WHEN applied to fasteners, hot-dip galvanising provides barrier protection and cathodic corrosion protection, and ensures safety and structural integrity. Although South Africa has the capability and capacity to manufacture these, volumes remain low. Robin Clarke, executive director of the Hot Dip Galvanisers Association of Southern Africa (HDGASA), says a clearer understanding of the benefits of hot-dip galvanising fasteners as well as the knowledge of and adherence to stringent industry standards, could grow the market and benefit the entire supply chain
The nuts and bolts of hot-dip galvanising
The conventional method of hot-dip galvanising , jigging components onto a flight bar and dipping them into molten zinc, is not feasible for small fasteners. Instead, they are placed in perforated baskets for hot-dip galvanising and then transferred to a centrifuge to spin off any unreacted zinc. This leaves a carefully controlled amount of zinc on the fastener with very good definition in the threaded area.
For the hot-dip galvanising of fasteners, the HDGASA supports and promotes adherence to the ISO 10684 and the SANS 121:2024 / ISO 1461:2022 standard. These specify a minimum coating thickness of 50 microns for threaded articles with a diameter exceeding 6mm; and 25 microns for those with diameters equal to or less than 6mm.
Clarke emphasises that engineers design structures with sufficient strength to perform their functions: “Corrosion control is integral to strength calculations. The use of the correct fasteners, particularly in a structural steel environment or highly corrosive climate, is critical. The considerations applied to corrosion control of nuts and bolts should, at all times, equal those applied to the main structural members.”
Fastening on to the benefits
A common misconception among users is that any zinc-coated item is hot-dip galvanised: “Hot-dip galvanising produces a significantly thicker zinc coating than zinc plating. Often fasteners specified as hot-dip galvanised are supplied as zinc-plated, resulting in reduced durability due to thinner coatings,” Clarke says.
A zinc-plated fastener typically has a coating thickness of around 10 microns, which is 2.5 to 5 times thinner than the minimum specified in hot-dip galvanising standards. It follows that hot-dip galvanising offers a 2.5 to 5 times greater level of corrosion control and service life.
Clarke also warns against using stainless steel fasteners to connect hot-dip galvanised steel components: “This mismatch of materials creates a galvanic corrosion cell which accelerates corrosion of the hot-dip galvanised component. In fact, the hot-dip galvanised component sacrifices itself to protect the stainless-steel nuts and bolts. The hot-dip galvanised steel retreats from the stainless steel, giving the incorrect impression that the hot-dip galvanised coating has failed.”
Price and aesthetics are also challenges, with the thin coating on a painted article appearing more attractive and more cost-effective. Clarke concedes that, with South African construction supply chains under pressure, there is always a temptation to resort to cheaper options, without sufficiently considering how these will perform from a safety and quality perspective.
Connecting over challenges
Furthermore, Clarke points out that concerns surrounding hot-dip galvanising of high tensile nuts and bolts which exceed a 1,000 mega-Pascals (MPa) tensile strength rating are to be re-evaluated: “While it is perceived that these are prone to hydrogen embrittlement during the galvanising process and should not be hot-dip galvanised, German companies are hot-dip galvanising tensile fasteners at volume. This is done by mechanically cleaning fasteners and flash pickling them, to minimise the potential for hydrogen ingress into the fastener,” he explains.
He also mentions that correct storage and handling of hot-dip galvanised fasteners is imperative. These should be stored and delivered to site with the nut secured onto the bolt: “The torqueing of hot-dip galvanised fasteners is usually done by the ‘turn of the nut’ method. The method of fastening is to turn the nuts to achieve a snug fit between plies. Once that is done, the nut is marked and then turned (depending on the size and length of the nut and the bolt) by a further third to two-thirds full turn.
