Question 6 of 6: Corrosion Control

Question 6: Corrosion Control (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.

6.1 — (a) How each coating type mitigates corrosion

(i) Ceramic (glass or cement) coatings. Vitreous enamel (fused glass) and cementitious linings work primarily as chemical and physical barriers: the fired glass or hydrated cement matrix is itself essentially inert in most service environments and, being continuous and well-bonded, simply excludes the electrolyte and oxygen from the underlying metal surface. Cement linings on ductile-iron and steel pipe add a second protective effect beyond a pure barrier: the high alkalinity of the hydrated cement (pore-water pH typically above 12.5) raises the local pH at the metal surface into the range where iron/steel is thermodynamically passive (the same passivation phenomenon quantified in Question 2), so the mechanism is barrier protection reinforced by alkaline passivation, not sacrificial or inhibitive action.

(ii) Zinc on steel (galvanizing). Zinc protects steel by two mechanisms operating together, which is what distinguishes it from a simple barrier coating. While intact, the zinc layer is a barrier exactly like (i). But because zinc is anodic to steel in the galvanic series, at any break, scratch, cut edge or coating defect that exposes bare steel, the zinc immediately adjacent becomes the anode of a small galvanic cell (exactly the mechanism of Question 3(a)) and is preferentially consumed, sacrificially protecting the exposed steel cathodically rather than letting it corrode. This self-healing behaviour at small defects is unique to a coating that is electrochemically active (anodic) relative to the substrate, and is why galvanizing remains protective even after minor mechanical damage, unlike a purely inert barrier coating.

(iii) Tin on steel (tinplate). Tin protects the opposite way: tin is cathodic to steel. While the coating is intact it is an effective barrier (and, for food-contact applications, chemically inert and non-toxic, which is why it is used for cans). But at any break in the coating, the exposed steel becomes the small anode against the much larger cathodic tin surface — the unfavourable small-anode/large-cathode area ratio of Question 3(a) — so a scratch in tinplate corrodes the underlying steel faster than bare, uncoated steel would. Tinplate therefore relies entirely on maintaining an unbroken barrier (helped, in sealed can interiors, by the local absence of free oxygen, which suppresses the cathodic reaction tin would otherwise support); it has no self-healing capacity at a defect, unlike galvanizing.

(iv) Inhibitors. Corrosion inhibitors are chemical additions to the electrolyte itself, present at only small concentration, that adsorb onto the metal surface (or react to form a thin protective film) and suppress the anodic reaction, the cathodic reaction, or both. Anodic (passivating) inhibitors (e.g. chromates, nitrites, molybdates) promote or stabilize a passive oxide film and raise the local anodic reaction's activation barrier; cathodic inhibitors (e.g. zinc salts, certain phosphates) precipitate a film that blocks the cathodic sites (often suppressing oxygen reduction or hydrogen evolution directly); and organic/mixed inhibitors adsorb as a monolayer that blocks both reactions and physically screens the surface. Because inhibitors work by shifting the electrochemical kinetics rather than by forming a mechanically robust independent barrier, they are best suited to closed, monitorable systems (cooling water loops, boiler feedwater, oilfield produced water) where concentration can be maintained and the environment does not dilute or wash the inhibitor away, and a caution applies specifically to anodic (passivating) inhibitors: if under-dosed, they can passivate most of the surface while leaving small unprotected sites that become intensely localized anodes (an unfavourable area ratio again), making the attack worse than no inhibitor at all.

(v) Organic paints. Paint and polymer coatings protect primarily as a physical/chemical barrier, by presenting a continuous, low-permeability film that greatly slows the diffusion of water, oxygen and ions to the metal surface and increases the electrolytic resistance of any path between anodic and cathodic sites on the surface. Many practical paint systems add a second mechanism at the primer coat: zinc-rich primers behave like galvanizing (sacrificial, self-healing at breaks) and chromate- or phosphate-pigmented primers release a passivating inhibitor into any moisture that does penetrate the film (acting like (iv) locally). Because a pure barrier coating has no self-healing ability, its performance depends heavily on surface preparation, film thickness/continuity and resistance to abrasion, UV and cathodic disbondment, which is why organic coatings are very often specified together with cathodic protection (Question 6(b)) on buried and submerged steel rather than relied on alone.

