Hot dip galvanising is a corrosion control mechanism. Yet, as with many things, hot dip galvanised articles are frequently – and inaccurately – judged on their surface appearance only. “All international standards related to the evaluation and specifications for hot dip galvanising actually state that aesthetics are not important. However, the common perception remains that if an article looks aesthetically pleasing, it must be of high quality. It is imperative that for the attainment of effective corrosion control, focus on the adherence to standards must be vigorously promoted,” advises Robin Clarke, executive director of the Hot Dip Galvanisers Association Southern Africa (HDGASA).
“In a medium-corrosion environment, carbon steel that is left unprotected will corrode at a rate of 25 to 50 microns per annum. In contrast, when hot dip galvanised, the zinc coating will only corrode at 0,7 to 2,1 microns per annum. So a well-galvanised piece of steel with a coating of 120 microns, in an environment where you are losing around one micron per annum, will have a coating service life of more 80 years,” Clarke explains.
In South Africa, the SANS 121 standard is the benchmark for testing of the coating, and the specifications are determined in this standard. “The perception that it is possible to apply 185 microns of zinc on a piece of steel, or 120 microns on another, to produce a ‘good, better or best’ product, is incorrect. The process, which is driven by chemical and metallurgical factors, does not allow one to do that. High-quality hot dip galvanising must be done to the same high standard, consistently and repeatedly,” he adds.
Supply chain links
The most important requirement for meeting the specifications in the SANS 121 standard is good steel. Another key factor is close collaboration between designer, fabricator and galvaniser: “Everyone must do their bit, so that we can predict the outcome and service life with a high degree of confidence. The corollary is: poor steel selection and poor design will lead to poor coating development, or even uncoated areas.”
This applies especially to hyper-reactive steels with high phosphorous levels or silicon content. While reactive steels may appear to be of good quality, they either produce very poor surface finishes, or chip, exposing the substrate. Designers and architects must therefore not only choose the right materials, but design to provide the galvaniser with the best chance of a good outcome.
Clarke also says that there are variations when it comes to finishing. Known as fettling, this requires the smoothing of zinc drainage spikes after steel has been galvanised. A galvanised article managed by the entire supply chain, and processed efficiently, does not require hours of fettling. The resulting surface finish is enhanced, with the attendant benefits of net cost reduction and customer expectations, which have been successfully met.
In Europe, there is a strong focus on process control from the fabricator through adherence to ISO 14713 prescriptions, and from the galvaniser on paying close attention to best practices for surface cleaning and fluxing prior to galvanising.
Even though official quality standards play down aesthetics, Clarke concedes that nothing detracts more from a high-quality galvanised object than poor welding seams, jagged pin holing, discontinuities, or the use of mismatching materials. For these reasons, he admits there is growing emphasis on achieving attractive finishes. “With a collaborative approach from engineers, architects, fabricators and galvanisers, we can achieve a high aesthetic standard consistently and repeatedly. However, this still requires the correct selection of good material to enable the fabricator to include the correct vent and rain holes, and to design so that there is only one submergence in the galvanising bath, with no oxide lines,” he notes.
Quality: the real challenge
As local steel production volumes fall and imports increase, quality issues predominate. While Clarke is not overly concerned that finished product galvanised offshore will fail to meet the global standards on which South Africa’s standards are also based, he is worried about an influx of poor-quality steel ahead of galvanising.
“If a merchant receives a mixed bag of steel, this will impact the entire supply chain. An architect may order the right quality steel and provide us with the certificates. However, when we test the steel, we will discover that the certificates do not accurately represent the entire material,” he notes.
Clarke has also encountered instances where reputable suppliers have inadvertently mixed up their own metallurgy. If the HDGASA spots a trend, this is traced back to the source and a solution is found. This underscores the important industry monitoring role of the HDGASA, and the importance of education and training.
Quality training
To this end, he calls on all hot dip galvanisers to strongly encourage designers, specifiers and fabricators to attend HDGASA training courses to better understand corrosion control.
”Our courses take participants through the fundamentals so that they can understand why the quality standards are written in the way they are. We also focus heavily on the science and on first principles, including the original concept of cathodic protection, and why zinc provides high levels of protection to a carbon steel substrate. Most importantly, our training courses help participants to understand how to achieve quality hot dip galvanising, ensuring they can consistently meet customers’ expectations of product service life,” Clarke concludes.
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