Corrosion of carbon steel and stainless steel by chloride ions
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What are the corrosion mechanisms of chloride ions on carbon steel and stainless steel, respectively? What is the chloride ion level?
This question is too difficult to answer. If it is a pressure-bearing type, it is recommended to refer to GB/T30579-2014.
In stainless steel tests, the chlorine content should not exceed 25 mg/l
Mechanism of chloride ions in destroying the passivation film of stainless steel: Metals in a passive state still possess a certain degree of reactivity; that is, the dissolution and restoration (re-passivation) of the passivation film are in dynamic equilibrium. When the medium contains active anions (such as chloride ions), the equilibrium is disrupted and dissolution becomes dominant. Chloride ions can selectively adsorb preferentially on the passivation film, displacing oxygen atoms, and then combine with the cations in the passivation film to form soluble chlorides. As a result, small pits (with diameters typically ranging from 20 to 30 μm) are formed at specific points on the newly exposed base metal. These small pits are known as pitting nuclei, and can also be considered the active centers where pitting occurs.
The mechanism involves the formation of hydrochloric acid, which reacts with iron to produce ferric chloride; simultaneously, ferric chloride combines with hydrogen sulfide, an impurity present in the metal, thereby causing further corrosion. The so-called indicator is not merely the chloride ion content in the medium; it should be considered taking into account temperature, concentration, flow rate, and pH value as well.
According to the standards referenced in TSG21, it should be no more than 25 ppm
"According to the standards referenced in TSG21, the limit is no more than 25 ppm. With such high requirements, there are few applications in engineering where conventional materials can be used. The so-called 25 mmp is a requirement for performance tests, aimed at avoiding secondary damage to the material.