Study of the formation of alkaline electroless Ni-P coating on magnesium and AZ31B magnesium alloy

In this work, alkaline electroless Ni-P coatings were directly formed on commercial purity magnesium and AZ31B magnesium alloy substrates using a process that avoided the use of Cr(VI) compounds. The study focused on two aspects of coating formation: (i) the effect of the substrate roughness on the kinetics of the electroless Ni-P deposition process on magnesium; (ii) the morphological and chemical evolution of the coating on both magnesium and the AZ31B alloy. For these purposes, gravimetric measurements, scanning electron microscopy (SEM), X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and open-circuit potential (OCP) measurements were employed. It is shown that a relatively rough substrate promotes the rapid formation of the Ni-P coating on the substrate surface in comparison with smoother substrates. Furthermore, the presence of fluoride ions derived from the NH4HF2 reagent in the electroless Ni-P plating bath leads to formation of MgF2 a few seconds after immersion in the bath. Subsequently, crystals of NaMgF3, with a cubic morphology, are developed, which later become embedded in the Ni-P matrix. The presence of fluorine species passivates the substrate during coating formation and hence restricts the decomposition of the electroless Ni-P plating bath, which can occur due to release of Mg2 + ions. Finally, according to gravimetric measurements, SEM and XRD, the plating process is initially faster on magnesium than on the alloy. © 2017 Elsevier B.V.

• Fecha de publicación: 2017
• Institución: Universidad de Medellín
• Repositorio: Repositorio UDEM
• Idioma: eng
• OAI Identifier: oai:repository.udem.edu.co:11407/4323
• Acceso en línea: http://hdl.handle.net/11407/4323
• Palabra clave: Coatings grown; Electroless coatings; Magnesium; Surface morphology; Alkalinity; Chromium compounds; Coatings; Magnesium alloys; Nickel; Rutherford backscattering spectroscopy; Scanning electron microscopy; Substrates; Surface morphology; X ray diffraction; AZ31B magnesium alloys; Electroless coating; Electroless Ni-P coating; Electroless Ni-P depositions; Electroless ni-p plating; Gravimetric measurements; Open circuit potential measurements; Rutherford backscattering spectrometry; Magnesium
• License: http://purl.org/coar/access_right/c_16ec