Question 7 of 7: Corrosion — True/False Analysis, Galvanized Steel, and Weld Decay in Stainless Steel

Question 7: Corrosion — True/False Analysis, Galvanized Steel, and Weld Decay in 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. Four T/F statements on general corrosion behaviour; a galvanized-vs-bare steel car-body corrosion scenario in NaCl; an 18Cr-8Ni-0.08C austenitic stainless steel (Type-304-like) welded structure showing localized attack $\tfrac14$ in from the weld.

Find. (a) True/false with justification for each. (b) The anodic/cathodic electron equations before and after galvanizing. (c) The corrosion mechanism, its equations, why the "stainless" steel corrodes, and a composition-and-paint-free fix.

Approach

All three parts are conceptual corrosion-electrochemistry questions, answered from the standard galvanic-series, passivation, and sensitization (weld-decay) mechanisms — no numeric calculation is required.

(a)(i) TRUE. For metals/alloys capable of passivation (stainless steels, aluminum, titanium), increasing the concentration of an oxidizing acid can shift the electrode potential into the passive region of the polarization curve, where a stable, protective oxide film forms and the corrosion current actually drops despite the higher oxidizer concentration — the classic active–passive transition. This is why concentrated nitric acid is often handled in stainless steel, while dilute nitric acid (active region) attacks it far more aggressively.

(a)(ii) TRUE. Grain boundaries are higher-energy, more disordered, more reactive regions of the microstructure than the grain interiors. During metallographic etching (a controlled galvanic-corrosion process), the grain boundaries act as the local anode and are preferentially dissolved, which is exactly why they appear as dark, recessed lines under the microscope — the etch itself is evidence of the grain boundary's anodic character.

(a)(iii) FALSE. Dissolved oxygen is the principal cathodic reactant for iron corroding in near-neutral water (oxygen-reduction reaction, $O_2+2H_2O+4e^-\to4OH^-$); more dissolved oxygen supports a faster cathodic reaction and therefore a faster, not unaffected, overall corrosion rate. This is also the basis of differential-aeration (concentration-cell) corrosion, where a poorly-aerated region becomes anodic relative to a well-aerated one.

(a)(iv) FALSE (the statement has the comparison backwards). Galvanic-couple severity depends on the anode-to-cathode area ratio: a small anode paired with a large cathode corrodes rapidly, while a small cathode paired with a large anode is comparatively unaffected. Aluminum is more active (anodic) than steel in the galvanic series, so aluminum rivets in a steel structure make the small rivets the anode against a large steel cathode — a small-anode/large-cathode pairing that corrodes the rivets rapidly and gives them a short life. Steel rivets in an aluminum structure reverse the roles: the small steel rivets become the protected cathode (they do not corrode at all), while the surrounding aluminum structure slowly corrodes instead. Steel rivets in an aluminum structure therefore last longer than aluminum rivets in a steel structure — the opposite of what the statement claims.

(b) Galvanized vs. ordinary steel in NaCl. Ordinary (bare) steel: the iron itself is directly attacked — anodic reaction $\text{Fe}\to\text{Fe}^{2+}+2e^-$, balanced by the cathodic oxygen-reduction reaction on adjacent iron surface, $O_2+2H_2O+4e^-\to4OH^-$ (near-neutral, aerated NaCl solution). Galvanized steel: zinc is more active (more anodic) than iron, so once the zinc coating is present, the zinc becomes the anode instead — $\text{Zn}\to\text{Zn}^{2+}+2e^-$ — while the same oxygen-reduction reaction, $O_2+2H_2O+4e^-\to4OH^-$, now occurs on the (cathodically protected) steel underneath. The zinc coating sacrifices itself so the underlying steel does not corrode, even where the coating is scratched through to bare steel, as long as zinc remains in electrical contact nearby.

(c)(i) Ion-electron equations for the weld-decay attack. In the chromium-depleted zone next to the weld (see (ii) below), the depleted material behaves electrochemically much like a plain low-alloy steel: anodic dissolution of the depleted matrix, $\text{Fe}\to\text{Fe}^{2+}+2e^-$ (with some $\text{Cr}\to\text{Cr}^{3+}+3e^-$ contribution from the remaining chromium), balanced by cathodic oxygen reduction on the adjacent, still-passive/Cr-rich material, $O_2+2H_2O+4e^-\to4OH^-$.

(c)(ii) Why the stainless steel is not "stainless" here (weld decay / sensitization). During welding, the region roughly $\tfrac14$ in from the weld (the heat-affected zone) is held for a period in the sensitization range, $450$–$850^{\circ}$C, as heat conducts away from the weld pool. In that temperature range, chromium diffuses to the grain boundaries and precipitates as chromium carbide, $\text{Cr}_{23}\text{C}_6$, which strips the immediately adjacent grain-boundary region of chromium, often to below the roughly $12\%$ minimum needed to sustain the passive Cr$_2$O$_3$ film. That narrow, chromium-depleted band corrodes preferentially (intergranularly) even though the bulk alloy, at its nominal $18\%$ Cr, is genuinely stainless — the alloy is "not stainless" locally because welding has locally destroyed the very chromium enrichment that makes it stainless.

(c)(iii) Prevention without changing composition or painting. Since switching to a low-carbon (304L/316L) or stabilized (321/347, Ti- or Nb-stabilized) grade would change the alloy's composition, and painting is explicitly excluded, the fix must be a post-weld solution anneal: reheating the welded assembly above roughly $1000$–$1100^{\circ}$C (redissolving the chromium carbides back into solid solution) and then rapidly quenching it through the $450$–$850^{\circ}$C sensitization range, so there is no time for the carbides to re-precipitate at the grain boundaries. This restores the chromium to solid solution everywhere, without altering the alloy's nominal composition and without any coating.