REACHing out for new materials
With hexavalent chromium compounds recently placed on the REACH list of restricted substances, the aerospace industry is searching for replacement materials for surface treatments. David Rickerby, Head of Surface Engineering at Rolls-Royce, UK, explains how the REACH CRVI HITEA project is addressing the issue.
The aerospace industry is heavily reliant on surface treatments containing hexavalent chromium to prevent corrosion and wear in products that have a design life in excess of 40 years. Hexavalent chromium compounds (HVCs) such as chromium trioxide (chromic acid) and chromates/dichromates are used in various chemical processes. Anodised coatings (notably chromic acid anodising [CAA]) have a columnar cellular structure of aluminium oxide, which by itself is a porous film. Sealing of the anodised film by filling the microcolumns with a dichromate-containing water seal further enhances its performance and provides self-healing corrosion protection not possible with other types of sealant. Similarly, the presence of strontium chromate particles in paint systems (particularly primers) fulfils the same purpose – when the film is damaged, the particles are able to leach to the damaged area, thereby maintaining corrosion protection. Also, it’s recognised that optimum paint adhesion on an anodised surface is when paint is applied to an unsealed surface, as the paint can key directly into the micro-columns.
In terms of tribological applications, hard chromium plating is used to provide specific sliding and hard wearing properties, for example hydraulic cylinders for landing gear. It is an extremely versatile process that allows manufacturers to coat the internal surfaces, which are inaccessible to other processes. The process can be tailored to offer outstanding low friction properties in both dry and, when combined with the fine crack network present in the coating, lubricated contacts. Conversion coatings are also used to protect components from corrosion, and these can be used in the field to make repairs where a component has suffered minor damage that penetrates the corrosion protection. The aerospace industry has decades of experience in both the production and performance of these coatings and therefore can use them with a very high level of confidence and predictability.
On the 1 June 2007, the EU introduced regulation concerning the registration, evaluation, authorisation and restriction of chemicals (REACH) within the European Economic Area (EEA), which includes Norway, Liechtenstein and Iceland. REACH applies to substances manufactured in or imported into the EEA. One of the aims of the legislation is to control the use of such substances via authorisation and encourage industry to substitute them for safer ones. HVCs are classified as substances of very high concern (SVHC) because they are carcinogenic, mutagenic or toxic for reproduction. The molecular entity that drives the carcinogenicity of the HVCs used in corrosion and wear applications is the Cr(VI) ion, which is released when the substances solubilise and dissociate. With this hazard classification, HVCs were included on the REACH Annex XIV Authorisation List in 2013, with a sunset date of September 2017. The sunset date is the deadline for placing on the market or use of the HVCs, unless a user is granted REACH Authorisation by the European Commission, taking advice from the European Chemical Agency (ECHA) and EU member states. However, for an authorisation to be granted, the applicant must also demonstrate that there are no suitable alternatives.
The implications for industry of the REACH legislation can be explained by the coating processes employed in the manufacture of just one component fitted to a modern civil airframe – an engine guide vane actuator. To be REACH compliant, the component must be redesigned, with alternative coatings investigated and implemented to ensure that performance is not impaired in any way. To satisfy this requirement, two key issues need addressing. First, coating properties must be defined and measured to prove that an alternative is fit for purpose in a given application. Second, testing methodologies must be identified that correlate with the current service lives experienced with existing technology.
It is against this backdrop that a 17-member consortium was formed, made up of industrial aerospace end-users, suppliers, paint application companies and UK universities. With support from a TSB/EPSRC R&D competition launched in 2012, Highly Innovative Technology Enablers for Aerospace (HITEA), the consortium formulated a REACHcompliant hexavalent chromium replacement for corrosion protection programme, for identification and evaluation of suitable alternative systems. This REACH CrVI HITEA programme is an opportunity for the UK to position itself as the leading exponent of REACH-compliant materials science, maintaining the competitiveness of the UK aerospace industry while offering broader exploitation opportunities within the UK surface treatment industry.
Technical aims of the REACH CRVI HITEA project:
Provide a performance database for current chromium-free processes and development of standardised wear and corrosion methodologies, to validate the reliability of new REACH-compliant coatings prior to more extensive component testing within the consortium.
Establish a fast, inexpensive and robust testing methodology for selecting the most promising chromium-free alternatives to industrial accelerated corrosion tests, such as the ASTM B117 salt spray test. This will quantitatively assess the practical performance of chromate-free coatings and produce a ranking between competing alternatives, and quantify the performance gap, if any, between chromium-based processes and their corresponding chromium-free alternatives. Develop a centralised data management system that allows the information to be communicated to end-users in a robust and useable form.
