The Corrosion lnhibition of a Metal Student Corrosion Experiment Ill A. I. Onuchukwu P.M.B. 3011, Kano Nigeria Corrosion inhibition has been one of the several measures adopted either for control andlor prevention of corrosion of metallic structures and components deployed in service. Thus, a n anodic or a cathodic inhibitor depends on the mode of action of the inhibitor with respect to the changes of the corrosion potential a t the metallcorrodent interphase. The inhibitors used are diverse; however, they are grouped according t o their nature: inorganic and organic inhibitors ( I , 2). The inclusion of these inhibitors in corrosive medium has been reported by several authors to reduce to a minimum the corrosion-susceptible material (2-4). The mechanism of their activities is yet to be understood. However, reports have shown that the effective inhibition by these chemical agents depends on metdinhibitor interaction and corrodentlinhibitor blockade of the metal surface (2. 5). Also. i t is known that the oseudo-ca~acitv . - of the metal/corr&ent interphase is changed by the presence of an inhibitor. Interestinnlv, this change at the interphase has been found in the a x o n of inhib:1ton in the cokosion of metals in most environments (2). Although the principles of application of inhibitors are briefly described in the literature, the fact that a detailed procedure is not included in laboratory manuals is possibly an oversight on our part in the training of electrochemists and material scientists. Inan attemot ro halance thisdeficiencv.and in the lieht of ,. favorable resdonse of colleagues on the earlier reports ?6-8), this paper presents the principles of corrosion inhibition of a
metal. This is the third part of the series of our materials science laboratory, whichis taught to the final-year students in the electrochemistry course a t our university. The objective of demonstrating principles of corrosion inhibition is achieved by an assessment of % inhibition of borax (disodium tetrahorate. N~.,BLOT.~OH~O) and ethvlene diamine ( N H ~ C H ~ C H in ~N the'inhibkion ~J of alum;num corrosion in alkaline medium. The teat metal is choscn hecause of 1 t s attendant loss in weight due to its rapid corrosion in potassium hydroxide medium (9). Experimental Aluminum alloy AA1060 has composition in weight 9% as detailed in reference 4 as follows: Si (0.35), Fe (0.1), Cu (0.15),Mn (0.051, and Al (balance). This aluminum sheet of thickness 0.045 cm was mechanically pressed cut into 2.0- X 2.0-cm coupons and the two faces had a total geometric surface area of 8.0 cm2.These coupons were degreased in absolute ethanol, dried in warm air, weighed, and stored in a moisture-free desiccator prior to laboratory session. The corrosion rate (mgIcm2h) of Al in various KOH concentrationshas been compared with other mineral acids in the earlier report (8).In this experiment (10-150 mM) of borax and ethylene diamine in 250 mL of 0.5 M KOH corrodent. The initial weights of these eouoons were recorded, introdwed mto the corradent inhihitur srlutimr, and made u, stand while inclined against the walls of the treakrm. These coupons were retrieved from the solur~onsafter 90 mi". washed, dried over warm air, and reweighed. A beaker containing the corrodent and a coupon without any inhibitor was the reference from which the weight law and percent inhibition of the inhibitors were assessed. Results and Discusdon The students observed that the corrosion reaction of aluminum in potassium hydroxide solution was characterized by vigorous effervescence and the pale aelatinous precipitate ai t h i corrodentlgas evolu~ioninterphase was d&l;ed as the precipitatediffused into the bulk solution. The followinrrea&ioni were suggested:
--
+
Al(s) + OH-(aq) + H20(1) AlO;(aq) XH,(~) (1) Al(8) + OH-(aq) + 5H20(l) [A1(OH)4.2H20]-(aq) + 3/,H,(g) (2)
-1
6
-1 3
They also observed that the presence of the inhibitors in the corrodent reduced the rapidity of the gas evolution, especially a t high concentration of the inhibitors. Also, on retrieving the couoons from the corrodentlinhihitor solutions. i t was shown ihat the solution containing borax produced's dark brown, smooth deposit, possibly aluminum metaborate, AIOB02. 2HnO-as reported recently ( l o ) ,on the coupon surface while the surface of the coupon from ethylene diaminelcorrodent was dull. From these observations and the data from the prior assessment of the corrosion rate, performed by weight loss determination in part I1 of the series (a), the percent inhibition was defined by
-1 0
log/rndl dm3
SMBnt resub, percm inhibition"6. inhibHa wnoentratton.
where I, Wi., and W, are..the percent inhibition and the Volume 66
Number 8 August 1989
681
weight losses of the coupons in inhihited and uninhibited solutions a t 28 "C, respectively. The typical plot obtained by the students (see figure) shows the variation of percent inhibition with corresponding concentration of the inhibitors. Following these observations as well as the explanations adduced by the students after a stimulating group discussion, the results were collated and summarizedas follows: (1) the alkaline solution was corrosive, especially in the corrodent without any inhibitor, and the corrosion is enhanced possibly due to soluhility of the reaction product; (2) the Hz evolution was inhibited by the presence of the inhihitors, (3) the percent inhibition is enhanced considerably by the increase in inhibitor concentration, and (4) the trend of percent inhibition of the inhibitors is borax greater than ethylene diamine. Considering that Evans (11) has found that the inhibition of aluminum in alkaline solution is a difficult prohlem, possibly because of the specific adsorption of hydroxyl ion on the metal surface, the students were satisfied by the percent inhibition of aluminum in boraxhorrodent solution. Thus, apart from the stimulating group interaction, the students
682
Journal of Chemical Education
were thrilled by this laboratory demonstration of the inhihition of aluminum corrosion in alkaline medium. Acknowledgment The author appreciates the suggestions from the 19871 1988 graduating students in chemistry and Mr. Adoyi of Faculty of Science Workshop for the preparation of the aluminum coupons. Llterature Clted 1. Talati, J. D.;Modi, R. M. Corm. Sci. 19?9,19,3S48. 2. Chapman, N.R.; Stevens, R. C. Pme. Conf. Development of Inhibitors, Landon. Oec. 19RI
3. Kobayaahi, K.; Shimieu, K.; Ohba. M.: Ohya, T. Plor. Evrap~onSymp. Car,. Inhibitors (6-SEIC), Ferrse, Italy, 1985.127-139. 4. Onuchukw, A. I.: Oppong-Basehie, F. K. Cams. Sci. 1986,26,91W26. 5. Onuchukw. A. I.; Lori, J. A. Corrm. Sci. 1984.24,813-841. 6. Onuchukw, A. I.: Mahelia. P. B. J. Chem. Edue. 1981.62.8W-811. 7. Onuehukw, A.I.: Obande, 0. D. J Chom. Educ. 1986.63.26%270. 8. Onuchukw, A. I. J. Chem. Edue.. in prerr. 9. Brown, 0. R: Whitley, J. 8. Electmchim. A d o 1987,32,546556. lo. Onuchukw.A.1. Mot.,. Che",.Phya.,inpres8. 11. Evans. U. R. Anhtroduction to Metallic Corm8ion;Amold: London. 1963,p 156.
