The World Corrosion Organization pointed out that according to incomplete statistics, the direct economic losses caused by metal corrosion in the world each year are as high as about 700 billion US dollars, and the losses caused by metal corrosion in my country account for about 4% of the gross national product.
The corrosion and rust of metals not only cause huge economic losses to social development, but also bring great inconvenience to people's daily production and life, as well as potential direct or indirect environmental pollution and safety hazards. Therefore, rust prevention of metal substrates has always been the focus of people's attention.
The simplest and most effective rust prevention method is to use rust inhibitors.
The advantage of rust inhibitors is that they are super efficient synthetic penetrants. They can penetrate rust, corrosive substances, and oil stains to easily remove rust and corrosive substances on screws and bolts. They have the properties of penetrating rust removal, loosening lubrication, corrosion resistance, and metal protection. They can also form and store a lubricating film on the surface of components, which can inhibit corrosion caused by moisture and many other chemical components. It can achieve rust prevention without destroying the surface appearance of the substrate, and can also increase the brightness of the surface under certain specific conditions.
The use process of rust inhibitors is simple, the types of substances in the formula are easy to obtain, and the preparation cost is low, and the scope of application is wide. From the perspective of problem solving, preparing for a rainy day and stopping the problem in its infancy is the best way and means to solve the problem. The emergence of rust inhibitors can effectively play this role.
Classification of rust inhibitors
According to the characteristics of the solution, rust inhibitors can usually be divided into; water-soluble rust inhibitors, oil-soluble rust inhibitors, emulsified rust inhibitors, vapor phase rust inhibitors and wax film rust inhibitors.
Water-soluble rust inhibitors: Water is used as the main solvent, and other rust inhibitors are added to form an aqueous solution. According to the mechanism of rust prevention, it can be specifically divided into three categories.
01 The metal and the rust inhibitor generate an insoluble and dense oxide film, thereby preventing the anodic dissolution of the metal or promoting the passivation of the metal, thereby inhibiting the corrosion of the metal. This type of rust inhibitor is also called a passivating agent, such as sodium nitrite, potassium dichromate, etc. It should be noted that when using it, sufficient dosage should be ensured. When the dosage is insufficient, a complete oxide film cannot be formed. On the small metal surface that is not covered, the corrosion current density increases, which can easily cause serious local corrosion;
02 The metal and the rust inhibitor generate insoluble salts, thereby isolating the metal from the corrosive medium and preventing it from rusting. For example: some phosphates can react with iron to generate insoluble iron phosphates; some silicates can react with iron and aluminum to generate insoluble silicates, etc.;
03 The metal and the rust inhibitor generate insoluble complexes, which cover the metal surface and protect the metal from corrosion. For example: benzotriazole and copper can generate chelate Cu(C6H4N3)2, which is neither soluble in water nor in oil, and can therefore protect the copper surface.
