Abstract
The dissolution behavior of a metal in a given environment is related to the competitive adsorption of available species onto the metal surface. Thus, for example, the effectiveness of passivators for mild steel can vary considerably with the nature and the concentration of the aggressive environment. This study was designed to evaluate the effects of four anions on the corrosion behavior of mild steel. The four anions selected were chloride and sulfate as aggressive ions, and nitrite and chromate as passivator ions. The aggressive anions were evaluated singly over a concentration range of four orders of magnitude (from 10 to 10,000 ppm) in order to obtain the effect of concentration in each case. The four anions, in groupings of two, were then studied over the same concentration range so as to ascertain the relative proportions of inhibiting and noninhibiting ions necessary to ensure passivity. Instantaneous corrosion rates measured by the linear polarization method were used as the criteria in comparing the effect of environmental changes. Multiple regression analysis techniques were utilized to obtain quantitative relationships between the concentrations of the four ions studied and the corrosion rates observed. The results are applied to a consideration of the mechanism of formation and maintenance of a passive film, and this is discussed in terms of electrochemical polarization, competitive adsorption, and passive film stability.
