Abstract
Cathodic plasma electrolytic deposition (CPED) is a plasma-assisted surface modification technique that enables the formation of ceramic coatings from an electrolytic precursor on conductive metallic substrates such as steel. This process presents a highly versatile anti corrosion treatment, as it is applicable to work pieces of complex geometries, can penetrate small gaps and has a small environmental footprint. In the present work, CPED is used in combination with an electrolyte mixture containing aluminium nitrate and PEG as an additive to deposit oxidic aluminium on stainless steel. During deposition, aluminium nitrate is decomposed in a cathodic plasma discharge. The deposition process is optimized towards its potential use for hydrogen barrier coatings. Here, the main objective is to achieve compact layers with as little pores and cracks as possible, showing good adhesion. The layer thickness needs to be adjusted in order to find a compromise between the need for a thin film showing less mechanical stress during pressure and/or temperature cycles, while still delivering a sufficient layer thickness to reduce hydrogen permeation to a tolerable level for the specific substrate and application. This is achieved by tuning the electrical parameters during deposition, additive concentration and the duration of the treatment. The samples are analysed comprehensively by means of electron microscopy, x-ray photoelectron spectroscopy, x-ray diffraction and profilometry to assess the quality of the coatings and to develop a detailed understanding of the processes involved during layer formation. It is found that the coating quality can be improved significantly by increasing the additive concentration, which is known to reduce the influence of the direction of the electric field. The aluminium content varies for different deposition voltages and tends to increase for longer treatment durations. The resulting optimized coating routine delivers homogeneous layers showing excellent adhesion and is applicable to work pieces of complex shapes.