Sulphur dioxide dissolves readily in water, which is then classed as a weak reducing acid (sulphurous acid H2SO3). Theoxidation of sulphur dioxide can be assisted by chlorine. Sulphuric acids and hydrochloric acids can be formed in aqueous (water) systems.
As a dry or liquefied gas, sulphur dioxide does not tend to be aggressive towards stainless steel. Grades with more than 18 to 20% chromium should be resistant to dry sulphur dioxide.
Corrosion risks to stainless steel
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 stainless steel 304 types, but a rate below 0.1mm/year on stainless steel 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.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)