EFFECT OF MICROSTRUCTURE ON CORROSION BEHAVIOR OF 9-PERCENT CHROMIUM 1-PERCENT MOLYBDENUM STEEL

The corrosion behavior of 9% Cr-196 Mo ferritic steel was studied usin g potentiodynamic polarization measurements in 0.5 M sulfuric acid (H2 SO4) with and without addition of chloride (Cl-) and arsenic oxide (As 2O3). The influence of various heat treatments and microstructure on t he electrochemical parameters and corrosion rate was investigated The steel showed characteristic features of active, passive, secondary pas sive, and transpassive regions. The open-circuit potential (OCP), prim ary passive potential primary and secondary passive ranges, and transp assive potential were about the same for all microstructures studied. Because of the inhibiting action of As2O3, the critical current densit y (i(crit)), Flade potential and primary (t(pass)) and secondary passi ve current densities (i(sec . pass)) were lower in H2SO4 containing As 2O3 than in H2SO4 alone. In both media, l(crit) and the Flade potentia l were very high for the annealed material consisting of proeutectoid ferrites and carbides. In the presence of Cl-, this steel was suscepti ble to pitting attack from localized breakdown of passivity. Passive b ehavior was observed in sodium chloride (NaCl) solution up to a concen tration of 0.005 M Cl-, while passivity could be sustained to a much h igher limit (up to a concentration of 0.1 M Cl-) in the H2SO4 medium. Critical pitting potentials (E(pp)) in a solution containing 0.001 M N aCl were 300 mV(SCE), 200 mV(SCE), and 140 mV(SCE) for quenched, norma lized, and annealed materials, respectively. When the normalized mater ials were tempered, pitting resistance decreased during the initial st ages of tempering. With further tempering, pitting resistance was rest ored. Pits nucleated preferentially within the ferrite grains in annea led materials, at the lath boundaries in quenched and normalized mater ials, and at prior austenite grain boundaries for tempered materials. The corrosion rate obtained from polarization and immersion tests decr eased with decreases in cooling rate from the austenitizing temperatur e. The fully tempered material was more resistant to aqueous corrosion . During cathodic polarization, the material was susceptible to hydrog en blistering. Severity of attack was highly dependent upon microstruc ture. Material that was austenitized and furnace-cooled (annealed) was more prone to failure by blistering. The difference in corrosion beha vior and susceptibility to blistering of various heat-treated material s was correlated to microstructural differences arising from heat trea tments.