Corrosion inhibitors

Corrosion is the degradation of metallic materials due to chemical or electrochemical reactions with the surrounding environment. This phenomenon can lead to structural losses, failures in industrial plants and environmental hazards. Corrosion inhibitors are a crucial solution to mitigate these effects, protecting metals and prolonging their useful life.

Types

Anodic Inhibitors

Anodic inhibitors work by promoting the formation of a passive film on the metal surface. This film, often made up of oxides or salts, prevents the metal from dissolving into the surrounding environment. The main mechanisms of action include:

• Passivation: Formation of a protective layer that reduces the reactivity of the metal.
• Adsorption: Inhibitors adsorb onto the metal surface, creating a physical barrier against corrosive agents.

Some of the most common include:

• Chromates: These are among the most effective, known for their ability to form a resistant protective film. However, due to their toxicity, their use is regulated.
• Nitrites: Used primarily in water treatment, they are less toxic than chromates and provide good passivating protection.
• Molybdates: Often used as an environmentally friendly alternative to chromates, they provide effective protection in a variety of corrosive environments.
• Phosphates: They are commonly used in paints and coatings to prevent corrosion.

Main Applications:

• Industrial Water Treatment: Nitrites are commonly added to cooling systems to prevent corrosion of pipes.
• Automotive Industry: Phosphates are used in the surface treatment of automotive components to improve paint adhesion and prevent corrosion.
• Shipbuilding: Molybdates are used in paints and coatings to protect ships from highly corrosive salt water.
• Petrochemical Plants: Chromates, although regulated, are used in highly corrosive environments where other inhibitors may not be sufficiently effective.

Cathodic Inhibitors

Cathodic inhibitors work primarily by reducing the reactivity of reducing species present in the corrosive environment. This process can occur through:

• Precipitation of Insoluble Compounds: They can react with ions in the environment to form insoluble compounds that are deposited on the metal surface, creating a protective barrier.
• Reduction of Reduction Potential: By modifying the electrochemical potential of the metal surface, cathodic inhibitors make it more difficult for metal ions to be reduced, slowing corrosion.

Cathodic inhibitors can be classified based on their chemical composition. Some of the most common include:

• Carbonates: Used for their ability to form protective layers of calcium carbonate on the metal surface.
• Phosphates: In addition to their use as anodic inhibitors, phosphates are also effective as cathodic inhibitors due to the formation of protective layers of metal phosphate.
• Zinc Salts: Known for their ability to form protective films through the precipitation of zinc hydroxide.
• Polyphosphates: Offer effective protection by forming insoluble complexes on the metal surface.

Main Applications:

• Water Treatment: Phosphates and polyphosphates are commonly used in drinking water and industrial water treatment systems to prevent corrosion of pipes and tanks.
• Oil and Gas Industry: Zinc salts are used to protect offshore structures and underground pipelines from highly corrosive seawater.
• Pulp and Paper Industry: Carbonates are used to prevent corrosion in paper plants, where the environment is highly corrosive due to the presence of acids and other chemicals.
• Power Generation: Phosphates are used in power plant cooling systems to prevent corrosion of metal surfaces exposed to cooling water.

Mixed Inhibitors

Unlike anodic and cathodic inhibitors, which act specifically on one of the two electrochemical processes that cause corrosion, mixed inhibitors act on both processes, reducing both the anodic and cathodic reactions. This characteristic makes them extremely versatile and effective in a wide range of industrial applications.

They act through a combination of mechanisms, which include:

• Formation of a Protective Film: They create a coating on the surface of the metal that prevents direct contact with the corrosive environment. This film can be composed of oxides, salts or organic compounds.
• Adsorption on the Metal Surface: The inhibitor molecules adsorb on the metal surface, forming a physical barrier that reduces electrochemical activity.
• Modification of the Electrochemical Potential: They alter the potential of the metal surface, making both anodic oxidation and cathodic reduction more difficult.

Mixed inhibitors can be divided according to their chemical composition. Some of the most common include:

• Silico-Phosphates: Inorganic compounds that combine the protective properties of silicates and phosphates.
• Benzimidazoles: Organic compounds containing functional groups that promote adsorption and the formation of protective films.
• Quaternary Amines: Organic molecules with amino groups that strongly adsorb to the metal surface.
• Imidazole Derivatives: Organic compounds known for their effectiveness in forming protective films and modifying the electrochemical potential.

Main Applications:

• Oil and Gas Industry:

  They are used to protect pipelines and tanks from both internal and external corrosion, caused by corrosive agents such as salt water and acid gases.

