Selection of stainless steels for handling sodium hypochlorite (NaOCl)

Introduction

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.

Corrosion resistance of stainless steels

The hypochlorite ion (OCl-) is aggressive to stainless steels, acting in a similar way to wet chlorine gas, and like the chloride ion (Cl-), is a dangerous pitting corrosion hazard.
Pitting or crevice corrosion can occur on most stainless steel grades in a 5% solution at ambient temperatures.
There is an additional risk of stress corrosion cracking (SCC) at higher temperatures.

Stainless steels 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.

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