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Steel low surface treatment anti-corrosion coating is a type of coating that can be widely applied to different surface treatment levels to achieve conventional anti-corrosion effects. Due to its special performance and application scenarios, it has received increasing attention from academic and engineering circles at home and abroad in recent years. Among various types of material corrosion, steel corrosion is the most common and causes the most severe losses. Hou et al. summarized the losses caused by corrosion in various countries, accounting for 3% to 5% of the annual GDP; In addition, the National Association of Corrosion Engineers (NACE) in the United States has estimated that the annual losses caused by corrosion worldwide reach $255 billion, which is over 3.4% of the world’s gross domestic product (GDP).

Steel is still the most widely used and widely used engineering structural material at present. Coating is the most important anti-corrosion technology for steel. For steel structures with ultra long service life, such as cross sea bridges, ocean platforms, and ocean going ships, due to the technical limitations of in-service online maintenance, there is an urgent need for coatings that can adapt to wet and rusted surfaces to be matched with them, in order to implement on-site coating and ensure the anti-corrosion effect of the project.

In order to slow down the corrosion of steel and extend its service life, it is often necessary to sandblasting or shot blasting the steel substrate before construction to remove the rust layer and enhance the adhesion between the coating and the steel surface. In most cases, the surface cleanliness after treatment needs to reach the surface treatment level Sa2.5 or above specified in ISO 12944:2019, NACE SP 0108-2008, and HG/T 5059-2016. For actual outdoor construction conditions, sandblasting and shot blasting are difficult to carry out in marine environments with humidity greater than 90%. On the other hand, due to safety hazards such as static electricity sparks, some special engineering fields cannot carry out sandblasting and rust removal, such as oil drilling platforms equipped with storage tanks and chemical pipeline systems. Therefore, domestic and foreign experts and scholars have conducted extensive research on low surface treatment coatings that can adapt to different degrees of substrate corrosion treatment.

Research status of low surface treatment anti-corrosion coatings

Low surface treatment coating is a type of coating that can be applied to the surface of substrates that have undergone simple surface treatment or have not been treated. Currently, low surface treatment coatings that have been applied can be suitable for steel substrates with surface treatment levels of St2 or higher. The conditions for low surface treatment substrates are mainly divided into wet, rusted, oily, or a combination of the above conditions. The rust layer is currently a difficult and urgent problem to deal with.

1.1 Rust coated paint

1.1.1 Composition of rust

Rust is a complex iron-based compound, and the common rust layer on steel can be divided into two layers according to its surface structure: the inner layer is a tightly bonded and highly active rust layer, which is the main cause of continuous corrosion of steel; The surface structure is a loose and porous water containing floating rust layer, which has a relatively stable structure. However, due to the loose and porous structure, it cannot suppress the development of rust towards the interior. Based on the research data of Mossbauer spectroscopy and X-ray diffraction (XRD), scholars have classified iron oxide into 16 crystal forms according to different crystal structures, chemical compositions, and iron valence states, as shown in Table 1. Usmen et al. pointed out that at the surface and room temperature, rust mainly exists in the form of Fe (III) oxides, while near the marine atmosphere, a portion of Fe (III) reacts with Fe (II) or other reducing agents to form mixed valence iron minerals. The XRD study by Kamimura et al. showed that the rust formed on low carbon steel (exposed for 15 years) and weathering steel (exposed for 32 years) consists of crystalline α – FeO (OH), γ – FeO (OH), and amorphous (amorphous phase>50% of total rust). Collazo et al. compared the differences in the phases and compositions of corroded crystals in the atmosphere and chloride rich environments (low carbon steel exposed to salt spray chambers). The main corrosion products in the atmosphere were γ – FeO (OH) and α – FeO (OH), with a small amount of highly active akaganeite [β - FeO (OH)] and spinel phase (γ – Fe2O3/Fe3O4) also present in chloride rich environments. Cates pointed out that the main crystal component in rust is γ – FeO (OH). Kanevskaya et al. pointed out that the basic corrosion product of steel is γ – FeO (OH), and under higher atmospheric exposure conditions, a small amount of Fe3O4 is also formed. Gancedo et al.’s study showed that both α – FeO (OH) and γ – FeO (OH) coexist in rust, with the latter being the most prominent. Tripathi et al. have identified that the initially formed rust contains a large amount of γ – FeO (OH), which transforms into non stoichiometric Fe3O4 over time. Yahya et al.’s report also mentioned that the main components of rust are γ – FeO (OH) and Fe3O4.

