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Over the years there have been significant studies in various parts of the world on the cost of corrosion and how it affects a country’s economy. NACE International, the worldwide corrosion authority, released its 2016 IMPACT study which examines the current role of corrosion management in industry and government, and establishes best practices. The Study estimates the global cost of corrosion to be as high as US$2.5 trillion. The IMPACT study emphasizes the need for strategic decision making across all industries where corrosion is concerned.
Simply defined, corrosion is the breaking down or destruction of a material, especially a metal, through chemical reactions. The most common form of corrosion is rusting, which occurs when iron combines with oxygen and moisture.
There are various types of corrosion, and they can be classified by the cause of the metal’s deterioration. They include the following:
- General Attack Corrosion
This affects the entire exposed surface of the metal, and it the most common type of corrosion.
- Localized corrosion
This type of corrosion specifically targets one area of the metal structure. Examples are Pitting (holes or cavities in the metal), Crevice corrosion and Filiform Corrosion.
- Galvanic Corrosion
This occurs when two different metals are located together in a corrosive electrolyte (e.g. Salt). A galvanic couple forms between the two metals, where one metal becomes the anode and the other the cathode. The anode, or sacrificial metal, corrodes and deteriorates faster than it would alone, while the cathode deteriorates more slowly than it would otherwise.
- Flow-Assisted Corrosion (FAC)
This happens when a protective layer of oxide on a metal surface is dissolved or removed by wind or water, exposing the underlying metal to further corrode and deteriorate. Example: Erosion-assisted corrosion.
- High-Temperature Corrosion
Fuels which are used in gas turbines, diesel engines and other machinery, which contain vanadium or sulfates can form compounds with a low melting point during combustion. Such compounds are very corrosive towards metal alloys such as stainless steel which is normally resistant to high temperatures and corrosion.
Corrosion is a major integrity threat to the global oil and gas industry, and corrosion management has become a core part of asset integrity management activities on oil and gas facilities. It is important for oil and gas assets which are susceptible to corrosion to have a proper corrosion management program to protect against the consequences of corrosion.
The lack of a proper Corrosion Management Program will increase an operator’s risk of asset failure and also reduce asset life due to corrosion. This will result in decreased safety on oil and gas facilities, higher environmental risks, more repairs and an increase in unplanned shutdowns, thus affecting profitability.
Corrosion Management (CM) addresses the threat of corrosion for existing and future assets across the lifecycles of the assets. General research has shown that the most significant success stories for Corrosion Management revolved around the adoption of adherence to set CM procedures, the integration of CM systems that are adopted into an organization’s overall policy, and the application of innovative technology methods used in corrosion prevention, monitoring, testing, and treatment.
Corrosion Monitoring uses a wide range of measurement techniques. Non-destructive Testing (NDT) methods are the most effective and broadly applied testing methods. Some suitable NDT methods for the monitoring of corrosion include:
- Visual Inspection
This is the first and simplest method of corrosion inspection. It involves a visual check of the surface condition of the material.
- Ultrasonic Testing (UT)
Ultrasonic Thickness Measurement and Flaw Detection is the non-destructive measurement of the local thickness of a solid element based on the ultrasound reflection principle. This is an effective method, but limited to near-surface defects and non-perpendicular defects.
- Radiographic Testing
This includes Digital Radiography, Computed Radiography and Real-time Radiography. This method is useful for detection of defects in weld joints and metal plates.
- Guided Wave Testing
This is an advanced form of Ultrasonic testing. Guided Wave Ultrasonic Testing (GWUT) is an effective screening UT method that is used in detecting general corrosion and wall loss. With GWUT, longer sections of pipe can be scanned that when conventional UT is used.
- Pulsed Eddy Current (PEC)
PEC is an electromagnetic method which is used to determine the wall thickness of electrical conductors through external coverings such as insulation, concrete, and heavy coating, up to 200mm. The suitability of PEC in testing for Corrosion Under Insulation makes it a popular method of corrosion testing.
When it comes to corrosion monitoring, the various inspection methods have their strengths and weaknesses, and they can be used to complement each other as combinations of inspection methods.
The selection of the appropriate method or mix of inspection methods is dependent on specific corrosion monitoring requirements, and it requires knowledgeable and experienced personnel.