Hot dip galvanising (HDG), a process that has been around for 185 years, offers corrosion-resistance properties found with few other methods, HDG expert Terry Smith has said.

In a webinar hosted by Creamer Media on June 30 on behalf of the International Zinc Association, he spoke about the benefits and shortcomings of using HDG for professional users, as well as provided case studies for various HDG applications.

The two ways HDG protects is by providing a slowly-corroding barrier that provides a predictable life. Therefore, the coating life is proportional to the coating thickness and, therefore, the thicker the coating, the longer it will last.

Secondly, where the coating is damaged zinc acts to sacrifice itself in preference to steel, making corrosion-creep impossible.

Smith said HDG performed to South African National Standard’s requirements takes place at about 450 ^oC between suitably cleaned steel components that are dipped in a bath of 98% molten zinc, which comprises about 2% or less of alloys and other elements, depending on the HDG application and type of steel used as a substrate.

“When steel gets to the temperature of 450 ^oC, a metallurgical reaction occurs, forming a cost-effective and corrosion-resistant barrier that generally will provide a service-free life in excess of about 50 years in a C3 [moderate conditions with some condensation] corrosion category,” he noted.

In terms of its characteristics, Smith said HDG produces an end-product that is easy to inspect as time goes on, which is important in structural assessments.

During the HDG process, there is no curing time and, therefore, once HDG has been performed, the coatings can be inspected and loaded onto vehicles for delivery in a short space of time.

Further, he said HDG was “honest” because if the steel is not clean the metallurgical reaction with molten zinc will not occur resulting in uncoated areas.

Such areas may be those where manufacturers have identified and marked printed numbers and letters, or areas where welding has taken place.

Another of HDG’s characteristics is that the process covers edges with the same volume of coating as elsewhere, thereby providing a uniformly applied coating and ensuring the entire component is covered and protected.

The covering of edges is more difficult to achieve with other corrosion-resistance methods, such as painting, even with a zinc-based paint.

In terms of preparing steel to be galvanised in a hot dip kettle, the steel thickness plays a big role, as does surface roughness in generating a thicker coating, Smith explained.

In certain circumstances, alloy additions can be added to a galvanising bath in order to unify the colour or create a “spangled” surface appearance. These additives include aluminium, tin, nickel, lead and bismuth.

“Phosphorus of greater than 0.2% in the steel can lead to delamination or peeling [of the HDG coating]” and while this is not added, is dependent on where the iron-ore is mined, said Smith.

In addition, the ideal steel grades should have between 0.15 and 0.25 percentage of silicon in their makeup, which provides for an acceptably thick coating of zinc.

In terms of HDG appearances, he pointed out that these could vary from a spangled look, to a matte or grey colour. “Sometimes there can be discolouration, and due to the size of items, components can cool in different ways, producing a blotchy appearance.”

Nonetheless, Smith said the thickness of the HDG coating achieved through the process is generally more than what the National Standard SANS121 requires and that, therefore, the longevity of the end-product should be longer than specified.