Based on the analysis of rust components in the above literature, it can be concluded that the main rust component in marine atmosphere is γ – FeO (OH), accompanied by a small amount of α – FeO (OH), β – FeO (OH), and Fe3O4. Chlorine in the corrosion layer usually exists in akaganeite [β - FeO (OH)], which can accommodate about 15% (molar fraction) Cl in its structure, but may also exist in the form of ferrous or ferric chloride or hydroxyferrous chloride [β - Fe2 (OH) 3 Cl].

The strong penetrability of chloride ions can open up corrosion pathways, allowing oxygen and water molecules to enter the matrix and accelerate the corrosion process of steel. The diversity of rust components and the unevenness of rust make its surface uneven, so it is required that low surface treatment coatings have sufficient penetration and adhesion, and can fully bond with the gaps between rust holes to prevent coating peeling; On the other hand, low surface treatment coatings should be able to remove and transform rust, preventing further corrosion into the interior of the substrate. The basic principle of low surface treatment coatings is to use the fluidity and permeability of resins to achieve penetration and strong adhesion to uneven corroded surfaces, and to wrap some corroded substances, blocking the development space of corrosion; Remove or transform internal rust by adding pigments and fillers to prevent further expansion of rust.

1.1.2 Classification of Rust Coating

According to the mechanism of rust treatment, rust coating can be divided into four types: permeable low surface treatment coating, conversion rust coating, stable rust coating, and mixed rust coating. Among them, the infiltration type utilizes the high fluidity and strong adhesion of film-forming resin to encapsulate the rust products, making them part of the coating; Conversion type and stable type utilize the dissolution and chelation of pigments and fillers in coatings to convert or form stable compounds of rust. The hybrid type combines the advantages of the above corrosion treatment methods, improving the stability and corrosion resistance of the coating while treating corrosion, and is currently a hot research direction for scientific and technological workers.

(1) Penetrating low surface treatment coating.

The generation of rust on steel is uneven and irregular, and its surface morphology can be considered as a collection of micro nano pore corrosion. There is a certain surface tension between the pores, which hinders the penetration and adhesion of the matrix resin. On the one hand, it seriously reduces the mechanical properties of the coating. On the other hand, the oxygen and water molecules in the internal pores can lead to the expansion of rust. Majumder et al. found that when the channel is filled with a liquid of appropriate surface tension and viscosity, adhesion is significantly enhanced. Penetrating rust coating itself does not react with the substrate rust. Its main function is to utilize the strong permeability and wettability of film-forming resin to the substrate, allowing it to reach the rust gap, seal the active substances such as rust in inert resin, prevent the expansion and continued generation of rust, and enhance the adhesion between the coating and the substrate. In summary, permeable rust coating is a typical physical shielding coating. The thinner the rust layer, the better the performance. It is commonly used on substrate surfaces with thin rust layers (<40 μ m) in industry, and its application is limited. Therefore, there are currently few types of pure permeable rust coating, and it is mainly combined with conversion and stable coatings.

