Does black steel rust -
corrosion mechanism and protection
Black steel rusts whenever moisture and oxygen are present on its surface at the same time. ISO 9223 defines the rate of this process for unalloyed steel in the first year of exposure: from 1.3 µm per year in category C1, that is in heated, clean interiors, up to 700 µm per year in category CX, which covers marine and extreme industrial environments. Everything that happens in warehouses and production plants falls between those extremes: typical halls and urban areas correspond to categories C2 and C3, where the thickness loss ranges from 1.3 to 50 µm per year.
The question of whether black steel rusts is therefore less useful than the question of how fast it rusts and with what consequences for a specific product.
Why rust does not protect steel
Atmospheric corrosion of carbon steel is an electrochemical process that takes place in a thin layer of electrolyte deposited on the metal surface. ISO 9223 describes this condition through the concept of time of wetness, the period during which the surface is covered by a moisture film capable of sustaining the reaction. To determine it, the standard uses the sum of hours per year in which relative humidity exceeds 80 percent at a temperature above zero.
The key difference between rust and the protective layers of other metals concerns volume and adhesion. Iron corrosion products occupy a distinctly larger volume than the steel they formed from, and that expansion causes the layer to crack and separate from the substrate. The layer remains porous and discontinuous, so it does not cut off access of oxygen and moisture to the fresh metal underneath. The process advances into the depth of the material.
By comparison, the patina on a zinc coating forms a compact, adherent layer that is insoluble in water and acts as a physical barrier. Zinc also protects steel cathodically: where bare metal is exposed, for example at a cut edge, a cell is created in which the zinc corrodes instead of the steel. Rust has no such properties and cannot have them.
The corrosion rate is governed by three factors covered in ISO 9223: the temperature and humidity regime, sulphur dioxide pollution and airborne salinity. Chlorides and sulphur compounds act as a catalyst, lowering the electrical resistance of the moisture film and accelerating the reaction. This is why the same steel grade behaves completely differently in a production hall inland and in a storage yard a few hundred metres from the sea.
| Category to ISO 9223 | Corrosivity level | Thickness loss of unalloyed steel in the first year |
|---|---|---|
| C1 | very low | up to 1.3 µm |
| C2 | low | 1.3-25 µm |
| C3 | medium | 25-50 µm |
| C4 | high | 50-80 µm |
| C5 | very high | 80-200 µm |
| CX | extreme | 200-700 µm |
Mill scale and corrosion
Hot rolled sheet in its as-rolled condition is covered with mill scale: a dark, adherent layer of iron oxides with a three-layer structure. Wustite lies next to the steel, magnetite above it and hematite on the outside.
Mill scale looks like a protective coating and is sometimes taken for one, but electrochemically it is cathodic to steel, that is more noble. As long as it covers the surface tightly, it slows corrosion down. As soon as it cracks or is damaged mechanically, a galvanic cell forms where the steel is exposed, and the steel acts as the anode. Small areas of exposed metal surrounded by extensive, intact scale are the most unfavourable case: a small anode works against a large cathode, and the result is intense pitting corrosion concentrated exactly at that spot. Material with partly detached scale therefore corrodes faster and more unevenly than material with a uniform surface.
This is one of the reasons why pickled steel is used for products that are painted, precision welded or cold formed, material in which the scale has been removed chemically in an acid bath and the surface protected with a film of oil. The selection of a grade for a given process is covered in the article on steel sheet grade designations.
Surface rust and material suitability
In warehouse practice the question is usually phrased differently: whether light surface rust on a sheet disqualifies a batch.
The surface condition of hot rolled products is governed by EN 10163. The standard distinguishes imperfections, that is discontinuities within the permitted depth limits and regarded as inherent to the manufacturing process, from defects, that is discontinuities exceeding those limits, as well as cracks, rolled-in scale and seams regardless of depth. It also defines classes A and B together with subclasses that specify the permitted repair methods. The difference between the classes concerns thickness: in class A the thickness remaining beneath a discontinuity and beneath a ground area may fall below the minimum permitted by the dimensional standard, in class B it may not. What matters here are the steel sheet thickness tolerances, because they set the lower reference limit.
The standard focuses on material discontinuities rather than on surface rust itself, and states explicitly that mill scale may conceal them. The degree of rusting is assessed before further processing in accordance with ISO 8501-1, which defines four grades of the initial surface condition.
| Grade to ISO 8501-1 | Surface condition |
|---|---|
| A | surface largely covered with adherent mill scale, practically free of rust |
| B | corrosion has started, mill scale is beginning to flake |
| C | mill scale corroded or able to be scraped off, slight pitting visible to the naked eye |
| D | mill scale corroded, general pitting visible |
The practical conclusion is that superficial rust without pitting, corresponding to grade B, does not determine the suitability of the material. What matters is whether the thickness has dropped below the lower tolerance limit under that rust and what process the material is intended for next. Surface preparation is required before painting in any case, and ISO 8501-1 defines the grades of abrasive blast cleaning: Sa 1 removes poorly adhering contamination, Sa 2 most of the mill scale and rust, Sa 2½ leaves only light staining in the form of spots or stripes, and Sa 3 gives a uniform metallic appearance. Surface assessment of material from the second sales channel is discussed in more detail in the article on surface defects of second-grade steel sheet.
