Inhibition of aluminium corrosion using <i>C. papaya </i> leaves extract in 1.0 M H <sub>2</sub>SO <sub>4</sub> was investigated by using gravimetric analysis at various concentrations and temperatures: 303 K, 313 K and 323 K. Characterization was done by using Scanning Electron Microscope (SEM) and Fourier Transform Infrared (FT-IR) spectroscopy. Results show that, inhibiting ability of the extract was due to its adsorption onto the metal surface through Langmuir adsorption isotherm. Thermodynamic (Gibbs energy, entropy and heats of adsorption) and kinetic parameters (activation energy and entropy of activation) were also determined. All of these agreed to physical adsorption of inhibitor onto the aluminium surface.
Although aluminium is thermodynamically reactive, it is the only metal with a property of protecting itself against corrosion by forming an amphoteric oxide film, which protects it against further attack when found under aggressive medium. In highly aggressive medium, of either alkalinity (pH > 9) or acidity (pH < 5), aluminium and its alloys corrodes by dissolving of its protective oxide layer [
Interest of investigating plants’ extracts as corrosion inhibitors is increasing nowadays due to the fact that, synthetic (inorganic) corrosion inhibitors such as chromates, phosphates, and molybdates have some kind of toxicity. Researches show that plants’ extracts are rich in phytochemicals with heteroatoms such as S, O and N which makes them active in developing inhibitive properties [
Inhibition efficiency,
% I E = ( 1 − W i n h W b l a n k ) × 100 (1)
Degree of surface coverage,
θ = 1 − W i n h W b l a n k (2)
where, % IE = Inhibition efficiency, Winh = Weight loss in presence of inhibitor, Wblank = Weight loss in absence of inhibitor and θ = Surface coverage of inhibitor [
The corrosion rate using the formula.
C R ( g ⋅ cm − 2 ⋅ h − 1 ) = W A t (3)
where; CR = Corrosion rate, A = Coupons surface area (cm2), t = immersion time in hours and W = Weight loss [
In case of activation energy values; equation below was used
ln C R = − E a R T + ln A (4)
where, ln C R versus 1 T is a straight line with a slope of − E a R which is used to calculate the activation energies [
Enthalapies of activation values are obtained by plotting the graph of
ln C R T versus 1 T , for each temperature. − Δ H 0 R values as slopes were used to calculate the enthalpies of activation and ln ( R N h ) + Δ S 0 R as intercepts to calculate entropies. Below is the equation used
C R = R T N h exp ( Δ S a 0 R ) exp ( − Δ H a 0 R T ) (5)
where h is the plank’s constant 6.626176 × 1034 and N is the Avogadro’s number, 6.02252 × 1023 mol−1.
Δ S a 0 is the entropy of activation and Δ H a 0 is the enthalpy of activation.
Determination of heats of adsorption;
ln ( θ 1 − θ ) versus 1 T is plotted for each concentration and − Q a d s 2.303 R as the slope is used to calculate the heats of adsorption. Below is the equation used
ln ( θ 1 − θ ) = log A + log K − Q a d s 2.303 R ( 1 T ) (6)
where: q is a degree of Surface Coverage [
Gibbs energy of adsorption is calculated using the formula below
Δ G = − R T ( 55.5 K ) (7)
where,
K = θ ( 1 − θ ) C
q = Degree of surface coverage, K = Adsorption equilibrium constant and C = Concentration [
In order to study the interaction (mechanism of adsorption) between the metal (aluminium) and inhibitor molecules; various adsorption isotherms have been tested. The adsorption taken place follows the Langmuir adsorption isotherm by having a high correlation coefficient. It obeys the following equation:
C θ = 1 K + C (8)
Other isotherms tested and equations used,
Frumkin Isotherm
( θ 1 − θ ) exp ( − 2 a θ ) = K C (9)
Temkin Isotherm
Freundlich Isotherm
log θ = log K + n log C (11)
where C = Concentration, q = Degree of surface coverage, a = molecular interaction parameter, n = Slope and K = Equilibrium Constant of adsorption [
C. papaya leaves used in this investigation were collected from farms of villagers around our campus, Tengeru area, Arumeru District in Arusha, Tanzania. They were air dried for two weeks, then ground by electrical grinder to a fine powder.