Weng Qiusong et al. used styrene butadiene styrene copolymer (SBS) modified acrylic resin to modify low relative molecular weight bisphenol F-type epoxy resin, increasing the flowability of the epoxy resin. The addition of modified acrylic resin can increase the active groups such as hydroxyl, carboxyl, and epoxy in the matrix resin. The increase in the content of active groups can significantly enhance the adhesion and encapsulation of the matrix resin to rust. Its neutral salt spray resistance reaches 1800 hours and can be recoated within 20 minutes, demonstrating excellent performance. Xiang Yonghua and others proposed a resin ratio scheme combining low viscosity phenolic epoxy resin and ethylene modified epoxy resin. Low viscosity phenolic resin endows the resin with good wetting and permeability, as well as excellent heat resistance; Ethylene based modified epoxy resin can not only encapsulate rust but also transform it, achieving the goal of multi mechanism rust prevention. Luo et al. proposed that the corrosion layer can be regarded as a natural micro/nano corrosion channel capillary structure, constructed a capillary structure model of the corrosion layer, and prepared an organic polyurethane resin (PU)/fluorinated polymer (PF) composite coating. Due to the fact that the PU/PF coating with deformable PF chains can spontaneously fill the micro/nano channels in the corrosion layer under capillary force, the composite coating has excellent penetration ability.

(2) Conversion type low surface treatment coating.

Conversion type rust coating refers to the use of a transfer rust agent in the coating to convert the corroded part of the substrate surface into an iron containing substance that does not damage the substrate, in order to achieve the purpose of rust conversion. Chromium containing compounds and lead containing compounds have good rust conversion effects, but they have been widely phased out due to the requirements of being green, environmentally friendly, non-toxic, and harmless.

The commonly used rust removers at present are acidic substances such as phosphoric acid, tannic acid, and gallic acid. These acidic substances can react with rust to dissolve it, and the phosphate groups, polyphosphate groups, phenolic hydroxyl groups, etc. can form relatively stable complexes with iron ions to prevent rust from corroding the substrate. Giudice et al. were the first to study the reaction between tannic acid substances and rust, indicating that tannic acid has a good conversion effect on rust. Collazo et al. constructed two corrosion environments to investigate the conversion efficiency of tannic acid phosphate composite rust agents on corrosion. The research results showed that in low chlorine environments (weathering cycle conditions), the addition of tannic acid phosphate composite rust agents had no significant effect on corrosion conversion; In a high chloride environment (5% NaCl solution), the addition of tannic acid phosphate composite rust inhibitor has a significant effect on rust transformation. Wang et al. explored the influence of the sharing of phosphoric acid and gallic acid in vinyl chloride acrylic acid lotion on the antirust performance. The experiment showed that the sharing of phosphoric acid and gallic acid can play its synergistic effect, achieve a better rust conversion level, and the adhesion performance of the coating has also been improved to a certain extent.

Although conversion type rust coatings such as tannic acid and phosphoric acid are relatively easy to apply in practical working conditions, there are also many problems, such as unfriendly to the environment, poor water resistance, and coating adhesion. In addition, Favre et al. found that the concentration of tannic acid is closely related to the conversion effect, and neither too high nor too low can achieve the expected effect. Almeida et al. found that the acid concentration in the corrosion conversion system has a significant impact on the corrosion effect. Collazo et al. found that if phosphoric acid still exists after rust transformation, it will penetrate into the interior of the substrate and cause internal metal corrosion, thereby affecting the anti-corrosion effect of the coating.

(3) Stable low surface treatment coating.

Stable rust coating refers to the use of film-forming agents in the coating to chelate with rust, converting it into a stable and dense complex, thereby achieving the goal of rust transformation. Compared to the film-forming agents of the conversion type rust coating, the film-forming agents of the stable type rust coating have weaker acidity and less corrosion hazard to the interior of the substrate; At the same time, stable rust coating has lower cost, better adhesion, and can penetrate into loose rust layers, complex or chelate with rust, passivating or transforming rust into stable iron oxide, becoming a filler in rust coating. Stable rust coating has relatively loose requirements for surface treatment of steel and low requirements for rust layer thickness. It can achieve good results on steel structure surfaces with uneven rust and significant differences in rust surface conditions. The main reason is that the stable film-forming agent in the coating has strong anchoring and adhesion abilities to rust, and has good encapsulation and complexation effects on various different rust components.

