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Corrosion





Sodium hypochlorite only exists in solutions. The solution can be unstable, giving off chlorine gas. Sodium hypochlorite is not stable as a solid chemical. The hypochlorites, although alkaline, are oxidising. Commercially concentrated Sodium hypochlorite is around 15-wt %. Household bleach solutions are around 5.25% sodium hypochlorite.

The hypochlorite ion (OCl-) is aggressive to stainless steel, acting in a similar way to wet chlorine gas, and like the chloride ion (Cl-), is a dangerous pitting corrosion hazard.

Corrosion resistance of stainless steel

Pitting or crevice corrosion can occur on most stainless steel grades in a 5% solution at ambient temperature. There is an additional risk of stress corrosion cracking (SCC) at higher temperatures. Stainless steel should not be considered suitable for storage or transport tank applications with concentrated (15%) hypochlorite solutions or bleaches (5%).

Contact with household bleach
Pitting corrosion has been reported from household bleach spills on stainless steel (304 type) sinks in domestic environments. If this occurs immediate dilution by rinsing should avoid pitting, but if left overnight, pitting can result.

Disinfecting or sanitising 304 or 316 stainless steel items with dilute hypochlorite solutions can be done with care, but it is important that the temperature and contact time is kept to a minimum and that the solution is thoroughly rinsed away afterwards.

Safe residual water chlorine levels for sterilization

As a guide, 15-20 ppm (mg/lt) residual chlorine solutions at ambient temperatures should be safe with 316 types for a 24-hour maximum contact time, if followed by rinsing.



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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