A phosphate coating is a porous layer; protection is completed by the soap or oil that fills its pores. A phosphated coil can therefore rust when stored badly even with a sound coating. We look at the causes, the condensation mechanism and stock conditions.
One of the most frequent complaints in the field is this: “we had it phosphated, but the wire rusted in the store.” Behind it there is usually not a poor coating but a gap in expectations about what the phosphate layer is for. Phosphate coating is described as a corrosion barrier, but on drawing lines its real function is something else, and its protective property is limited on its own.
This article covers why and how much the phosphate layer protects, the real causes of rusting, how condensation works, and how to set up stock conditions.
A zinc phosphate layer consists of crystals growing out of the metal surface and is porous by nature. That porosity is not a defect but the layer's real function: it takes drawing soap and oil into itself and holds them at the surface. We explain this in what is phosphating.
The same porosity works against you on the corrosion side. If the voids are left empty, moisture and oxygen can reach the metal directly. In other words the phosphate layer is not a closed barrier keeping water out on its own; most of the protection comes from the second layer filling the pores — soap or protective oil. What the phosphate contributes is holding that film at the surface and protecting for far longer than bare metal would.
Rusting is not an accident; almost every case met in the field reduces to a handful of known causes.
Condensation arises because the moisture air can carry depends on temperature. The moment the surface temperature falls below the dew point of the ambient air, water becomes liquid on the surface. This needs no rain or splashing; it happens in an enclosed store too.
In practice the riskiest scenario is this: a coil that cooled overnight is left in an environment that warms up and gains humidity in the morning. Because the mass of steel heats slowly, the surface stays colder than the air for a while, and water condenses on it in exactly that window. The same happens when a coil left outside in cold weather is brought into a heated indoor space.
| Condition | Recommendation | Rationale |
|---|---|---|
| Environment | Enclosed, free of rain and dust | Direct water contact ends the protection quickly |
| Relative humidity | Below 60% as far as possible | High humidity increases the risk of condensation |
| Temperature | An environment with low swing | Water condenses on a surface falling below the dew point |
| Floor | On pallets, no contact with the floor | Moisture rising from concrete hits the lower windings |
| Protective film | Soap or protective oil applied | If the pores are left empty the metal is effectively exposed |
| Packaging | VCI or breathable wrapping onto a dry surface | Wrapping a wet surface does not protect, it traps |
| Stock rotation | First in, first out (FIFO) | The oldest coil is the riskiest coil |
We have deliberately not written “shelf life of so many months” in the table: the time phosphated semi-finished product will last is not a single number. The same coating can sit for months without trouble in a dry store at steady temperature, while by the sea, in a semi-open area whose temperature swings through the day, the surface can go within weeks. The sound approach is to derive your own time band through periodic surface checks in your own store.
Shelf life is a property of the store, not of the coating. The same coil lives two different lives in two different stores.
| Symptom | Likely cause | First check |
|---|---|---|
| Widespread rust on the outer windings of the coil | Ambient humidity, condensation, no packaging | Store humidity and the sequence of packaging |
| Rust on inner windings and between windings | Inadequate drying, water trapped | Drying temperature and line speed |
| Scattered spot rust marks | Inadequate rinsing, acid residue | Rinse water conductivity and refresh rate |
| Linear rust on the lower windings | Contact with the floor, moisture bridge | Use of pallets and the stacking arrangement |
| Coating light grey and dusty, rusts immediately | Coarse crystals, low coating weight | Activation bath and the g/m² measurement |
| Rust shortly after phosphating | No protective film applied | The soap/oil stage and the waiting time |
The last two rows point straight at the bath side: if the crystal structure is coarser than expected, the activation bath is the starting point; if the acid balance has drifted, phosphate bath maintenance is.
A phosphated surface is at its most vulnerable in the wait between coating and the next operation. The shorter that interval, the lower the risk. Setting the line up in planning so that phosphated material does not sit in intermediate stock for long is more effective than any precaution taken afterwards.
Where waiting is unavoidable, the surface should not be left unprotected: a soaped surface provides temporary protection in itself, while protective oil is preferred for longer stock. How stable the soap film stays on the surface is covered in dry drawing soap film stability.
Phosphate coating is a porous conversion layer; its real function is to hold soap, not to provide permanent corrosion protection. Rust on phosphated wire generally comes from three places: acid residue left on the surface, inadequate drying and condensation in the store. The soap or protective oil applied over the layer, a dry store at steady temperature and packaging done at the right moment are the three factors that determine practical shelf life.
Kimfosan manufactures zinc phosphate coatings (KİMSOL) and drawing soaps (Kimkal). If you are seeing stock-related surface problems on phosphated semi-finished product, talk to our technical team with your bath records and storage conditions to hand. In production since 1982, Kimfosan offers technical support tailored to your process conditions from its plant in Kartepe / Kocaeli, Türkiye.
Not entirely. A zinc phosphate layer is porous and does not on its own form a closed barrier keeping water out. Most of the protection comes from the soap or protective oil filling the pores. The phosphate + film pair gives serious temporary protection compared with bare steel, but does not substitute for a permanent coating such as paint or galvanising.
The most common causes are acid residue left on the surface because the final rinse was inadequate, water trapped between the windings because drying was insufficient, no protective soap or oil film being applied at all, and condensation in the store. The coating itself is usually not the source of the problem.
It would be wrong to give a single figure; the time depends far more on storage conditions than on the coating. A coil kept in a dry environment with low temperature swing and proper packaging can last months without trouble, while in a humid area whose temperature swings through the day it can show surface problems within weeks. The soundest approach is to determine your own safe period through periodic checks in your own store.
Condensation occurs when the surface temperature falls below the dew point of the ambient air. To prevent it, keep the store temperature free of swings, bring coils arriving from outside to ambient temperature before packaging, close packaging only when the surface is dry, and stack coils on pallets with no contact with the floor.
The shorter the wait, the lower the risk; ideally phosphated material is not held in intermediate stock for long. Where waiting is unavoidable, the surface should not be left unprotected: a soap film should be applied, and for longer stock a protective oil is preferred.
Coating weight is the criterion that reduces phosphate coating quality to a single number. The steps of gravimetric measurement, sampling discipline, the likely causes of deviations, and how to monitor the trend.
Read →Phosphate coating is not a single tank but a connected chain: degreasing, rinsing, activation, phosphating, soap and drawing. This guide summarises each stage end to end — what it does, its typical parameters and how it affects the next one.
Read →In dry drawing the lubricating film forms when powder picked up from the soap box is pressed onto the surface at the die entry by pressure and heat. We look at why that film breaks at high speed, the symptoms it shows, and the discipline required on the soap box side.
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