Material Selection For Sour Service Environment

Let us consider the properties of hydrogen sulfide (H2S) gas which is also known as sour gas or sulfuretted hydrogen. It is possibly the second-most deadly gas after carbon monoxide that man can encounter in the industrial environment. Common in all sectors of the petroleum industry, this gas can be found in any location where decaying matter is present. Knowledge of the properties of H2S is important to understand the role of this gas.

Materials Selection
The materials selection process should reflect the overall philosophy regarding design life, cost profile, inspection and maintenance philosophy, safety and environmental profile, failure risk evaluations and other specific project requirements. Materials selection should be optimized and provide acceptable safety and reliability. At a minimum, the following should be considered:

• Corrosivity, taking into account specified operating conditions including start up and shut-down conditions;
• Design life and system availability requirements;
• Failure probabilities, failure modes and failure consequences for human health, environment, safety and material assets;
• Resistance to brittle fracture;
• Inspection and corrosion monitoring; and
• Access for maintenance and repair.

For the final materials selection, the following additional factors should be included in the evaluation: priority should be to select materials with good market availability and documented fabrication and service performance; and the number of different materials should be minimized considering stock, costs, interchangeability and availability of relevant spare parts.

In the best engineering judgment, the deviations from materials selection guidance given above may be taken if an overall cost, safety and reliability study points to an alternative choice.

The selected material must be evaluated for its corrosivity. At a minimum, this would include the following: CO2-content; H2S-content; oxygen content and content of other oxidizing agents; operating temperature and pressure; organic acids, (pH); halide, metal ion and metal concentration; velocity, flow regime and sand production; biological activity; and condensing conditions.

As is evident, there are various factors that must be considered for material selection for any system, some of these may be already defined in the project documents and other factors might require further investigations. However, since the present discussion is limited to the material selection for sour service system we will concentrate on that.

What is meant by sour service? What could be considered sour service is based on the definitions and guidance provided by industry specifications like NACE MR 0175/ISO 15156 and also on Canadian specification CSA Z 662. The approach of NACE is not so much on defining sour service but on the question of how severe is the possibility of sulfide stress corrosion cracking in the material. It may be noted that, irrespective of which approach is taken as basis for the determination of sour service conditions, they complement each other and the end result to the material selection process is same.

The CSA Z 662 (Table 1) includes a list of approved materials and allows for the non-listed materials to be used as per the approval process given in the NACE procedures. NACE MR 0175 does not include a list of approved materials but gives ways to establish suitability of the material for sour service.