Moving Forward Every Day — 2009.12.28

The reference answer provided by this site is: Basic requirements for the performance of anti-corrosion coatings. 1. Adhesion: The adhesion of the coating, especially its wet adhesion, enables it to replace water and oxygen at the interface, thereby enhancing the protective properties of the coating. Moreover, the coating must remain firmly attached to the substrate throughout its use in order to ensure its corrosion-resistant properties. 2. Barrier property (reducing the penetration of water and oxygen): The low permeability of the paint film to water and oxygen reduces the substitution of water and oxygen at the polymer groups, thereby providing better protection. 3. Stability against corrosive media: The stability of a corrosion-resistant coating against corrosive media refers to its ability to neither be decomposed by such media nor undergo harmful reactions with them chemically ； It is also not dissolved or swelled by the medium physically.

According to the theory of electrochemical corrosion, the protective effect of the coating formed by anti-corrosion coatings on metals includes both physical and chemical mechanisms: (1) physical shielding against corrosive media; (2) the resistive effect of the coating layer; (3) the corrosion-inhibiting and passivating action of pigments; (4) cathodic protection. Based on the protection mechanism of coatings, anti-corrosion coatings should meet the following basic requirements: (1) The film-forming substance must have high chemical stability against corrosive media; (2) The coating should exhibit good adhesion to both the substrate and wet surfaces; (3) It should possess good physical and mechanical properties. These properties are closely related to the chemical structure of the film-forming substance, such as the molecular weight and molecular weight distribution of the polymer, the internal chain structures, side-chain groups, cohesion, intermolecular forces, and cross-linking density. The molecular structure of polymers has a significant impact on the physical and mechanical properties of coatings; (4) The coating should provide good shielding against the penetration of water, oxygen, ions, etc. Studies by Potvin and others have shown that there is a close relationship among the pores in the coating, water penetration, and the corrosion rate of the metal beneath the coating. Pores and structural voids present in the coating film are the main reasons for its water and air permeability. The water absorption of the coating film is caused by polar groups such as hydroxyl and amino groups in the molecular structure of the film-forming substances, low-molecular-weight water-soluble impurities, and certain hygroscopic pigments. Oxygen can pass through the coating simultaneously with water, and when water passes through the coating and accumulates beneath it, it increases the permeation and diffusion of oxygen and ions. This reduces the adhesion between the coating and the metal substrate. The depolarizing and oxidizing effects of oxygen in electrochemical corrosion lead to the destruction of the coating structure and an increase in the rate of electrochemical corrosion. The diffusion of ions within the coating increases its electrical conductivity, thereby raising the intensity of the corrosion current. Once metal corrosion occurs beneath the coating, the volume expansion of the corrosion products, along with the balance in osmotic pressure between the ionic solution under the coating and the external medium, will cause the coating to bubble, develop rust through, or even peel off, thereby losing its protective function for the substrate. Therefore, a corrosion-resistant coating’s ability to effectively shield against corrosive agents is a fundamental requirement for suppressing metal corrosion and maintaining durable anti-corrosion performance. Basic requirements for corrosion-resistant coatings in reinforced concrete: 1. The surface of the concrete should be free of contaminants, and cleaned using water or high-pressure air. 2. Cracks, honeycombing, pitting, and other defects visible on the concrete surface shall be repaired in accordance with the design requirements. Areas on the concrete surface such as tiny pores where paint cannot be applied should be filled with putty. 3. The concrete surface must be dry before applying the coating. 4. The paint used shall come with a product certificate, a certification document, physical and mechanical property specifications, as well as a product manual. 5. The preparation and application of the coating shall be carried out strictly in accordance with the requirements specified in the product instructions; no thinners shall be added to the coating during the application process. 1# tl1380843633

