Applications of polyaniline anti-corrosion coatings

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Applications of Polyaniline Anti-corrosion Coatings by Zhang Zuo-hua, Director of Ranghuiyun Anti-corrosion Technology and Moderator of the 51 Anti-corrosion Forum. Polyaniline (PANI) was initially used as a polymer conductive material in fields such as chemical power sources, sensors, anti-static applications, and stealth materials. Since DeBerry reported in 1985 that polyaniline is a new type of metal passivator, countries around the world have carried out research and development in the field of corrosion protection using conductive polymers, and have successively introduced antistatic anti-corrosion coatings. The research team led by Professor Wang Jixiao from Tianjin University has conducted in-depth studies on the preparation, characterization, and applications of conductive polymers. They have published numerous research papers and filed multiple patents; the production of polyaniline nanospheres, nanowires, and nanosheets has reached a scaleable and commercial level, with technology that is among the best in the world. This has helped to overcome China’s lagging position in the development of polyaniline-based nanomaterials for use in anti-corrosion coatings. Traditional metal protection methods include the use of corrosion inhibitors, electroplating with inert materials, passivating metals to form a dense passive film, cathodic protection, and coating. From a practical perspective, corrosion inhibitors are easy to use but have low efficiency ； Electroplating with inert precious metals yields good results but is costly ； Cathodic protection is suitable for certain specific environments ； Coating protection is a convenient, simple method that is widely used. However, the protective effectiveness of a coating depends on its barrier properties; to improve these properties, flaky materials and/or oxidizing metal salts are often added to the coating material. The addition of flaky materials and/or oxidizing metal salts cannot prevent defects in the coating, such as porosity in the coating and low density; these factors make it difficult to prevent corrosive agents like air, water, and salt ions from penetrating the coating, thereby leading to metal corrosion ； The inorganic flaky materials in the coating (such as glass flakes, mica, basalt flakes, etc.) have poor affinity for the resin, which facilitates phase separation and the formation of interfacial microcracks. This accelerates the penetration of corrosive agents, leading to metal corrosion. Even if inorganic scales are surface-treated with coupling agents, there is still a difference in their coefficient of thermal expansion compared to resin materials; this leads to phase separation, the formation of pores, and the creation of pathways for corrosive agents, thereby causing metal corrosion. The addition of oxidizing metal salts can also pose a risk to human health due to heavy metal contamination. The polyaniline anti-corrosion coating provides corrosion protection through the following mechanism: First, it utilizes the redox properties of polyaniline to form a dense oxide film layer on the surface of the metal substrate (a passivation film), and the formation of this passivation film **slows down the corrosion of the metal** ； Secondly, also based on the redox properties of polyaniline, as the oxidized metal forms a dense passivation film, polyaniline is reduced; the reduced polyaniline acts as an insulator, preventing the transfer of corrosion-related charges ； Third, based on the reversible doping/dedoping property of polyaniline, when appropriate ions are used for doping, these ions can promote the formation of a passivation film or can themselves form an adsorption layer on the metal surface to provide corrosion protection ； Fourth, the electrical conductivity of polyaniline creates an electric field on the metal surface; the direction of this field is opposite to that of electron transfer, thereby hindering the transfer of electrons from the metal to the oxidizing substance, thus forming a barrier to electron transfer. When the structure and state of polyaniline are properly controlled, the addition of a small amount of polyaniline can significantly enhance the protective capacity of the anti-corrosion coating against metals, as well as the mechanical properties of the coating. Therefore, polyaniline anti-corrosion coatings exhibit excellent overall performance; for example, the series of polyaniline anti-corrosion coatings developed by Professor Wang Jixiao’s team at Tianjin University can withstand salt spray for over 12,000 hours, as well as rapid temperature fluctuations (alternating between -80°C for 30 minutes and 230°C for 30 minutes) for more than 24 times. They are suitable for use in various highly corrosive environments involving high concentrations of acids, alkalis, and salts. Polyaniline anti-corrosion coatings exhibit excellent edge protection properties and resistance to scratches; the coating can withstand scratches of 1–2 mm, thereby greatly slowing down the spread of rust. Engineering applications in highly corrosive environments such as the secondary cooling chamber of rolling mills (where the corrosion rate is ≥12 mm/Y, which is more than 60 times the highest corrosion rate for C5 and more than 17 times that for CX) and low-temperature heat exchangers (with a corrosion rate of ≥12 mm/Y, again more than 60 times the highest corrosion rate for C5 and more than 17 times that for CX) demonstrate that coatings containing polyaniline possess excellent protective properties in environments involving acids, bases, salts, etc. Figure 1: Corrosion status of 10mm H-shaped steel in the secondary cooling chamber during one year, the site where anti-corrosion work was carried out, and the condition of the steel structure 14 months after the construction. Figure 2: Corrosion status of the steel structures at Sinopec Puguang Natural Gas Purification Plant (in a high-hydrogen sulfide and high-humidity environment) within less than 4 months of trial operation, the site where anti-corrosion work was carried out, and the condition of the steel structures 9 years after the construction. Figure 3: Comparison of the site where anti-corrosion work was carried out on Xianggang’s gas pipelines and their condition 1 month after the construction. By controlling the structure of the coating, the polyaniline anti-corrosion coating has a wide temperature range of operation; it exhibits excellent