CORROSION INHIBITOR

Corrosion generally takes place where water and corrosive chemicals are present, with the rate affected by variable parameters including:

• CO₂ and H₂S content
• Temperature
• pH
• Water cut
• Salinity
• Pressure
• Solids levels
• Liquid and gas velocity

To mitigate carbon steel corrosion, corrosion inhibitors, H₂S scavengers and biocides are applied.

In water injection system, corrosion can also occur due to the presence of oxygen in the injected water. However, the oxygen can be removed from the water fairly easily using a combination of chemical or mechanical methods. Chemical oxygen scavenger is used to remove the oxygen, normally applied in conjunction with De-Aeration tower. Bacterial corrosion in water injection system is also common, but can be mitigated by the regular application of biocides.

Forms of corrosion

Corrosion can manifest itself in many forms that can result in failures. The most common forms of corrosion can be categorized within seven types:

• Localized pitting corrosion
• Galvanic corrosion
• Under deposit corrosion
• Stress corrosion
• Erosion corrosion
• Corrosion fatigue
• Uniform corrosion

Impact of corrosion

The failure to control corrosion rates can have catastrophic consequences such as equipment failure, which can result in environmentally-damaging leaks. Extensive repair and maintenance operations often require expensive system shout downs. There are also safety implications of failing to control corrosion with the potential to harm personnel.

Corrosion inhibitor

A corrosion inhibitor is a chemical which, when added in small amounts to a corrosive environment decreases the rate of corrosion of a metal in that environment. Corrosion inhibitors can be subdivided into four groups:

1. Inorganic anodic inhibitors mitigate corrosion by disrupting or blocking electrochemical reaction at the anodic sites on a metal surface. Commonly used to control corrosion in cooling tower water, heating / cooling mediums, and glycol dehydration, they are not recommended to control corrosion due to CO₂ or H₂S or in water injection systems.
2. Inorganic cathodic inhibitors function by forming a protective film at the cathodic sites that polarizes the metal by restricting access by dissolving oxygen to the metal surface. Used with the same application and restrictions as anodic inhibitors, this group is regarded as less effective.
3. Neutralizing / volatile inhibitors are suitable where the phase is condensate water, and are particularly effective where the main cause of corrosion is CO₂. Increasing system pH reduces corrosion and forms a protective FeCO3 film on metal surfaces.
4. Organic inhibitors work by forming a protective film over the entire metal surface and address corrosion due to CO₂, H₂S and organic acid. They are not effective in preventing corrosion due to oxygen.

In addition to four inhibitors categories, scavengers can also be used to remove H₂S from oil and gas system and oxygen from water injection system.

Corrosion inhibitor applications:

Corrosion inhibitors can be applied anywhere in a production system: downhole at the wellhead, production manifold, gathering lines, gas or oil export pipeline, and also in a water injection system. Corrosion inhibitors are normally applied either continuously or via batch treatment.

For batch treatment in wells, an inhibitor is typically diluted in a carrier solvent (e.g. diesel or condensate) and pump down the well or pipeline. Fore closed water system or heating /cooling mediums, anodic inhibitors are normally used to prevent oxygen corrosion. In all application, corrosion monitoring data informs the user when additional treatment are required.

Inhibitor treatment considerations:

Before deploying an organic corrosion inhibitor, a number of factors need to be considered. Firstly, the method of inhibitor application itself, which depends on several parameters including system conditions, injection location, and availability of chemical injection equipment.

Solubility is also a consideration. Inhibitors are generally classified as being oil soluble and water soluble.

There are two classes of corrosion inhibitors:

• Oil soluble corrosion inhibitor
• Water soluble corrosion inhibitor

Their emulsification and foaming tendencies at the required dose rate are essential properties that must be taken into account, and must be assessed to ensure product stability under typical storage temperature for prolonged period, and at injection temperature.

The compatibility of corrosion inhibitors with the metallurgy of the chemical injection system needs to be assessed, as does their compatibility with other production chemicals in the system. Conversely, checks should be performed to ensure that other chemicals present do not affect corrosion inhibitor performance.

Corrosion inhibitor range

These corrosion inhibitors offer an extremely comprehensive portfolio of corrosion inhibitors suitable for application to address a wide range of corrosion challenges including:

• Oxygen corrosion
• Sweet (CO₂) corrosion
• Sour (H₂S) corrosion
• Organic acids corrosion
• Under-deposit corrosion
• Galvanic corrosion
• Erosion corrosion

The corrosion inhibitor product range comprises the following types:

• CO₂ corrosion inhibitors
• H₂S corrosion inhibitors
• Hydro test products
• Cooling water corrosion inhibitors
• Environmentally- acceptable corrosion inhibitors
• Batch corrosion inhibitors
• High temperature corrosion inhibitors
• Gas corrosion inhibitors
• Multifunctional products (combined scale/ corrosion inhibitors)
• Oxygen scavenger
• H₂S scavenger
• Oil / gas pipeline corrosion inhibitors
• Subsea/Deepwater corrosion inhibitors