Performance in other environments

Galvanizers Association

Interior environments

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 food processing plants. Corrosion may also arise on exposed ungalvanized steelwork intended for interior environments if there is a delay in assembly or construction.

Immersed: cold water

Waters containing scale-forming salts can form a protective layer on the inside surfaces of galvanized water distribution systems, hence coatings may have their lives enhanced 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 a suitable paint system (e.g. two coats of bituminous solution or a glass flake epoxy system).

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 zinc anode as “back-up” to the zinc coating.

Immersed: sea water

Sea water is more aggressive than freshwater. For UK waters, which are cold and have naturally dissolved corrosion inhibiting chemicals present, the corrosion rate normally lies in the range 10 – 15 µm/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 provided you have a relatively large surface area of galvanized steel to the other metal. Bimetallic corrosion can occur more readily in immersed conditions or in locations where rainwater is unable to drain easily or dry out at the contact surfaces. Guidance is given in PD 6484: ‘Commentary on corrosion at bimetallic contacts and its alleviation’.

Underground

The life of a buried galvanized coating can vary depending on, for example, the type of soil – the most important factors being soil pH, drainage characteristics, resistivity and whether it has been disturbed (contact Galvanizers Association for further advice). A pH range of 5.5 to 12.5, i.e. weakly acidic to alkaline, is favourable. Soils containing ashes and clinkers are especially harmful. In many cases, the application of a bituminous solution or a glass flake epoxy system over the zinc coating is beneficial – particularly where galvanized steel is buried in the ground or at the point where it emerges from the ground into an atmospheric environment. 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.

High temperature

Galvanized coatings will withstand infrequent exposure to temperatures of approximately 200°C without any significant 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.

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 but best practice is to isolate 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.

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Performance in other environments

Interior environments

Immersed: cold water

Immersed: hot water

Immersed: sea water

In contact with other metals

Underground

In contact with chemicals

High temperature

In contact with wood

In contact with building materials

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