Nickel alloys critical pitting potential

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... 

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

Nickel, containing 0.6 rf-electron vacancy per atom (as measured magnetically), when alloyed with copper, a nontransition metal containing no rf-electron vacancies, confers passivity on the alloy above approximately 30-40 at.% Ni. Initiation of passivity beginning at this composition is indicated by corrosion rates in sodium chloride solution (Figs. 6.12 and 6.13), by corrosion pitting behavior in seawater (Fig. 6.13), and, more quantitatively, by measured values of /critical and passive (Fig. 6.14), [41-43] or by decay (Flade) potentials (Fig. 6.15) [44] in IN H2SO4. 

---