Corrosion behaviour of VTS, STS and DTS inhibitors in formic and acetic acids

Transcription

1 Indian Journal of Chemical Technology Vol. 9, November 2002, pp Corrosion behaviour of VTS, STS and DTS inhibitors in formic and acetic acids M A Quraishi*, D Jamal & M Luqman Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering & Technology. Aligarh Muslim University, Aligarh India Received I Jalluary 2002; revised received I August 2002; accepted 4 October 2002 Three organic inhibitors namely, I-Vanillin thiosemicarbazone (VTS), I-Salicy laldehyde thiosemicarbazone (STS). I-Dimethyl aminobenzaldehyde thiosemicarbazone (DTS) were synthesized to investigate their inhibiting actions on corrosion of mild steel (MS) in aqueous solutions containing 20% formic acid and 20% acetic acid by weight loss and potentiodynamic polarization methods. All of these compounds have shown good inhibition efficiency (IE) in formi c acid solutions. IE of these compounds has been found to vary with the concentration of the compounds, temperature and immersion time. The adsorption of these compounds on the steel surface from the acids has been found to obey Temkin's adsorption isotherm. The values of activation energy (Ea) and free energy of adsorption (Gads ) indicated physical adsorption of these compounds on the steel surface. The potentiodynamic polarization experiments revealed that all the compounds are mixed type inhibitors.. Corrosion studies on metals in organic acid solutions are scarce in comparison with similar studies in minerai acids'-4. Mild steel is used in fabrication of reaction vessels, storage tanks, etc. by industries, which either manufacture or use organic acid as reactant. Influence of three condensation products of thiosemicarbazides and organic aldehydes namely, 1- vanillin thiosemicarbazone (VTS), I-salicylaldehyde thiosemicarbazone (STS), I-dimethyl aminobenzaldehyde thiosemicarbazone (DTS) on corrosion of mild steel in 20% formic acid and 20% acetic acid has been reported here. The selection of these inhibitors is based on the considerations such as (a) these can be synthesized conveniently from relatively cheap raw materials, (b) compounds contain nitrogen with presence of non-bonding electron pairs and (c) readily polarizable sulphur atoms in the compounds are likely to induce greater adsorption of the compounds on the metal surface leading to higher efficiency. Experimental Procedure Inhibitors The inhibitors were synthesized in the laboratory following the procedure described earlier 5, characterized through their spectral data and their purity was *For correspondence ( maquraishi@rediffmail.com; Fax: ) confirmed by thin layer chromatography (TLC ). Name, structural formulas, melting points and molecular weight of the condensation products are given in Table 1. Table I-Name and molecular structures of the compounds used 1. R I =OCH 3, R 2 =OH,R 3 =H (I-Vanillin thiosemicarbazone. VTS); mol.wt = 228: melting point 136 C 2. R1=H, R 2 =H, R 3 =OH (l-salicyaldehyde tihosemicarbazone, STS); mo l. wt = 198: melting point 143"C 3. R 1 =N(CH 3 )2' R 2 =H,R 3 =H (I -Dimethyl aminobenzaldehydethiosemicarbazone, DTS ): mol.wt =225; melting point 127 C Electrolyte Analytical grade formic acid (MERCK), acetic acid (NUMEX) and doubled distilled water were used for preparing test solutions for all the experiments. All the xperiments were carried out in 20% formic and 20% acetic acid solutions.

