What is corrosion? What are the conditions for rusting? Explain the methods of preventing corrosion.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define with an example. Corrosion is the gradual destruction of a metal by chemical or electrochemical reaction with its surroundings. Rusting: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O.
2 · Conditions for rusting. Iron rusts only when both air (oxygen) and water are present. Salt and acidic gases speed it up.
3 · Draw the activity. [Three labelled test tubes with nails: A in tap water open to air (rusts); B in boiled water under oil, corked (no rust); C with anhydrous CaCl₂, corked (no rust)]
4 · Methods of prevention.
Alloying: stainless steel
Galvanisation: zinc coating on iron
Electroplating: chromium or nickel by electrolysis
Anodising: thicker oxide layer on aluminium
Cathodic protection: a more active metal (Mg, Zn) corrodes instead
Painting, oiling and greasing keep air and water away
5 · Real-life link. Galvanised roofing sheets, chrome-plated taps and painted iron gates are all protected from corrosion.
6 · Close in one line. Thus keeping air and water away, or sacrificing a more active metal, stops corrosion.
Scoring tip: Give one example with every method. The three test-tube sketch quickly shows the conditions for rusting.
