How do you stop your garden gate or your steel garden furniture breaking out in unsightly brown scabs? Why do some steel garden accessories last for decades without showing any sign of rust at all, yet others rust within a year or two?
All you need to cause steel to rust is oxygen – keep the oxygen away from the steel, and it will not rust. Water on steel forms a small electric cell, like a battery, and speeds up the process enormously. So we need to do two things to prevent rust – exclude oxygen and exclude water.
The cheapest and most effective way of achieving these two goals is with galvanising or molten zinc spray. Both of these methods use zinc, because zinc is relatively cheap, and it is a Stop Rust sacrificial metal, meaning that if you put zinc and steel in close proximity in the presence of water, the zinc will sacrifice itself by corroding before the steel does.
Galvanizing is the process of coating steel with a thin layer of zinc. The procedure is to remove all existing rust from the item (by shot blasting or immersing in an acid/alkali bath) before briefly immersing the item in a bath of molten zinc (approximately 450ºC), resulting in a smooth coating of zinc approximately 50 – 100 microns thick. To provide extra protection, the item is given a chromate passivating coat.
The zinc spray process (also referred to as metal spray or flame spray) involves shot blasting to remove all rust, followed by spraying with a flame gun which melts pure zinc wire, then uses compressed air to atomise and propel the molten droplets onto the steel item, much like an aerosol paint. The melting, atomising and propelling is a continuous process, so the operator literally spray paints the item with a fine spray of molten zinc droplets. The droplets solidify and adhere strongly on contact with the cold steel item, forming an even, textured coating of pure zinc, approximately 100 – 200 microns thick.
Both of these processes coat the item with sacrificial zinc, but equally importantly, they are always preceded by the complete removal of all existing rust. This perfectly rust free surface provides an excellent surface for bonding with the molten zinc, resulting in an extremely tough metal coating, which is highly resistant to all but the most extreme abuse.
If you buy any steel accessories for your garden, make sure that they have been galvanised or zinc sprayed, or they will surely rust. Powder coating over galvanised or zinc sprayed steel offers excellent protection. Beware, powder coating over bare steel does not – any minute pinhole, crack or chip in the powder coating will admit oxygen and water.
Do not despair if you have rust on your existing garden furniture or other items. Give them a few more decades of life by having them re coated. Zinc coating businesses will shot blast all old paint, rust or other coatings off your items and process them as they would new items.
If galvanised steel is left unpainted, it will assume a dull grey appearance after a few months. If you don’t mind this colour, fine. Otherwise, if you plan to paint the item, rather have it zinc sprayed. Zinc sprayed items have a textured surface, similar to sandpaper, which provides an excellent key for primer and paint, resulting in paintwork that is far more durable than galvanising, and will never peel or flake.
Take precautions before you buy steel garden accessories – ensure they have a zinc coating, and they will give you a lifetime of service.