Orange spots under the balm, where water and fatty acids reached bare steel

Tinplate corrodes wherever water or fatty acids reach steel at a scratch or seam. How to spot it early, which tins are lined, and the storage humidity to hold.

Orange freckles appearing under the balm, or a brown halo around the rolled edge of a tin that looked perfect at filling, are not a defect in the balm. They are steel corroding at a place where the tin coating was never continuous, driven by water that should not have been in the package. This page covers where that water comes from, why an ageing oil phase makes the attack worse, and how to decide which of the affected units can be salvaged.

• Tin coating 1.1 to 11.2 g/m2 per side
• Critical humidity about 60%
• Needs liquid water
• Rusted tins are scrap

What a balm tin is actually made of

The slip-lid tin used for nearly every craft balm is electrolytic tinplate, not tin. It is low-carbon steel strip, usually 0.15 to 0.30 mm thick, given a thin electrodeposited tin layer on both faces, then passivated with a chromium treatment and coated with a trace of oil to help it through the press. The specifications are ASTM A623 in the United States and EN 10202 in Europe, and both express the tin as a coating weight per side rather than a thickness: commonly anywhere from 1.1 to 11.2 grams per square metre, which at tin's density of 7.3 g/cm3 works out at about 0.15 to 1.5 microns. Craft tins sit at the low end of that range.

Half a micron of anything is not a barrier. Electrodeposited tin at commercial coating weights is not pore free even before forming, and forming makes it worse: drawing the shell stretches the coating over the base radius, and curling the rim cracks it. Where a tin has a separate base disc joined by a rolled seam, the seam involves cut edges, and a cut edge is bare steel by definition. That is why rust appears at the underside radius, the rolled seam and the curled lip long before it appears in the middle of a flat wall.

Three further constructions turn up in balm packaging. Chromium and chromium-oxide coated steel, sold as ECCS or tin free steel, has almost no corrosion resistance of its own and is only used with an organic coating over it. Lacquered tinplate carries an internal film, historically epoxy-phenolic and increasingly polyester or acrylic where a supplier is avoiding bisphenol A, which is what actually keeps contents off the metal. And aluminium containers do not rust at all, because aluminium forms a self-repairing passive oxide, though they still pit in the presence of chlorides and are attacked by alkali. The formats and their trade-offs are compared in the packaging guide, and the tin against tube question in lip balm tube versus tin.

Corrosion needs liquid water, so where did it come from

Iron corrodes by an electrochemical reaction that requires an electrolyte. Without a film of liquid water on the surface there is no ion transport, no anode and cathode, and no rust, which is why oiled steel tools keep and why a properly anhydrous balm is chemically incapable of rusting a tin. Atmospheric corrosion of iron follows the same rule at a distance: below roughly 60 percent relative humidity the adsorbed water layer is too thin to support the reaction, and above it the rate climbs steeply. That 60 percent figure is the critical humidity, and it drops considerably when hygroscopic contamination such as chloride or sulphate is present on the surface, which is why coastal workshops and rooms that see salt or cleaning residues have more trouble.

There is one detail worth knowing because it inverts what people assume about tin. In a sealed, deaerated food can with acidic contents, tin is anodic to steel and sacrifices itself to protect the base metal, which is the whole basis of unlined food cans. In the presence of oxygen the polarity reverses, and tin becomes cathodic to steel. A cosmetic tin is open to air every time it is used, so it lives in the second case: at every pore and scratch, the small area of exposed steel is anodic against a large area of tin, and a small anode against a large cathode is the worst possible geometry. That is why the damage is intense and localised, appearing as discrete spots rather than a general dulling.

The five ways water gets into a dry product

Take these in order of how often they turn out to be the answer. The wider consequences of moisture in a dry product, corrosion aside, are in water contamination in balm.

Condensation from temperature cycling. The dominant cause and the most often missed. At 21 C and 60 percent relative humidity the dew point is about 13 C, so any surface colder than 13 C brought into that room grows a water film. A tin taken from a fridge, a cold delivery van, an unheated garage or an outside stall wets on both faces within seconds, and the film sits under the balm at the tin wall where nothing will evaporate it. Fridge storage of finished balm, recommended surprisingly often, is a reliable way to produce this fault.

Incompletely dried botanicals. Fresh or partly dried plant material carries a great deal of water, and an oil infusion made with it delivers that water into the product as fine droplets. This is the same root cause as the fault in mould in balm, and the drying and wilting protocol in herbal infused oils exists for exactly this reason. Honey and other aqueous additions do it deliberately: honey runs at roughly 17 to 20 percent water, so even a one percent addition puts measurable moisture into the package, as covered in honey in balms.

Containers washed and not dried. Rinsing tins before filling and leaving them upside down on a rack traps a bead of water in the rim curl that no visual check will find. If tins must be washed, dry them in an oven at 60 to 70 C for twenty minutes and fill them the same day.

Humid storage of empty stock. Empty tins corrode on the shelf, before they ever meet a balm. This produces the confusing case of a brand new tin with rust already in the seam.

Steam and splash in the workshop. A double boiler releases a great deal of water vapour, and empty tins laid out beside it collect it. The environmental controls in workshop hygiene and the more general treatment in moisture control cover the layout that avoids this.

Free fatty acids, and the loop that feeds itself

Water starts the corrosion. The oil phase decides how fast it proceeds. Triglycerides hydrolyse slowly to free fatty acids, a process measured as acid value in milligrams of potassium hydroxide per gram of fat, and the number rises over a product's life. Freshly refined oils typically test below 0.5 mg KOH/g, unrefined and cold-pressed oils often sit between 0.5 and 4, and home-made infusions made over plant material tend to be higher still, because the material contributes both moisture and lipase.

