Uniform corrosion, also known as general corrosion, is the uniform loss of metal over an entire surface. However, it is not regarded as the most serious form of corrosion of stainless steel because it is relatively easy to predict. This type of corrosion is perhaps the most well known corrosion type in existence.
Uniform corrosion is characterized and expressed as a mass loss per unit area and unit of time, e.g. mm/year. If the corrosion rate is less than 0.1 mm/year the stainless steel is normally considered to be resistant.
Problems with uniform corrosion on stainless steel are most likely to occur with exposure to strong acid and hot alkaline environments. Generally, inorganic acids are more aggressive than organic, with hydrochloric acid being one of the worst.
| Inorganic acids | Organic acids | Alkaline solutions |
|---|---|---|
| Sulphuric acid (H2SO4) | Formic acid (HCOOH) | Sodium hydroxide (NaOH) |
| Phosphoric acid ((H3PO4) | Tartaric acid | - |
| Hydrochloric acid (HCl)* | Acetic acid (CO3COOH) | - |
| Nitric acid (HNO3)** | Lactic acid | - |
* Pitting occurs at low concentration/low temperature
** Passivation at low concentrations
Acids can also be divided into reducing (non-oxidizing) and oxidizing acids:
| Reducing | All organic acids and most inorganic acids; H2SO4 in concentrations up to about 90%. |
|---|---|
| Oxidizing | Above all HNO3; H2SO4 in concentrations over about 90% |
As a rule, the higher the concentration and temperature of the solution, the more aggressive it becomes and the sooner problems with uniform corrosion are likely to occur. Also, impurities in the solution can have a drastic influence.
Influence of alloying elements
Depending on the type of environment, alloying elements have a varying influence on the uniform corrosion resistance of stainless materials:
Reducing acids
- Positive: Cr, Mo, Cu (in inorganic acids)
Oxidizing acids
- Positive: Cr, Si (in H2SO4 in concentrations over about 90%)
- Negative: Mo, C, P
Alkaline solution
- Positive: Cr, Ni
Facts in brief about uniform corrosion
- Uniform corrosion is the most well known corrosion type
- Rates of uniform corrosion are expressed as a mass loss per unit area and unit of time
- Uniform corrosion on stainless steel occurs mostly with exposure to strong acid or hot alkaline environments