Defending Against Corrosion: A General Introduction to Anti-Rust Additives

What is an anti-rust additive in lubricant and metalworking fluid?

 

Anti-rust additives are chemical compounds incorporated into lubricants and metal working fluids (MWF) to protect metal surfaces from corrosion. The anti-rust additive is also called: rust inhibitor, rust preventive, and corrosion inhibitor. They are wily used in engine oils and MWFs.

 

Classification of anti-rust additives

 

Generally, according to the characteristics of the solution, Rust Preventives can be divided into water-soluble Rust Preventives, oil-soluble Rust Preventives, emulsified Rust Preventives, gas-phase Rust Preventives, and wax film-type Rust Preventives.

 

Water Soluble Antirust Additives

 

Water is used as the main solvent, and the aqueous solution is formed by adding other rust-preventive chemicals. According to the anti-rust mechanism, it can be divided into three categories.

 

• The metal and the antirust chemical form an insoluble and dense oxide film, which prevents the anodic dissolution of the metal or promotes the passivation of the metal, thereby inhibiting the corrosion of the metal. Such antirust additives are also called passivators, such as sodium nitrite, potassium dichromate, etc. It should be noted that when used, sufficient dosage should be ensured. When the amount is insufficient, a complete oxide film cannot be formed, and the corrosion current density increases on the small metal surface that is not covered, which is easy to cause serious local corrosion.
• The metal and the anti-rust additive form insoluble salts, so that the metal is isolated from the corrosive medium and free from rust. For example: Some phosphates can react with iron to form insoluble iron phosphate salts; some silicates can react with iron and aluminum to form insoluble silicates.
• Metal and rust inhibitor form insoluble complexes, which cover the metal surface to protect the metal from corrosion. For example benzotriazole and copper can form a chelate complex Cu(C6H4N3)2, which is neither soluble in water nor oil, so it can protect the surface of copper from corrosion.

 

Oil-soluble rust preventives

 

It can also be called an oil-soluble corrosion inhibitor. Most are long carbon chain organic compounds with polar groups. The polar groups in its molecules are closely adsorbed on the metal surface by the action of charge; the non-polar group long carbon chain hydrocarbons are facing the outside of the metal surface and can be miscible with oils so that the molecules of the rust inhibitor are aligned on the metal surface to form an adsorptive protective film, which protects the metal from the erosion of water and oxygen. According to their polar groups can be divided into five categories:

 

• Sulfonates, the general chemical formula is (R—SO3) metal surface treatment additives. Generally, alkali metal or alkaline earth metal salts of petroleum sulfonic acid are used, such as barium petroleum sulfonate, sodium petroleum sulfonate, barium dinonylnaphthalenesulfonate, etc.
• Carboxylic acid and its soaps, the general chemical formula is R—COOH and (R—COO)nMm. The carboxylic acids used as rust inhibitors include fatty acids of animal and vegetable oils, such as stearic acid, oleic acid, etc., and carboxylic acids synthesized from oxidized petroleum grease, alkenyl succinic acid, etc. and naphthenic acid from petroleum products. The polarity of the metal soap of carboxylic acid is stronger than that of the corresponding carboxylic acid, so the anti-rust effect is better, but the oil solubility is smaller. And it will be hydrolyzed when it meets water, and its stability is poor when dispersed in oil, and sometimes it will precipitate out of oil.
• Esters, the general chemical formula is RCOOR'. Lanolin and beeswax are natural ester compounds, and they are also good metal antirust sealing materials. Esters of polyols have good antirust effects, such as pentaerythritol monooleate and sorbitan monooleate (Span-80), which are good metal antirust agents and are widely used.
• Amines, the general chemical formula is R—NH2, such as octadecylamine. However, the antirust effect of simple amines in mineral oil is not good enough, and amine salts or other compounds formed by amines and organic acids are commonly used, such as octadecylamine oleate, cyclohexylamine stearate, etc.
• Sulfur and nitrogen heterocyclic compounds, which are sulfur-containing or nitrogen-containing heterocyclic rings and certain derivatives, are also good metal rust inhibitors, such as imidazoline alkyl phosphate salts, benzotriazoles and α-mercaptobenzothiazoles, etc. Imidazolines can be used for rust prevention of ferrous and non-ferrous metals.

 

Emulsion rust preventives

 

There are two types of emulsified antirust agents: one is the suspension of oil particles in water, that is, oil-in-water emulsion, which is usually milky white; the other is the suspension of water particles in oil, that is, water-in-oil emulsion, which is usually a transparent or translucent liquid.

The emulsified antirust not only has antirust performance, but also has lubricating performance and cooling performance, so it is often used as a lubricating coolant for metal cutting. The emulsifier in the emulsified antirust agent used to be processed by saponification of vegetable oil (such as rapeseed oil, castor oil, etc.) in the past. In recent years, oleic acid triethanolamine, sulfonated oil or nonionic surfactants are used. In order to enhance the antirust performance, a certain amount of water-soluble antirust additives, such as sodium nitrite and sodium carbonate, sodium nitrite and triethanolamine, can also be added when adding water to make an emulsion. In addition, in order to prevent and slow down the odor and deterioration of the emulsion, a small amount of antifungal agent can also be added, such as phenol, pentachlorophenol, sodium benzoate, etc.

 

Vapor phase rust preventives

Also known as gas-phase corrosion inhibitor, it forms a protective layer on the metal surface by the gas slowly volatilized at room temperature to achieve the purpose of rust prevention. Its anti-rust principle is very similar to that of moth balls. It is characterized by automatic and continuous sublimation at room temperature, and volatilizes into gas. When the gas reaches saturation, it can protect metals such as steel from rust. Inorganic compounds such as ammonia and ammonium carbonate, as well as urea, urotropine, and cyclohexylamine carbonate, are used for steel rust prevention.

 

Wax film-type rust preventives

 

The wax film rust prevention is mainly through the formation of a film on the surface of the substrate by paraffin, stearic acid and long-chain fatty ammonium salts. Then compound lanolin magnesium soap, volatile solvent oil and some acid fat.

 

Application of rust preventives

 

In the actual application of anti-rust agents, several anti-rust agents are often used in a compound formula to enhance their anti-rust effect.

The antirust agent can be applied to: the lubrication and antirust of general guide rails, pin shafts, hydraulic props, bearings, suspension chains, etc., especially suitable for equipment and parts that cannot be cured by heating, such as large equipment, non-metallic materials, etc. It is also suitable for the lubrication and rust prevention of machinery left in the open air, naval equipment (aircraft, ships) that are often exposed to seawater smoke, and mechanical equipment that is often in acid, alkali, high and low temperature environments.

Some anti-rust agents are also suitable for spraying on the machining surface and grinding surface of various carbon steel, alloy steel, copper, aluminum, magnesium and their alloys, cast iron, non-metallic materials, etc., and can also be sprayed on the surface of metal material workpieces that have been sandblasted, phosphating, passivated, anodized, and sulfur-nitrocarburized.

 

 

Summarize

 

Through the classification of the original action principles and the summary of advantages and disadvantages of various anti-rust chemicals, it can be seen that there are many types of anti-rust additives and a wide range of applications, which can basically meet the current anti-rust requirements of various industries. However, with the strict requirements on environmental protections, people pay great attention to their own health. Environmentally friendly antirust additives will surely become the direction of development and advancement of the antirust chemical industry.