Zinc vs Tin

Both metals spend most of their working lives as a thin layer on top of steel, and they protect it by opposite mechanisms. Zinc is electrochemically less noble than iron, so it corrodes in the steel's place. Tin is more noble than iron, so it protects only by covering — and where the cover is broken, it makes things worse.

If a coating will be scratched, dented or drilled, zinc. If it will stay intact and must be food-safe and solderable, tin. Then there is the exception that swallows the rule, which is what happens inside a sealed can.

Zinc corrodes on purpose

Galvanising works because zinc gives itself up. Couple zinc to steel in the presence of moisture and the zinc becomes the anode of the cell; it dissolves and the steel does not. A galvanised gutter with a saw cut through the coating still has protected steel at the cut face, because current flows from the surrounding zinc to the exposed iron. The protection reaches across a gap of a few millimetres, which is exactly the scale of ordinary handling damage.

That mechanism sets how galvanised coatings are specified. Life is very nearly proportional to coating thickness and inversely proportional to how aggressive the atmosphere is, so specifications are written in microns or grams per square metre against a corrosivity category rather than in years. The same principle, scaled up, gives the sacrificial anodes bolted to ship hulls, buried pipelines and the inside of domestic water heaters — lumps of zinc whose entire purpose is to be consumed.

The polarity reverses inside a sealed can

Tinplate is a very thin electrodeposited film — often under a micrometre — on steel strip. In open air with oxygen present, tin sits on the noble side of iron, so a scratch creates a small exposed anode connected to a large cathodic area, which is the worst possible geometry. The steel under a scratched tin coating rusts faster than bare steel would. Anyone who has watched a dented can rust from the ding outward has seen it.

Inside a sealed can of acidic food, the arrangement inverts. Oxygen is scavenged during processing, and the organic acids in fruit — citrate, malate — form complexes with tin(II) ions that pull tin's effective potential below iron's. Tin becomes the sacrificial partner and slowly dissolves into the contents while the steel stays intact. That reversal is why plain, unlacquered tinplate was a workable container for acidic products for well over a century, and it is also why food regulations set a limit on tin concentration in canned goods. Most modern cans are lacquered internally anyway, and a large share of food packaging has moved to chromium-coated tin-free steel or to aluminum.

Tin is the metal that joins things

Tin melts at a little over 230 °C, low enough that alloys of it wet and bond metals without damaging what they are attached to, and that is the basis of its largest single market.

The eutectic tin-lead solder that the electronics industry used for decades melts at 183 °C. European RoHS rules took lead out of most consumer electronics from July 2006, and the replacement — typically 96.5% tin with small additions of silver and copper — melts around 217 to 220 °C. Thirty-odd degrees does not sound like much until it is applied to every component and every laminate on a board, and the reliability consequences of that shift occupied the industry for years.

Whiskers, and the price of removing lead

Lead was doing something else in solder that nobody had properly appreciated: it suppressed whiskers.

Left to itself over months or years, a pure tin plating pushes thin single crystals up out of the surface, driven by compressive stress locked inside the deposit. They can reach millimetres in length and a few micrometres across, and they conduct. In fine-pitch electronics a whisker is a short circuit that appears years after manufacture, passes every test the board ever had, and leaves almost no evidence once the current vaporises it.

The failure record is real and expensive: whiskers have been implicated in satellite losses, including the 1998 failure of the Galaxy IV communications satellite, and in relay failures at nuclear plants. NASA maintains a public database of the cases. The mitigations — alloying the plating, adding a nickel underlayer, conformal coating, annealing — are managed rather than solved. Zinc plating grows whiskers too, but is used in applications with millimetres of clearance rather than micrometres.

Tin pest, and two stories that are probably not true

Below 13.2 °C the ordinary metallic form of tin is thermodynamically unstable and converts to a grey, brittle, semiconducting form with a diamond-cubic structure and about a quarter more volume. The metal crumbles. It is called tin pest, and it is genuinely observed — though the transformation needs nucleation, runs fastest well below freezing, and is strongly suppressed by small amounts of bismuth, antimony or lead.

Two famous anecdotes are attached to it and neither survives scrutiny. The tin buttons of Napoleon's army are said to have disintegrated during the retreat from Moscow in 1812; no surviving button from the campaign shows the transformation, and much of the army's clothing used bone or brass fasteners. Tin-soldered fuel cans are said to have failed on Scott's Antarctic expedition in 1912; the leaking cans are documented, but the cause has never been established and several plainer explanations fit. Both stories are repeated as fact far more often than the evidence supports, and the honest version is that tin pest is real and these two cases are folklore.

Brass, bronze, zamak and pewter

Alloyed, the two metals part company completely.

Zinc's great alloy is brass — copper with typically a third of its mass as zinc — and its great manufacturing niche is die casting. Zamak alloys melt low enough for hot-chamber machines, which gives die lives measured in hundreds of thousands of shots, wall sections thinner than aluminum can hold, and surface detail crisp enough to plate directly. Locks, zips, carburettor bodies and model cars are zinc for that reason rather than for any property of the metal in service.

Tin's alloy is bronze, and it was scarce enough in the ancient world to shape trade routes across Europe and Asia, because copper is common and tin is not. Its modern casting role is pewter and organ pipes. Tin's oddest bulk use is not an alloy at all: the float glass process runs a ribbon of molten glass across a bath of molten tin to make it flat, and a substantial fraction of the world's tin sits in those baths.

Both metals also live in chemistry. Organotin compounds are the dominant heat stabilisers for PVC, and tributyltin was the most effective antifouling paint ever formulated before it was banned worldwide, having been found to cause reproductive damage in marine snails at concentrations of nanograms per litre. Zinc oxide is essential to vulcanising rubber and is the mineral in physical sunscreens.

Biologically the two are not comparable. Zinc is an essential element — the catalytic ion in carbonic anhydrase, the structural ion holding zinc-finger proteins in shape, and a few grams of it are distributed through your body. Tin has no established role in human biology.

Scale, price and where they come from

Zinc is produced at something over thirteen million tonnes a year, mostly as a co-product of lead-zinc sulfide mining, with China, Peru and Australia leading. Tin production is under four hundred thousand tonnes — a thirty-fold difference in scale — and it trades at roughly ten times zinc's price per tonne.

Tin's supply is also more fragile and more politically exposed. Production is concentrated in China, Indonesia, Myanmar, Peru and the Democratic Republic of the Congo; a mining suspension in Myanmar's Wa region in 2023 moved the world price on its own, and tin is one of the four designated conflict minerals subject to supply-chain due diligence rules.

Coating, casting or joining

• Protecting steel that will be scratched or cut — zinc, for the sacrificial action.
• Protecting steel inside a sealed acidic container — tin, which reverses polarity and protects there.
• A die casting with fine detail and long tool life — zinc alloy.
• Joining two metals below 250 °C — tin, in an alloy chosen for the temperature you can afford.
• Fine-pitch electronics with a long design life — tin, but with whisker mitigation written into the specification.
• Anything that will be held below freezing for years in pure form — not unalloyed tin.