Corrosive Inhibitive Study of Stainless Steel in Hydrochloric Acid Using Eco-Friendly Sesmum Indicum Oil

Introduction

Stainless steel’s strength, resistance to corrosion and low maintenance make it the ideal material for a wide range of applications such as pharmaceutical industry, architecture, medical, energy, heavy industry, construction, automotive, transportation, food and catering.

Corrosion is chemical or electrochemical reaction between materials, usually a metal, and its environment that produces a deterioration of the material. The annual loss due to corrosion has been estimated for any country at about 4 percent of gross domestic product.

In stainless steel there are some amounts of chromium which can be able to forms a passive film, this passive film can protect metal from corrosive medium. Corrosion inhibitors can protect passive film from corrosive medium. Due to chloride ions pitting corrosion is occur in stainless steel which is a great problem, face by engineers. Generally the loss of equipment is done by pitting corrosion. Many organic compounds have been studied as corrosion inhibitor for different metals ^1-8. Larger amount of organic compounds have been use as inhibitor but they are poisonous and expensive, some eco-friendly, less costly and green inhibitors are substitute of organic inhibitors ^9-12. Due to above reason in the present study eco-friendly Sesmum indicum oil has been tested for corrosion inhibitive property for the stainless steel metal in 0.5N hydrochloric acid.

Experimental

Weight Loss Method

Material Preparation

Stainless steel coupons with major amount of iron (80%) and lesser amount of chromium (14%) with rectangular size 3×2×0.2 cm. were used for mass loss method. These coupons washed with acetone and double-distilled water, dried in electrical oven and then stored in dessicator. The same metal was used for polarization studies in which metal was coated with araldite and only 1.0 cm² area was opened for electrochemical study.

Solutions

 0.5N HCl was prepared in AR grade chemical for mass loss and polarization study.  The amount of Sesmum indicum in corrosive medium was of 1.0g to 6.0g in 1000ml for mass loss method and 1.0g to 5.0g in 1000ml corrosive medium for polarization studies.

CR (Corrosion Rate) Computation

 The data of mass loss was determined using following method reported earlier ¹³. Experiment was done in repeatedly in test solution. All of first the coupons of stainless steel weighted and after this all are dipped in test solutions, which containing 0.5 N HCl with absence and presence of oil by the hooks, which are made by glass. The stainless steel coupons got back from the tested solution. The coupons rubbed for clean and after this dried in electrical oven, again weight and loss of weight was evaluated. This method have been repeated for  different immersion times 1hour, 3 hours, 6 hours, 15 hours and 24 hours at room temperature (298K).

The corrosion rate (CR) was evaluated by following formula:

In above formula d = density of the coupon (gcm^-3), W = mass loss of metal (gdm^-3), A = area of the coupon (square inch) and t = time (hrs).

The %IE with eco-friendly Sesmum indicum oil calculated using the following formula:

Where W_o= Mass loss of the stainless steel specimens in the absence of Sesmum indicum oil   and W_i= Mass lossof the stainless steel specimens in presence of Sesmum indicum oil.

The θ which is surface coverage was evaluated by following formula (3)

Electrochemical procedure

The galvanostatic polarization was done with three-electrode polarization cell. Polarization method have a working electrode which was prepared by stainless steel metal with 1cm² open area, auxiliary electrode which was  prepared by platinum, reference electrode which was prepared  by saturated calomel electrode.

Experimental results

The inhibitor efficiency (IE %) of Sesmum indicum oil, degree of surface coverage (θ) by Sesmum indicum oil and CR (corrosion rate) with variable amount  of the inhibitor in 0.5N HCl for1hour, 3hours, 6hours, 15hours and 24hours immersion periods and different kinetic and thermodynamic parameters were evaluated by weight loss method. Electrochemical parameters were evaluated by galvanostatic polarization method. Different parameters evaluated from mass loss procedure are presented in tables 1 to 5.  In 0.5N HCl at 1g/lit of Sesmum indicum oil, there is 75.65% inhibition observed for immersion time 1 hour. On increasing the concentration 1 to 6 g/lit, the inhibition increases up to 89.56%. When the duration of immersion varied from 1 hour to 24 hours inhibitor efficiency decreases.

