What is sweet corrosion? A quick introduction.
Corrosion is a common challenge in many industries, particularly those dealing with oil, gas, and petrochemical processing. One of the lesser known yet equally detrimental forms of corrosion is “sweet corrosion.” This type of corrosion can lead to severe damage in pipelines, storage tanks, and other metal structures if not properly managed. Using stainless steel components in environments where sweet corrosion occurs can mitigate damage and extend the service life of equipment.
What is sweet corrosion?
Sweet corrosion, also known as carbon dioxide (CO₂) corrosion, occurs in environments where carbon dioxide is present, particularly in oil and gas production. When CO₂ dissolves in water, it forms carbonic acid (H₂CO₃), which can aggressively attack metal surfaces, especially carbon steel. This process can lead to significant metal loss, pitting, and cracking. Carbon dioxide corrosion differs from “sour corrosion,” which involves hydrogen sulfide (H₂S) and tends to be more localized and aggressive. While generally less aggressive than sour corrosion, it can cause widespread damage if not adequately addressed.
Which industries encounter carbon dioxide corrosion?
Oil and Gas Production and Transport
- Oil Wells and Gas Reservoirs: prevalent in oil wells and gas reservoirs where CO₂ is naturally present in the hydrocarbons being extracted. When these fluids are brought
to the surface, the CO₂ can dissolve in water, forming carbonic acid and leading to corrosion in pipelines and well tubing. - Pipelines: Pipelines that transport oil and natural gas containing CO₂ are particularly susceptible to sweet corrosion. The internal surfaces of these pipelines can be exposed to carbonic acid, resulting in metal loss and potential leaks.
- Processing Facilities: Refineries and gas processing plants that handle CO₂-rich fluids are also at risk. Carbon steel equipment, such as separators, heat exchangers, and storage tanks, can experience CO₂ corrosion if not adequately protected.
Geothermal Energy Systems
In geothermal energy systems, CO₂ is often present in the steam and hot water extracted from underground reservoirs. When this CO₂ mixes with water, it can form carbonic acid, leading to CO₂ corrosion of metal components in piping and equipment used in geothermal plants.
Chemical and Petrochemical Industries
- Chemical processes that involve CO₂ as a reactant or byproduct can also create conditions for CO₂ corrosion. Storage tanks, reactors, and heat exchangers in these industries are at risk, especially when water is present.
How does sweet corrosion occur?
The sweet corrosion process involves several stages:
1. Formation of Carbonic Acid: When CO₂ gas comes into contact with water, it dissolves and reacts to form carbonic acid (H₂CO₃). This acid lowers the pH of the environment, making it more corrosive.
2. Electrochemical Reaction: Carbonic acid dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). The hydrogen ions react with the metal surface, leading to the formation of iron carbonate (FeCO₃), a protective layer that can slow down the corrosion process. However, this protective layer can be unstable and easily disrupted, especially under high flow rates or varying temperatures.
3. Corrosion of the Metal: When the protective iron carbonate layer is damaged, the underlying metal is exposed to further attack, leading to pitting and metal loss.
Why is sweet corrosion a problem?
Carbon dioxide corrosion poses several risks to industries, particularly in oil and gas operations:
• Equipment Failure: Over time, CO₂ corrosion can lead to the thinning of metal components, causing leaks, ruptures, and potential equipment failure. This can result in costly repairs, unplanned shutdowns, and safety hazards.
• Operational Costs: Mitigating sweet corrosion often involves frequent maintenance, corrosion inhibitors, and protective coatings. These measures, while effective, can be expensive and may not always provide long-term solutions.
• Environmental Impact: Corrosion-induced leaks can result in environmental contamination, particularly in oil and gas operations. This can lead to regulatory penalties and damage to a company’s reputation.
In this older report you can read about incidents related to corrosion related incidents back from 1965 to 2012, “Corrosion-Related Accidents in Petroleum Refineries – Lessons learned from accidents in EU and OECD countries” .
Why using stainless steel in sweet corrosion environments?
Stainless steel is widely regarded as the best material choice for environments prone to CO₂ corrosion. Here’s why:
1. Corrosion Resistance: Stainless steel contains a minimum of 10.5% chromium, which forms a passive oxide layer on the surface. This layer acts as a barrier, protecting the metal from corrosive agents like carbonic acid. In CO₂ corrosion environments, stainless steel’s resistance to CO₂ attack is significantly higher than that of carbon steel.
2. Durability and Longevity: Stainless steel’s corrosion resistance translates to a longer service life for components exposed to sweet corrosion. This means reduced maintenance costs and fewer replacements, leading to long-term savings.
3. Strength and Reliability: Stainless steel maintains its mechanical properties in a wide range of temperatures and pressures, making it suitable for harsh conditions often encountered in oil and gas production. It can withstand the mechanical stresses associated with high-flow rates, which can exacerbate corrosion in less durable materials.
4. Reduced Need for Corrosion Inhibitors: Using stainless steel can reduce or eliminate the need for chemical corrosion inhibitors, which are commonly used to control sweet corrosion in carbon steel systems. This not only cuts operational costs but also reduces the potential environmental impact of chemical use.
5. Versatility: Stainless steel comes in various grades, such as 304, 316, and duplex stainless steels, each offering different levels of corrosion resistance and mechanical properties. This versatility allows engineers to select the most suitable grade for specific conditions, including the concentration of CO₂, temperature, and pressure.
What stainless steel grade do I need to prevent CO₂ corrosion?
While stainless steel is a superior material for CO₂ corrosion environments, it’s crucial to select the appropriate grade based on the specific operating conditions:
• Austenitic Stainless Steels (e.g., 304, 316): These are the most commonly used grades for their excellent corrosion resistance and mechanical properties. Grade 316 contains molybdenum, which enhances resistance to pitting and crevice corrosion, making it suitable for environments with higher CO₂ concentrations.
• Duplex Stainless Steels: Duplex grades combine the best properties of austenitic and ferritic stainless steels, offering higher strength and improved resistance to stress corrosion cracking. They are ideal for high-pressure, high-temperature environments where sweet corrosion is a concern.
Sweet corrosion is a pervasive issue in industries dealing with carbon dioxide-rich environments. Whereas inhibitors and organic compounds are excellent corrosion mitigators for industries with big installations, material selection for smaller components are of the essence. For Stainless Steel 316 components, such as bolts, autolok nuts, handles, clamps, hinges and band clamps, Anemo Engineering offers multiple solutions. Get in touch if you need help with your selection of the most adequate solution for your application or download the catalogue
There is much more to CO₂ corrosion.
Read these articles if you want to learn more:
