Active-passive type alloys potential ranges

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

Reference has been made to the observation that both anionic and cationic species in the environment can influence the anodic polarization of active-passive types of metals and alloys. Specific examples have related to the effect of pH as it influences the stability and potential range of formation of oxide and related corrosion product films. The effect of pH, however, cannot be treated, even with single chemical species, independent of the accompanying anions. For example, chloride, sulfate, phosphate, and nitrate ions accompanying acids based on these ionic species will influence both the kinetics and thermodynamics of metal dissolution in addition to the effect of pH. Major effects may result if the anion either enhances or prevents formation of protective corrosion product films, or if an anion, both thermodynamically and kinetically, is an effective oxidizing species (easily reduced), then large changes in the measured anodic polarization curve will be observed. 

The authors also noted that following potentiodynamic polarization from the corrosion potential to 0 mV at a scan rate of 1 mV/s, XPS analysis was still able to detect significant quantities of surface nitride. This is illustrated in Figure 11. The most active of the alloys studied, type 304, was determined to have dissolved approximately 20 monolayers. This suggested that the nitride may form a kinetic barrier that is protected by the oxide passive film from rapid protonation to ammonia and ammonium in the active range of potential. In the same study the nitride phase formed on Ni had little effect on anodic behavior in 0.1 M HCl,... 

---