Status and Development of Mainstream Corrosion Inhibitors(1)

In recent years, the research of environment-friendly water-based rust inhibitors has received wide attention at home and abroad, especially the development of long-lasting water-based rust inhibitors has become a trend.

Water-based rust inhibitor overview

In the production, storage, and transportation of metal materials and products, rust will occur within a short period if they are not treated with rust prevention. The appearance and performance of finished steel products will be adversely affected. Rust will even cause the scrapping of metal products. There are many ways of rust prevention treatment for metals, but the effect of rust prevention should be considered in terms of rust prevention time, corrosive environment, and cost. For short-term rust prevention (a few months), oil-based or water-based rust inhibitors are generally used; for medium-term rust prevention (a few years), organic coatings are usually used for anti-corrosion treatment; for long-term rust prevention (more than ten years), hot-dip galvanizing (or aluminum) and organic coating are generally utilized for anti-corrosion treatment.

At present, rust prevention oil is mainly used for the inter-process and long-time sealed transportation of metal materials and products. Still, the subsequent process requires oil removal treatment, but removing rust prevention oil is quite tricky. With the increasing tension of oil resources and the increasing awareness of environmental protection, coupled with problems such as oil quality not easily decomposed, it researches developing water-based rust preventive agent of great importance. 

1. Water-based metal antirust agent, the direction of future efforts

The traditional water-based rust inhibitor (also called rust-proof water) refers to adding a certain amount of rust inhibitor to the water solvent to prevent chemical or electrochemical effects from occurring. Most traditional water-soluble inorganic antirust agents, including those still in use today, use passivation techniques such as sodium nitrite, potassium dichromate, borate, aluminate, tungstate, etc., to generate insoluble passivation film layers or reaction film layers to act as rust inhibitors. This kind of inorganic rust inhibitor is inexpensive. However, after this kind of rust inhibitor is applied to the metal surface, the liquid film is easy to flow. After the water evaporates, a layer of white frost will be formed on the metal surface, which will lose the excellent effect of rust prevention. The rust prevention performance will not reach the mark of rust prevention oil. Although some progress has been made in reducing the amount of nitrite in China, the effect of rust prevention is not satisfactory, and the problem of public health has not been eliminated. Although chromic acid and dichromate are more effective in rust prevention, they also have environmental pollution and toxicity problems, and their use is now prohibited. The development of non-toxic, environmentally friendly water-soluble antirust agents has become a concern at home and abroad.

In recent years, the research of environment-friendly water-based antirust agents has received wide attention at home and abroad, especially the development of long-lasting water-based antirust agents has become a trend. At present, domestic and foreign water-based antirust agents have been different from the traditional inorganic passivation-type antirust water, which mainly relies on the passivation effect of nitrite and so on to slow down the corrosion of metals. In recent years, based on green organic synthetic chemistry and self-assembly chemistry, many environmentally friendly water-soluble organic rust inhibitors have been developed and achieved an excellent anti-corrosion effect. Water-soluble organic rust inhibitors are mainly adsorbed on the metal surface through the physical and chemical adsorption of organic molecules added in the solution to change the state of the metal surface and play a role in rust prevention; also by adding water-soluble organic film-forming agents, corrosion inhibitors, co-solvents and other components of the compound, which can form a skinny organic film layer on the metal surface to block the contact between corrosive media and the metal, and the film added in the corrosion inhibitor added in the film layer also plays the role of blocking corrosion. This type of water-based antirust agent is similar to organic anticorrosive coatings, but there are essential differences with traditional organic anticorrosive coatings. The currently developed environmentally friendly water-based antirust agents have the following characteristics.


 (1) It is non-flammable, non-toxic, and environmentally friendly.

 (2) It can form a continuous and dense protective layer on the metal surface and isolate the contact between oxygen and water vapor in the air and the steel surface to achieve the purpose of rust prevention.

 (3) Compared with traditional water-based inorganic antirust water, it has better anti-corrosion performance and can be used for the short and medium-term protection of metal materials and products.

 (4) The protective film layer formed by this antirust agent on the metal surface is skinny, maintaining the metal base color and combining well with the metal base. It can be used in the subsequent processing or treatment process usually without further treatment, eliminating the tedious steps of traditional antirust oil cleaning, and even if the protective film layer needs to be removed, it can be removed by simple methods such as alkali or acid washing, which is convenient to use and less polluting.

