ELECTROCHEMICAL POLARIZATION METHOD TO DETECT AGING EMBRITTLEMENT OF 321 STAINLESS-STEEL

Type 321 austenitic stainless (UNS S32100) steel boiler tubes used at 550-degrees-C for nearly 20 y in power plant are tested by electrochem ical polarization. The second anodic current peak increases with servi ce time. The second anodic current peak is caused by the formation of chromium-rich carbides and sigma phase at grain boundaries. A normaliz ed second current peak value lp/lo read from electrochemical polarizat ion curves is used to estimate the aging embrittlement degradation. A close relation between the normalized anodic current density value and the service time is observed. From these results, a nondestructive me thod for evaluating the degree of aging embrittlement of 321 stainless steel is derived.