Effect of oxygen and oxidizers on the electrochemical behaviour of a-brass in aqueous solutions of different p H

The electrochemical behaviour of a-brass (Cu:Zn ~ 70:30 ) in naturally aerated, oxygen saturated and oxygen free acidic, neutral and basic solutions of constant pH has been investigated. The effect of oxygen and oxidizers on the corrosion behaviour of the alloy in the different solutions has been dis­ cussed. The effect of oxygen or oxidizers on the corrosion parameters of the alloy is dependent on the solution pH. The removal of oxygen or oxidizers from acidic solutions increases the corrosion resistance of the alloy. In these solutions, the presence of oxygen or oxidizers enhances disproportionation reac­ tions which lead to the breakdown of the passive film formed on the ~lloy surface. The preferential dis­ solution of zinc has been found to destroy the passive film, especially in acidic solutions. X-ray photoe­ lectron spectroscopic investigations (XPS) of the alloy surface after long immersion time in solutions containing Cr2O~- ions have shown that chromium is not incorporated in the barrier film. Scanning electron microscopy (SEM) does not show remarkable difference between the polished surface and the surface after immersion in the test solutions. In-acidic solutions containing dichromate ions increased corrosion rates have been measured and clear corrosion patterns in the scanning electron micrographs have been recorded. The wide range of industrial applications of brass alloys is behind the extensive effort~ which have been directed towards the understanding of the corrosion and passivation behaviour of these alloys in different media. Studies of the electrochemical behaviour of brasses are in essence of the study of the dezincification processl-6. Many attempts have been made to understand the mechanism of this process. It was found that the alloy dissolution mode is dependent on the zinc content in the alloy and its polarization conditionl,2,7. Different tech­ niques, electrochemical, radiometric and optical, have been used to investigate the electrochemical behaviour of the alloy and to understand the selec­ tive leaching of zinc 1,8,9. The corrosiQn and pas­ sivation processes were found to depend on the alloy composition, the corrosion medium and the presence of oxygen or oxidizers in the electro­ lyte6,IO,II. In the present work, the electrochemical behaviour of a widely used brass alloy (Cu:Zn ~70:30) was investigated in solutions of different pH, in order to understand the effect of oxygen and oxidizers on the corrosion and passivation processes taking place at the electrode/electrolyte inter­ face. It was important to specify the stability of the passive film formed on the alloy surface in differ­ ent solutions.