 (5) Compared with the traditional thicker anti-corrosion organic coating, organic rust inhibitors form only a fragile protective film on the metal surface, which is still a temporary or short- to medium-term protective coating and is suitable for storage, transportation, and intermediate processing of metals and products. Some of the rust inhibitors with good performance can achieve several years of rust prevention effect indoors. Even some of them can achieve similar protection performance as traditional organic thick coatings outdoors.

(6) Water-based rust inhibitors are very convenient to use. They can be used to prevent various metal materials, components, products, and even instruments and equipment using immersion, spraying, brushing, etc., with low processing costs.

 2. The development status of foreign environment-friendly water-based antirust agents

Many non-toxic and biodegradable green products have been applied in practice, but the relevant technologies are the technical secrets of significant companies and are less publicly reported. The search found that more than 1,200 foreign patents on rust inhibitors were retrieved, of which about 100 were on water-soluble or water-based rust inhibitors.

At present, the focus of research in water-based rust inhibitors is mainly on the polymerization reaction on the surface of the protected material to generate a dense anti-corrosion polymer film, i.e., a temporary or short- to the medium-term protective film layer. A U.S. patent introduced a water-based rust inhibitor, which is 270g potassium hydroxide, 180g octanoic acid, 20g ethylenediaminetetraacetic acid sodium tetrahydrate, 30g dodecyl dicarboxylic acid, 14g fatty acid potassium, and the appropriate amount of toluene triazole added to 440g of distilled water to dissolve, and then dilute 50 times with distilled water can be used. This rust preventive water is not toxic, friendly to the environment, and has strong rust resistance, and can be used to protect not only steel products but also copper and aluminum from rusting, and can be added to metal cutting fluids and rust preventive paint. When the clean metal is immersed in it, the metal ions generated by the metal reaction can promote the resin to settle on the metal surface to form a resin film, and then after post-treatment with an aqueous solution containing amine-based resin and amine-based compound, a dense and efficient rust prevention film is formed on the metal surface.

Such as the United States CORTEC company, Germany INTECHCHEMIE, Australia DB company, Inverun company, Japan NEOS company are professional manufacturers of environmentally friendly water-based rust inhibitors, have formed a series of products, and have corresponding agents in China. Among them, especially the technology of American Goethe is at the world's highest level, and many green products are widely used in the market. The company's products have been introduced into the Chinese market, and more than ten companies are agents for the company's water-based rust inhibitor products.

   

3. The development status of China's environment-friendly water-based antirust agents

China's research on environmentally friendly water-based antirust agents started late, and there are few green products with market competitiveness. However, the primary research in related areas has developed rapidly in recent years. Most of the analyses are focused on the compounding antirust agents of ethanolamine and acid, polyol ester antirust agents; metal surface self-assembling antirust agents; silane coupling antirust agents; gas-phase antirust water, etc.

(1) Rust inhibitors for compounding ethanolamine and acid

Ethanolamine includes monoethanolamine, diethanolamine, and triethanolamine, and the acid compounded with them can be inorganic acid and organic acid. Alcohol amine and acid are compounded at room temperature to produce alcohol amine salt. The amide generated by heating monoethanolamine and diethanolamine with a carboxylic acid is also a very effective antirust agent. A very dilute solution of alkyl alcohol amide can prevent steel from rusting and has good resistance to hydrolysis, and at the same time has a thickening effect on antirust water, thus avoiding the loss of antirust agent from the metal surface and making the antirust agent firmly attached to the metal surface. The nitrogen and oxygen atoms in the molecules of organic carboxylic acid alcohol amides and alkyl alcohol amides have lone pairs of electrons, which can act with metal surfaces with d-space orbitals such as iron to generate complex films, preventing molecules such as oxygen and water from coming into contact with metal surfaces.

(2) Polyol ester rust inhibitors

Sorbitan monooleate is a kind of rust inhibitor with excellent performance, and there are also pentaerythritol esters, sugar esters, and so on. Inositol hexakisphosphate contains 24 oxygen atoms, 12 hydroxyl groups, and six phosphate groups in one molecule, which is easy to form several stable chelate rings when complexed with metal and create a dense transparent monomolecular film on the metal surface rapidly, thus effectively resisting the corrosion of metal, the rust prevention period can be more than 1a, suitable for steel and non-ferrous metals between processes and long-term sealing rust prevention treatment, and can also It can also replace the phosphate treatment before metal coating and avoid water pollution caused by the discharge of phosphorus-containing wastewater. The antirust agent is extracted from food crops and requires deionized water for preparation, so production costs are a problem.