1 Basic requirements for water-based coatings as anti-corrosion coatings. According to the theory of electrochemical corrosion, the protective effect of the coating formed by anti-corrosion coatings on metals includes both physical and chemical mechanisms: (1) physical shielding against corrosive media; (2) the resistive effect of the coating film; (3) the corrosion-inhibiting and passivation effects of pigments; (4) cathodic protection. Based on the protection mechanism of coatings, anti-corrosion coatings should meet the following basic requirements: (1) The film-forming substance must have high chemical stability against corrosive media; (2) The coating should exhibit good adhesion to both the substrate and wet surfaces; (3) It should possess good physical and mechanical properties. These properties are closely related to the chemical structure of the film-forming substance, such as the molecular weight and molecular weight distribution of the polymer, the internal chain structures, side-chain groups, cohesion, intermolecular forces, and cross-linking density. The molecular structure of polymers has a significant impact on the physical and mechanical properties of coatings; (4) The coating should provide good shielding against the penetration of water, oxygen, ions, etc. Studies by Potvin and others have shown that there is a close relationship among the pores in the coating, water penetration, and the corrosion rate of the metal beneath the coating. Pores and structural voids present in the coating film are the main reasons for its water and air permeability. The water absorption of the coating film is caused by polar groups such as hydroxyl and amino groups in the molecular structure of the film-forming substances, low-molecular-weight water-soluble impurities, and certain hygroscopic pigments. Oxygen can pass through the coating simultaneously with water, and when water passes through the coating and accumulates beneath it, it increases the permeation and diffusion of oxygen and ions. This reduces the adhesion between the coating and the metal substrate. The depolarizing and oxidizing effects of oxygen in electrochemical corrosion lead to the destruction of the coating structure and an increase in the rate of electrochemical corrosion. The diffusion of ions within the coating increases its electrical conductivity, thereby raising the intensity of the corrosion current. Once metal corrosion occurs beneath the coating, the volume expansion of the corrosion products, along with the balance in osmotic pressure between the ionic solution under the coating and the external medium, will cause the coating to bubble, develop rust through, or even peel off, thereby losing its protective function for the substrate. Therefore, a corrosion-resistant coating’s ability to effectively shield against corrosive agents is a fundamental requirement for suppressing metal corrosion and maintaining durable anti-corrosion performance. 2 The relationship between the wet adhesion of a coating and its corrosion resistance. Wet adhesion refers to the ability of a coating to maintain its adhesive strength under wet conditions. Current research indicates that the failure of organic coatings usually occurs in three ways: 1) poor adhesion between the coating and the substrate, leading to peeling; 2) the coating being eroded by acids, bases, salts, and other substances, thereby losing its ability to protect the substrate; 3) defects in the coating such as pinholes, or poor impermeability of the coating, which allows ions to diffuse through it, resulting in a decrease in the coating’s resistance and thus electrochemical corrosion. Research shows that the wet adhesion of the coating is a key factor determining coating failure. Practice has shown that the corrosion-resistant effect of the coating only becomes effective in wet conditions (where electrochemical corrosion occurs). To achieve high wet adhesion, metal surfaces are often polished or phosphated, and resins with strong wet adhesion, such as those containing amino groups, are selected; or organic silicon couplers, aluminate couplers, zinc acetate couplers, and other types of couplers are added to the coating to improve its wet adhesion. The work of R.A. Prior Funke and others indicates that wet adhesion is related to the glass transition temperature (Tg) of the coating resin, the molecular structure of the coating resin, as well as the concentration of the polymer in the coating transition layer. Resin molecules contain polar groups that facilitate adhesion; however, only when these polar groups interact with the steel surface, that is, only when their bonding force to steel is greater than their bonding force to water, can they arrange themselves neatly on the steel surface, thereby enhancing wet adhesion. Therefore, the basic system for heavy-duty anti-corrosion primers for steel remains the epoxy·amine system. On the one hand, the resin contains an appropriate amount of polar groups such as hydroxyl groups, amino groups, and ether bonds; in particular, amino groups can form strong hydrogen bonds with the steel surface, while a small amount of hydroxyl groups prevents the formation of a water film. Ether bonds offer good resistance to hydrolysis, ensuring that water does not affect the wet adhesion of the coating. On the other hand, the aryl groups in the main chain confer rigidity, and this rigid structure of the resin molecules helps to improve wet adhesion. Funkeil pointed out that improving the corrosion resistance of steel coatings will still depend on the modification of resins, so as to enable the polar groups in the resin molecules to bond fully with the substrate, ensuring full coverage of the substrate surface and the formation of a complete thin layer, thereby enhancing the wet adhesion of the coating film. 1# tl1380843633

