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Volume 3 Paper 21


Inhibition of the Corrosion of Mild Steel in Hydrochloric Acid by (4-Amino-2-Methyl-5-Pyrimidinyl Methylthio) Acetic Acid and its Precursor.

O.K. Abiola* and N.C. Oforka

Department of Pure and Industrial Chemistry, University of Port Harcourt, P.M.B. 5323, Port Harcourt, Nigeria. 
*Corresponding author: E-mail:

Abstract

The inhibition of corrosion of mild steel in HCl solution by [4 � amino �2 methyl �5 � pyrimidinyl methyl thio ] acetic acid (AMMPTA), 3-[4 � amino �2 methyl � 5 � pyrimidyl methyl ] �5- [2 � hydroxyethyl]-4-methylthiazoliumchloride hydrochloride (AMMPTC) or Thiamine chloride (Vitamin B1) and thioglycollic acid (TGA) has been studied.�� Weight loss and hydrogen evolution measurements reveal that AMMPTA exhibits a higher maximum inhibition� efficiency than� AMMPTC and TGA. Generally, inhibition was found� to increase� with increase in inhibitor� concentration,� half-life, activation energy and decrease in the first-order rate constants at 30 and 40oC. Physical adsorption� mechanism� has been proposed� for the inhibitors and the� difference in the inhibition behaviour of the compounds has been explained on the� basis of molecular weights and molecular structures.

Keywords: Corrosion inhibitor; Pyrimidinyl, methyl thio; Thiazolium; mild steel; physisorption.

Introduction

Corrosion of metals is a major industrial problem that has attracted a lot of investigators in recent years [1-3]. Corrosion inhibitors are of great� practical importance, being extensively employed� in minimizing� metallic waste in engineering� materials� [2].�� Several� N � and S � containing organic compounds have been� used as inhibitors [3-5].� The corrosion� inhibition is a surface process which involves the� adsorption of the� organic� compounds on metal surface [4,5]. The inhibition efficiency of organic compounds depends on molecular size, mode of� interaction� with the metal surface [3-5]. The adsorption depends mainly on the electronic structure of the molecules. [4 � amino � 2 � methyl �5 � pyrimidinyl methyl thio] acetic acid� (AMMPTA) and 3 � [4 � amino � 2 � methyl � 5 � pyrimidyl methyl] � 5 � [2 -� hydroxyethyl] � 4- methyl thiazolium chloride hydrochloride (AMMPTC) or Thiamine Chloride (Vitamin B1) are potential corrosion �inhibitors, since these compounds� contain nitrogen� and sulphur� [6-8]. However, studies on use of chelating agents as inhibitors� especially those bearing nitrogen� and� sulphur as coordinating� atoms, are few� in the� literature. AMMPTA is unique� since� it contains a wide� variety of coordination sites,� namely, the heterocyclic nitrogen, the amino group, the carboxyl group and sulphur� donor atom, in addition to being amphoteric [ 6,8 ]

AMMPTC� (Vitamin B1) consists of a pyrimidine ring and a� thiazollium ring bridged by methylene� group is already known for� its versatility in� complex formation owing to its varied coordination� sites [ 7 ].� AMMPTC�� and their derivatives� have opened a wide field� of study in coordination� and bioinorganic� chemistry [ 8,13 ]. Thioglycollic  acid (TGA) has a wide range of application,� being used in prevention of corrosion of copper, in the cosmetics and oil industries [ 9,10,13 ]. The aim� of this investigation was to evaluate the effect of� molecular size, changing functional and structural� groups on the protection� efficiency imparted� by the molecules. In this investigation, weight loss and hydrogen� evolution techniques were used� to study corrosion of mild steel in HCl� solutions containing AMMPTA, AMMPTC and TGA. The inhibitor efficiency at each concentration� of inhibitor used was calculated using the equation ( 1 )

����������� %P = ��x 100����������������������� (1)

where kun and kin are the rate constants in absence and presence of the inhibitor respectively.

