Corrosion of metals by nitric acid: effects of concentration and temperature

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◆Effect of concentration: Pure dilute nitric acid or concentrated nitric acid decomposes upon exposure to light at room temperature, producing NO2, O2, and H2O. Nitric acid (even dilute nitric acid) has a strong oxidizing property; the hydrogen produced when it reacts with metals is immediately oxidized to water by the excess nitric acid. Therefore, the products of the reaction between nitric acid and metals are nitrates, water, and nitrogen oxides. Even the most reactive metals do not produce hydrogen gas when reacting with nitric acid. The relationship between the corrosion rate of carbon steel in nitric acid and the acid concentration is very similar to that in sulfuric acid. Carbon steel does not readily passivate, and it continues to suffer from hydrogen-depolarized corrosion in dilute nitric acid. The corrosion rate initially increases as the acid concentration rises; when the acid concentration reaches a certain value (30%), the corrosion rate reaches its maximum ； When the acid content exceeds 30%, the corrosion rate drops rapidly ； When the acid content increases to 50%, the increased oxidizing capacity of nitric acid puts carbon steel in a passive state, resulting in a minimum corrosion rate; further increases in acid concentration have little effect on the corrosion rate ； And when the acid content increases to over 85%, the corrosion rate rises again. The relationship between the corrosion rate of stainless steel in nitric acid and the acid concentration is very similar to that of lead in sulfuric acid. Due to its strong passivation ability, stainless steel can be passivated even in dilute nitric acid with weak oxidizing properties; as a result, it exhibits excellent corrosion resistance to such acid. An increase in acid concentration has little effect on the corrosion rate. However, in concentrated nitric acid with an azeotropic composition of 68.4% or higher, the extremely strong oxidizing power of the acid causes excessive passivation of the stainless steel, leading to an increased corrosion rate. Even high-purity stainless steel in the unsensitized state cannot avoid intergranular corrosion in fuming nitric acid. The corrosion of aluminum in nitric acid follows a pattern similar to that of carbon steel in nitric acid: below 30% nitric acid concentration, as the acid concentration increases, the hydrogen ion concentration rises, thereby intensifying hydrogen depolarization corrosion ； When the acid content increases to over 30%, the corrosion rate decreases due to passivation. An important characteristic of aluminum is that it does not suffer from passivation corrosion even in very concentrated nitric acid; in nitric acid with an 80% concentration, aluminum’s corrosion resistance is **better than that of stainless steel ； However, in dilute nitric acid, the corrosion resistance of aluminum is much worse than that of stainless steel. ◆The effect of temperature: Carbon steel can be passivated in nitric acid with a 50% concentration at a temperature of 25°C; however, when the temperature rises to 75°C, it becomes difficult to passivate it even in nitric acid with a 90% concentration. It can be seen that an increase in temperature accelerates the corrosion of carbon steel by nitric acid. For stainless steel, when the temperature of nitric acid is not too high, it is in a passivated state; raising the temperature has no significant effect on the corrosion rate. However, at higher temperatures, the stainless steel becomes over-passivated, which results in an increased corrosion rate. For example, 18-8 type low-carbon stainless steel (0Cr18Ni9Ti) that has undergone solution treatment (quenching-annealing) has an erosion rate in nitric acid with a concentration of 65% at temperatures below 90°C of less than 0.125 mm, and temperature variations have little effect on the erosion rate ； And when the temperature reaches the boiling point (121°C at standard pressure), the annual corrosion rate increases to 0.5 mm; at 127°C, the annual corrosion rate is 2.54 mm ； At 138°C, the annual corrosion rate is 5.1 mm. That is, when the temperature rises by only 17°C above the boiling point, the corrosion rate increases by a factor of 10 ； When the temperature increases by 67°C, reaching 188°C, the annual corrosion rate becomes 127 mm, an increase of 250 times.