corrosion
What is corrosion?
Does corrosion mean rusting?
What is the process of corrosion?
How can corrosion be prevented or controlled, and how does galvanization protect steel from rusting?
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corrosion, wearing away of a material, often a refined metal such as iron or copper, because of chemical reactions, mainly oxidation (see oxidation-reduction and oxide). It occurs whenever an exposed surface chemically reacts with atmospheric agents such as moisture and oxygen. This process is accelerated by warm temperatures and the presence of acids and salts.
Corrosion of metals is a spontaneous natural process in which the metal is converted into more thermodynamically stable formsātypically oxides, hydroxides, or sulfides, the forms in which metals naturally occur as ores before refinement. Although metals are most commonly affected, corrosion can also degrade nonmetallic materials, including polymers, ceramics, and composite materials.
Chemical process of corrosion in metals
Corrosion in metals proceeds through a series of electrochemical reactions. These occur in a thin surface film formed from atmospheric moisture and other substances present in the environment, such as oxygen and pollutants. This creates an electrolytic medium that allows the flow of ions. In this medium, different regions of the metal act as anodes and cathodes. At the anode, metal atoms undergo oxidation, losing electrons and entering the environment as cations. The released electrons are gained by dissolved oxygen or other oxidizing speciesāsubstances that readily accept electronsāto form anions.
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These redox reactions (chemical reactions involving the transfer of electrons) lead to the gradual formation of corrosion products such as rust or patina. Some products are porous and nonprotective. Iron, for example, forms rust, a porous oxide layer that promotes further corrosion. Other products are stable and adherent and act as protective films. Metals such as aluminum, copper, and zinc develop compact adherent oxide layers that limit further attack, and silver forms a superficial sulfide tarnish that generally does not impair the underlying metal.
Types of metallic corrosion
Corrosion of metals can be categorized by appearance, the materials affected, and the chemical byproducts formed, as shown in the following table:
| type | description | commonly affected materials | key indicators |
|---|---|---|---|
| uniform | thinning of the metal across the entire exposed surface, eventually resulting in significant metal loss | iron, steel, copper | red rust (iron oxide) or green patina (copper carbonate) over the entire surface |
| galvanic/bimetallic | corrosion of the more active metal when it is in electrical contact with a less-active metal in the presence of an electrolyte | steel with brass or aluminum with steel in seawater | thick white (zinc oxide) or gray crusty deposits (aluminum oxide) on the active metal |
| crevice | material deterioration within a localized environment because of accumulated corrosive substances | underneath washers, nuts, clamps, loose gaskets | concentrated metal salts and acidic liquid trapped in the gap |
| pitting | localized attack where the protective passive film is damaged or inconsistent, forming cavities or holes that may penetrate the metal | stainless steel, aluminum, nickel | white, powdery spots (aluminum oxide) or dark metallic scabs (metal hydroxides) |
| intergranular | attacks along the internal boundaries between tiny crystal regions within the metal | austenitic stainless steel, chromium-depleted alloys | weakened grain boundaries caused by precipitated chromium carbide particles |
| selective leaching/dealloying | selective corrosion and consequent removal of one of the elements in an alloy | brass, gray cast iron | weak, porous metal structure due to dezincification of brass or graphitic corrosion of gray cast iron |
| erosion | accelerated corrosion of a metal surface when turbulent movement of a corrosive fluid intensifies the attack | blockages, inlet ends, pump impellers | stripped metal particles that leave a raw, grooved surface |
| stress cracking | cracking of the material under the combined action of mechanical stress and a corrosive environment | high-strength steel, stainless steel | no visible byproduct, as the metal appears clean but is fractured |
Prevention and control
Corrosion damages infrastructure, such as industrial facilities and transportation systems, necessitating continual and costly maintenance, repairs, and replacements. Some materials resist corrosion naturally, whereas others can be treated for protection. Strategies to limit environmental exposure and interrupt electrochemical processes can be categorized into four primary methods.
- Cathodic protection: This method prevents corrosion of structural metals such as iron, steel, and other alloys by converting the structure into a cathode, either through the attachment of sacrificial anodes made of a more active metal such as zinc or magnesium, which corrode preferentially, or by applying an impressed current from an external power source to supply electrons and suppress oxidation.
- Materials selection: Choosing the appropriate material, such as stainless steels, aluminum alloys, or titanium alloys that form protective surface films, can significantly reduce corrosion in harsh environmental conditions.
- Galvanization: Steel is dipped into molten zinc, which provides both a barrier and sacrificial anode protection. If the coating is damaged, the surrounding zinc corrodes preferentially, protecting the exposed steel. Another method is sherardization, in which iron or steel is heated in a sealed container with finely divided zinc, forming zinc-iron alloys and an outer layer of pure zinc that provides a uniform, corrosion-resistant coating.