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By adding alloying elements to the base metal that promote passivation, a material with excellent corrosion resistance is obtained once the addition amount reaches a certain proportion. When chromium is added to iron, and the chromium content reaches over 12%, it becomes stainless steel, which possesses high corrosion resistance in oxidizing environments due to the formation of a passivation film on its surface. Adding nickel to chromium steel can expand the passivation range and also improve mechanical properties. Chromium-nickel stainless steel, containing 18% chromium and 9% nickel, is the most widely used corrosion-resistant alloy in industrial and domestic applications. For example, when 14% silicon is added to iron, high-silicon iron with excellent acid resistance is obtained; its surface is covered by a silica protective film that provides good resistance to hot sulfuric acid, nitric acid, mixed acids, and similar substances. When the nickel content in nickel-copper alloys is above 30–40%, cupronickel alloys with 10–30% nickel and Monel alloys with 70% nickel and 30% copper can be obtained. These alloys exhibit better corrosion resistance in certain environments compared to pure copper and pure nickel. A series of nickel alloys are well-known for their corrosion resistance; for example, nickel cast iron has excellent alkali resistance. Nickel-molybdenum-chromium alloys are among the few alloys that can resist high-temperature non-oxidizing acids such as hydrochloric acid. Nickel-aluminum-chromium-iron alloys can resist high-temperature oxidizing acids, hypochlorites, seawater, etc., and perform better than ordinary stainless steels. Adding trace amounts of cathodic noble metals with low overpotential to certain active metals can promote passivation. For example, stainless steel and titanium are active in sulfuric acid at certain concentrations and temperatures; by adding 0.1–0.15% palladium or platinum to the base metal, numerous microcathodes are formed on the surface of the alloy, which facilitates the operation of local corrosion cells. As a result, the anode current increases rapidly, and the passivation region is quickly reached, thereby enhancing the corrosion resistance of the alloy.