INTRODUCTION:

As a result of excellent mechanical properties and its availability, mild steel is extensively used as a construction material in many industries. However, when this mild steel is exposed to the corrosive industrial environment, it easily corrodes. Normally acid solutions such as hydrochloric acid are widely used in areas like acid pickling, industrial cleaning, oil well cleaning etc. Mild steel is one of the most used industrially important metals¹.

Corrosion is an electrochemical process by which metallic surfaces react with their environment causing the metal to lose its material properties due to surface deterioration².Use of corrosive chemicals in industries and water as solvent are unavoidable, which could lead to the dissolution of the metal. To protect the metal surface from the aggressive environment, various techniques such as corrosion protection coating, cathodic protection, anodic protection and inhibitors are available at the industrial level. Among the available measures for corrosion prevention, use of corrosion inhibitors is preferred widely since the quantity of inhibitor needed for observing the fruitful result is usually less. Various synthetic organic and inorganic chemicals have been studied as corrosion inhibitors for mild steel in different aqueous media. Synthetic inhibitors being toxic in nature are less preferred, which has made the exploration of natural compounds which have a strong affinity towards the metal surface important. This group of compounds called as green corrosion inhibitors are organic compounds that prevent corrosion by adsorbing onto the metal surface. The polar functions of these molecules with S, O or N atoms, heterocyclic compounds and p electrons are believed to be responsible for corrosion inhibition capacity of green corrosion inhibitors. Many synthetic chemicals which are benign to the environment have also been studied as an alternate to the toxic organic chemicals. The corrosion inhibitors of plant origin are highly preferred because they are readily available, inexpensive with an advantage of environmentally benign nature. There are many studies which report the use of leaves, bark, fruits and vegetables of different plants as green corrosion inhibitor for mild steel in various aggressive media³.

Bredelia ferruginea as a common occurring plant was employed for mild steel corrosion inhibition in aqueous medium. Bredelia ferruginea belongs to the family of Euphorbiaceae which is commonly found in Savannah regions. It is usually a gnarled shrub which sometimes reaches the size of a tree in suitable condition. Its common names are Kizni (Hausa), Marehi (Fulani), Iralodan (Yoruba), Ola (Igbo); and KensangeAbia (Boki). Its habitat is the Savannah, especially in the moister regions extending from Guinea to Zaire and Angola. The tree is 6 - 15 m high, up to 1.5 m in girth and bole crooked branching low down.

MATERIALS AND METHODS:

The mild steel used for the research work was obtained from metal’s Dealer in Akure and the chemical composition was determined at Universal Steel, Ogba, Lagos State, Nigeria. It was sectioned into uniform dimension of 3cm× 2.5cm × 0.3 cm in the Mechanical Engineering workshop of The Federal University of Technology, Akure.

Bredelia ferruginea leaf and bark were obtained at Odo Osun farm settlement in Nigeria. The Sample of the leaf and the bark of Bredelia ferruginea were washed with distilled water, oven dried, pulverized, sieved through a mesh sieve 850micron. 100g was soaked in ethanol for 72 hours and filtered. The filtrates were further subjected to evaporation in order to leave the sample free of the ethanol. The crude extract obtained was used in preparing different concentrations of the extract by dissolving 0.2, 0.6, 1.0, and 1.4g in 1 L of 1M HCl respectively. Qualitative and quantitative phytochemical screening of the ethanol extract of bark and leaf of Bredelia ferruginea were carried out as described by Pandeyet al.⁴

After initial weighing, the specimen in triplicate were immersed in 100ml 1M HCl solution in the absence and presence of different concentrations 0.1 -1.4g/l of plants extracts with the aid of glass hooks at different temperatures viz 303, 313, 323 and 333k for 2 hours. The thermostatic water bath was set to the appropriate temperature and after 2hrs of immersion, the specimens were removed, washed, dried completely and their final weights were noted. From the initial and final weights of the mild steel, the weight loss, the corrosion rate (ghr^-1cm^-2), inhibition efficiency (%), and surface coverage of the plant extracts was calculated at different concentration of the inhibitors in HCl using the formulae;

