The effects of dissolved ozone on the corrosion behavior of nickel-based chromium-molybdenum alloys in artificial seawater

Ozone is currently being considered as an environmental-friendly alternative to chlorine-based marine biocides. The goal of this study was to determine the effects of dissolved ozone on the corrosion behavior of nickel-based Cr-Mo alloys in artificial seawater. Alloys 276, 22, 625, 59 and 690 (UNS N10276, N06022, N06625, N06059 and N06690) were studied, and the results were compared with those obtained for aerated solutions.; It was found that although these alloys undergo uniform transpassive dissolution on crevice-free surfaces in ozonated artificial seawater, the corrosion rates were less than 10 {dollar}mu{dollar}m/y for all alloys. Oxidation of transpassively dissolved nickelous ions caused the formation of a nickel oxide precipitate in ozonated solutions.; An anomalous form of crevice corrosion was displayed by Alloy 276 in ozonated artificial seawater. The corrosion was concentrated in regions containing stagnant solutions and in areas where the geometry of the sample created a boundary layer. This "boundary layer corrosion" (BLC) was typified by uniform corrosion that produced an electropolished finish beneath a brown iron silicate corrosion product. The highest uniform corrosion rates of these areas were 48-72 {dollar}mu{dollar}m/y.; The mechanism of BLC is driven by acidification of a stagnant interfacial area due to the oxidation of transpassively dissolved nickelous ions and passivity enhancing elements. The boundary layer allows only limited diffusion of ozone into the layer, and prevents the nickelous ions and acidified corrosion products from being removed, which, in turn, causes the destabilization of passivity and increased corrosion of these areas.; The occurrence of BLC in nickel-based alloys was related to the weight percent of the alloying elements which enhance passivity, due to their ability to form acidic components when transpassively dissolved. The pitting resistance equivalent number (PREN) calculated using the formula: PREN = Cr + 3.0Mo + 1.65W, indicated that at values of the PREN greater than 50, the alloy becomes susceptible to BLC. At PREN values less than 50, accelerated classical crevice corrosion underneath tight crevices occurs.