The commonly used stable film-forming aids include chromate, phosphate, and other substances containing phosphate and chromate groups. Chromates and phosphates can hydrolyze to produce corresponding chromate and phosphate ions, both of which can react with active rust to generate heteropolyacid compounds for the purpose of rust stabilization. Traditional chromium lead composite anti-corrosion pigments have good anti-corrosion properties, but due to their high toxicity and potential harm to the environment and human health, they have gradually been replaced by some non-toxic and environmentally friendly pigments.

The most commonly used stable film-forming aid at present is aluminum tripolyphosphate, which is a coordination compound formed by aluminum ions and phosphate ions. Its molecule contains three O=P bonds and two HO-P bonds, and its density of active sites is much higher than other phosphates. Therefore, it has stronger chelating power and iron ion capture ability than phosphates, and higher rust removal efficiency. The rust prevention mechanism of aluminum tripolyphosphate is as follows: firstly, the hydrolysis of aluminum tripolyphosphate can dissociate the tripolyphosphate ion, which can form a relatively stable chelate with iron; The orthophosphate ion formed by further hydrolysis of tripolyphosphate ion has a strong binding effect with iron ion (Figure 1). Song et al. combined aluminum tripolyphosphate with acrylic resin and epoxy resin for the anti-corrosion of low-carbon steel surfaces, demonstrating good anti-corrosion effects. Sorensen et al. mixed aluminum tripolyphosphate with talc and other materials to prepare acrylic coatings for long-term anti-corrosion of pipelines and steel plates. Kong Huaying combined aluminum tripolyphosphate with polymethyl methacrylate microspheres to form an excellent anti-corrosion pigment with epoxy resin, and the coating exhibited good corrosion resistance.

However, excessive content of aluminum tripolyphosphate can lead to agglomeration and flash corrosion. Zhang Heng pointed out that aluminum tripolyphosphate has a certain degree of water absorption, which accelerates the penetration of water molecules and instead accelerates the corrosion of the substrate. Li Shanshan proposed that aluminum tripolyphosphate itself has a certain acidity, and the released H+can also accelerate the corrosion of the substrate. The traditional single phosphate stabilized film-forming agent can no longer meet the increasingly high requirements for rust removal and corrosion resistance. The combination of multiple rust stabilizing substances and the discovery of new substances have become new research ideas. Feng et al. used hydroxyethylidenediphosphonic acid (HEDP) and tannic acid modified aluminum tripolyphosphate as film forming additives, and styrene acrylic acid lotion as film forming resin to prepare water-borne antirust coatings. Through the synergistic effect of three reagents, the anti rust conversion performance and corrosion resistance were improved, and the electrostatic adsorption of HEDP and tannic acid on aluminum tripolyphosphate was utilized to improve the agglomeration problem of aluminum tripolyphosphate, thereby improving the early anti rust performance deficiency of aluminum tripolyphosphate.

(4) Hybrid low surface treatment coating

With the deepening of research on rust coating, single principle rust coating can no longer meet higher requirements for rust removal and corrosion prevention. The combination of multiple principles of rust coating has become the main direction of current research. Wang Jiao et al. modified acrylic resin with fluorosilicon containing phosphate groups and successfully prepared a wet to rust coating that can be applied to highway guardrail coating by adding substances such as aluminum tripolyphosphate and nano silica. At a thickness of 150 μ m, the neutral salt spray test (NSS) can reach 1200 hours. Yang Yuqi et al. studied the effect of the combination of aluminum tripolyphosphate with phosphoric acid, tannic acid, butanol and other substances on rust conversion. The experiment showed that the addition of various rust stabilizing fillers can significantly enhance the removal and conversion of rust, achieving good performance indicators. Chen Peng et al. used a copolymerization reaction of ethylene propylene diene monomer resin, SBS resin, epoxy and heterocyclic compounds, and graphene as a corrosion-resistant filler to prepare a wet and rusted coating that can be applied in marine atmospheric environments. Its lifespan is generally 10-15 years in highly corrosive environments, and it has a high level of performance.

