Martensite transformation and surface cracking of hydrogen charged and outgassed type 304 stainless steel

The effects of cathodic hydrogen charging and outgassing on martensite tran sformation, surface cracking and anodic dissolution behavior of type 304 st ainless steel have been investigated. Cathodic charging and aging results i n the formation of a considerable amount of epsilon and alpha' martensites. Optical observation combined with X-ray diffraction analysis shows that th ere are critical charging conditions for the martensite transformation. The minimum charging current density to induce the phase transformation is 0.2 mA cm(-2). Surface cracks are observed on the specimens, which are hydroge n charged for a longer time than the critical charging time for the matensi te transformation. The surface cracks mainly initiate during aging instead of during cathodic charging and propagate along martensite/austenite interf aces, grain boundaries and twin boundaries. The anodic dissolution tests in 0.5 M H2SO4 + 0.1 M HCl solution reveal that an obvious increase in the we ight loss is due to the preferential dissolution of martensite phases and t he weight loss decreases with the outgassing time. (C) 2000 Elsevier Scienc e S.A. All rights reserved.