Oil-soluble rust inhibitor
It can also be called oil-soluble corrosion inhibitor, most of which are long carbon chain organic compounds with polar groups. The polar groups in its molecules are tightly adsorbed on the metal surface by the action of electric charge; the non-polar group long carbon chain hydrocarbon faces the outside of the metal surface and can be miscible with oils, so that the rust inhibitor molecules are oriented on the metal surface to form an adsorptive protective film, which protects the metal from water and oxygen corrosion. According to its polar groups, it can be divided into five categories:
01 Sulfonates, with a chemical formula of (R—SO3), metal surface treatment agents, generally use alkali metal or alkaline earth metal salts of petroleum sulfonic acid, such as barium petroleum sulfonate, sodium petroleum sulfonate, barium dinonylnaphthalene sulfonate, etc.;
02 Carboxylic acids and their soaps, with a chemical formula of R—COOH and (R—COO)nMm. Carboxylic acids used as rust inhibitors include fatty acids of animal and vegetable oils, such as stearic acid, oleic acid, etc., as well as synthetic carboxylic acids such as oxidized petroleum grease and alkenyl succinic acid, and petroleum product cyclohexane acid, etc. The polarity of metal soap of carboxylic acid is stronger than that of the corresponding carboxylic acid, so it has better anti-rust effect, but it is less oil-soluble and will hydrolyze in water. It has poor stability when dispersed in oil and sometimes precipitates from the oil;
03 Esters, the general chemical formula is RCOOR′. Lanolin and beeswax are natural ester compounds and are also good metal anti-rust sealing materials. The esters of polyols have a good anti-rust effect, such as pentaerythritol monooleate and sorbitan monooleate (Span-80), which are good metal rust inhibitors and are widely used;
04 Amines, the general chemical formula is R—NH2, such as octadecylamine. However, the rust prevention effect of pure amines in mineral oil is not good enough, and amine salts or other complexes generated by amines and organic acids are commonly used, such as oleic acid octadecylamine, stearic acid cyclohexylamine, etc.;
05 Sulfur and nitrogen heterocyclic compounds are sulfur-containing or nitrogen-containing heterocyclic rings and certain derivatives, which are also good metal rust inhibitors, such as alkyl phosphates of imidazoline, benzotriazole and α-mercaptobenzothiazole, etc. Imidazolines can be used for rust prevention of ferrous and non-ferrous metals.
Emulsified rust inhibitors
There are two types of emulsified rust inhibitors:
01 One is a suspension of oil particles in water, that is, water-in-oil emulsion, which is usually milky white;
02 The other is a suspension of water particles in oil, that is, oil-in-water emulsion, which is usually a transparent or translucent liquid.
Emulsified rust inhibitors have both rust prevention properties and lubrication and cooling properties, so they are often used as lubricating coolants for metal cutting.
In the past, the emulsifier in the emulsified rust inhibitor was usually made of vegetable oils (such as rapeseed oil, castor oil, etc.) through saponification. In recent years, oleic acid triethanolamine, sulfonated oil or non-ionic surfactants have been used.
In order to enhance the anti-rust performance, a certain amount of water-soluble rust inhibitors, such as organic acids and triethanolamine, can be added when adding water to prepare the emulsion.
In addition, in order to prevent and slow down the odor and deterioration of the emulsion, a small amount of mildew inhibitors, such as phenol, pentachlorophenol, sodium benzoate, etc., can be added.
Gas phase rust inhibitor
Also known as gas phase corrosion inhibitor, it forms a protective layer on the metal surface by slowly evaporating gas at room temperature to achieve the purpose of rust prevention.
Its anti-rust principle is very similar to moth-proofing with camphor balls. It is characterized by automatic and continuous sublimation at room temperature and evaporation into gas. When this gas reaches a saturated state, it can provide anti-rust protection for metals such as steel.
Inorganic compounds such as ammonia water and ammonium carbonate, as well as urea, hexamethylenetetramine carbonate, etc. are used for steel rust prevention.
Wax film type rust inhibitor
Wax film rust prevention is mainly achieved by forming a film on the surface of the substrate with paraffin, stearic acid and long-chain fatty ammonium salts, and then compounding lanolin magnesium soap, volatile solvent oil and some acid fats.
Application of rust inhibitor
In the actual application of rust inhibitors, several rust inhibitors are often used in a composite formula to enhance their rust prevention effect.
Rust inhibitors can be used for:
Lubrication and rust prevention of general guide rails, pins, hydraulic struts, bearings, suspension chains and other parts, especially for equipment and parts that cannot be heated and cured, such as large equipment, non-metallic materials, etc.
It is also suitable for machinery that is shelved in the open air, naval equipment (aircraft, ships) that are often exposed to seawater smoke, and lubrication and rust prevention of mechanical equipment that is often in acid, alkali, high and low temperature environments.
Some rust inhibitors are also suitable for spraying on machined and ground surfaces of various carbon steels, alloy steels, copper, aluminum, magnesium and their alloys, cast iron, non-metallic materials, etc. They can also be sprayed on the surfaces of metal workpieces that have been sandblasted, phosphated, passivated, anodized, and sulfur-nitrogen-carbonized.