In each of the batch experiments conducted at specific temperatures, three liters (3 L) of 1.0 M H2SO4 were firstly prepared and then used to prepare the extract and the corrosive media. Inhibitor (the extract) preparation was done by mixing two liters (2 L) of the acid with 200 g of the C. papaya leaves powder into two different one liter (1 L) pyrex conical flasks, in a ratio of 10 g of the leaves powder per 100 mls of the acid. This ratio was taken because on testing various ratios, it was found to be most efficient. The mixture obtained was boiled at 90˚C using a heating plate for three hours with a stir (Thermometer was used to maintain such temperature). Then it was left to cool at room temperature for 24 hours and then filtered by firstly using a sieve, centrifuged and finally with Whatman No. 1 filter papers. After filtering, the obtained solution was a corrosion inhibitor (itself with 100 v/v% concentration). From that inhibitor, other concentrations (20, 40, 60, and 80) v/v% were prepared by diluting it with the remained one liter (1 L) of 1.0 M H2SO4 at various ratios [
The coupons with 2 cm × 2 cm × 0.12 cm in dimensions were filed by using emery papers (# 400, 600, 800, 1000, and 1200), washed with ethanol, dried with acetone, and then stored in desiccators until used. Elemental composition of aluminium used to prepare the coupons (
In this method a Denver analytical balance with an accuracy of 0.0001 g was used. Coupons’ weights were measured before and after they have been immersed in the corrosive media. Distilled water, ethanol, acetone and desiccators were used to wash and dry the coupons several times before taking measurements. In every batch experiment, each coupon was immersed for 24 hours in a 150 ml beaker. The total volume of the medium in each beaker was made up to 100 ml. The experiments were done at concentrations of a blank, (20, 40, 60, 80 and 100) v/v% and temperatures of 303 K, 313 K and 323 K. All beakers were put in water baths to control such temperatures. The experiments were repeated three times and the average weights lost were recorded and then well analysed.
Element | % | Element | % |
---|---|---|---|
Al | 98.94 | Cu | 0.004 |
Mg | 0.35 | Cr | 0.0029 |
Si | 0.34 | P | 0.002 |
Fe | 0.26 | Mn | 0.0017 |
Ti | 0.0097 | Ag | 0.0004 |
Ni | 0.0008 | Pb | 0.0004 |
V | 0.0069 | Co | 0.0003 |
Zn | 0.004 |
Conc. (v/v%) | Weight loss (g) and Inhibition efficiency | CR (gcm−2h−1) (10−5) | |||||||
---|---|---|---|---|---|---|---|---|---|
303 K | I.E% | 313 K | I.E% | 323 K | I.E% | 303 K | 313 K | 323K | |
Blank | 0.012 | 0.0359 | 0.0856 | 5.580 | 16.695 | 39.807 | |||
20 | 0.0049 | 59.17 | 0.015 | 58.32 | 0.0428 | 50.00 | 2.279 | 6.975 | 19.903 |
40 | 0.0038 | 68.33 | 0.013 | 63.88 | 0.0378 | 55.84 | 1.767 | 6.045 | 17.578 |
60 | 0.0034 | 71.67 | 0.0122 | 65.98 | 0.0354 | 58.64 | 1.581 | 5.673 | 16.462 |
80 | 0.0034 | 71.67 | 0.0118 | 67 | 0.036 | 57.94 | 1.581 | 5.487 | 16.741 |
100 | 0.0036 | 70.00 | 0.0126 | 65.9 | 0.0384 | 55.14 | 1.674 | 5.859 | 17.857 |
Surface morphology was examined using a Field Scanning Electron Microscope
(FE-SEM, Hilachi, S-4700). BRUKER FTIR spectrometer in transmittance mode with a spectral range of 4000 - 500 cm−1 was used to ascertain functional groups of the extract associated with corrosion inhibition action.
Kinetic and thermodynamic parameters include: activation energy― E a ( J / mol ) , enthalpy of activation― + Δ H ( kJ / mol ) , entropy of adsorption― Δ S 0 ( J / mol ) , heat energy of adsorption― Δ Q a d s ( kJ / mol ) and Gibbs energy of adsorption― Δ G 0 ( J / mol ) were determined and recorded in
1/Temperature using Equation (4). Their slopes − E a R obtained were then used
Conc. v/v% | Ea kJ/mol | +DH kJ/mol | DS0 J/mol | DQads (kJ/mol) | DG (kJ/mol) | ||
---|---|---|---|---|---|---|---|
303 K | 313 K | 323 K | |||||
Blank | 80.01 | 77.40 | −70.75 | ||||
20 | 88.18 | 85.58 | −51.46 | 14.96 | −3.51 | −3.53 | −2.74 |
40 | 93.50 | 86.42 | −35.92 | 21.67 | −2.76 | −2.34 | −1.51 |
60 | 95.38 | 92.78 | −30.59 | 23.51 | −2.14 | −1.52 | −0.73 |
80 | 96.03 | 93.42 | −28.52 | 24.64 | −1.42 | −0.89 | 0.12 |
100 | 96.33 | 93.73 | −27.02 | 26.00 | −0.65 | −0.18 | 1.03 |
in calculations (
Enthalpy of activation and entropy of adsorption were determined by plotting graphs of ln (Corrosion rate/Temperature) vs 1/Temperature using Equation (5),
then − Δ H 0 R , as slopes to calculate the enthalpy of activation and ln ( R N h ) + Δ S 0 R
as intercepts to calculate the entropy of adsorption (
Heats of adsorption were determined by plotting graphs of Log [Surface
coverage/(1-Surface Coverage)] using Equation (6), and − Q a d s 2.303 R , as slopes
were used in calculations (
The increase in activation energy of the metal dissolution was due to the increase in the inhibitor concentration and its adsorption. This is because the inhibitor adsorption on the metal surface hindered the metal dissolution [
Increasing in entropy (from most negative to less negative values), shows the increase in disorder at the metal and solution interface which is due to the desorption of water molecules from the surface of the metal by molecules of the inhibitor [
centration of the inhibitor, shows the exothermic nature of its adsorption [
Isotherm | Correlation coefficient value (R2) at various temperatures | ||
---|---|---|---|
303 K | 313 K | 323 K | |
Frumkin | 0.561 | 0.794 | 0.383 |
Freundlich | 0.786 | 0.869 | 0.565 |
Temkin | 0.787 | 0.873 | 0.553 |
Langmuir | 0.997 | 0.998 | 0.995 |
achieved through physical or chemical mechanism. In the current study all parameters agreed to physical adsorption of the inhibitor on the metal surface.
