The invisible shield
In guide 1 we watched iron hand its electrons to oxygen and dissolve into rust; in guide 2 the galvanic series ranked which metal loses that argument. So here is a puzzle. Chromium and aluminum are both reactive metals — they sit well toward the active, eager-to-corrode end of that ranking, more willing to give up electrons than iron is. By rights a chromium-rich or aluminum part should corrode briskly. Yet a stainless knife shrugs off lemon juice for years and an aluminum window frame stands outdoors for decades. The trick that rescues them is passivation: the metal grows a microscopically thin oxide skin that all but shuts down its own anode reaction. It is the single most useful idea in the whole story of metallic corrosion.
That skin is the passive film, and its magic is how it works rather than how thick it is. It is astonishingly thin — roughly 1 to 5 nm, a few dozen atoms deep — invisible, transparent, and welded tight to the metal. It does not protect by being armour; it protects by being a near-perfect barrier that starves the anodic dissolution, dropping the corrosion current by a factor of a thousand or more. Picture not a suit of plate mail but a coat of cling film so tight, so seamless, and so instantly re-forming that the water can barely reach the metal underneath to react with it. A passive film beats corrosion by choking off the reaction, not by outmuscling it.
The crucial property is self-healing. Scratch clean through the film to bare metal and, in air or aerated water, the fresh surface re-oxidizes and re-seals within moments — the scab that reforms the instant you pick at it. This is exactly why stainless survives scuffs, machining, and daily knocks that would ruin a coat of paint, which cannot heal itself (that contrast returns in guide 5). But note the fine print, because the whole second half of this guide hangs on it: the film only re-forms if oxygen is around to feed it. Starve the wound of oxygen and it stays open.
Active or passive: the polarization curve
Here is the honest paradox, and it is worth sitting with. Thermodynamically, chromium and aluminum genuinely want to corrode — their electrode potentials put them near the active end, more reactive than iron. So passivity is not the true immunity of a noble metal like gold, which resists because it hardly reacts at all. Passivity is a kinetic trick: the metal is desperate to react, and the film simply refuses to let the reaction get going. That distinction matters, because a thermodynamically driven process that is held back only by a barrier can always be un-held — knock the barrier away and the eagerness comes flooding back.
You can watch the barrier click into place. Imagine slowly pushing the metal's potential upward — driving it to be ever more anodic — while measuring the corrosion current. At first the current climbs with the potential: this is the active state, bare metal dissolving faster and faster. Then, at a particular potential, something remarkable happens — the current suddenly collapses by a factor of 10^3 to 10^4 as the film precipitates and seals the surface. The current stays down at a tiny passive current density, often around 1 microamp/cm^2, across a wide band of potential called the passive range. Push the potential much higher still and the current climbs again — the transpassive region, where the film breaks down or oxygen starts to evolve.
ANODIC POLARIZATION of a passivating metal (stainless, Al, Ti) E (potential, more anodic upward) ^ | TRANSPASSIVE .......... film breaks / O2 evolves | \ | PASSIVE range | | (film holds) ....... | i_pass ~ 1 uA/cm^2 | | (tiny, near-vertical line) | E_pp ------------. | | ACTIVE | | | (bare metal \ | | dissolving) \__| <- peak current i_crit | / +-------------------+----------------------------> i (log scale) Drive E up: current rises through the ACTIVE nose to i_crit, the film clicks shut, current COLLAPSES ~1000-10000x to i_pass, and stays low across a wide window. That low-current window is the whole reason a stainless steel barely corrodes.
The whole art of stainless, then, is to keep the metal sitting in that wide low-current passive window. The good news is that ordinary environments — air, aerated tap water — hold it there for free, with no effort from you. The sobering news is that a strongly reducing acid, or an oxygen-starved crevice, can drag the potential down into the active nose, and then the identical steel corrodes briskly. Passivity is a state the metal is in, not a permanent property it owns — and a metal can be knocked clean out of it.
Why stainless is stainless: the 12% rule
What separates a protective film from useless rust is the quality of the oxide. Plain iron's oxide — rust — is porous, loosely bonded, and flakes off, forever baring fresh metal to corrode again (why it fails is the subject of guide 4 and the Pilling-Bedworth ratio). Chromium's oxide, Cr2O3, is the opposite: dense, tightly adherent, and self-healing. Dissolve enough chromium into iron and the surface film becomes chromium-rich and protective instead of flaky red rust. That single swap is what turns ordinary steel into stainless steel.
