Development of novel coatings for the protection of high strength steels (NSIRC 217)
Due to its toxicity and carcinogenic effects, cadmium (Cd) has been a “substance of very high concern” to the EU. The aerospace and defence sector specifies cadmium on many safety-critical high strength steel (HSS) components and it is likely that this will continue for a few more years. However, new products will be required to implement Cd alternatives. There have been many research projects to develop suitable replacements for all applications of cadmium with varying degrees of success.
Electrodeposition of Zn-Ni coatings has been undertaken by industry for several years. However, due to issues with intellectual property (IP), variability and commercial competitiveness, industry as a whole has not benefitted. It is unclear as to which deposition parameters give the best coating performance. Some literature gives deposition parameters and performance data based on short-term laboratory tests. However, these tests are often too short or in unrealistic conditions to be of much practical use. Therefore, the development of coatings, testing, and evaluation following industrial standards is an important objective.
The project seeks to develop electrodeposited coatings with corrosion performance similar to or better than cadmium. Some of the candidates include Ni, Zn, Zn-Ni and Al. Of the above, Ni is cathodic to carbon steel which implies that any breach or coating defect would lead to preferential corrosion of the substrate. Zn and Al coatings on the other hand are anodic to steel and hence will protect steel in the case of any breach. However, due to the high activity of Zn the corrosion rate of Zn is sometimes unacceptable. A compromise is reached by Zn-Ni coatings (10-15% Ni) with an acceptable corrosion rate.
As a first step, the project will look at the literature data on Zn-Ni and Al coatings used for the protection of high strength steels and make an informed choice regarding the processes, parameters and test protocol. The project will also investigate the use of ionic liquids and deep-eutectic solvents (DES) for the deposition of coatings. Electrodeposition of Zn, Cr, Ag, Ni, Zn–Sn alloys and Cu composites from this DES has been demonstrated previously. Modification of the above composition has also been made to show beneficial effects. These electrolytes have been used to deposit single metals in most cases. The work on the deposition of Zn-Ni alloys by deep eutectic solvents is rare. However, no work has been reported on the behaviour of the coatings in service conditions following a standard.
Zn-Ni coatings will be developed using aqueous and non-aqueous solvents as electrolytes. Optimisation of the coating production will also be attempted. For aqueous systems, chlorides of Zn and Ni will be taken in requisite amounts and electrolysed. As Al cannot be deposited from aqueous solutions, ionic liquids (IL) will be employed. The electroactive solutions will be prepared by dissolving the aluminium source in the selected ionic liquid. Deposits will be prepared by a potentiostatic technique.
Two types of tests will be carried out: screening tests possibly using salt spray, and final embrittlement/re-embrittlement tests using a standard. ASTM F519 outlines a procedure for hydrogen re-embrittlement testing (in test solution), which may be used as an alternative to ASTM F1624. A scanning electron microscope (SEM) will be employed for examination of the coating surface morphology and an energy dispersive X-ray spectrometer (EDS) will be used for chemical composition analysis of the elements in the coating. Coating-cross sectional analysis will be carried out to analyse the coating microstructure and measure the coating thickness. SEM/EDS will be employed for this purpose.
NSIRC is a state-of-the-art postgraduate engineering facility established and managed by structural integrity specialist TWI, working closely with lead academic partner Brunel University, the universities of Cambridge, Manchester, Loughborough, Birmingham, Leicester and a number of leading industrial partners. NSIRC aims to deliver cutting-edge research and highly qualified personnel to its key industrial partners.
Candidates should have a relevant degree at 2.1 minimum, or an equivalent overseas degree in Chemistry, Materials, Physics, Chemical Engineering or a related subject. Candidates with suitable work experience and strong capacity in coatings development are particularly welcome to apply. Overseas applicants should also submit IELTS results (minimum 6.5) if applicable.
This project is funded by TWI and academic partners. Up to £24k per annum scholarship is available which will help cover tuition fees and provide a STIPEND for the duration of three years.