In this article we explain what determines the rate of corrosion, how the available protection methods differ and which one to choose for a warehouse, agricultural or livestock building.
In brief
The rate of corrosion depends on the environment, which EN ISO 9223 divides into categories from C1 (very low corrosivity) to CX (extreme). The most durable and lowest-maintenance protection for building frames is hot-dip galvanising. For components with a wall thickness over 6 mm, EN ISO 1461 requires an average of at least 85 µm of zinc, which in a typical C3 atmosphere lasts about 40 years without maintenance.
Painting is cheaper initially, but needs renewing and is easily damaged during transport and erection. In very aggressive conditions a duplex system is used, i.e. zinc with a paint top coat.
Why does a steel structure corrode?
Corrosion is a reaction of iron with oxygen and water. Steel rusts fastest when its surface stays damp for a long time, i.e. when the relative humidity exceeds approx. 80% and the temperature is above zero. The process is accelerated by pollutants: sulphur dioxide in industrial air, chlorides from road salt or sea salt and, in livestock buildings, ammonia and constant moisture from the animals.
Rust appears first where water lies or the coating is thin or damaged: on edges, welds, at connections, at column bases and where the steel meets the floor slab. Good corrosion protection must therefore cover the entire surface of the component, including hard-to-reach areas.
Corrosivity categories: how to assess the building's environment?
Choosing the protection starts with assessing the environment in which the structure will work. EN ISO 9223 and EN ISO 12944-2 describe six atmospheric corrosivity categories. For each of them, the annual zinc thickness loss is given, among other things, which makes it possible to estimate the life of a zinc coating.
| Category | Corrosivity | Typical environments | Zinc loss (µm/year) |
|---|---|---|---|
| C1 | very low | heated buildings with clean air: offices, shops | up to 0.1 |
| C2 | low | unheated buildings where condensation may occur (warehouses, sports halls); rural areas with clean air | 0.1–0.7 |
| C3 | medium | production rooms with high humidity (dairies, laundries, breweries); urban and industrial atmospheres | 0.7–2.1 |
| C4 | high | chemical plants, swimming pools; industrial areas, coastal zones | 2.1–4.2 |
| C5 | very high | buildings with almost permanent condensation and heavy pollution; high-humidity industry, coast | 4.2–8.4 |
| CX | extreme | offshore structures, highly aggressive industrial environments | 8.4–25 |
Based on EN ISO 9223 and EN ISO 12944-2. Examples are for information only.
Livestock buildings such as cattle sheds and piggeries do not have a single category assigned in the standard. Because of moisture, ammonia and condensation, designers assess them individually and often adopt category C3 or C4 – and even higher in zones exposed to manure and slurry.
Methods of corrosion protection for steel structures
Hot-dip galvanising
Hot-dip galvanising involves immersing the finished, cleaned component in a bath of molten zinc at approx. 450°C. The zinc not only coats the steel but reacts with it, forming iron-zinc alloy layers permanently bonded to the substrate. The bath reaches everywhere: edges, corners, holes and the inside of hollow sections.
A zinc coating protects steel in two ways. Firstly, it seals it off from moisture and oxygen. Secondly, it acts cathodically: if a small scratch forms on the surface, the zinc around it corrodes instead of the steel and prevents rust from developing. Minimum coating thicknesses are specified in EN ISO 1461:
| Steel thickness | Minimum local thickness | Minimum mean thickness |
|---|---|---|
| over 6 mm | 70 µm | 85 µm |
| over 3 mm up to 6 mm | 55 µm | 70 µm |
| from 1.5 mm to 3 mm | 45 µm | 55 µm |
| under 1.5 mm | 35 µm | 45 µm |
EN ISO 1461 requirements for non-centrifuged components.
Durability follows directly from the coating thickness and the corrosivity category. If zinc is lost at a rate of up to approx. 2 µm per year in a C3 atmosphere, an 85 µm coating will last about 40 years before maintenance needs to be considered.
Painting (paint systems)
Steel structures are painted in accordance with EN ISO 12944. Durability depends on careful surface preparation, usually abrasive blast cleaning to Sa 2½, followed by several coats: primer, intermediate coat and top coat. The standard specifies how long such a system should protect the steel until the first major maintenance:
- L (low): up to 7 years,
- M (medium): 7 to 15 years,
- H (high): 15 to 25 years,
- VH (very high): over 25 years.
Paint offers any colour and is cheaper initially, but it has weak points. The coating is easily scratched during transport and erection, and corrosion can develop under the paint at every such spot. Edges and the inside of sections are also harder to protect, and the system has to be renewed periodically, which in an operating building means scaffolding and downtime.
Thermal spray metallising
Metallising involves spraying molten zinc or aluminium onto a cleaned surface. It is used for components that do not fit into the galvanising bath and for repairs on site. In typical modular buildings it is rarely needed.
Duplex system
A duplex system combines hot-dip galvanising with painting. The paint protects the zinc from the weather, and the zinc protects the steel where the paint is damaged. The durability of such a system is significantly greater than the sum of the two coatings' lives taken separately. Duplex is chosen for C4 and C5 environments, for high-humidity livestock buildings and when the structure must be a specific colour.
