O-Substituted Phenols as Corrosion Inhibitors for Aluminium-Copper Alloy in Sodium Hydroxide

The inhibition of corrosion of A1-4% Cu alloy (Al B26 S) in solutions of sodium hydroxide, by o-substituted phenols, has been studied with respect to concentration of alkali and of inhibitor, period of immersion and temperature. In general, at constant alkali concentration, inhibitor efficiency increases with increase in concentration of inhibitor but, at constant inhibitor concentration, decreases with increase in alkali concentration. The inhibitive efficiency remains almost constant with temperature in the range 20–50°c, and with time of immersion up to one hour.

The general order of efficiency in 0·1 M NaOH at an inhibitor concentration of 1% (s as follows: 0-allylphenol < guiacol < o-nitrophenol < phenol ≤ o-cresol ≤ o-aminophenol < catechol < o-chlorophenol < salicylaldehyde. In general, −OH, −N0₂, −Cl, and −CHO groups in the ortho- position increase the efficiency of phenol, while −OCH₃, −NH₂, and −CH₂ CHCH₂ groups decrease the efficiency. The most reliable inhibitors over the concentration range studied are catechol, o-chlorophenol, and salicylaldehyde.

Except catechol, all the inhibitors raise the open circuit potential of B26 S aluminium to slightly more noble values, indicating the polarisation of local anodes.