Experimental

Material� Preparation

Every sheet was 0.04cm� in thickness and 97.8% purity were machined into coupons of dimension�5.0x 4.0cm, and� 3.0 x 2.0cm. A 0.015cm� diameter hole were drilled on the centre� of the shorter sides of all the coupons for� suspension. These� coupons were used as supplied without further polishing, but were�degreased in absolute ethanol and dried� in acetone. The coupons were stored� in a desiccator in the� absence of moisture before their use for the� investigation. [4- amino � 2- methyl �5 � pyrimidinyl methyl thio] acetic acid (AMMPTA) was� prepared from 3-[ 4 - amino �2 � methyl � 5 � pyrimidyl� methyl ] � 5 -�� [2- hydroxyethyl ] � 4 � methylthiazolium (AMMPTC)� and thioglycollic acid [TGA] according to the method of Bonvicinol and Hennessy [18]. AMMPTA was recrystalized� twice from hot� water and dried in an oven. The inhibitor concentrations of 1 x 10-6, 1x 10 � 5 �, 1x 10-4, 5 x 10-4 and 1 x 10-3 M were prepared� in 0.5 M HCl solution at 30 and 40°C. The�� prepared inhibitor solutions were used� for all measurements. All reagents were of analar grade and doubly distilled� water was used for the preparations of all solutions.

Weight Loss Determination �

Mild steel coupons of 4.0 x 5.0 x 0.04 cm were used for weight loss measurements. The total geometric surface area of coupons exposed is 40.0cm2, and the average weight is 7.11 to 7.14 grams. The coupons were suspended through a hole 0.015cm in diameter. Five 250ml beakers, which separately contained 0.1, 0.2, 0.3, 0.4, and 0.5M HCl solutions, were maintained at 30 and then 40°C, constituting first set of experiment(s). Previously weighed mild steel coupons were each suspended in each beaker through a 0.015 cm� in diameter hole. The coupons of this first set of experiments were retrieved from their corrodent at 24 hours� interval progressively for 168 hours (7 days). They were then chemically cleaned in 20% sodium hydroxide containing 200gl-1 of zinc dust in order to remove the corrosion product. After rinsing in distilled water and absolute ethanol, the coupons were dried and weighed. The weight loss was calculated in grams as the difference between the initial weight prior to immersion, and weight after removal of the corrosion product. Each reading reported is an average of two readings recorded to the nearest 0.0001g on a Mettler AE 166 (Delta range) analytical balance. Experiments were repeated with the introduction of five different concentration (1x10-6M to 1x10-3M) of AMMPTA, AMMPTC and TGA in 0.5M HCl solution at 30 and 40 0C. Fifteen 250ml beakers which separately contained 1x10-6, 1x10-5, 1x10-4, 5 x 10-4 and 1x10-3M of inhibitor concentrations were maintained at 30and then 40°C. Previously weighed coupons were then placed in the corrodent inhibitor solutions containing one mild steel coupon. As before, each coupon was retrieved from the solution at 24 hours intervals, progressively for 168 hours (7 days). The difference in weight of the coupons was again taken as the weight loss.

Hydrogen evolution technique

The gasometric assembly for the measurement of hydrogen evolution from the corrosion of mild steel was designed following the� method employed by Onuchukwu et al [11] and Ekpe et al [3]. Mild steel coupons of 3.0 x 2.0 x 0.04 cm were used in the experiments. The two faces had 12.0cm2 total geometric surface area and the average weight is 2.30 to 2.34 grams. The volume of the HCl used in each experiment is 100ml. A 100ml solution of 8M HCl was introduced into the reaction vessel connected to a burrete through a delivery tube. The initial volume of air in the burrette was recorded. One mild steel coupon was dropped into 8M HCl solution and the reaction vessel quickly closed. Variation in the volume of hydrogen evolved with time was recorded every I minutes for 30 minutes. Each experiment was conducted on a fresh specimen of mild steel coupon. The hydrogen evolved displaces the fluid in the gasometric set-up, which is read directly (cm3). The same experiment was repeated in the presence of the inhibitors having the concentration of 1 x 10-6 M to 1x10-3 M as used in the weight loss determinations but with a corrodent concentration of 8M� HCl solution.

Results and discussion

Effect of corrodent concentration

Figure 1 Variation of corrosion rate (mg dm-2 per day) with corrodent concentration for mild steel coupons in HCl solutions at 30°C without inhibitor

Figure 1 shows the variation of corrosion rate with corrodent concentration at 30°C. It is observed that the corrosion rate (mg dm-2day-1) of mild steel increases with increased concentration of HCl. This observation is in agreement with several investigators [3,5] and may be due to increasing concentration of an ion active in corrosion reaction [12]. From figure 2 a linear variation is observed for the plot of logarithm of weight loss against time. This confirms a first order reaction kinetics with respect to the mild steel in HCl solution

Effect of inhibitor concentration on inhibition efficiency

Figure 3 shows the variation of inhibition efficiency with various compounds investigated (AMMPTA, AMMPTC and TGA) Generally, inhibition efficiency of the compounds is in this order AMMPTA > AMMPTC> TGA at all temperatures studied. This trend of inhibition effectiveness is also confirmed from the hydrogen evolution measurements (figure 4) studied at very high corrodent concentration (8M HCl). At 8M HCl solution, the effect of dissolution could exceed that of adsorption of the inhibitors on the mild steel due to high chloride ion concentration, thereby resulting in a reduction in inhibition efficiency when compared with 0.5M HCl for AMMPTA and AMMPTC. 

