Zinc protection for underbody components

Advanced steel alloys continue to gain favor in the auto industry, allowing original equipment manufacturers (OEMs) to reduce metal volumes and weights without sacrificing crashworthiness or performance. However, replacing bulky, mild steel components with thin, high-strength steel alternatives creates a new engineering challenge.

With less mass, corrosion protection becomes crucial. With a 50mm mild steel suspension component, for example, losing 5mm to rust throughout a 10-year lifespan is often acceptable because the remaining 45mm of material is still enough to structurally support the vehicle. With a 10mm high-strength steel replacement part, losing 5mm of mass to rust in 10 years would mean losing half of the component.

“Trends toward high-strength steel will lead to [OEMs] increasing their corrosion standards,” says Sean McKeon, vice president for BASF’s North American OEM Coatings business. “There’s less thickness to the steel, so they have to increase their corrosion standards to meet that.”

BASF and Swiss zinc coating specialist Thermission have introduced Zinc Thermal Diffusion (ZTD), a heat-treat coating process that deposits thin layers of zinc onto steel substrates, creating an intermetallic protective layer that could extend service lives of existing steel components and support further decreases in component masses.

Physical process

Virtually every component on modern vehicles goes through electrocoating (e-coat), in which entire vehicle bodies are submerged in zinc-phosphate tanks to impart ultra-thin layers of that crystalline coating on the metal. Thermission USA CEO Andreas Hackstedt says e-coat is sufficient for sheet metal or 10-year lifespans for thicker steel structural parts, but it doesn’t provide the protection needed for the new generation of lightweight components.

“Zinc diffusion is completely independent of e-coating. With ZTD, we apply a metal layer of zinc onto the substrate between 8µm and 10µm thick, and the OEM will still e-coat that with 10µm to 15µm of zinc phosphate,” Hackstedt says. “Then, you have a superior layer that meets the higher corrosion standards that automakers are mandating.”

The ZTD process uses a thermal chamber that heats the component to about 700°F, along with aluminum and zinc powdered metals. As the temperature rises, the powdered metals vaporize, and the chamber rotates, infusing the metallic powders onto the steel component. Though hot enough to vaporize the powders, the treatment process is cool enough to not change the metallic structure of the underlying steel.

“This is a strategic process, something to apply to the most sensitive parts,” Hackstedt says. “Our target is underbody components. They get hammered by impingements – stone and gravel bombardments that can destroy the coatings on top of it. And those are some of the most safety critical components on the vehicle. The intermetallic zinc-iron structure created by the process protects the crucial interface between the e-coating and the base steel.”

BASF and Thermission are in the process of setting up ZTD demonstration systems in North America. Several OEMs have discussed using the process, but so far, the technology has not been used in finished vehicles. McKeon says he believes the process will be part of the toolbox in supplier plants that provide structural components to OEMs. He adds that it will not be as widespread as e-coat systems that exist in every assembly plant.

“Top tier suppliers will be the early adopters, especially the ones working on body structures,” McKeon says. “Preventing corrosion on these parts is a safety concern – the frame, control arms, suspension components – you don’t want those breaking, or you’re going to lose control.”

Corrosion challenges

Protecting components with thin wall gauges is the primary target for ZTD technology, but there are other corrosion issues that it addresses. Most underbody assemblies are multi-part components made of various stamped parts that have been welded together. As part walls get thinner, those weld points connect less metal. In addition, the heat from the welding process can alter the microstructure of the steel. These conditions can create potential weak points that corrosion could exacerbate.

“The Thermission ZTD process will induce the zinc into the body of the part and into the welds. Weld corrosion and edge corrosion [corrosion that forms on the edges of components instead of the main walls] are the main challenges with e-coating today with standard zinc phosphate,” McKeon explains.

Another problem that has gotten more extreme as vehicles get lighter is galvanic corrosion – metal corrosion that occurs when different materials meet.

“If you look at the Ford F-150, they’re using a lot of aluminum in the body panels, but the frame is high-strength steel,” Hackstedt says. “Somewhere on the vehicle, you have a connection between steel and aluminum. If you don’t protect that connection, you create galvanic corrosion, like a battery effect.”

Typically, only one metal corrodes at the connection point, so OEMs could coat the steel component with zinc to eliminate the issue, he adds.

Supporting the trend

Even with the increases in high-strength steel and lightweight aluminum in recent years, e-coating is still sufficient to protect most vehicles. However, Lepkowski says, industry trends call for cars to be even lighter in the coming years, and there are few ways to do that without targeting vehicle underbodies.

“You’re getting to the end of what zinc phosphate e-coat can do,” he adds. “If you want to get past 10 years with steel or mixed metal, or if you’re going to cut component gauge thickness even more, you’re going to have to look for something different.”

Hackstedt is blunter in his assessment.

“There is no other choice,” he states. “When you get past the combustion-engine cars and look at the move to hybrids and electric vehicles, the goal is for those vehicles to have long travel ranges on a single charge. So, OEMs are going to have to take more weight out, and they can’t do that without addressing corrosion.”

BASF SE
www.basf.com

Thermission AG
www.thermission.com

About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or rschoenberger@gie.net.