Table 1: %IE, CR and surface covered by metal evaluated  by  mass loss experiment with stainless steel in hydrochloric acid (Experiment period : 1h)

Concentration of oil (gram/liter)	Loss of metal (gram)	Efficiency of inhibitor (%)	CR (mmpy)	Surface covered by inhibitor ( θ )
0.00	0.0115	–	9.093	–
1.00	0.0028	75.65	2.214	0.7565
2.00	0.0025	78.28	1.97	0.7828
3.00	0.0021	81.73	1.66	0.8173
4.00	0.0018	84.34	1.56	0.8434
5.00	0.0015	86.95	1.18	0.8695
6.00	0.0012	89.56	0.94	0.8956

 

Table 2: %IE, CR and surface covered by metal evaluated by mass loss experiment with stainless steel in hydrochloric acid (Experiment period: 3h)

Concentration of oil (gram/liter)	Loss of metal (gram)	Efficiency of inhibitor (%)	CR  (mmpy)	Surface covered by inhibitor ( θ )
0.00	0.0150	–	3.95	–
1.00	0.0051	66	1.34	0.66
2.00	0.0045	70	1.18	0.70
3.00	0.0040	73	1.05	0.73
4.00	0.0036	76	0.94	0.76
5.00	0.0030	80	0.79	0.80
6.00	0.0024	84	0.63	0.84

 

Table 3: %IE, CR and surface covered by metal evaluated  by  mass loss experiment with stainless steel in hydrochloric acid (Experiment period: 6h)

Concentration of oil (gram/liter)	Loss of metal (gram)	Efficiency of inhibitor (%)	CR  (mmpy)	Surface covered by inhibitor ( θ )
0.00	0.0180	–	2.37	–
1.00	0.0071	60.55	0.93	0.6055
2.00	0.0068	62.22	0.89	0.6222
3.00	0.0064	64.44	0.84	0.6444
4.00	0.0055	69.44	0.72	0.6944
5.00	0.0047	73.88	0.61	0.7388
6.00	0.0038	78.88	0.50	0.7888

 

Table 4: %IE, CR and surface covered by metal evaluated  by  mass loss experiment with stainless steel in hydrochloric acid (Experiment period: 15h) 

Concentration of oil (gram/liter)	Loss of metal (gram)	Efficiency of inhibitor (%)	CR  (mmpy)	Surface covered by inhibitor ( θ )
0.00	0.0225	–	1.18	–
1.00	0.0095	57.77	0.50	57.77
2.00	0.0086	61.77	0.45	61.77
3.00	0.0081	64.00	0.42	64.00
4.00	0.0073	67.55	0.38	67.55
5.00	0.0067	70.22	0.35	70.22
6.00	0.0061	72.88	0.32	72.88

 

Table 5: %IE, CR and surface covered by metal evaluated  by  mass loss experiment with stainless steel in hydrochloric acid (Experiment period: 24h)

Concentration of oil (gram/liter)	Loss of metal (gram)	Efficiency of inhibitor (%)	CR  (mmpy)	Surface covered by inhibitor ( θ )
0.00	0.0410	–	1.35
1.00	0.0181	55.85	0.59	0.5585
2.00	0.0172	58.04	0.56	0.5804
3.00	0.0163	60.24	0.53	0.6024
4.00	0.0154	62.43	0.50	0.6243
5.00	0.0148	63.90	0.48	0.6390
6.00	0.0140	65.85	0.46	0.6585

 

Weight loss measurements indicate that the dissolution rate decreases to significant extent due to the presence of oil especially at high concentrations. The inhibition efficiency is maximum on immersion time 1 hour at 6g/lit Sesmum indicum.  The maximum inhibition efficiency for immersion time 24 hours at 6g/lit Sesmum indicum is 65.85%.The graph which is present  in figure 1 shows that  %IE  is varies with  the amount of the Sesmum indicum oil with variable experimental times like 1 hour, 3 hours, 6 hours, 15 hours and 24 hours.

Figure 1: Increasing of %IE with concentration of Sesmum indicum oil for stainless steel (immersion time periods: 1 hour, 3 hour, 6 hour and 24 hour) in 0.5N HCl.   Click here to View figure

 

Corrosion kinetics of stainless steel in hydrochloric acid with inhibitor

Figure 2 present a graph  which shows  log W_f (weight of stainless steel at time t) is change with time, the evaluated data putted  in to the rate law for first order reaction. The plots are linear, which concluded  that the reaction with  stainless steel in test solution  follows first order kinetic ^14-15.  The rate constants measured from the  figure 2 are concluded  in table 6.

The half-life (t_1/2) evaluated from equation giving below.

(t _½)=0.693/ k                                                        (5)

Table 6:  Kinetic parameter study  of protection  and destruction of  stainless steel in acidic medium with and without of oil.

Sesmum indicum oil Amount (gram/liter)	(k) (s-1)	(t ½) (s)
0.00	1.15×10-7	5.02×106
1.00	4.6×10-8	1.50×107
2.00	2.3×10-8	3.0×107
3.00	2.3×10-8	3.0×107
4.00	2.3×10-8	3.0×107
5.00	2.3×10-8	3.0×107
6.00	2.0×10-8	3.4×107

 

Figure 2: Chemical kinetic of  stainless steel in hydrochloric acid absence and presence of oil.   Click here to View figure

 

The half life were evaluated by above formula are shows  in table 6 which clearly shows that rate constant inversely proportional of half life which conclude the protection of metal by inhibitor.

