The black-brown appearance of molybdenum foil primarily results from surface oxidation or other chemical reactions, with specific causes as follows: 1) High-Temperature Oxidation: Above 600°C in air, molybdenum reacts with oxygen, forming volatile MoO₃, initially creating a thin black-brown oxide layer, with severe cases leading to surface powdering. 2) Slow Room-Temperature Oxidation: Prolonged exposure to air or humidity forms a thin oxide film (MoO₂ or MoO₃), appearing gray-black or black-brown, losing metallic luster. 3) Processing Residues: During rolling or cutting, lubricants or impurities decompose at high temperatures, forming carbon-based compounds that adhere to the surface, appearing black-brown. 4) Impurity Effects: Low-purity foil (<99.9%) contains carbon or sulfur impurities, reacting with oxygen at high temperatures to form complex oxides, darkening the surface. 5) Surface Defects: Improper rolling or acid washing causes micropores or scratches, readily adsorbing oxygen or contaminants, forming localized black-brown spots. 6) Improper Storage: Without inert atmosphere protection, moisture or corrosive gases (e.g., SO₂) in air trigger surface discoloration. High-purity foil (≥99.95%) oxidizes more slowly but still requires vacuum or inert gas protection. Preventive measures include polishing (roughness Ra<0.1 μm), acid washing to remove oxides, and inert atmosphere storage. Quality control uses spectral analysis and microscopic inspection to identify oxide causes and ensure performance.
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