Basic Diagram of Corrosion Process on Steel
Corrosion of steel is a process by which the metal deteriorates and deteriorates over time due to exposure to various environmental factors such as moisture, chemicals, and oxygen. It is a common problem for many steel structures and components and can lead to loss of strength, structural integrity, and, ultimately, failure.
Steel corrosion occurs when the metal reacts with its environment to form rust, an iron oxide. The presence of salts, acids, or other aggressive substances in the environment can accelerate this reaction. Over time, the rust can continue to spread and cause further damage to the steel, reducing its overall strength and stability.
Preventing and mitigating steel corrosion is vital to ensure the longevity and safety of steel structures and components. Various methods can be employed to slow down or stop corrosion, such as applying coatings, using corrosion-resistant alloys, or controlling the environment.
The basic process of corrosion of steel can be described as follows:
- Initiation: The first stage of corrosion is initiation, where the surface of the steel is exposed to corrosive agents, such as water or oxygen. This exposure can cause small cracks, pits, or other defects to form on the steel surface.
- Propagation: Once initiated, corrosion can then spread, or propagate, throughout the metal. As the deterioration continues, the cracks or pits grow more prominent, and the metal begins to dissolve and degrade. This can lead to the formation of corrosion products, such as rust.
- Passivation: In some cases, the corrosion process can slow or even stop when a protective layer of corrosion products forms on the steel surface. This protective layer is called a “passive film,” It can limit further corrosion by preventing the steel from being exposed to the corrosive agents.
- Breakdown: The passive protective film can become unstable under certain conditions, such as changes in pH, temperature, or exposure to aggressive environments. When this happens, the passive film can break down, and the corrosion process can restart and propagate.
Corrosion on bridge supports
The corrosion rate can depend on many factors, including the type and concentration of corrosive agents, the presence of impurities in the steel, the surface conditions of the steel, and environmental factors such as temperature, humidity, and mechanical stress.
Corrosion is a primary concern for oil and gas pipelines as it can cause significant damage to the pipeline, leading to leaks, reduced flow capacity, and in severe cases, pipeline failure. The importance of controlling corrosion in oil and gas pipelines is due to the following reasons:
- Safety: Pipeline corrosion can result in leaks and spills, posing a significant risk to public safety, the environment, and wildlife.
- Economic loss: Corrosion can lead to reduced flow capacity, which results in a loss of revenue for the pipeline operator and a decrease in the supply of oil and gas to customers. Additionally, the cost of repairing or replacing damaged pipelines can be high.
- Environmental impact: Leaks and spills from pipelines can have serious ecological consequences, including contamination of soil and groundwater, damage to wildlife and habitats, and harm to public health.
- Regulatory compliance: The oil and gas industry is subject to strict pipeline safety and environmental protection regulations. Pipeline operators are required to implement measures to prevent and control corrosion.
Therefore, it is crucial for oil and gas pipeline operators to implement effective corrosion control and management strategies to minimize the risk of pipeline failure and ensure their pipelines’ safety, reliability, and efficiency. This typically involves regular monitoring and maintenance of the pipeline, including applying protective coatings, cathodic protection, and other corrosion mitigation measures.
The Federal Energy Regulatory Commission (FERC) regulates the safety of natural gas pipelines under 45 CFR Part 192, which establishes standards for the design, construction, operation, and maintenance of pipelines. Along with 45 CFR Part 195, “Transportation of Hazardous Liquids by Pipeline,” is a regulation from the Pipeline and Hazardous Materials Safety Administration (PHMSA). According to these regulations, pipeline operators are required to perform various corrosion control and inspection activities to ensure the integrity and safety of their pipelines. Some of the corrosion inspections that must be performed under 45 CFR Part 192 & 195 include:
- External corrosion control: External corrosion is a significant concern for pipelines, and operators must implement measures to prevent it. This can include applying protective coatings, cathodic protection, and monitoring for corrosion.
- Internal corrosion control: Internal corrosion can occur when the pipeline is exposed to corrosive substances such as water, acids, or salts. Operators must implement measures to control internal corrosion, including monitoring the chemical composition of the pipeline’s contents, controlling the flow rate and velocity, and using corrosion inhibitors.
