Augmentation of Corrosion Inhibition Property of Thiourea by Aliphatic Amines in Presence of Aggressive Cl

With the growth of industrialization and modernization, it has now become essential for any country to construct and develop the basic infrastructure like highways, high speed rails, metro trains, bridges, schools, colleges, hospitals, strong air vehicle network etc. Development of these basic requisites has created a strong demand for production of steel. And we are doing so. We are now 4th largest steel producer in world. But production of steel and its products is only one step towards a sustained growth. We are facing strong challenges in maintaining the quality of steel products. Atmospheric corrosion of almost all parts of these structural elements can cause serious losses to lives, a great loss of money and times as well. Atmospheric corrosion can aggressively accelerate the rate of degradation of steel during their manufacturing, processing, storage and transportation. In these cases, traditional methods to prevent corrosion are not suitable which provide scope of Original Research Article Saini et al.; IRJPAC, 11(1): 1-10, 2016; Article no.IRJPAC.22529 2 vapour phase corrosion inhibitors in industries, defense and daily life. Corrosion inhibition property of Thiourea is tested with aliphatic amines i,e. 1,3-Diaminopropane, N,N,N,NTetramethylethylenediamine and Ethylamine for mild steel in different aggressive atmospheric conditions of chloride ions and sulphurdioxide by Vapour pressure determination test, Weight loss test, Salt spray test and Sulphur dioxide test at 50°C . It is found that percentage corrosion inhibition efficiency of combinations are in order of Thiourea+N,N,N,N-Tetramethylethylenediamine > Thiourea+Ethylamine > Thiourea+1,3-Diaminopropane. Chloride ions affect the barrier layer of corrosion inhibitor on surface of mild steel coupon and produce porosity on mild steel surface. Mechanism of action of corrosion inhibitors and effect of aggressive corrodent on the surface of mild steel are supported by morphology of mild steel studied by Metallurgical research microcopy and Scanning electron microcopy. Results of scanning electron microscopy provide the evidence of crevice corrosion, formation of barrier layer of vapours of vapour phase corrosion inhibitors and penetration of that barrier layer by chloride ions on mild steel surface. It is found that percentage corrosion inhibition efficiency of Thiourea + N,N,N,N-Tetramethylethylenediamine is very high (80.98%) in weight loss test but become little bit low in NaCl and sulphurdioxide environments by aggressive action of chloride and sulphate ions respectively.


INTRODUCTION
With large number of outdoor structures such as buildings, fences, bridges, towers, automobiles, ships and innumerable other applications exposed to the atmospheric environment, there is no wonder that so much attention has been given to the subject. Many variables influenced the corrosion characteristics of an atmosphere. Relative humidity, temperature, sulphur dioxide content, hydrogen sulphide content, chloride content, amount of rain fall, dust and even the position of the exposed metal exhibit marked influence on corrosion behavior, geographic location is also a factor. Atmospheric corrosion depends not only on the moisture content present but also on the dust content and the presence of other impurities in the air, all of which have an effect on the consideration of moisture on the metal surface and the resulting corrosiveness. Air temperature can also be a factor [1][2][3]. Corvo [4] and Moricelli et al. [5] studied the relationship between chloride ion concentrations with corrosion rate in atmospheric conditions. Ericsson showed that NaCl can cause corrosion at very low concentration because it can induce corrosion by SO 2 on a carbon steel surface [6]. NaCl can enhance 14 times rate of corrosion by SO 2 at 90% relative humidity at 22°C. In another report of Blucher et al., they have investigated adverse effect of CO 2 on corrosion of aluminium [7]. Vuorinen et al. [8] and a list of authors have worked on organic compounds as vapour phase corrosion inhibitors (VPCI). Organic substances have been studied as VPCI for mild steel were morpholine derivatives and diaminohexane derivatives, fatty acid thiosemicarbazides [9], cyclohexylamine and dicyclohexylamine [10,11], amine carboxylates [12], diethyl ammonia caprylate [13], ammonium caprylate [14], benzoic hydrazide derivatives [15], polyamines [16], bispiperidiniummethyl-urea and β-amino alcoholic compounds [17]. Apart from organic substances, natural compounds like wood bark oil [18] and thyme [19,20] have also been used as VPCIs. Cano et al. recently have proposed mechanism of inhibition of dicyclohexamine nitrite and dicyclohexamineisonitrite against corrosion due to vapours of acetic acid and formic acid on carbon steel [21]. Zubielewicz et al. [22] studied the electrochemical behavior of mixed anodic inhibitors. Batis et al. [23] evaluated the performance of two primers, first natural rust converter and other on organic primer coating containing VPCI against atmospheric corrosion for reinforcing steel. Lyublinski studied synergistic corrosion management systems by use of corrosion inhibitors [24]. It has been reported in the literature that the corrosion inhibition efficiency of the mixed vapour phase corrosion inhibitors is significantly increased in comparison to that of individual inhibitors in a corroding system [25]. Influence of combinations of sodium dodecyl sulphate-Zn 2+ -HEDP system towards corrosion inhibition of carbon steel was studied by Rajendran et al. [26] and found that this combination gave synergistic effect and provides complete protection in low chloride ion water. Venugopalan et al. [27] studied effects of HEDP, gluconate and zinc on the corrosion inhibition of carbon steel. They developed this ternary inhibitor formulation for carbon steel, which acts synergistically and gives maximum protection towards corrosion for carbon steel. Similar synergistic effects of HEDP with other combinations have been reported in the literature [28][29][30][31].

