Passivity of Aluminium and pH Values

As an illustration, the dissolution rate in a solution of sodium hydroxide at 0.1 g l is 25 times higher than in an ammonia solution at 500 g l, although both solutions have very similar pH values, 12.7 and 12.2, respectively. 

The same applies in acid solutions at a given pH value, solutions of hydrochloric acid or hydrofluoric acid are much more aggressive towards aluminium than solutions of acetic acid. 

Like all passive metals, aluminium is prone to pitting corrosion in aqueous media close to neutrality. Under these conditions, pitting corrosion depends more on the quantity of anions, such as chlorides, than on variations in the pH value of the aqueous medium. 

In organic media, the resistance of aluminium and its alloys also depends on the stability of the natural oxide film. If this film is damaged or destroyed, for example, in contact with certain organic acids at their boding point, the underlying aluminium may be attacked. This may result, in certain cases, in extremely fast and violent reactions (see Chapters F.2, F.3 and F.6). 

Smith C.A., The corrosion story. Part 3, Zinc, lead and aluminum, Anti-Corrosion, vol. 24, 1977, p. 13-15. 

---