INTRODUCTION
While the vast majority of hot dip galvanizing is used to protect steel from atmospheric corrosion, there are always particular applications where hot dip galvanized coatings will come in contact with chemicals, food products or minerals in the course of their transport and storage. While there is an enormous number of possibilities, this chapter of the Specifiers Manual reviews the effects of contact with the more commonly encountered bulk materials on zinc (galvanized) coating.

Table 1 is a corrosivity classification based on annual rate of coating loss that is used in Table 2 as the material’s corrosion rating. Table 2 lists the type of chemical, its form (solid, liquid, vapour) and its concentration and its corrosion rating on zinc based on annual estimated coating loss. It should be noted that this corrosivity classification has been arbitrarily allocated to chemicals and materials in contact with galvanized steel, as a method of classifying their corrosivity with respect to each other, and is not related to the corrosivity classifications of atmospheres, that is covered in detail in Australian Standards AS/NZS 2312 and AS 4312.

Table 1

CHEMICALS – INORGANIC AND ORGANIC
Table 2

OTHER MATERIALS
Galvanized steel comes in contact with a wide range of bulk materials, including grains, fruit and other farm produce, as well as minerals such as coal, iron ore and many commercial minerals.

Galvanized coatings have been widely used in the coal industry for both coal handling and treatment. The corrosivity of coal with respect to galvanized coatings is generally very low, although there is an exception where ex-mine high sulfur coal is stored in bulk and is subject to rainwater leaching through the coal stacks. This can give rise to low pH run-off ground water that can be aggressively corrosive to both zinc and steel.

Because all grains need to be dry for transport and storage, hot dip galvanized coatings perform well for this purpose. The relatively hard and abrasion resistant hot dip galvanized coating also provided an additional performance benefit in for grain handling.

The use of galvanized steel for the bulk handling of some fruit and products such as sugar cane has some limitations, as because of the acidic nature of the juices associated with the fruit. This may reduce the service life of the galvanized coating where liquid fruit residues can accumulate on the galvanized surfaces.

Iron ore itself is relatively benign in contact with galvanized steel. Studies of structures used in WA’s Pilbara iron ore operations indicate that corrosion rates of galvanized steel in contact with iron or are very low.

The performance of galvanized coatings with other ores and commercial minerals will depend on the specific nature of the material in general, and its moisture content in particular. Many sulfide ores that are produced by floatation processes (copper, lead, zinc…) may also contain chemical residues from the floatation process that can impact on the durability of the galvanized coatings with which they come in contact

SUMMARY
In general, galvanized coatings will perform well in contact with most petroleum-based products and minerals such as coal and iron ore. Most organic chemicals, with the exception of organic acids and a few specialised products, are benign to galvanized coatings, while the majority of inorganic chemicals are corrosive to zinc and galvanized coatings.
Fertilisers and detergents tend to be corrosive to zinc, although there are some exceptions as shown in Table 2. Building materials such as cement and mortar, and plaster, particularly gypsum plaster, can be corrosive to zinc while damp (during curing) but are benign once dry or cured.

The majority of information in the chapter has been derived from Zinc: Its corrosion resistance, by C.J.Sundler and W.K Boyd, published by the International Lead Zinc Research Organization Inc, New York, 1986.

A further useful reference is ‘Corrosion Resistance of Zinc and Zinc Alloys’ by Frank C Porter published by Marcel Decker Inc, New York, for the International Lead Zinc Research Organization Inc, North Carolina, 1996.