- Corrosion monitoring: Operators must regularly monitor their pipelines for signs of corrosion, using tools such as corrosion coupons, electrical resistance probes, and ultrasonic thickness measurements.
- In-line inspection: Operators must use in-line inspection tools to assess the pipeline’s condition and detect corrosion, cracks, and other anomalies. This may include devices such as smart pigs, which use sensors to gather data on the pipeline’s condition.
- Hydrostatic testing: Operators may be required to perform hydrostatic testing, where the pipeline is filled with water and pressurized to a certain level, to check for leaks and assess the pipeline’s ability to withstand pressure.
- Risk-based inspection: Operators must use a risk-based inspection (RBI) program to prioritize examining high-risk pipeline segments and ensure that critical areas are thoroughly inspected regularly.
These inspections are intended to ensure that operators are regularly monitoring their pipelines and taking necessary steps to control and prevent corrosion from maintaining the integrity and safety of their pipelines. Failure to comply with these regulations can result in fines, penalties, and other enforcement actions.
Corrosion coupons are metal samples placed in pipelines to monitor the corrosion rate. The coupons are periodically retrieved from the channel and then inspected in a laboratory to determine the amount of metal loss and the corrosion rate. The following steps describe the basic process of examining corrosion coupons from pipelines:
- Retrieval: Corrosion coupons are retrieved from the pipeline and cleaned to remove any debris or deposits that may interfere with the inspection process.
- Weight measurement: The weight of the coupon is measured before and after exposure to the pipeline environment to determine the amount of metal loss due to corrosion.
- Visual inspection: The coupons are visually inspected to determine the type and distribution of corrosion products on the surface, including rust, pits, and corrosion scales.
- Thickness measurement: The thickness of the coupon is measured using a micrometer or other precision instrument to determine the extent of metal loss due to corrosion.
- Analysis of corrosion products: The corrosion products on the surface of the coupon may be analyzed to determine the type of corrosion and the factors contributing to it, such as the presence of certain chemicals or conditions in the pipeline environment.
- Calculation of corrosion rate: The amount of metal loss and the exposure time are used to calculate the corrosion rate, expressed in terms of mils per year or other units.
This information is used by pipeline operators to assess the effectiveness of their corrosion control measures and to make decisions about how to manage and mitigate corrosion in their pipelines. By regularly monitoring the corrosion rate, operators can ensure the integrity and safety of their pipelines and prevent the formation of leaks and other safety hazards.
Placement of Corrosion Coupons in a Pipeline
Several standards are used to inspect pipeline corrosion coupons, including:
- NACE International: NACE International, The Corrosion Society, is a professional organization that provides standards and guidelines for assessing and managing corrosion. NACE International publishes several standards related to the inspection of pipeline corrosion coupons, including NACE TM0177-2017 “Standard Test Method for Determining Corrosion Rates and Related Information from Electrochemical Measurements on Operating Plant and Laboratory-Exposed Coupons.”
- ASTM International: ASTM International is a standards development organization that provides technical standards for various materials and processes, including the inspection of pipeline corrosion coupons. ASTM G1-03(2018) “Standard Practice for Preparing, Cleaning and Evaluating Corrosion Test Specimens” provides guidelines for preparing, cleaning, and evaluating corrosion coupons to obtain accurate and reliable results.
- API Standards: The American Petroleum Institute (API) is a trade organization that represents the oil and natural gas industry and provides standards and guidelines for inspecting and managing corrosion in pipelines. API RP 11G2 “Recommended Practice for In-Service Inspection of Corrosion Coupons in Oil and Gas Production” provides guidelines for examining and interpreting corrosion coupons in the oil and gas production industry.
These standards provide guidelines for the preparation, inspection, and analysis of pipeline corrosion coupons to ensure that the results are accurate and consistent. Using these standards helps to ensure that corrosion coupons are collected, handled, and analyzed in a standardized and consistent manner, providing valuable data for assessing and managing corrosion in pipelines.