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Title: Electrochemical corrosion evaluation of aluminium-based coating alternatives to cadmium plating
Author: Fasuba, Omoniyi
ISNI:       0000 0004 5356 5486
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2014
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Electroplated cadmium has been employed for decades as a coating material to provide protection for steels against corrosion because of its combination of inherent corrosion resistance in atmospheric conditions, bi-metallic corrosion characteristics, lubricity and good electrical conductivity. However, it is a toxic metal and known carcinogenic agent, which is plated from an aqueous bath containing cyanide salts. For these reasons, the use of cadmium has been banned in Europe for most industrial applications. Nevertheless, the aerospace industry is still exempt due to stringent technical and safety requirements associated with aeronautical applications, until a reliable and acceptable replacement is found. Aluminium-based coatings are potential candidates to replace cadmium because of their low toxicity and high corrosion resistance in aggressive media. The corrosion properties of commercially available aluminium-based coatings: Al-Zn flake inorganic spin, Al-based slurry sprayed and arc sprayed Al coatings deposited on mild steel were investigated. Al, AlCr and AlCr(N) coatings deposited by Electron Beam Physical Vapour Deposition (EBPVD) technique on M2 and 17/4 PH steel substrates at 3000 C were also investigated. The range of electrochemical techniques used to evaluate the corrosion properties of the coatings include: Open-circuit Potential (OCP), Potentiodynamic Polarisation (PDP), Electrochemical Impedance Spectroscopy (EIS), Galvanic coupling and Electrochemical Noise (ECN) measurements. The structure and composition of the coatings were characterised by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) spectroscopy. For the commercially available Al-based coatings, the Al-based slurry coating exhibited an open-circuit potential closer to the mild steel substrate than other coatings, as well as a low corrosion current density and a more positive corrosion potential. In terms of galvanic compatibility with the steel substrate, both the Al-Zn flake inorganic spin coating and the Al-based slurry sprayed coating show low galvanic current, in comparison with the arc sprayed Al coating and cadmium. This behaviour confirms their superior cathodic protection capability and galvanic compatibility over other coatings. Good correlation between data from the ECN, OCP, potentiodynamic polarisation and EIS measurements was found; in particular electrochemical noise resistance (Rn) correlates well with polarisation resistance (RP) and corrosion current density (icorr). However, magnitudes of elements in EIS results vary widely when compared to those of the potentiodynamic polarisation tests. Furthermore, analysis of the current-potential noise time data reveals uniform corrosion for cadmium and Al-Zn flake inorganic coating, passivation for Al-based slurry sprayed coating and localised corrosion for arc sprayed Al coating. However, the polarisation results revealed localised corrosion for Al-Zn flake inorganic coating, indicating that the corrosion mechanism of the coating is mixed (i.e. both localised and uniform). For the EBPVD Al-based coatings, OCP and PDP measurements show that the incorporation of chromium and nitrogen in the coatings enhanced the corrosion resistance. However, this also resulted in the ennoblement of the corrosion potential, hence the AlCr and AlCr(N) PVD coatings deposited on M2 steel could not preserve their sacrificial character, while the same coatings deposited on 17/4 PH steel remained sacrificial with respect to the substrate. This difference is due to the respective open-circuit potential of the two substrates. Good agreement between data from the different electrochemical techniques was found. The AlCr(N) coatings show strong passivation, the AlCr coating deposited on M2 steel displayed localised form of corrosion, while its counterpart deposited on 17/4 PH corroded uniformly. This discrepancy lies in the difference in their coating morphology and Cr content. Thus, the corrosion behaviour of the EBPVD Al-coatings deposited on M2 steel is strongly dependent on the chromium and nitrogen content. Overall, the AlCr coating deposited on the 17/4 PH steel demonstrates combined barrier and sacrificial protection capacity. Characteristic charge, q and frequency of corrosion events, fn could not distinguish properly between the different types of corrosion associated with the EBPVD Al-based coatings, indicating that these parameters cannot be relied upon as true indicators of corrosion mechanisms. Finally, it was shown that utilising the localisation index (LI) and the roll-off slope of power spectra density (PSD) to discriminate between different types of corrosion needs further investigation, especially with regards to passive systems, where ECN measurements are often contaminated with extraneous noise sources, leading to false estimates of values obtained from these parameters.
Supervisor: Yerokhin, Aleksey ; Leyland, Adrian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available