(3) Self-assembled rust inhibitor for metal surfaces

Some organic molecules in solution can spontaneously adsorb on the metal surface, forming a layer of well-oriented, tightly arranged hydrophobic single-molecule coating, which can effectively prevent the transfer of water molecules, oxygen molecules, and electrons to the metal surface so that the critical potential of oxidation of the base metal is shifted, and the oxidation-reduction current of the metal surface is significantly reduced, thus playing a protective role for the metal, this process is the rust inhibitor molecule in This process is the self-assembly of rust inhibitor molecules on the metal surface. The surface of the iron electrode is modified by self-assembly of n-decyl thiol, which can effectively resist the corrosion of SO2.

(4) Silane coupling antirust agent

Silane coupling agent, according to its chemical structure, can be divided into two categories: monosilane and silane coupling agent, the general formula of the two are Y-(CH2)n-Si-(OR)3 and (RO)3-Si-(CH2)n-Y-(CH2)n-Si-(OR)3, where Y is the functional group, RO-is hydrolyzable alkoxy. In recent years, silane coupling agents have been used as rust inhibitors for metal materials and are expected to replace chromate passivation and conventional phosphating processes. When used as a rust inhibitor, the silane is first allowed to undergo hydrolysis. The resulting silanol undergoes a condensation reaction with oxides or hydroxides on the metal surface to produce Si-O-Me covalent bonds, with Me representing the metal being protected. In contrast, the remaining -SiOH groups adsorbed on the metal surface undergo a condensation reaction to form a dense silane film. It is found that silane coupling agents such as 1,2-diethoxysilyl ethane can significantly improve the rust resistance of metals. As silane coupling agent is cheap and easy to obtain, good environmental protection, excellent rust resistance, can be treated with iron, aluminum, and many other metals, so its research and application have shown great potential.
 
The preliminary search found that there are more than 370 patents on rust inhibitors in China, among which there are 60-70 patents on water-based rust inhibitors or water-soluble rust inhibitors. The earliest patent was already filed in 1985, and new patents and methods have appeared year by year since then.


In addition, there are some product information reports.

Environmentally friendly multi-functional long-lasting water-based antirust agent: Compared with antirust oil, the water-based antirust agent is easy to prepare and use, inexpensive, safe to process, and transfer to the following process without removing oil reducing workload, etc. However, most of the ordinary water-based antirust agents on the market are mainly composed of nitrite. Recent studies have shown that this product is an indirect carcinogenic substance, which has caused widespread concern in the world. Although some alternative products without nitrite or low nitrite content have appeared on the market at home and abroad, the rust prevention period is relatively short. Generally, it does not exceed 15 days in the natural environment, challenging to meet the market requirements. The CJ-type water-based metal rust preventive agent developed by a Chinese company is a non-toxic, tasteless, light yellow transparent liquid without any stimulation to the human body, which is neutral and can be dissolved with water in any proportion. CJ-type non-toxic and environmentally friendly long-lasting rust inhibitor can be used as inter-process and long-term closed rust prevention for various metal materials and corrosion inhibitor for metal cutting fluids, cleaning fluids, and refrigeration circulatory systems.

New environmental protection water-based metal rust inhibitor: With the rapid development of metal material surface treatment and institutional processing industry towards multi-functional, high-speed, robust, and precision, the selection of metal rust inhibitor has put forward higher and higher requirements. Especially since the 1980s, due to the further strengthening of people's awareness of the ecological environment, higher standards have been put forward for metal antirust products, which must meet environmental protection requirements and have excellent antirust functions. The oleic acid type oil-soluble antirust agents commonly used in recent years are unsuitable for use under challenging water conditions because of their limited solubility in water and sensitivity to electrolytes. The new water-based metal rust inhibitor successfully developed by a Chinese company selects natural, non-toxic chemical product phytic acid as the main component of the rust inhibitor and compounds it with synergists and non-ionic surfactants, which can form a dense and firm phytate protective film on the metal surface to prevent oxidation and corrosion of the metal for a long time. It is reported that the product has been added to metal surface cleaning fluid, cutting fluid, phosphating fluid, and chrome-free passivation fluid, all of which have achieved satisfactory results.

