Co-Inhibition Characteristics of Sodium Tungstate with Sodium Nitrate on Low Carbon Steel in 0.085M Hcl

Emirates Journal for Engineering Research, 17 (2), 1-6 (2012)

(Regular Paper)

CO-INHIBITION CHARACTERISTICS OF SODIUM TUNGSTATE WITH SODIUM NITRATE ON LOW CARBON STEEL IN 0.085M HCL.

Ogundare, O.1, Umoru, L.E.2, Olorunniwo, O. E. 2, Ige, O. O. 2

1 Engineering Materials Development Institute, Akure Nigeria. 2Department of Materials Science and Engineering, Obafemi Awolowo University, Ile- Ife, Nigeria

(Received January 2012 and Accepted September 2012)

ﻗﻴﻤﺖ هﺬﻩ اﻟﻮرﻗﺔ the co-inhibitive characteristics ﻟﺘﻨﺠﺴﺘﺎت اﻟﺼﻮدﻳﻮم ﻣﻊ ﻧﺘﺮات اﻟﺼﻮدﻳﻮم ﻓﻲ ﺣﺎﻣﺾ اﻟﻬﻴﺪروآﻠﻮرﻳﻚ M.0.085 وﻗﺪ أﺟﺮﻳﺖ اﻟﺪراﺳﺔ ﺑﺎﺳﺘﺨﺪام ﺗﻘﻨﻴﺔ اﻟﺠﺎذﺑﻴﺔ. ﺗﻢ دراﺳﺔ ﺗﺄﺛﻴﺮ إﺿﺎﻓﺔ single inhibitor and co-inhibitors ﻓﻲ ﻣﻌﺪﻻت اﻟﺘﺂآﻞ. أﻇﻬﺮت اﻟﻨﺘﺎﺋﺞ أن ﺗﻨﺠﺴﺘﺎت اﻟﺼﻮدﻳﻮم وﺣﺪﻩ أﻋﻄﻰ 47IE٪ وﻧﺘﺮات اﻟﺼﻮدﻳﻮم اﻧﺘﺠﺖIE 57٪ ﻋﻨﺪﻣﺎ ﺗﺴﺘﺨﺪم وﺣﺪهﺎ ﺑﻌﺪ 336 ﺳﺎﻋﺔ ﻣﻦ اﻟﺘﻌﺮض. ﺗﻢ اﻟﺤﺼﻮل ﻋﻠﻰ آﻔﺎءة 54٪ ﻋﻨﺪ 150 ﺟﺰء ﻓﻲ اﻟﻤﻠﻴﻮن ﻣﻦ ﻧﺘﺮات اﻟﺼﻮدﻳﻮم، و 50 ﺟﺰء ﻓﻲ اﻟﻤﻠﻴﻮن ﻣﻦ اﻟﺼﻮدﻳﻮم ﺗﻨﺠﺴﺘﺎت ﺑﻌﺪ 336 ﺳﺎﻋﺔ ﻣﻦ اﻟﺘﻌﺮض. اﺳﺘﻨﺎدا إﻟﻰ اﻟﻨﺘﻴﺠﺔ اﻟﺘﻰ ﺗﻢ اﻟﺤﺼﻮل ﻋﻠﻴﻬﺎ ﻳﺠﺐ ان ﻳﻜﻮن ﺗﺮآﻴﺰ ﺗﻨﺠﺴﺘﺎت اﻟﺼﻮدﻳﻮم أﻗﻞ ﻣﻦ ﺗﺮآﻴﺰ ﻧﺘﺮات اﻟﺼﻮدﻳﻮم ﻟﻠﺤﺼﻮل ﻋﻠﻲ آﻔﺎءة ﻣﺎﻧﻊ ﻓﻌﺎﻟﺔ.

This paper has evaluated the co-inhibitive characteristics of sodium tungstate with sodium nitrate in 0.085 M hydrochloric acid. The inhibition study was carried out using the gravimetric technique. The uninhibited solution of 0.085 M hydrochloric acid served as the control. The effect of addition of single inhibitor and co-inhibitors at 50 (parts per million) ppm, 100 ppm, and 150 ppm on corrosion rates (CR) (mpy) and inhibition efficiencies (IE) was investigated. The results showed that sodium tungstate alone gave 47% IE and sodium nitrate produced 57% IE when used alone after 336 hours of exposure. The inhibitive efficiency of 54% was obtained at 150 ppm of sodium nitrate and 50 ppm of sodium tungstate after 336 hours of exposure. Based on the result obtained sodium tungstate must be in lower concentration than sodium nitrate for effective inhibitive efficiency. Keywords: Corrosion, inhibition, co- inhibitors and corrosion inhibition efficiency.

