Question 4 of 6: Environmental Cells

Question 4: Environmental Cells (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.

4.1 — (a) Crevice corrosion: conditions, mechanism, and engineering solutions

Conditions. Crevice corrosion requires a narrow, shielded gap wide enough to admit and hold a stagnant electrolyte but narrow enough to restrict the exchange of that electrolyte with the surrounding bulk solution — typically on the order of a few micrometres to a few hundred micrometres. It attacks a passive alloy (a stainless steel, a nickel alloy, aluminum, titanium) in an electrolyte that contains an aggressive anion, chloride above all. Common geometric sites are gasketed and bolted or riveted lap joints, under washers and fastener heads, beneath surface deposits, marine fouling or corrosion product, under loose paint or tape, and inside the small clearance of a threaded connection — anywhere a stagnant volume of solution can sit in electrical and ionic continuity with the bulk surface but is not freely flushed by it.

Mechanism. Corrosion begins uniformly, at a low rate, both inside the crevice and on the freely exposed surface. Cathodic oxygen reduction consumes the small volume of dissolved oxygen trapped in the crevice, and — because the crevice geometry restricts diffusive resupply of fresh oxygenated solution — that oxygen is not replenished, while the open surface stays well aerated. The crevice interior therefore becomes progressively oxygen-depleted relative to the bulk, and this is a purely local aeration difference: the crevice is the anode of a differential-aeration cell and the open, oxygenated surface is the cathode (the same "oxygen concentration cell" logic that underlies the Nernst dependence of $E_{O_2/H_2O}$ on $p_{O_2}$ derived in Question 1). Anodic metal dissolution inside the crevice continues, and the resulting metal cations hydrolyze in the confined, poorly exchanged volume (e.g. $M^{n+}+nH_2O\rightarrow M(OH)_n+nH^+$), which drives the crevice solution acidic. The falling pH and the migration of chloride ions into the crevice to balance the accumulating positive charge together produce a small, highly acidic, chloride-concentrated occluded cell that can no longer repassivate; once this autocatalytic condition is established the attack accelerates and is largely self-sustaining, penetrating the metal locally while the surrounding open surface remains essentially unattacked.

Engineering solutions. (i) Eliminate crevice geometry in design — welded rather than bolted or riveted joints, full-penetration welds without backing-strip crevices, sloped rather than flat-bottomed vessels that do not trap stagnant liquid or deposits; (ii) where a crevice-forming joint cannot be avoided, use non-absorbent gasket materials and design for drainage so liquid does not pool; (iii) select an alloy with higher pitting/crevice resistance (higher chromium, molybdenum and nitrogen content, ranked by the pitting resistance equivalent number, $PREN=\%Cr+3.3\%Mo+16\%N$) for the specific chloride level and temperature expected; (iv) keep surfaces clean of deposits and fouling through regular inspection and cleaning, since a deposit is itself a crevice-forming shield; (v) apply cathodic protection, which raises the whole structure's potential below the level needed to sustain the local acidification/repassivation-failure cycle; and (vi) reduce chloride content, temperature, or dissolved-oxygen level in the service environment where the process allows.

4.2 — (b) Pitting corrosion: conditions, mechanism, and engineering solutions

Conditions. Pitting attacks the same class of passive-film-forming alloys as crevice corrosion, but needs no pre-existing geometric crevice: it initiates directly on an otherwise open, apparently intact passive surface exposed to an electrolyte containing an aggressive anion (again, chloride is the classic case), typically once the applied or free-corrosion potential exceeds a critical pitting potential, $E_{pit}$ — the potential (identifiable on a polarization curve like Question 2's, but with a sharp current upturn well below the uniform transpassive potential) above which the passive film can no longer resist local chloride attack. Susceptible sites are inhomogeneities in the passive film itself: sulfide inclusions, second-phase particles, mechanical damage or scratches, and grain boundaries.

Mechanism. Chloride ions adsorb preferentially at a local film weak point and are incorporated into the oxide, locally thinning it and increasing its ionic conductivity; once a microscopic breach occurs, bare metal beneath is exposed to the electrolyte and dissolves rapidly at that single point while the surrounding film stays passive and cathodic — an extreme, self-initiated version of the same unfavourable area ratio (a vanishingly small anode against a huge passive cathode) that drives galvanic and crevice attack. Just as in a crevice, the tiny occluded pit volume cannot exchange with the bulk solution: metal-cation hydrolysis acidifies the pit interior, chloride migrates in to balance charge, and the pit becomes self-propagating and autocatalytic, growing preferentially downward (into the section) rather than sideways because gravity and diffusion favour a denser, more concentrated solution collecting at the pit bottom. The result is deep, narrow, often hemispherical or undercut penetration with a disproportionately small loss of surface area and mass — the most dangerous form of localized attack for a pressure boundary or a structural member, because a component can appear almost undamaged by visual/weight-loss inspection while perforated or critically weakened at a pit.

Engineering solutions. (i) Select an alloy whose pitting resistance equivalent number is comfortably above the service chloride level and temperature (higher Mo and N content in particular raise $E_{pit}$); (ii) keep the operating potential below $E_{pit}$, e.g. by cathodic protection or by avoiding oxidizing contaminants/excess free chlorine that would raise the free-corrosion potential into the pitting range; (iii) maintain a smooth, clean, mechanically undamaged, well-passivated (e.g. acid-passivated per ASTM A967) surface, since scratches and embedded iron particles are common initiation sites; (iv) reduce chloride concentration and temperature where the process allows, since $E_{pit}$ falls (susceptibility rises) with both; (v) avoid stagnant, low-flow conditions that let chloride and dissolved species concentrate locally, and avoid deposits/fouling for the same reason as in crevice corrosion; and (vi) specify inclusion-controlled ("clean") steel making practice to minimize sulfide-inclusion pit initiation sites.