Galvanised steel

Things to think about before galvanising

There is far more to galvanising than most realise. It is not simply about coating any piece of metal in zinc for protection. The specifications of the final product need to be carefully considered so that the right thicknesses, lengths and tolerances are achieved in order to avoid any potential issues that result in end-user rejection.

A number of factors can influence the galvanising process and as such, these should be considered before materials are delivered.

Checking steel specs

Have you checked the material data sheet to determine whether steel being galvanized is made up of reactive or non-reactive properties?

In general, it is possible to hot dip most steel alloys. The composition of the steel can, however, have an influence on the structure of the zinc layer, thickness, appearance and resilience of the finished product.

Steel with high phosphorous or silicon concentration can cause a reaction when coming into contact with zinc during the galvanising process. In instances where this occurs, steels are regarded as reactive. The more reactive the steel, the more rapidly the iron-zinc alloy layers form. This results in higher alloy layers in the galvanised coating than would occur in steel with lower or no reactive properties, making it thicker and rougher, which can affect the appearance, even when a duplex coating is applied.

Learning about Galvanising finishes

Have you familiarized yourself with the galvanising process and the different surface conditions that arise after galvanising?

When most people think of a galvanised finish, they think of a shiny, silver coating. However, this is not always the case. There are a variety of factors than can influence the finish and appearance of hot dip galvanising coatings, some can be controlled and others cannot.

Cooling rates can influence the appearance of the finished galvanised product. Thinner material generally cools faster and often stays shinier than thicker materials which stay hotter for longer and have a matte grey colour as a result.

When steel has unusually high levels of certain chemicals such as carbon, manganese, phosphorus and silicon, galvanised coatings  tend to be comprised of zinc-iron layers with little or no free zinc layer. This results in a darker, matte grey colour.

Is the material clean, free of paint, grease, bitumen, welding spatter?

In hot dip galvanising, steel is immersed in a zinc bath which houses molten zinc at high temperatures. When steel materials are immersed into the 450 ºC molten zinc, it forms an alloy with the steel, and a indestructible, protective layer is formed.

A flawless anti-corrosive layer can only be achieved on a perfectly clean material surface. So if the steel has not been adequately cleaned prior to galvanising, and any residual paint, grease, bitumen and welding spatter remain, these will appear as blackened areas that are noticeable on a sleek silver zinc coating. Not the finished product you're looking for by any means!

If you find that your materials are not up to standard, have stubborn dirt or paint, bitumen or welding spatter that traditional acid pickling just won't remove and you need your jobs done in a hurry, chat to us about our shot blasting cleaning method. Not only does this allow for more vigorous and rapid cleaning than traditional acid pickling, but also creates a rough surface that allows for better zinc adherence and a higher quality galvanised finish.

Are there sufficient drainage holes?

In order to ensure full interior and exterior cleaning and galvanising, it is necessary to ensure that there are sufficient drainage holes in all fabrications. These can be plugged once the galvanising process is complete should this be necessary.

Drainage holes help to avoid air locks and allow air to be evacuated from within and around the fabricated material when immersed in cleaning solutions and molten zinc as well as to ensure that these drain out and away from the material in question.

The ventilation process also ensures that cleaning solutions and rinse waters do not become trapped and turn to steam when coming into contact with molten zinc. If this occurs the trapped moisture can result in uncoated localised surfaces and, in instances where these flash to steam, the resulting pressure can cause ruptures in fabrications.

Addressing the above prior to sending material for galvanising will reduce time wastage and costs, avoid rejected items, and ultimately ensure a smoother overall service. If you have any questions, or require us to assist with specialist consulting services, feel free to chat to us today.