Corrosion rate (1)

W = Weight loss in g

A = area in cm²

(2)

(3)

Where CR_LI = Corrosion rate of the steel in inhibited solution

CR_Lb = corrosion rate of the steel in blank solution

The enthalpy (∆H^o) and entropy (∆S^o) of activation of corrosion process was calculated from the equation:

CR = (^RT/nh) exp (^∆S/_R) exp (^∆H/_RT) (4)

where

h is the Planck’s constant,

N is the Avogadro’s number,

T is the absolute temperature,

R is the universal gas constant, /

∆S^o is the entropy of activation, and

∆H is the enthalpy of activation.

Straight lines were obtained with a slope of (-∆H^o/R) and an intercept of (lnR /Nh + ∆S^o/R) from which the values of ∆H^o and ∆S^o were calculated.

Kinetic Studies:

In order to investigate the order of the reactions, the experiment was carried out at room temperature at various concentration of 0.2-1.4g/l ethanol extract in 1M HCl. The pre-weighed coupons were immersed in 100cm³ of the respective inhibitor/blank solutions. The coupons were retrieved from the solutions and weighed at 1 day interval for 5 days. The order of reaction and half life were determined from the results obtained.

Temperature Variation:

Effect of temperature gives more understanding of the thermodynamic and isotherm mechanism. The pre-weighed coupons were dipped in 100cm³ of the various concentrations of the inhibitors/blank solution ranges from 0.2-1.0g/L ethanol extract in 1M HCl and maintained at 303-333 K in a thermostated water bath for 2h. after which it was retrieved, rinsed in water and latter in ethanol. The results obtained were fitted into the Temkin, Langmuir, and Freundlich and the correlation coefficients (R²) were used to determine the best fitted isotherm.

Infra Red Measurement:

Pulverized mild steel specimen was immersed in a solution of hydrochloric acid containing the plants extract for 24h to form the adsorption product of mild steel and extract. FT-IR spectrum was recorded for the extracts and the adsorption product. These spectra were recorded in a Perkin-Elmer-1600 spectrophotometer using KBr pellet.

Photo Micrograph Analysis:

The surface analysis using scanning Electron Microscope provides more information on the inhibition mechanism of the extract on the surface of mild steel. Mild steel specimens were immersed in 100ml 1M HCl solution in the absence and presence of 1.4g/L of inhibitor for 24 hours at romm temperature.The surface morphology of mild steel before immersion, after immersion without inhibitors and the presence of inhibitors were examined. Scanning electron microscope images obtained from Nikon Eclipse ME600 model

RESULT AND DISCUSSION:

The IR spectra of the extract of BF (bark) and BF (leaf) and the dried solid adsorption product of mild steel powder are shown in Figures 3.1- 3.4. The ethanol extract of Bredelia ferruginea (Bark) shifted from 3423.76 cm^-1 to 3369.75 cm^-1, sharp peak at 1620.26 cm^-1shifted to 1629.90 cm^-1, also the peak at 1035.81 cm^-1 shifted to 1020.38 cm^-1(Figure 3.1 and 3.2) and Bredelia ferruginea (leaf) shifted from 3392.90 cm^-1 to 3346.61cm^-1, sharp peak shifted from 1624.12 cm^-1to 1618.33 cm^-1, also the peak at 1037.74 cm^-1 shifted to 1018.45 cm^-1(Figure 3.3 and 3.4).

The shift in the frequencies confirmed that the active phytochemical constituents present in the inhibitor molecules bind to the metal surface to form a protective metal-inhibitor complex to reduce the further dissolution of metal in the aggressive media. A broad peak at 3423.76cm^-1 is attributed to polymeric O-H group, the peak at 2820.32cm^-1 corresponds to C-H stretching frequency and the peak at 1620.26cm^-1 has been assigned to C=O. The presence of the functional groups C-N and C-O-C in Bredelia ferruginea extracts is confirmed from the bands at 1244.13 cm^-1, C-H bending frequency is noted at 1369.50 cm¹.