1. INTRODUCTION potential and high cost[2]. This is due to the fact that tungstate ions alone cannot shift the potential of the Corrosion is the reaction between a material and its metal into the passive region substantially[3-5]. This environment that produces a deterioration of the fact justifies the small quantity of sodium tungstate material and alters its mechanical properties. The (50 ppm) used throughout this work. Hence, its actual corrosion process that takes place on a piece of inhibitive potential can be fully maximized when bare mild steel is very complex due to factors such as used in the presence of oxidizing inhibitor such as variations in the composition or structure of the steel, sodium nitrate, sodium silicate, sodium chromate and presence of impurities due to the high instances of the sodium nitrite. recycled steel, uneven internal stress, or exposure to a non-uniform environment[1]. Co-inhibitive effects have been investigated on molybdate and phosphate as inhibitors in synthetic Inhibitors are substances added in small tap water environments[6] and on potassium concentrations to particular environments with the permanganate with sodium tungstate for the aim of slowing down the rate of corrosion at such protection of carbon steel[3]. They proposed that due sites. Co- inhibition is achieved when two or more to the similarity in chemical structure and periodicity inhibitors are used at the same time to effect better between chromates and other group VI ions, inhibition efficiency than a single inhibitor used molybdate and tungstate are to be reliable separately. replacement. Among the non–oxidizing passivating The major use of inhibitors in acid solutions is in inhibitors, tungstate seems to be better than pickling processes, oil well cleaning, descaling and molybdate in terms of its inhibition efficiency and industrial acid cleaning. Anodic Inhibitors such as applicability under wide experimental condition, sodium tungstate are found to be the most effective including pH. This was reported in their research and readily available but for its low oxidizing work[3,4]. To improve the inhibitive capacity of

1 Ogundare, O., Umoru, L.E., Olorunniwo, O. E. and Ige, O. O. molybdate, it must be in co-inhibition with other The inhibitive property of palm kernel oil (PKO), inhibitors (including organic and inorganic)[6]. For an inorganic inhibitor, on crude oil pipeline has been tungstate ions to act as anodic inhibitors (due to its investigated and modeled (non-linear regression low oxidizing ability and high cost); it must be in the analysis) to forecast and predict it[13]. presence of dissolved oxygen or oxidizing agent or The aim of this study is to evaluate the corrosion inhibitor. It should be noted that tungstate ions alone inhibition efficiencies of sodium tungstate and cannot shift the potential of the metal into the passive sodium nitrate and their co-inhibitive characteristics region substantially and this view was supported by as it relates to their application in acid pickling, oil earlier findings[3-6]. However, tungstate has been well cleaning, descaling and industrial acid cleaning. adjudged to be highly effective even in highly reducing environment[5,7]. A recent work by[11] has reported the evaluation the co-inhibitive 2. EXPERIMENTAL characteristics of sodium tungstate with sodium The low carbon steel sheet and the HCl used for this silicate in 0.085 M hydrochloric acid. The effect of research work were obtained commercially. The low palm oil as corrosion inhibitor on ductile iron and carbon steel was spectrometrically analyzed at the mild steel has been successfully reported Materials laboratory, Ajaokuta Steel Complex recently[12]. It was concluded that palm oil, being Limited, Ajaokuta, Nigeria and the composition of non-toxic, has met one of the stringent requirements the steel was as presented in Table 1. for corrosion inhibitors and its application.

