Identifying New Alternatives for Chromium Plating
by Per Møller, Technical University of Denmark, Lyngby, Denmark and Lars Pleth Nielsen, Danish Technological Institute, Aarhus, Denmark
Editor’s Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2014 in Cleveland, Ohio on June 10, 2014. A printable PDF version is available by clicking HERE.
Many industries are looking for alternatives to hard chromium like thermochemical diffusion and PVD. However, it is hard to find one replacement coating fitting all applications where hard chromium is normally the preferred solution. This is simply because hard chromium is not only price competitive but also has several unique properties, such as good corrosion resistance, high hardness, high abrasive wear resistance and good low friction properties, which are all desirable at the same time. In this paper, we present a new iron-phosphorus alloy with 1-6 wt% P produced by an electroplating process, followed by electrodeposition of a topcoat of tin (Sn) of a few microns thickness, subsequently diffusion annealed at 220°C. Characterization of the new coating has revealed that some of the properties known from hard chromium such as low friction, high hardness, excellent resistance against adhesive wear, good ability to retain lubricants and high durability in neutral salt spray tests can actually be matched by this new FeP/Sn alloyed coating.
Keywords: hard chromium plating, substitutes, alternatives, REACH, iron plating, iron-phosphorus, tin, diffusion annealing
Chromium is a hard metal, and is too brittle to be used as a construction material in its pure form. Nevertheless, it finds widespread application as an alloying element in steel and as a surface coating in the form of chromium plating. Stainless steel contains a significant amount of chromium (minimum 10-12 wt%). Additional elements such as nickel can be added to improve the mechanical properties, whereas, e.g., molybdenum can be added in order to improve the corrosion resistance.
Chromium plating can be used in decorative applications to obtain a highly reflective glossy surface (called bright chromium plating) or for engineering applications to obtain a hard, wear-resistant and corrosion resistant surface. The type of chromium plating applied critically depends on the functionality and application of the component in question. Hard chromium coatings have numerous applications and are used on rollers, hydraulic cylinders and various other industrial components and tools.
Hydraulic cylinders are commonly coated with 20-30 µm of hard chromium to counteract abrasive wear. During operation, dust will settle and accumulate on the chromium plated surface. The dust will later be trapped by the oil gasket when the piston moves in and out of the cylinder pipe. The type of wear can be classified as two- or three-part abrasive wear, depending on whether or not the abrasive particle is incorporated into the sealing gasket.
The main difference between decorative and hard chromium is the layer thickness, which is highly dependent on the specified function of the coating. Normally, bright chromium plating will vary from 0.25 to 1.0 μm, and typically hard chromium plating will typically vary between 20-30 µm, but can in special cases be as much as 1000 μm.
The sunset date for using chromium trioxide (Cr+6) in Europe is September 21, 2017. After this date, it will be no longer permissible to use hexavalent chromium for any production in the surface treatment field without having an authorization. March 21, 2016 is the latest date to apply for such an authorization. If an authorization is required, it is specific to a substance and its application(s). It is important to note that there are no application-specific exceptions from authorization granted. This implies that any use of chromium trioxide will be subject to authorization. This also means that each application of a substance requires its own authorization. In the case of chromium trioxide, decorative and functional chromium plating will be separate applications for each type of device, implying that each new product will require its own specific authorization. At present, it is not clear if further detailing into specific applications of hard chromium is required. For example, it may be necessary to specify the sector where hard chromium plating is used in segments, such as aerospace applications, hydraulic pistons, automotive engine components, etc.
Basically each chromium plater needs to be authorized to use chromium trioxide for his own applications. However, authorization for a particular use can be shared throughout the supply chain. For example, if the importer of chromium trioxide applies for, and is granted, authorization for the electroplating of hard chromium for a specific application, although this is not the importer’s own use, then each hard chromium plating shop using material from this importer is compliant with the pre-authorization requirements without having the necessary authorization itself.
As the process of authorization is very complex, costly and time consuming, the industry is advised to collaborate along the relevant supply chains. Close and well organized cooperation of all concerned users of chromium trioxide - importers, formulators and platers - is the key to achieving sound application documents and consistent information.
The reason that hexavalent chromium needs an authorization is the REACH Annex XIV list. REACH is the abbreviation for Registration, Evaluation, Authorization and restriction of CHemicals. REACH entered into force on June 1, 2007, with a phased implementation over the next decade. The regulation also established the European Chemicals Agency, which manages the technical, scientific and administrative aspects of REACH (Fig. 1).