Storage corrosion and condensation
Storage corrosion rarely results from long storage as such. It results from condensation. When cold material is brought into a warmer room, the air temperature right at the steel surface drops below the dew point and moisture settles on the sheet. Tightly stacked sheets and the wraps of a coil trap that moisture between the layers, where there is no air circulation and nothing supports evaporation.
The rules that limit this are simple and common to steel products regardless of surface type: transport in covered and sealed vehicles, storage under a roof and above floor level, dry spacers that allow air to flow between packs, stacking with a slight slope so that water can run off, and no tight wrapping in film, which traps condensate underneath. Material that stays dry does not corrode.
Temporary and permanent protection
Protection of black steel rests on two levels. In the short term, oil films applied on the line protect the product during transport and short storage, together with control of storage conditions. In the long term, when the product is to work in a corrosive environment, a coating is required.
The selection of a coating system is governed by ISO 12944, which links the corrosivity category of the environment with the required durability of the protection and on that basis specifies the surface preparation, the type of paint, the number of layers and the dry film thickness. The higher the corrosivity category and the longer the assumed maintenance-free period, the more elaborate the system. An alternative to painting is zinc protection, in which the metal layer acts both as a barrier and as a sacrificial anode.
Weathering steels
Weathering steels covered by EN 10025-5, known on the market under the name corten, work on a different principle. Additions of chromium, copper, phosphorus and nickel cause the layer of corrosion products to stabilise and form a compact patina that hinders further access of oxygen and moisture, slowing corrosion down markedly.
The patina forms only under cycles of alternating wetting and drying. Where the surface stays permanently wet, submerged or buried, the mechanism does not work and the material behaves like ordinary structural steel, so it requires painting. Industry guidance rules out the use of uncoated weathering steel within a band of up to 2 km from coastal waters, unless chloride deposition stays below category S2 to ISO 9223, that is below 300 mg per square metre per day. Industrial atmospheres with a high sulphur dioxide concentration are treated in the same way, as are environments classified as C5 and CX, for which this material is not intended.
Criteria for selecting the surface condition
The decision to buy black, pickled or galvanised sheet comes down to three questions: in what environment the product will work, what processing awaits the material and how long it will be stored before use. Black steel in its as-rolled condition is the most economical choice for products that are painted after fabrication and processed soon after delivery. Pickled and oiled steel has the advantage wherever mill scale interferes with welding, forming or painting. Hot dip coated sheet solves the corrosion problem at the material stage rather than at the finished product stage.
On one side of the calculation stands the price per tonne, on the other the cost of protection, the cost of storage losses and the risk of downtime. Available steel grades, coating types and formats of coils, slit strip and sheets are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.
See how we can help
Send us the specification of the material you currently use: grade, thickness, width, surface condition, coating type, preferred delivery form and batch size. We will verify availability and present the best terms we can offer. If the surface condition in use turns out to be specified beyond what the process actually needs, we will point to the material that matches the real requirements of the product.
Frequently asked questions
How fast does black steel rust in storage?
It depends on the conditions, not on the storage time as such. In a dry, heated hall corresponding to category C1 to ISO 9223 the thickness loss does not exceed 1.3 µm per year. In an unheated hall with condensation and in outdoor conditions the rate rises to several dozen µm per year.
Does mill scale protect sheet against corrosion?
As long as it is tight and undamaged, it slows corrosion down. Once it cracks it works the other way round: as a layer cathodic to steel it causes accelerated pitting corrosion where the metal is exposed, most intense at small exposed areas surrounded by intact scale.
Is surface rust grounds for a complaint?
Superficial rust without pitting is usually not a defect within the meaning of EN 10163. What matters is whether the thickness of the material after cleaning stays within the tolerance of the dimensional standard and whether the surface condition matches the class stated in the order.
What is the difference between black steel and pickled steel?
Black steel has a surface with the mill scale left after hot rolling. Pickled steel has gone through an acid bath that removes the scale and is protected with oil, which gives a uniform surface prepared for welding, forming and painting.
Does weathering steel need no protection?
In moderate environments, with cyclic wetting and drying, the patina stabilises on its own and additional protection is not needed. Under permanent wetness, in atmospheres with a high concentration of chlorides or sulphur dioxide and within a band of up to 2 km from coastal waters the mechanism does not work and a coating is required.
What limits storage corrosion most effectively?
Keeping the material dry: transport in a covered vehicle, storage under a roof above floor level, dry spacers, air circulation between packs and no tight wrapping in film, under which condensate collects.
Sources
- ISO 9223, Corrosion of metals and alloys, corrosivity of atmospheres, classification and assessment
- ISO 8501-1, Preparation of steel substrates before application of paints, visual assessment of surface cleanliness
- ISO 12944, Paints and varnishes, corrosion protection of steel structures by protective paint systems
- EN 10163-2, Delivery requirements for surface condition of hot rolled steel plates, wide flats and sections
- EN 10025-5, Hot rolled products of structural steels, technical delivery conditions for structural steels with improved atmospheric corrosion resistance
- The Steel Construction Institute, Corrosion protection and Weathering steel
- ArcelorMittal, Introduction to the corrosion process, Steel Academy
- Galvanizers Association, Corrosion protection