1.2 Wet coating paint

Wet coating coatings are mainly suitable for high-pressure water jet rust removal surfaces and coatings on surfaces with water film or in humid environments. Water molecules adhering to the substrate surface can significantly reduce the adhesion of the coating, and at the same time, form micro corrosion cells on the substrate surface, accelerating the corrosion of steel. In response to the anti-corrosion requirements in humid and heavily corrosive environments such as marine and industrial environments, the development of wet coating coating products has a wide range of practical applications. The function of wet coating is mainly achieved by selecting appropriate curing agents or corresponding additives.

The bonding ability between film-forming resin and curing agent and the substrate surface determines the actual effect of wet coating. When the iron affinity of the mixture of film-forming resin and curing agent is greater than that of water, the substrate can be combined with the coating, and water molecules will evaporate through the micropores of the coating or with the solvent to achieve the purpose of water removal; In addition, suitable water removing agents can also achieve the expected effect. Alcohol and ether solvents with good compatibility with water can bring water molecules out of the coating. Surfactants and other amphiphilic substances are also used to achieve wet coating. Suitable hydrophilic groups of surfactants can dissolve water molecules and displace them from the substrate surface during the reaction, achieving the purpose of water removal. Zhao Limin et al. proposed that hyperbranched polyether polyester resins terminated with phenolic hydroxyl and carboxyl groups can effectively achieve wet coating requirements. The end groups of phenolic hydroxyl and carboxyl groups are hydrophilic groups, which can wet the substrate surface and dissolve the moisture on the substrate surface in the initial stage, and bring water molecules out of the substrate surface when the groups participate in the reaction, achieving the purpose of water removal.

1.3 Oil coated paint

Oil coating is mainly used to solve the adhesion of oily substances on the surface of the substrate. The difficulty of oil coating is how to make the coating have high adhesion. The wetting state of a substance depends on the surface tension and surface roughness of the liquid. At 25 ℃, the surface tension of deionized water (DI) is 72.0 mN/m. Oil has a relatively low surface tension, typically ranging from 21.1 (octane) to 27.0 mN/m (cetane). As the temperature increases, the surface tension of all liquids decreases.

The adhesion between oily substances and substrate surfaces is tighter and more difficult to remove, making it the most challenging and least researched field among the three low surface treatment coatings. Currently, there are few literature that can provide a detailed description of the principles and solutions of oil coating. Here, a possible application direction for oil coating is proposed – the use of superoleophilic substances for oil coating. Superoleophilic substances are currently mainly used in oil-water separation, based on the principle of using lower surface energy to increase their affinity for oily substances on the substrate surface, thereby achieving the ability to bind with oily substances on the substrate surface. Zhang et al. used epoxy resin (EP) as the base resin and adhesive, and employed silica nanoparticles and dodecyltrimethoxysilane (DTMS) to enhance nanoscale roughness and reduce surface free energy, respectively, to prepare a super oleophilic and super hydrophobic coating for oil-water separation. This method is also expected to be used for oil coating, thereby achieving high adhesion oil coating.

Latest technological progress in low surface treatment coatings

In recent years, significant progress has been made in the research of low surface treatment coatings, and new types of low surface treatment coatings with advantages such as low environmental pollution, strong weather resistance and photo aging resistance, and good corrosion resistance are constantly being developed.

