Testing times ahead

As previously reported in Aerospace Manufacturing, the removal of chrome-based compounds from aircraft coatings is going to cause a few headaches for manufacturers ahead of REACH (Registration, Evaluation, Authorisation and registration of CHemicals) regulations coming into full effect. While chrome compounds provide a well established and universal level of performance for parts being treated for corrosion protection, alternative formulations have yet to establish a clear path, although progress is being made.

Typically used in conjunction with a seal coat, the key advantage of chrome as used in metallic coatings, is to act as a galvanic cell, which when exposed to the environment undergoes an electrochemical reaction that reduces it from a trivalent to a hexavalent oxidation state, essentially sacrificing itself in place of the substrate. The importance of this is to ensure that components are protected if and when external coatings fail.

CERAM's two main areas of business consist of design consultancy and the testing and analysis of materials and surface treatments. As such the company's activities have frequently taken it into the area of coating development, working with both airframers and their suppliers. The effects of the restriction, set to come into full effect in 2013 will be wide ranging, so what methods are being used to develop new formulations?

Principal consultant for aerospace and defence Dr Chris Pickles explains: “The complex chemistry involved in reformulating anti-corrosion coatings depends critically on the developer's ability to understand the interfacial phenomena occurring between the various ingredients, between the coating ingredients and the substrate layer and between the coating ingredients and the environment it is being exposed to. “In formulating alternative systems it is clear that the precise chemical and microstructural makeup of the coating is of crucial importance in order to optimise the galvanic mechanism by which protection is achieved. The complete characterisation of metal surfaces requires a variety of approaches, including the examination of chemical structures, topographical profiles and microstructures.”

Part of the problem lies in the fact that without chrome, the large range of high performance applications for such coatings, from critical ‘hot end' engine components to outer aircraft surfaces will require increasingly tailored solutions with different chemical compositions and microstructures, so there will be a great deal of activity as new ideas are developed.

Pickles continues: “In terms of coating structure, we would use surface analysis techniques like X-ray spectroscopy and mass spectrometry as applied to surfaces. The X-ray technique will give you information on any metallic pigments in there, their oxidation states and their abundance so there's a number of ways we can investigate the surface chemistry. Surface mass spectrometry analyses sputtered material and gives information on molecular composition. In terms of the microstructure we'll use electron microscopy to look at grain structure and pigment particle size.
Every one of these aspects is potentially important. Quantitative 3D surface topographical profiling has also been applied to the specification and control of substrate surface roughness.

“Quantitative surface elemental spectroscopy operating in high resolution mode can distinguish between the two oxidation states of chromium with high precision and this analysis is routinely used in the characterisation of such coatings. Moreover, similar capability exists for all metallic elements to which it could otherwise apply and this will be important in supporting the development of non-chromium coatings.” The application of new products is also something highly dependent on chemical composition with new systems incorporating alternative curing and deposition methods to make the most of new formulations. Despite the many variables however, the importance of alternative coatings is well understood and supported by many industry leaders with the first products, albeit with some performance shortfalls, beginning to filter into the market. Much work still needs to be done however, to ensure disruption can be avoided before the restriction comes fully into force. www.ceram.com