Sulphur attack (Sulphidation)

At high temperatures sulphur compounds react with stainless steels to form complex sulphides and/or oxides.

Sulphur also reacts with nickel and forms nickel sulphide which, together with nickel, forms a low melting point

eutectic. This causes very severe attack unless the chromium content is very high. Steels with low nickel contents

should be used in environments containing sulphur or reducing sulphur compounds. For this reason the chromium

steels exhibit good resistance to sulphidation.

In reducing environments such as hydrogen sulphide or hydrogen sulphide/hydrogen mixtures, stainless steels are

attacked at even relatively low temperatures compared to the behaviour in air. Table 5 shows examples of the

corrosion rate for some stainless steels in hydrogen suphide at high temperatures. Table 6 shows corresponding

data for some austenitic stainless steels in a mixture of hydrogen sulphide and hydrogen. The beneficial effect of a

high chromium content is clear from the tables.

In oxidizing - sulphidizing environments such as sulphur dioxide (SO2) the relative performance of stainless steels

is similar to that in air, but the attack is more rapid and therefore more serious. The scaling temperature typically

decreases by 70-125°C compared to that in air. The decrease is smallest for the chromium steels .