Figure 2 Variation of log ∆ w with time for various mild steel coupons in HCl� solutions at 30oC without inhibitor

 

Figure 3 Variation of percentage inhibition efficiency (%p) with inhibitor concentration for mild steel coupon in 0.5M HCl containing various inhibitors at 30° and 40°C (click the image for an enlarged view)

Figure 4  Variation of percentage inhibition efficiency (%p) with inhibitor concentration for mild steel coupon in 8M HCl containing various inhibitors � data obtained from H2 evolution measurement.

Figure 5  (4-amino-2methyl-5-pyrimidinyl methylthio) acetic acid (AMMPTA)

Figure 6  3-[4- amino -2- methyl -5-pyrimidylmethyl] -5- [2-hydroxyethyl] -4-methylthiazolium chloride hydrochloride (AMMPTC). Thiamine chloride or Vitamin B1

Table 1 �Kinetic data for mild steel in 0.5M HCl containing AMMPTA

Inhibitor conc.

Inhibitor efficiency

K30(day-1)

K40 (day-1)

Half-life t1/2 (day)

Activation energy (kJ mol-1)

Average activation energy (kJ mol-1)

M

30°C

40°C

 

 

30°C

40°C

 

 

1x10-6

25

15

0.0109

0.01815

63

38

40.21

 

1x10-5

47

35

0.00756

0.0190

91

49

48.59

 

1x10-4

80

56

0.00297

0.00945

232

73

91.28

 

5x10-4

83

60

0.00296

0.00849

281

81

97.69

73.89

1x10-3

86

67.7

0.00217

0.00694

318

99

91.68

 

Table 2 Kinetic data for mild steel in 0.5M HCl containing AMMPTC

Inhibitor conc.

Inhibitor efficiency

K30(day-1)

K40 (day-1)

Half-life t1/2 (day)

Activation energy (kJ mol-1)

Average activation energy (kJ mol-1)

M

30°C

40°C

 

 

30°C

40°C

 

 

1x10-6

20

8

0.0114

0.01983

60

35

43.66

 

1x10-5

36

28

0.00912

0.01556

75

44

42.13

 

1x10-4

54

33

0.0065

0.01429

106

48

62.13

61.54

5x10-4

70

53

0.00406

0.01003

170

69

71.32

 

1x10-3

78

55

0.00313

0.00961

221

72

88.47

 

Table 3 Kinetic data for mild steel in 0.5M HCl containing TGA

Inhibitor conc.

Inhibitor efficiency

K30(day-1)

K40 (day-1)

Half-life t1/2 (day)

Activation energy (kJ mol-1)

Average activation energy (kJ mol-1)

M

30°C

40°C

 

 

30°C

40°C

 

 

1x10-6

-8

-7

0.0155

0.0231

44

30

31.47

 

1x10-5

-5

-16

0.0150

0.0250

45

28

39.76

 

1x10-4

37

20

0.0089

0.0164

77

42

48.20

46.07

5x10-4

45

29

0.00797

0.0154

86

45

51.94

 

1x10-3

59

32

0.00691

0.0146

100

47

58.99

 

Application of the principles of chemical kinetics to the results

Tables 1-3 give the kinetic data obtained in the presence of AMMPTA, AMMPTC and TGA� respectively. The rate constants at 30°C (K30) and 40°C (K40) were used to calculate the activation energy of the system using the integrated form of the Arrhenius equation, i.e

Log

From tables 1-3, the rate constants at 30°C (K30), 40°C (K40) show a general decrease with increased inhibitor concentration, inhibition efficiency, half-life (t1/2) and activation energy of the metal - corrodent - inhibitor systems. Similar trend in kinetic data has been reported by several investigators [3,5,12] and� indicates that a good inhibitor is one that is able to increase the time of conversion of metals to corrosion products. The half-life (t1/2) of the mild steel obtained in the absence of the inhibitors in� 0.5 M HCl is 47.7 day at 30°C and 32.2 day at 40°C respectively. The rate constant. K30 and K40without inhibitor are 0.0145 and 0.0214� day-1 �respectively while 30.7 kJmol-1 is obtained as the average activation energy of the metal - corrodent system without inhibitor at 30°C and 40°C in 0.5M HCl solution. Tables 1-3 show increase in half-life (t1/2) when inhibitors are present, which further support the� assertion that the compounds are corrosion inhibitors for mild steel in HCl solution.

