Describe the main difference between dry corrosion and wet corrosion. Discuss any two factors which affect the corrosion of metal components and show how corrosion can be controlled in each case. Explain why the zinc coating of galvanized wire netting is eaten away before the wire it is protecting.

QUESTION 6

a) The main difference between dry corrosion and wet corrosion lies in the presence of an electrolyte.

• Dry corrosion (or direct chemical corrosion) occurs in the absence of a liquid electrolyte, typically at high temperatures. It involves a direct chemical reaction between the metal surface and gases (like oxygen, sulfur dioxide) in the environment, forming an oxide or other compound layer.
• Wet corrosion (or electrochemical corrosion) occurs in the presence of a liquid electrolyte (e.g., water, salt solution) at ambient temperatures. It involves an electrochemical process where anodic and cathodic reactions occur simultaneously on the metal surface, leading to the formation of corrosion products.

b) Two factors affecting the corrosion of metal components and their control:

1. Presence of Oxygen and Moisture: Oxygen and moisture are essential for most forms of wet corrosion. Higher concentrations of oxygen and humidity accelerate the corrosion rate.
   • Control: This can be controlled by applying protective coatings (e.g., paint, galvanizing) to create a barrier between the metal and the environment. Dehumidification or storing metals in dry environments also reduces moisture.
2. Presence of Dissimilar Metals (Galvanic Corrosion): When two different metals are in electrical contact in the presence of an electrolyte, the more active (anodic) metal will corrode preferentially.
   • Control: This can be controlled by electrically insulating dissimilar metals from each other. Alternatively, cathodic protection can be used, where a more active sacrificial anode is intentionally connected to the metal to be protected, or an impressed current system is used.

c) The zinc coating of galvanized wire netting is eaten away before the wire it is protecting due to sacrificial protection. Zinc is more electrochemically active (anodic) than iron (steel). When both metals are exposed to an electrolyte (like moisture), the zinc preferentially corrodes, acting as a sacrificial anode and protecting the underlying steel cathode from corrosion.

QUESTION 7

a) Descriptions of four (4) material properties:

• (i) Elasticity: The ability of a material to return to its original shape and size after the removal of an applied load. Deformation in this region is temporary and non-permanent.
• (ii) Plasticity: The ability of a material to undergo permanent deformation without fracturing when subjected to stress. Once the load is removed, the material does not fully return to its original shape.
• (v) Ductility: The ability of a material to be drawn or stretched into a wire under tensile stress without fracturing. It is a measure of a material's ability to deform plastically.
• (vi) Malleability: The ability of a material to be hammered, pressed, or rolled into thin sheets without fracturing. It indicates a material's capacity for plastic deformation under compressive stress.

b)

• Example of a ductile material: Mild steel
• Example of a malleable material: Gold

c) Essential differences in composition between:

• (i) Plain carbon steels and alloy steels:
  • Plain carbon steels primarily consist of iron and carbon (up to about 2.1 wt%), with small amounts of other elements like manganese, silicon, sulfur, and phosphorus. Their properties are mainly determined by carbon content.
  • Alloy steels contain significant amounts of additional alloying elements (e.g., chromium, nickel, molybdenum, vanadium) beyond those found in plain carbon steels. These elements are added to improve specific properties like strength, hardness, toughness, or corrosion resistance.

QUESTION 8

a) A typical tensile test curve for an annealed low-carbon steel plots stress (y-axis) against strain (x-axis).

• The curve starts with a linear region, representing the elastic range, where stress is proportional to strain (Hooke's Law).
• The point at the end of this linear region, or just before the onset of permanent deformation, is the elastic limit.
• Immediately following the elastic limit, there is a point where plastic deformation begins, known as the yield point. For low-carbon steel, this often appears as a distinct upper and lower yield point.
• Beyond the yield point, the material undergoes significant permanent deformation, which is the plastic range. In this region, the material continues to deform even with little or no increase in stress (yielding), followed by strain hardening where stress increases again.
• The curve eventually reaches a maximum stress (ultimate tensile strength), after which it begins to neck (localize deformation). The point where the specimen finally breaks is the breaking point of the specimen.

(Imagine a graph with Stress on the y-axis and Strain on the x-axis. The curve would start linearly, then flatten slightly at the yield point, rise again, and then drop sharply to the breaking point.)

b) The Charpy and Izod impact tests measure the energy absorbed by a material during fracture under impact loading. They differ in specimen proportions, notch profile, method of support, and criteria for determining the impact number.

• Specimen Proportions and Notch Profile:

  • Charpy Test: Uses a square cross-section specimen (e.g., 10mm x 10mm) with a V-notch or U-notch located in the middle of the specimen. The notch is typically 2mm deep.
  • Izod Test: Uses a square or circular cross-section specimen (e.g., 10mm x 10mm square or 11.4mm diameter circular) with a V-notch located near one end. The notch is typically 2mm deep.

• Method of Support:

  • Charpy Test: The specimen is supported horizontally at both ends, acting as a simply supported beam. The hammer strikes the side opposite the notch, in the middle of the specimen.
  • Izod Test: The specimen is clamped vertically at one end, acting as a cantilever beam. The hammer strikes the free end, on the notched side, above the notch.

• Criteria for Determining the Impact Number:

  • Both tests measure the energy absorbed by the specimen during fracture, which is indicated by the height the pendulum swings after breaking the specimen. This absorbed energy is the "impact number" or "impact strength." Higher absorbed energy indicates greater toughness.

(Imagine two simple diagrams: one showing a horizontal bar supported at both ends with a notch in the middle, being struck from above. The other showing a vertical bar clamped at the bottom with a notch near the top, being struck from the side.)