Compile information on new specifications for these alternative material systems, which have been shown by the methodologies adopted within the project to be effective in preventing corrosion and wear, in a form that will benefit other end-users.
Corrosion testing is an essential step in the development of new protective treatments and to validate performance before final deployment. The test time can be reduced by increasing the aggressiveness of the environment or, for electrochemical tests, by applying a significant electrical perturbation. However, this may trigger corrosion mechanisms that would not be observed during service life, thereby compromising the predictive capability.
REACH CrVI HITEA is focused on developing advanced corrosion testing methods, linear polarisation resistance (LPR), electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ENA), with the aim of improving the predictive capability of accelerated testing, but retaining the capability of obtaining fundamental information. ENA was selected for this approach because it is unique among the electrochemical techniques, since it does not require the application of a probing signal. In ENA, two identical specimens are immersed in the test environment and electrically connected through an external circuit. As a result of corrosion, the current and potential fluctuations are recorded. Using ENA, the consortium has been able to rapidly optimise and assess the performance of a variety of novel surface treatments, including a new family of chromium-free, environmentally friendly anodising treatments.
Given that existing test data on the performance of the wear and corrosion material solutions has been generated over the last 40 years, and that the range of data sources from both within the consortium and externally generated was complex, it was imperative that an effective material information management system was selected. The chosen platform, GRANTA MI, offers significant benefits, including the ability to compare the proposed chromium-free alternatives identified within the consortium against relevant lists of restricted or authorised materials, ensuring that the next generation of materials systems are sustainable in the long term.
By consolidating the external reference data on material properties and processing available to each partner, the project is able to support effective decision making in the specification and use of alternative coating solutions.
To date, the consortium has completed and populated the knowledge repository, adding more than 500 records for the current CAA and chromate conversion coatings (CCC). The partners are now testing the REACH-compliant alternatives with a wide range of paint suppliers and coating companies. All these systems will be compared to the current chromated systems and more than 1,000 data sets will be entered into the database to facilitate cross-comparisons and down selection of the systems to progress to the next round of testing.
In terms of developing a direct replacement of electrolytic hard chromium plating, the consortium has identified a number of alternative processes, which are currently being assessed by a range of tribological and electrochemical tests designed to down select the most viable systems for progression through to technology readiness level four – described as an ‘alternative validated in a representative environment using a production-capable process’.
It is likely that the chromium-free alternatives developed under this project will find applications in other industrial sectors and that the sound scientific understanding on which these developments are based will allow for effective deployment of material systems into a broad product range. While thermal spraying can be an excellent substitute for hard chromium plating, there are a number of components that, as a result of their size, shape or material properties, require alternative process technologies, and these are also being assessed within the REACH CrVI HITEA project.
As well as providing functional in-service technologies, a number of hexavalent chromium compounds are required for special chemical processes used in general manufacturing operations. Because there is no in-service need for the use of these chemicals, they are often overlooked when discussing REACH. In many cases, these processes are specified by customers and the fixed processes are carried out by suppliers. The customer often has little experience of alternatives, with suppliers retaining the expertise. This increases the rate at which customers are moving to outcome-based specifications, with suppliers empowered by their experience. This allows suppliers to streamline their own fixed processes to align internal methods for all of their customers.
One example of common substance use across the aerospace industry is trichloroethylene, which is used for general cleaning and degreasing. This substance is also being controlled by REACH and has a sunset date of April 2016. Suppliers are able to substitute with the most practical alternative as long as the customer specification is met. REACH is a phased approach to substance regulation and substances are constantly being reviewed for control. REACH is transparent in its target of substances, which allows time before they become unavailable. Examples include refractory ceramic fibres for insulation, cadmium in plating and braze alloys, boric acid for plating and brazing operations as well as 4,4’-diaminodiphenylmethane (MDA) used in composite processing. Many of these can be seen in use in the image above.
Cadmium was a known health, safety and environmental issue well before REACH and is still present on some parts manufactured for legacy products. The legislation will prevent the continued use of this technology, and elimination from legacy products will be necessary. Cadmium plating provides corrosion protection and is used in a number of applications. Because there are many alternatives to cadmium plating and no one-to-one replacement, phase-out will require a case-by-case assessment of each application. This will incur significant costs due to the volume of engineering resource required by the end-users.
Whatever REACH and other substance regulation mandates on the industry, there is no doubt that industrial collaboration is a worthwhile approach to identifying alternatives. The REACH CrVI HITEA example shows that it can work, but also that it is important to secure access to a broad range of complementary skills to ensure confidence in a successful outcome to these complex engineering change projects.
For more information, email David Rickerby, email@example.com