Corrosion Control Of Metals Biology Essay

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Urine (CU) addition was evaluated at five different temperatures in the range from 30-70 C by weight loss measurements. CU acts as a good inhibitor for the corrosion of mild steel in 1.5 M H2SO4. The

value of inhibition efficiency increases with increasing both inhibitor concentration and solution

temperature. Approach: The adsorption of CU constituents on the mild steel surface obeys the

Langmuir adsorption isotherm suggesting a monolayer adsorption of CU species. Thermodynamic

parameters for CU adsorption and mild steel corrosion were evaluated. The negative values of (DGoads)

emphasize the spontaneity of the adsorption process and stability of the adsorbed layer. Results: The

estimated high, positive value of DHoads ensures that CU species is adsorbed chemically on mild steel

surface. All values of E*

app for mild steel corrosion in inhibited solutions were lower than that for the

uninhibited solution indicating the occurrence of chemisorption mechanism. Conclusion: The surface

morphology of mild steel in absence and presence of inhibitor revealed that with increasing both CU

concentration and solution temperature, mild steel surface is modified and looks smooth. Good

correlation between the inhibitor constituents and its inhibitory action was obtained.

Key words: Corrosion, thermodynamic, chemisorption, sulfuric acid, environmentally friendly

inhibitor, inhibitor constituents, natural inhibitors, plant resources, non-toxic, reaction

constants, natural products

INTRODUCTION

Etre, 2006), artemisia oil (Benabdellah et al., 2006),

Zenthoxylum alatum extract (Chauhan and

Corrosion control of metals is an important activity Gunasekaran, 2007), Fenugreek Leaves extract (Noor,

of technical, economical, environmental and aesthetical 2007), Justicia gendarussa (Satapathy et al., 2009) have

importance. Thus, the search for new and efficient been reported to be good inhibitors for steel in acid

corrosion inhibitors has become a necessity to secure solutions. As noticed, all the previous natural inhibitors

metallic materials against corrosion. Over the years, were obtained from plant resources. In recent works

considerable efforts have been deployed to find suitable (Noor, 2004; 2008), Camel s Urine (CU) obtained from

compounds of organic origin to be used as corrosion animal origin was reported as corrosion inhibitor for

inhibitors in various corrosive media, to either stop or mild steel in HCl solutions. Camel s urine can be

delay the maximum attack of a metal (Umoren et al., classified as environmentally friendly inhibitor, because

2008). Nevertheless, the known hazard effects of most microbiological study on CU proved its high efficiency

synthetic organic inhibitors and the need to develop against a number of pathogenic microbes when

cheap, non-toxic and environmentally benign processes compared with some antibiotics. Moreover, the

have now made researchers to focus on the use of effective constituent of CU was isolated and tested as

natural products. These natural organic compounds are anticancer agent which is labeled as PM 701 (Moshref

either synthesized or extracted from aromatic herbs, et al., 2006)

spices and medicinal plants. Generally speaking, inhibitors are found to protect

Recently, various natural products from plant steel corrosion in acid solutions by adsorbing onto

origins e.g., Zenthoxylum-alatum fruits extract steel surface. Adsorption isotherms such as Langmuir

(Gunasekaran and Chauhan, 2004), Telfaria (1917) adsorption isotherm, adsorption isotherm,

Occidentalis extract (Oguzie, 2005), Khilla extract (El-Flory (1942) and Huggins (1942) adsorption isotherm

Corresponding Author:

Ehteram A. Noor, Department of Chemistry, Science Faculty for Girls, King Abdulaziz University,

Jeddah, Saudi Arabia Tel: +00966(0505537707) Fax: +00966(022652112)

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Am. J. Applied Sci., 8 (12): 1353-1362, 2011

and Frumkin (1964) adsorption isotherm are used to

elucidate the inhibition mechanism of inhibitors. If

the adsorption isotherm for a given inhibitor is

specified at different temperatures, thermodynamic

parameters for the adsorption process would be

estimated, giving a good help to suggest the

inhibition mechanism. Moreover the thermodynamic

activation parameters for the corrosion process are

also important to explain the adsorption phenomenon

of inhibitor.

In the present study the authors attempt to study the

inhibitory action of CU for mild steel corrosion in 1.5M

H2SO4 at five different temperatures (30-70 C) by

using weigh loss method. Various thermodynamic

parameters for inhibitor adsorption as well as for mild

steel corrosion in absence and presence of different

concentrations of CU were estimated and discussed.

MATERIALS AND METHODS

Specimens: The experiments were performed with mild

steel rods of the following composition; C: 0.250, Mn:

0.480, Si: 0.300, Ni: 0.040, Cr: 0.060, Mo: 0.020, S:

0.021, P: 0.019 and the remainder is Fe.