Research on relevant literature in the past 10 years has found that the selection of rust conversion agents in most low surface treatment coating schemes is concentrated on aluminum tripolyphosphate and modified aluminum tripolyphosphate, supplemented with various inorganic substances. On the one hand, it can enhance the mechanical properties of the coating, and on the other hand, it provides physical shielding for the coating, improving its corrosion resistance. In the past two years, research on low surface treatment coatings has mainly focused on the application of phytic acid and gallic acid. Phytic acid is an organic solvent with a molecular formula of C6H18O24P6 and a relative molecular weight of 660. Its basic structure consists of a central carbon ring and six phosphate groups on the periphery. Its unique structure allows phytic acid to form relatively stable compounds with rust, while its weak acidity and high stability prevent corrosion of the interior of the substrate. Chang Liang studied the rust removal effects of four rust conversion agents, namely phytic acid, tannic acid, sulfosalicylic acid, and phosphoric acid, on rusted steel plates with a rust removal grade of St2. The results showed that phytic acid had the best rust removal effect, and the addition of phytic acid could improve the adhesion and corrosion resistance of the coating during rust removal. Tao Junjie et al. also studied the application of phytic acid in anti-corrosion coatings. Phytic acid has higher rust removal efficiency and better rust removal effect compared to phosphoric acid, tannic acid, etc. It can also alleviate the corrosion of the substrate interior by film-forming agents. Wang Rongxiang et al. studied the effects of different types of phytates on the rust removal efficiency of substrates, among which potassium phytate had the best protective effect on rusted substrates. Experiments have shown that compared with phosphoric acid and tannic acid, phytate has a better corrosion inhibition effect on rusted carbon steel substrates; This is mainly due to the presence of multiple phosphate groups in phytate molecules, which can chelate with Fe3+and make the rust layer denser. Meanwhile, the addition of phytate can improve the adhesion of the coating and enhance its shielding performance.

Gallic acid is one of the earliest discovered organic acids by humans, composed of multiple phenolic hydroxyl and carboxyl groups. It has the characteristics of simple structure, easy availability of raw materials, stable properties, and weak acidity. Lei et al. found that gallic acid esters formed by the reaction of gallic acid with 2,3-butanediol have good rust removal properties. Gallic acid esters contain phenolic hydroxyl groups with flexible long chains, which can penetrate into rust layers, chelate with Fe2+and Fe3+, and form Fe-O-C structured complexes (Figure 2), not only encapsulating rust substances, but also firmly binding iron ions. It is precisely for this reason that the adhesion and corrosion resistance of epoxy resin corrosion coatings are significantly improved. Feng et al. studied the modification method of gallic acid and grafted hydroxyimine into 3,4,5-trihydroxybenzoic acid (gallic acid) to obtain a novel rust inhibitor -3,4,5-trihydroxybenzoic acid? 2- ([hydroxyimino) methyl] benzoic acid, salt spray and other test results show that it has good corrosion resistance and can form a relatively dense passivation film on the surface of the substrate, preventing the penetration of water molecules and chloride ions. Jia et al. synthesized polymerizable lotion through ring opening reaction of glycidyl methacrylate and natural gallic acid, and explored its mechanism of rust conversion; Raman spectroscopy, XPS, SEM and other studies have shown that the coating with 4% gallic acid modified substance has the best corrosion conversion ability, which can convert the porous rust on the substrate surface into relatively stable layered stacking of α – FeO (OH) phase and Fe3O4 phase, preventing the corrosion medium from spreading from the surface to the metal substrate and improving corrosion resistance.

Problems and Solutions of Low Surface Treatment Anti Corrosion Coatings

2.1 Issues related to water-based low surface treatment coatings

The water-based low surface treatment coatings are mainly composed of water-based lotion and rust transfer and stabilization additives. At present, the resins that have been successfully applied to water-based low surface treatment coatings include water-based oil, alkyd, acrylate and acrylic polyurethane modified resin. Similar to solvent based low surface treatment coatings, phosphoric acid, phytic acid, and phosphate functional components are widely used as rust stabilizing agents.

