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It rather depends on the type of metal being used and how it is protected. Mild steel will corrode quite well in any water chlorinated or not which is why it is generally protected by either coatings or electrolytic protection. The oxygen dissolved in the water or as we are considering dissolved chlorine can take electrons from iron atoms turning them into ions which are then insoluble as the familiar brown ferric oxide (hydroxide after absorbing water). Paint or other coating protect by restricting access to the metal surface by the water.

Other metals can be used to protect such as zinc, either as a coating such as galvanising or separate anodes such as are used on ships but these depend on continuous immersion. In this case the zinc preferentially reacts with the oxidant protecting the steel but is itself protected by its oxide film that is harder than iron oxide and inhibits the reaction. On ships etc the anodes are periodically replaced.

Crucially the point is that low levels of chlorine up to 2 mg/l say do not add much to the corrosive nature of the water compared to the oxygen that is already dissolved and it wouldn’t be very drinkable if there wasn’t any.

Hardness and alkalinity also play a part in the propensity to corrode and there is a derived measure that is calculated from the hardness alkalinity and ph called the Langellier Saturation Index. If this is positive scale will tend to form and no corrosion negative and corrosion may occur. Generally the aim is to be very slightly scale forming.

Aluminium and stainless steel both form protective oxide coatings that self repair. In stainless there is also the confusion of chloride crevice corrosion. Chloride ion is different to chlorine molecules and in high concentrations such as seawater and where fresh water concentrates dissolved solids such as cooling systems can cause corrosion of stainless steels. Typically this occurs in crevices and cracks such as areas where items are joined. This phenomenon is controlled by selecting the correct steel for the duty and eliminating crevices as well as limiting chloride concentration. I believe that it is this confusion between low levels of chlorine and high levels of chloride that leads to this sort of query.

Here is a link to the British stainless steels association website that states that up to 2 mg/l for 304 SS there is no effect.

Article: Selection of stainless steels for handling chlorine (Cl2) and chlorine dioxide (ClO2)

Steve Oxtoby