Browsing by Author "Adediran, A.A."

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Corrosion Protection by Novel Conversion Coatings on Structural Al 606
(King Mongkut's University of Technology North Bangkok, 2023) Oki, M.; Adediran, A.A.; Anyim, I.; Onokohwomo, C.O.; Bosa, C.
Chromate conversion coatings have witnessed limited acceptability in recent times. The coatings contain Cr (VI) species that have been classified as environmental hazards and injurious to human health. Thus, the use of environment-friendly and non-carcinogenic novel inorganic-inorganic hybrid conversion coatings are being explored. Vanadate (VCC), hybrid Vanadate/Molybdate (HCC) conversion coatings on Al6061 have been classified in terms of corrosion and adhesion performance with reference to the untreated alloy. Natural exposure tests in the atmosphere and stagnant near-neutral 3.5% sodium chloride solution, as well as potentio-dynamic polarization measurements showed that the corrosion rate for HCC is lower for Vanadate, which in turn outclassed the ‘bare’ alloy. However, clusters of passive incipient pits were revealed on the former after 120 h of exposure in stagnant chloride solution. Both conversion coatings outperformed the untreated aluminium alloy after atmospheric corrosion and adhesion tests.
Corrosion rates of green novel hybrid conversion coating on aluminium 6061
(Elsevier B.V., 2020) Oki, M.; Adediran, A.A.; Ikubanni, P.P.; Adesina, O.S.; Adeleke, A.A.; Akintola, S.A.; Edoziuno, F.; Aleem, A.
The use of chromate conversion coatings have been limited by several protocols as a result of their carcinogenicity and toxicity towards humans and the environment. Searches are ongoing for chromate replacement in coating baths and processes. This paper describes the comparison among the corrosion rates of a novel hybrid conversion coating derived from water extracts of hibiscus sabdariffa calyx in conjunction with ammonium molybdate, a molybdate conversion coating and the so-called chromate conversion coating. Potentiodynamic polarization measurement in 3.5 wt% sodium chloride solution was employed in ranking the coatings as sabdariffa molybdate being more corrosion resistant than chromate, which in turn out performed molybdate.
Electron-optical and auger electron spectroscopy studies of a zirconium conversion coating on Aluminium
(Institute of Metals and Technology, Ljubljana, 2019) Adediran, A.A.; Oki, M.; Akintola, S.A.; Ogunsemi, B.; Akinlabi, E.T.
The techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultramicrotomy and energy dispersive X-ray analysis (EDX) were employed to examine conversion coatings on aluminium developed from a zirconium nitrate/fluoride solution. The coating developed slowly on a microscopic metal substrate with islands of zirconium-rich centres within the coating matrix. According to the TEM assessment, the coating thickness was 30 nm for specimens treated for 60 s and 50 nm for those treated for 900s. The population of the zirconium-rich centers ranged from 2.8 × 1012 m–2 to 6 × 1012 m–2 over the treatment period. According to Auger-electron-spectroscopy in-depth analyses, the coating is composed of three diffused layers with an outer region of zirconium oxide followed by a layer of aluminium oxide/hydroxide and an inner layer at the metal/coating interface comprising compounds of aluminium and fluorides. The corrosion-resistance and paint-adhesion characteristics of the zirconium conversion coating are superior to those of bare aluminium.
Scanning and transmission electron microscopy examinations of composite hybrid chromate and chromate phosphate conversion coatings exposed in hot 100% relative humidity environments
(Elsevier B. V., 2023) Ikubanni, P.P.; Oki, M.; Adediran, A.A.; Akintola, S.A.; Adeleke, A.A.
Conversion coatings are sine qua non in the finishing of aluminium alloys. They may be put into service without coating of paints. However, for aesthetics in architectural applications, paint overcoat is common. Chromate phosphate coatings are relatively less toxic than chromate which gained the pride of place until recently. Thus, aluminium specimens coated with hybrid chromate/phosphate conversion coatings have been subjected to 100% relative humidity for 480 h at 313K in comparison to chromate coatings and bare aluminium. Scanning electron microscopy examination revealed that initial dried river bed morphologies on coated specimens were obliterated. The friable outer coatings collapsed into the cracks/environment leaving a relatively smooth surface after exposure. Additionally, transmission electron microscopy of sections before and after exposure revealed similar texture and morphological striations nearly parallel to each other throughout the length of the coatings to the metal/coating interface. These were not disrupted as well as the metal/coating interface which remained relatively smooth with no visible corrosion products after exposure. Thus, the coatings formed effective barrier between the substrate and the relatively harsh environment. From EDS, elemental compositions were the same albeit, spectral intensities remained relatively constant after humidity exposure regimes. The coatings are made up of either Cr, O and Al for chromate and Cr, P, O and Al for chromate/phosphate. At deliberately breached regions, the coatings prevented spread of corrosion and paint delamination over the substrates. On the other hand, bare aluminium was decorated with mounds of hydrated aluminium oxide/hydroxide after exposure as revealed in the SEM/TEM.