EFFECT OF SOME ANIONS ON THE GALVANIC CORROSION OF AI- Cu COUPLE

Abstract

A general survey of the previous literature concerning the effect of some anions on the corrosion of aluminium, copper and aluminium-copper couple was given. From this survey it appeared that some work was needed to throw more light on the effect of these anions on the corrosion of AI, Cu and AI-Cu couple. Therefore, it was aimed to study the electrochemical behaviour of AI, Cu and AI-Cu couple in Na2 S and Na2C03 under freely aerated and oxygenated conditions in absence and presence of 3.5 % NaCl at room temperature. The general experimental techniques, preparation of the chemicals and treatment of the electrodes were discussed in the experimental part.

The outcome of the results can be summarized as follows:

I- Behaviour in Na2S solutions. 1) From the open circuit potential(O.C.P.)measurements, two features of E-t curves of Cu electrode can be seen by increasing the concnetrations of freely aerated Na2 S. At C < 1o-3 M S2-, oxide film thickening takes place revealed by the increased potential to more positive direction. At C

> 1o-3 M s2

dissolution of oxide film and fonnation of Cu2 S accompained

by potential decrease to more negative values.

Bubbling of oxygen through Na2 S solutions promotes oxide film

thicknening at C < 1o-3 M S2

while at C > 1 o-3 M, the presence of oxygen

facilitates the oxidation of S2- in the medium probably to sulphate according to : s2- + 202 = sal- and the formation of an oxide film of Cu2 0 on the electrode surface leading to positive increase of potential after a period of time which increases with increase ofNa2 S concnetrations.

The presence of 3.5% NaCl with freely.aerated Na2 S produces more aggressiveness to the medium, specially at .lower Na2 S concentrations and causes a shift in potential of the Cu electrode to more negative values.