6.2 — (b) Cathodic protection: common features, and sacrificial vs. impressed-current selection

Common features of any cathodic protection (CP) system. Every CP system works by the same principle established in Questions 1–3: it forces the structure to be protected to become entirely a cathode, by supplying enough external electron (DC) current to it that the structure's potential is polarized negative of its free-corrosion potential, ideally to or beyond the potential at which the anodic metal-dissolution reaction is thermodynamically or kinetically suppressed (commonly assessed against the $-850$ mV vs. Cu/CuSO₄ criterion for buried/immersed steel). Every installation therefore needs: an anode (or anode bed) that supplies the protective current and is itself consumed or otherwise able to sustain the reaction; a continuous, low-resistance metallic path from anode to structure; a continuous electrolyte (soil or water) connecting anode and structure; electrical continuity of the structure being protected (bonded joints, continuous rebar mat, etc., since an isolated, unbonded section receives no current); and reference electrodes plus periodic potential surveys to confirm the whole structure has actually reached the protection criterion, since both under-protection (attack continues) and over-protection (coating disbondment, hydrogen embrittlement of susceptible steels — the same over-current risk against SCC noted in Question 5(a)) are undesirable.

Sacrificial (galvanic) anode systems use a metal more active than the structure (zinc, magnesium, or aluminum alloy anodes for steel) connected directly to it; the anode's own free-corrosion driving force supplies the protective current, with no external power source, and the anode is consumed at a rate set by Faraday's law (directly analogous to Question 2(b)'s corrosion-rate calculation, run in reverse to size and predict anode life). Because the driving voltage is limited to the small potential difference between the anode metal and the structure (a few hundred millivolts to about a volt), the current output and throw (protective range) of a single anode are both modest, current output cannot be adjusted after installation, and the system is best suited to well-coated structures with a relatively low, uniform current demand and to sites without a convenient AC power supply, since it needs none.

Impressed-current systems use a DC rectifier connected to the local AC power grid (or a solar/other remote power source) to drive current from relatively inert, long-lived anodes (high-silicon cast iron, mixed-metal-oxide-coated titanium, graphite) through the electrolyte to the structure. Because the rectifier can supply a driving voltage of tens of volts, a single impressed-current anode bed can protect a much larger area or a poorly coated/bare structure, output current is adjustable to match seasonal or condition changes, and far fewer, more widely spaced anode installations are needed — at the cost of requiring a reliable power supply, periodic monitoring/maintenance of the rectifier, a real risk of over-protection or of interference (stray-current corrosion of nearby unrelated buried structures) if not carefully designed, and generally higher installation complexity and cost per site.

Remotely located buried pipeline — favours sacrificial anodes (though long pipelines often use both). A cross-country pipeline is typically well-coated (so total current demand per unit length is low), runs through remote terrain with no reliable AC power along most of its length, and its failure consequence (a long, hard-to-access, low-density asset) is best served by simple, low-maintenance, distributed protection that needs no operator intervention or grid connection — sacrificial magnesium (in low-conductivity soils, where magnesium's higher driving voltage overcomes the higher soil resistivity) or zinc anodes bonded periodically along the line are standard, sized by Faraday's-law consumption rate against the design life, exactly the calculation of Question 2(b). Impressed current is still commonly used at specific points (station yards, casings, higher-current-demand sections) where power is available, but is impractical as the sole method along tens or hundreds of kilometres with no grid access.

Urban concrete/rebar structure — favours impressed current. A reinforced-concrete structure in a city (a parking structure, a bridge deck) has grid power readily available on site, a large steel surface area (the full embedded rebar mat) with a current demand that is both larger and much less predictable than a coated pipeline's (concrete cover thickness, chloride ingress from de-icing salt, and carbonation all vary across the structure and change with time), and a strong incentive to be able to adjust the applied current as those conditions evolve and as periodic potential surveys are carried out — all of which favour an impressed-current system's higher available driving voltage, adjustability, and ability to protect a large, variably-demanding area from a modest number of anode installations (e.g. conductive coatings, titanium mesh, or discrete anodes embedded in the concrete cover). A galvanic system would need an impractically large mass and number of sacrificial anodes to deliver comparable current to a large rebar mat from the small driving voltage available, and would offer no way to compensate as demand grows with continuing chloride contamination — exactly the throw/adjustability limitation described above.