Table 1: Chemical Composition of experimental low carbon steel Element Fe C Si Mn P S Co Mo Ni Al Cu % Wt 99.00 0.117 0.033 0.011 0.0258 0.014 0.005 0.014 0.19 0.59 0.055

One hundred and seventy five coupon samples were cut from the low carbon steel sheet using the 3. RESULTS cutting saw into 50 mm X 30 mm X 0.5 mm each and In Fig.1 the inhibition efficiencies of low carbon steel a hole of 4mm diameter was drilled at the center top immersed in 0.085M HCl with sodium tungstate are of each coupon to facilitate its suspension. The presented. It must be emphasized that the corrosion samples were mechanically polished using emery rate is inversely related to the percent inhibition papers with 240, 320, 400 and 600 grits. This was to efficiencies (%IE). It is evident that as the amount of eliminate scaling, surface contaminant and oxide film the inhibitor increases the inhibition efficiencies on the surface of the metal. Subsequently, the sample decrease that is the concentration of the inhibitor is surfaces were degreased by swabbing with acetone. independently proportional to the inhibition The degreased surface is then air- dried. efficiencies. Interestingly at 144 hours of exposure The dimension of each coupon sample was the percent inhibition efficiencies are the same with a carefully taken and recorded in order to take care of value of about 57%IE. It is noted that before the minutest difference in the exposed surface area. The period the efficiencies increases but immediately after coupons were afterwards rinsed under tap water, the performance was decreasing after 144 hours. This dried, weighed on digital mettler balance and then may be attributed to the dissolution of the barrier immersed in the 0.085 M HCl already prepared. formed by the inhibitor. The concentration of the sodium tungstate at 50 ppm showed the highest IE Immersion tests during fourteen days using which is about 45% after 288 hours of exposure and gravimetric technique were carried out according to about 40% and 35% respectively for 100 ppm and ASTM G 1-90, by complete immersion of the seven 200 ppm. samples in the media using a large labeled plastic bowl and suspending them on sample rack. A coupon was removed from each solution medium at intervals of 48 hours, held under a stream of tap water and the surface scrubbed with a rubber stopper to remove the corrosion product. This was followed by cleaning with acetone to halt the minute electrochemical reactions and form a barrier against atmospheric corrosion. The coupon was finally air-dried and re- weighed.

2 Emirates Journal for Engineering Research, Vol. 17, No.2, 2012 Co-inhibition characteristics of sodium tungstate with sodium nitrate on low carbon steel in 0.085M HCl.

20 100ppm Na2WO4

50ppm Na2WO4

Percent Inhibition Efficiencies (%IE) 10 150ppm Na2WO4

0 0 100 200 300 400 Time of exposure (hrs)

Figure 1. Percent Inhibition Efficiencies of low carbon steel immersed in 0.085 M hydrochloric acid inhibited with sodium tungstate

relationship between the concentration and the While Fig. 2 shows the inhibition efficiencies of efficiency. Hence samples with 150 ppm have the mild steel as immersed in the medium and inhibited highest efficiencies and the lowest occurred at 50 with sodium nitrate. It is observed that the ppm. The efficiency also increases with the period of efficiencies increase as the concentration of the exposure but the corrosion rate is almost the same at inhibitor which shows that there is a linear

50ppm NaNO3

100ppm NaNO3 90

80 150ppm NaNO3

70 200ppm NaNO3

30 Percent Inhibition efficiencies (%IE)

0 0 100 200 300 400 Time of exposure (hrs)

Figure 2. Percent Inhibition Efficiencies of low carbon steel immersed in 0.085 M hydrochloric acid inhibited with sodium nitrate.

Emirates Journal for Engineering Research, Vol. 17, No.2, 2012 3 Ogundare, O., Umoru, L.E., Olorunniwo, O. E. and Ige, O. O.

50ppm Na2WO4 + 50ppm NaNO3

50ppm Na2WO4 + 90 100ppm NaNO3 80 50ppm Na2WO4 +

70 150ppm NaNO3

30 Percent Inhibition 20 50ppm Na2WO4 +

efficiencies (%IE) 200ppm NaNO 10 3

0 0 100 200 300 400 Time of exposure (hrs)

Figure 3. Percent Inhibition Efficiencies of low carbon steel immersed in 0.085 M hydrochloric acid co-inhibited with sodium tungstate and sodium nitrate.

