Electrochimica Acta 55 (2010) 804–812 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Surface analytical characterization of passive iron surface modified by alkyl-phosphonic acid layers A. Paszternák ∗ , I. Felh ˝ osi 1 , Z. Pászti, E. Kuzmann, A. Vértes, E. Kálmán 1 , L. Nyikos Chemical Research Center of the Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri út 59-67, Hungary article info Article history: Received 10 June 2009 Received in revised form 7 September 2009 Accepted 7 September 2009 Available online 15 September 2009 Keywords: Corrosion protection Passive iron Alkyl-phosphonic acid CEMS XPS abstract Surface modification with self-assembled alkyl-phosphonic acid layers is a powerful technique for cor- rosion protection of different metals. In the present study, passivated, native (covered with air-formed oxide) and metallic iron surfaces were used as substrates. The influence of the oxide on the phosphonate layer formation; the corrosion protective effect of the phosphonate layer and the structure of the modified surfaces were investigated. The layer formation process was monitored directly in the phosphonate- containing solution. It was found that the phosphonate layer formation process depends on the conditions applied during iron passivation, and the oxide layer has an important role in the stability of the protective layer, while the bare metallic iron surface is disadvantageous for phosphonate bonding. Conversion electron Mössbauer spectroscopy (CEMS) and X-ray photoelectron spectroscopy (XPS) proved the presence of phosphonate layer on the top of the passive iron surface, with the thickness around monolayer. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Layers of amphiphilic organic molecules that form sponta- neously on substrate surface by adsorption from solution have been intensively studied as an advantageous technique of surface modification and functionalization. Protection of metal surfaces by organic self-assembled monolayers (SAMs) is a new way to fight against corrosion. Papers have been published already on the application of alka- nethiol SAMs on iron [1,2] and copper [3,4] surfaces. Thiols form a well-ordered layer on metallic surfaces. Organosilicon protective layers were also extensively investigated [5] similarly to phospho- ric acids [6–8], carboxylic acids [9,10], hydroxamic acids [8,11], imidazole compounds [12] and sulfonic acids [8], which bond preferably to oxidized surfaces. Highly ordered films of alkyl-phosphonic acids can be used also to modify the chemical and physical properties of different metal surfaces in a wide range of applications including corrosion protection [6,7,13–17]. The protective effect of phosphonate lay- ers for iron has been discussed in our previous papers [18–22]. Alkyl-phosphonic acid molecules might be able to replace envi- ronmentally restricted, e.g., chromate surface treatments. The phosphonate layer formation proved to be a spontaneous process ∗ Corresponding author. Tel.: +36 1 438 1166; fax: +36 1 438 1164. E-mail address: paszti@chemres.hu (A. Paszternák). 1 ISE member. that could be accomplished by simple immersion into the aque- ous solution of the phosphono compounds. Significant corrosion protection was observed, markedly decreasing the anodic dissolu- tion of substrate iron. Our previous studies were done on passive (potentiostatically passivated [19]) and on native (covered with air- formed oxide [21]) iron. It was proved that the presence of oxide at the iron surface plays a significant role in the phosphonate layer formation. It has also been shown by other authors that phosphono groups interact strongly with several metal oxides, mostly through the formation of stable M–O–P bonds [11,23]. The aim of the present study is to characterize the influence of the surface oxide on the phosphonate layer formation. In order to reveal the role of the oxide, several samples has been prepared, and potentiostatic passivation has been performed in borate buffer solution in the potential range between -400 and 800 mV (versus SCE). Passivated samples were compared also to native (covered with air-formed oxide) and metallic (cathodically reduced) iron surfaces. The layer formation process has been monitored directly in the phosphonate-containing solution by measuring the open circuit potential and the electrochemical impedance spectra. The corrosion protection effect of phosphonate layer has been inves- tigated in a neutral solution of 0.1 mol dm -3 of NaClO 4 . Attention was also paid to the investigation of the phosphonate layer struc- ture. The structure and composition of surface film on modified iron were determined with conversion electron Mössbauer spec- troscopy (CEMS) and X-ray photoelectron spectroscopy (XPS). The question was: Is the alkyl-phosphonic layer really a monolayer on the pre-passivated iron surface? 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.09.023