Question 7 of 8: Corrosion Statements; Galvanized Steel; Weld-Zone Attack on Stainless Steel

Question 7: Corrosion Statements; Galvanized Steel; Weld-Zone Attack on Stainless Steel (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.

Given. (b) Ordinary vs. Zn-galvanized steel sheet, dilute NaCl electrolyte. (c) 304-type austenitic stainless steel (18Cr-8Ni, 0.08%C), welded, corrosion observed 1/4 in from the weld.

Find. (a) Correct/incorrect + justification for each statement. (b) Ion-electron (half-cell) equations before/after galvanizing. (c) Half-cell equations, cause of attack, and a non-compositional, non-coating fix.

1. (a)(i) Correct. A concentrated oxidizing acid can drive certain active-passive metals (Fe, Cr, Ni, Ti, stainless steels) into their passive state by building a thin, dense, adherent oxide film that dramatically lowers the corrosion current — the classic example is iron in concentrated nitric acid, which is far less aggressive toward iron than dilute nitric acid because the concentrated acid passivates the surface rather than dissolving it actively.
2. (a)(ii) Correct. Grain boundaries are regions of higher lattice energy (disorder, segregated impurities, higher dislocation density) than the grain interiors, making them chemically more reactive/anodic relative to the grain interiors (cathodic) during etching — this differential (galvanic microcell) attack is precisely the mechanism that reveals the grain structure under the microscope; without this anodic/cathodic contrast, etching would not selectively darken the boundaries.
3. (a)(iii) Incorrect. Dissolved oxygen is essential to the dominant corrosion mechanism for iron/steel in near-neutral water: it is the oxygen-reduction reaction, $O_2+2H_2O+4e^-\rightarrow4OH^-$, that serves as the cathodic reaction consuming the electrons released by iron dissolution ($\text{Fe}\rightarrow\text{Fe}^{2+}+2e^-$); removing dissolved oxygen (deaeration) is in fact one of the standard practical methods of suppressing corrosion in near-neutral aqueous systems (e.g. boiler feedwater treatment), which is direct evidence that oxygen strongly affects, rather than has "no effect on," the corrosion rate.
4. (a)(iv) Incorrect. Aluminum is anodic (less noble) relative to steel in the galvanic series, so an Al rivet joining a large steel structure sets up a small-anode/large-cathode galvanic couple that corrodes the aluminum rivet rapidly (the area effect concentrates essentially all the corrosion current onto the small anodic rivet area). The reverse pairing (steel rivets in an aluminum structure) makes the small steel rivet the cathode and the large aluminum structure the anode — the rivet itself is well protected (cathodically), and attack is instead spread thinly over the much larger aluminum area. The statement has the comparison backwards: steel rivets in aluminum last longer (as rivets) than aluminum rivets in steel, not the other way around.
5. (b) Ordinary (bare) steel in dilute NaCl. Anodic (at bare-metal scratches/defects): $$\text{Fe}\rightarrow\text{Fe}^{2+}+2e^-.$$ Cathodic (oxygen reduction, near-neutral aerated solution): $$O_2+2H_2O+4e^-\rightarrow4OH^-.$$ Once bare steel is scratched, the exposed iron itself becomes the anode and corrodes directly — there is no sacrificial protection.
6. Galvanised (Zn-coated) steel in dilute NaCl. Zinc is anodic to iron, so once the coating is scratched and both metals are exposed to the electrolyte, zinc becomes the anode and is preferentially consumed, while the exposed steel is cathodically protected: $$\text{Zn}\rightarrow\text{Zn}^{2+}+2e^- \quad(\text{anodic, at the zinc}),$$ $$O_2+2H_2O+4e^-\rightarrow4OH^-\quad(\text{cathodic, now at the protected steel surface}).$$ This is the entire point of galvanizing: even after the coating is locally breached, the steel underneath is protected sacrificially rather than corroding itself.
7. (c)(i) Weld-zone ion-electron equations. The attack occurs in the base metal adjacent to (not in) the weld, in a chromium-depleted band. There, the depleted (low-Cr) steel is anodic and dissolves: $$\text{Fe}\rightarrow\text{Fe}^{2+}+2e^- \quad(\text{anodic, chromium-depleted band}),$$ with oxygen reduction as the cathodic reaction (occurring preferentially on the still fully passive, Cr-rich weld metal and unaffected base metal): $$O_2+2H_2O+4e^-\rightarrow4OH^-\quad(\text{cathodic, passive Cr-rich regions}).$$
8. (c)(ii) Why the "stainless" steel is not stainless: weld decay / sensitization. During welding, the heat-affected zone (HAZ) adjacent to the weld dwells for a time in the $450$–$850^\circ$C sensitization range. In this range, carbon (present at 0.08%, above the very low threshold needed for this to occur) diffuses to and precipitates as chromium carbide ($\text{Cr}_{23}\text{C}_6$) along the austenite grain boundaries. Because chromium diffuses far more slowly than carbon, the carbide-forming reaction locally strips chromium from a narrow band of matrix immediately adjacent to each grain boundary, pulling its local Cr content below the ≈12% minimum needed to sustain the passive (chromium-oxide) film. This narrow, Cr-depleted, no-longer-passive band — sitting in the HAZ a short distance from the weld (the ¼ in reported here, consistent with where the HAZ cooled through the sensitizing range for the longest time) — is what corrodes preferentially: the alloy is "stainless" everywhere except this locally depleted band.
9. (c)(iii) Prevention without changing composition or painting. Since the composition (and hence carbide-forming tendency) cannot change, the fix must remove the metallurgical condition that caused sensitization: a post-weld solution anneal — reheating the welded assembly to ≈1050–1100°C (well above the carbide solvus) to redissolve the grain-boundary $\text{Cr}_{23}\text{C}_6$ back into solid solution, followed by a rapid quench through the 450–850°C sensitization range so the carbides do not have time to reprecipitate. This restores a uniform, fully passivating Cr distribution everywhere, including the former HAZ, without altering the alloy's composition or applying any coating/paint.