Selection of Stainless Steel for Handling Chlorine Cl2 and ClO2
Chlorine is a powerful oxidising agent. It normally occurs as a gas but can be liquefied under pressure at ambient temperature. As a dry gas it does not tend to be aggressive towards stainless steel, but as either a damp gas or dissolved in water it can be a corrosion hazard.
Chlorine readily forms chlorides when in contact with gases such as methane, hydrogen sulphide and ammonia. Hydrochloric acid HCl can also be formed by these reactions.
Chlorine dissolves readily in water forming hydrochloric and hypochlorous (HOCl) acids, which is very corrosive mixture.
Chlorine can also assist in the oxidation of dissolved gasses, such as sulphur dioxide (SO2), forming sulphuric and hydrochloric acid in water. It is these oxidising properties that make chlorine an aggressive component in waters.
Corrosion resistance of stainless steel
Chlorine in contact with water and as a dissolved gas, sometimes found in water treatment applications, is potentially aggressive to stainless steel.
Localised crevice & pitting corrosion attack is a hazard in water and stress corrosion cracking (SCC) can be an additional hazard in damp chlorine gas, if the temperature is high enough.
Condensates formed over chlorinated water in storage tanks have been known to result in staining or pitting to stainless steels. Improvements to ventilation in such situations should help reduce the risk of attack.
Chlorine as a sterlizing or sanitising agent
When using chlorine as a sterilizer or sanitiser in contact with type 316 stainless steel items, a maximum of 15-20 ppm (mg/lt) 'free' chlorine is suggested, for maximum times of 24 hours, followed by a thorough chlorine free water flush. As with any additions, thorough dilution around the injection point is important to avoid localised 'over-concentration' problems.
Residual chlorine levels in waters of 2ppm maximum for 304 and 5ppm for 316 types should not normally be considered a crevice corrosion hazard.
Chlorine dioxide as a sanitiser in contact with stainless steel
Chlorine dioxide (ClO2), occurs naturally as a gas, but is normally dissolved in water, as the gas is highly explosive. Although a powerful oxidiser, unlike chlorine it does not breakdown to release chlorides. The chlorine and oxygen work together tending to form chlorites in the oxidation process. Although the chlorite can break down to form chlorides, it is a weaker oxidising agent than the chlorine dioxide and so can be expected to be less of a hazard to stainless steel when used as a water sanitiser.
Related References:
Selection of Stainless Steel fo Handling Sulphur Dioxide SO2 and Sulphur Trioxide SO3
Selection of Stainless Steel for Handling Phosphoric Acid H3PO4
Selection of Stainless Steel for Handling Hydrofluoric Acid HF
Selection of Stainless Steel for Handling Citric Acid C3H4OH (COOH)3
Selection of Stainless Steel for Handling Ammonia NH3
Selection of Stainless Steel for Handling Chlorine Cl2 and Chlorine Dioxide ClO2
Selection of Stainless Steels For Handling Hydrochloric Acid HCl
Selection of Stainless Steel for Handling Sulphuric Acid H2SO4
Selection Stainless Steel for Handling Sodium Hydroxide NaOH
Selection of stainless steels for handling acetic acid (CH3COOH)
Selection of stainless steels for handling sodium hypochlorite (NaOCl)
Selection of stainless steels for handling nitric acid (HNO3)
NACE MR 0175/ISO 15156 for Corrosion Resistant Alloys for Sulphide Service
Selection of stainless steels in water supply and waste water treatment
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