Mechanism of flash rusting on coatings and solutions
Thread Content
Flash rusting: The formation of flash rusting is an electrochemical process. Since water-based coatings are conductive in nature, they create channels for charge migration between areas with different potential differences; as a result, corrosion occurs within a short time during the film-forming process of the water-based coating, leading to flash rust. It is clear that the longer the wet paint film remains in place, the more severe the flash rusting becomes, which explains why flash rusting is more pronounced in conditions of low temperature and high humidity.
Before the coating dries completely, other anti-rust measures such as zinc powder, barrier effects, and inertness do not yet exert their protective functions. When water-based coatings are used on metal surfaces, corrosion can occur during the drying process due to the presence of water and oxygen; this is known as flash rusting. Especially on high-carbon steel or cast iron parts.
The effect of the pH value of water-based coatings on different metal substrates: It is not the case that a higher pH value means greater stability and less corrosion of the metal substrate. Water-based coatings using different resin systems exhibit varying pH values of their paint films on different metal substrates, such as carbon steel. In water-based alkyd and epoxy ester coatings, an h value of around 4, 5–5 provides relatively stable protection against flash rust corrosion. This indicates that certain metal carbon steel substrates are sensitive to alkalis. Therefore, acidic systems are more suitable for the coating protection of metal carbon steel substrates. Galvanized steel sheets and cold-rolled steel sheets are both suitable metal substrates for coating with alkaline paints for protection. These materials help to maintain an electrical charge balance with 2,2-N-dimethylethanolamine in aqueous systems, thereby ensuring that the electrical charge potential of the metal substrates is roughly equal to that of the aqueous acrylic amino topcoat. Achieve a balance in the potential space, so that the resin in the aqueous system can be absorbed promptly and effectively onto the metal substrate to form a hydrocarbon film. Thereby achieving corrosion-resistant protection.
On different metal substrates, the speed and severity of flash rusting during the application of water-based coatings vary, which leads to differences in the choice and amount of flash rust inhibitors used. Materials such as cast iron are relatively difficult to work with; at humidity levels above 85%, sodium nitrite is almost ineffective. Similarly, aluminum sheets also tend to develop flash rust when used as a substrate, but this is a material that is relatively easy to deal with. As for most metal substrates such as steel, dealing with flash rust is not a difficult issue; however, it is necessary to strike a balance among flash rust inhibition, corrosion prevention inside the tank, and long-term corrosion resistance. This can be achieved by using alkynol groups in combination with sodium benzoate, or by combining sodium nitrite with other flash rust inhibition additives. Due to the complexity of the structure of water-based resins, anti-rust inhibitors available on the market are also evolving; traditional oil-based anti-rust inhibitors such as polyphosphates and nitrites cannot be effectively used in current water-based coating technologies. Water-based coatings contain a large amount of water in their structure, and water is the biggest enemy of metal substrates when it comes to corrosion; it is the H bonds in water that react with the ions in the metal substrate, leading to rapid corrosion. To address the issue of water-based coatings inhibiting corrosion on metal substrates, we generally add reactive and functional solvents or certain special functional additives to the formulation in order to prevent such corrosion; for example, coating chemists often incorporate amine-based active diluents containing hydroxyl groups. Such as N,N-dimethylethanolamine (DMEA) as a neutralizing agent to neutralize polyurethanes, water-based acrylic amino topcoats, or water-based epoxy ester anti-rust paints. The addition of a small amount of metal-based rust-inhibiting agents virtually eliminates the occurrence of flash rust, whereas emulsion-type acrylic coatings neutralized with amines such as ammonia, ethanolamine, and cyclohexylamine are prone to flash rust even when the pH value is adjusted to around 9.5. By using DMEA or AMP-95 to adjust the pH value to around 11, flash rust virtually no longer occurs in emulsion-type acrylic anti-rust paints, even under 90% humidity conditions.
Based on their film-forming mechanism, corrosion inhibitors can be classified into oxide film-type corrosion inhibitors, precipitation film-type corrosion inhibitors, and adsorption film-type corrosion inhibitors.
Oxide film-type corrosion inhibitors are substances that prevent further corrosion of metals by reacting with the cations on the metal surface to form a dense and strongly adhering oxide film there. Common oxide film-type corrosion inhibitors include chromates, dichromates, and sodium nitrite. Since these types of inhibitors have a passivating effect on metals, they are also known as passivators. Its disadvantages are that it is not environmentally friendly or its cost is too high; furthermore, due to its reducing nature, its storage performance over time is poor, and it may exhibit thixotropy or gelation in certain oxidized and dried systems. Precipitate film-type corrosion inhibitors work by the interaction of inhibitor molecules with ions in the corrosive medium; this occurs on the metal surface, where a protective precipitate film is formed. This film effectively separates the metal from the corrosive agents, thereby providing corrosion protection. Such products generally offer good rust prevention effects. However, due to their rapid release rate and the presence of a large number of metal ion groups, it is necessary to verify the compatibility of the system and its long-term stability. Moreover, since these products are in salt form, using large amounts of them can affect their resistance; in addition, under harsh weather conditions, rusting may occur (either because the surface cannot dry properly or because the paint film allows water to penetrate) ； Adsorption film-type corrosion inhibitors achieve their corrosion-inhibiting effect through the principles of molecular forces. The polar groups in these inhibitors, which are primarily composed of elements with high electronegativity, adsorb to the charges on the metal surface, thereby forming an adsorption film between the metal and the corrosion-inhibiting medium. The advantage of such products is that they do not form salts, have little impact on tolerance, and can be modified to produce products with good stability and compatibility, offering excellent adaptability in multiple systems, such as ANTICO ARCH100S.
It is still very difficult for water-based anti-corrosion coatings to achieve the performance of oil-based coatings.
However, it is now being used more and more in the construction industry, while the usage conditions in the chemical industry may be even more demanding
In Hainan Province, the anti-corrosion standards seem to prohibit the use of water-based coatings