Electrochemistry - SS2 Chemistry Past Questions and Answers - page 4

Corrosion is a natural electrochemical process that occurs when metals react with their environment, leading to the deterioration and degradation of materials. It involves the oxidation of metals in the presence of an electrolyte, typically moisture or water, and the subsequent formation of metal oxides, hydroxides, or salts.

Different types of corrosion can occur depending on the specific conditions and factors involved:

1.    Uniform Corrosion: This type of corrosion occurs evenly across the surface of a metal. It is often caused by exposure to moisture or aggressive chemicals and can result in a general thinning or loss of material. An example is the rusting of iron due to atmospheric oxygen and moisture.

2.    Galvanic Corrosion: Galvanic corrosion occurs when two dissimilar metals are in contact in the presence of an electrolyte, leading to the accelerated corrosion of the less noble metal. This occurs due to the formation of a galvanic cell, where the more active metal acts as the anode and undergoes oxidation, while the less active metal acts as the cathode. For instance, the corrosion of steel nails when in contact with aluminium roofing material is an example of galvanic corrosion.

3.    Pitting Corrosion: Pitting corrosion is characterised by the localised attack on a metal surface, resulting in the formation of small pits or holes. It occurs due to localised chemical or electrochemical reactions, often in the presence of chloride ions or acidic environments. Pitting corrosion can be highly damaging and can lead to significant material loss. An example is the pitting corrosion of stainless steel in chloride-rich environments.

4.    Crevice Corrosion: Crevice corrosion occurs in confined spaces or crevices where oxygen and other reactants have limited access. It is commonly found in joints, gaskets, or areas where stagnant water can accumulate, leading to localised corrosion. The corrosion of metal pipes near pipe fittings is an example of crevice corrosion.

Corrosion prevention and protection techniques aim to inhibit or control the electrochemical reactions that lead to material degradation. Several methods are employed to prevent corrosion and protect materials:

1.    Barrier Protection: Barrier protection involves creating a physical barrier between the metal surface and the corrosive environment. This prevents direct contact and inhibits the transfer of moisture or corrosive substances to the metal surface. Examples include painting, coating with polymers, or using corrosion-resistant materials such as stainless steel or aluminium.

2.    Cathodic Protection: Cathodic protection involves making the metal to be protected a cathode by connecting it to a more easily corroded metal (sacrificial anode) or applying an impressed current. This shifts the electrochemical reaction on the protected metal to a reduction reaction, preventing its oxidation. Galvanising, where a layer of zinc is applied to steel, is an example of sacrificial anode cathodic protection.

3.    Alloying: Alloying involves adding small amounts of other elements to a metal to form alloys. The alloying elements improve the corrosion resistance of the metal by forming stable oxides or passive layers that protect the metal surface. Stainless steel, which contains chromium and nickel, is an example of alloying to enhance corrosion resistance.

4.    Environmental Modification: Modifying the environment around the metal can prevent or reduce corrosion. This can be done by controlling the temperature, humidity, pH, or the concentration of corrosive substances. For example, controlling the pH of water or using corrosion inhibitors in cooling systems can mitigate corrosion.

5.    Coating and Plating: Applying protective coatings or plating on the metal surface provides a physical barrier against corrosion. Common coating methods include electroplating, hot-dip galvanising, or using chemical conversion coatings. An example is the application of a layer of zinc on iron or steel to prevent rusting.

6.    Proper Design and Maintenance: Designing structures or equipment to minimise areas where moisture or corrosive substances can accumulate, ensuring proper drainage and ventilation, and regular inspection and maintenance can help prevent corrosion. By eliminating or reducing conditions conducive to corrosion, the overall longevity of the material can be increased.