Since the inhibitor works through adsorption onto the metal surface, various adsorption isotherms were tested includes: Frumkin, Freundlich, Temkin and Langmuir adsorption isotherms using Equations (8)-(11) respectively.
The adsorption of the inhibitor onto the metal surface seems to obey Langmuir adsorption isotherm by having a high correlation coefficient of approximately equal to 1. This isotherm goes with assumption that, there is no lateral interaction between the adsorbed species and the adsorbent.
FT-IR test was performed to the C. papaya leaves powder, inhibitor prepared by
using 1.0 M H2SO4 and the corrosion product (adsorbed film). In case of the leaves powder a little amount of the sample was tested and in corrosion inhibitor few drops were tested. A test concerning a corrosion product was done by firstly preparing the aluminium coupon. The coupon was prepared by being abraded
with emery papers, then washed with ethanol and dried with acetone. The prepared coupon was immersed in 60 v/v% inhibitor (optimal concentration) in 100 mls of the medium (150 mls beaker was used), for 24 hrs at 30˚C. Then after, a coupon was scrapped to get a corrosion product which was FT-IR tested. The results obtained here were the FT-IR spectra of the C. papaya leaves, inhibitor and the corrosion product shown in Figures 6-8 respectively.
inhibitors which have been reported. The shift of these frequencies from inhibitor to the adsorbed ones can be attributed to the interaction of inhibitor functional groups with the metal surface for the adsorption to take place.
For example O-H vibration shifted from 3334.5 cm−1 to 3329.84 cm−1 and that of C=O from 1634.65 cm−1 to 1635.23 cm−1 [
Uncorroded, uninhibited and inhibited coupons were examined by using Scanning Electron Microscope (SEM). Uninhibited and inhibited coupons were dipped in the medium (100 ml of 1.0 M H2SO4) for 24 hours at 30˚C before analysis. For
inhibited coupon, inhibitor was added at the optimal concentration of 60 v/v%.
Leaves powder | Inhibitor prepared using H2SO4 | Corrosion product | |||
---|---|---|---|---|---|
Frequency (cm−1) | Assignment | Frequency (cm−1) | Assignment | Frequency (cm−1) | Assignment |
3281.43 | -OH or N-H Stretch | 3334.5 | O-H Stretch | 3329.84 | O-H Stretch |
2917.52 | -C-H Stretch | 1634.65 | C=O Stretch | 1635.23 | C=O Stretch |
2849.24 | 1189.41 | C-O or C-H Stretch | 1183.08 | C-O or C-N Stretch | |
1625.62 | C=O stretch | 1049.60 | 1048.00 | ||
1405.52 | C-H bending | ||||
1316.22 | |||||
1244.02 | C-O or C-N stretch | ||||
1031.06 |
From
1) C. papaya extracts investigated was found to be among of effective green corrosion inhibitors to the aluminium corrosion under acidic medium. Its action through physiosorpion of the extract phytochemicals created a corrosion barrier at the metal (aluminium)-solution (acid) interface.
2) The difference in kinetic and thermodynamic parameters in the absence and presence of the extract proved its adsorption.
3) Adsorption isotherms test as a mean to understand the model of adsorption agreed to Langmuir adsorption isotherm.
4) The difference in SEM images of the aluminium surface by the one with inhibitor to be smoother than the surface of without it is also a proof of the inhibitor adsorption as well as its activeness.
This work was supported by the Government of Tanzania through Nelson Mandela African Institution of Science and Technology.
Kasuga, B., Park, E. and Machunda, R.L. (2018) Inhibition of Aluminium Corrosion Using Carica papaya Leaves Extract in Sulphuric Acid. Journal of Minerals and Materials Characterization and Engineering, 6, 1-14. https://doi.org/10.4236/jmmce.2018.61001