Galvanised Steel – Is it sustainable

Steel is essential for housing, infrastructure, transport, manufacturing, and agriculture. Everywhere you look, you’ll see galvanized steel. It’s used as frames for our buildings, safety on our roads, and support for the bridges we cross. Hence we must never waste it or waste the resources used to produce it by throwing it away when its original function is no longer valid. Instead, we must repurpose steel, especially if the world wants to move away from traditional business models of use-throw-away-remake to a more environmentally friendly, sustainable circular economy that aims to eliminate waste and repurpose resources.

Galvanizing is highly sustainable and produces minimal waste – any zinc that does not instantly form a coating on the metal remains in the galvanizing bath for re-use. The non-ferrous properties of the metal make it indefinitely recyclable without loss of physical or chemical properties.

When steel is galvanized, it’s dipped in a bath of molten zinc, where it alloys with the iron in the steel to form zinc/iron alloy layers. These layers form the basis of the coating, which is then overlain with free zinc as it’s withdrawn from the galvanizing bath. The result is a robust, durable, and corrosion-protective finish that will last for many years.

Without a galvanized coating, steel would corrode, and its lifespan would be short. However, with a galvanized coating, steel can last between 34 and 170 years before the base steel is exposed. This means that steel can be used for the original purpose for which it was fabricated, and – once the project is dismantled – the steel is still good enough to be used elsewhere.

Once a structure – such as a house or fencing – reaches its end of life, the steel can easily be repurposed if it has been hot dip galvanized. This is because the galvanized coating protects the steel from impact and abrasion when disassembled and reassembled.

Galvanizing plants are self-contained, with steel going in at one end and the final product coming out at the other.

Modern galvanizing plants also use zinc very efficiently throughout the galvanizing process. For example, excess metal from the dipping process deposits back into the galvanizing bath. Zinc that oxidises on the surface is removed as ash and recycled, and dross from the bottom of the bath is routinely removed and has a high recycling value.

Other process consumables such as hydrochloric acid and flux solutions also have important recycling or regeneration routes. Spent hydrochloric acid solutions can be used to produce iron chloride for treating municipal wastewater, for example. Closed-loop flux recycling is also used in many plants, and improved monitoring and maintenance of flux tanks reduce the volume of solids for disposal.

When compared to other coating technologies galvanizing uses low volumes of water, with plants rarely discharging wastewater. Any wastewater generated can be treated and reused, with only small volumes of stable solids requiring external disposal.

While not considered a particularly energy-intensive sector, the galvanizing industry also has set targets for energy efficiency and improved energy management. New technology has seen improvements in burner efficiency, bath lid efficiency, and reuse of waste heat to warm pre-treatment tanks. Plant emissions are also carefully controlled to ensure neighbouring communities are not adversely affected.

Finally, galvanized steel fits nicely into a circular economy because it can be melted and used again with no loss of quality if there is no immediate need for repurposing.

The zinc coating can also be reused. Zinc and steel are recycled in well-established steel recycling processes. The zinc particulates are returned, without loss of properties, to zinc production plants, where they are transformed into zinc ingots that are reused in the galvanizing process.