Surface finishing of ferrous and nonferrous metal components is getting increasingly important, especially for underhood applications. Higher temperatures make materials more prone to corrosion or can even take the protective surface layer of a component to its limits. In this case, the components will be additionally organically surface treated or protected by an additional coating layer, which adds up to a multilayer design. Often the use of stainless steel is an expensive last resort under extreme operating conditions. Multimaterial, lightweight engineering can be another cause for corrosion as the direct contact of different metals can result in bimetal corrosion caused by differing electrochemical potentials.
High structural loads that require high-strength-steel (HSS) grades can also be an issue as HSS requires special care during surface treatment. First, the process temperature must be limited to avoid structural changes, and secondly, water-based electroplating baths can lead to hydrogen embrittlement (ISO 8044) of steel, which can seriously impact the material's properties.
Typically steels with a strength of >1050 MPa (152 ksi) mark the critical limit for electrogalvanizing. Surface finishing can increase the choice of material: if a suitable protective plating process is available for corrosion protection, it will be possible to use standard steel grades instead of more costly stainless steels.
From a technical point of view, electroplating with aluminum is advantageous as the characteristic aluminum-oxide film provides excellent corrosion protection. An aluminum electroplating process, developed by Aluminal Oberflächentechnik GmbH & Co. KG since 2002, meets many of the high expectations for surface coatings, according to the company.
One of the key advantages is a new water-free electroplating bath that restricts the use of water to the pre-treatment. The actual surface layer is deposited on metallic components in a chamber filled with inert gas. An electric current feeds an aluminum anode that sets free aluminum. The aluminum then travels to the component surface via the organic solvent. Currently, the emissions-free, low-temperature process is being used for components on racks or in drums.
“By using this technology, hydrogen embrittlement is completely avoided,” said Walter Bernardi, Managing Director of Aluminal. “We regularly plate spring steel with material strengths of 2000 MPa without the need for a subsequent heat treatment of the components. In comparison to electrogalvanized zinc or nickel platings that typically reach their temperature limit at around 130°C, our aluminum plating withstands up to 400°C,” Bernardi added.
As the plating is electrodeposited, it has the high purity level typical of this process: The Aluminal plating has an aluminum content of >99.99% and therefore behaves like the chemical element. “At a minimum thickness of just 6 µm, salt spray tests according to ISO 9227:2006 showed a 30 to 50% increase in corrosion resistance in comparison to standard electrogalvanizing technology,” said Bernardi. If required, the plating can additionally be passivated.
Materials that are currently electroplated on automotive series production level include regular steel grades, stainless steel (rich in Cr and Ni), all HSS with a strength level of >1050 MPa, steels that contain lead, and cast iron. The company, a member of the Friedhelm Loh Group, has been using the process for BMW and Volkswagen components for more than two years now. The plating process meets customer specific requirements as exemplified e.g. in the U.S. Army MIL-DTL-83488D, the VW TL 243:2003-02, and the BMW Group Standard GS 90010: 2004-02 or DIN EN 15646.
Use cases include a pipe loop for an air-conditioning system (BMW) and a number of coated HSS retaining rings for VW Group. Volvo uses the coating for high-strength stainless steel retaining clamps. Since the beginning of 2008, several chassis components for a steering application and steel springs are aluminum electroplated for two more German vehicle manufacturers. Future applications could be with components that are exposed to green diesel fuel as the zinc-free plating is resistant to rape seed methyl ester (RME).
At the company’s technology center, called Technikum, specific coating processes for nonferrous metals, aluminum alloys, and zinc die-cast components are also being developed, but these materials require special pre-treatment and/or a material specific process control and are currently limited to the Technikum level. The two existing production lines at Montabaur can plate up to 2 million m2 of surface annually.
From a manufacturing point of view, the Aluminal plating offers another potential benefit, said Bernardi: “You can work the plated material up to the failure limit of the substrate without the aluminum plating coming off. As a consequence, the corrosion protection can be applied, for instance, before deep drawing of the actual part.”