Galvanization or galvanizing (or galvanisation or galvanising) is the process of applying a zinc coating to iron or steel to protect it from rust. Here is an explanation of the main galvanization processes, the uses of galvanization, whether galvanized steel rusts, and the health risks associated with the process.
Galvanization Processes
There are multiple methods of galvanizing metal.
- Hot-dip galvanizing: This process deposits a thick layer of zinc or a zinc alloy onto iron or steel. The metal surface is first prepared by caustic cleaning, rinsing, pickling in hot dilute sulfuric acid, and rinsing again. The part is dipped in a flux solution (usually an aqueous zinc-ammonium chloride solution) and dried before being dipped in a bath of molten zinc. Hot-dip galvanization forms a highly corrosion-resistant coating of uniform thickness. If damage occurs, the surrounding zinc coating acts as a sacrificial anode, which corrodes before the underlying iron or steel. Heat from the process confers a soldering effect.
- Electrogalvanizing: In electrogalvanizing or electroplating, electric current passes through an electrolyte solution applied to steel, depositing a thin layer of zinc metal. While electrogalvanizing confers less corrosion protection, the thinner coating makes it a better choice for detailed or smaller parts. The process does not require heat and deposits a controlled zinc thickness.
- Sherardizing: Sherardizing or thermal diffusion galvanization deposits a zinc diffusion coating on iron-based or copper-based materials. Tumbling and heating metal parts with zinc powder causes zinc to diffuse into the metal surface, forming a zinc alloy. Because there are no liquids, the process is also called “dry galvanizing.” Sherardization is preferable when it’s important to avoid hydrogen embrittlement or to prepare a surface for powder coating or painting.
Does Galvanized Steel Rust?
Galvanized steel does rust, but galvanization can protect the underlying metal for many years. The duration of protection depends on the galvanization method and on environmental conditions. Salts, acids, and bases attack zinc. Deep scratches and other mechanical damage may expose the underlying metal, giving rust a foothold.
Uses of Galvanization
Galvanized steel has many uses:
- Vehicles: Automobiles and bicycles are made from galvanized metal.
- Water: Galvanized metal buckets and troughs transport and store water for people and animals. Drinking water may be collected from galvanized roofs. Some plumbing pipes used galvanized metal, although this practice has been largely discontinued.
- Construction: Tools, wires, nuts, and bolts are often galvanized. Galvanized steel finds use in buildings, fences, stairways, roofing, and ladders.
- Communications: Phone wiring and equipment boxes use galvanized steel to reduce maintenance.
- Energy industry: Solar and wind power use galvanized parts because they resist environmental exposure.
Is Galvanizing Toxic?
Finished galvanized steel products, such as buckets, nails, or roofing, do not present a risk of toxicity to people, animals, or plants. It’s safe to collect rainwater for drinking using galvanized containers and to use galvanized tubs to water pets, livestock, and plants. Galvanized planters are generally considered safe for growing vegetables and herbs. The plants do absorb some zinc from the metal, but if the amount is excessive, the plants will show indications of zinc toxicity before harvest.
However, the zinc coating eventually becomes damaged. This exposes the underlying metal, which may be toxic. Galvanized steel pipes and containers used for drinking water should be replaced when damage is apparent or at least every 50 years. Old galvanized containers are unsafe for growing crops because their coating is more likely to be damaged, potentially causing absorption of undesirable metals (e.g., chromium, nickel) from the underlying metal.
Galvanized metal isn’t safe for use with acidic foods (e.g., fruit juice, soda, wine, tomatoes, and vinegar). The acid dissolves the zinc coating. Similarly highly basic or alkaline liquids (e.g., laundry detergent, lye, bleach) shouldn’t be stored in galvanized containers.
Sometimes people use large galvanized containers to store food for parties or get-togethers. A galvanized trash can is fine as a giant ice chest, but it’s an unwise choice for holding liquids or hot food. Never cook in galvanized metal or using galvanized utensils. According to the American Galvanizers Association, around 392 °F (200 °C), heat causes galvanized metal to release zinc fumes. A small amount of zinc is essential to human, animal, and plant nutrition, but too much zinc is toxic.
The galvanization process, particularly hot-dip galvanization and sherardizing, poses safety risks. Both of these processes involve molten zinc, so workers need protection from zinc, zinc oxide, and magnesium oxide fumes. Inhaling vapor can cause a flu-like illness called metal fume fever.
History
Galvanization takes its name from 18th-century Italian scientist Luigi Galvani. However, Galvani didn’t actually work with zinc coatings. Galvani demonstrated that muscles were stimulated by electricity. In modern usage, to be “galvanized” is to be stimulated into action. While “galvanize” and “galvanized” have multiple definitions, “galvanizing” refers only to zinc coatings. The first use of the process pre-dates Galvani. Indian armor from the 17th century uses galvanized iron.
References
- Magalhães, A. A. O; Margarit, I. C. P; Mattos, O. R (1999). “Electrochemical characterization of chromate coatings on galvanized steel”. Electrochimica Acta. 44 (24): 4281–4287. doi:10.1016/S0013-4686(99)00143-7
- Murray, James A. H.; et al. (eds.) (1989). The Oxford English Dictionary. VI (2nd ed.). Oxford University Press. ISBN 0 19 861218-4.
- Natrup, F.; Graf, W. (2014). “Sherardizing: Corrosion Protection of Steels by Zinc Diffusion Coatings”. In Mittemeijer, Eric J.; Somers, Marcel A. J. (eds.). Thermochemical Surface Engineering of Steels: Improving Materials Performance. Elsevier Science. ISBN 978-0-85709-652-4.
- Porter, Frank C. (1991). Zinc Handbook. CRC Press. ISBN 978-0-8247-8340-2.