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Pit initiation on passivated metal surfaces: crystallographic orientation and environmental effects Guo, Ruijin

Abstract

The susceptibility to pitting corrosion of tin and zinc was found to be dependent on the crystallographic orientation of the surface in chloride solutions. For tin single crystals, the(111) face showed the lowest critical pitting potential among five differently oriented surfaces. The critical pitting potential of zinc single crystals decreased with the surface orientation in the following order: (1010) > (1120) > (0001). The critical pitting potentials for zinc and aluminum were found to be lower in chloride solutions than in bromide solutions. The effects of the solution pH, the presence of buffers, and halide ion species on the pitting of polycrystalline nickel were investigated in 1.0 M halide solutions. It was found that the critical pitting potential (Ecp) was independent of the pH of unbuffered solutions in the pH region of 4.5 to 10.5. However, Ecp was greatly increased at pH 12.5, and pitting corrosion was totally retarded at pH 14 in 1.0 M NaC1 solution. The addition of Na2CO3/NaHCO3 orNa3PO4/Na2HPO4 buffer to the pH 10.5 solution raised the critical pitting potential of nickel. Furthermore, Ecp was lower in the chloride solution than in the bromide solution. Using published thermodynamic data for halide complexes, potential-pH (E-pH) and halide concentration - pH (X-pH) diagrams were constructed for H20-halide-metal (Sn, Zn, Al and Ni) systems. With increasing halide concentration, the formation of halide complexes is thermodynamically favoured and the zone of passivity is diminished. These new diagrams are unique and useful in the understanding of pit initiation. A pitting theory is proposed, which emphasizes the change in local solution chemistry and the formation of halide complexes during the pit initiation process. Pits will be nucleated when conditions are met for the formation of stable halide complexes in the local region. This theory successfully accounts for the effects of the solution pH, buffers, halide ions and the crystallographic orientations of single crystals on pitting behavior. It may also explain other aspects such as the effects of temperature, alloying elements, and fluid flow on the pitting corrosion of metals.

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