Figure 3: IR spectrum of ethanol extract of Bredelia ferruginea (Leaf).

Figure 4: IR spectra of dried solid adsorption product of ethanol extract of Bredelia ferruginea (Leaf) and mild steel powder

Table 1.0: Phytochemical screening of BF bark and BF leaf extract

	Bark	Bark	Leaf	Leaf
Parameter	Qualitative	Quantitative	Qualitative	Quantitative
Alkaloids (%)	++	4.27 ± 0012	+	2.07 ± 0.42
Tanins (mg/g)	+++	8.33 ± 0.06	++	5.37 ± 0.16
Sapinins (%)	+++	10.83±0.12	+++	9.85 ± 0.50
Glycoside (mg/g)	++	4.32 ± 0.04	++	5.4 ± 80.2
Saponins Glycoside	+	ND	+	ND
Flavonoides (%)	+++	11.70±0.54	+	9.20±0.23
Anthraquinone (mg/g)	-ve	-ve	-ve	-ve
Steroides (%)	+	ND	-ve	ND

+++ = presence in appreciable amount, ++ = presence in moderate amount, + = presence in trace amount, -ve = negative, ND = not determined. (values are express in mean ± standard deviation).

Leaf composition result revealed that Alkaloids, tannins, saponnin, glycosides, and flavonoids, has 2..27 0.42, 5.370.16, 9.85.0.50,5.480.2,and 9.20 0.20mg/g respectively, The yield of these compounds as well as the corrosion inhibition abilities vary widely depending on the part of the plant and its geographical location and the efficiency is justified by the phytochemical compounds present therein. The chemical structures of phyto-constituents contained electron rich bond or hetero atoms that facilitate their electron donating ability, hence the inhibition of the corrosion steel by ethanoic extracts of Bredelia ferruginea is attributed to the phyto-constituents of the extract. The corrosion rates of mild steel in the absence and presence of Bredelia ferruginea (bark) and (leaf) extracts of various concentrations were shown in Figure 5 and 6 respectively. The result obtained showed that the rate of corrosion of mild steel in 1M HCl decreased with increase in the concentration of ethanol extract of bark and leaf but increased with increase in immersion time. It shows that increase in concentration of the inhibitors in the acidic media resulted to adsorption of more active ions from the plant extract on the surface of the specimen and less contact of the specimen with the acidic environment which lead to decrease in corrosion rate of mild steel.

Fe 5: Bredelia ferruginea bark

Figure 6: Bredelia ferruginea leaf

Effect of extracts concentrations on inhibition efficiency of mild steel in 1M HCl in the presence Bredelia ferruginea (bark) and (leaf) extract are shown in Figure 7 and 8 respectively. The result obtained showed that the inhibition efficiency of the extracts increase with increasing concentration of ethanol extract of both inhibitors and it was more efficient in bark than leaf but decreased with immersion time. High inhibition efficiency of 96.15% and 94.58% were observed after 24 hours of immersion in both bark and leaf respectively, this suggests that BF adsorption on the mild steel surface was completed within 24hours, afterwards the aggressive action of the acid medium was increasingly felt than the adsorbed inhibitor, leading to reduced inhibition efficiency with increased exposure time.

The effect of temperature and concentration of extracts on the corrosion rate of mild steel in free acid and in the presence of different concentrations of Bredelia ferruginea both bark and leaf extracts was studied in the temperature range of 303K to 333K as shown in Figures 9 and 10. It was found that the rates of corrosion of mild steel in free acid solution and in the presence of different concentrations of the inhibitor decrease but increase with increase in temperature. This is expected because as temperature increases, the rate of corrosion of mild steel also increases as a result of increase in the average kinetic energy of the reacting molecules.