Sodium sulphate (anhydrous)
Minimum assay 99.9%, Molecular weight 142.04, Grade A.R., Source Himedia Lab. Pvt. Ltd. Mumbai. ) and sulphuric acid were also used to make solutions and washing of metallic specimens.

Weighing balance
Single pan analytical balance, Precision 0.01 mg, Model AB 135-S/FACT, Source Mettler Toledo, Japan.

Metallurgical research micrograph
Trinocular inverted metallurgical microscope, CXR II Laomed, Mumbai, India, connected with a computer and printer.

Vapour pressure determination test
A definite amount of exactly weighted VPCI was placed in a single neck round bottom flask fitted with a rubber cork in the neck having a glass capillary of 1.0 mm diameter in the center of rubber cork. Then the flask was kept in electrically controlled air thermostat maintained at the constant temperature of 50°C for 10 days. Change in weight of VPCIs was observed by analytical balance and vapour pressure of investigated VPCI was determined by weight loss of VPCI for time of exposure by equation 1. ).

Weight loss test
Mild steel coupons were mechanically polished successively with the help of emery papers grading 100, 200, 300, 400 and 600µ and then thoroughly cleaned with plenty of triple distilled water, ethanol and acetone. Then coupons were dried with hot air blower and stored in desiccators over silica gel. Weight loss tests were carried out in an electronically controlled air thermostat maintained at a constant temperature of 50°C. After recording the initial weights of mild steel coupons, they were kept in different isolated chambers of air thermostat having fixed amount of VPCI at a constant temperature of 50°C for 24 hours of exposure time. A uniform thin film of VPCI was adsorbed onto the metal coupon surface after 24 hours of exposure. Then these coupons were transferred to a digitally controlled humidity chamber maintained at 85% humidity at a constant temperature of 50°C for 10 days. Blank coupons untreated with VPCI were also kept in humidity chamber for the same duration in the same corrosive environment. After exposing the coupons for 10 days, coupons were taken out from the humidity chamber and washed initially under running tap water. Loosely adhering corrosion products were removed with the help of rubber cork and coupon was again washed dried and then weighed again. Corrosion rate in miles per year (mpy) and percentage corrosion inhibition efficiency (PCIE) were calculated by using equations 2 and 3 respectively.
Where, W = weight loss of mild steel coupon in 10 days of experiment (in mg), D = density of mild steel (in g/cm 3 ), A = area of mild steel coupon (in sq. inch), T = exposure time of vapours of tested inhibitor on mild steel coupon (in hour).
Where, PCIE = Percentage corrosion inhibition efficiency, CR o = corrosion rate in absence of inhibitor and CR = corrosion rate in presence of inhibitor.