Inhibitor: The camel s urine sample is extracted from

female camel (one humped) with age around 4-5 years,

early in the morning. Physically, the fresh extracted

urine appears clear, amber yellow and watery.

Solutions: The aggressive solution (1.5M H2SO4) was

prepared by dilution of analytical grade reagent with

deionized water. The required concentrations (1, 2, 6,

10 and 14 v/v %) of inhibitor were prepared by diluting

with 1.5 M of H2SO4 solutions.

Corrosion rate measurements: Weight loss method

was employed for mild steel corrosion rate

measurements in absence and presence of various

concentrations of CU at different temperatures. Prior to

each experiment, the mild steel specimen of 1.0 cm in

diameter and 5.0 cm in length was abraded with a series

of emery study from 220-1000 grades. Then, it was

washed several times with deionized water then with

ethanol and dried using a stream of air. After weighing

accurately, it was immersed in 100 mL flask, containing

50 mL of solution. After 90 min, the specimen was

taken out, washed, dried and weighed accurately. The

test was performed in absence and presence of different

inhibitor concentrations and different temperatures (3070

C). The rate of weight loss was calculated (rWL, mg

cm.2 min.1) as follows Eq. 1:

W - W

rWL = 1 2 (1)

S.t

Where:

W1 and W2 = The specimen weight before and after

immersion in the tested solution

S = The surface area of the specimen

t.

= The end time of each experiment

The corrosion rates in the absence ( roWL ) and

presence (rWL) of an inhibitor are used to evaluate its

inhibition efficiency by using the following Eq. 2:

IE% = (1 -r

oWL ) 100 (2) rWL

Surface morphology studies: Characteristic features

of mild steel surface after immersion in 1.5 M H2SO4

in absence and presence of low (1%) and high (10%)

concentrations of CU at 30 and 70 C were

investigated by optical micrographs using microscope

of the type (Leitz METALLUX3 microscope

WETZLAR, Germany).

RESULTS

Table 1 represents the corrosion rates of mild steel

in 1.5 M of H2SO4 solution in absence and presence of

various concentrations of CU (1-14 mL%).

Figure 1 show the relationship between logr and

logCinh at different temperatures which is in accordance

with the following Eq. 3 (Noor and Al-Moubaraki, 2003):

log r= log r+ Blog C inh

(3)

Where:

Cinh = The concentration of the inhibitor

r = The corrosion rate when the concentration of

inhibitor becomes unity

B = A constant for the studied reaction

Table 1: Mild steel corrosion rates in 1.5 M H2SO4 in absence

and presence of different concentrations of CU at different

temperatures

Corrosion rate 105 (g cm-2 min-1)

Cinh (mL%) 30 40 50 60 70

0 0.739 2.295 4.483 12.294 31.22

1 0.493 0.750 1.505 2.001 3.408

2 0.459 0.627 0.912 1.112 1.743

6 0.241 0.249 0.418 0.572 0.853

10 0.156 0.177 0.241 0.467 0.637

14 0.134 0.126 0.176 0.203 0.303

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Am. J. Applied Sci., 8 (12): 1353-1362, 2011

Table 2: Kinetic parameters (B, r and r2) for mild steel corrosion in

1.5M H2SO4 solution containing CU at different

temperatures

2

t (oC) -B r (g cm-2 min-1)

r

30o 0.5 0.564 0.96

40o 0.7 0.847 0.98

50o 0.8 1.574 0.99

60o 0.8 2.028 0.94

70o 0.8 3.381 0.96

Fig. 1: Dependence of mild steel corrosion rate on the

concentration of CU in 1.5M H2SO4 at different

temperatures

Fig. 2: Effect of CU concentration on the inhibition

efficiency of mild steel corrosion in 1.5M

H2SO4 solution at different temperatures

The later parameter gives a measure for the

inhibitor performance. The kinetic parameters (B and

r ) and correlation coefficient (r2) were estimated from

the straight lines shown in Fig. 1 and listed in Table 2.

Figure 2 shows the variation of IE% with the

concentrations of CU at different temperatures. Most of

the corrosion inhibition is achieved between 1% and

6% of CU with only small improvements at 10% or

higher. In general, the inhibitor efficiency was observed

to be increased with increasing both CU concentration

and solution temperature.

The adsorption of inhibitor species, Inh, on a metal

surface in aqueous solution should be considered as a

place exchanger reaction:

Inh + nH O U Inh + nHO (4)

aq 2ads ads 2aq

where, n is the number of water molecules displaced by

one molecule of inhibitor.

