Corrosion Evaluation of a Metal in Aqueous Media Student Corrosion Experiment A. I. Onuchukwu and 0. D. Obande Bayero University, P. M. B. 301 1, Kano, Nigeria
Tested experiments and laboratory manuals in corrosion evaluation of metals in electrochemistry are not as plentiful as those in the traditional areas of chemistry. One reason for this is that electrochemistry is not sufficiently taught in most chemistry departments despite the industrial demand, particularly in petroleum and chemical industries, for chemists with corrosion experience. Experiments are needed that would acquaint students with unique properties of metal behavior in environments and that could be taught in existing electrochemistry, material science, and physical chemistry courses. The experiment described herein was part of the series of the laboratory section of the electrochemistry and corrosion science courses being taught at Bayero University but could be readilv adapted to other laboratorv courses of related interest. he experiments include: surfice preparation of test material, potentiostatic polarization for an accelerated corrosion evacuation, and group treatment of the experimental data. The experiment can be done with readily available equipment; however, with a potentiostat, the work would be tedious. Interest in studying corrosion evaluation of material stems from the need to prevent the deterioration of metals from the design stage of the industrial plant through proper material selection. And i t enables the designer to determine
the suitahle environment for the material in order t o prolong the life of the structurelplant. These preliminary evaluation studies, if successfully completed, would reduce the millions of dollars spent on replacement of material in industries ( I ) . Also, this experiment provides the students with a number of valuable laboratory skills as well as increased awareness of electrochemistry, corrosion evaluation of metals, and group interaction generally found in "real life" research efforts.
Figure 1. Electrolytic cell assembly f a corrosion pdentlostatlc polarization Studies at 30%
Figwe 2 CDnosmn polarmtion ol carbon stwi n 6.5% NaCI. A . am 2 M H2S04.A , deaerated elenrolytes at 30% iR corrected
The surface preparation of the commercial steel is the same as reported recently (2,3)with the following composition (wlo): C = 0.30; Mn = 0.65; S = 0.62; P = 0.02; and balance Fe. This steel foil of 0.10-cm thickness and 1.0 cm2 geometric area was annealed a t 520°C and quenched in paraffin. The surface was polished with emery cloth, degreased in absolute acetone, and finally dried in warm air after ultrasonic final washing in absolute ethanol. In order to facilitate current flow, a steel wire was spot welded onto the 1.0 cm2 steelspecimen (Fig. 1). The spot-weldedpoint and wire were insulated with hydrophobic epoxy. The corrodents were 6.5% NaCl and 2 M H2S04solutions prepared from bi-distilled water. These reagents were of analaR grades. Students wore rubber gloves and worked in groups.
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The steel test electrode specimen, already prepared and stored in a dessicator prior to the laboratory class, is the working electrode (w.;.); auxiliary electrode (a.e.) is the graphite rod, while reference electrode (1.e.)-saturated calomel electrode (s.c.e.), is housed in a luggin capillary from the w.e. The electrolytic cell assembly Figure 1,with a provision for NOeas used to deoxveenate the solution was used in this stud; ?he circuit was Gmpleted, Figure 1.The open circuit voltage (o.c.v.) was established after an electrochemical surfarecleaningof the steel electrode in the 02-gas-freeelectrolyte. This was acwmplished according to work of Frumkin et al. ( 4 ) , i.e by a galvanostic HZrharging pulsecurrent of 1.0A cm-'for 10 ms nsing a potentiostat model 703C-3A ronnected to Philips PM 516.5 wave generator and Tektronix model 5130X oscilloscope. Evaluation of Corrosion Susceptlblllty of the Steel From the log,,,i (mAlcm2)versus over-voltage (mV) profilrs, Figure 2, in the deaerated media investigated, the studentsevaluated thecorrosionsusceptibility ot thesteel specimen and observed the following: (1) the steel was more susceptible to corrosion in acid for the same over-voltge; (2) there was a pseudo-passive region in acid for the specimen; and (3) the chloride solution maintained a steady and proeressive corrosion of the steel. Reasons for these obsewa;ions varied among the students after the group discussion. The authors collate the results and summarize the reasons adduced by the students as follows: (1) the acid produced more vigorous reaction than the chloride medium, but was partiall; inhibited (pseudo-passivity) because of the accumulation of reaction product(s) a t the interphase; this, they
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argued, could be responsible for the observed passivity in the acid medium: (2) the steel electrode from ootical examination (X120) after corrosion suffered fro& graphitization which is not reactive with acid; consequently, the icorr (mA/cm2) could not be sustained; and (3)the chloride medium exhibited low reactivity, yet there was no passivity. This the students exolained was due to eraohite ~enetrationof chloride ion by pit formation mechanism (5) maintain the anodic dissolution of the steel, Figure 2. One of the major conclusions is that the corrosion susceptibility in any medium depends not only on the metal composition, but also on the nature of the medium. As demonstrated previously (6, 7) the students were very much interested in the experimental procedure and group interaction as well as the conclusions adduced. Thus, the corrosion evaluation experiment can he adopted in physical chemistry laboratories to acquaint the students with the behavior of metals in various environments prior to deployment of such metals for specific purposes.
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Acknowledgment The authors are grateful to Part 111Chemistry Students, Bayero University, who performed the experiments in their electrochemistry course and offered useful suggestions. Lherature Clted
