Localized Corrosion — Forms and Prevention: 21-Mat-A7 Environmental Degradation of Materials, May 2018, Q2

Question 2: Localized Corrosion — Forms and Prevention (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Approach. Each row names the mechanism that drives that form of attack and gives three prevention routes that interrupt it — material choice/alloying, design/geometry, or environment control — mirroring the worked example's own three-part structure (alloy, heat treatment, composition control).

Localized corrosion — mechanism and three prevention methods each
Type	Prevention methods (with explanation)
Intergranular (worked example)	1. Alloy with Ti or Nb ("stabilised" grades) so these stronger carbide-formers tie up carbon before Cr can, preventing Cr-depletion at grain boundaries. 2. Solution-anneal/quench after any weld or hot-work step so Cr carbides never precipitate in the sensitizing range (450–850 °C). 3. Specify a low-carbon ("L"-grade, <0.03 wt.%C) stainless so there is too little carbon to deplete the boundary of Cr below the ~12 wt.% passivity threshold.
Crevice	1. Eliminate crevices by design — welded rather than bolted/riveted joints, full-penetration welds with no backing-strip gap, solid (not laminated) gaskets. 2. Use crevice-resistant alloys with high Cr, Mo and N (super-austenitic or duplex stainless, Ni-Cr-Mo alloys) — molybdenum in particular raises resistance to the acidified, chloride-concentrated crevice electrolyte. 3. Keep surfaces clean and periodically remove deposits/sediment (which themselves create crevices) and, where a crevice cannot be avoided, apply cathodic protection to hold the whole assembly below the crevice's own corrosion potential.
Pitting	1. Select alloys with a high Pitting Resistance Equivalent Number (PREN $\approx$ %Cr + 3.3%Mo + 16%N) — molybdenum and nitrogen both raise the chloride pitting potential. 2. Reduce aggressive-ion exposure: lower chloride concentration, remove oxidizing metal-ion contaminants (e.g. cupric or ferric ions) that can locally initiate pits, and avoid stagnant chloride-containing water. 3. Apply cathodic protection or an inhibitor (e.g. nitrite, chromate where permitted) to hold the surface potential below the alloy's pitting potential $E_b$, and specify smooth, defect-free surface finishes since pits nucleate preferentially at inclusions and scratches.
Galvanic	1. Avoid coupling metals far apart in the galvanic series, or select a compatible pair close together in the series for the service electrolyte. 2. Keep the cathode (nobler metal) small relative to the anode (active metal) — an unfavourable area ratio (small anode, large cathode) concentrates the anodic current density and accelerates attack, so design for a large-anode/small-cathode ratio instead. 3. Electrically insulate the dissimilar-metal joint (dielectric unions, isolating gaskets/washers) or apply a coating to the cathodic member only (a coating flaw on the anodic member is self-limiting; a flaw on the cathodic member concentrates attack at the flaw).

Concept & Theory

Topic. The shared "occluded-cell" logic behind crevice, pitting and galvanic attack, and how each prevention route interrupts a different link in that chain.

Key relations. Occluded-cell autocatalysis: local O$_2$ depletion $\rightarrow$ metal-ion hydrolysis ($M^{n+}+nH_2O\rightarrow M(OH)_n+nH^+$) $\rightarrow$ acidification $\rightarrow$ Cl$^-$ migration into the site to balance charge $\rightarrow$ accelerated dissolution; PREN for pitting resistance ranking; galvanic series for couple compatibility; area-ratio rule $i_{anode}\propto A_{cathode}/A_{anode}$.

Why this works. Crevice and pitting corrosion are really the same autocatalytic mechanism at two different starting points — a crevice is a geometric occluded cell that exists before corrosion starts, while a pit is a chemically self-created occluded cell that starts from a random local breakdown of an otherwise intact passive film. Galvanic corrosion is a different mechanism (a macroscopic mixed-potential couple between two dissimilar electrodes) but the design remedies converge on the same three levers: remove the geometric/electrochemical driving force, choose more compatible/resistant materials, or intervene with an external potential (cathodic protection) or barrier (coating/inhibitor).

Common pitfall. Coating the anodic (active) member of a galvanic couple rather than the cathodic member — a pinhole in an anode coating creates a disastrously unfavourable small-anode/large-cathode ratio at the flaw, whereas the same flaw on a cathode coating is comparatively harmless.

Source. Fontana, Corrosion Engineering, 3rd ed., Ch. 3 (eight forms of corrosion); Jones, Principles and Prevention of Corrosion, 2nd ed., Ch. 7 (crevice, pitting) and Ch. 8 (galvanic).

Question 2 of 8: Localized Corrosion — Forms and Prevention

Question 2: Localized Corrosion — Forms and Prevention (20 marks)