Hot-dip galvanising vs. painting: comparison
| Criterion | Hot-dip galvanising | Painting |
|---|---|---|
| Durability in a C3 atmosphere | several decades without maintenance | depending on the system, from a few to over 25 years until first maintenance |
| Bond with steel | metallurgical, iron-zinc alloy | adhesive, dependent on surface preparation |
| Damage resistance | high; small scratches protected cathodically | lower; steel is exposed at a scratch |
| Edges and inside of sections | fully coated | difficult to coat evenly |
| Life-cycle costs | one-off, no periodic maintenance | lower initially, then periodic renewal |
| Quality control | simple coating thickness measurement | thickness and adhesion measurement of each coat |
| Colour | natural silver-grey, dulls over time | any RAL colour |
In buildings that are expected to serve for decades without maintenance shutdowns, hot-dip galvanising is usually the better option, even if it requires a slightly higher initial outlay. We explain why the quality of the structure, not just the cladding, plays such a big role in steel buildings in the article steel building vs. sheet-metal building.
Where is corrosion protection particularly important?
- Cattle sheds, piggeries and poultry houses. Moisture, ammonia and condensation mean that cattle buildings, pig farms and poultry houses are among the most demanding agricultural buildings.
- Fertiliser and bulk material stores. Mineral fertilisers form aggressive solutions in contact with moisture, which is why bulk material storage buildings require durable protection, especially at the bottom of the columns.
- Riding arenas. A damp sand surface and arena watering increase humidity in the building. We write about this in the article on building an indoor riding arena.
- Cold stores and food-industry buildings. In cold stores and freezer stores and buildings for the food industry steam condenses on the structure, and the rooms are often washed with pressurised water.
- Workshops and car washes. Road salt brought in on vehicle chassis in winter and water from car washes mean that a car workshop building works in a more aggressive environment than it may seem.
What to watch out for when erecting and using a galvanised structure?
- Do not weld or cut components after galvanising. Every such spot loses its protection. Holes and connections should be made before galvanising, and minor damage is repaired with zinc-rich paint or zinc spraying in accordance with EN ISO 1461.
- Use hot-dip galvanised fasteners. Bolts with a thin electroplated coating will corrode faster than the structure. Also avoid direct contact between zinc and copper, which accelerates zinc corrosion.
- Keep column bases away from moisture. Columns should not stand in water, manure or arena sand. A plinth and properly built foundations for a steel building.
- Store components with spacers. Freshly galvanised components stacked tightly in damp conditions may develop a white deposit known as white rust. It is usually superficial, but it is better to prevent it by ensuring ventilation.
- Carry out inspections. Once a year, it is worth inspecting column bases, connections and places where water or dirt collects.
How do we protect structures at MatBud?
All our steel structures available from stock and modular buildings are made from hot-rolled, hot-dip galvanised IPE I-beams. We drill the erection holes before galvanising, so on site the components are bolted together rather than cut or welded, and the coating remains intact. We give a 10-year warranty on the zinc coating and offer galvanised buildings for the price of painted ones.
A warranty is not the same as durability. Ten years is the period for which we take responsibility for the coating, while its actual service life in typical conditions is several decades. You can read more about how ready-made structures shorten construction and reduce the risk of errors in the article on ready-made steel structures, and about frame types and their applications in the article on types of steel building structures.
Summary
Corrosion protection is chosen to suit the environment in which the building will operate. In most warehouse, industrial and agricultural buildings, hot-dip galvanising offers the best ratio of durability to cost: it protects the entire surface, tolerates minor damage and requires no maintenance for decades. Painting works where colour matters and conditions are mild, while in aggressive environments duplex is the best choice.
Planning a building? Get a quote in the configurator or contact us. We will advise you on the right structural protection for the building's intended use.
FAQ
How long will a hot-dip galvanised structure last?
It depends on the coating thickness and the environment. An 85 µm coating on components over 6 mm thick lasts about 40 years without maintenance in a typical C3 atmosphere. In more aggressive environments, e.g. livestock buildings, this period is shorter and a duplex system is worth considering.
Can a galvanised structure be painted?
Yes – that is exactly what a duplex system is. However, the zinc surface must be properly prepared, e.g. by light sweep blasting, and paints designed for zinc substrates must be used. Paint applied without preparation may peel.
What is the difference between hot-dip galvanising and electro-galvanising?
Electro-galvanising is applied electrolytically and produces a coating only a few to a dozen or so micrometres thick. It is used mainly for small components used inside buildings. Hot-dip galvanising is several times thicker and suitable for structures exposed to outdoor conditions.
Is zinc-rich paint the same as hot-dip galvanising?
No. Zinc-rich paint is a good primer and repair product, but it does not form an alloy with the steel and is thinner. It does not replace hot-dip galvanising of the whole structure.
What is white rust and is it dangerous?
White rust is a white, powdery deposit on fresh zinc that appears when components lie in damp conditions for a long time without air circulation. It is usually superficial and does not affect the protection of the steel, but it indicates poor storage.
Is a painted structure enough for an agricultural building?
Sometimes, for a dry machinery or straw store – but it needs periodic maintenance. In cattle sheds, piggeries, poultry houses and fertiliser stores, a painted structure quickly loses its protection, so hot-dip galvanising or duplex is the better choice there. See our agricultural buildings.
