XII Iberian Meeting of Electrochemistry XVI Meeting of the ...

XII Iberian Meeting of Electrochemistry & XVI Meeting of the Portuguese Electrochemical Society PD 09
Effect of chloride and sulphate ions on the electrochemical
behaviour of an austenitic stainless steel, the H A Z and the
welded metal in phosphoric acid medium at different
temperatures
Blasco-Tamarit, E.; García-García, D.M.; Ibáñez-Ferrándiz, M.V.;
García-Antón, J.
Ingeniería Electroquímica y Corrosión (IEC). Departamento de Ingeniería Química y Nuclear.
E.T.S.I.Industriales. Universidad Politécnica de Valencia. P.O. Box 22012, E-46071 Valencia.
Spain.
jgarciaa@iqn.upv.es
Industrial phosphoric acid solutions are chemically complex and their analysis changes
from one plant to another, depending on the quality of phosphate ores and the process
utilized. Phosphoric acid produced according to the wet process, in which the phosphate
ores are attacked by sulphuric acid, is the most important of such chemicals. During the
wet process, impurities as chlorides and sulphides are incorporated to the phosphoric
acid, which becomes this acid in the most aggressive industrial phosphoric acid. So that,
severe corrosion problems on the equipments used in this industry take place. In this
sense, the austenitic stainless steels are a good choice to be used in phosphoric media
due to their high corrosion resistance.
The objective of the this work was to study the effect of the impurities (chlorides and
sulphates) on the electrochemical behaviour of a highly alloyed austenitic stainless steel
(UNS N08031) used as base metal, the welded metal obtained by TIG (Tungsten Inert
Gas) welding using a Nickel-base alloy (UNS N06059) as filler metal, and the Heat
Affected Zone (HAZ) of the base metal. The materials have been tested in two solutions:
pure 5.5 M phosphoric acid solution and 5.5 M phosphoric acid solution polluted with
379.2 ppm of chloride ions and 2wt% of H 2 SO 4 . Tests were developed at 25 ºC, 40ºC,
60ºC and 80ºC.
Open Circuit Potential tests and potentiodynamic anodic polarization curves were
carried out to obtain information about the electrochemical behaviour of the materials.
Corrosion potentials and corrosion current densities were obtained from Tafel analysis.
The critic potentials and the passivation current densities of the studied materials were
also analysed.
The most important influence of impurities was observed on the critic potential obtained
from the potentiodynamic curves. More active values of critic potential were obtained in
the polluted solution than in the pure solution. This fact means that the presence of
impurities makes the rupture of the passive film easier.
Acknowledgments: We wish to express our gratitude to Programa de Apoyo a la Investigación y
Desarrollo de la Universidad Politécnica de Valencia (PAID-06-09 and to the Ministerio de
Asuntos Exteriores y Cooperación of Spain (PCI Mediterráneo C/8196/07, Ref. C/018046/08, and
D/023608/09).
September, 811, 2010. ISEL - Lisbon 91