Comparison of corrosion inhibition behaviour of the inhibitors

From� figure 3 and 4 and tables 1-3 AMMPTA exhibits a better corrosion inhibition tendency than AMMPTC and TGA. The structures of the compounds are given in figures 5 to 8 as in the literatures [6-8 ]. The highest inhibition efficiencies were exhibited by AMMPTA. These are 86 and 70% from weight loss and hydrogen evolution measurements respectively. AMMPTC exhibit 78 and 66% respectively while TGA exhibit� 59 and 50% from weight loss and hydrogen evolution measurements respectively, were obtained at 1x10-3 M inhibitor concentration at 30°C.� At 40°C low values of inhibition efficiencies are recorded for all the inhibitors� signifying that the compounds are all physically adsorbed on the mild steel coupons for the inhibition to be effective. The average activation energy values of the inhibitors reported as 73.9 kJmol-1 for AMMPTA, 61.5 kJmol-1 for AMMPTC and 46.6 kJmol -1 for TGA confirm the assertion that the inhibition of the corrosion of mild steel is by physical adsorption mechanism. This observation is in good agreement with several investigators [3,5 ]. These reports state that the heat of chemical adsorption should be greater than 80 kJ mol-1 and less than this value signifies a physical adsorption mechanism. Also, for a chemical adsorption mechanism, inhibition efficiency increases with increase in temperature [12 ], whereas increase in inhibition efficiency with decrease in temperature is suggestive of a physical adsorption mechanism (tables 1-3 and fig. 3). Since we have proposed physical adsorption mechanism for the present inhibitors, a multi - layer protective coverage is expected on the entire mild steel surface by the inhibitors for the inhibition to be effective. However, we observe that AMMPTC with a higher molecular weight (337 gm) than AMMPTA (214 gm) is less effective than AMMPTA. The difference in the inhibitory actions of AMMPTA and AMMPTC could be explained in terms of the ability of AMMPTA to form chelate structure with metal ion wherein four co-ordinate bonds are involved, fig. 9 as reported by Adeyeno et al� [6,8,13 ]

Although we have proposed a physical adsorption mechanism for the present inhibitors, this may not rule out completely the contributory effect of the structure (cyclic structure versus� linear structure, electron donating or with drawing groups, geometric factors and inhibitor solubility, which in the present investigation play a very less significant role and is not considered sufficiently important for the purposes of our discursion. However, the effect of the structure could have been of interest in the case of chemical adsorption mechanism where there is usually a dative link between the inhibitor and the metal [3,12 ]

H3C � S � CH2 � COOH

Figure 7��� Thioglycollic acid�� (TGA)

The difference in the inhibition actions of AMMPTA and AMMPTC could be explained by the comparison of their coordination sites. The comparison of the donor properties of AMMPTA is more straight forward than in the parent AMMPTC (Thiamine) -molecule� where the pyrimidine ring is conjugated with the thiazolium ring [8 ]. The aromatic nature of the pyrimidinyl moiety of AMMPTA, tends to make the 2 - 4-, and 6- positions naturally electron - deficient by virtue of the powerful electron with drawing effect of the ring nitrogen atom of the pyrimidyl ring which are metal to each other. Their separate effects reinforce each other and the resultant effect is the withdraw of the lone pair of the electrons on the substituent amino group at 4 - position of the pyrimidyl ring which in effect leads to electron deficiency and positively charged metal ion will prefer the 5-position of the thioacetic and substituent site to the ring nitrogen and the amino group� figure 5�� [8,13 ]. And the lone pair of electrons on both the sulfur� and the carboxyl groups provide a good site of coordination at the high pH and in acidic medium.

Conclusion

The corrosion of mild steel in HCl solution without inhibitor increases with increase in acid concentration and temperature. The inhibitors used in this investigation (AMMPTA, AMMPTC and TGA) inhibit the acid corrosion of mild steel to various degrees. The order of inhibition efficiency (%) is AMMPTA > AMMPTC > TGA. On the basis of� activation energy, AMMPTA and its precursor obey the mechanism of physical adsorption. The inhibition efficiency increases with increasing inhibitor concentration and decreasing temperature.

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Acknowledgements

The appreciation of the authors goes to Dr. O. Owolabi of the University of Port Harcourt for his immense contribution. The authors are highly indebted to the University of Port Harcourt for the use of facilities in the Department of Pure and Industrial Chemistry and the graduate supervision assignment of one of the authors.