Corrosion Performance of alloys in Various Acid Mixtures

In various processes, mixtures of various acids or acids and salts occur. Corrosion resistance in these conditions is sometimes predictable qualitatively. In some condition, anomalous effects can be developed. However, it is impossible to mention the corrosion rates of alloys in several acid mixtures within the constraints.

The corrosion resistant wrought nickel alloy families include commercially pure nickel, nickel-copper alloys, nickel – chromium – molybdenum alloys and nickel – chromium – iron – molybdenum alloys. Similar alloys categories are feasible for cast alloys. Nickel and nickel alloys for example stainless steels offer an extreme level of corrosion resistance. Although nickel can accommodate larger magnitudes of alloying elements, mainly chromium, molybdenum, copper and tungsten in solid solution comprising of iron. So nickel base alloys can be used in more vigorous environments and offer supreme resistance to general corrosion, pitting, crevice and intergranular corrosion and stress corrosion cracking.

Sulfuric and Nitric acid solution – Alloys comprising of chromium and attain active passive behavior, inclusion of nitric acid or nitrates to sulfuric acid will decrease the corrosion rate. In nonchromium alloys such as Hastelloy B2 and Monel alloy 400, inclusion of nitric acid will increase the corrosion rates. The nitrate reduction reaction enhances the redox potential in sulfuric acid solution, the redox potential in sulfuric acid solution, the redox potential of sulfuric acid solution is controlled by hydrogen ion reduction reaction. In nonpassivating alloys for instance, Hastelloy B2 in which the corrosion current increases monotonically with potential, increase in potential increases the corrosion rate. In passivating alloys, increase in potential can move the alloy from active state to the passive state, hence decreasing the corrosion rate. In high nitrate concentrations, the passive current density increases that increases the corrosion rate. For alloy C76, an increase in corrosion rate is only observed with nitric acid addition. It is feasible that for lower concentrations of HNO3 a reduction in corrosion rate could be noticed.

Sulfuric acid and hydrochloric acid-Inclusion of alkali chloride salts or hydrochloric acid to sulfuric acid increases the corrosion rates of all alloys. In deaerated conditions, Hastelloy B and Hastelloy B2 are the most versatile with alloys C276 and HastelloyC22 bar. Normally the higher molybdenum concentration offers the better performance of alloy in mixture of sulfuric acid and hydrochloric acid.

Nitric acid and Hydrochloric acid mixtures- The influence of nitric acid to hydrochloric acid are similar to influence of nitric acid to sulfuric acid. Although in HNO3 + HCl mixtures, pitting causes corrosion, instead uniform corrosion that occurs in mixture of sulfuric acid and nitric acid. Additionally, nominal variations in HCl content can create wide changes in corrosion rates.

Nitric acid and Hydrofluoric acid mixtures – The inclusion of nitric acid to hydrofluoric acid decreases the corrosion rate initially however above 10% HNO3, the corrosion rate increases. Increasing HF content results into increased corrosion rate. Although unlike to HCl inclusion, higher chromium alloys normally showed nominal rates, irrespective of molybdenum concentration. Intergranular corrosion was also noticed in various alloys. In these cases, higher temperature resulted into increase in corrosion rates.