Corrosion Chemistry Notes
→ When metals come in contact with atmosphere then gases and water vapour present in atmosphere attack at the surface of metals and some undesired compounds like metal oxide, carbonates, sulphides, sulphates etc., are formed. This process is known as corrosion.
→ “The process in which oxides, carbonates, sulphides, sulphates etc., are formed by attack of atmospheric gases or water vapour on the surface of metals and metal corroded slowly, then this process is called corrosion.”
- Tarnishing of silver.
- Formation of green layer on copper surface.
- Rusting of iron etc.
Types of Corrosion :
It is of following four types:
- Soil Corrosion-Pipe lines, electric wires etc., which are embedded in soil undergo corrosion and such corrosion is called soil corrosion.
- Microorganism Corrosion-When substances are corroded due to metabolic activity of microorganism then this type of corrosion is called microorganism corrosion.
- Tearing Corrosion-It is generally a local corrosion which is generally produced from dust, cracking after paint and water.
- Galvanic Corrosion-Most common corrosion is Galvanic corrosion. When two metals like Zn and Cu are attached electrochemically then that metal which lies above in electrochemical series corrodes in the presence of electrolyte.
- Such type of corrosion is called Galvanic corrosion.
Factors Helping in Corrosion :
- Metals form cell in the presence of impurities as a result corrosion takes place rapidly. Rust does not occur in pure iron while rust is formed in impure iron rapdily.
- The rate of corrosion is high in the presence of electrolytes so iron corrodes rapidly in salty water than pure water.
- If strains are present at surface of metal then that surface is corroded rapidly. It is the reason that the corners of boxes are broken rapidly.
- Those metals like copper, iron etc., are more active, corrode rapidly and those metals like Au, Ag, Pt, Pd etc., are less reactive, undergo less corrosion.
- The process of corrosion takes place rapidly in the presence of air and moisture. Corrosion takes place more rapidly in the presence of CO2 and SO2 in air.
- Example : Rust occurs easily in iron in the presence of moist air while rusting of iron does not occur in dry air and vacuum.
Electrochemical Theory of Corrosion :
→ We can explain electrochemical theory of corrosion by example of rusting of iron. Oxygen and CO2 gas of atmosphere dissolved in water drops present at the surface of iron metal.
CO2 + H2O → H2CO3
→ Here, impure iron acts as cathode and pure iron acts as anode and aqueous solution present at surface in which O2 and CO2 are dissolved, acts as electrolyte. So; an electrochemical cell is formed at the surface of iron. The reactions take place in cell are as follows:
- At anode : Iron undergoes oxidation and Fe2+ ions go in solution.
Fe → Fe2+ + 2e– (Oxidation)
- At Cathode : H+ ions obtained by ionisation of H2CO3 and H2Oformed by adding H2O and CO2,accept electron.
Methods of Prevention of Corrosion :
Barrier Protection It is simplest method for protection of corrosion. In this method, iron surface can not in contact with atmospheric moisture, oxygen and CO2.
We can use following methods for barrier protection :
- By painting at the surface of iron.
- By using thin layer of oil or grease at the surface of iron.
- By coating of a metal like Ni, Cr, Cu etc., which does not undergo corrosion, at the surface of iron by electroplating
- By coating of Fe2 O3, iron phosphate or other chemicals at the surface of iron. These substances form insoluble layer at the surface of iron which acts as barrier.
→ Sacrificial Protection – In this method, the surface of iron is covered with a layer of more electropositive metal. So, oxidation of iron does not take place. Thus, iron prevents from corrosion due to sacrifice of more electropositive metal.
→ Its best example is galvanization, In this method, a thin layer of zine covers the surface of iron which protects rusting of iron. Since, zinc is placed above in electrochemical series. So, its oxidation takes place first and zinc is disintegrated and iron is protected.
→ Cathodic or Electrical Protection When underground iron pipes or tanks are used then we protect them from corrosion by this method. In this method, the bridges of pipe or tank or water etc., are attached with a block of more electropositive metals like Mg, Al, Zn etc. This metal block acts as anode and loses electrons.
→ The electrons obtained from anode move to cathode where the iron ions react with these electrons to convert into iron metal again and so here iron protects from corrosion. More electropositive metals are changed time to time so that iron is protected.
→ By Anti-rust Solution-Alkaline sodium phosphate and chromate solution act as anti-rust solutions. When iron metal objects are dipped in boiled phosphate solution then an invisible thin protective layer is formed on objects which protects iron from corrosion.