Abstract
The corrosion of 3S aluminium (Al–Mn alloy) and its inhibition in solutions of phosphoric acid has been studied. The extent of corrosion increases with increase in acid concentration up to 10·67 M, and with temperature. The activation energy for the corrosion process remains constant at all molarities in any particular range of temperature, but increases with rise in temperature; e.g., 12·2 kJ/mol (20°–30°c); 20·4 kJ/mol (30°–40°c); 83·1 kJ/mol (40°–50°c).
At constant acid concentration inhibitor efficiency increases with increase in the concentration of the inhibitor, but at constant inhibitor concentration the efficiency decreases with increase in acid concentration. At 0·5% inhibitor concentration in 0·033 M acid the efficiency of the inhibitors increases in the order: n-n-diethylaniline < o-chloraniline < p-chloraniline < p-anisidine < o-toluidine < m-chloraniline < o-anisidine < m-anisidine < n-n-dimethylaniline < p-toluidine < m-toluidine ≤ methylaniline ≤ aniline. The anilines appear to function as inhibitors by adsorption on the metal surface through the nitrogen atom or through the delocalised π-electrons of the aromatic ring.
Maximum inhibition is achieved when the pH of the inhibited acid lies in the range 3·85–6·80, but where the solution is required for use at low pH values, then O- and p-toluidines and m-anisidine and m-chloraniline appear to be the most suitable inhibitors.
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