CHLORIDES STRESS CORROSION CRACKING IN GAS SERVICE

Chlorides Stress Corrosion Cracking in Gas Service

[Institute's Name]

Chlorides Stress Corrosion Cracking in Gas Service

Abstract/Summary

The Health and Safety Executive has a strategy to reduce the incidence of hydrocarbon releases in the US oil and gas sector. One source of such releases is corrosion related failure of equipment. In the oil and gas production with increasing depth of the deposit taking the corrosive conditions. As media are chloride-rich waters and gas mixtures with H 2 S and CO 2 at high pressures and temperatures. The materials used here are high-strength alloy steels and nickel base alloys. For safe operation, the corrosion properties to be checked. Relevant published literature was reviewed and additional information was obtained from discussions with other corrosion experts in the field.

Introduction

Stress corrosion cracking is transgranular (through the grain structure) or intergranular (along the grain boundaries of the microstructure) cracking in materials under the simultaneous influence of a purely static tension or with superimposed low-frequency Zugschwellspannung and a specific plea. Also, tensile stresses in the form of residual stresses are effective here. (Iverson 1968, pp.617-618)

Cracks across the grains (transgranular SCC) or along the grain boundaries (intergranular SCC).

Stress corrosion cracking (SCC) results from the combined action of three factors: Tensile stresses in the material, a corrosive medium (esp. chloride-bearing or hydrogen-sulphide environment) and elevated temperature (normally above 60°C for chloride-induced SCC). Cases where chloride induced SCC has occurred at lower temperatures than 60°C exist. The most common media where stress corrosion cracking occurs are chloride containing solutions, but in other environments, such as caustics and polythionic acid, problems with SCC may also appear. Some enviroments that may cause stress corrosion cracking of stainless steels are listed below.

Some environments where stainless steels are prone to stress corrosion cracking:

Acid chloride solutions

Seawater

Condensing steam from chloride waters

H 2 S + chlorides

Polythionic acid (sensitised material)

NaCl-H 2 O 2

NaOH-H 2 S(Iverson 1968, pp.617-618)

The mechanism of stress corrosion cracking is not well understood. This is mainly due to the specific features of SCC being the result of a complex interplay of metal, interface and environment properties. As a result of this different combinations of solution and stress are seldom comparable and the most reliable information is obtained from empirical experiments. During SCC the material does not undergo general corrosion and the phenomenon is sometimes considered to be one of activation/passivation interaction. It has been found that cracks often initiate in trenches or pits on the surface, which can act as stress raisers. The isolated times for pit initiation, pit growth, crack initiation and fracture may, however, differ considerably between different materials.

In some cases crack initiation has been associated with the formation of a brittle film at the surface. The film developed at grain boundaries might, for instance, have lower ductility due to a different metal composition than the bulk material. At a certain film thickness and under stress this brittle film will crack and expose the underlying metal. New film growth will proceed with subsequent continued crack growth and so ...