Study of ceramic‐like aluminum oxide thin films developed using plasma electrolytic oxidation applied on austenitic steels

Abstract

Austenitic steels 304 L and 316 L are used extensively as nuclear structural materials. The objective of this study is to determine if their corrosion behavior can be improved by developing an Al‐containing surface layer by a complex surface treatment, including substrate modification by growth of oxide layers using anodic oxidation, cyclic voltammetry and autoclaving in water, as well as a micro‐arc oxidation (MAO) treatment in 0.1 M NaAlO2 aqueous solution. The processes and structures obtained by MAO under various experimental conditions were characterized by X‐ray photoelectron spectroscopy, scanning electron microscopy, X‐ray diffraction and energy dispersive X‐ray spectrometry analysis. The corrosion behavior, studied by Tafel potentiodynamic polarization, was correlated to the microstructure and the chemical composition of the surface layers which are strongly dependent on the treatment parameters. An overall description of the electrochemical processes involved in the growth of aluminum coating, the surface properties improvement, together with some consideration about the new materials development for energy technologies are presented. Copyright © 2016 John Wiley & Sons, Ltd.

Growth of Anodic Layers on 304L Stainless Steel Using Fluoride Free Electrolytes and Their Electrochemical Behavior in Chloride Solution

“…According to Yang et al [ 36 ], a hole that permits direct contact between the substrate and the electrolyte decreases the anticorrosive performance since it acts as a channel, allowing a pitting corrosion mechanism to develop. In addition, the electrochemical behavior of the anodic layers can also be associated with the absence of a stable barrier layer in the metal/oxide interface, as mentioned by Andrei et al [ 28 , 37 ]. They advised the formation of a barrier layer of iron oxide without Ni and Cr before the advanced anodizing treatments (MAO) to obtain coatings on austenitic stainless steel using aqueous solutions of NaAlO 2 .…”

Aluminum Oxide Ceramic Coatings on 316l Austenitic Steel Obtained by Plasma Electrolysis Oxidation Using a Pulsed Unipolar Power Supply

“…• the voltage in the direct current PEO mode (320 V in [30,32]) is applied permanently on the sample, while in PEO pulse mode, a time interval of one period is applied, after which it is zero. In our experiment, the pulse voltage applied is over 500 V, much higher than in DC, which allows for higher temperatures in electrical discharge in the microarc and the synthesis of a larger amount of Al 2 O 3 ; • between the two impulses, the oxidation layer material cools and crystallizes the molten portions of the layer, which makes it possible to obtain layers of oxide with much higher thickness than in the continuous PEO mode.…”

“…In the case of non-valve metals (base metals) as steels, it can be used as two methods to develop a porous oxide barrier layer required for initiation of the micro-discharge oxidation processes [30][31][32]:…”

“…Recently, the feasibility of using PEO in aqueous NaAlO 2 for preparation of ceramic-like aluminum oxide films on the surface of austenitic stainless steels was proved [32]. After preliminary treatment by the autoclave oxidation, which produces a thick layer of magnetite suitable for the formation of the barrier layer over the 316L steel substrate, ceramic-like layers with a thickness of ≈ 30 µm, made of aluminum oxide were obtained by micro-arc oxidation in 0.1 M NaAlO 2 and 0.05 M NaOH aqueous solution, using DC, constant voltage conditions, 320 V. Ceramic-like aluminum oxide coatings were non-uniform, porous, and discontinuous.…”