Environmental Effects

Corrosion is the deterioration of a metal by chemical or electrochemical reaction with its environment.

Galvanic corrosion

When two dissimilar metals are in electrical contact in the presence of an electrolyte (water, moisture), a galvanic cell forms. The more active metal (anode) corrodes and the more noble metal (cathode) is protected.

• Anode: $M \rightarrow M^{n+} + n e^-$
• Cathode (in neutral water): $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$

The metal's position in the galvanic series decides which is the anode (e.g., Mg, Zn, Al are anodic to steel; steel is anodic to copper, stainless steel).

Severity increases with larger potential difference, small anode area compared with cathode area, and high conductivity of the electrolyte.

Example: steel bolts used on a copper plate or an aluminium sheet riveted with steel rivets in sea water.

Prevention: select metals close in the galvanic series, insulate them, keep the anode area large, apply coatings, or use sacrificial anodes.

Stress corrosion cracking (SCC)

SCC is brittle cracking of a normally ductile metal under the combined action of tensile stress (applied or residual) and a specific corrosive environment.

Cracks form at the crack tip where the protective film is broken by stress, and the exposed metal dissolves, so the crack advances (intergranular or transgranular) with little overall metal loss.

Example: season cracking of brass cartridge cases, and chloride SCC of 304 stainless steel pipes.

Prevention: stress-relief annealing, shot peening, changing the alloy, controlling the environment.

(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).