Henkel's BONDERITE Two-Step (Flex) Process enables seamless transition to increased aluminium content

Aluminium does not take naturally to zinc phosphate. The metal that automakers have raced to adopt for lightweighting sits awkwardly inside a chemistry designed, decades ago, for steel. Conventional zinc phosphate baths tolerate perhaps 25 to 30 percent aluminium in a mixed metal line before the process turns unstable, throwing off sludge and quality problems that ripple through the paint shop. That mismatch, and the rise of aluminium bonnets, battery trays and body structures, is the problem Henkel's BONDERITE Two-Step (Flex) Process was built to solve.

David George, Surface Treatment Technical Manager at Henkel, describes it plainly. "The Two-Step Process, a patented Henkel process, is a hybrid, or ‘bridge’ technology that combines traditional zinc phosphate conversion coating, with zirconium - or thin-film - conversion coating technology."

Two chemistries, one production line

The logic of the Two-Step Process is elegantly simple once explained, if intricate in execution. Zinc phosphate remains highly effective on steel-based substrates, cold rolled, hot rolled and zinc coated variants such as electro galvanised and hot dip galvanised steel. Aluminium, however, is left almost untouched at that stage.

Mike Kripl, Senior Principal Application Engineer at Henkel, puts the underlying chemistry in blunter terms. "The way I usually explain it is that traditional zinc phosphate doesn't like aluminium, and aluminium doesn't like zinc phosphate. Zirconium, on the other hand, works very well with aluminium."

Henkel's answer is to lightly etch the aluminium with zinc phosphate and further down the line, apply a zirconium bath that conversion coats the aluminium and provides a final sealing treatment for the steel-based substrates. So, it performs a dual-function.

Depending on the metal mix, the Two-Step Process can accommodate around 75 to 80 percent aluminium, according to Kevin Miller, Principal Engineer at Henkel who puts the current figure at "eighty percent." Full zirconium, or thin-film, technology can, by contrast, process the entire spectrum up to 100 percent.

The two-thirds sludge story

The commercial and environmental case for Two-Step rests substantially on sludge. Ordinary phosphating produces sludge as a natural by-product, but aluminium generates significantly more of it than steel, which is one of the reasons conventional zinc phosphate struggles as aluminium content climbs.

Removing the zinc phosphate stage from the aluminium's journey removes that additional burden entirely. As George explains, "By eliminating zinc phosphate coating on the aluminium, that additional sludge simply isn't created."

The numbers, where Henkel has them, are striking. Kripl cites one customer whose production volumes held broadly steady, output in fact rising, while its sludge output fell dramatically. "The plant reduced annual sludge generation from approximately 213,000kg to around 67,000-68,000kg."

For plants running the older technology, that reduction often necessitated a laborious process of manually removing zinc phosphate detritus on a regular basis simply to keep up with maintenance. Hannah Reister, Account Manager for BEV Automotive Components, has witnessed the contrast directly.

"I've seen customers operating competitor systems where the zinc phosphate tank has to be emptied every weekend to remove accumulated sludge. By switching to the BONDERITE 2798 Flex Process, customers can significantly reduce the amount of preventative maintenance required on the line."

Built for battery trays, and their quirks

Nowhere is the pressure on pretreatment sharper than around the electric vehicle battery enclosure, now among the largest single aluminium applications on any production line. Complex geometries and tight racking bring their own challenges, chiefly around what Kripl calls “solution flow”. Enclosures racked too closely together can starve internal sections of adequate cleaning and rinsing.

Miller offers a vivid illustration of the process at work on such parts. "I saw an excellent example of that at one of our customers’. They were processing aluminium battery trays, and during cleaning we could still see faint suction-cup marks left by the material handling equipment," he says.

The marks, a legacy of the trays' handling rather than any defect, might have telegraphed through the e-coat had they persisted. They did not. "As the parts progressed through the pretreatment process, the marks became progressively less visible. Once they passed through the BONDERITE 2798 stage, that final etching completely removed them."

A bridge, by design

Henkel is candid that the Two-Step Process was never meant to be a final destination. George is explicit that “it was always intended as a bridge technology," a way of letting existing zinc phosphate lines absorb far more aluminium without wholesale reinvestment. Converting a conventional line to Flex, Kripl notes, "is relatively straightforward, provided the line has enough process stages." The harder journey comes later.

"Moving from the Two-Step Process to a full thin-film or zirconium process is more involved,” he says, “even though zirconium offers significant advantages, including ambient operating temperatures and very little sludge generation."

That gap exists because zinc phosphate, whatever its limitations with aluminium, remains forgiving of variable incoming parts, oils and lubricants in a way full zirconium is not. "Zinc phosphate offers more latitude when dealing with variations in incoming parts, oils and lubricants," George says.

"A full zirconium process doesn't provide quite the same level of flexibility." The Two-Step Process, by retaining the front half of a conventional zinc phosphate line, keeps that robustness intact while still opening the door to aluminium. It is, in other words, indeed a ‘bridge’, but one decked with efficient savings and whose structure flexes to adapt to its oft-unpredictable environment.

Turning practical challenges into process advantages

The limits of Two-Step are, by most accounts, more practical than technical. On the OEM side, George reports few significant obstacles. Component suppliers, working through Henkel's automotive components business, face a narrower but real challenge, namely whether their existing lines have room for an additional stage to house the zirconium sealer. Where that stage does not exist, Kripl says, "implementing the Two-Step Process becomes, of course, more difficult."

Kevin Miller's team addresses the other recurring headache; cleaning, through dedicated studies for customers introducing new part geometries - work he describes as central to keeping the wider process robust.

One case illustrates both the difficulty and the payoff. A component supplier processing around 85 percent aluminium on one line had struggled with an incumbent's technology for the best part of a year, its process failing to clean or coat properly. Miller's team, initially brought in to support a separate line, ran a trial batch of parts from the troubled one. "Within three attempts, simply by adjusting cleaning times, we had optimised the process and the parts were coming out perfectly." The full line was converted soon after.

The long road to zirconium

Henkel's own history with zirconium dates back to around 2007, when it became the first pretreatment supplier to introduce the chemistry to OEM assembly plants. Two body pretreatment lines in North America still run the two-step process, alongside several European OEMs, but George expects OEM plants generally to keep converting to full zirconium as their tightly controlled manufacturing environments make that transition easier to manage than it is for component suppliers.

For Tier suppliers, adoption tends to lag, and Miller describes persuading them as "a gradual process," complicated by the sheer variety of parts, metals and contaminants a typical component line handles. What unites both markets, he adds, is a broader shift toward lower operating temperatures across cleaning, pretreatment and e-coat curing, driven increasingly by sustainability rather than cost alone.

Asked to distil the Two-Step Process into a single message for OEMs and Tier suppliers, Henkel's team returned, more than once, to the same theme. "For me, it's sludge reduction," says Kripl. Miller agrees. "Waste treatment is a major challenge for many of our component customers, and this process dramatically reduces that burden." There's also a significant cost-saving element through reduced sludge generation. While waste disposal isn't eliminated entirely, disposal costs can be reduced dramatically compared with conventional zinc phosphate.

Ultimately, Henkel’s Two-Step Process is a practical pathway for manufacturers to modernize at their own pace – enabling improved sustainability and operational efficiency without requiring an immediate overhaul of existing production lines.