Question 6 of 7: Corrosion-Rate Reasoning; Weld Sensitization; Concentration Cells

Question 6: Corrosion-Rate Reasoning; Weld Sensitization; Concentration 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.

Given. Iron specimen in tap water; five separate modifications to consider (a); stainless steel welding scenario (b); an oxygen (differential-aeration) concentration cell (c).

Find. (a)(i)–(v) increase/decrease + reason. (b) Mechanism of weld-zone corrosion susceptibility. (c) Why the low-concentration region corrodes.

Approach

Each part is explained from first-principles electrochemistry: corrosion rate tracks how easily the anodic (metal dissolution) reaction can proceed and how conductive/aggressive the electrolyte is; galvanic coupling and impressed current are judged by relative nobility and the direction of forced electron flow; the sensitized-weld and concentration-cell questions both turn on local depletion — of chromium in one case, of dissolved oxygen in the other.

1. (i) Adding NaCl. Increases the corrosion rate. Dissolved Cl$^-$ raises the electrolyte’s ionic conductivity (lowering solution resistance and allowing more corrosion current to flow between anodic and cathodic sites) and, being a small, aggressive anion, can locally penetrate/break down passive oxide films, further accelerating attack (a mechanism underlying pitting corrosion).
2. (ii) Dry-cell-imposed electron flow into the iron. Decreases the corrosion rate. Forcing electrons into the iron makes it more cathodic, suppressing the anodic (metal-dissolution, Fe→Fe$^{2+}$+2e$^-$) reaction and instead favouring cathodic reactions (e.g. O$_2$ or H$^+$ reduction) at its surface — this is exactly the principle of impressed-current cathodic protection.
3. (iii) Placing nickel in contact. Increases the corrosion rate of the iron. Ni is more noble (more cathodic) than Fe in the galvanic series; the couple forces Fe to become the anode of a galvanic cell, so it corrodes preferentially and at an accelerated rate (galvanic corrosion), while the nickel is protected.
4. (iv) Adding chromate ion. Decreases the corrosion rate. Chromate is a classic anodic (passivating) inhibitor: it forms a stable, adherent, protective oxide film on the iron surface that blocks the anodic dissolution reaction.
5. (v) Freezing the water. Decreases the corrosion rate sharply. Corrosion is an electrochemical process requiring ion transport through a liquid electrolyte; once frozen, ionic mobility (and dissolved-oxygen transport to the metal surface) is essentially halted, starving both the anodic and cathodic half-reactions.
6. (b) Post-weld stainless steel corrosion (sensitization). In the heat-affected zone (HAZ), material briefly passes through $\approx450$–$850^\circ$C during welding — exactly the range where chromium carbides (Cr$_{23}$C$_6$) precipitate preferentially at grain boundaries. Because chromium diffuses to the boundary faster than it can be replenished from the grain interior, a thin zone immediately adjacent to each boundary is left depleted below the $\approx11$–$12\%$ Cr needed to sustain the passive Cr$_2$O$_3$ film. These Cr-depleted zones become anodic relative to the (still Cr-rich, still passive) grain interiors, producing severe, highly localized intergranular corrosion (weld decay). It is mitigated by using low-carbon grades (304L/316L, too little carbon to deplete the boundaries significantly) or stabilized grades (321/347, alloyed with Ti/Nb that preferentially tie up carbon before Cr-carbide formation).
7. (c) Concentration (differential-aeration) cells. Where dissolved oxygen concentration is higher, the cathodic reaction O$_2$+2H$_2$O+4e$^-\to$4OH$^-$ proceeds readily, holding that region at a more positive (noble) potential; where oxygen is depleted (under debris, in a crevice, beneath a rust deposit, or below the waterline), that reaction starves, leaving the local potential more negative (active). The resulting potential difference drives current through the electrolyte from the low-O$_2$ region (anode, where metal dissolves) to the high-O$_2$ region (cathode) — so it is specifically the oxygen-starved region that corrodes, exactly the mechanism behind crevice corrosion and corrosion under deposits.