Methods for preventing corrosion of steel parts

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There are many processes to prevent steel corrosion. The most common anti-corrosion method is to apply non-metallic and metallic protective coatings on the metal surface, or to form a chemical conversion coating on its surface through special treatment processes. Based on the substance that constitutes the protective layer, it can be divided into the following 3 categories. (1) Non-metallic protective layer: Applying organic and inorganic compounds to the metal surface. (2) Metal protective layer: A metal or alloy is applied (plated) on the surface of steel as a protective layer to slow down the rate of corrosion. (3) Hot-dip coating: The metal material or product to be protected is immersed in molten metal, resulting in the formation of a protective metal coating on its surface. Metals that can exist in a liquid state are generally those with relatively low melting points, as well as corrosion- and heat-resistant properties, such as Al and Zn. Tractors contain various structural components and cast parts, and their corrosion resistance is directly related to the quality of the product. Traditional anti-corrosion methods for tractor and construction machinery products involve painting or electroplating. However, these methods provide poor protection against corrosion for parts that are in relative motion during use, parts that are under pressure, and parts that are prone to impact. Finding ways to protect such parts from corrosion is a problem faced by many companies. In relevant literature, there are many reports comparing salt spray resistance using standard panels with different coating processes. However, there are fewer reports on conducting salt spray resistance tests using actual parts treated with various anti-corrosion processes. This study conducts salt spray tests on actual steel parts treated using several common anti-corrosion processes, and analyzes the results; it provides references for improving the anti-corrosion performance of products. 1. Selection of corrosion protection treatment processes: In accordance with the corrosion protection requirements of typical mechanical manufacturing enterprises, 10 common and typical corrosion protection treatment processes were selected for salt spray corrosion resistance tests. Based on the performance criteria for salt spray corrosion resistance in common anti-corrosion processes, the test duration was set at a maximum of 600 hours; the cold-rolled sheet used in the test was of the same material as that of the cold-rolled parts. Process A: Sandblasting – dipping in iron red epoxy primer. The cast iron gasket is subjected to shot blasting, resulting in a surface roughness of around Ra50. It is then coated with an iron oxide epoxy primer with a viscosity of 18 s, produced by a company in Jiangsu; the coating process involves sandblasting – medium-temperature phosphating – and application of the iron oxide epoxy primer. The cast iron gasket is subjected to shot blasting, resulting in a surface roughness of around Ra50; it is then phosphated using a zinc-based process at 50°C, followed by coating with an iron oxide epoxy primer. Process C: Sandblasting – Phosphating – Standard cathodic electrophoretic coating. The cast iron gasket is subjected to shot blasting, resulting in a surface roughness of around Ra50. It is then phosphated using a zinc-based process at a temperature of 50°C, and coated with a conventional cathodic electrophoretic coating with a thickness of 20 um. Process D: Sandblasting – Phosphating – High-edge, highly corrosion-resistant cathodic electrophoretic coating. The cast iron gasket is subjected to shot blasting, resulting in a surface roughness of around Ra50. It is then phosphated using a zinc-based process at a temperature of 50°C, and covered with a high-edge, highly corrosion-resistant cathodic electrophoretic coating with a thickness of 20 um. E process: Sandblasting – Hot-dip galvanizing. The cast iron gasket is subjected to shot blasting, resulting in a surface roughness of around Ra50; a zinc coating with a thickness of 110 um is applied to the surface. Process F: Electro-galvanizing. A zinc layer with a surface plating thickness of 10um. G process: Castings are sprayed with acrylic topcoat. Acrylic enamel with a surface coating thickness of 20um is applied. H process: \"Hot-dip galvanizing + topcoat spraying\" treatment and \"electrophoretic primer + topcoat spraying\" process. The part processing technique involves first coating the surface with a zinc layer 110 um thick, and then spraying an acrylic topcoat on the surface ； The part processing procedure involves first applying a cathodic electrophoretic coating with a thickness of 18 um on the surface, and then spraying an acrylic topcoat on it. Process I: Cold-rolled sheet – sprayed with iron red epoxy primer. J process: Cold-rolled sheet — Phosphating — Spraying iron red epoxy primer. The surface is first treated with a zinc-based phosphating process at a temperature of 50°C, and then coated with an iron oxide epoxy primer. 