*Updated 2024

What considerations are necessary when corrosive chemicals/substances will contact hot-dip galvanized steel?

The AGA has published multiple resources (see the publications Specifiers Guide or Performance of Hot-Dip Galvanized Steel Products) listing chemicals, substances, and environments hot-dip galvanized steel can successfully be used in or with. However, there are some chemicals and substances where caution should be used if galvanized steel will be specified for a specific application. Although it is impossible to list the performance of HDG in contact with all chemicals, there are some chemicals and substances specifiers often ask about concerning the corrosion rate of galvanized steel.

Generally, hot-dip galvanized steel performs well in solutions with a pH between 5 and 12, but other factors such as aeration, agitation, ion levels, and temperature also affect the corrosion rate of the galvanized coating.

In Chemical Solutions

Meanwhile, the following is a list of chemicals and substances where additional considerations are considered:

• Bleach, even when mixed with water, is corrosive to galvanized steel but is sometimes considered for cleaning of galvanized surfaces or roofing by pressure washing. A dilute solution of bleach (1% or less) can be appropriate under the right procedures, but is easily corrosive to galvanized steel. The key to minimizing damage to the galvanized coating when cleaning with chemicals and pressure washing is to let the chemical solution sit as little as possible on the surface, followed by thorough rinsing and drying of the galvanized steel. It should also be noted bleach solutions can dull the galvanized surface. An alternative cleaning solution is a mixture of one part ammonia with 10 parts water and then rinsing the galvanized coating with fresh water and allowing it to thoroughly dry. An ammonia solution can sometimes result in a golden appearance after cleaning. When pressure washing galvanized steel, always ensure the pressure is less than 1,450 psi to prevent mechanical damage of the coating.

• Chlorine, the base for many commonly used bleaches, is also corrosive to galvanized steel. This means galvanized steel will exhibit higher corrosion rates when exposed to chlorinated water; however, when appropriately specified, both hot-dip galvanized structural steel and Duplex Systems (paint or powder coating over HDG) have demonstrated suitable longevity for waterpark, pool, and aquatic facility projects.

• Acidic foods including tomatoes and fruit juices can aggressively attack the galvanized coating, so storing of these types of foods is not recommended in galvanized containers. Additionally, The U.S. Food and Drug Association (FDA) restricts the use of galvanized steel in contact with acidic foods.

• Greases, oils, waxes, and fats based on animals or vegetables should be avoided. However, hot-dip galvanizing stands up well to lubricants of mineral origin which are stable and free from acidity in most cases.

• Gasoline and diesel fuels are generally not corrosive to zinc coatings. However, water and/or sulfur found in refined oil fuels may form zinc compounds which can then clog filters and other small orifices.

• Aluminum cleaners are extremely aggressive to galvanized steel and many product data sheets for these types of cleaners state they should not be used on zinc-coated steel.

• Road salts are used for preventing/reducing icing and aggressive to galvanized steel storage containers when wetted, so care should be taken when storing road salts in galvanized containers or trailers. In the absence of excessive heat and moisture, galvanized steel performs very well in the presence of dry salts (hence its continued use for corrosion protection of dry salt storage facilities).

• Quicklime (calcium oxide) is a widely used chemical compound used for mixing mortar, acting as a flux in smelting processes, and treating wastewater. When dry, quicklime is not excessively corrosive to galvanized steel, but when it combines with water it creates calcium hydroxide, which is extremely corrosive to galvanized steel. In addition, the reaction of quicklime with water is an exothermic reaction (meaning it releases heat), which will further add to the corrosion of the galvanized coating.

• In the absence of excessive heat and moisture, galvanized steel performs very well for storage of dry fertilizer containing anhydrous ammonia and phosphate-containing compounds. Additionally, galvanized steel is successfully specified in fertilizer manufacturing plants. However, fertilizers stored the presence of moisture and/or heat can be very corrosive to galvanized steel because it's the presence of ammonia and phosphate ions which accelerate the corrosion rate. Such ions can only form if present in a solution or exposed to moisture. As expected, liquid fertilizers have a very high corrosion rate, but damp granular fertilizers are especially corrosive as well. In terms of dampness, as little as 3% moisture by weight is enough water when combined with granular fertilizers to attack the galvanized coating. On a project basis, it is helpful to determine whether there is a risk of excessive heat, moisture, or humidity in the storage environment which may result in the breakdown of the dry fertilizer compounds into ionic form. However, if storage is in a moderate or dry environment and simply placed nearby or above damp fertilizer, this will only have a minor impact in coating longevity, especially if damp for relatively short durations over the project life.

• Bird droppings may become very acidic in the right type of climatic conditions and this could affect any type of corrosion control coating on steel. However, the AGA has not heard of any problems with hot-dip galvanized steel installations experiencing any corrosion problem due to bird droppings. In fact there are many examples of hot-dip galvanized steel usage where birds use the metal surface as a perch and there have been no reported instances of excessive corrosion due to their droppings.

• Acidic woods or resinous/sappy woods such as cedar, pine, fur, and larch are aggressive to hot-dip galvanized steel and known to stain the HDG surface. Instead, dry, non-resinous wood species like poplar, ash, and spruce are recommended woods in contact with galvanized surfaces. Additional reference information regarding the corrosive effects of additional wood types are provided in Table 1.

• Often when hot-dip galvanized steel is placed onto dunnage, it is not completely cooled. This can char the wood dunnage and leave burn or discoloration marks on the steel which can be an additional aesthetic concern. For architecturally exposed structural steel (AESS) projects, this might be something to avoid. For such projects, using a test sample prior to making the choice of wood may prove sensible.

Hot-Dip Galvanized Steel in Contact with Treated Wood

Table 1: Corrosive Effects of Different Types of Wood on Steel and Zinc

This list of chemicals and compounds that can cause excessive consumption (or even complete destruction) of the galvanized coating is by no means exhaustive. There are many obscure chemicals and compounds specifiers would like to store or place in contact with galvanized steel. The book Corrosion Resistance of Zinc and Zinc Alloys by Frank Porter has corrosion rate listings for inorganic chemicals, organic chemicals, and detergents on galvanized steel and is a fantastic resource for researching whether galvanized steel is appropriate for a particular application.

Inorganic Chemicals Organic Chemicals Detergents, Cleaners, Soaps, Synthetic Detergents

One last thing to consider is even though some of these chemicals can be very corrosive to galvanized steel when combined with high-performance paint, epoxy, or powder coating, the duplex system of galvanized steel and top coating can still offer significant cost savings over protective coatings or stainless steel.

Please contact the AGA Technical Department at 720-361-4485 with questions concerning the corrosion rate of particular chemicals or substances on galvanized steel.