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Key Points Note
The oxidation of metals, especially the process of iron turning into red rust, is explained through the movement of electrons.
1. About Metal Oxidation
Reason for Rusting: Metals rust due to their combination with oxygen in the air, leading to oxidation.
Properties of Metals: Metals reflect visible light and shine because they possess free electrons. The vibration of these free electrons indicates that the surface is highly energetic and active. This activated surface seeks stability by chemically bonding with oxygen in the air.
Contrast with Plastics: Contrarily, in plastics, atoms share electrons to form polymers, creating large molecular structures. Materials like PE (Polyethylene), PP (Polypropylene), and fluororesins have crystalline properties and are excellent in terms of chemical resistance and weatherability. However, due to their low-energy surfaces, they present challenges in paint adhesion.
2. The Oxidation Process of Iron
2-1. Formation of Iron Ions
When iron comes into contact with water, iron atoms dissolve into iron ions, and free electrons e are released, creating an electron flow.
2-2. Direction of Electric Current
In the aqueous solution, the current flows from the damaged area (part A) to the intact surface area (part C), and within the iron, it flows from C to A. Part A becomes the anode and part C the cathode, creating a potential difference.
2-3. Reduction Reaction
Electrons e are supplied to part C, and dissolved oxygen takes these electrons e to form OH-, a reduction reaction corresponding to the oxidation occurring at part A.
2-4. Formation of Iron Oxide
In the aqueous solution, Fe2+ reacts with OH- to form iron hydroxide, which reacts with oxygen in the air to become stable iron oxide. This state is known as red rust, representing the stabilization of active iron.