Discuss metal surface treatment methods and their characteristics

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Before being exposed to the operating environment, metals are treated with passivators or film-forming agents (such as mixtures of chromates, phosphates, alkalis, nitrates, and nitrites), which result in the formation of a stable and dense passivation film on the surface, thereby **increasing** its corrosion resistance. Its difference from the corrosion inhibitor protection method is that no additional corrosion inhibitors are required in the subsequent operating environments (such as the atmosphere or water); through anodic treatment, a denser film can be formed on the surface of aluminum than that formed in the atmosphere. Such membranes exhibit excellent corrosion resistance in mild corrosive environments (atmosphere and water). The bluing of steel parts’ surfaces is another widely used example. Overall alloying is relatively expensive; a surface alloying method can be used to infiltrate the steel surface with alloying elements that are prone to passivation, such as chromium, molybdenum, and silicon. Typically, the steel components are placed in a mixture of powdered chromium, aluminum, and silicon, or in metal vapor, and then heated for deposition. The surface deposit forms a passivation film in oxidizing environments, and its resistance to high-temperature oxidation as well as certain corrosion resistances are superior to those of the underlying steel. Due to its thin protective layer and low wear resistance, its lifespan is shorter than that of the overall alloy, making it unsuitable for long-term exposure to highly corrosive media. A newer surface treatment technique is ion implantation, in which elements such as boron, carbon, phosphorus, silicon, nitrogen, molybdenum, palladium, platinum, or other precious metals are ionized and accelerated using an ion implanter. These high-energy ions then collide with the base metal and penetrate into its surface, forming an amorphous alloy layer of certain depth and concentration. It possesses much higher corrosion resistance than the base metal, and has now been applied to small components.