English
## I. Essential Process of Metal Corrosion
1. Definition: A process in which metals undergo phase transformation, form new substances and suffer damage under the action of media.
2. Thermodynamic Driving Force: Energy input is required for metal smelting. The free energy of elemental metals is higher than that of their ore compounds, so metals tend to spontaneously release energy and convert into thermodynamically more stable corrosion products. Hence, corrosion is a spontaneous process. Taking iron as an example, iron oxide, the corrosion product, has the same composition as natural hematite; the essence of corrosion is iron returning to its ore form.
3. General Reaction Formula: Metallic material + components of corrosive medium → corrosion products. Two conditions must be satisfied simultaneously for corrosion to occur: new phases are generated from the reaction between metal and medium, and the free energy of the entire system decreases.
## II. Three Major Types and Characteristics of Metal Corrosion
1. Physical Corrosion
Damage caused by simple dissolution of base metal in liquid metals without redox reactions. A typical example is the corrosion of iron pots by molten zinc during hot-dip galvanizing, which generates zinc-iron intermetallic compounds.
2. Chemical Corrosion
Direct redox reactions between metals and non-electrolytes, with instantaneous direct electron transfer at the interface and no electric current generated. It mostly occurs in anhydrous organic solvents and dry high-temperature gas environments, accounting for a small proportion in actual working conditions.
3. Electrochemical Corrosion (the dominant form of corrosion in industry)
(1) Core Mechanism: When metals come into contact with conductive electrolytes, oxidation and reduction reactions proceed synchronously at spatially separated anodic and cathodic regions respectively. Electrons flow through the metal interior and ions migrate through the medium to form a complete circuit accompanied by electric current, which is essentially a short-circuited corrosive galvanic cell.
(2) Two Types of Corrosive Galvanic Cells
- Bimetallic Galvanic Cell: Electric current forms when two dissimilar metals are immersed in the same electrolyte and connected via a metallic path.
- Concentration Cell: Potential difference arises on the surface of the same metal due to varying electrolyte concentrations, driving corrosion.
(3) Four Indispensable Elements of a Corrosive Cell
Anode (where oxidation and metal dissolution/corrosion take place), cathode (which gains electrons and is protected), conductive electrolyte (acidic, alkaline or saline aqueous solution), and a metallic circuit connecting the anode and cathode. Corrosion ceases if any single element is missing.
(4) Three Complete Stages of Electrochemical Corrosion
- Anodic Process: Metal ions dissolve into the electrolyte, leaving electrons on the substrate.
- Cathodic Process: Oxidants in the medium (most commonly H⁺ and O₂) capture electrons, corresponding to hydrogen evolution corrosion and oxygen absorption corrosion respectively.
- Current Circulation: Electrons flow from the anode to the cathode, while anions and cations migrate directionally in the solution. Blockage of any step halts overall corrosion. Corrosion is localized exclusively at the anodic zone with negligible loss at the cathode.
## III. Corrosion Mechanisms of Steel Materials in Different Environments
Core reason for steel’s high susceptibility to corrosion: Steel readily reacts with media such as air and water at ambient temperature, categorized into five working conditions:
1. Dry High-temperature / Dry Steam Environments: No liquid water film exists, only chemical corrosion occurs, and iron reacts with water vapor to generate ferrous oxide.
2. Humid Atmospheric Environments (The Most Prevalent): Once the relative air humidity exceeds the critical value, a water film condenses on the steel surface and triggers oxygen-absorption electrochemical corrosion. Ferrous hydroxide forms initially, which is gradually oxidized into complex hydrated iron oxides (rust forms layered structures: ferric iron in the outer layer, triiron tetroxide in the middle layer, and ferrous iron in the inner layer). Impurities (SO₂, NaCl) drastically reduce the critical humidity and accelerate corrosion.
3. Marine Atmosphere: Salt deposits on steel surfaces enable conductive water film formation even at low humidity, leading to electrochemical corrosion.
4. Industrial Atmosphere: Sulfur dioxide in air forms an acidic water film, inducing electrochemical corrosion under low-humidity conditions.
5. Steel Surfaces Covered with Rust or Dust: Capillary action generated by pores in rust layers and dust allows water film condensation even below saturation humidity, sustaining continuous electrochemical corrosion.
### Overall Core Conclusions
1. Corrosion is a spontaneous energy-reduction process of metals, classified into physical, chemical and electrochemical types. Electrochemical corrosion is the primary cause of steel failure.
2. Electrochemical corrosion relies on an intact galvanic cell system, with corrosive damage concentrated at the anode.
3. The vast majority of steel corrosion in natural environments originates from electrochemical corrosion initiated by liquid water films on surfaces. Humidity, salt content, acidic pollutants and surface rust/dust all drastically accelerate rust formation.