Uniform corrosion occurs when the passive layer on a stainless steel surface partly or completely breaks down.
The corrosion then propagates at a rate determined by a combination of the corrosive environment and the alloy composition. Uniform corrosion or general corrosion occurs on stainless steel in acid environments or hot alkaline solutions. Severe environments from a corrosive point of view are high concentrations of hydrochloric or hydrofluoric acid in which the corrosion may propagate at a rate that can be detrimental to a construction.
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Uniform corrosion rate is also affected if the acid contains oxidising or reducing chemicals. Reducing impurities, for example hydrogen sulphide, may increase the corrosion rate. An oxidising acid that has a positive effect at lower concentration is nitric acid. It promotes passivation of the stainless steel surface at some concentrations rather than initiating uniform corrosion.
Nevertheless, there is a limitation of how oxidising the environment may be from a corrosive point of view. A hot concentrated nitric acid can dissolve the passive layer and transpassive corrosion may occur. The mechanism of transpassive corrosion is not similar to uniform corrosion as the passive layer will be oxidised to a more soluble species but the consequence is the same, the passive layer breaks down partly or completely and uniform corrosion occurs. Higher alloyed stainless steel grades are more resistant to uniform corrosion than the lower alloyed stainless steel grades.