2 Specimens The mild steel samples having composition, (Wt%): 0.14% C, 0.35% Mn, 0.17% Si, 0.025% S, 0.03% P and balance Fe were used for the experiment. Weight loss studies The mild steel sample of size 2.0 x 2.0 x cm were used for weight loss measurement studies. Weight loss measurement studies were carried out at 20 C for 48 h in 20% formic and 20% acetic acid solutions. The experiments were performed as per ASTM method descri bed previousll. Electrochemical polarization measurement For potentiodynamic polarization studies, mild steel strips of same composition, coated with commercially available lacquer (Iakme) with an exposed area of 1.0 cm 2 were used and the experiments were carried out at temperature (30 ± 2 C). Equilibrium time leading to steady state of the specimens was 30 min. Sweep rate in potentiodynamic experiment was 1 my Is. Potentiodynamic polarization studies were carried out using an EG & G Princeton Applied research (PAR) potentiostatlgalvanostat (model 173), a universal programmer (model 175) and a X-Y recorder (model RE0089). A platinum foil was used as an auxiliary electrode and a saturated calomel electrode (SCE) was used as reference electrode. Results and Discussion Weight loss studies The values of percentage inhibition efficiency (I.E.) and corrosion rate (C.R.) obtained from weight loss studies are given in Table 2 a & b. It is seen that all of these compounds inhibit corrosion of mild steel in 20% formic acid and 20% acetic acid to varying degrees at all concentrations under study. It was observed to increase progressively with increase in concentration of the added inhibitor. Maximum inhibition efficiency of each compound within the range of chosen concentration was achieved at 500 ppm and a further increase in concentration did not cause any appreciable change in the performance of the inhibitor. Fig. 1 shows the influence of temperature on I.E. for DTS (best performing inhibitor) at 500 ppm of concentration at 48 h of immersion time. It is observed that the I.E. increases with the increase in temperature from 30 to 60 C. The increase in I.E. at Indian J. Chem. Techno!., November 2002 Table 2a-Corrosion parameters for mild steel in aqueous solution of 20% formic acid in absence and presence of different concentrations of various inhibitors from weight loss measurements at 20 C for 48 h Inhibitor conc. Weight loss IE CR (ppm) (mg) (%) (mmpy) 20% Formic acid VTS STS DTS Table 2 b-corrosion parameters for mild steel in 20% acetic acid in absence and presence of different concentrations of various inhibitors from weight loss measurements at 20"C for 48 h Inhibitor conc. Weight loss IE CR (ppm) (mg) (%) (mmpy) 20% Acetic acid VTS STS DTS Q

3 Quraishi et al.: Corrosion behaviour of VTS, STS and DTS inhibitors in formic and acetic acids u.i 100 _r_ Formic acid -.-Acetic acid higher temperatures indicates that the inhibitor is more effective at higher temperatures up to 60 C. The variation of the I.E. with immersion time for DTS at 500 ppm is shown in Fig. 2. It is found that there is no significant change in I.E. with the increases in immersion time from 48 to 96 h. The values of activation energy (Ea) obtained from Arrhenius equation 7,8 and free energy of adsorption (Gads) calculated using the following equations are given in Table 3. Gads =-RT In (55.5 K) Temperature tc) Fig. I-Variation of inhibition efficiency with solution temperature in (I) 20% formic acid and (2) 20% acetic acid for 500 ppm concentration of DTS u.i 60 90_r_ Formic acid -'-Acetic acid Immersion time (Hour) Fig. 2-Variation of inhibition efficiency with immersion time in (1) 20% formic acid and (2) 20% acetic acid for 500 ppm concentration of DTS and K is given by: K = 8 /C 0-8) where 8 is degree of coverage on the metal surface, C is concentration of inhibitor in mole/l, K is equilibrium constant, R is a constant and T is temperature. It is found that the Gads value is less than-40 k l/mol (-9.56 k Cal/mol) indicating that DTS is physically adsorbed on the metal surface 9. The low and negative value of Gad s indicated the spontaneous adsorption of inhibitor on the surface of mild steel 10. It was also found that value of activation energy of the inhibited system (DTS) was lower than that of uninhibited system. Putilova ll has indicated that this type of inhibitor is effective at higher temperatures. Application of adsorption isotherm In order to understand the mechanism of corrosion inhibition, the adsorption behaviour of the organic adsorbate on the metal surface must be known. The degree of surface coverage (8) for different concentration of inhibitors in 20% formic acid and 20% acetic acid has been evaluated from weight loss values. The data were tested graphically by fitting to various Table 3-Activation energy (Ell) and free energy of adsorption (dg ads ) for mild steel in 20% formic acid and 20% acetic acid in the absence and presence of maximal concentration of the inhibitor Inhibitor cone. (ppm) Eo (kj.mor i ) -dg ads (kj.mor i ) 20 C 30 C 40 C 50 C 60 C 20% Formic acid DTS % Acetic acid DTS

