Start date and planned duration: January 2019, 36 months
- Develop REACH compliant coatings based on zinc-nickel and aluminium for cadmium replacement.
- Benchmark the coatings against cadmium coatings firstly by a general salt spray screening test such as ISO 9227 and secondly by using a hydrogen re-embrittlement test such as ASTM F519-17a.
The project seeks to replace cadmium coatings by using electrodeposited coatings of materials with corrosion performance similar to or better than cadmium. Zn and Al coatings are anodic to steel and hence protect steel in the case of any breach. However, due to the high activity of Zn its corrosion rate is sometimes unacceptable. A compromise is reached by using Zn-Ni coatings with an acceptable corrosion rate. Zn-Ni coatings usually contain between 10-15% Ni .
Methods used for the production of coatings could include thermal spray, cold spray, CVD, PVD and electrodeposition. However, taking the complexity of the shapes to be coated (non-line-of-sight areas), cost of coating production and coating thickness requirement into account, a fluid-based process such as electrodeposition would be preferred.
Electrodeposition of Zn-Ni coatings has been used in industry for several years. However, due to issues with IP, variability in testing and commercial competitiveness, industry as a whole has not benefitted. It is unclear which deposition parameters give the best coating performance. Some publications provide deposition parameters, while others give a good indication of the performance based on short-term laboratory corrosion tests. However, these tests are often too short or in unrealistically harsh conditions to be of much practical use. The academic community developing such techniques is often not concerned about industrial standards, cost, ease of application and repeatability. Therefore, the development of coatings, testing, and evaluation following industrial standards should be an important objective. As a first step, the project will look at the literature data on Zn-Ni coatings and make an informed choice regarding the process, parameters and test protocol.
A similar approach will be undertaken for electrodeposition of aluminium.
The project will address the challenges associated with electrodeposition, including selection of electrolyte, electrodes, current density, pressure and temperature. In addition, the use of ionic liquids for aluminium raises further challenges related to the use of liquids that require high temperature, may be expensive, and can be hazardous and/or difficult to handle in air.
Benefits to Industry
Using ionic liquids and deep eutectic solvents will permit superior coatings with performance comparable with cadmium coatings. These will overcome the drawbacks associated with the current state-of-the-art alternatives. Qualification of coatings to ISO9227 and ASTM F519 (standards used by the aerospace industry to qualify cadmium replacement coatings), will allow the results to be exploited directly by industry.
Co-operation with the I-UK “No Cadmium” project CRUPPAIL provides access to the UK supply chain through project partners including the Surface Engineering Association.
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