1. Adhesion: The adhesion of coatings, especially wet adhesion, enables them to replace water and oxygen at the interface, thereby enhancing the protective properties of the coating. Moreover, the coating must remain firmly attached to the substrate throughout its use in order to ensure its corrosion-resistant properties. 2. Barrier property (reducing the penetration of water and oxygen): The low permeability of the paint film to water and oxygen reduces the substitution of water and oxygen at the polymer groups, thereby providing better protection. 3. Stability against corrosive media: The stability of a corrosion-resistant coating against corrosive media refers to its ability to neither be decomposed by such media nor undergo harmful reactions with them chemically ； It is also not dissolved or swelled by the medium physically.

Metal corrosion is a chemical or electrochemical reaction between the metal surface and the media in its surrounding environment, which gradually progresses from the surface inward, causing the metal to be damaged by the environment and lose its original properties. There are various classifications of metal corrosion. Based on the corrosive medium, it can be classified into atmospheric corrosion, seawater corrosion, soil corrosion, and chemical medium corrosion ； Based on the mechanism of the corrosion process, it can be divided into chemical corrosion and electrochemical corrosion. For those working in paint corrosion prevention, corrosion is often classified into two types: \"wet corrosion\" and \"dry corrosion\". Wet etching is the action of various media on metals in the presence of water or water vapor ； Dry etching refers to the direct action of chemical substances on metals as well as high-temperature oxidation, etc. Atmospheric corrosion, corrosion by water and seawater, electrolyte corrosion, etc., all fall under \"wet corrosion\". Corrosion-resistant coatings can protect metal substrates from corrosion through shielding, corrosion inhibition, and cathodic protection. (1) Shielding effect: The shielding function of a coating is to isolate the substrate from the environment in order to prevent it from being corroded. For metals, according to the principles of electrochemical corrosion, corrosion of the metal beneath the coating requires the presence of water, oxygen, and ions, as well as a pathway for ion movement (conduction). Therefore, to prevent metal corrosion, the coating must be able to block water, oxygen, and ions from penetrating through the coating and reaching the metal surface; thus, the shielding effect depends on the coating’s impermeability. (2) Corrosion inhibition: When the coating contains chemical rust-inhibiting pigments, the presence of water causes corrosion-inhibiting ions to be released from these pigments; these ions then polarize one or both electrodes of the corrosion cell through various mechanisms, thereby suppressing corrosion. Therefore, the corrosion-inhibiting effect can compensate for the shortcomings of the shielding effect, while the shielding effect in turn prevents the loss of corrosion-inhibiting ions, ensuring a stable and long-lasting corrosion-inhibiting effect. (3) Cathodic protection effect: By adding metal powders that can corrode the base metal preferentially to the coating, the base metal can be protected from corrosion; this is precisely how zinc-rich primers protect steel. Based on the functions of the aforementioned anti-corrosion coatings, the shielding effect requires that the coating be impermeable to water, oxygen, and ions, while the corrosion-inhibiting effect demands the presence of a certain amount of water; some corrosion-inhibiting pigments also require the presence of oxygen. To fulfill both functions, balance must sometimes be considered, but impermeability remains a fundamental requirement for anti-corrosion coatings. Furthermore, in all cases, the coating is required to be durable, as durability is what confers practical value. To be durable, the coating must be stable against environmental agents, adhere firmly to the substrate, and have considerable adaptability to applied stresses. Therefore, impermeability, stability to media, adhesion, and mechanical strength are the basic requirements for anti-corrosion coatings.

