Sodium benzoate and other metal benzoates as corrosion‐inhibitors in water and in aqueous solutions

Sodium benzoate has been shown to be an effective inhibitor of the corrosion of mild steel in distilled water, a moderately hard mains‐water and very dilute (e.g. 0·03%) sodium chloride solutions. The concentration of benzoate required for inhibition is greater (0·5%) for machined than for emeried surfaces (0·1% in favourable conditions) and for mains water or chloride solutions (1·0 or 1·5%) as compared with distilled water (0·5%). Movement of the solution, or saturation with oxygen, assists inhibition, but a pH below 6 causes breakdown. Comparisons with sodium chromate show that sodium benzoate is less efficient; it is, however, a ‘safe’ inhibitor since it does not lead to intense localized corrosion when the concentration is just below the minimum for protection. The following benzoates have also been shown to possess inhibitive properties: potassium, lithium, zinc and magnesium. Zinc is partly, and copper and aluminium completely, protected in 0·05% sodium benzoate solution at room temperature. An unusually high rate of hydrogen gas evolution occurs in dilute sodium chloride solutions containing insufficient sodium benzoate for complete inhibition. A tentative explanation is suggested. The detailed mechanism of the protective action of sodium benzoate is not yet established, but electrode‐potential measurements and film‐stripping experiments provide evidence for the view that anodio inhibition produces and maintains a continuous film. Electron‐diffraction examination of the stripped film has so far yielded definite evidence of γ‐ferric oxide (or Fe3O4) only.