4 Indian 1. Chern. Techno!.. November CD OJ (/).vrs.sts OTS , Log C (ppm) 2.9 -JOO GJ u Vl -400 >,. -;;; c :; - sao (; ' 1 0!l) OJ 0.6 :::l 0.4 (/).vrs.sts OTS GJ u Vl >... B 0 "- - ) SOD (j O l , Log C (ppm) Fig. 3-Temkin's adsorption isotherm plots for the adsorption of various ir.hibitors in (a) 20% formic acid and (b) 20% acetic acid. on the surface of mild steel (I, VTS; 2, STS; 3, DTS) isotherms. A straight line was obtained on plotting e vs log C (Figs 3 a & b) suggesting that the adsorption of the compounds from 20% formic acid and 20% acetic acid on mild steel surface follow Temkin's adsorption isotherm. Potentiodynamic polarization studies The potentiodynamic polarization behavior of mild steel in 20% formic acid and 20% acetic acid in the presence and absence of different inhibitors at 500 ppm concentration is shown in Fig. 4. Various corrosion parameters such as corrosion potential (Ecorr), corrosion current density (Icorr) and % I.E. are given in Table 4. It is observed that presence of azathiones lowers leorr values. Maximum decrease in leorr was observed in case of I-dimethyl aminobenzaldehyde thiosemicarbazone (OTS) indicating that OTS is most effective corrosion inhibitor among the studied con- Curn;nl densit y ( lil A C III ) Fig. 4--Potentiodynamic curves for mild steel in (a) 20% formic acid and (b) 20% acetic acid in the presence and absence of various inhibitors containing 500 ppm of concentration of the inhibitors used (1. Blank; 2. VTS; 3, STS; 4, DTS) Table 4--Electrochemical polarization parameters for the corrosion of mild steel in 20% formic acid and 20% acetic acid containing maximal concentration of various inhibitors at 30±2 C Inhibitor cone. (ppm) E eorr (mv) 20% Formic acid -416 VTS -404 STS -412 DTS % Acetic acid -404 VTS -430 STS -440 DTS -422 leorr (rna cm 1) IE (%) densation products. Similar trend was observed by weight loss method. It is also observed from the table that there is no significant change in Eeorr values of 482

5 QlIrai shi "I,iI.: COl r",inll hclta\'iour of \11'5. STS alltl DTS inhibilm\ ill for illi c and ;Icel ic acids inh ibitl'd ;lild uninhi biled :--) sie ms suggcsl ing that th e inhibitors arc mi xed type in nature. T he inliibilion or corro,iun of mild stee l in the formic aeid and acetic acid by lhe condensat ion product s can ik' ex plai ned on t he bas is of molecular st ructure. II is ;Jpparent from the molecular st ru cture that these L\) mpounds are able tu get adsorbed on th e metal surll ee through lone pair or electrons or N- and S-atoms and IT-electrons of benzene ring. It is scen from T;lhl e 2 lhat inhibition efficiency f ollows th e> nrdcr: DTS > STS > VTS T!lC hi gher IE of DTS as compared to oth er inhi bi ILlrs may be attri bu ted to better electron donor properties or -N-(CH,)c group as compared to OH and OCH, groups of STS and VTS. Conclusions (i) The condensation products behaved as good corrosion inhibitors in 20% formic acid and 20% acetic acid. (Ii) They inhibited th e cormsillil ui.jl1 k: s!l.:e l in ac id solut ions by adsorption mechanisl ll and the adsorpti ()n of lhesc compounds on lhe met", Sll.- (" ce obeyed Temkin's adsorption isolhcrill. (iii) A ll the compounds behaved,i" Inl \ cd type inhibitors. References I Sc hm itt G. Brit Corms..I. I ') ( tl)x-i ) Ih5. :2 Sdi llc I. Ohkawa H & Hallk T. Co,.,." ',VI. :'2 I Il)X2) 111 :1. :1 Sdillc 1 & Cilinda A. Cormslfl/I. -I(l llt) -Ij l) i. 4 Singh M M & Glipia A. Mot ('h('lff I'll.'. -(('II ')IJ(,) Desai M N. Shah M B. Shalt C B & I k sai 5 M. Corms Sci. 26 ( 1986) ASTM. SWJldord P['{{c1ice jill' 1,{f/JO['{{IIIIT IlII lfl('l'lioll COITIIsimi Tl'.I/illg u(mew/s. G : (1990) li9. 7 Sc horr M & Yahalom J. Corms Sci. 12 (1972) X67. 8 Vashi R T & Champancri \I A. IlIIliffll.I Choll Tee/llilil. -I ( 1997) Brinic S. Grllbac Z. Babic R & MCl i k() - H lik() v ic M. S'" EliI' SYIlIP Corms IlIiJib. I ( 1995) I GO lllma G K & Wahdan M H. 1If(Ii{lil.I CiJl'11f Teelllwi. :2 ( 1995) 107. II Plitilova 1 N. Balezin S A & Barallik U P. Mewllie Corrosi(J/1 IlIiJiiJitors (Pergamon Press. New York)