21-Mat-A7 Environmental Degradation of Materials · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2018 — 10-Met-A7, Corrosion and Oxidation. Three hours, open book, approved Casio/Sharp calculator only. Eight questions of 20 marks each; the rubric states that the first five questions as they appear in the answer book constitute a complete paper (100 marks). All eight are answered here, because this set is a study resource rather than an exam script. The rubric also flags that answers take one of three forms — essay, calculation, or a comparison table — and marks clarity and organisation accordingly.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
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.
Given. Two entries already filled (Saturated Calomel at $+0.241$ V vs SHE; Ag-AgCl at $+0.222$ V vs SHE, biological media) and the Standard Hydrogen half-reaction $2H^++2e^-=H_2$.
| Name | Half-cell reaction | Potential (V vs. SHE) | Applicable environment |
|---|---|---|---|
| Saturated Calomel (SCE) | $Hg_2Cl_2+2e^-=2Hg+2Cl^-$ | +0.241 | General laboratory/field use; saturated KCl filling solution |
| Standard Hydrogen (SHE) | $2H^++2e^-=H_2$ | 0.000 (by definition) | Primary reference standard; impractical for routine field use (needs $H_2$ gas bubbling at 1 atm, $[H^+]=1$) |
| Ag-AgCl | $AgCl+e^-=Ag+Cl^-$ | +0.222 | Biological media; seawater and marine structures (robust, non-toxic filling solution) |
Three features the galvanic series has that the standard EMF/electrochemical series lacks:
Crevice corrosion begins in a narrow, stagnant gap (a gasket face, a lap joint, under a deposit or fastener head) where the electrolyte inside the crevice cannot exchange freely with the bulk solution. Initially the metal surfaces inside and outside the crevice corrode uniformly and at the same low rate, both supported by oxygen reduction as the cathodic reaction. Because the crevice geometry restricts diffusion, however, the small volume of oxygen trapped inside is consumed by that corrosion reaction faster than it can be replenished from the bulk, so the oxygen concentration inside the crevice falls essentially to zero while the bulk surface remains fully aerated.
Once oxygen is depleted inside the crevice, the cathodic reaction there shuts down, but metal dissolution (the anodic reaction, e.g. $M\rightarrow M^{n+}+ne^-$) does not require oxygen and continues. The crevice interior is now a net anode, electrically coupled to the still-cathodically-active, oxygen-rich external surface, which is a much larger area — an unfavourable small-anode/large-cathode geometry that concentrates the corrosion current into the small crevice area. The metal cations produced inside the crevice hydrolyse in water, $M^{n+}+nH_2O\rightarrow M(OH)_n+nH^+$, which acidifies the trapped solution. To preserve local electroneutrality against the rising positive charge from both the metal cations and the new $H^+$, aggressive anions — chloride in particular — migrate into the crevice from the bulk solution, driven by the electric field set up between the anodic crevice and the cathodic external surface.
The result is a crevice electrolyte that becomes progressively more acidic and more concentrated in chloride than the bulk solution, and both changes make the metal dissolve faster inside the crevice, which produces still more metal ions, still more acid, and still more chloride migration — each cycle of the reaction makes the local conditions more aggressive and further accelerates the very same reaction that produced them. This self-reinforcing, ever-worsening feedback loop — the products of the reaction (H$^+$ and concentrated Cl$^-$) are also the reaction's own accelerants — is precisely what "autocatalytic" describes, and it is why crevice corrosion, once initiated, propagates at a rate that keeps increasing rather than settling to a steady value, unlike general uniform corrosion.