attack in the form of pits and cavities. The corrosion 196 hours of exposure at an efficiency value of about products were also found to be widespread and 15%. At 288 hours of exposure the %IE are as concentrate more around the edges of the pits and follows: 27%, 34% and 37% respectively for 50 ppm, along the grain boundaries. The micrograph of the 100 ppm and 150 ppm sodium nitrate. sample inhibited with 150 ppm sodium tungstate is as The co-inhibition of sodium tungstate and sodium shown in Plate 2. It has uniform corrosion products nitrate in 0.085M HCl was exhibited in Fig. 3 and it and it is found to be concentrated around the grain can be seen that the concentration of sodium tungstate boundaries and the edges of the cavities. The is fixed at 50 ppm while that of sodium nitrate localized corrosion attacks are widespread and non- increases from 50 ppm to 200 ppm at an interval of uniform but less severe when compared with that as 50 ppm. As the sodium nitrate increases it is observed shown in Plate 1. that the inhibition efficiencies increases up till 150 The microstructure of the sample inhibited with ppm after which it decreases hence the best %IE is 150 ppm sodium nitrate is as shown in Plate 3 and the obtainable at 150 ppm NaNO and 50 ppm Na WO . 3 2 4 localized corrosion attacks are less pronounced, non- Also the IE decreases as the period of exposure uniform and the corrosion products are widespread, increases for all the concentrations of the co- and they almost covered the sample surface. Plate 4 is inhibitors. However in the case of using the inhibitors the micrograph showing the effect of co-inhibition of singly the 50 ppm Na2WO4 and 150 ppm NaNO3. It revealed a IE increases with time. It is observed that the use less pronounced and distributed localized corrosion of sodium tungstate to co-inhibit with NaNO3 IN attack in size, shape and severity and the corrosion 0.085m HCl gives the optimum percent inhibition products are uniform and widespread. efficiencies in the ratio of 1:3. The microstructural evaluations of the corroded samples were done after 336 hours of exposure to the environment. Physical observation of the uninhibited sample suggests that the deterioration was of great magnitude such that the steel sheet is near self- tearing. Plate 1 is the micrograph of the uninhibited sample immersed in the medium and it is characterized by severe and widespread localized

4 Emirates Journal for Engineering Research, Vol. 17, No.2, 2012 Co-inhibition characteristics of sodium tungstate with sodium nitrate on low carbon steel in 0.085M HCl.

Plate 1: Micrograph of the uninhibited sample in 0.085 M Plate 4: Micrograph of the sample immersed in 50 ppm HCl acid medium after 336 hours of exposure. X 200 Sodium Tungstate and 150 ppm Sodium Nitrate after 336 hours of exposure. X 200

4. DISCUSSION It must be noted that as the corrosion rate of materials increases it implies that the effectiveness and efficiency of inhibitor in the environment becomes poorer. Hence for an inhibitor to perform creditably well the corrosion rate of the material must be comparably low in the medium[2]. It is observed from figure 1 that as the concentration of sodium tungstate increases the %IE decreases this can be attributed to the fact that adsorption of inhibitor species at low coverage is more effective in producing inhibition than adsorption at high surface coverage. It is assumed that the tungstate ions were adsorbed onto the sample surfaces and formed diffusion barrier against the metal dissolution[2]. It is Plate 2: Micrograph of the sample inhibited with 150 ppm also known that the presence of localized corrosion Sodium Tungstate after 336 hours of exposure. X 200 attack and low coverage of corrosion product aided the corrosion rate and thereby reducing the %IE[7]. It is observed that localized corrosion attacks increase with the concentration of sodium tungstate but reduces with increase of sodium nitrate hence lower concentration of Na2WO4 is favoured as inhibitor for mild steel in 0.085M HCl. It was reported that the adsorption effect and the inhibitive tendencies of the nitrate ions become greater with increase in concentration of participating inhibitor species[7]. This is an indication of the continuous weakening of the medium by the corrosion process. So the inhibitive tendencies of sodium nitrate in 0.085M HCl increase with increase in concentration, hence the corrosion rate at 50 ppm of NaNO3 is higher than that at 200 ppm. It is also noted that the inhibition efficiencies increases with time of exposure and this followed the trend that Plate 3: Micrograph of the sample inhibited with 150 ppm corrosion rate decreases as the period of exposure is Sodium Nitrate after 336 hours of exposure. X 200 prolonged and this is due to increasing weakness of

