A common misconception is that corrosion may not be a problem for interior steelwork which is protected from the elements. If there is frequent condensation on its surface, corrosion of inadequately protected steel will be significant. Under these conditions, hot dip galvanizing may give more than 40 years life. Hot dip galvanizing has also been used extensively to protect interior steelwork in harsh environments such as swimming pools and breweries. Corrosion may also arise on exposed ungalvanized steelwork intended for interior environments if there is a delay in assembly or construction.
Immersed: cold water
Most waters contain scale-forming salts which can form a protective layer on the inside surfaces of galvanized water distribution systems, hence coatings can have their lives enhanced usually to greater than 40 years. If these salts are not present, as is often the case in soft waters, longer life can be obtained by applying two coats of bituminous solution (to BS3416 Type II for drinking water).
Immersed: hot water
The scale-forming properties of water are also important in hot water giving normal life expectancies of over 10 years. Above 60°C, zinc may become cathodic to steel in some waters and no longer provide sacrificial protection if the coating is damaged. Where this situation could arise, sacrificial protection can be assured by installing a magnesium anode as “back-up” to the zinc coating.
Immersed: sea water
Sea water is more aggressive than freshwater. For UK waters the corrosion rate normally lies in the range 10 – 15µm/yr for continuous immersion. Tidal immersion, regular sea spray or immersion in warm tropical sea water can lead to an increased corrosion rate.
In contact with other metals
There is only slight additional corrosion of zinc as a result of contact with the majority of metals in most atmospheric conditions. Bimetallic corrosion can occur in immersed conditions or in locations where rainwater is unable to drain easily or dry out at the contact surfaces. Guidance is given in BSI’s PD6484: ‘Commentary on corrosion at bimetallic contacts and its alleviation’.
The life of a buried galvanized coating can vary depending on, for example, the type of soil – its acidity and whether it has been disturbed (contact Galvanizers Association for further advice). A pH range of 5.5 to 12.5, i.e. weakly acid to alkaline, is favourable. Soils containing ashes and clinkers are especially harmful. In many cases, the application of a bituminous solution (to BS3416 Type 1) over the zinc coating is beneficial – particularly where galvanized steel is buried in the ground or at the point where it emerges from concrete. Galvanized steel can be safely embedded into concrete. For even greater protection in soils, thicker galvanized coatings can be specified.
In contact with chemicals
Contact with chemicals requires special consideration. A wide range of chemicals are compatible with galvanized steel. Prolonged or frequent contact with acids and strong alkalis is not advisable.
Galvanized coatings will withstand continuous exposure to temperatures of approximately 200°C and occasional excursions up to 275°C without any effect on the coating. Above these temperatures there is a tendency for the outer zinc layer to separate, but the alloy-layer, which comprises the majority of the coating, remains intact. Adequate protection may often, therefore, be provided up to the melting point of the alloy layer (around 530°C).
In contact with wood
Very acidic woods such as oak, sweet chestnut, western red cedar and douglas fir can be used in conjunction with galvanized steel, provided they are isolated from direct contact.
In contact with building materials
Damp mortar, cement and plaster have a slight etching action upon galvanized coatings whilst drying or setting. This effect ceases once the action has finished.
Further information on these subjects is available in EN ISO 14713-1.