• Water Treatment: They are used in cooling water treatment systems and closed circuits to prevent corrosion of pipelines and heat exchangers.
• Chemical Industry: Used to protect process equipment from corrosion caused by acids, bases and other aggressive chemicals.
• Shipbuilding:

  Mixed inhibitors are used to protect ship structures and offshore platforms from seawater and atmospheric agents.

Organic Inhibitors

These inhibitors contain organic compounds that interact with the metal surface, forming a protective layer that prevents contact with corrosive agents. Due to their versatility and the ability to be tailored to specific applications, organic inhibitors are widely used in various industries. Many organic inhibitors are biodegradable and less toxic than traditional inorganic inhibitors.

They act through several mechanisms, including:

• Adsorption on the Metal Surface: Inhibitor molecules adsorb on the metal surface, forming a protective film that reduces the electrochemical reactivity of the metal.
• Complex Formation: Organic inhibitors can form insoluble complexes with metal ions, creating a barrier that prevents further corrosive reactions.
• Passivation: Some organic inhibitors promote the formation of a passivating layer that reduces the rate of corrosion.

Organic inhibitors can be classified based on their chemical structure and the functional groups present in the molecules. Some of the most common include:

• Amines: Including alkylamines and ethanolamines, they are known for their effectiveness in protecting against corrosion in aqueous environments.
• Alcohols: Used for their ability to form protective films on the metal surface.
• Carboxylic Acids: Fatty acids and their salts are often used as corrosion inhibitors due to their ability to adsorb to the metal surface.
• Imidazole Derivatives: Compounds containing the imidazole group are particularly effective in forming protective films and modifying the electrochemical potential.
• Thiols: Compounds containing sulfhydryl (-SH) groups that form strong bonds with the metal surface, offering effective protection.

Main Applications:

• Oil and Gas Industry: Amines and imidazole derivatives are used to protect pipelines and tanks from corrosion caused by hydrogen sulfide and carbon dioxide.
• Water Treatment: Carboxylic acids and their salts are used in cooling systems and closed circuits to prevent corrosion of pipelines and heat exchangers.
• Chemical Industry: Alcohols and thiols are used to protect process equipment from corrosion caused by acids, bases and other aggressive chemicals.
• Automotive Industry: They are added to coolants and protective coatings to prevent corrosion of metal components.

Inorganic Inhibitors

Inorganic inhibitors are chemical compounds that can act as anodic, cathodic or mixed inhibitors, and are valued for their effectiveness and stability, offering long-term protection against corrosion, in a wide range of environmental conditions.

They can be classified according to their specific function:

Anodic inorganic inhibitors

• Chromates: They form a passive film of chromium oxide on the metal surface. Although very effective, their use is regulated due to their toxicity.
• Molybdates: They act similarly to chromates, forming protective layers of molybdenum oxide. They are an environmentally friendly choice compared to chromates.
• Nitrites: Used mainly in cooling systems, they form protective iron oxides on the metal surface.

Cathodic inorganic inhibitors

• Zinc Salts: They precipitate as zinc hydroxide on cathodic sites, reducing the rate of reduction reactions.
• Phosphates: They form protective layers of metal phosphate that reduce both anodic and cathodic reactions.
• Mixed inorganic inhibitors
• Silico-Phosphates: They combine the protective properties of silicates and phosphates, acting on both anodic and cathodic reactions.
• Polyphosphates: Form insoluble complexes that provide both anodic and cathodic protection.

Main Applications:

• Water Treatment: Nitrites, phosphates, and polyphosphates are commonly used in cooling systems and boilers to prevent corrosion of pipes and heat exchangers.
• Oil and Gas Industry: Molybdates and zinc salts are used to protect offshore structures and pipelines from corrosion by seawater and sulfur compounds.
• Automotive Industry: Phosphates are used in the surface treatment of automotive components to improve paint adhesion and prevent corrosion.
• Shipbuilding: Although regulated, chromates are used in some applications because of their effectiveness in protecting marine structures from salt water.
• Power Generation: Inorganic inhibitors such as nitrites and molybdates are used in power plant cooling circuits to prevent corrosion of metal components.

Innovations

Recently, research has focused on more environmentally friendly and sustainable corrosion inhibitors. Inhibitors based on natural extracts and bio-based compounds are emerging as promising alternatives to traditional inhibitors. In addition, the use of nanomaterials to create more effective and durable coatings is an area of ​​growing interest.