Discuss the characteristics of various chemical conversion methods for steel surfaces and their applications in corrosion protection

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Let’s discuss the characteristics and anti-corrosion applications of various chemical conversion methods for steel surfaces. After oil removal and rust removal, it is sometimes not possible to apply paint to the steel surface immediately; to prevent re-rusting and to enhance the adhesion of the paint coating, certain chemical treatments are used to form a protective film on the steel surface. The common methods include oxidation, phosphating, and passivation, and their characteristics and applications are as follows； 1. Oxidation: Through oxidation treatment, a protective oxide film can be formed on the surface of steel. The inner layer of this oxide film is FeO, while the outer layer is Fe₂O₃; it also exhibits different colors. Therefore, this process is also known as “bluing” treatment. There are three methods: thermal oxidation, alkaline oxidation, and acidic oxidation, among which the alkaline oxidation method is the most commonly used. Depending on the oxidizing agent used, the color of the oxide film on the steel surface varies. If the oxidant is an alkali metal nitrate, potassium chromate, potassium nitrite, oxalate, etc., the colors of their oxide films are respectively dull dark black, black with a slight reddish tint, light blueish-black (glossy), and beautiful blue. 2. Phosphating: This process involves immersing metal parts in an aqueous solution prepared from phosphoric acid or phosphates (such as metaphosphates, zinc dihydrogen phosphate, or zinc phosphate), along with zinc nitrate, at room temperature or under heated conditions. The interaction between the metal and these acids and salts results in the formation of a water-insoluble gray film on the surface; this treatment method is known as phosphating. The general ratio of phosphate to zinc nitrate is 1:3–4 at room temperature, and around 1:2 when heated to (65–70°C). It can be appropriately selected according to the requirements of the workpiece. There are many methods for phosphating, which can be divided into immersion type, spraying type, and electrochemical phosphating, etc. Each type of phosphating is further divided into three categories: high-temperature phosphating, medium-temperature phosphating, and low-temperature phosphating. Phosphating treatment includes more than a dozen processes such as pre-treatment for phosphating, the phosphating itself, and post-treatment after phosphating. During the phosphating process, in addition to testing the various components of the bath solutions and the phosphating process itself, it is also necessary to examine the corrosion resistance of the phosphated parts and the phosphating coating. This testing should be carried out in accordance with the specifications in Standard GB6807‑2001; if the results are not satisfactory, the parts can be cleaned with acid (sulfuric acid) or alkali (caustic soda) before undergoing phosphating again. The phosphating film is generally 1 to 10 micrometers thick. During the formation of this film, only minimal dissolution occurs in the steel substrate, so the dimensions of the parts after phosphating remain unchanged. The zinc phosphate phosphating film consists of needle-like crystals, which enhance the adhesion between the coating and the metal. It also possesses excellent corrosion resistance, heat resistance, and electrical insulation properties. Since the equipment used for phosphating treatment is simple to operate, inexpensive, and allows for high production efficiency, it has been widely used, especially in the treatment of ordinary carbon steel. Since the phosphating film has good insulating properties, when phosphating components that are to be electrophoretically painted, an appropriate formula must be used to ensure that the phosphating film is fine and thin; moreover, the voltage should be increased appropriately during electrophoresis to achieve good results. 3. Passivation: The process of using a chromate solution to react with a metal’s surface, thereby forming a trivalent or hexavalent chromium layer, is called passivation, also known as chromating. It is commonly used in the treatment of aluminum, magnesium, and their alloys; it can also form a chromium layer on steel. However, it is rarely used alone and is usually employed in combination with phosphating to seal the pores in the phosphating layer, thereby passivating the exposed steel within that layer and suppressing the corrosive effect of residual phosphating accelerants, thus enhancing the protective properties. For passivation, a potassium dichromate solution (2–4 grams per liter) is generally used; sometimes 1–2 grams of phosphoric acid is also added. The part is immersed in this solution at 80–90 degrees Celsius for 2–3 minutes before being removed and washed with water. To reduce the time required for surface treatment, lower the labor intensity, and improve efficiency, various integrated treatment methods have been developed and applied. These include the “two-in-one” treatment of oil removal and rust removal, the “three-in-one” treatment of oil removal, rust removal, and phosphating, and the “four-in-one” treatment of oil removal, rust removal, phosphating, and passivation. All of these methods have proven to be effective in practical manufacturing applications. Its formula, as well as the treatment temperature and time, vary depending on specific requirements.