anti-corrosion properties in environments ranging from -65°C to 180°C. It also shows strong resistance to corrosion in acidic, alkaline, saline environments, as well as in environments with alternating acid and alkaline conditions, thus providing long-term anti-corrosion protection. Used in combination with various topcoats, it has broad application prospects as a long-lasting anti-corrosion material in various fields such as cross-sea bridges, port docks, offshore facilities, marine platforms, storage tanks and pipelines, waste treatment equipment, gate fences, mining facilities, coal mining equipment, high-voltage transmission towers, pipe pile jetties, and steel structure pipe racks. In particular, in harsh environments like bay bridges, marine platforms, wind power generation, and photovoltaic power generation systems, it is considered a next-generation product in the field of anti-corrosion. Applications of polyaniline anti-corrosion coatings abroad include: railway bridges in Germany, overpasses and baseball stadiums in Japan, steel structures for wastewater treatment in Poland, crude oil pipelines in Germany, Airbus propulsion systems in France, container ships in Denmark, as well as filtration equipment and mining equipment in German chemical plants ； In China, the polyaniline anti-corrosion coating developed by Professor Wang Jixiao’s team from Tianjin University and produced by Shanghai Pingnai Company is used in applications such as the coating of the steel structures in the secondary cooling chamber of Casting Workshop 1 at Laigang Plant II (operating conditions: temperature 85°C, relative humidity 100%, saturated salt content, high light intensity, vibration; corrosion rate ≥ 12 mm/year); the anti-corrosion treatment of low-temperature heat exchangers at Jiangsu Supor Group (operating conditions: temperature -5°C, 20% saline solution; corrosion rate ≥ 12 mm/year); and the anti-corrosion protection of the underground corridors in the sulfur storage and handling areas of Sinopec Dazhou Natural Gas Purification Plant (operating conditions: temperature 20°C–40°C, hydrogen sulfide concentration 6–18 PPm, relative humidity 45%–100%). The application method of polyaniline anti-corrosion coating is not very different from that of traditional anti-corrosion coatings; the following points should be paid attention to: 1. Polyaniline anti-corrosion coating is a two-component material, which must be thoroughly mixed in the correct proportions, along with appropriate pretreatment. Under normal conditions, the cured paint must be used within 2 hours; otherwise, it will gel and become unusable. 2. Polyaniline anti-corrosion coatings should be applied within 4 hours after sandblasting at normal temperatures; when the relative humidity is ≥80%, application should take place immediately to prevent flash rusting. The subsequent coating should be applied after the previous coating has dried on the surface; the drying time varies from 3 to 12 hours, while complete curing takes 10 days. The maximum interval between applications should not exceed 3 days. 3. The polyaniline anti-corrosion coating achieves its optimal stability within two weeks; the surface cannot be coated if the coating is not yet dry or if it is wet. The process steps for applying polyaniline anti-corrosion coatings in actual construction are similar to those of traditional coating methods and are not complicated. The specific procedure is as follows: 1. Surface treatment. Remove dust, dirt, oil stains, and loose materials from the surface of components or facilities; the surface should be dry and clean. It is recommended to use sandblasting or shot blasting for rust removal. If sandblasting is not possible due to limited conditions, mechanical grinding can be used instead. The rust removal quality requirements shall meet those specified in the latest version of GB8923 standards: sandblasting to Sa2.5 level and grinding to St3 level. 2. Coating of ingredients. After mixing the two components of the polyaniline anti-corrosion coating thoroughly, stir them mechanically for 5–10 minutes according to the specified ratio, followed by 10 minutes of ultrasonic treatment. The cured coating can be applied using methods such as brushing, rolling, dipping, or spraying. 3. Natural curing. After applying the polyaniline anti-corrosion coating, it should be left to cure naturally for a certain period of time. It is recommended to immerse it at room temperature or in an environment with a high temperature (above 100°C) for 7 days before putting it into use; if the ambient temperature is below 10°C, the curing time should be extended to 10 days before use. Under normal ambient conditions, it can be used once the surface is dry. 4. Matching topcoat. Under normal conditions, polyaniline can achieve long-term corrosion resistance without the need for a topcoat, as long as it is fully immersed or not exposed to strong ultraviolet light. When the polyaniline coating is used in environments with strong ultraviolet exposure, or when decorative or warning functions are required, different topcoats must be used, and the application of these topcoats must take place after the polyaniline coating has dried. Table of excellent physical and chemical properties of polyaniline. Polyaniline anti-corrosion coatings are regarded as the next generation of anti-corrosion coatings in the field of corrosion protection. Its excellent comprehensive performance and long-lasting resistance to salt spray make it revolutionary for use in the anti-corrosion of offshore platforms and marine facilities, as well as for the protection of other steel components; it will bring significant opportunities for development in materials used for marine anti-corrosion and steel component protection. The author also suggests that further improvements and enhancements be made to polyaniline anti-corrosion coatings in terms of their application during construction; aspects such as superconducting mixing, humidity control, tolerance for surface treatment, and extended operation time require continuous innovation by researchers, so that this high-performance anti-corrosion coating can truly become the \"newest generation of anti-corrosion products\". The author believes that the introduction of polyaniline anti-corrosion coatings will change traditional concepts of corrosion protection. Their long-lasting corrosion resistance and extended salt spray resistance will enable them to play an unexpected role in specialized anti-corrosion applications and in various other fields.