Nickel alloys potential/anodic current density

The technique may be understood in terms of metallic passivity, i.e. the loss of chemical activity experienced by certain metals and alloys under particular environmental conditions as a result of surface film formation. Equations 15.2 and 15.3 suggest that the application of an anodic current to a metal should tend to increase metal dissolution and decrease hydrogen production. Metals that display passivity, such as iron, nickel chromium, titanium and their alloys respond to an anodic current by shifting their polarisation potential into the passive regon. Current densities required to initiate passivity are relatively high [Uhlig and Revie 1985] but the current density to maintain passivity are low, with a consequent reduction in power costs [Scully 1990]. 

A schematic summary of the alloying metals that affect the anodic polarization curve of stainless steel is shown in Fig. 4.16. The addition of 8% nickel to an alloy containing 18% chromium forms austenitic structure SS Type 304. The addition of Mn and N increases the stability of austenitic steel. The chromium content of stainless steel affects the anodic polarization curves as shown in Fig. 4.16. Nickel promotes repassivation in a corrosive environment, but concentrations higher than 30% reduces the passivation current, the critical current density, and increases the critical pitting potential. Nitrogen... 

In acidic media, the metals iron, nickel and chromium have passivation current densities that increase in the order Cr < Ni < Fe. In Figure 6.11, the anodic polarization curves for the three metals in 0.5 M sulfuric acid (25 °C) are compared. Chromium has lower values of both ip and Ep than the other two metals. By alloying increasing amounts of chromium to steel one therefore improves the corrosion resistance. Experience shows that above a chromium concentration of 12 to 13%, a steel passivates spontaneously in contact with aerated water. It becomes "stainless", meaning it does not rust easily. Figure 6.12 gives the corrosion potential of different... 

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