Going chrome-free

The entire dynamic for coating aircraft materials is being rearranged as the REACH regulations come into force. One such example is the banning of chrome-based compounds, which will have wide implications for the way critical structural parts are protected. As the European Union's REACH (Registration, Evaluation, Authorisation and restriction of Chemicals) directive continues to be felt across the manufacturing industry, innovations in chemical technologies have had to come thick and fast. Of the sectors affected, coating producers will be one of the hardest hit and in the area of turbine coatings, the requirement to move away from widely used, but potentially carcinogenic chromium-based coatings is an important step change that will need to be made in the near future.

As things stand, there is a degree of confusion over the way in which rules are to be implemented with the prospective restrictions open to a certain amount of interpretation as well as opposition coming from organisations such as the Aerospace & Defence Industries Association of Europe. The belief within the industry is however, that use of sodium dichromate – a precursor of many chrome-based products – will be severely limited in the near future. Registering for use is not only a complex and expensive process, but will also mean the publication of intellectual property. Thus, with the urgent need to develop alternatives, Birmingham-based coating and paint manufacturer, Indestructible is about to embark on a £160,000, 15 month research project, funded in part by Advantage West Midlands and the European Regional Development Fund, to remove the substance from its current range of aircraft coatings.

At the heart of Indestructible's R&D programme - which involves industry partners Rolls-Royce, Messier-Dowty and research, testing and consultancy company CERAM as a sub-provider - is its Ipcote range. Developed initially at the request of Rolls-Royce during the 90s for use in gas turbines and compressors for high temperature erosion and corrosion resistance, Ipcote now consists of a host of basecoats, sealcoats and diffusion products which have since seen approval by Pratt & Whitney Canada, Snecma, Turbomeca, ITP Spain, MTU Aero Engines, and is currently being trialled by General Electric.

The standard Ipcote basecoat in its current formulation - which contains only 37ppm hexavalent chrome, is typically applied at a thickness of 50µm and can be cured either at 560°C, 350°C with additional bead peening or polishing depending on the application, or at260°C with an extended dwell time. This yields a typical corrosion resistance of 3,000 hours. A smooth surface alternative for improved engine efficiency, Smoothcote, which makes uses of aluminium powder consisting of smaller particles (1µm as opposed to 5µm), is also available, offering a denser and more erosion resistant product for improved performance. As an interim measure, organic chrome free version of these systems have become available, however this necessarily contains solvents, which bring their own environmental problems and are more expensive to produce.

The big concern overall is that viable replacements will not reach the market in time to prevent the compromise of aircraft programmes. In terms of a practical solution, several main factors will need to be addressed in order to reach a tangible and commercially viable replacement. The aircraft industry has become accustomed to the performance levels delivered by chrome containing products as they offer the key properties of good adhesion, corrosion resistance and the ability for molecules to crosslink and promote a stable oxide film. As such, chrome is a key part of the overall formulation and has typically been used in all aspects of turbine coatings, be it in pre-treatments, primers, basecoats or sealcoats.

Within a typical turbofan, 15-20 different coating products are specified to protect against heat and corrosion. For example, Ipcote is relied upon to protect the central shaft of an engine, a component that carries the load of the engine and also provides the interface with the major rotating parts. The normal procedure is to apply a conductive, sacrificial basecoat with a non-conductive sealcoat on top to seal any porosity and to give a smoother finish. Typically made from highly corrodible steel, engine shafts rely entirely on the coating to protect it from condensation and oil contamination, the effects of which are exaggerated by the perpetual heating and cooling. Engine oil for example can typically pick up between 400-800ppm moisture, a figure that increases substantially under adverse conditions. A further consideration is that as weight drives development and materials improve, the decreasing thickness of shaft walls has placed additional pressure on coating products.

Of course, integrity is everything, but there are a variety of cases where current alternatives cannot match the requirements of certain applications, compromising service lives and the safety of components. To give an example of the change in performance, one particular test using a former Ipcote product containing 5% chromic acid ensures that the product will last for between 4,000 and 5,000 hours in a salt spray environment. Current products with already vastly reduced chrome contents due to previous environmental legislation are typically tasked with achieving only 1,000 hours. While this can be achieved, chrome-free products still fall short of this necessary time span.

In addition to this, current chrome-free products have a usable life of around four to six weeks and must be applied within this timescale to ensure the formulation performs as it should. There is also the fact that some products that do meet or come close to the necessary performance requirements are not stable enough to be stored for extended periods of time, meaning that it is impractical to apply such products in the field. Additionally, while chrome compounds offer largely universal characteristics across a variety of materials, this is not the case for many proposed alternatives, complicating and greatly extending the amount of materials that would need to be investigated and tested for certification.

With every challenge however, lies opportunity and Indestructible's position in the market as an SME and the progress it has made, has drawn the attention of several important manufacturers. Technical sales manager, Graham Armstrong explains: “For one project with Eurocopter, we are looking at magnesium and aluminium treatments for components like gearboxes. Eurocopter approached us 18 months ago because it wasn't getting much help from the larger companies. We were able to change its systems to totally chrome-free with the exception of the primer we supply, which we're working on. Indestructible has formulated some samples and there's now a whole matrix of trials to be done on various panels and multi-substrates and looking at steels, various grades of aluminium and magnesium. Altogether this will probably be three to four years in gestation before it goes live on a helicopter, but we were chosen because we will be able to produce a custom product.”

Amid its own research and similar developments with other major manufacturers, it is hoped that the reformulation of all coatings within the Ipcote range will have been completed within the next few years.

www.indestructible.co.uk