Selection of stainless steels for handling sulphur dioxide (SO2) and sulphur trioxide (SO3)

Introduction

Sulphur dioxide dissolves readily in water, which is then classed as a weak reducing acid (sulphurous acid H2SO3).
The oxidation of sulphur dioxide can be assisted by chlorine.
Sulphuric and hydrochloric acids can be formed in aqueous (water) systems.

Corrosion risks to stainless steels

As a dry or liquefied gas, sulphur dioxide does not tend to be aggressive towards stainless steels.
Grades with more than 18 to 20% chromium should be resistant to dry sulphur dioxide.

A 20% concentration of sulphur dioxide dissolved in water at 20 degC can be expected to give a corrosion rate between 0.1 and 1.0 mm/year on 304 types, but a rate below 0.1mm/year on 316 types and hence is 'mildly' corrosive.

If exposed to the air sulphuric acid (H2SO4) is formed from the sulphurous acid and so sulphur dioxide dissolved in water can be hazardous, where sulphuric acid has been allowed to form.

In damp vapours where oxidising conditions exist sulphur dioxide can be aggressive, depending on the dew points of acid and water at the service temperature under consideration.

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