Free fatty acids are weak organic acids. In the presence of even a trace of water they lower the local pH at a pore, dissolve the protective oxide, and complex the iron that comes off as iron soaps, which removes the corrosion product from the surface instead of letting it build a passivating layer. The attack therefore accelerates rather than self-limits. Then the loop closes: dissolved iron is a strong pro-oxidant, catalysing the breakdown of lipid hydroperoxides at concentrations of a few parts per million, so the metal that the acid released speeds up the oxidation described in rancidity and oxidation, which generates more acid. A rusting tin and a rancid balm are usually the same event seen from two directions, which is why the two faults so often arrive together and why rancid balm is worth reading alongside this page.

Where to look first

Inspect in this order, because it separates a packaging problem from a product problem in about five minutes.

1. The underside of the tin. Turn it over. The drawn base radius and the stamped centre are where the tin coating was stretched thinnest, and external rust here points at storage humidity rather than at anything you put inside.
2. The rolled seam and the curled lip. Run a fingernail round the inside of the curl and along any base seam. Rust in the curl with a clean flat wall is a cut-edge or forming problem, and it is the classic signature of an unlined tin.
3. The balm surface against the wall. Look for orange staining that follows the wall line rather than sitting in the middle of the puck. Corrosion happens at metal, so stains that begin at the wall are rust and stains that begin in the middle are almost certainly something else.
4. The lid rim. Repeated opening scrapes the coating on the sealing surface, so an in-use tin can develop rust there that a retained sample never shows.
5. Unfilled stock from the same delivery, stored in the same room. This is the decisive test. If empty tins are also spotting, your storage conditions are the cause and no reformulation will help. If empty stock is clean and only filled units corrode, the water came in with the product.
6. Retained samples of the same batch in a different container. If the balm in a glass jar is clean and the same balm in tinplate is not, the balm is dry enough and the packaging is the variable.

Is it rust, or something that looks like it

The middle rows matter commercially. Orange balm in a tin is far more often pigment or an infused oil than corrosion, and adhesive bleed at the rim is dealt with under labels peeling off balm tins. Scrapping a batch on a visual impression is expensive, so confirm the pit before you write anything off.

Rescue what can be rescued

The container is not recoverable. There is no way to reinstate a tin coating in the field, and cleaning a rusted tin leaves bare steel that will corrode faster next time. Rusted tins are scrap, including the visually clean ones from the same shelf if you cannot tell them apart.

The balm is a separate judgement. If the corrosion is confined to the outside of the tin and the underside, and the product smells clean, the balm is unaffected: scoop it into new containers, or remelt the batch to about 70 C, strain it through a 200 micron mesh and repot. Remelting is safe for the balm itself. If rust has formed on the inside wall in contact with the product, discard it. The balm has been in contact with dissolved iron, which shortens its oxidative life whatever it smells like today, and you cannot see how far the staining runs into the fat.

If the rescued balm goes back on sale, shorten its stated best before rather than carrying the original date across, because the oxidative clock has been advanced by an unknown amount. How to arrive at a defensible date is set out in shelf life testing.

Storing empty tins so they survive to be filled

Most rust that appears on new stock was made on the shelf.

Hold the storage room below 60 percent relative humidity and check it with a hygrometer rather than by feel. A note taken twice a week is enough, and it is the most useful environmental measurement a small workshop can make.

Keep stock off concrete floors. A slab stays cooler than the room and wicks moisture from the ground, so a box sitting on it runs at a higher local humidity and a lower surface temperature than the air, which is the condensation case again. Shelving, or a pallet with 100 mm of clear air underneath, solves it, and keep boxes away from outside walls for the same reason.

Leave tins in their original polythene until the day of filling. The bag is a vapour barrier and costs nothing to keep.

Testing before a customer does it for you

Two honest caveats. Salt spray testing under ASTM B117 is the standard people reach for and it is the wrong tool here: it is a severe screening method for coatings in a chloride environment, and its correlation with service life in a neutral, largely dry cosmetic package is poor. And accelerated heat tests do not accelerate corrosion by the same factor as they accelerate oxidation, so a container that passes twelve weeks at 40 C has not demonstrated twelve months on a shelf. The limits of that reasoning are set out in accelerated ageing, and the fill-and-store method in packaging compatibility.

The decision rule, and what this page cannot settle

There is no published corrosion data for anhydrous cosmetic balms in tinplate. Everything above is standard packaging metallurgy and food-can corrosion science applied to a package nobody has formally studied in this use, so the mechanisms are solid and the timescales are not. Anyone quoting a number of months to first rust for a given tin and formula is guessing.

The rule that follows is conservative and cheap. If your product contains only refined or well-dried oils, waxes and butters, is filled into dry containers, and is stored and sold in a controlled room, unlined tinplate is fine and always has been. If it contains an infusion, a resin, honey, anything with a plant in it, or if it will go to outdoor markets, into cars and into cold rooms, specify a lacquered lining or move to aluminium and treat the extra cost as insurance. The sourcing question to put to a supplier is not "are these good quality tins" but "what is the coating weight, and is there an internal lacquer, and of what type", which is the sort of specification discussed in sourcing ingredients. A supplier who cannot answer is reselling a container they have not specified, and you are the one carrying the risk.

Reviewed and updated 6 September 2026. Spotted an error? Tell us and we will fix and log it.