Corrosion Resistance in Chemical Environments

Acidic environments are involved in most of the severe corrosion problems encountered with steels, as with most metals, although alkaline environments can also be responsible for increased corrosion. The corrosion rate of steels in acids depends upon the composition and concentration of acid, as well as temperature. The corrosion rate of steels in hydrochloric acids will continuously increase with increasing acid concentration. In sulfuric acids, however, the corrosion rate increases until a level of concentration where passivity is reached, see Figure 33. If the passive film is damaged by mechanical or chemical means, the corrosion rate will significantly increase in concentrated solutions.

Nitric acid readily attacks carbon and low alloy steels. Austenitic stainless steels, as well as aluminum alloys, form strong adherent oxide films. This makes them the most applicable metals for use with nitric acid.

Sodium and potassium hydroxides have similar effects on steels. The uniform corrosion rates are generally ≤ 2 mils/yr, for all concentration levels. The problem with exposure of low alloy steels to these materials is the susceptibility to SCC, sometimes referred to as caustic embrittlement. The relation of temperature and sodium hydroxide concentration to observed cracking is shown in Figure 34.