The stability of a corrosion-resistant coating against corrosive media refers to its chemical resistance to being decomposed by such media, as well as its resistance to undergoing any harmful reactions with them； It is not physically dissolved or swelled by the medium. Most anti-corrosion coatings are only used in neutral to slightly alkaline aqueous media and organic solvents with low polarity. However, with enhanced and thicker coating systems, it is possible to achieve long-term resistance to highly corrosive media. The effect of inorganic acids on coatings is mainly to hydrolyze certain polar groups in the polymers within the coating, to cause addition reactions and isomerization at double bonds, and to dissolve and decompose the pigments and additives present in the coating, ultimately causing the coating to lose its protective function. Organic acids accelerate the progression of harmful reactions due to the swelling and dissolving properties of polymers, thereby intensifying the erosive effect. The action of alkaline solutions is mainly hydrolysis; they form salts with the acidic groups in the polymers, making the coating more hydrophilic and even causing it to swell and soften. Water is the most common corrosive agent. In addition to its ability to cause hydrolysis and penetration damage to polymers, it also exerts a synergistic destructive effect in combination with other substances present. The damaging effect of salt solutions lies in the increase in ion concentration; the penetration of ions leads to a decrease in the resistance of the coating. Some ions, such as chloride and sulfate ions, can also interfere with the function of corrosion-inhibiting pigments beneath the film, thereby promoting corrosion of the metal underneath the coating. In terms of resistance to medium corrosion, carbon-chain polymers are superior to hybrid-chain ones. It is better for the hydrogen atoms on the carbon chain to be replaced by fluorine and chlorine atoms. Those with high saturation and low polarity are better than those containing more double bonds and polar groups. To protect the substrate, the coating must remain firmly attached to it throughout its use. Apart from the reactive primer, the adhesion of the coating relies mainly on physical attraction between molecules, known as secondary forces or van der Waals forces. Among them, hydrogen bonding is the strongest form of attraction, but such forces only occur at molecular distances; therefore, the primer should have good wetting properties to enable full contact with the substrate. The main factors that affect adhesion during use are as follows. ① Water accumulation at the coating-metal interface: Since water has a greater affinity for metals than ordinary polymers do, it can interpose itself between them, replacing the polymer adsorption. Interfacial water may originate from the water film originally adsorbed on the metal surface during construction, affecting the initial strength of the coating; it can also enter through the coating surface or via cracks during use, gradually reducing the adhesion. Therefore, under high-temperature conditions, the adhesion decreases more rapidly. ②The accumulation of internal stress occurs due to volume contraction of the coating as a result of solvent evaporation during the later stages of curing, further cross-linking during use, and the leaching out of small molecules. In repeated cycles of heat and cold, as well as dry and wet conditions, the different rates of expansion and contraction between the coating and the substrate lead to repeated relative displacement at the interface, which in turn generates destructive stress. When the internal stress builds up to a level greater than the adhesion force, the coating peels off. If it is less than the adhesion force but greater than the cohesion force, the coating will crack. It has been measured that the internal stress resulting from volume contraction can reach as high as 9.8×103 kPa, indicating the significant impact of this phenomenon. The formation of internal stress is also closely related to the structure of polymers; soft coatings with low modulus can eliminate internal stress through changes in molecular conformation, whereas highly cross-linked rigid coatings cannot. Flaky or fibrous pigment fillers can cause micro-cracking between the polymers, thereby locally releasing stress and reducing the internal stress of the coating. The conventional mechanical properties of coatings include hardness, flexibility, impact resistance, and wear resistance. Considering the coating as a viscoelastic system, its mechanical properties can comprehensively reflect the extent of deformation caused by external forces; since these mechanical properties are related to temperature, when discussing how to enable the coating to withstand external forces over the long term, stress-strain behavior and the glass transition temperature should be considered first. The mechanical properties of polymers are determined by the glass transition temperature (Tg), which in turn is influenced by the structure of the polymer molecules. Rigid molecular chains and strong secondary bond forces can increase the Tg, while the addition of fillers has little effect on Tg. The mechanical properties of a coating depend on the relationship between the mechanical stresses it is subjected to and the strain distribution within the polymer structure. The stress-strain properties of the coating are related to the type of polymer, as well as the type and concentration of pigments and fillers. In the range where the pigment filler concentration is below the critical pigment volume concentration, as the concentration increases, the tensile strength of the coating film rises while its elongation decreases. The elongation rate of the tensile strength of the coating film provides a good indication of its performance in use. The low elongation rate and high tensile strength indicate that it is hard and tough, suggesting good wear resistance ； The high elongation rate and low tensile strength indicate that it is a soft, elastic membrane ； A high value for both indicates a tough elastic membrane. Therefore, when developing or selecting anti-corrosion coatings, the stress-strain properties should be used as evaluation criteria.

