First-choice option for steel protection
Perry Metal Protection’s new multi-million dollar hot dip galvanising plant in Hamilton offers a level of capacity and capability not previously seen in New Zealand. Stefan Richter drove to Te Rapa to get a first impression of the new facility that will be operational as of 16 January 2012.
It inevitably occurs every time iron or steel, atmospheric oxygen and water come together. Iron oxide, better known as rust, is formed in an electrochemical process on the surface of the steel, and the red-brown colour we perceive indicates that the metal has started to degrade. One of the possibilities to prevent steel from rusting is to completely envelope its surface in a protective coat of zinc to prevent any contact with the corrosive environment. That is exactly what happens during the hot dip galvanising process in which the two partners zinc and steel form a metallurgically bonded coating that provides outstanding abrasion resistance and long-term protection against rusting. Perry Metal Protection, a division of the Perry Group, have been utilising this process since the early 1970s and offer their services at five locations in New Zealand: Auckland, Hamilton, Tauranga, Wellington and Christchurch. The company has just opened their newest facility on the site of its existing galvanising facility in Te Rapa/Hamilton which now houses the largest zinc bath in New Zealand.
“We have been in the business for 37 years, and now our existing factory is outgrown and so we needed to build a new one. It took us three years to research, plan, design and finally build this new facility,” says Duane Baguley, General Manager, Perry Metal Protection. “And with an overall investment of 8 million NZD we were asking for a significant amount of money, which required a quite stringent approval process from our board to secure the funding.” The new plant will be able to treat steel structures up to 18 metres in length. “Currently the maximum size suitable for galvanising using the double dip method in the country is about 14 metres,” says Duane, “and in our existing facility we can dip structures up to 11 metres. We are therefore taking a quantum leap forward compared to the maximum dimensions of steel structures that can be treated by our competitors.”
Perry Metal Protection service the construction industry by galvanising structural steelwork and pipes, and they also treat cow sheds, agricultural sprayers and more for the agricultural industry, and help the leisure industry by making trailers more durable. But you could as well approach the company to get your garden gate or a beam covered with a zinc layer to protect it from rusting. The General Manager says that the new plant has doubled the capacity of the existing plant. “We strive to provide a better service to our existing customer base,” says Duane, “But we also believe that the ability to treat larger steel structures will provide a greater range of choices and opportunities for the construction industry that haven’t previously existed.”
Perry Metal Protection has employed a business development manager to promote galvanising as the first-choice option for steel protection. “His mission is to get out into the market place and talk to the specifiers, the architects, design engineers and fabricators,” explains Duane. “We have found out that there is a reluctance to specify hot dip galvanizing due to lack of information available to the architects and engineers. And when a specifier or engineer does not have enough information and/or experience with this technology, then they are not going to specify and utilise this form of protective coating. So we highlighted the need to provide further education on the advantages of hot dip galvanising.” In the field of metal protection the two main systems “competing” with hot dip galvanising are paint systems and zinc arc spraying, with the latter being an alternative method of applying a zinc protective coating. “When it comes to achieving the specifications and a reproducible result the galvanising process is much simpler than any of these two other procedures,” comments Duane.
The galvanising process
According to Wikipedia it was a French chemist by the name of Paul Jacques Malouin who in 1742 described a method of coating iron by dipping it in molten zinc in a presentation to the French Royal Academy. And in 1837, another French chemist, Stanislas Sorel, obtained a patent for a method of coating iron with zinc to protect it from rusting. “The electrochemical basics of galvanising haven’t changed since that time,” says Duane, “What has changed and improved are the process technology, the materials handling and the environmental control mechanisms we put in place.”
Galvanising does not simply apply a coat of zinc to the steel item, the coating does not just stick to the surface like paint does. Zinc and steel undergo an electrochemical reaction and form an alloy which is metallurgically bonded to the steel. To achieve this durable bond galvanisers utilise five process steps.
The steel structure to be treated is dipped in a hot caustic solution bath where grease, oil, paint, organic materials and other surface contaminants are removed.
The next bath contains hydrochloric acid solution and rids the steel of rust and mill scale. After this step the steel is rinsed with water to remove any pickling acid residues.
This process step is necessary to remove the oxide film that forms after the acid treatment. A further oxidation before galvanising the steel item would affect the quality of the zinc-steel bond to be formed and jeopardise the complete and consistent coverage of the steel surface. The flux solution usually is 30 % zinc ammonium chloride with wetting agents, which is kept above 65 degrees Celsius. After this process step the item is dried.
The steel is fully immersed in molten zinc which is kept at around 450 degrees Celsius. Draining and venting holes provide an escape route for hot air and allow for the molten zinc to reach the inside surface of hollow spaces in the steel structure respectively for the excess zinc to leave those spaces after the structure is taken out of the zinc bath. The period of immersion does not determine the thickness of the alloy layer formed. Anyway, the galvaniser has to wait for the steel item to heat up to the temperature of the molten zinc, which can take up to several minutes when massive structures are immersed.
The steel item is quenched to cool the item and passivate the zinc surface to prevent early oxidation. After this step the item is ready to be deburred, inspected and, if desired, painted.
The benefits of galvanising
“We at Perry Metal Protection believe that galvanising constitutes the premium-protection system for steel, and there is a lot of research data available which proves that this established process provides effective corrosion protection,” says Duane. The list of benefits that the General Manager provides is quite impressive. The process of applying paint to a surface is much more dependent on variables such as temperature, humidity and air quality than the galvanisation process. “And when the painter is having a bad day and does not clean the surface properly, or applies the paint unevenly, then the overall result can be severely affected,” says Duane. “Galvanising on the other hand is not subjected to the skills of a specific person; it is an industrial process with reproducible results and consistent quality outcomes.”
It is also impossible to apply paint to the inside of a hollow space, resulting in an unprotected surface. Another problem with paint systems is known as under-film corrosion. This term refers to the possibility of rust forming beneath the paint coat on the steel surface, for example after an impact damage has occurred. The steel might degrade undetected until the component ceases to function. “Because the zinc forms a durable bond with the steel, galvanising makes for a tougher and more abrasion resistant coating when compared to paint system. Transport and erection damage is therefore less likely to occur as the treated item can take a lot of knocks and punches.” This is also the reason why galvanised surfaces are low in maintenance. “Most paint coatings require frequent inspection and renewal, and when the coating is breached, corrosion begins at the affected area and quickly spreads,” says Duane. “Galvanisation is the ultimate solution, especially for applications in coastal areas, where rusting and corrosion are boosted by salt air and water. We think it absolutely necessary to use galvanised components more often in these so called sea spray zones.”
Duane reports that there are light poles in Australia that have been up for 150 years – untouched. “In New Zealand it would be very easy to achieve a lifecycle of 25 to 30 years without any problems,” says the General Manager. This fact has to be taken into account when talking about the costs of various steel protection systems. “Sometimes galvanising is higher in first cost than paint systems, but inevitably zinc beats paint in the area of life cycle costs because galvanising needs less maintenance and lasts longer.” Last but not least, zinc is also a very sustainable solution. ”All of the by-products we create in the galvanising process, namely ash and dross, are marketable products that can be sold on and re-processed. And zinc itself is fully recyclable.”
Perry Metal Protection’s new hot dip galvanisation plant is operational as of 16 January, and the old factory on the same site was decommissioned over the Christmas break. “Hot dip galvanising is a proven process to protect steel from rusting,” says Duane. “The NZ Transport Agency and KiwiRail trust this technology and have all their light poles, their guard rail systems, their gantry signs and their overhead line masts galvanised. Many Government agencies specify hot dip galvanisation because they know they get the best result for the best total price.”