In order to access the effect of temperature on the corrosion and corrosion inhibition process, weight loss experiments were carried out in the temperature range 303-333K in 1M HCl in the absence and presence of different concentrations (0.2-1.4g/L) of Bredelia ferruginea (bark) and Bredelia ferruginea (leaf) extracts. It was found that after 2 hours immersion period, the corrosion rate in both uninhibited and inhibited acids increases with rise in temperature, Inhibition efficiency was also found to increase but decreased with increase in temperature at all the concentrations of the extract studied.

Figure 7: Bredelia ferruginea bark

Figure 8: Bredelia ferruginea leaf

The correlation coefficient (R²) of the adsorption isotherm data showed that Langmuir isotherm is best fitted into the experiment with R² ranges from 0.982- 0.999 for all the extract and at different temperature. The isotherm best applicable at 323K for ethanol extract of Bredelia ferrugunea (Stem bark) and 323K for ethanol extract of Bredelia ferruginea (leaf) with correlation of 0.999 and 0.999 respectively as indicated in Tables 3.0 and 4.0. The R² values are very close to unity, indicating strong adherence to Langmuir adsorption isotherm. The application of Langmuir isotherm to the adsorption of extract of Bredelia ferruginea (Bark) and Bredelia ferruginea (leaf) on surface of mild steel indicates that there is interaction between the adsorbate and adsorbent. For the Freudlich, the correlation coefficient (r²) ranges from 0.962–0.994 for ethanol extract of Bredelia ferruginea (stem bark) and ranges from 0.813- 0.994 for ethanol extract of Bredelia ferruginea (leaf) respectively. This showed those Freundlich isotherm models are favorable for the corrosion inhibition of mild steel but best fitted by the Stem bark of BF with 0.994 at 333 K which is very close to infinity.

Table 3.0: Langmuir, Freundlich and Temkin adsorption isotherm parameters obtained from the corrosion Data for mild steel in 1.0 M HCl containing Bf extract (Stem bark)

Temperature (K)	Intercept	Slope	K_ads	R²
Langmuir adsorption isotherm parameters
303	0.192	1.169	5.208	0.992
313	0.172	1.399	5.813	0.999
323	0.206	1.571	4.854	0.999
333	0.277	1.562	3.611	0.996
Freundlich adsorption isotherm paramerers
303	-0.142	0.211	-7.042	0.962
313	-0.211	0.199	-5.001	0.981
323	-0.254	0.204	-3.937	0.987
333	-0.271	0.233	-.3.703	0.994
Temkin adsorption isotherm parameters
303	0.724	0.307	1.3812	0.947
313	0.631	0.251	1.5847	0.991
323	0.557	0.226	1.7953	0.995
333	0.538	0.249	1.8587	0.989

Table 4.0: Langmuir, Freundlich and Temkin adsorption isotherm parameters obtained from the corrosion Data for mild steel in 1.0 M HCl extract (Leaf)

Temperature (K)	Intercept	Slope	K_ads	R²
Langmuir adsorption isotherm parameters
303	0.187	1.258	5.347	0.996
313	0.191	1.442	5.263	0.997
323	0.227	1.672	4.405	0.999
333	0.352	1.687	2.841	0.971
Freundlich adsorption isotherm parameters
303	0.166	0.206	-6.024	0.994
313	0.213	0.251	-4.694	0.997
323	0.283	0.204	-3.533	0.999
333	0.222	0.203	-3.105	0.971
Temkin adsorption isotherm parameters
303	0.682	0.283	1.4662	0.994
313	0.571	0.204	1.7513	0.576
323	0.557	0.226	1.7953	0.995
333	0.538	0.247	1.8587	0.989

The rate constant parameters; rate constant and half-life are recorded in Table 5.0. The plots showed a linear variation and slope, k, which confirms a pseudo-first order reaction kinetics with respect to the corrosion of mild steel in 1.0M HCl solution in the absence and presence of Bredelia ferruginea extracts.

