The 5 Major Types of Oil-Soluble Rust Inhibitors and Their Mechanisms

Currently, there are many types of oil-soluble rust inhibitors available on the market. Based on their polar groups, they can be classified into five major categories: sulfonates, carboxylic acids and their soaps, esters, amines, and other organic compounds.

This is a type of oil-soluble corrosion inhibitor that was applied early on and remains widely used today. The most commonly used are petroleum sulfonates. Petroleum sulfonates are mixtures of various sulfonates. Their main components are complex alkylbenzene sulfonates and alkylnaphthalene sulfonates, followed by aliphatic hydrocarbon sulfonates, cycloaliphatic hydrocarbon sulfonates, and their oxides.

It is an oil-soluble anionic surfactant widely used in industry and can be used as an emulsifier in cutting oils. High molecular weight petroleum sulfonates are used as rust inhibitors in metal rust preventive oils. Commonly used ones include barium petroleum sulfonate, sodium petroleum sulfonate, and calcium petroleum sulfonate. Based on the content of metal hydroxides in them, petroleum sulfonates can be divided into three categories: neutral sulfonates, basic sulfonates, and overbased sulfonates.

• Neutral Sulfonates: Exhibit good resistance to saltwater corrosion and have excellent sweat displacement properties. However, their rust prevention effect on copper and copper alloys is relatively poor. They are often used in combination with copper corrosion inhibitors like benzotriazole. These sulfonates are frequently used for inter-process rust prevention of parts and long-term sealing of products. Among them, barium salts offer the best rust prevention.
• Basic Sulfonates: Possess excellent neutralizing and floating dispersion properties. They are often used in internal combustion engine oils, diesel engine lubricating oils, and some fuel oils.
• Overbased Sulfonates: Mainly used in internal combustion engine and diesel engine lubricating oils as floating dispersants to prevent the generation of carbon deposits.

Sulfonates are early and widely used rust inhibitors, with their usage dating back to the 1930s. Based on their metal components, they can be divided into barium salts, calcium salts, sodium salts, and magnesium salts.

Based on their source, they can be categorized into petroleum sulfonates and synthetic sulfonates. Calcium salts are most commonly used as detergents. As rust inhibitors, barium salts are the most frequently used sulfonates, followed by calcium and sodium salts. Based on their base number, sulfonates can also be divided into neutral and basic sulfonates.

Protective Mechanism of Sulfonates:

The polar part of the sulfonate can adsorb onto the metal surface while simultaneously repelling moisture from the surface. The non-polar part can form a barrier film, providing an excellent protective effect.

Carboxylic acids are among the earliest studied oil-soluble corrosion inhibitors. Corrosion inhibitors belonging to this category include:

1. Animal and vegetable fatty acids and their metal soaps: Such as capric acid, caprylic acid, lauric acid, myristic acid, palmitic acid, oleic acid, ricinoleic acid, stearic acid, and their metal soaps.
2. Synthetic fatty acids and their soaps: Such as oxidized petrolatum, oxidized ceresin, high carbon acids derived from the oxidation products of certain petroleum fractions, and their metal soaps.
3. Synthetic multi-polar high-molecular carboxylic acids: Such as C12–C18 alkenyl succinic acid, hydroxy fatty acids (e.g., alpha-hydroxy stearic acid), phenoxy stearic acid, nonylphenoxyacetic acid, N-oleoyl sarcosine, and the organic amine salts of these acids. There are many reports abroad regarding the use of derivatives prepared by reacting alkenyl or alkyl succinic acid with organic amines like ethylene oxide or alkanolamines as corrosion inhibitors.
4. Petroleum acids in natural petroleum products: Such as naphthenic acids and their soaps.

A single saturated monobasic fatty acid has poor corrosion inhibition properties and is rarely used directly. Because the polarity of a single carboxyl group is insufficient and its adsorption is weak, its metal soaps are often used instead. Examples include aluminum stearate and zinc naphthenate.