2. Comparative analysis of experimental results shows that the coating process involving only the use of iron red epoxy primer on the castings, or simply applying topcoat alone, results in very poor corrosion and rust resistance. When the same coating process is used for cast iron parts and cold-rolled sheets, there is a significant difference in their resistance to salt spray corrosion; the cold-rolled sheets exhibit much better resistance to salt spray corrosion than cast iron parts. Using the same rust-inhibiting primer, if phosphating is not applied, cast iron parts develop extensive rusting after just 24 hours of salt spray testing, while cold-rolled sheets only show localized bubbling corrosion after 168 hours; if phosphating is applied to both materials, cast iron parts exhibit localized rusting after 168 hours of salt spray testing, whereas cold-rolled sheets show no signs of rusting even after 240 hours. The phosphating process has a significant impact on enhancing the corrosion and rust resistance of cast iron parts; it can greatly improve the parts’ resistance to salt spray corrosion. Using a similar coating process, cast iron parts that were not phosphated showed extensive rusting after just 24 hours of exposure to salt spray; whereas the same parts, after being phosphated, also experienced severe rusting after 168 hours of salt spray exposure, but the overall condition of their surface was much better than that of the cast iron parts that were not phosphated. As long as the coating is not damaged by impacts, the use of high-corrosion-resistant, edge-protection cathodic electrophoretic paint can meet the long-term rust prevention requirements for cast iron parts. In cases where there is relative movement or impact between parts during use, the \"hot-dip galvanizing process + surface painting process\" can effectively address the problem of rust on the damaged coating areas of these parts. The hot-dip galvanizing process enhances the corrosion resistance of castings; its corrosion resistance is superior to that of conventional cathodic electrophoretic processes. After being treated with this process, cast iron parts do not develop red rust even after 600 hours of exposure to salt spray. The salt spray resistance of electro-galvanized parts is much lower than that of parts treated using the high-edge cathodic electrophoretic coating process. Different cathodic electrophoretic coatings have a significant impact on the corrosion and rust resistance of cast iron parts. Generally, cast parts coated with a cathodic electrophoretic primer exhibit localized rusting after 96 hours of salt spray testing, and extensive rusting after 240 hours. However, when a cathodic electrophoretic coating with enhanced corrosion resistance is used, only minor pitting appears on the surface of the samples after 600 hours of salt spray testing” ； The main reason is that the rust resistance at the edges of ordinary electrophoretic paint films is poor, and the surfaces of typical cast iron parts are relatively rough, with countless \"edges\" on their surface. Therefore, for such castings – especially those with high surface roughness – it is necessary to use electrophoretic coatings with enhanced edge protection properties for coating. The main factors affecting the salt spray corrosion resistance of cast iron parts through phosphating processes and high-corrosion-resistant, high-edge-closure cathodic electrophoretic coatings are the composition of the cast iron parts and their surface roughness. 2.1 Influence of composition: The contents of non-ferrous elements such as carbon (C), sulfur (S), and phosphorus (P) in cast iron are all higher than those in carbon steel, by about one order of magnitude. The presence of these non-iron elements causes iron molecules to easily form countless galvanic cells with them, leading to electrochemical corrosion. Under the same temperature conditions, the rate of electrochemical corrosion is much higher than that of chemical corrosion. 2.2 Influence of surface roughness: The surface roughness of castings is generally above Ra12.5; in some cast iron parts, it can reach Ra25 or even Ra50. The surface roughness of carbon structural steel is generally below Ra3.2, while that of hot-rolled steel is usually between Ra6.3 and 12.5. High roughness, on the one hand, makes it easier for condensed water and CO2 to adhere, thereby promoting corrosion ； On the other hand, surface roughness also affects the protective coverage of the surface coating. If only a layer of iron red epoxy primer is applied to the surface of the casting, due to the leveling properties of the coating itself, it is difficult to form an effective protective coating film on the protruding parts of the rough surface of the casting. After cast iron parts are treated with a phosphating process, the phosphating film formed on their surface effectively adsorbs protective substances such as coatings, thereby creating a protective layer composed of both the phosphating film and the electrophoretic paint, even on the protruding areas of the rough surface. Applying high-edge electrophoretic paint to the surface of cast iron parts that have been treated with phosphating results in the formation of a repeated protective layer composed of the phosphating film and the electrophoretic coating, thereby achieving an excellent level of protection. 3. Conclusion: Through a comparative study of the salt spray resistance test results for several steel anti-corrosion processes, it is evident that different anti-corrosion methods have a significant impact on the corrosion resistance of steel components. In practical applications, it is necessary to select the appropriate anti-corrosion method based on the operating environment and requirements of the product. When the requirements for corrosion resistance are the same, cast iron parts require corrosion protection treatments that offer greater resistance to corrosion than those used for cold-rolled steel materials. For parts treated using the same anti-corrosion process, those made of cast iron have a much lower anti-corrosion performance compared to those made of cold-rolled steel. For products or parts that are prone to being scratched, worn, or subjected to relative movement during assembly and use, conventional coating or electroplating methods are insufficient to meet anti-corrosion requirements. It is recommended to use hot-dip galvanizing or hot zinc infiltration as anti-corrosion techniques, as these can effectively prevent the coating on the parts from being damaged and corroded. For castings (especially those with a surface roughness Ra > 25), conventional coating processes make it difficult to coat the sharp edges of the rough surface, resulting in poor corrosion resistance; however, the use of phosphating and high-edge electrophoretic coating processes can **improve their resistance to salt spray corrosion.