Basic requirements for applying anti-corrosion coatings: ① The substrate must undergo strict and thorough surface treatment. Steel and equipment generally require treatments such as rust removal, oil removal, pickling, and phosphating. The first two treatments are necessary for any painting, while the latter two depend on the specific circumstances. ②Required coating thickness. The thickness of the anti-corrosion coating must exceed its critical thickness to provide protection; generally, 150–200 μm is an appropriate value. ③Control environmental factors such as temperature and humidity at the painting site. ④Control the coating interval time. If the primer (such as epoxy polyamide paint) is left for too long before the topcoat is applied, the topcoat will have difficulty adhering, affecting the adhesion between the layers. Furthermore, the recoatability between coatings should be considered. The durability and corrosion resistance of the coating layer are generally proportional to the thickness of the layer. The required thickness of the coating layer varies by region as follows: ① In rural areas, it is 75μm (2–3 coats applied) ； ②General industrial areas: 125μm (3–4 coats of paint) ； ③Quite harsh corrosive environment, over 250μm (5–6 coats of paint) ； ④Areas exposed to seawater immersion or splashing: 500 μm or more (apply 6–7 coats of paint).

Corrosion-resistant, sun-resistant, does not crack over time, high coverage, good durability, easy to apply

Requirements for the performance of anti-corrosion coatings: Anti-corrosion coatings can fulfill their role in protecting the base metal only when they meet the requirements regarding basic properties such as impermeability, stability against various media, adhesion (including adhesion in wet conditions), and mechanical strength.

According to the theory of electrochemical corrosion, the protective effect of the coating formed by anti-corrosion coatings on metals includes both physical and chemical mechanisms: (1) physical shielding against corrosive media; (2) the resistive effect of the coating layer; (3) the corrosion-inhibiting and passivating action of pigments; (4) cathodic protection. Based on the protection mechanism of coatings, anti-corrosion coatings should meet the following basic requirements: (1) The film-forming substance must have high chemical stability against corrosive media; (2) The coating should exhibit good adhesion to both the substrate and wet surfaces; (3) It should possess good physical and mechanical properties. These properties are closely related to the chemical structure of the film-forming substance, such as the molecular weight and molecular weight distribution of the polymer, the internal chain structures, side-chain groups, cohesion, intermolecular forces, and cross-linking density. The molecular structure of polymers has a significant impact on the physical and mechanical properties of coatings; (4) The coating should provide good shielding against the penetration of water, oxygen, ions, etc. Studies by Potvin and others have shown that there is a close relationship among the pores in the coating, water penetration, and the corrosion rate of the metal beneath the coating. Pores and structural voids present in the coating film are the main reasons for its water and air permeability. The water absorption of the coating film is caused by polar groups such as hydroxyl and amino groups in the molecular structure of the film-forming substances, low-molecular-weight water-soluble impurities, and certain hygroscopic pigments. Oxygen can pass through the coating simultaneously with water, and when water passes through the coating and accumulates beneath it, it increases the permeation and diffusion of oxygen and ions. This reduces the adhesion between the coating and the metal substrate. The depolarizing and oxidizing effects of oxygen in electrochemical corrosion lead to the destruction of the coating structure and an increase in the rate of electrochemical corrosion. The diffusion of ions within the coating increases its electrical conductivity, thereby raising the intensity of the corrosion current. Once metal corrosion occurs beneath the coating, the volume expansion of the corrosion products, along with the balance in osmotic pressure between the ionic solution under the coating and the external medium, will cause the coating to bubble, develop rust through, or even peel off, thereby losing its protective function for the substrate. Therefore, a corrosion-resistant coating’s ability to effectively shield against corrosive agents is a fundamental requirement for suppressing metal corrosion and maintaining durable anti-corrosion performance.