Abstract
The action of sodium nitrite as a corrosion inhibitor for mild steel in neutral aqueous solution has been studied in relation to the surface preparation of the steel, the presence of aggressive salts in the solution, and temperature.
Surface preparation of mild steel has little effect on the minimum concentration of nitrite required for protection in distilled water. In solutions containing aggressive anions a linear relation exists between the logarithm of the nitrite concentration and the logarithm of the maximum concentration of aggressive anion that will permit inhibition. In solutions of low nitrite concentration the order of aggressiveness of anions is sulphate > chloride > nitrate; the order changes with increase in nitrite concentration. Corrosion in non-inhibiting nitrite/aggressive anion solutions is frequently of a severely localised form. A 5-fold increase in nitrite requirement for inhibition of abraded mild steel in distilled water is found between 5° and 70°, with a further large, rather indeterminate, increase above 70°.
Of the three inhibitors studied in Parts I-III, chromate and nitrite are slightly moreeffective than benzoate for the protection of an abraded surface, and considerably more so for a grit-blasted surface. In general, nitrite is the most tolerant towards the presence of chloride, but it is the most dangerous if excess chloride is present. The increase in inhibitor requirement with increase in temperature from 5° to 70° is very approximately the same for all three inhibitors, that is, about 5– to 10–fold, with all inhibitors needing much higher concentrations at about 90°.
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