the medium with time[9]. Therefore sodium nitrate

used alone gives a better oxidizing ability, exhibiting

Emirates Journal for Engineering Research, Vol. 17, No.2, 2012 5 Ogundare, O., Umoru, L.E., Olorunniwo, O. E. and Ige, O. O. better inhibitive efficiency than sodium tungstate 6. Li Y., Gong X., Peng, K. and Xie X., (2002) A used alone. study of the inhibitive behaviour of molybdate when used in combination with phosphate in The co-inhibitive efficiencies of sodium tungstate synthetic tap-water environments, Journal of and sodium nitrate are found to be optimum at a Anti-Corrosion Methods and Materials, 49, 4, concentration ratio of 1:3. Also it is found out that 252-255. Na WO must be maintained at a very low 2 4 7. Pu, C.C and Wu, J.K. (2003) Inhibition of concentration and for this study the concentration is chloride pitting corrosion of stainless steels by 50 ppm. The corrosion rate decreases as the Sodium Nitrite, Journal of Corrosion Prevention concentration of the sodium nitrate increases until it and Control, 50, 4, 155-160. get to about 150 ppm but at concentration greater than 8. Abdallah, M., Nagar, M.El., Megahed, H.E., this the %IE decreases. It is assumed partly that the Radius, D. and Mabrouk, E.M., (2003) Corrosion adsorbed iron tungstate and iron nitrate were Kinetics of nickel alloys in the H SO and the responsible for film formation over the surface of the 2 4 effect of dihydrazide derivatives as corrosion specimen and hence corrosion inhibition. Also partly inhibitors, Journal of Corrosion Prevention and it may be attributed to the fact that the oxidizing Control, 50, 4, 173-180. power of the medium has been altered in such a way 9. Loto, C.A and Mohammed, A.I, (2000) The that the amount of tungstate ions cannot act as effect of cashew juice extract on corrosion diffusion barrier against metal dissolution. inhibition of mild steel in HCl, Journal of Corrosion Prevention and Control, 47, 2, 50-53. 5. CONCLUSION 10. ASTM G 1-90, (1993) “Standard Practice of Preparing, Cleaning and Evaluating Corrosion It can be concluded that NaNO3 as inhibitor for low carbon steel in 0.085m HCl is recommended for use Test Specimens” Annual Book of ASTM at a higher concentration while a low dosage of Standards, Vol 3.02 ASTM Philadelphia, P.A. 11. O.Ogundare, L.E. Umoru, O.E. Olorunniwo, Na2WO4 must be applied. The co-inhibitive efficiencies of sodium tungstate and sodium nitrate A.R. Adetunji, (2009) Evaluation of corrosion will give optimum performance at a ratio of 1:3. co-inhibition characteristics of sodium tungstate with sodium silicate on low carbon steel in REFERENCES 0.085M hydrochloric acid. Nigerian Journal of Materials Science and Engineering, 1, 1, 43 – 47. 1. Fontana M.G and Greene, N.D., (1983) 12. A.A. Daniyan, O. Ogundare, B.E. AttahDaniel, Corrosion Engineering” 2nd Edition, McGraw and B. Babatope, (2011) Effect of palm oil as Hill, Tokyo. corrosion inhibitor on ductile iron and mild steel. 2. Saeed, M. Tariq, (2004) Corrosion inhibition of The Pacific Journal of Science and Technology, carbon steels in sulfuric acid by bicyclic 12,2, 45 -53 isoxazolidines, Journal of Anti-corrosion Methods and Materials, 51, 6, 389-398. 13. O.K. Ukoba, U.S. Anamu, O. Ogundare, M.C. 3. Shibli, S.M.A and Saji, V.S. (2003) Evaluation of Ibegbulam, and O.A. Akintunlaji, (2011) A co-inhibition characteristics of potassium Model to Predict the Inhibitive Property of PKO permanganate with sodium tungstate for the on Crude Oil Pipeline. The Pacific Journal of protection of carbon steel, Journal of Corrosion Science and Technology, 12,2, 39 - 44 Prevention and Control, 50, 23, 136- 146. 4. Saji, V.S and Shibli, S.M.A, (2002) Synergistic inhibition of carbon steel corrosion by sodium tungstate and sodium silicate in neutral aqueous media, Journal of Anti-corrosion Methods and Materials, 49, 6, 433-443. 5. Azambuja, D. S., Martins, E. M.A. and Millers, I. L., (2003) Corrosion Behaviour of Iron AISI 304 stainless steel in Tungstate Aqueous Solution containing chloride , Journal of Brazilian Chemical Society, 14, 4, 570-567.

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