Thermodynamic Parameters of Stainless Steel in Hydrochloric Acid With Inhibitor

The graph of C/ θ vs C presents linearity in figure 3. This conclude that adsorption of oil molecules on stainless steel metal surface follows Langmuir isotherm. According to Langmuir adsorption isotherm degree of surface coverage (θ) of stainless steel and the concentration of the inhibitor in hydrochloric acid are related to following formula (6).

C/ θ = 1/K_ad + C                                                            (6)

Average values for free energy of adsorption (ΔG_ads), were calculated using the following equations 7.

ΔG_ads = -RT In (55.5K)                                         (7)

K = equilibrium constant, R = gas constant and T = temperature. It is found that the ΔG_ads values for the studied compound  are less than -40 kJ mol^-1, indicating that the Sesmum indicum oil are physically adsorbed on stainless steel. The less and negative value of ΔG_ads shows the spontaneous adsorption reaction of the oil on the stainless steel ^16-18_.

Table 7: Adsorption parameters of Sesmum indicum oil on the stainless steel surface in hydrochloric acid for different experimental periods.

Experimental Time periods	Kad	Slope	-∆Gads	R2
1 hour	2.71	1.0	25.39	0.9979
3hour	1.77	1.1	20.02	0.9943
6 hour	1.27	1.1	17.17	0.9861
15 hour	1.63	1.2	19.22	0.9964
24 hour	2.10	1.4	21.91	0.9983

 

Figure 3: Langmuir adsorption isotherm with stainless steel in hydrochloric acid with   different amount of oil for different experimental time   Click here to View figure

 

Galvanostatic Polarization Studies

The polarization data of stainless steel working as a cathode and  an anode in absence and presence of  Sesmum indicum oil with hydrochloric acid  isshown in figure 4. The electrochemical polarization parameters concluded  from figure 4 like  corrosion current I_corr, Tafel constantb_a,  b_c andE_corr  aretabulated in table 8.

Figure 4: Polarization curve of stainless steel in 0.5N HCl without and with of oil concentrations.   Click here to View figure

 

Table 8: Electrochemical polarization parameters for stainless steel in hydrochloric acid without and with Sesmum indicum oil.

Sesmum indicum oil Concentration (gram/liter)	-Ecorr (V)	Tafel slopes (V.dec-1)		Icorr(A.cm-2)	IE%
		bc	ba
0.00	0.429	7.601	6.765	3.163×10-4	–
1.00	0.432	4.153	8.542	1.103×10-4	65.13
3.00	0.430	8.647	14.88	5.929×10-5	81.25
5.00	0.443	9.504	6.506	5.207×10-6	98.35

 

A decrease in the corrosion current and increase in the cathodic and anodic Tafel slopes have been observed with the grater in the amount of oil. E_corr values have not significant change with the addition of inhibitor.The inhibition efficiencies determined from the values of corrosion current are found to be in comparable with those obtained from weight loss measurement. The inhibition efficiencies determined by following equation:

(Inhibitor efficiency ) IE%= 1-(I¹_corr / I_corr)×100                                  (8)

In the present equation   I_corr = current densities with Sesmum indicum oil in 0.5N hydrochloric acid I¹_corr =   current densities without of Sesmum indicum oil in hydrochloric acid.  The presence of passive region in the graph as shown in figure 4 may be due to the protection layers of chromium and iron oxide formed from their respective metals present in stainless steel. Chloride ions which are present in hydrochloric acid destroy the protective layers which are made by oil and generate pits on the stainless steel surface. I _critical is maximum when no inhibitor was added to 0.5N HCl (uninhibited solution) the values of I _critical decreases with increased concentration of inhibitor suggesting inhibitor is capable of reducing corrosion.

Conclusions

By the evaluated data from weight loss and polarization study the following conclusion can be explain:

The eco friendly Sesmum indicum oil was concluded as a best inhibitor for stainless steel in 0.5N hydrochloric acid.

Inhibition efficiency directly proportional with the amount of Sesmum indicum oil content up to 6g/liter to reach 89.56% in 0.5N HCl at room temperature.

Kinetics analyses of data showed linear variations, which confirm a first order kinetics in hydrochloric acid.

The above research concluded that adsorption of oil molecules on stainless steel metal surface follows Langmuir isotherm in hydrochloric acid medium.

The less and negative value of DG_ads shows the spontaneous adsorption reaction of the Sesmum indicum oil on the stainless steel surface.

Obtained results were justified from the galvanostatic polarization method.

The values E_corr obtained from galvanostatic polarization method are mostly constant, by this we can conclude that Sesmum indicum oil work as mixed type in hydrochloric acid and  retard both the reaction in equal amount on anodic and cathodic ends.

Acknowledgements

The authors gratefully acknowledge the Head of department, Chemistry, University of Rajasthan, Jaipur and Prof.(Dr.) S. L. Surana  Director (Academic), Swami Keshvanand Institute of Technology, Management & Gramothan, Jaipur for providing necessary facilities.

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