German Automotive Supplier Buckles Up for a Greener Coating Process

Mandatory in new vehicles in Germany since 1976, the seat belt saves lives every day. Although not a new development, the seat belt’s metal parts have succumbed to more stringent requirements through the years. Newer requirements ensure mechanical strength and long-term stability.

“For three-point seat belts, metal components must be not only resilient but also permanently stable under variable climatic conditions. At the same time, there are high expectations for their decorative appearance,” explains Thomas Haberfellner, chief operating officer at Schlötter GmbH & Co. KG (Geislingen, Germany), a 110-year-old plating business with locations in six different countries.

These standards are generally fulfilled by means of galvanically separated protective layers made from nickel and chromium. However, the hexavalent chromium compounds used for chrome separation have faced scrutiny under the European Union’s Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) regulation.

Therefore, to implement environmentally friendly materials and practices, automotive seat belt supplier ZF Lifetech (Alfdorf, Germany) upgraded its in-house galvanizing facility and switched from a hexavalent chromium system to an advanced trivalent chromium-based system in September 2023. The conversion was made in close collaboration with Schlötter.

Seat belt coating method

ZF Lifetech, with a parent company that supplies drive and chassis technology for the auto industry, produces seat belts for almost all auto manufacturers. Producing about 40 million seat belt parts per year, the 1,600-employee company introduced the trivalent chromium-based system in response to pending regulatory requirements for European chemicals.

The visual appearance of Schlötter’s electrolytic seat belt coating used at ZF Lieftech is on par with chromium coatings from hexavalent chromium systems. Source (All Images) | Schlötter GmbH & Co. KG

Because a seat belt’s buckle components are exposed to corrosion and friction when in use, these metal parts are coated with an electroplated nickel and chrome layer, which is then overmolded with plastic.

Before coating, the metal parts are punched, deburred, hardened and pre-finished by vibratory grinding. The advanced vibratory finishing process at ZF Lifetech reduces processing times and prepares the surfaces for optimal galvanic coating. In the fully automated galvanizing lines, the parts are degreased, nickel plated and finally chrome plated to fulfill demanding requirements for corrosion resistance and aesthetic appearance.

Improving electrolyte systems

Schlötter has been researching and developing electrolyte systems for trivalent chromium for many years and, as a result, has created a viable alternative to traditional hexavalent chromium systems called Slotochrom DR 2340. Electrolytic post-treatment creates the necessary passivity of the layers, which ensures the required level of corrosion resistance. This method requires a bath with a rectifier and an additional rinsing device.

Unlike other hexavalent chromium-free processes, a color tone identical to the hexavalent chromium process is achieved with a negative b* value. With layer thicknesses in the range of 0.2-0.4 micrometer required by automotive OEMs, the color values are L* 81, a* -0.75, b* -0.4 and are veil-free. L* is the lightness of the color, ranging from 0 to 100, where 0 is black and 100 is white; a* is the red-green value, where negative values indicate green and positive values indicate red; and b* is the blue-yellow value, where negative values indicate blue and positive values indicate yellow. 

A few adjustments to the plant equipment were necessary to prepare for the new system. ZF Lifetech, with Schlötter’s guidance, installed an ion exchanger for bath maintenance and automatic dosing. During the electrolysis process, bath components are consumed, and changes occur in the electrolyte’s pH value. In addition, drag-out occurs because of product throughput. The components required for pH value correction and bath replenishment are automatically refilled after amp-hour throughput to ensure constant operation and consistent results.

Comparing former vs. new finishing systems

Since implementing Schlötter’s trivalent chromium bath and its Slotochrom DR 2340 post-treatment for finishing metal parts, ZF Lifetech reports a positive shift in safety and plant cleanliness because unhealthy perfluorooctane sulfonic acid (PFOS)-containing wetting agents are no longer necessary to reduce bath mist. Now, the additives are dosed continuously from liquid products, and the time-consuming handling and dust from trivalent chromium scales are no longer necessary.

“The collaboration with Schlötter was critical for the success of this project,” says Harald Häfele, head of the surfaces department at ZF Lifetech. “With this new approach, we are supporting our customers’ environmental protection efforts and our own sustainability goals.”