41. Chemical Contact with Galvanized Coatings

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

Annual rate of coating loss - Microns Corrosivity classification
<2 Very low
<2-5 Low
5-10 Moderate
10-25 High
>25 -100 Severe
>100 Extreme

Table 2

Chemicals - Inorganic and Organic

Material Concentration Phase Corrosivity classification
Acetic acid 6% Solution Severe
  0.1 g/l in air Vapour Severe
Acetone 100% Liquid Very low
Ammonium sulfate (fertilizer) 100% Solid Severe
Superphosphate (fertiliser) 100% Solid Very low
Agricultural lime 100% Solid Very low
Nitrate-based fertilisers 95% (5% moisture) Granulated solid Severe
Urea 100% Damp solid Moderate
Aluminium chloride 25% Solution Extreme
Anhydrous ammonia 100% Liquid Low
Ammonium chloride 10% Solution Severe
Ammonium sulfate 10% Solution Severe
Gypsum plaster 100% Dry Moderate
Plaster of Paris 100% Damp Low
Cement 100% Damp Very low
Clay 100% Solid Very low
Quartz sand 100% Solid Zero
Red brick 100% Solid Zero
Portland Cement sand mortar 100% Solid Moderate
Calcium chloride (Detergent) 20% Solution High
Citric acid 2% Solution Extreme
Sodium carbonate (Detergent) 2% Solution Severe
Sodium-based detergents 0.5% Solution Severe
Commercial soaps and syndets (no phosphates) 0.2-0.5% Solution High
Commercial soaps and syndets (no phosphates) 0.2-0.5% Solution Extreme
Ethanol 100% Solution Low
Ethylene glycol 50% Solution Moderate
Formaldehyde 0.1g/l in air Vapour Moderate
Gasoline 100% Liquid Low
Glycerin 100% Liquid Very low
Magnesium chloride 1.2% Solution Severe
Methanol 100% Liquid Very low
Methyl ethyl ketone 100% Liquid Very low
Naphtha 100% Liquid Severe
Fuel oil/crude oil 100% Liquid Very low
Phenol 100% Solid Low
Potassium chloride Any concentration Solution Extreme
Potassium dichromate 15% Solution Low
Potassium fluoride 5% Solution Very low
Potassium nitrate 0.5-10% Solution Moderate
Sodium carbonate 0.5% Solution Severe
Sodium chloride 3% Solution Extreme
Sodium hydroxide 0.5% Solution Severe
Trichloroethylene 100% Liquid Extreme
Non-acidic organic chemicals – Alcohols, aldehydes, Aryls, polyethylene glycols, esters, Ethers, Plasticisers, glycol ethers, ketones, monomers, acrylics, vinyl esters, alkyl amines, nitriles etc. 100% Liquids Very low

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


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.

Most commercial minerals are not particularly corrosive to zinc coatings. However, some like manganese dioxide produced in this plant will permanently stain zinc coatings.
These galvanized conveyor stringers have been largely unaffected after being covered in coal spillage for over 10nyears at North Goonyella.
Galvanized coatings perform well in contact with petroleum products such as diesel fuel.
File name; Fertiliser Blockline: Most fertilisers are aggressive to zinc coatings and with few exceptions, may cause rapid corrosion of galvanized coatings, particularly when damp.
Galvanized coatings work well in storage and handling of dry grains, but not with other horticultural products such as fruit with acidic juice.
Galvanized coatings work well in storage and handling of dry grains, but not with other horticultural products such as fruit with acidic juice.