4. Forms of Corrosion

4.9 Sulfide Stress Corrosion Cracking

Sulfide Stress Corrosion Cracking (SSCC)

Sulfide stress cracking effects high strength effects high strength carbon steels above Rockwell hardness 22 especially in petroleum production and petroleum refining. SSCC is a specialized case of hydrogen cracking as it may be possible that sulfide stress corrosion cracking and hydrogen cracking proceed simultaneously.


Iron is oxidized to the ferrous form at the anode and hydrogen sulfide undergoes a two step dissociation at the cathode as shown below

At the anode                F Fe2+ + 2e

At the cathode            H2S + H2O H+ + HS- + H2O

                                   HS- + H2O H+ +S- + H2O

Product combination    2e + 2H+ + Fe2+ + S- 2Ho + FeS (trolite)

The net reaction is         Fe + H2S FeS + 2Ho

As shown by the above reaction, not only FeS is formed bu other sulfides such as FeS2 (Pyrite), Fe7S8 (pyrrhotite) and Fe9S8 (Kansite) may also be formed. At low H2S concentrations (0.0009 to 0.1 psi) the sulfide film is generally pyrite, trolite and some kansite and a lower rate of corrosion is observed. The increasing concentration of kansite leads to an increase in the corrosion rate.

Other factors affecting SSCC:

Examples of Chloride induced SCC are shown in figures below. The samples are taken from a pump transporting hydrocarbons.

A sleeve showing signs of Chloride induced stress corrosion cracking. The crack is at right angles to the retaining screw point which is the point of maximum stress.

Another example of Chloride induced SCC corrosion cracking.