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Atmospheric Corrosion Resistance
of Galvanized Coatings


Advisory Note
GEN/12/1
Revised May 2004

Scope

This note is intended to succinctly provide guidance on the service life of the common galvanizing grades over a range of atmosphere exposure conditions. Extrapolated service life estimates are taken from Australian and ISO corrosion rates.

General

No commercial coating has proven to be more serviceable and of such predictable performance in the Australian atmosphere for protecting steel than hot dip galvanizing. Its good performance is due to its inherent corrosion control, high resistance to mechanical damage and, in contrast to conventional paints and other organic coatings, its inertness to the high incidence of sunlight and elevated temperatures prevailing over most of the Australian continent.

The service life of any particular galvanized item is a function of:

•  The thickness of the zinc alloy coating

Service life is directly proportional to galvanizing thickness, which, in turn is normally a function of the thickness of the metal; thicker steel, increasing the layer of zinc.

The in-line galvanizing process however accurately restricts zinc coating thickness to retain ductility for subsequent manufacturing operations with its lesser protective thickness often supplemented by painting.

•  The nature of the environment

In terms of environment, the corrosion resistance of zinc is classified in both international and Australian standards, largely as a function of the distance from the marine influence, as summarised in Table 1. The range of corrosion rates within some of these climatic zones is quite wide and for a more accurate appreciation of the likely corrosivity within any particular zone refer to AS/NZS 2312 2 .

In addition to the guidance provided in Table 1, the following aspects of the serviceability of galvanized coatings should be taken into account.

Predictability of Service Life

Extensive studies 3 have established that, in the long term, the service life of any galvanized structure is essentially proportional to the thickness of the galvanizing for any particular atmospheric environment and the corrosivity of that environment can be assessed from short-term durability data, typically taken over 1-2 year periods. Importantly, unlike paints which tend to shrink away from sharp corners and can be difficult to apply to complex shapes, galvanizing ensures an essentially even coat over all surfaces accessed by the molten zinc. This provides for more accurate service life predictions.

Impact of Industrial Pollution

The influence of industrial pollution in Australia on the service life of galvanizing is very low. This is due in part to the environmental pollution constraints introduced from the early 1970's, notably in the reduction of sulphur dioxide emissions. Thus any accelerated attack on galvanizing, due to acid, highly alkaline or other adverse chemical agents, is likely to be confined to a discrete area around an emission source. For guidance on any particular chemical pollutants specific references, such as Slunder and Boyd 3 or Porter 4 , should be consulted. For added protection in aggressive environments refer to the GAA guide (GSR 1)

Microclimates

Corrosion rates may increase significantly above the values shown in Table 1 due to quite localised microclimates, such as severe coastal service, where the surface is exposed to airborne salts or other pollutants, but not subjected to the cleansing influence of rainwater and the lack of good ventilation and drying - refer AS/NZS 2312 2 . To maximise the service life of galvanizing, a disciplined recognition for the potential for such microclimates to occur, and where practicable avoiding them, should be taken. This includes avoiding crevices; depressions and laps that can collect water, and can cause corrosion rates higher than shown in Table 1. In some instances, where the microclimate is significantly more severe than that for the rest of the structure, the area may best be overpainted, refer - GAA guide (GSR 1)

Dissimilar Metals

The effect of dissimilar metals in contact with galvanizing in atmospheric service may also need consideration. Stainless steel and aluminium fixings are commonly used in contact with galvanizing, and, except in very corrosive locations, are most satisfactory. However, fixings of copper and its alloys can accelerate the corrosion of galvanizing in corrosive situations, when in direct electrical contact. Corrosion products of copper and its alloys can also accelerate the corrosion of galvanizing.

When to Reinstate

Galvanizing protects a steel substrate until the zinc has largely corroded away. Consequently any maintenance painting can be deferred until about 2% of areas exhibit "red rusting" 2 . In contrast, for conventionally painted steel, maintenance painting should not be deferred beyond an area of 0.2 - 0.5% "red rusting", as corrosion of the steel can progress unobserved under a paint film. Costs of ultimate maintenance painting to extend galvanizing life are normally lower than for reinstating painted steel, because the residual galvanizing is strongly adherent and readily abrasively blasted back to a sound substrate. Taken a little earlier in its cycle, galvanizing may require little more than washing before painting.

Table 3 Corrosion Rate and Estimated Typical Service Life

(Time to First Maintenance of Galvanizing in Various Environments)

Note 1   This table is an extrapolation of well established corrosion rates and is supported by case history evidence in Australia, where service life records of 50 years are common and up to 110 years are recorded. The corrosion rates are consistent with both AS/NZS2312 2 and EN ISO 14713 6 . Calculated service lives assume that corrosion commences when the remaining zinc coating is reduced to 1/3.

Note 2   Because the actual galvanizing thickness applied is usually well above the specification minimum, the service lives quoted in rain exposed locations are likely to be conservative.

Note 2.   Coastal zone is defined as between 50 metres to 1 Km inland from sheltered seas and between 1 Km and 10-50 Km from surf beaches depending upon prevailing winds and topography.

BIBLIOGRAPHY

                    1.   ISO 9223 Corrosion of metals and alloys - Corrosivity of atmospheres -                                       Classification.

2.   AS/NZS 2312:2002 "Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings".

3.   CJ Slunder and WK Boyd, "Zinc: Its Corrosion Resistance" Int. Lead Zinc Res. Org. In. Aug. 1983.

4.   Porter F. "Zinc Handbook, Properties, Processing and Use in Design", Marcel Dekker Inc. Y 1991.

5.   Galvanizers Association of Australia "Guide to Adopting Paint Systems for Galvanized Steel"

6.   EN ISO 14713, Protection against corrosion of iron and steel in structures - Zinc and aluminium coatings - Guidelines (ISO 14713:1999)