Corrosion is a natural process due to which a metal suffers gradual decay when it interacts chemically with its environment. During this interaction the surface of metal is coated with compounds such as oxides, sulphides and carbonates.
A common example is rusting of iron, where iron reacts with oxygen and water present in the atmosphere to form hydrated iron (III) oxide. Once iron begins to rust, the process will continue as rust is porous and allows both air and moisture to come into contact with fresh metal underneath. Hence, all the metal will corrode given enough time.
The process of corrosion of iron appears to require two important chemicals, oxygen and water, which are involved in the following electrochemical reactions. Iron loses electrons to form Fe(II) ions.
Fe²⁺ then react with more oxygen and water to form iron(III) hydroxide which is rust.
Corrosion can be avoided by coating the metal surface with barriers that prevent contact of metal surface with the atmosphere. For this purpose, the metal surfaces are protected by either greasing with oil or painting. Other methods are galvanizing, electroplating and sacrificial protection.
Galvanization is a common method to protect iron or steel from rusting. It involves the dipping of an iron sheet in a bath of molten zinc. A layer of zinc metal gets deposited on the iron sheet and prevents rusting. Tin cans are actually steel cans which have been coated with a thin layer of tin.
In sacrificial protection a more reactive metal like zinc or magnesium is used which will corrode in place of the less reactive metal like iron. Magnesium is often employed to protect steel in buried fuel tanks and pipe lines and ship hulls submerged in sea water.
The difference in the activity of the two metals (Mg and Fe) causes a current to flow between them, producing corrosion on the more active metal protecting the less active metal iron.
1. What conditions are required for rusting of iron?
✅ Answer: Both oxygen and water (moisture) are essential. Rusting occurs when iron is exposed to air (O₂) and water simultaneously.
2. If a sample of iron and a sample of zinc come into contact, which will corrode? Explain your answer.
✅ Answer: Zinc will corrode because it is more reactive than iron. In galvanic contact, the more active metal (zinc) acts as anode and gets oxidized (sacrificial protection), while iron is protected.
➕ Which other electrolyte can be used in hydrogen‑oxygen fuel cell?
✅ Answer: Apart from phosphoric acid, common electrolytes include potassium hydroxide (KOH) solution (alkaline fuel cell), molten carbonates, or solid oxides. KOH is widely used.
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