The Corrosion Behavior of Nickel 270 in Phosphate-Containing Solutions

The corrosion behavior of Ni 270 in 1M H₃PO₄, NaH₂PO₄, Na₂HPO₄, and Na₃PO₄ solutions, in the 25 to 100 C temperature range, was studied. Potentiokinetic and potentiostatic polarization curves were drawn in the four solutions. Anodic kinetics of Ni 270 increased with increasing temperature and increasing acidity of the solution.

Corrosion potentials in 1M H₃PO₄ and 1M NaH₂PO₄ solutions were in the active corrosion range at all tested temperatures. Active to passive transition took place by successive steps at 25 and 50 C in both solutions. The first retardation in the anodic kinetics of Ni 270 seemed to be related to NiO formation and the second one to Ni₃O₄ formation. At 75 and 100 C only one active peak was observed. Peak potential values were close to Ni/Ni₃O₄ redox potentials for the 1M H₃PO₄ solution and to Ni/NiO redox potentials for the 1M NaH₂PO₄ solution. Passivation potential values for both solutions in the 25 to 100 C range were close to the corresponding Ni/Ni₂O₃ redox potentials.

Potentiostatic polarization curves of Ni 270 in 1M Na₂HPO₄ solution showed that corrosion potentials were in the passive range at 25, 50, and 75 C. An active peak was obtained at 100 C, followed by a passive region. The corrosion potential of Ni 270 in the 1M Na₃PO₄ solution was in the passive zone in the 25 to 100 C temperature range.

Film growth mechanism of the oxide film in the passive range was estimated from current density decay curves at constant potential. The initial state of anodic film growth follows a logarithmic or an inverse logarithmic law for most of the testing conditions. Film thickness values were also estimated in 1M H₃PO₄ and 1M NaH₂PO₄ solutions at 25 and 50 C.