The threshold is sharp and worth memorising: you need roughly 11 to 12% chromium dissolved in the iron before a continuous protective film forms. Below that, the film is patchy and the steel rusts; above it, the steel is stainless. That number is the definition. A worked feel for it: the workhorse 304 grade is nicknamed '18-8' — about 18% chromium and 8% nickel — comfortably over the line, while 316 adds 2 to 3% molybdenum to better shrug off chloride. One caveat to file away, because it is the whole crux of the last section: the chromium must be in solid solution — dissolved and free in the metal — to count. Chromium that is locked up in some other compound does nothing to protect the surface.
Aluminum and titanium play exactly the same game. Aluminum is even more reactive than chromium, yet an aluminum drinks can or window frame barely corrodes because a tight Al2O3 film seals it the instant it is exposed; anodizing simply grows that natural film deliberately thicker, for extra durability or colour. Titanium's TiO2 film is so stable and so kind to living tissue that ship fittings, chemical plant, and hip implants are made of it. The unifying lesson is a delightful inversion of intuition: all three are reactive metals that survive precisely by reacting instantly and sealing themselves. Passivity is not nobility — it is a reactive metal that has learned to wear its own oxide as armour. See aluminum alloys for how far that trick is pushed in practice.
When the shield fails: chloride, pits, and crevices
Because the passive film is only nanometers thick, it is locally fragile — and it has one great enemy above all others: the chloride ion. Chloride is small, aggressive, and everywhere — in seawater, road salt, swimming-pool water, coastal air, even sweat on your fingers. It adsorbs on a weak spot in the film and punches a tiny hole through it, exposing a speck of bare metal. Now the geometry turns vicious. That pinhole is a minute anode, and it is surrounded by the entire vast, still-passive surface acting as cathode. This is pitting corrosion.
Recall the worst-case area ratio from guide 2: a pinhead anode driving the corrosion for a whole plate of cathode concentrates all the damage into one deep, narrow pit. Inside the pit the chemistry turns autocatalytic — metal ions pile up, chloride migrates in to balance the charge, the trapped solution turns acidic, and the film cannot re-form because oxygen can no longer reach the bottom of the hole. So the pit drills relentlessly downward, often hidden under an innocent-looking surface, and can perforate a tube while 99% of the metal around it still looks mirror-bright. Crevice corrosion is the same drama in an occluded gap — under a gasket, a washer, a bolt head, or a surface deposit — where stagnant, oxygen-starved liquid strips the film and cannot let it heal. Both failures are the same story: the film goes locally, and the wound cannot breathe.
Sensitization: the welding trap
One failure mode deserves its own spotlight, because it is a manufacturing trap that catches good steel made careless by heat. Take an austenitic stainless like 304 and hold it in the 500 to 800 degrees C range — precisely the temperature that the band of metal just beside a weld passes through and lingers in as it cools. In that window, chromium and carbon combine into chromium carbide (Cr23C6), which precipitates along the grain boundaries — the mismatched seams where two crystal grains meet, like floor tiles laid at slightly different angles, always the favourite site for such particles to settle. This heat-caused change is called sensitization.
The damage is quiet and devastating. Because that carbide is chromium-rich, forming it robs the thin band of metal right beside each boundary of its chromium, dragging it below the 12% threshold — remember, only chromium in solid solution protects. Those chromium-depleted ribbons lose their passivity while the grain interiors stay safely protected, so corrosion carves narrow channels along every grain boundary and the steel can literally fall apart grain by grain. Because it appears a short distance from a weld, it earned the nickname 'weld decay.' This is precisely the intergranular corrosion named in guide 2 — now explained down to its mechanism.
- Pick a low-carbon grade — 304L or 316L, carbon under 0.03% — so there is barely any carbon to feed the carbides, and few can form.
- Or choose a stabilized grade — 321 (with titanium) or 347 (with niobium) — whose additions grab the carbon first, leaving the chromium in solution to protect the surface.
- After any high-temperature step, solution-anneal at about 1050 degrees C and quench fast, re-dissolving the carbides before they can settle back at the boundaries.
- For chloride service, step up in alloy — molybdenum-bearing 316, or a duplex or high-nickel grade — to widen the passive window and resist pitting.
- Finally, passivate the finished part and keep its surface clean, aerated, and free of embedded iron — the film heals itself only if you let oxygen reach it.
Step back and the whole picture snaps into focus. Passivation is why an entire class of reactive metals — stainless steel, aluminum, titanium — outlives its lowly place in the galvanic series, and it is a genuinely different defence from the coatings and sacrificial anodes of guide 5, because the metal grows this armour itself and heals it itself. But keep it honest to the end: the film is a nanometer-thin, kinetic barrier stretched over a metal that still wants to corrode, and chloride, crevices, reducing acids, and sensitization can each breach it. Design for passivity, then respect its limits — choose the right grade, keep oxygen on the surface, and keep chloride and tight crevices away. The next guide leaves water behind entirely and meets the enemy that is red-hot air itself: high-temperature oxidation and the growth of scale.