Compared to solvent based low surface treatment coatings, water-based low surface treatment coatings have better flowability and wetting properties, which can better penetrate into the interior of rust and encapsulate and seal it. For water-based low surface treatment coatings, there are several issues that urgently need to be addressed: firstly, the corrosion resistance and comprehensive performance of water-based low surface treatment coatings are inferior to solvent based coatings, and most water-based coatings cannot meet the increasingly high performance requirements. The fundamental reason is the hydrophilicity and instability of water-based coatings; Secondly, rust stabilizers are mostly acidic substances with a large number of active groups. If the amount of functional components added is small, the reaction with rust will not be complete; The addition of a large amount of functional components can disrupt the stability of water-based film-forming systems and accelerate metal corrosion due to their acidic characteristics.

Li Shiyu mixed phosphoric acid and tannic acid, adjusted the pH of the rust inhibitor with alcohol amine, and added it to water-based vinyl chloride acrylic resin. The long-term anti-corrosion ability of the water-based vinyl chloride acrylic rust conversion primer was significantly improved, with a corrosion current density of 0.003 43 μ A/cm2 after 30 days and an adhesion force increased from 1.03 MPa to 3.06 MPa. Gao Lijun and others proposed an environmentally friendly technology that uses water as a solvent and soluble salts as solutes to stabilize the rust layer on the surface of weathering steel. Li et al. developed a water-based composite conversion agent that can effectively improve the adhesion and electrochemical performance of coatings. Compared with commercially available rust conversion agents, the adhesion of coatings increased from 1.6 MPa to 4.8 MPa, and the neutral salt spray time increased from 24 hours to 120 hours.

2.2 Flash rust caused by rust conversion agents

Flash rust is one of the most common problems encountered in the application of low surface treatment coatings. Flash rust is a point like corrosion phenomenon that occurs before the water-based components in water-based coatings come into contact with the metal surface, as shown in Figure 3. The speed at which flash rust occurs on metal substrates is: cast iron>carbon steel>galvanized sheet (polished)>tinplate (polished).

The possibility of flash rust is higher during construction in humid climate environments, and this problem mainly occurs on corroded or recently activated metal surfaces. Flash rust is closely related to the construction environment. Construction in low temperature and high humidity environments, or coating spraying too thick with relatively slow water evaporation, can all lead to slow coating drying and flash rust. Aluminum tripolyphosphate, tannic acid, phosphoric acid, etc. are still commonly used rust removers in industry, but their strong acidity greatly increases the possibility of flash rust on the substrate.

At present, researchers have proposed a combination of various acidic substances and a modification scheme for aluminum tripolyphosphate to address the issue of flash rust. Wang studied the compounding schemes of various acidic substances and found that compounding tannic acid and phosphoric acid can improve the problem of flash rust. The modification of aluminum tripolyphosphate involves the weak chemical reaction between the active hydrogen in aluminum tripolyphosphate and alkaline or amphoteric substances under certain conditions. Wu Longchao et al. studied the scheme of modifying aluminum tripolyphosphate by compounding zinc oxide and calcium oxide, and the experiment proved that its corrosion resistance was improved. Chen Xing, Ning Hong, and others have modified aluminum tripolyphosphate with molybdate to enhance the corrosion resistance of the coating.

2.3 Curing issues of low surface treatment coatings

The selection of curing agents is an important issue for low surface treatment coatings. The curing agent for low surface treatment coatings should have the characteristics of low viscosity, good permeability, low volatility, low toxicity, good compatibility with film-forming resins, and fast curing speed. Among them, low viscosity curing agents and low viscosity fluids crosslinked with curing agents and resins can ensure that the coating covers the rust gaps and covers the rust, thereby achieving the goal of rust coating through penetration. In addition, the curing speed is also one of the important factors determining the quality of the curing agent. Due to the increase of acidic substances such as rust conversion agents, the stability of the system will be greatly reduced. Lower substrate surface treatment levels and longer curing times may lead to flash rust. Therefore, it is particularly important to choose the corresponding curing agent for the appropriate resin.

