----- Author to whom all correspondence should be addressed. Inhibition of corrosion of aluminium in hydrochloric cobalt tetrasulphophthalocyanine

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1 ndiap Journal of Chemical Technology Vol 2, March 1995, pp nhibition of corrosion of aluminium in hydrochloric cobalt tetrasulphophthalocyanine acid by V N Sivasankara Pillaib', Jayan Thomas" & P S Harikumar< "Department of Applied Chemistry, Cochin University of Scienc~ and Technology, Cochin , ndia bschool of Environmental Studies, Cochin University of Science and Technology, Cochin , ndia <Centre for Water Resources Development & Management, Calicut , ndia nhibition of corrosion of aluminium in 2M HCl by cobalt tetrasulphophthalocyanine (CoTSP) has been investigated by gasometry, thermometry, weight loss and polarization techniques. nhibitive property is exhibited in concentrations as low as 10-7 M but reaches its maximum in concentrations exceeding 2 x 1O-4M. The activation energy of the dissolution process is found to increase with increasing inhibitor concentration and Arrhenius activation energies show that the inhibition process is activation controlled. Heteromacrocyclic compounds are effective corrosion inhibitors because of their strong chemisorption onto the metal surface. Cobalt tetrasulphophthalocyanine (CoTSP) (Fig. 1) is a unique planar macrocyclic pigment soluble in water and stable in acid and alkaline media. t has a variety of electrochemical and surface adsorption properties 1-3by virtue of its molecular structure. The strong chemisorption of metal tetrasulphophthalocyanines (MTSP) is attributed to the co-ordination of four sulpho groups with the surface atoms of the metal and the interaction of the n-molecular orbital of the macrocyclic ligand with the metal d electrons4.5 The electron density and planarity of heteromacrocycles are the major factors which determine the surface adsorption while the solubility of MTSP makes them good chemisorbing materials. n the present work corrosion inhibition of aluminium in 2M HCl by CoTSP has been studied and the inhibition activity has been measured by weight loss, thermometry, gas evolution and polarization techniques. Oi /} Fig. -Planar macrocyclic pigment Author to whom all correspondence should be addressed. Experimental Procedure The test pieces (20 x 20 x 1 rom) aluminium strips (lndal 2S of the constitution, Si-O.3%, Fe-O.5%, Ti-0.01 %, B-0.005%, Ga-0.02% and the rest A) were polished with 4/0 grade emery paper, degreased with alkaline solution (15 gl NaZC g/l Na3P04) and washed with a copious amount of distilled water. After washing with acetone they were dried by pressing between filter paper folds. A lresh test piece was used for each measurement. Cobalt tetrasulphophtl).alocyanine (CoTSP) was prepared and purified by a reported procedure6. Solutions of different concentrations were prepared froll a lo-z Mstock solution of CoTSP in water. To measure the inhibition activity by thermometric method the test pieces of aluminium were bent to U shape and put in a reaction vessel which was insulated with foamed polystyrene to prevent heat loss. The temperature of the system was measured using a sensitive mercury thermometer, lower end of which rested on the dome of the metal specimen. The dissolution of aluminium in hydrochloric acid is shown to become considerable at concentrations > 1.5 M7. Hence 2M HCl containing varying amounts of CoTSP was used as the medium. For weight loss method prepared test pieces we~e weighed and immersed in 50 ml of test solution at room tenlperature for a period of 15 min. They were then washed with distilled water, dried and weighed.

