Zinco UK (Hereford, United Kingdom), a provider of advanced technical services for the hot-dip galvanizing (HDG) industry, developed new ultrasonic technology with the University of Warwick (Coventry, United Kingdom) to test the integrity of galvanizing kettles loaded with molten zinc.
HDG coats steel with zinc by dipping the metal into a molten zinc bath. The resulting galvanized steel is widely used in applications that require corrosion resistance.
But these steel kettles need to be checked periodically to monitor their corrosion rates to avoid a spill of molten zinc. The corrosion rates can vary depending on volumes, dip sizes, and the amount of production at each plant, the developers explain. Thus, solutions such as the ultrasonic probe have arisen.
“We’ve been measuring kettles since 2007 and we wanted to develop a better system,” says David Watkins, managing director of Zinco UK. “So we decided to work with Warwick to develop a better method.”
Placement of the Probe
Their method involves mounting a probe on an aluminum framework that sits on top of the zinc kettle at a galvanizing plant. The probe is then inserted into the zinc by a vertical motor drive or manual operation.
In this process, the probe is connected to a laptop computer, and the depth of probe inserted into the molten zinc is programmed by a touchscreen controller. The probe contains a high-temperature ultrasonic sensor that launches ultrasonic waves into and from the molten zinc. The internal components of the ultrasonic transducer inside the sensor are designed to survive the high temperatures without any cooling.
The probe is typically submerged into the zinc for about six hours, Watkins explains. Once the probe is pulled to the side of the kettle wall, the signal is interpreted and saved. Readings are generally taken from the hottest end, since this is usually the location of the most corrosion.
After the probe is used, it cools down naturally and has a thin layer of zinc at its tip. Each probe has a design life of 200 operational hours, or in excess of 30 inspections, as long as it is properly maintained and not subjected to large thermal shocks.
When a customer places an order, the manufacturer ships the equipment and sends a technician to the site, if necessary. The technician takes a grid measurement of the entire kettle, Watkins explains, to provide a profile of the corrosion. The data information is then collated and sent in a detailed report to the customer, which is saved for future use.
Hot-Dip Galvanizing Process
HDG protects against corrosion by providing a barrier against corrosive compounds. Since zinc is anodic to steel, the HDG also acts as a sacrificial anode if the galvanized coating is damaged. If areas of underlying steel are exposed, the surrounding zinc will provide sacrificial cathodic protection to the unprotected sites by corroding preferentially. The zinc is consumed as it sacrifices itself to protect the bare steel.
The HDG process results in a top surface layer of pure zinc for galvanic and barrier protection, as well as several intermetallic zinc alloy layers that form as the zinc coating is applied under high temperatures. These layers are metallurgically bonded with the steel to form a coating that provides protection.
Limitations of Prior Testing
Previous monitoring techniques involved the use of external probes that required draining the zinc from the kettles to another device, Watkins explains. From there, the kettle would be left to cool to a point that physical sampling of the kettle’s surface could be conducted.
“The traditional methods involved emptying the zinc kettle and then undertaking an ultrasonic test measurement of the kettle to profile the corrosion,” Watkins says. “This obviously would mean considerable downtime in production.”
According to the developers, the new probe can provide accurate thickness measurements of kettles—even while immersed in zinc at 450 °C.
While Watkins acknowledges that other probes are on the market, he says they require the use of argon gas. This method, on the other hand, does not.
“We have always felt that [using argon gas] was an operational limitation, as you have to transport the gas from site to site,” Watkins explains. “We were also unsure as to the effect of corrosion of the sensor.”
Warwick scientists Mark Potter and Steve Dixon—specialists in non-contact ultrasonic nondestructive testing (NDT)—formed spin-out company Sonemat in 2005 to focus on the growth of ultrasonic solutions for NDT issues that include crack detection, preventive maintenance, wall thickness loss monitoring, and rapid inspection. In 2014, the spin-out company and technical services provider established a formal relationship to collaborate on the technical development of the probe.
Since 2014, the manufacturer and school have successfully tested the probe in field applications throughout Europe as well as locations in Africa, the Middle East, and Malaysia.
According to Watkins, about 3,000 galvanizing kettles around the world can potentially benefit from this type of inspection. He says the probe can provide significant safety data while also maximizing the return on investment for important pieces of equipment.
Source: The University of Warwick, www2.warwick.ac.uk. Contact David Watkins, Zinco UK—Email: david.watkins@hereford.galvanizers.co.uk.