Protective Treatment

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Steel, if exposed to moisture and oxygen will rust but in practice the important factor is how fast it will rust. In dry heated interiors no special precaution are necessary. Where precautions are required modern durable protective coatings are available which, when used appropriately, provide extended maintenance intervals and improved performance.

The Corrosion process

The principle factors that determine the rate of corrosion of steel in air are

1. The type and amount of atmospheric pollution, e.g. sulphur dioxide, chlorides, dust
2. The ‘time of wetness’, i.e. the proportion of total time during which the surface is wet, due to rainfall, condensation etc
3. The temperature

Within a given environment corrosion rates can vary significantly. For example steel may corrode more on a particular side of a structure because it is in the shade and therefore remains wet for longer. The wind may carry pollution predominately to one face of the structure. Because of these variations in atmospheric environments the rate of corrosion cannot be generalised. However environments and corresponding corrosion rates are broadly classified in BS EN ISO 12944 Part 2 and ISO 9223.

The effect of design on corrosion prevention

In dry heated interiors such as offices, shops schools, hotels, residences, airport terminals and hospitals etc no special precautions are necessary. In external or wet environments design can have an important bearing on the corrosion of steel structures. The prevention of corrosion should therefore be taken into account at the design stage. The main points to be considered are:

• To avoid the entrapment of moisture and dirt
  The key here is to avoid the creation of cavities and crevices; so welded joints are preferable to bolted joints. Lap joints should be avoided or sealed where possible. Additionally drainage holes to prevent standing water may have to be incorporated.
• Coating application
  The design should ensure that the selected protective coatings can be applied efficiently. Typically this might involved ensuring adequate access for painting or adding drain/vent holes to sealed components which will be subject to hot dip galvanizing.

The application of protective coatings

Surface preparation

The surface preparation of steel is concerned with the removal of mill-scale, rust and other contaminants to provide a satisfactory substrate for coatings and is generally considered to be a two stage process.

The first stage of any surface preparation is to remove residues of grease, oil and marling inks. The second stage is to remove any mill scale and rust and is generally done by either hand and power tool cleaning or abrasive blast cleaning.

Painting

Painting is the principle method of protecting structural steelwork from corrosion.

Paints are made by mixing together pigments (the coloured part), binders (the film forming component) and solvents (which dissolves the binder). Paints are usually applied one coat on top of another and each coat has a specified function.

Following surface preparation the paint is applied in the following stages:

1. Stage 1 – The primer is applied directly to the cleaned steel surface,
2. Stage 2 – The intermediate coats (or undercoat) are applied to build the total film thickness of the paint system. Generally the thicker the coating the longer the life and this may involve the application of several coats
3. Stage 3 – The finishing coats provide the first line of defence against the environment and also determined the final appearance in terms of gloss and colour.

Hot dip galvanizing

The most common method of applying a metal coating to structural steel is by hot-dip galvanizing.

Following surface preparation the galvanizing process involves the following stages:

1. Stage 1 – The cleaned steel is immersed in a fluxing agent to ensure good contact between the steel and the zinc during the galvanizing process
2. Stage 2 – The steel is dipped in to a bath of molten zinc at a temperature of about 450˚C. The steel reacts with the molten zinc to form a series of zinc/iron alloys on the surface of the steel.
3. Stage 3 – As the steel is removed from the zinc a layer of zinc is deposited on top of the alloy layers.

Weathering steels

Weathering steels are high strength, low alloy weldable structural steels that possess good weathering resistance in many atmospheric conditions without the need for protective coatings. They contain up to 2.5% of alloying elements, e.g. chromium, copper, nickel and phosphorous. On exposure to air they form an adherent protective rust patina. This acts as a protective layer that, with time, causes the corrosion rate to reduce until it reaches a low terminal level, usually between 2-5 years. As a result a weathering steel structure should only require periodic inspection to ensure that it continues to perform well.

Further detailed information is given in

The prevention of corrosion on structural steelwork, Corus. Available from www.corusconstruction.com

BS EN ISO 12944, Paints and varnishes. Corrosion protection of steel structures by protective paint systems, BSI.

BS 773, Code of Practice for Cleaning and Preparation of Metal Surfaces, BSI

For information on the specification of galvanizing contact the Galvanizing Association at www.galvanizing.org.uk