Civil's Guide
Steel Durability and Exposure
Durability of Steel
Over the lifetime of a steel frame structure, the members will start rusting over time. This is due to the prescence of water and oxygen in the air, which causes a chemical reaction on the surface of the steel producing rust \(Fe_2O_3\). The rate of corrosion depends on the atmospheric pollution (i.e, there is a lot of cholride present in costal and marine areas which will accelerate the corrosion rate).
When designing any steel structure and it’s elements, it is important to take into consideration it’s durability aspect and location within the country. Keep in mind the types of protective measures that can be used for steel (i.e, paint coatings, galvanised steel, weathering steel and stainless steel). In addition, bi-metallic corrosion can be issue at connections where you have 2 different types of metals in contact with each other.
Corrosion and Fire protection
Corrosion and fire protection is achieved through paint coatings, galvanised steel and alloys
Paint Coatings
When applying paint coatings on a steel memeber, this creates a barrier between the surface of the memeber and atmosphere, so water and oxgen does not penetrate casuing corrosion. When applying paint coatings, the surface needs to be cleaned (achieved through blast cleaning), before applying a primer coat (wet and adheres to substrate and acts as inhibitor), and then apply an intermediate coats (builds film thickness), before applying the finishing coat (Resistance to the environment and provides aesthetics). The paint is generally applied via brus, roller and sprayed. This can either be done in a shop before transported to site or site applied. It should be considered that shop applied coats can be easily damaged during transportation (barrier is broken) and needs to be rectified on site.
Galvanised Steel
Galvanised steel is the process of applying zinc coating to the steel. This coating acts as a barrier between the steel and atmosphere. Initially, the steel member needs to be cleaned and treated. The steel is immersed in a fluz solution, which helps prevent further oxidation before galvanising begins. The steel is then dipped in a bath of molten zinc, which produces a zinc-iron alloy. This process is can be done for strucural steel members and small components such as fasteners.
Weathering Steel
Weathering steel is a high strength, low alloy steel which forms an adherent protective rust ‘patina’ that reduces corrosion. Cor-ten is the name given to weathering steel and the low rate of corrosion can mitigate the need for protecttive painting and achieve a high design life (120 years for bridges) with minimal maintenance. The standard for weathering steel is located in BS EN 10155. The benifits of weathering steel are listed below:
Durability Exposure conditions
The tables below defines the ‘environment category’ which is based on BS EN 12944-2 and BS EN ISO 9223. The corrosivity class should be defined during the start of your project (i.e, based on location of building)
Steel material properties
| Corrosivity category | Low-carbon steel Thickness loss \((\mu m)\) | Examples of typical environments (informative only) | |
|---|---|---|---|
| Exterior | Interior | ||
| C1 very low | \(\leq 1.3\) | – | Heated buildings with clean atmospheres, e.g. offices, shops, schools, hotels |
| C2 low | \(>1.3\) to \(25\) | Atmospheres with low level of pollution: mostly rural areas | Unheated buildings where condensation can occur, e.g. depots, sports halls |
| C3 medium | \(>25\) to \(50\) | Urban and industrial atmospheres, moderate sulphur dioxide pollution; coastal areas wih low salinity | Production rooms with high humidity and some air pollution, e.g. food-processing plants, laundries, breweries, dairies |
| C4 High | \(>50\) to \(80\) | Industrial areas and coastal areas with moderate salinity | Chemical plants, swimming pools, coastal ship and boatyards |
| C5 Very High | \(>80\) to \(200\) | Industrial areas with high humidity and aggressive atmosphere and coastal areas with high salinity | Buildinngs or areas with almost permanent condensation and high pollution |
| CX extreme | \(>200\) to \(700\) | Offshore area with high salinity and industrial areas with extreme humidity and aggressive atmosphere and sub-tropical and tropical atmospheres | Industrial areas with extreme humidity and aggressive atmosphere |