Uniform corrosion occurs when all, or at least a large section, of the passive layer is destroyed. Corrosion reactions then cause a more or less uniform removal of metal from the unprotected surface. Attack by uniform corrosion on stainless steels occurs mainly in acids or in hot alkaline solutions (i.e. sodium and potassium).

Stainless steels generally show good resistance in oxidizing acids such as nitric acid, but are not always able to maintain their passive layer in non-oxidizing acids (like hydrochloric and hydrofluoric acids). Substances, which are neither reducing nor oxidizing, may also affect the corrosivity of acid solutions. Most important are halides, such as chlorides and fluorides, the presence of which increases the corrosivity of both organic and inorganic acids. Even small amounts of halides may affect the corrosion resistance of stainless steels. The resistance to uniform corrosion is generally improved by higher chromium content, since chromium is essential for ensuring the passivity of stainless steels. Nickel is also important as it helps reduce the corrosion rate of depassivated steel. The presence of molybdenum and copper also has a positive effect and enhance the steel’s resistance.

In selecting material, it is important to consider all variations in temperature and chemical composition that might occur in the process environment.


3. Uniform corrosion on the outside of a steam tube that has been exposed to sulphuric acid.