Progress in Organic Coatings 41 (2001) 121–127 Corrosion inhibition under heat transfer of 904L stainless steel in phosphoric acid by benzotriazole A. Bellaouchou ∗ , B. Kabkab, A. Guenbour, A. Ben Bachir Laboratoire d’Electrochimie-Corrosion, Faculté des Sciences, Avenue Ibn Batouta, BP 1014, Rabat, Morocco Received 17 July 2000; received in revised form 5 September 2000; accepted 18 October 2000 Abstract The inhibition of the corrosion under heat transfer of the stainless alloy 904L by benzotriazole (BTA), in a phosphoric acid composed by 40% H 3 PO 4 + 4% H 2 SO 4 + 300 ppm Cl - has been studied, by using electrochemical and spectroscopic techniques. Results obtained by polarisation measurements show that BTA affects both anodic and cathodic processes. With the increase in concentration of BTA the corrosion rate decreases and the inhibition efficiency increases. The highest inhibitory efficiency is obtained at 10 -1 M in BTA. The inhibitory effect is related to the chemisorption of BTA at the surface of the 904L alloy. Analyses of the surface by SIMS and X-rays confirm this mechanism. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Corrosion; Alloy 904L; Benzotriazole; Phosphoric acid; Heat transfer 1. Introduction The main stages of making phosphoric acid by wet pro- cesses (WPA) generally involve an attack of ore phosphates by sulphur acid, a filtration of the pulp and a concentration of acid. Corrosion problems occurring in WPA plants have been examined previously [1–3]. Phosphoric acid is used at different concentrations. Ammonium phosphate is produced through phosphoric acid at 40%. Commercial concentration is at 52–54% in P 2 O 5 , and this concentration is widely used in manufacturing liq- uid fertilisers. The high rate in P 2 O 5 is a potential advantage that concerns transportation. The most widely used process for phosphoric acid concentration is indirect heating followed by an air-tight evaporation. In this process, stainless steel exchangers, where vapour runs internally (T = 140 ◦ C), are submerged into the phosphoric acid to be concentrated (T = 80 ◦ C). The heat exchanger is the element of the concentration unit exposed to the most severe conditions and requires, there- fore, most maintenance. The heat transfer through the tubes speeds up the kinetics of metal solution exchange reaction, which is manifested by a significant increase in the corro- sion rate. Thus, in the unit of concentration of the industrial phosphoric acid, equipment is strongly damaged due to the ∗ Corresponding author. Fax: +212-7-775440. E-mail address: guenbour@fsr.ac.ma (A. Bellaouchou). severe corrosion and temperature conditions. Indeed ma- terials are submitted to the aggressiveness of the medium (presence of Cl - ,S 2- and F - ions) and to the heat transfer imposed by the technology of concentration. In a previous work [4–6], we developed a methodology for studying the corrosion process of materials under heat transfer in industrial phosphoric acid. We have finalised an experimental set-up that simulates these conditions and allows also a precise fundamental study of different types of heat exchange [4]. Azole-type organic compounds have been widely used as good corrosion inhibitors [7–13]. Amongst these com- pounds, BTA is an effective corrosion inhibitor for copper [14–18] and iron [12,13,19]. BTA forms a chemisorbed protective polymeric film on the copper surface [14,18,20]. Recently, the inhibition of corrosion of stainless steel in var- ious acidic media by different types of organic compounds have been studied [21–25]. However, a limited number of studies have been devoted to triazole derivatives as corro- sion inhibitors of stainless steel in acidic media (H 3 PO 4 , H 2 SO 4 , HCl). The aim of the present work is to evaluate the inhibitory properties of BTA on corrosion under heat transfer of stain- less steel in phosphoric acid solutions. The electrochemical behaviour of 904L stainless steel in phosphoric acid in the absence and in the presence of BTA at different concentra- tions has been studied with potentiokinetic polarisation and surface analyses spectroscopic techniques (SIMS, X-rays). 0300-9440/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0300-9440(00)00157-0