2 100 NDAN J. CHEM. TECHNOL., MARCH 1995 To measure the inhibition efficiency by gasometric method the dissolution of aluminium in 2M Hel with and without the inhibitor was studied by hydrogen,evolution method in the temperature range C. The test pieces were prepared as above. n the gasometric reaction vessel the temperature was controlled at ± 0.1 C by circulating thermo stated water around the reaction vessel8. For polarization studies aluminium sheet of size 50 x 25 x 0.5 mm3 was used as the electrode. This was polished and cleaned. A coating of wax was applied on the specimen using a mask leaving behind only a small area (2 cm2) exposed to the medium. A platinum sheet was used as auxiliary electrode and saturated calomel electrode was used as the reference. Measurements were done in the test solution (75 ml) in an undivided cell. A scanning potentiostat/galvanostat (EG & G PAR Model 362) was used for the polarization studies. The test solutions were flushed with nitrogen for 30 min. Both anodic and cathodic polarization measurements were taken. Results and Discussion The inhibitor efficiency, 1, is calculated from the weight loss method using the relation, Tabie -nhibitor efficiency for the dissolution of aluminium 2S in 2M HCl at various concentrations of CoTSP Concentration CoTSP,M x X x X 10-7 of nhibition after 30min, % Wt loss pure - Wt lossinhx 100 % 1= Wt losspure The values are displayed in Table 1. Fig. 2 shows the weight loss-time relationship for different concentrations of CoTSP and also in its absence. The dissolution is markedly inhibited by the presence of CoTSP. The induction period is enhanced in presence of the inhibitor. This may be due to the protection of the oxide layer against dissolution by the inhibitor. Thermometric method was used to evaluate the reaction number and percentage inhibition8,9. The reaction number is defined as, Tm, ~ are the maximum and initial temperature, respectively, and' t' is the time in minutes taken to reach Tm. The percentage inhibition is taken as, (RNpure- R~nl,) X 100 %1= RN pure... (1)... (2)... (3) n Fig. 3 the variation of temperature with time for different concentrations of CoTSP is shown. t is seen that the effes:tiveness of CoTSP as an inhibitor is distinct only in concentrations.exceeding 10-5M. Below this concentration, the inhibition efficiency sharply fats. The reaction number vs log C plot (Fig. 4) also shows that RN is only moderate up to 1O-5M CoTSP. At concentrations below this RN increases rapidly (Table 2). This is in agreement with the temperature-time plot and the gravimetric results... 7~ u o ~ ~~ Q. f '" >- 4~ 3~ Timt',min Fig. 2- Weight loss-time plot for the dissolution of aluminium in presence and absence of CoTSP. Medium 2M HCl 0-0 HCl;.-. lo-mmhcl;.-. 1O-7MCoTSP; x- x 1O-6MCoTSP; A-A 1O-7MCoTSP; D-D 1O-4MCoTSP Timt',min Fig. 3 - Temperature-time plots for the dissolution of aluminium in different concentration of CoTSP in 2M HC 0-0 HCl;.-. x1o-7m CoTSP; ~-~ xlo-6m CoTSP; A - A 1 X 10-5 M CoTSP; 0 - D 1 X 10-4 M CoTSP ' \., il f U'llll'' H

3 0.' T c e 0.1 ou..0.7.q e ::J C o. c.2 "UD. o r 0.4 SVASANKARA PllA et al: NHBmON OF CORROSON OF AWMNUM 101 / /201 > - 0> :: ~ i ':..,10.: E~O 40 ~ 20 /j /00 0., log C,moll - Fig. 4-Plot of log CCOTSPversus reactioft number for the dissolution of aluminium in 2M HC Table 2-Dependence of reaction number and percentage inhibition on the concentration of CoTSP, medium 2M HC Concentration 1;o;'iol' C Tmax, C Time, RN CoTSP,M min 2MHC x x x x Table 3 - Percentage reduction in RR at various concentrations of CoTSP, medium 2M HC Temp,oC Time, min Concentration of CoTSP, movl..j 100 E~10 't:l > i5. tlo :r 5 40 E ::J ~ Time.min Fig. 5 - Volume of hydrogen evolved-time curves obtained at various temperatures in 2M HC only C; C; (:; A-A 25"C so lxlo-4 lxlo-5 lxlo-6 lxlo-7 Time,min Fig. 6- Volume of hydrogen evolved-time curves obtained at various temperatures in presence of 1 x 1O-4M CoTSP medium 2MHC o C; - 40 C; C; A - A 30 C The inhibitor efficiency is also evaluated by gasometry by the percentage reduction in reaction rate according to the relation, % Reduction in RR = _R_fT_ee_-_R_in_h X 100 RfTee... (4) where ReTee and Rinh are rates of aluminium dissolution in the absence and in the presence of CoTSP, respectively, both being measured for the same reaction time (Table 3). The rate of hydrogen evolution was determined at 25, 35, 40 and 45 C at different concentrations of CoTSP in 2M HC. Typical results are illustrated in Figs 5 and 6. The plot of volume of hydrogen evolved vs time at different temperatures shows that the rate increases slowly during the initial period. Thermometric method also shows an incubation period for dissolution. This incubation corresponds to the breakdown of the oxide film present on metaj. surfacelo and the dissolution of aluminium oxide in acid is not accompanied by hydrogen evolution. The post-incubation reaction of aluminium is linearly related to reaction time and this behaviour is characteristic of zero order reactions8 The equation for the zero order reaction may be written in the form, x= kt where x is the fraction of aluminium converted into products at time t, which in turn is directly proportional to volume of hydrogen evolved at time t and k is the rate constant of the reaction. The value of k can be calcu-

