Corrosion, Degradation and Surface Protection
Corrosion, Degradation and Surface Protection tests whether you can identify the electrochemical cell, quantify the driving force and the rates, and prescribe protection that survives the design life. Expect scenario questions set in marine, buried, process and building-services environments.
The cell and the forms
Anode, cathode, electrolyte and a metallic return path are all required; remove one and the cell stops. Standard electrode potentials give the thermodynamic driving force; the practical galvanic series in seawater gives behaviour. Learn the morphology: uniform loss, galvanic attack governed by the area ratio (a small anode facing a large cathode is the dangerous case), crevice under gaskets and deposits, pitting where chloride exceeds the breakdown potential, intergranular attack after sensitisation, dezincification of brass, erosion-corrosion and cavitation at high flow, fretting at clamped joints, and stress corrosion cracking - chloride SCC of austenitic stainless above roughly 60 C, caustic SCC of carbon steel at welds, ammonia SCC of copper alloys. Sour service adds hydrogen embrittlement, controlled through NACE MR0175/ISO 15156.
Protection
Barrier coatings need surface preparation to ISO 8501-1 Sa 2 1/2, verified dry-film thickness to ISO 19840, and stripe coats at edges and welds. Galvanising and zinc-rich primers give sacrificial protection at coating breaks. Cathodic protection polarises the structure negative; buried steel is normally protected at about -850 mV against copper/copper-sulphate, and overprotection must be avoided because it generates hydrogen on high-strength steels. Design detail - drainage, no pockets, insulated dissimilar joints, compatible sealants - is cheaper than any coating.
Worked example
A 316L cooling-water header at 70 C with 300 ppm chloride and high residual weld stress cracks longitudinally beside a weld. Diagnosis: chloride stress corrosion cracking. Remedies in order of preference: reduce temperature below the threshold, reduce chloride, stress-relieve, re-specify duplex 2205 or a super-austenitic grade, and replace chloride-bearing insulation. Increasing wall thickness does not help, because crack growth is stress-controlled rather than load-controlled.
Traps
Ignoring the anode-to-cathode area ratio; believing stainless is immune; forgetting coatings fail at edges, welds and damage first; overprotecting with CP and inducing hydrogen cracking; confusing erosion-corrosion with cavitation; omitting reference electrodes and a monitoring plan from a CP design.