Interaction between Scientific Research and Chemical Education Development of a Corrosion Experiment Claudio 0. Higa, Sylvia L. F. A. da Costa, Silvia M. L. Agodtinho Universidade de SBo Paulo, Caixa Postal 20780, 05498-S5o Paulo, Brasil The teachmg of chemistry, especially of a theoretical subject, is much more agreeable to students when it is illustrated with practical examples derived from research work or industrial activity. At the university, professors and researchers giving &emistry classes occasionally discuss results from their own scientific investigations to motivate the students and make them more aware of the present state of scientific knowledge. This work presents the results of an investigation of the effect of the corrosion inhibitor henzotriazole on copper corrosion through weight-loss experiments. BTA has been used for more than 30 vears as a corrosion inhibitor for comer .. and copper-based alloys in different mediaI.2. In previous works3c we reported that BTA effectively inhihits the corrosion of copper by ferric ions in deaerated 0.5 M HzS04 solutions a t 25.0 f 0.5 "C. A rotating disk electrode, a t which transport conditions are well established, was used t o obtain the experimental data. The rate of the corrosion process is controlled by diffusion of the ferric ions t o the electrode surface, both in the absence and presence of BTA. The main purpose of the present work is t o develop a corrosion experiment based on the above system using more simple and cheaper equipments.
Presented at the Ninth International Conferenceon Chemical Education. Sio Paulo. Brazil. July 1987. Walker, R. Corrosion 1973, 29, 290. FOX,P. G.; Lewis, G.; Boden, P. J. Corros. Sci. 1979, 19, 457. Da Costa. S. L. F. A. Dissertation, Universidade de Sio Paulo. Brazil. 1987. Da Costa. S. L. F. A.; Agostinho, S. M. L.; Chagas. H. C.; Rubim. J. C. Corrosion 1987, 43, 149.
Experlrnental The equipment assembly is shown in Figure 1. The capper eleetrode was a commercial copper foil (4.95 x 2.50 x 0.05 em). The copper foil was mechanically polished with emery paper, rinsed with distilled water and alcohol, air dried, and weighed. The copper foil
n
'
'
Figure 1. Equipmem assembly. A, copper foil; 6, beaker; C, magneticstirrer + magnetic bar; D, nylon string; E, rubber stopper; F. damp; G, support.
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Number 5 May 1989
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Eflect of ETA on Corrosion of Copper In 1.0 M H,SO,
+ 0.02 M
Fe(lll)* Run
Anb (g)
1
0.0337
2 3 4
5
c, x
10' (M)
0.332 0.662 4.59 6.22 7.80
Am (s)
0.0288 0.0269
0.0152 0.0096
0.0065
Figure 2. Langmuir adsorption isotherm forBTA on copper in W presence of 0.02 M Fe(II1)+ 1.0 M H2SO+ was suspended by a nylon string that was passed trough a central hole in the upper side of the copper sample, and a knot was tied at the ruhher stopper. The nylon string Length was such that the sample was completely submerged in the solution. After each corrosion experiment, the disk was rinsed again with distilled water and alcohol,air dried, and reweighed. An analytical balance (precision of 0.0001 g) was used to measure the weight loss of the electrode. The electrolyticcell was a beaker of 800 mL capacity. The volume of electrolyte used was 600 mL. Analytical reagent-grade chemicals and distilled water were used to prepare the solutions. The work solution consisted of 1.0 M HzSOl as the supporting electrolyte,0.02 M Fe(II1)ions as the oxidant agent, and BTA as the corrosion inhibitor. The BTA concentrationranged from 0 to 6 mM. All the experiments were conducted at room temperature (21 1 "C).
*
Results and Discussion The table presents the results obtained from weight-loss experiments for the corrosion of copper, in the absence and presence of BTA; Am, represents the weight loss of copper foils in the absence of BTA, and Am, represents the weight loss of copper foils in the presence of concentration C, of BTA. T h e time interval of each experiment was 30 min. Each Am, value corresponds to a set of experiments where the inhibitor concentration was gradually increased. I t is important to notice that the same stirring conditions must be kept constant both in the absence and presence of BTA. The results in the table show that BTA is an effective inhibitor for conoer corrosion in thismedium. For BTAl = 6 mM, the copp&surface is completely blocked by ihe pa& atine film. These results are in -eood aereement with ~ r e v i ous work3p4. From the weight-loss experiments, the degree of surface coverage (0) can be obtained by
-
8=
Am, - Ami Amo
Langmuir adsorption isotherm5,
Atkins. P. W. Physical Chemistry: Oxford University: Oxford. 1986.
442
Journal of Chemical Education
where K.A. = eauilibrium adsomtion constant. was tested for the p&&ati& film. ~ ~ u a t i o 1 nand s 2 were combined to exoress the Lanemuir ahsorotion isotherm in the followine form:
where the relation AmJAmi is determined with more precision than 0. Fieure 2 oresents the deoendence of AmJAm on C;.The inteicept o i Figure 2 (1.0) is in accord with eq 3, the mean s l o. ~ .. eives e a n adsorotion eouilibrium constant of 4.5 X lo3 M-I. This result is ingood agreement with the previous work (8.8 X 10'' M-'1". if we consider the differences in ex~erimen. tal conditions.1n this work the solutions were not dkaerated, the room temverature was maintained around 21 "C. the concentrationbf supporting electrolyte was 1.0 M, and the metal samples were of commercial copper.
1. This type of experiment can be applied to students of general chemistry laboratory. From weight loss experimenu, the qualitative inhibiting effect of BTA can he verificed in a short period of time (see table). 2. The L a n m u i r adsorotiun isotherm can be built from the experimental results obtained by different group of students of physical chemistry laboratory. I t is important to keep the following parameters constant: H&04 concentration, Fe(II1) concentration, copper of the same lot, and the same stirring condition of solution.
