Conversion of refined metal to a chemically stable form (oxides, hydroxides, or sulfides). It happens because of a reaction with oxidants like oxygen and sulfur. In the oil and gas industry, it is mainly because of water content, carbon dioxide, hydrogen sulfide, and microbiological activities.
Requirements for Corrosion
Three conditions are necessary for corrosion to happen
- Metal that corrodes (example iron)
- Oxygen (usually from oxygen)
- An electrolyte (usually water)
Corrosion Failure Potential
- Stress corrosion cracking (SCC) is the growth of crack formation in corrosive environments which leads to mechanical failure under tensile load.
- Sulfide stress cracking (SSC) is caused in the oil and gas industry because of the presence of H2S in production fluids
- Internal corrosion problems are difficult to detect, so they are come to know after a failure happens.
- According to NACE report, corrosion causes a loss of 2.5 trillion US dollars annually which is 3-4% of global GDP
The following steps can be implemented to mitigate the corrosion.
- Selection of Suitable Material
In the oil refineries, condition constraints are high temperature and low pressure while in gas processing plants, these are high pressure and lower temperature. Choosing the right material for the structure is a capital solution and must include the study of variations of aggressive conditions in the future. The material must be selected carefully and monitored and treated continuously to meet performance criteria.
Considerations for Metal Selection
- Cost of investment (Short term VS. Long term)
- Properties of material needed
- Application of material
- Product of material (e.g. Fittings, Valve)
- Material VS. Material comparisons to find the best solution
- Technical advice
- Industry requirements and regulations
2. Coating and Lining
Coating and lining are useful techniques to mitigate the chances of corrosion on or in the process equipment. Paint application is very common to avoid corrosion on the surface of the equipment. In the oil and gas industry, working conditions change, so existing materials are to be replaced keeping into consideration the economic feasibilities. Moreover, the internals of the vessels is lined with corrosion resistive material to avoid failure of the vessels. Stainless steel covers a wide range of alloys, each with a particular set of corrosion resistance and mechanical properties.
3. Use of Inhibitor
Inhibitors are chemicals used to protect metal surfaces from corrosion. They act either by merging into metal surfaces or by reacting with impurities of the environment. They reduce the anodic/cathodic process simply by blocking the active metal sites. They also increase the potential of metallic surfaces to form passive layers like metallic oxide film.
4. Cathodic Protection Technique
It is the process of conversion of metal to be protected into the cathode of an electrochemical cell. This electrochemical cell may be naturally established or may be established through an external source. For the protection of exterior surfaces, both coating material and CP methods can be used together to achieve the best results. CP method is also used for interior surfaces of water storage tanks and water circulating systems.