The Benefits of Aluminium-Zinc Coated Steel
Steel is an important aspect of economic activity in most countries. Its use extends to almost all sectors of the economy, from Engineering, Construction and Railways to Shipbuilding, Automotive and Consumer Goods. However, steel has an inherent weakness in that when used unprotected and exposed to the environment, it corrodes very easily. Therefore protective coatings are fundamental to steel corrosion prevention, and there are two types of coatings which provide a barrier between the valuable metal and the elements:
• Aluminium-Zinc Coating (AZ)
• Galvanised Coating (GI)
AZ vs GI
The mild steel substrate is continuously hot dipped in a formulation of Aluminium (55%), Zinc (43.5%) and Silicon (1.5%). The process of applying an Aluminium-Zinc alloy in a hot dip delivers superior oxidation-resistance therefore extending the service lifespan of a steel roof by up to four times that of galvanised steel.
Offering some defence against corrosion-causing elements, galvanised metal is a sheet of steel which has been coated with a thin layer of Zinc oxide (99.8% Zinc). Galvanising offers almost twice the service life of the steel substrate.
There is a visible difference between the two coatings as an Aluminium-Zinc alloy metal appears to almost have a matte finish while galvanised steel has a unique shiny spangle appearance.
Galvanised metal corrodes linearly, and eventually, the Zinc coating is completely depleted, whereas an Aluminium-Zinc alloy experiences some corrosion, but on a smaller and less significant scale. An Aluminium-Zinc alloy metal is less prone to rusting than normal galvanised steel, and less likely to completely break down, even after a few decades.
Solar Reflective Index Compared:
The solar reflective index (SRI) is a measure of the constructed surface’s ability to reflect solar heat, as shown by a small temperature rise. It is defined so that a standard black surface (reflectance 0.05, emittance 0.90) is 0 and a standard white surface (reflectance 0.80, emittance 0.90) is 100. The solar reflective index incorporates both solar reflectance and thermal emissivity in a single value.
A roofing surface with high solar reflectance and high thermal emittance will reflect solar heat and release absorbed heat readily. High thermal emittance material radiates thermal heat back into the atmosphere more readily than one with a low thermal emittance. The table below shows that the SRI value of a roof with an Al-Zn coating is almost double that of one with a galvanised coating.
Solar Properties of Roofing Materials
The Safal Group sells its 55% Al-Zn coated steel product under the brand name Zincal®. Zincal® uses a patented metallic coating which provides long lasting corrosion protection to the underlying mild steel core.
Zincal® is suitable to be used in heat-sensitive applications such as gas heater parts and toaster components – this is mainly due to the fact that Zincal® does not discolour in prolonged exposure to temperatures up to 350°C.
Zincal® can be used in applications which require spot welding, seam welding and arc welding during fabrication. The Zincal® products would require higher welding currents, welding times and electrode forces for resistance welding. To ensure the corrosion properties are enhanced, it is normally advised that the uncoated areas be coated with metal spray Zinc, Aluminium-Zinc or organic coatings. Please note that any weld should be coated with either a galvanised or Al-Zn coating, otherwise the weld will rust.
Safal Steel’s Colorplus® range is applied to coated steel to provide some additional corrosion protection through the chemical pre-treatment process and then the primer coat which lies beneath the paint layer. The paint layer is baked on at high temperatures, providing an additional mechanical barrier against the elements. However, it must be remembered that the primary purpose of pre-painting steel is its aesthetic, the criteria for a quality colour coating therefore being its resistance to fading or chalking in order to retain its fresh appearance for years to come.
Sourcing your Aluminium-Zinc coated steel from a licensed supplier is the only assurance for maximising its service life. The Safal Group remains the foremost producer of 55% Al-Zn coated steel on the continent and currently has three steel coating plants: in South Africa (Safal Steel in Cato Ridge), Tanzania (ALAF in Dar es Salaam), and Kenya (Mabati Rolling Mills in Mombasa).