Salt spray test
After exposing the pre-weighted mild steel coupons to VPCI in air thermostat for 24 hours, they were transferred to salt spray chamber having 3.0% NaCl solution maintained at 50°C for duration of 10 days along with blank coupons. After exposing coupons for 10 days, coupons were treated in same manner as treated in weight loss test to remove corrosion products and then CR and PCIE were calculated.

Sulphurdioxide test
SO 2 test was carried out on the mild steel coupons as in weight loss test. SO 2 gas was prepared by dissolving 0.04 g of sodium thiosulphate in 30mL aqueous solution of 1.0% NH 4 Cl and 1.0% Na 2 SO 4 solution and 0.5 mL of 1.0N H 2 SO 4 was added to the flask. Initially preweighed and mechanically polished mild steel coupons were placed in air thermostat maintained at 50°C for duration of 10 days. Definite weight of VPCIs in a petridis and flask, which is the source of SO 2 , was placed in the isolated chambers of air thermostat containing mild steel coupons. After exposing coupons for 10 days, CR and PCIE were calculated.

Metallurgical Research Microscopy (MRM)
This test was employed for the surface study of mild steel coupons to know about nature and type of corrosion. Micrograph of mild steel coupon treated with investigated VPCI were subjected to porosity study and morphology of surface which provided the information about the number of pores, size of pores, percentage porosity and the area covered by the pores on the surface of coupon. Percentage porosity (PP) and total objects (TO) shows the roughness of surface. On the other hand maximum perimeter and maximum area object ratio (A/O) provide the information about the size and depth of the pores on the surface of mild steel after the corrosion test.

Scanning Electron Microscopy
Morphology of the selected samples was observed by Scanning Electron Microscopy to provide the evidence in support of type of corrosion. In this technique, the sample was studied at different resolutions on the different spots on the mild steel coupons for complete information about the inhibition mechanism.

Morphology of the selected samples was
Microscopy to provide the evidence in support of type of corrosion. In this technique, the sample was studied at different resolutions on the different spots on the mild steel coupons for complete information about the inhibition mechanism.

Combinations
of thiourea with 1, 3-Diaminopropane, N,N,N,N-Tetramethylethylenediamine and Ethylamine are tested due to presence of adsorption attacking site like lone pair donar atoms and π electrons system in the structures of these amines a shown in Table 1.

Vapour Pressure Determination Test
Vapour pressures of combinations of thiourea with different amines are given in Table 2.

Weight Loss Test
Results of this test are given in Table 3 which shows that thiourea perform as an efficient VPCI with all combinations for mild steel in atmospheric corrosion. Weight loss and corrosion rate of combinations are very low as compared to that of thiourea due to increase in vapour pressure and vapour density of the combinations. PCIE of the different combinations is in following order: Thiourea+N,N,N,N Tetramethylethylenediamine > Thiourea+Ethylamine > Thiourea+1,3 Diaminopropane. Thiourea + N,N,N,N Tetramethylethylenediamine show very high percentage corrosion inhibition efficiency due to presence of strong +I effect of four methyl groups near lone pair donar N atom which enhance the Lewis basic strength to form layer of acid base complex on surface of mild steel coupon and to neutralize the acidic environment of atmos around coupon as shown in Fig. 1  system in the structures of these amines as

Vapour Pressure Determination Test
Vapour pressures of combinations of thiourea with different amines are given in Table 2.
Results of this test are given in Table 3

Salt Spray Test
Direct contact of NaCl salt on the surface of mild steel coupon and its hydrolysis products accelerate the corrosion rate due to which percentage corrosion inhibition efficiency is little bit a low than that of weight loss test as shown in Table 4.
To determine the effect of salt hydrolysis product and type of corrosion on mild steel in presence of chloride ion is studied by morphology of mild steel coupon with the help of metallurgical research microscopy which are given in Fig. 2 and Table 5.

Sulphurdioxide Test
Results of effect of acidic nature of sulphurdioxide on mild steel coupon are given in Table 6.