Corrosion properties of steel protected by nanometre-thick oxide coatings Belén Díaz a , Emma Härkönen b , Jolanta S ´ wiatowska a , Antoine Seyeux a , Vincent Maurice a,⇑ , Mikko Ritala b , Philippe Marcus a,⇑ a Laboratoire de Physico-Chimie des Surfaces (LPCS), Chimie ParisTech – CNRS (UMR 7045), Ecole Nationale Supérieure de Chimie de Paris, 11 rue Pierre et Marie Curie, F-75005 Paris, France b Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland article info Article history: Received 25 October 2013 Accepted 22 January 2014 Available online 31 January 2014 Keywords: A. Low alloy steel B. EIS B. ToF-SIMS C. Oxide coatings C. Atomic layer deposition abstract A comprehensive study of the corrosion properties of low alloy steel protected by 40–50 nm aluminium and tantalum mixed oxide coatings grown by atomic layer deposition is reported. Electrochemical and surface analysis was performed to address the effect of substrate surface finish and whether an oxide mixture or nanolaminate was used. There was no dissolution or breakdown for nanolaminate alumina/ tantala stacks in acidic NaCl solution. Localised corrosion (pitting) took place when defects exposing the substrate pre-existed in the coating. Substrate pre-treatment by brushing and H 2 –Ar plasma was instrumental to block or slow down pit initiation by reducing the defect dimensions. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The interest of producing thin layers on top of metallic sub- strates has been mostly associated to engineering applications such as wear resistance, microelectronics, optics and solar cells devices [1–4]. The ability of producing adherent and dense films of defined stoichiometry and structure has supported the expansion of several chemical vapour deposition and physical vapour deposition techniques suitable for such films development. One of the very important aims of thin layers development of various natures, mostly oxides and nitrides, was also enhanced corrosion protection [5–17]. Markedly improved corrosion resistance is commonly obtained with layers of thickness in the range 1–10 lm, necessary for formation of efficient barrier coatings. A great challenge in the development of the corrosion performances of metallic parts would be to produce nanometre-thick layers (less than 100 nm), in order to reduce dimensional modifications and weight and cost increments [18]. One of the critical points that should be considered to create high quality ultra thin films would be an improvement of the surface finish of the substrate prior to coating deposition. Recent studies have confirmed that most significant reduction of corrosion rates, reaching more than 3 orders of magnitude, of low alloy and stainless steel substrates can be obtained with oxide layers about 50 nm thick, prepared by Atomic Layer Deposition (ALD) and Filtered Cathodic Arc Deposition (FCAD) methods [19–24]. ALD is based on self-limiting reactions on a substrate sur- face [25]. This enables growth of thin films with high conformality and precision on challenging 3D morphologies. Thus ALD coatings can encapsulate also rough surfaces, but if such surface contains mechanically fragile features, defects may form afterwards by a fracture of such site. Nanometre-thick coatings made of pure or mixed aluminium and tantalum oxides have shown significantly improved corrosion properties in comparison to most of the previ- ously reported data [19–22,24]. However these thin layers still contain some defects providing a direct pathway for ions of the corrosive environment to the substrate. Some improvements have been accomplished in order to reduce the surface fraction exposed to aggressive environment, using hydrogen–argon plasma pre- treatment of the substrate surface prior to coating deposition [26]. The alloy substrates may be exposed to aggressive environment during service life, through the remaining channel defects of the coating, and thus analysis of the corrosion properties is essential. The chemical stability of the coating is another issue to be consid- ered in order to ensure the durability of the protection. Our previ- ous studies on pure Al 2 O 3 or Ta 2 O 5 nanometre-thick coatings deposited on low alloy steel substrates showed that despite having the best sealing properties, alumina coatings do not provide the best corrosion protection to steel substrates due to poor stability http://dx.doi.org/10.1016/j.corsci.2014.01.024 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. Tel.: +33 1 46348025. E-mail addresses: vincent-maurice@chimie-paristech.fr (V. Maurice), philippe-marcus@ chimie-paristech.fr (P. Marcus). Corrosion Science 82 (2014) 208–217 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci