Cathodic Protection against Corrosion An Experiment J. R. Jocelyn ParB'
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Independent Research Laboratories, The J. R. J. Pare Establishment for Chemistry (International)Limited, 1245 Walkley Road, Suite 1103, Ottawa, ON, Canada K1V 955 Krzysztof Jankowski Depatiement de Chimie, Universite de Moncton, Moncton. NB. Canada E1A 3E9
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The word "corrosion" often brines to mind the nicture ~ ~of -rust, destruction, or junkyard; however, one should not forget tbat iron and its by-products are not the only substances presenting this phenomenon. This form of oxidation is the general cause of alteration and deterioration of most natural and manufactured materials and as such it is of prime interest to the chemical community. We would like, therefore, t o present an experiment in electrochemistry tbat could be included in the third or fourth year curriculum of physical chemistry students. I t is most suitahle for a full year laboratory functioning on a self-pace basis where critical exchanees between student and supervisor are more numerous &d valuable, although, it could well be used as a freshman chemistry experiment, with discussions a t a lower level. Our approach came from the growing concern about rusting cars. Cathodic protection appeared to be a rust inhibitor that had been overlooked in practical applications to the car industry. We found it most convenient to start on a brief renew of the simple definitions of electrochemical terms (anode, cathode, pile, and battery) in spite of the fact that this laboratory was intended for advanced undergraduate students that had already had a course in electrochemistry and its applications, since the concepts of corrosion potential, dissolution current, passivity, polarization, and transpassivation must he familiar to the student. These elementary definitions are followed by a seminar describing cathodic protection itself. The choice of the sacrificial anode must be made by the student himself. Therefore. it is important to nrovide a eeneral table of redox potentials('l'able i j . From these data, it-should he obvious to the student that onlv zinc ( Z ~and I maenesium (Mg) are good candidates if account is taken of the economic and environmental background: the student is lookine for an be application to car corrosion. The final choice evident from Table 2. Now that the student has decided his pathway, he can proceed to the actual experiment. Once he has completed the experimental section descrihed below (about 6 months), he can attempt to determine the actual corrosion current that will be dependent upon manv factors such as geometry, time and so on &d compare it to that of his colleagues. Conclusions can then be drawn with respect to the eventual use of such cathodic protectors for cars,-where the geometry is very complex and where the locations of the an-
will
A companion paper which serves as an introduction to this experi. ment was released as a communication at the 47th Annual Congress of the Association Canadienne-Franpise pour I'Avancement des Sciences held in Montreal, OC, Canada, May 1979 (paper number 10 of the Metallurgical Engineering Section). Author to whom corres~ondenceshould be addressed. ~ . Hladky, K.. and Richardson,J. A.. in Callow. L. M., ~ a w s o n , 'L., "Online Surveillance and Plant Condition Monitoring." S.C.I.. London.
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1977.
King, R. A,, Nabizadeh. H.. and Ross, T. K., Corr. Prevention and Control, 24, 11 (1977). Ashworth, V., Grant, W. A., and Procter, R. P. M., in "Ion lmplantation for Materials Science and Technolwv." - ed. Hirvonen. Academic Press, London. 1980. 798
Journal of Chemical Education
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