Cashew phenol modified amine curing agent is a good curing agent for bisphenol A epoxy resin. The benzene ring contained in it can reduce the viscosity of the overall coating, the hydroxyl group can increase the wettability of the coating, and the phenolic hydroxyl group and unsaturated carbon chain provide efficient low-temperature curing ability for the coating.

3 Development directions of low surface treatment anti-corrosion coatings

At present, low surface treatment coatings are difficult to meet the requirements of efficient and green use. With the advancement of science and technology and the development of the coating industry, low surface treatment coatings are gradually moving towards multifunctionality, green environmental protection, and low cost, giving rise to many new directions for the development of low surface treatment coatings.

3.1 High performance, low thickness, and low surface treatment coatings

The current research focus on low surface treatment coatings is on the selection and modification of rust removers, while external corrosion resistance is often overlooked, relying solely on the accumulation of film thickness to achieve the goal of corrosion prevention. Many commercially available rust conversion agents often only have a single function of rust conversion and cannot be combined with external anti-corrosion resins or fillers. Aluminum tripolyphosphate, as the most commonly used type of rust conversion agent in industrial production, has poor solubility in aqueous solutions and poses a challenge to the stability of coating performance due to its pH less than 7. How to achieve the synergistic effect of rust remover and anti-corrosion filler is one of the important research directions for low surface treatment coatings.

3.2 Application of Organic and Biobased Corrosion Inhibitors

The focus and difficulty of treating low surface treatment coatings lies in the conversion of rust and the removal of impurities such as water and oil molecules. Corrosion inhibitors can be divided into anodic corrosion inhibitors, cathodic corrosion inhibitors, and mixed corrosion inhibitors according to their functional positions. Anodic corrosion inhibitors are divided into two types: oxidizing and non oxidizing. Oxidative corrosion inhibitors refer to corrosion inhibitors that can passivate metals under anaerobic conditions, such as chromate ions; Non oxidizing corrosion inhibitors require oxygen to passivate metals, which can quickly form protective oxide films with oxidized metal materials, such as phosphates. Corrosion inhibitors can be divided into inorganic inhibitors, organic inhibitors, and bio based inhibitors. Inorganic corrosion inhibitors are commonly used rust prevention and removal agents, such as chromate, dichromate, phosphate, etc. Organic corrosion inhibitors are compounds containing one or more polar groups (with O, N, P, S atoms and π electrons), including sulfates, alcohols, ethers, amines, amides, ammonium salts, carboxylate salts, heterocyclic nitrogen compounds; Biobased corrosion inhibitors have the characteristics of natural extraction, non-toxic and harmless, and environmentally friendly. Gece summarized the types and mechanisms of action of drug-related corrosion inhibitors, which can provide assistance for the future application of bio based corrosion inhibitors in low surface treatment coatings.

3.3 Integrated anti rust coating

In actual coating conditions, the methods of “primer+topcoat” or “primer+intermediate coat+topcoat” are most commonly used. Multi layer coating can enable each layer to perform its corresponding function, increase the thickness of the coating, and achieve the combination of multiple functional effects. For low surface treatment coatings, the research focus is mainly on the primer, which is a combination of resin and rust removal and stabilization agents used to achieve the transformation, removal, and coating of rust; The use of intermediate paint is mainly to increase the thickness of the coating, thereby enhancing the corrosion resistance and weather resistance of the coating; On the one hand, topcoat can further improve the performance of the coating, and on the other hand, topcoat can provide functional options for the entire coating. However, multi-layer coating can affect the overall adhesion of the coating. Lei et al. compared the differences between the integrated gallic acid anti rust coating and the tannic acid multi-layer coating and phosphoric acid multi-layer coating obtained on the market. The experiment showed that the integrated coating can bond more tightly with the substrate, and the adhesion of the coating is significantly higher than the other two coatings. However, the multi-layer coating form of “primer+topcoat” or “primer+intermediate coat+topcoat” can cause gaps between layers, reduce the adhesion of the coating, and easily cause detachment due to long-term salt spray.