4 102 NDAN 1. CHEM. TECHNOL., MARCH 1995 Jated from X- curve at any given temperature (Table 4). The value of k increases with increase in temperature and decreases with increase in inhibitor concentration. ncrease in activation energy is due to increased adsorption of the inhibitor on the metal surface. Surface coverage 0 of the inhibitor on the metal surface was determined using the relation, ~ 0= 1- V o <D CD... [] o log C, mol-1... (5) where Vj and V are the corrosion rates in solution with and without inhibitor, respectively9. The isothermal variation of 0 with concentration conformed to the Langmuir adsorption isotherm which may be written in the form, GOads log (ell - e) = log BC RT... (6) where B is the adsorption equilibrium constant and GOads is the free energy of adsorption. Fig. 7 is the plot of log (011-0) vs log C. The slope of this straight line is free energy of adsorption Fig. 7- Dependence of log( 0/1-0) on the concentration of CoTSP Activation C C energy, 35 C kj Heat mol-1of Ea kj adsorption, mol-4 Rate Surface constant, K GOads' and is found to be 38.9 kj mol-. Similarly the values of heat of adsorption for various concentrations of CoTSP is calculated from the log( 011-0) vs 11Tplot and is givenin Table 4. The effect of different concentrations of CoTSP on the polarization curves of aluminium is shown in Fig. 8. The effect of inhibitor on the anodic and cathodic reactions can be determined using the relation, a=l-~ '0 eorr e= }-~ i~orr... (7)... (8) where a and e are the efficiency of anodic and cathodic reactions and ioeorr is the corrosion current density without inhibitor. The values of ia and ie were obtained from the log (current density) vs potential plot. The pola"ization curves clearly indicate that the inhibitor slows down the hydrogen evolution. Table 5 gives the percentage inhibition from polarization data. The surface micrographs of the sample,in presence of various concentrations of CoTSP do not indicate pitting and also confirms that the attack of CoTSP on the surface of the metal is of a general nature covering the metal surface evenly. Aluminium shows identical behaviour towards CoTSP in oxygenated and deoxygenated soluhons. The catalysing effect of CoTSP for dioxygen reduction does not interfere with its corrosion inhibition efficiency. The non-conducting aluminium oxide layer on which CoTSP is adsorbed inhibits the CoTSP-mediated electron transfer from aluminium to dioxygen. The inhibition efficiency of CoTSP in HCl medium is superior to that of p-diketo compounds6 and phenyl azo-derivatives9. The surface coverage of this compound on aluminium is 0.85 which is much higher than the values for similar inhibitors. The activation energy for corrosion is also high. Table 4- Rate constant, surface coverage, activation energy and heat coverage of adsorption for the dissolution of aluminium in 2M HCl at different concentrations of CoTSP. ""ll,;"li. U, j '" '!,~.' '! '!' '

5 SNASANKARA P1LAl et al: NHBmON OF CORROSON OF AWMNUM 103 1& > E 900 UJ 700 & & -' O.t log i,ma cm-2 Fig. 8-Polarization curves obtained at different concentnations of CoTSP. Medium 2M HC.-. HC; ~-~ lxlo-7m CoTSP;.-. lxlo-smcotsp;o-o lxlo-5mcotsp; x - x 1 x 1O-4MCoTSP Table 5-Comparison of the efficiency of CoTSP determined by thermometric. weight loss, hydrogen evolution and polarization techniques in 2M HC at vanous concentrations Conc Reduction nhibition, % Reduction in RR, nhibition, % M in RN, % by weight loss % by polarization method method 1 X CoTSP exhibits its inhibiting propel1y in concentrations as low as 1.0 x 10-7M and the inhibition efficiency reaches saturation at concentration exceeding about 1.0 x 10-4 M. The solutions prepared at concentrations exceeding 10-4 will not be stable for a longer period since aggregation of CoTSP will take place at higher concentrations. Other inhibitors are effective only at concentrations exceeding 1.0 x 10-3M. Even at lower and higher concentrations it does not accelerate corrosion. Moreover, phthalocyanines are non-toxic and well tolerated in living systemsl2 Phthalocyanine compounds are not acted upon by strong acids. strong bases or even mild oxidants and reductants. Since it is is stable towards optical radiation, formation of decomposition products is also ruled out. Thus CoTSP satisfies the criteria desired for a corrosion inhibitor. Ac~nowledgement P S Harikumar expresses thanks to Department of Science and Technology, Government of ndia for the financial support. References McLendon G & Martell A E, lnorg Chern, 16 (1977) RoHman L D & wamoto R T, J Arn Chern Sac, 90 (1968) Nikolic B Z, Adzic R R & Yeager E B, J E/ectroanal Chern, 103 (1979) AssorJ,] Chern Phy, 43 (1965)i Fleisher E B & Wang J H, J Arn Chern Soc,82 (1960' Weber J H & Busch D G, norg Chern, 4 (1965) Aziz K & Shams AM, CorrosSci, 5 (1965) 489. R Hassan S M, Moussa M N H, Taha F M & Fouda A S, Corros Sci, 21 (1981) Fouda A, Nader H A E, Moussa M N D & Shehata, Bull Chern SocJpn, 61 (1988) Desai M N, Patel R R & Shah D K, J ndion Chern Soc, 50(1973) Gruen L C & Blagrove RJ,AustJ Chern, 26 (1973) Moser F H & Thomas A L, Phthalocyanine cornpounds (Reinhold Publishing Corporation, New York), 1963,302.