Table 5.0 The values of Rate Constant (K) and Half-life (t1/2) for mild steel in 1 M HCl in the absence and presence of different concentrations of Bredelia ferruginea

Conc. (w/v)	rate constants of Ms (day^-1)	Half life of Ms (days)	Conc. (w/v)	rate constants of Ms (day^-1)	Half life of Ms (days)
Stem bark			Leaf
0.00	0.4448	1.55913	0.00	0.4444	1.55941
0.20	0.4951	1.39959	0.20	0.4790	1.44671
0.60	0.5319	1.30264	0.60	0.5343	1.29705
1.0	0.8084	0.85729	1.00	0.5734	1.20849
1.40	0.8636	0.80243	1.40	0.6034	1.14852

SEM micrographs obtained from mild steel surface after 24hours of immersion in 1.0M HCl for the SEM micrographs of mild steel, in 1 M HCl without the extracts and metals with Bredelia ferruginea bark and leaf extract are shown in Figure 11, 12 and 13 respectively. It can be clearly observed that the mild steel surface morphology immersed in the acidic medium without inhibitor, Figure 11 was attacked with evidence of pits and cracks by the corrosive environment when compared with the ones immersed in the medium containing inhibitors (Figure 12 and 13). It can be concluded that the extract of Bredelia ferruginea bark and leaf which served as inhibitors at a concentration of 1.4g were able to lower the corrosion rate by the acidic environment as a result of the formation of films layer on the metal surface and leaf extract

Figure 11: SEM micrographs of mild steel, in 1 M HCl without the extracts

Figure 12: SEM micrographs of mild steel in 1 M HCl with the Bredelia ferruginea bark extracts

Figure 13: SEM micrographs of mild steel, in 1 M HCl without the Bredelia ferruginea leaf extracts

CONCLUSION:

The crude extracts of Bredelia ferruginea both bark and leaf act as inhibitor for mild steel corrosion in acidic medium, Inhibition efficiency of the extract which were 96.15% and 94.58% for bark and leaf respectively increased with increase in concentration of the inhibitors but decreased with increase in immersion time and temperature.

The extracts were found to obey Langmuir, Frendlich and Temkins adsorption isotherm from the fit of the experimental data at all the concentrations and temperatures studied. The values of Ea obtained in the presence of the extract were higher compared to the blank acid solution which further support the physical adsorption proposed. The values of free energy of adsorption obtained are low and negative, which reveals the spontaneity of the adsorption process. The positive values of enthalpy of adsorption suggest that the reaction is endothermic and the adsorption of the inhibitors on the metal surface takes place. The negative values of entropies imply that the activated complex in the rate determining step represents an association rather than a dissociation step. The functional groups from the FT-IR spectra of the extract which showed that the effective adsorption of the inhibitor is due to the donation of lone pair of electrons on oxygen to the vacant d-orbitals of the metal which leads to the formation of the metal complexes. The result of the SEM analysis of the mild steel revealed that the inhibitors molecules form protective layer on the surface of mild steel and prevent from the further corrosion.

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Received on 17.10.2019 Modified on 25.10.2019

Asian J. Research Chem. 2019; 12(6):287-294.

DOI: 10.5958/0974-4150.2019.00053.1

*Bredelia ferruginea* Stem Bark Bredelia ferruginea Leaf
Conc (g/L)	Ea (k j mol^-1)	ΔH (k j mol^-1)	ΔS (k j mol^-1)	Ea (k j mol^-1)	Δ H (k j mol^-1)	ΔS (k j mol^-1)
Blank	21.012	17.423	-4.324	21.317	17.423	-4.311
0.20	29.007	24.451	-3.287	26.901	23.072	-3.598
0.60	31.094	25.333	-3.085	34.771	27.878	-2.411
1.00	38.313	33.794	-2.018	33.507	28.333	-2.741
1.40	37.873	34.121	-2.019	37.088	32.435	-2.841