of forged DSS.