They also have a relatively wide range of applications as rust inhibitors. Long-chain fatty acids inherently possess certain rust-preventive properties, and fatty acid metal salts generally offer better rust prevention than the original fatty acids. Commonly used carboxylic acids and their salt-based rust inhibitors include alkenyl or alkyl succinic acid, zinc naphthenate, and magnesium lanolin soap.

Alkenyl or alkyl succinic acid offers outstanding rust prevention, requires a small dosage, and is insensitive to water. It is widely used in hydraulic oils, turbine oils, and slideway oils. An addition amount of just around 0.02% is generally sufficient to pass rust prevention tests. When used in combination with barium sulfonate, its resistance to salt spray and hot, humid conditions is even better.

Ester corrosion inhibitors include two major categories: natural compounds and synthetic esters.

Commonly used natural compounds include lanolin and its soaps. This is an early class of oil-soluble corrosion inhibitors with good rust prevention performance. Another is beeswax, a natural surfactant that also offers good corrosion inhibition; however, due to its high cost and sourcing difficulties, it is rarely used.

Synthetic esters have weak polarity and high solubility in oil; therefore, they are only effective when added in large amounts. The corrosion inhibition effects of ethyl stearate, octadecyl laurate, and ethyl ricinoleate are not very good. Introducing additional polar groups into the ester molecule can significantly reduce the ester’s solubility in oil. The most prominent among these is Sorbitan Monooleate (Span 80). In addition, there are glycerol monooleate, pentaerythritol oleate, etc.

Ester corrosion inhibitors are rarely used alone. They are often compounded with other corrosion inhibitors to enhance rust prevention or act as co-solvents for other corrosion inhibitors. Their disadvantage is that they easily oxidize into acids at high temperatures, causing metal corrosion, so they are not suitable for high-temperature applications.

Because their inherent polarity is relatively weak, large amounts generally need to be added to be effective. Therefore, suitable polar groups are often added to ester compounds to increase their polarity and improve the rust prevention effect. Span 80 (Sorbitan Monooleate) is one of the most widely used ester-based rust inhibitors.

These can be divided into monoamines, diamines, and polyamine compounds. Generally speaking, the rust prevention effect of branched-chain aliphatic amines is poorer than that of straight-chain aliphatic amines.

Amines have poor solubility in mineral oils. However, after neutralizing into salts with oil-soluble petroleum sulfonic acids, alkyl phosphoric acids, or N-oleoyl sarcosine, their oil solubility can be significantly improved. Common amine salt rust inhibitors include N-oleoyl sarcosine octadecylamine salt and heptadecenyl imidazoline alkenyl succinate.

Organic amine rust inhibitors have good moisture resistance, acid neutralization, and water displacement properties. However, they exhibit a certain degree of corrosivity towards copper, zinc, and lead.

Other frequently used rust inhibitors include imidazoline succinates, oil-soluble derivatives of 2-mercaptobenzotriazole, alkyl phosphate salts of imidazoline, and benzotriazole.

Benzotriazole (T706) is the most commonly used corrosion inhibitor for non-ferrous metals (like copper and silver), and it also provides a certain rust-preventive effect on carbon steels. Because benzotriazole is poorly soluble in mineral oil, it is generally mixed and blended with co-solvents before being added to the mineral oil.

With the development of industry and scientific technology, the types, functions, and inhibition principles of corrosion inhibitors have evolved and innovated. There are numerous types of corrosion inhibitors with various mechanisms. However, due to the influence of factors such as cost-effectiveness and raw material availability, the number of corrosion inhibitors that can be put into actual production is limited. Further research on the inhibition mechanisms of oil-soluble corrosion inhibitors and the optimal combinations of inhibitors is beneficial for selecting the right inhibitors in industrial production.

Driven by the needs of technological development, eco-friendly corrosion inhibitors have become the primary direction for future advancement. Additionally, corrosion inhibitors often fail to achieve the expected inhibition effect when acting together. Therefore, researching and developing new, highly universal, and eco-friendly corrosion inhibitors holds high academic and practical application value.