Performance in various environments

The excellent corrosion resistance of galvanized coatings in the atmosphere and in most natural waters is due to the formation of a protective layer or patina which consists of insoluble zinc oxides, hydroxides, carbonates and basic zinc salts, depending on the environment. When the protective patina has stabilized, reaction between the coating and its environment proceeds at a greatly reduced rate resulting in long coating life.

 

In the atmosphere

The appraisement of the protective life of a galvanized coating in a particular location must take into account factors such as climatic conditions, the presence in the atmosphere of contaminants introduced by urban or industrial activity, and chlorides in the air due to proximity to the sea.  For more information on how these factors affect the durability of the hot dip galvanized coating, you can download our Atmospheric Corrosion Resistance of Hot Dip Galvanized Coatings advisory note.

Environments which appear to be generally similar often produce considerable differences in corrosive conditions due to relativity minor variations such as the effects of prevailing winds, proximity to corrosive effluents and general atmospheric conditions.

 

In warm dry atmospheres zinc is very stable. The patina formed during initial exposure remains intact preventing further reaction between the galvanized coating and the air, and protection continues indefinitely.

 

In the presence of atmospheric moisture the zinc oxide film is quickly converted to zinc hydroxide, and carbon dioxide normally present in the air reacts to form basic zinc carbonates. These stable inert compounds resist further action and ensure long life for the protective galvanized coating.

 

In rural areas the life of galvanized coatings may be reduced due to the effects of aerial spraying of fertilizers or insecticides. In dry form most fertilizers and insecticides are harmless to zinc coatings but when wetted by rainwater or irrigation spray water, aggressive solutions can be formed which will attack galvanized coatings until washed off by further wetting.

 

Near the sea coast the rate of corrosion is increased by the presence of soluble chlorides in the atmosphere. The performance of galvanized coatings relative to other protective systems is outstanding however, particularly when used as part of a duplex galvanizing-plus-paint system.

 

In industrial areas the presence of atmospheric impurities such as sulphurous gases and chemicals results in the formation of soluble zinc salts. These are removed by moisture, exposing more zinc to attack. In light industrial areas galvanized coatings give adequate protection, but in the extremely corrosive conditions of heavy industrial areas it is desirable to reinforce galvanized coatings with a paint system resistant to the prevailing chemical attack.

Galvanized steel test piece has had circular areas of the coating removed before exposure in a severe industrial environment.  Sacrificial protection provided by the surrounding zinc coating has prevented corrosion of circles up to 3 mm diameter and minimised corrosion of 5mm circle.  Larger circles also exhibit corrosion-free annular areas adjacent to the surrounding coating.

 

Effect of temperature

Hot dip galvanized coatings will withstand continuous exposure to temperatures of approximately 200oC and occasional excursions up to 275oC 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. Adequate protection may often, therefore, be provided up to the melting point of the alloy layer (around 650oC).

 

Under Water

General. The corrosion rate of zinc under immersed conditions can be high in acidic solutions below pH 6 and alkaline solutions above pH 12.5. Between these limits the rate of corrosion is much lower.

In mains supply water of pH 6 to pH 8, calcium carbonate is normally present and this is precipitated onto the galvanized coating as an adherent calcium carbonate scale, together with zinc corrosion products, forming an impervious layer. When sufficiently dense, this layer virtually stops corrosion of the coating, resulting in very long life in many domestic water systems.

Other factors may interfere with this scale deposition. If the water has a high concentration of uncombined carbon dioxide, the protective scale is not formed and full protection never develops. The characteristics of the water supply should be taken into account in the design of domestic water systems. The presence of even small quantities of dissolved copper of the order of 0.1 parts per million in the water may cause corrosion by rapid pitting as discussed under galvanic corrosion.

In unfavourable waters, galvanized steel may require the added protection of galvanic anodes or suitable paint coatings.

Pure water. When newly galvanized articles are immersed in pure water, such as rainwater, there are no dissolved salts present to form the film of insoluble compounds which normally protects the coating from further action. Where practical this condition can be corrected by the addition to the water of controlled amounts of salts during initial immersion. Most natural waters contain sufficient dissolved salts to prevent initial attack and galvanized tanks and equipment give excellent service.

Effect of water temperature. In cold water of normal composition galvanized coatings are most effective and the rate of consumption of the coating is very low. This has resulted in almost universal use of galvanized steel for tanks for water storage and transport.

At about 60ºC to 65ºC the rate of corrosion of galvanized coatings increases and continued corrosion resistance depends on early formation of adequate non-flaking scale. Hard water in hot water systems will deposit a scale of calcium and magnesium carbonates on the galvanized surface, nullifying the temperature effect. Soft water may not deposit a protective scale. In such cases galvanized coatings are unsuitable for hot water systems.

Sea water. Galvanized coatings perform relatively well in submerged sea water conditions which are severely corrosive to most protective systems. Dissolved salts present in sea water react with zinc to form a protective layer minimizing corrosive action.

The addition to the galvanized coating of a suitable paint system is recommended in areas of severe sea water exposure, particularly in the splash zone. Such duplex systems provide the best available protective coating for steel in sea water.  Suitable paint coating systems are listed in ‘Painting galvanized steel.

 

 

Underground

The corrosion behaviour of buried galvanized steel varies greatly with the type of soil. Knowledge of local conditions is therefore essential in estimating the life of galvanized steel pipes. Generally galvanized steel lasts considerably longer than uncoated or painted steels but performance is best in alkaline and oxidizing soils, where 600g/m2 galvanized coating will give an additional life of about 10 years to steel pipes. Highly reducing soil is most aggressive and may consume zinc coatings at more than 13μm per year.

The life of galvanized steel underground is extended by the use of paint coatings, bituminous compounds, tape wraps or concrete encasement.

About Hot-Dip Galvanizing

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