Corrosion protection and corrosion rate

(a) Corrosion protection methods

1. Protective coatings: paint, enamel, plastic, or metallic coatings. Galvanising (zinc on steel) protects both as a barrier and sacrificially, since zinc is anodic to iron even if scratched. Tin plating protects only as a barrier.
2. Cathodic protection: make the structure the cathode.
   • Sacrificial anode: connect Mg, Zn or Al blocks to buried pipes or ship hulls; they corrode instead of the steel.
   • Impressed current: a DC source drives current from an inert anode to the structure.
3. Inhibitors: small additions (chromates, phosphates, amines) to the environment that form a protective film or slow the electrode reaction.
4. Material selection and alloying: use stainless steel, aluminium, or Cu alloys; design to avoid crevices, galvanic couples, and stagnant water.
   • Other: anodising of aluminium, controlling temperature/humidity.

(b) Corrosion rate

Volume of metal lost:

$$V = \frac{W}{\rho} = \frac{0.45}{7.87} = 0.05718\ \text{cm}^3$$

Thickness lost over 240 h:

$$x = \frac{V}{A} = \frac{0.05718}{12} = 0.004765\ \text{cm} = 0.04765\ \text{mm}$$

Hours in a year $= 8760$, so scaling to a year:

$$CPR = 0.04765 \times \frac{8760}{240} = 0.04765 \times 36.5 = 1.74\ \text{mm/year}$$

This is the same as the standard formula $CPR = \dfrac{87.6\,W}{\rho A t}$ with $W$ in mg, $\rho$ in g/cm$^3$, $A$ in cm$^2$, $t$ in hours: $\dfrac{87.6 \times 450}{7.87 \times 12 \times 240} = 1.74$ mm/yr.

Answer: corrosion penetration rate = 1.74 mm/year (about 68 mpy).