CORROSION SCIENCE 865 CORROSION–Vol. 52, No. 11 Submitted for publication May 1995; in revised form, February 1996. * Equipe de Métallurgie Physique, Laboratoire des Matériaux (URA CNRS 445), Ecole Nationale Supérieure de Chimie (INP), 118 Route de Narbonne, 31077, Toulouse, France. ** IRSID, Creusot Loire Industrie, CRMC, BP 56, 71202 le Creusot, Cedex, France. *** Laboratoire de Chimie Physique Appliquée, Departement de Chimie, Av Ibn, Batouta, BP 1014, Rabat, Morocco. † Trade name. 0010-9312/96/000189/$5.00+$0.50/0 © 1996, NACE International Passive State Behavior of Special Austenitic and Ferritic Stainless Steels in Phosphoric Acid Polluted by Sulfide Ions S. El Hajjaji, L. Aries, N. Pebere, F. Dabosi,* J.-P. Audouard,** and A. Ben Bachir*** ABSTRACT The passive state behavior of the ferritic Z1 CD 29-4 stain- less steel (SS) in industrial phosphoric acid (40 wt% H 3 PO 4 with 1,000 ppm chloride [Cl – ] and 330 ppm sulfate [SO 4 2– ]) polluted with 20 ppm sulfide ions (S 2– ) was compared to that of the austenitic Z2 CNDU 25-25 SS, which is known for its high corrosion resistance. Comparison was made using electrochemical techniques, electron spectroscopy for chemi- cal analysis (ESCA), and secondary ion mass spectroscopy (SIMS). The presence of sulfide ions (S 2– ) increased the dissolution rate of the austenitic SS to near that of the ferritic SS. Degradation of the protective passive film on the austen- itic SS was attributed to formation of compounds of nickel and copper at the expense of chromium and molybdenum. Behavior of the two SS in the passive state was not very different. KEY WORDS: austenitic stainless steel, chloride, corrosion rate, ferritic stainless steel, films and film formation, phosphoric acid, sulfide, sulfate INTRODUCTION In the wet process of phosphoric acid (H 3 PO 4 ) manu- facturing, impurities such as chloride (Cl – ), fluoride (F – ), sulfate (SO 4 2– ), and sulfide (S 2– ) ions combined with solid particles in suspension significantly in- crease corrosion rates of metals and alloys. 1-9 Stainless steels (SS) generally are used because of their high corrosion resistance in such media. 1,3,7 S 2– ions increase the corrosion rate of special SS, and they modify the nature and/or kinetics of the formation of surface films that control the behavior of the alloy. 3,9 Chromium and molybdenum as alloying elements improve the corrosion resistance of SS. In the presence of S 2– , the Z1 CD 29-4 ferritic SS exhibits better behavior than some special austenitic SS. 9 The objective of the present work was to study passive state behavior of this ferritic SS compared to the austenitic Z2 CNDU 25-25 SS in H 3 PO 4 solutions polluted by S 2– . EXPERIMENTAL Chemical composition of the austenitic Z2 CNDU 25-25 and ferritic Z1 CD 29-4 SS studied are given in Table 1. Electrochemical measurements were performed using a Tacussel † corrosion unit including a PJT † potentiostat/galvanostat and an IMT1 † program- mable interface. The experimental setup for impedance measurements was composed mainly of a Schlumberger † electrochemical interface and a Model 1250 † frequency response analyser. Complex imped- ance of the working electrode was plotted in Nyquist representation. Results were obtained at 600 mV SCE in the frequency range 10 mHz < f < 60 kHz. The integration time was set to five cycles.