Passive Films on 90Cu‐10Ni Alloy: The Mechanism of Breakdown in Chloride Containing Solutions

Abstract

The complex electrochemical behavior of 90Cu-10Ni alloy in borate buffer solutions containing chloride ions involves Cu oxide formation, passivity onset, and pitting corrosion. The nature of the processes occurring was studied using electrochemical techniques complemented with SEM and EDXA. The overall process can be described by two competing processes: formation and growth of the duplex oxide passive film and formation of the CuC1 layer. The CuC1 layer is poorly protective and when the applied potential exceeds a certain critical value, pitting corrosion takes place. The change in both the kinetics and the morphology indicates that at least three stages should be distinguished in the pitting process: adsorption of C1-ions, pit nucleation, and pit growth. The physical interpretation of the proposed model of passive film breakdown is consistent with its duplex structure, and is discussed in terms of the various conditions leading to nucleation and growth of pits. Experimental kinetic parameters of the passive film breakdown are presented.

Prediction of Corrosion of Advanced Materials and Fabricated Components

“…This is in agreement with findings by Milosev and coworkers for the case of pitting and repassivation potentials of Cu-Ni alloys (Milosev and Metikos-Hukovic, 1991;Metikos-Hukovic and Milosev, 1992).…”

“…Chloride ions cause the breakdown of the passive film on the Cu-Ni alloys leading to localized corrosion (Milosev and Metikos-Hukovic, 1991;Metikos-Hukovic and Milosev, 1992). The structure, composition and thickness of the passive film influence the extent of localized corrosion .…”

“…This chloride ion incorporation is thought to modify the oxide lattice. The Cl -ions replace the O 2-ions in the lattice and enhance cation diffusion through the oxide layer (Milosev and Metikos-Hukovic, 1991). The charge imbalance caused by the ion substitution is neutralized by ejection of a Cu + ion into the solution creating a cation vacancy (Milosev and Metikos-Hukovic, 1991 The copper ion dissolution causes the formation of a porous film over the dissolution site (Hettiarachchi and Hoar, 1979).…”

“…The alloy exhibited passivity in this Cl -concentration range due to oxide growth by migration under a high electric field (Zaky and Assaf, 2002). The breakdown of this film is believed to occur when the chemisorbed chloride ions on the surface react with Cu, promoting its dissolution according to the following equations (Milosev and Metikos-Hukovic, 1991):…”

Kinetic measurements during transient film growth on zinc

“…The first straight line may be considered to correspond to the period of pit nucleation, while the second line to that of pit growth. 30 The increase of current transient after t i can be represented by the Engell-Stolica equation for pitting corrosion: 17…”