 Environmentally friendly water-based antirust agent for magnetic materials: NdFeB magnetic materials are iron-based alloys, and antirust is essential in the process of production. At present, there are two kinds of antirust products: oil-based and water-based. The oil-based antirust agent makes the surface of the material slippery and difficult to remove and brings inconvenience to the following process, so it is rarely used now. Water-based antirust agents are easy to use and inexpensive. Still, the products sold in the market generally contain carcinogenic toxic substances such as nitrite and chromate, which are harmful to the health of operators and cause severe pollution to the ecological environment after discharge. According to the chemical and electrochemical corrosion characteristics of magnetic alloy materials and market demand, a Chinese company has developed an environmentally friendly water-based metal antirust agent KJ-2, which is made by using inositol hexaphosphate, a potent metal chelator, as the main component, and several other water-based additives. Inositol hexaphosphate is a natural, non-toxic organic chemical product extracted from food crops. When used as a rust preventive agent for magnetic materials, it can rapidly chelate with metals on the surface and form a dense monomolecular complex protective film, which can effectively inhibit the corrosion of metals. The treated material surface has better bonding with the plating or coating. The product can also be widely used in ferrous, non-ferrous metals, and other alloy materials between rust prevention treatment and alternative phosphate treatment before painting.

 In China, as early as 1996, some research institutions researched water-based and water-soluble antirust agents and gradually expanded from the initial water-soluble antirust agents to gas-phase antirust agents' study some other products. Currently, there are more than 20 research institutions engaged in this area of research.

 Water-based rust inhibitors can replace the main body of mechanical oil as rust prevention oil, but also eliminate the traditional oil removal, phosphating, and other processes, not only to save energy but also to reduce operating costs but also to avoid the environmental pollution caused by the discharge of waste liquid, is an alternative to rust prevention oil and phosphating solution, a variety of metal materials and products can be treated with this type of rust inhibitor at different stages, industrial demand is enormous, the market outlook is optimistic.

 In addition, multi-functional water-based rust inhibitor research and development and application are becoming a trend, such as oil removal, rust removal "two in one," oil removal, rust removal, rust prevention products are increasing, such as the United States Goethe company has developed a series of products. For example, Goethe has developed a series of products. Through emulsifying the oil layer, penetrating and transforming the rust layer, they play a protective role for metal products. Research and development of good antirust properties, stable nature, low price, reduce pollution, the full-featured, easy-to-use water-based metal antirust agent is the direction of future efforts.

 Copper and its alloys have excellent strength, good mechanical processing properties, electrical conductivity, thermal conductivity, weldability, and high thermodynamic stability. They are widely used in machinery, electric power, electronics, transportation, transportation, national defense, and other fields. Copper has high thermodynamic equilibrium. Usually, due to the oxide film formed by copper and its alloy itself, the corrosion of copper is inhibited, and it shows good corrosion resistance. However, its more severe corrosion of copper and its alloys can occur in higher humidity, corrosive media (such as air containing sulfur dioxide, water holding oxygen, oxidizing acids, and in liquids containing CN-, NH4+, etc. form complex ions with copper).

 To reduce or prevent the problems caused by corrosion, the application of vapor phase corrosion inhibitors in the manufacture, storage, transportation, and transport of copper and other metal products is a proven method that has gained widespread use. Vapor phase corrosion inhibitor molecular weight is negligible. At room temperature can be volatile components with corrosion inhibition. Diffusion, migration, adsorption to the metal surface can bring protection to the metal to avoid or slow down metal corrosion.

 Copper and its alloys commonly used corrosion inhibitors include various organic and inorganic compounds, with organic compounds dominating. Before the 1990s, copper corrosion inhibitors are mainly thiourea, aldehydes, amines, benzoic acid, and other organic compounds and their derivatives and heterocyclic compounds such as thiazole. The copper corrosion inhibitors applied in China are mainly benzotriazole and mercaptobenzothiazole and their products. Domestic and foreign research on copper corrosion inhibitors focuses on synthesizing new organic compounds and the compounding of corrosion inhibitors, such as imidazole, thiazole, carbazole, aniline, thiourea, and derivatives of triphenylmethane and the compounding of these corrosion inhibitors.

 In the 21st century, copper corrosion inhibitor research has made significant progress. There are many new substances as copper corrosion inhibitors and play a perfect corrosion inhibition effect. Research on the synergistic effect between different corrosion inhibitors and the realization of corrosion inhibitor compounding is an essential trend in the development of corrosion inhibitors, the extraction of natural ingredients, the synthesis of new organic substances, and the study of corrosion inhibitor compounding has become a meaningful way to develop copper corrosion inhibitors. With the increasing concern about the environmental pollution and environmental hazards caused by using a large number of chemicals, toxic, difficult to degrade the corrosion inhibitor has been increasingly restricted in use. Therefore, high-efficiency, low-toxic, or even non-toxic green corrosion inhibitors have become the development trend of copper corrosion inhibitors. The following will briefly introduce several types of specific environmentally-friendly copper corrosion inhibitors that have been widely used or researched, and developed.