Question
Stainless steel (an alloy of iron with chromium and nickel) doesn’t rust under conditions that would severely corrode pure iron. Use d-block chemistry to explain: (a) why chromium prevents rusting, (b) why iron alone rusts so easily, and (c) why pure chromium is even more corrosion-resistant than stainless steel.
Solution — Step by Step
Iron has the electron configuration . It readily loses 2 or 3 electrons to form Fe or Fe. In moist air, the redox couple Fe/Fe (E° = −0.44 V) means iron is easily oxidized by atmospheric O.
The product FeO·xHO (rust) is flaky and porous — it doesn’t adhere to the iron surface. Fresh iron is constantly exposed to oxygen, so rusting continues until the entire piece is consumed.
Chromium () also oxidizes — but the oxide formed (CrO) is dense, transparent, adherent, and impermeable. Once a thin layer (a few nanometres) forms on the surface, it acts as a barrier preventing further oxidation. This is called passivation.
In stainless steel (typically 10–20% Cr), the chromium oxide layer covers the entire surface, protecting the underlying iron from oxygen and water.
Pure chromium has 100% chromium, so the passivation layer is uniform and dense everywhere. In stainless steel, regions with low Cr content (or grain boundaries) can be vulnerable. So chromium plating gives better protection than chromium alloying.
This is why bathroom fixtures, car bumpers, and laboratory tools are often “chrome-plated” — a thin (1–10 μm) layer of chromium gives them shiny, corrosion-free finishes.
Many d-block metals (Cr, Ni, Ti, Al though it is a p-block metal) form passivating oxides. Iron does not — the difference is the structural compatibility of the oxide lattice with the underlying metal lattice. Pilling-Bedworth ratio quantifies this: ratios near 1 give protective oxides; far from 1 give flaky oxides.
Why This Works
Passivation depends on the oxide being:
- Dense and non-porous (no oxygen can penetrate).
- Adherent (sticks to the metal, doesn’t flake off).
- Insulating (no electron transport to allow further oxidation).
CrO satisfies all three. FeO satisfies none. The choice of alloying element to confer corrosion resistance is essentially “find a passivating oxide that bonds well to the host metal.”
Three corrosion-resistance principles for d-block metals:
- Cr passivates. This is why stainless steel is protected.
- Ni gives strength + corrosion resistance. Hence Cr-Ni alloys.
- Mo improves pitting resistance. Hence “Type 316 stainless” containing Mo.
Alternative Method
Look up standard reduction potentials. Cr/Cr is −0.74 V (more easily oxidized than Fe), but the kinetics are dominated by the oxide layer. Thermodynamics says Cr “wants” to oxidize more, but kinetics says it can’t — the oxide layer is too good an insulator.
Students think corrosion resistance is purely about reduction potentials (thermodynamics). No — kinetic barriers (passivation) often dominate. Aluminium’s standard potential is even more negative than chromium’s, but it doesn’t corrode in air because AlO also passivates.
Final answer: Stainless steel resists rust because chromium forms a dense, adherent CrO passivating layer that blocks further oxidation. Pure chromium does this even better. Iron alone forms flaky FeO that doesn’t protect the underlying metal.