Design for low maintenance
Low maintenance is important in reducing operational costs and environmental impacts. It can be facilitated by use of robust forms of construction, efficient and durable painting or protection systems and easily maintained servicing and façade systems.
Steel construction products make use of a range of corrosion protection systems. The choice of coating system depends upon both the type of steel, the product and the application. They include:
Coatings for structural steelwork
A large proportion of structural steelwork is used internally, frequently hidden, in dry buildings with low risk, corrosive atmospheres. Under such conditions, there is no need to provide corrosion protection, either during manufacture, or during the life of the building.
Weathering steels are high strength, low alloy, weldable structural steels that possess good weather resistance in many atmospheric conditions without the need for protective coatings. They contain up to 2.5 per cent alloying elements, e.g. chromium, copper and nickel. On exposure to air, a protective rust patina forms that adheres to the surface of the steel. This layer causes the rate of corrosion to slow so that after two to five years, corrosion almost ceases.
The most common metallic coating used to protect steel construction products is hot dip galvanising. Very simply, the process involves coating the surface of the steel with a very thin coating of a corrosion-resistant metal, usually zinc or an aluminium/zinc alloy.
These coatings provide a continuous, impervious metallic barrier that does not allow moisture to contact the steel. Without moisture, there is no corrosion. The effectiveness of the coatings is directly proportional to the coating thickness. A major advantage of this is that it allows predictions of ultimate life to be made on the basis of interim assessments of coating thickness.
The most important contaminant for zinc is sulphur dioxide (SO) and it is the presence of SO which largely controls the atmospheric corrosion of zinc. Levels of sulphur dioxide (SO) in the atmosphere have reduced considerably over the last 20 years and as a consequence, the life of galvanized coatings has increased significantly as shown below.
There are two principal methods for galvanising steel construction products.
Continuous galvanising in which coils of steel are continuously unwound and fed through cleaning and annealing sections before entering a molten zinc bath at speeds of up to 200 m/min. The steel then undergoes a series of mechanical or chemical treatments depending on the customer requirements for the finished product is used to protect a large number of steel construction products. These include:
- Cladding and roofing products (that may also have an organic coating)
- Structural floor decking products
- Light gauge products such as purlins, rails, etc
The typical weight of zinc applied to such products is 275 g/m2 of surface area.
Hot-dip or general galvanising is the most common method of applying metal coatings to structural steel. The process simply involves dipping the cleaned steel product into a bath of molten zinc at a temperature of around 450°C. At this temperature, the steel reacts with the molten zinc to form a series of zinc/iron alloys integral with the steel surface. The minimum average coating for structural steelwork is a zinc thickness of 85 µm.
Steel construction products that are corrosion-protected include:
- Hot-rolled and hollow structural sections
- Lintels
- Road barriers and other street furniture such as gantries, signs and lamp posts
The specification of hot-dip galvanised coating for structural steelwork is covered by BS EN ISO 1461.
Many flat steel construction products, such as cladding and roofing products, have an organic coating for increased durability and enhanced appearance. A range of different coatings is available depending upon the product and the application. Coating thicknesses vary from 25 to 200 µm.
The most common organic coating used for corrosion protection for building envelopes in the UK is Colorcoat HPS200®. HPS200 has been formulated for superior durability, retention of hue and gloss, and mould resistance. Its performance is guaranteed in northern and southern Europe by Corus' Confidex guarantee.
The performance of HPS200 is defined by the 'Minimum Period to Re-paint Decision', which is the length of time before a building owner needs to consider whether to re-paint the cladding
Properly protected and well-maintained exposed steel structures are highly durable even under extreme corrosive conditions. The Forth rail bridge is still going strong after 140 years of service!
Today, modern, durable, cost effective coatings are available which, when used appropriately, allow extended maintenance intervals and improved performance with less impact on the environment.
A range of generic coatings are available for protecting structural steel. These include:
- Air drying paints such as alkyds
- One-pack chemical resistant paints such as acrylated rubbers
- Two-pack chemical resistant paints such as epoxy and urethane
Paint coatings are made by mixing and blending three components:
- Pigments
- Binder
- Solvent
The solvents used in structural steel coatings have traditionally been volatile organic compounds (VOCs). VOCs have several potentially adverse environmental impacts that include health risks during application of the coating and the formation of low-level ozone that can cause respiratory diseases and damage crops, etc.
Concern over the environmental impacts of VOCs in coatings has led to the development of a range of new coatings with a higher solids content and reduced VOCs content or in the case of water-based paints, no VOCs at all.
