Corrosion behaviour of Pb-Sn binary alloys in acid solutions

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Corrosion behaviour of Pb-Sn binary alloys in acid solutions
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  Surface Technology, 14 (1981) 257 - 264 257 CORROSION BEHAVIOUR OF Pb Sn BINARY ALLOYS IN ACID SOLUTIONS M. A. FAWZY, G. H. SEDAHMED* and A. A. MOHAMED Chemical Engineering Department, Alexandria University, Alexandria (Egypt) (Received April 7, 1981) Summary The corrosion behaviour of Pb-Sn alloys in HNO, and acetic acid was studied by the weight-loss technique and by recording the variation in the corrosion potential with time. It was found that the rate of corrosion of the alloy in acid decreases with increasing tin content. Thiourea was found to give a high degree of inhibition of the corrosion of Pb-Sn alloys in both HNO, and acetic acid, while o-nitroaniline was found to inhibit corrosion in HNO, only. 1. Introduction Although much work has been done on the corrosion behaviour of metallic lead and tin, little attention has been paid to the corrosion be- haviour of Pb-Sn alloys despite their technical importance. The object of the present work was to investigate the corrosion behaviour of binary Pb-Sn alloys in acid media and to explore the possibility of inhibiting their corrosion by using organic inhibitors. The rates of corrosion were de- termined by measuring the weight loss and recording the variation in the corrosion potential with time. 2. Experimental technique The alloys were prepared from pure lead and tin. The required weights of lead and tin were mixed and melted in a Pyrex tube in an inert nitrogen atmosphere. After melting was complete, the alloy was allowed to cool and * Present address: Chemical Engineering Department, McMaster University, Hamilton, Ontario, Canada. 0376-4883/81/0000-0000/ 02.50 0 Elsevier Sequoia/Printed in The Netherlands  the test tube was then broken to obtain the alloy in the form of a rod which was machined to give test samples 2 cm in diameter and 1.5 cm long. Before each run the samples were treated with emery paper to remove any surface roughness, the flat surfaces of the cylindrical sample were isolated with a thin layer of wax and finally the sample was weighed using an analytical balance. The sample was placed vertically in a beaker of volume 250 cm3 containing 200 cm3 of the corrosive solution for a minimum time of 30 min. This time was sufficient to give a measurable weight loss. A longer immer- sion time was used for dilute corrosive solutions. After immersion the sample was washed with distilled water, dried and reweighed. The rate of corrosion was obtained by dividing the weight loss by the sample area and the corrosion time. The weight-loss technique was also used to evaluate the inhibition efficiency of thiourea and o-nitroaniline. The inhibition efhciency was calculated from the formula inhibition efficiency = R R Lx 100 R where R and Ri are the corrosion rates in the absence and the presence respectively of the inhibitor. The corrosion behaviour of the Pb-Sn alloys was also examined by plotting the corrosion potential of the test sample against time. The cor- rosion potential of the test sample was measured against a saturated calomel electrode using a high impedance valve potentiometer. All experiments were carried out at 20 C. All the reagents used were of AnalaR grade. 3. Results and discussions Figures 1 and 2 show the effect of the alloy composition on the rate of in corrosion of Pb-Sn alloys in HNO, and acetic acid respectively. It can be seen that the rate of alloy corrosion decreases with increasing tin content the alloy. Before discussing the kinetics of alloy corrosion it would be helpful tc write down the reactions involved during dissolution. The dissolution of Pb-Sn alloys in acid solutions takes place through the formation of num- erous galvanic cells on the alloy surface. The anodic reaction of these galvanic cells is alloy dissolution, while the cathodic reaction depends on the nature of the medium. In HNO, the anodic reaction is Pb-Sn -+ Pb2+ +Sn” +4e- and the cathodic reaction is 6H+ +2NO,- +4e- + 2HN0, +2H,O In acetic acid the anodic reaction is Pb-Sn+PbZ++Sn2++4e-  259 r ‘b 12 I 60 Fig. 1. Effect of the tin content on the rate of corrosion of Pb-Sn alloys in HNO, at various concentrations: A 0.07 N; 0, 0.05 N; x , 0.02 N. and the cathodic reaction is 4H+ +4e- -+ 2H, The observation that the rate of alloy corrosion decreases with increas- ing tin content in the alloy may be due either to the slowness of the cathodic reactions on tin or to the slowness of the dissolution of tin itself. The latter reason is more probable since it is well known that the cathodic hydrogen evolution reaction is slower on lead than on tin, i e the hydrogen overpotential on lead is higher than that on tin [l]. The present observation  ^_ ._ __ ^^ 0 U 4v bU ‘1. Tin in the alloy WJ Fig. 2. Effect of the tin content on the rate of corrosion of Pb-Sn alloys in acetic acid at various concentrations: 0, 1 N; x, 0.5 N; 0, 0.1 N 410 400- - v , E n A 37o7 5 10 I5 20 25 30 Tvme,mtn Fig. 3. Effect of the HNO, concentration on the corrosion potential of 20 Pb-SO Sn: 0, 0.1 N; x, 0.06N; A, 0.05N; 0, 0.01 N.  261 that the rate of alloy dissolution decreases with increasing tin content is in agreement with previous studies of the anodic dissolution of lead and tin where it was found that the exchange current for tin dissolution in seawater is less than that for lead dissolution [2]. Figures 3 and 4 show the effect of changes in the concentrations of HNO, and acetic acid respectively on the corrosion potential of the alloy. In both cases the corrosion potential is shifted to a more negative value with increasing acid concentration. This shift in the corrosion potential and the corresponding increase in the rate of alloy corrosion with increasing acid concentration is attributed to the decrease in the concentration polarization at the cathode caused by the rapid diffusion of H+ or NO,- ions to the cathode surface at high concentrations of acetic acid’and HNO,. Increasing the H + or NO, - ion concentration also displaces the reversible cathodic potential to a more positive value. This contributes to a further shift of the corrosion (mixed) potential to a more negative value as illustrated in Fig. 5. 530 I 510 490- ; 2 i I a0 470- 450- L< 3oo I I IO 20 30 Time min A r-x ” ” v ” V ” I\ A \ A X A c 0 Fig. 4. Effect of the acetic acid concentration on the corrosion potential of 40°/0Pb-600/0Sn: 0, 0.7 N; x, 0.5 N; 0, 0.3 N; A 0.07 N; 0, 0.03 N.
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