When the equilibrium of the process described in

Eq. 4 is reached, it is possible to plot the degree of

surface coverage (q) as a function of inhibitor

concentration at constant temperature by different

mathematical expressions which are called adsorption

isotherms models. Several adsorption isotherms were

tried and was found the best description of the

adsorption behavior of the studied inhibitor is by the

Langmuir adsorption isotherm Eq. 5:

Cinh. 1

=+ Cinh. (5)

q Kads.

where, Kads is the equilibrium constant of adsorption

process. The plot of Cinh versus Cinh for CU at different

q

temperatures gives a straight line as shown in Fig. 3. It is

found that all the linear correlation coefficients (r2) are

approximately equal to 1.00 and all the slopes are very

close to unity. From the intercepts of the straight lines, Kads

values at different temperatures were obtained.

It is well known that the free energy DGads of

adsorption is related to Kads by Eq. 6 (Noor and Al-

Moubaraki, 2008):

DGads.

log K =- logC - (6)

ads. HO

2 2.303RT

where, CH O is the concentration of water molecules

2

and must have the same unit as that used for inhibitor.

The standard free energies of CU adsorption ( DGo )at

ads

different temperatures were calculated. A plot of DGads

versus T in Fig. 4 gave the heat of adsorption (DHads)

and the entropy of adsorption (DSads) according to the

thermodynamic basic Eq. 7 (Babakhouya et al., 2010):

DG =D H - TDS (7)

ads ads ads

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Am. J. Applied Sci., 8 (12): 1353-1362, 2011

Table 3:

Adsorption parameters ((Kads, DGads, DHads and (DSads) for

CU on mild steel surface in 1.5M H2SO4 solution at

different temperatures

t ( C) Kads DGads DHads DSads

.....

(mL1L) (kJ mol1) (kJ mol1) (J K1mol1)

30o 0.0404 -9.330

40o 0.1702 -13.40

50o 0.1941 -14.18 60.01 231

60o 0.4735 -17.10

70o 0.7738 -19.03

Fig. 3: Langmuir isotherm for adsorption of CU on

mild steel surface in 1.5M H2SO4 at different

temperatures

Fig. 4: The variation of DGads with T

Table 4: Activation parameters ( E# , DH# and DS#) for mild steel

app

corrosion in 1.5M H2SO4 solution in absence and presence

of different concentrations of CU

inh. C #

app E DH# DS#

mL% (kJ mol-1) (kJ mol-1) (J K-1mol-1)

0 79.17 76.49 -90.82

1 41.92 39.24 -217.44

2 27.97 25.29 -264.04

6 28.87 26.19 -267.45

10 30.73 28.06 -264.51

14 18.01 15.33 -308.12

The thermodynamic data obtained for CU

using the adsorption isotherm are collected in

Table 3.

The thermodynamic activation parameters were

calculated from Arrhenius-type plot (Eq. 8) and

transition state equation (Eq. 9) (Faiku et al.,

2010):

E#

log r= log A - app. (8)

2.303RT

r R DS# DH#

log( ) =[(log( )) + ( )] .

(9)

T hN 2.303R 2.303RT

where,

E# , DH# and DS# are the apparent activation

app

energy, the enthalpy of activation and the entropy of

activation. A is the frequency factor which has the same

unit as that of the corrosion rate.

Figure 5 shows the typical plots of logr versus 1

T

r 1

while Fig. 6 shows the plots of log versus ; straight

TT

lines with good correlation coefficients were obtained.

All thermodynamic activation parameters were

estimated and listed in Table 4.

Figure 7 gives the dependence of both E# and DH#

app

of mild steel corrosion in 1.5 M H2SO4 on the

concentration of CU.

Figure 8 illustrates thhe optical micrographs for

mild steel surface before and after immersion for

90 min in 1.5 M H2SO4 at 30 and 70 C. While

Fig. 9 and 10 illustrate the structural features of

mild steel surface in 1.5M H2SO4 in absence and

presence of 1 and 10% of CU at 30 and 70 C,

respectively.

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Am. J. Applied Sci., 8 (12): 1353-1362, 2011

Fig. 5: Arrhenius plots for mild steel corrosion rates in

1.5M H2SO4 in absence and presence of

different concentration of CU

Fig. 6: Transition state plots for mild steel corrosion

rates in 1.5M H2SO4 in absence and presence of

different concentration of CU

Fig. 7: Dependence of both apparent activation energy

and enthalpy change of mild steel corrosion in

1.5 m H2SO4 on the concentration of CU

Fig. 8: Micrographs for mild steel surface before (A)

and after immersion for 90 min in 1.5M H2SO4

at 30 C (B) and 70 C (C)

DISCUSSION

Effect of CU concentration on mild steel corrosion at

different temperatures: The collected data in Table 1

can be summarized as follows:

At constant temperature, mild steel corrosion rate

tends to decrease dramatically with increasing CU

concentration. This result indicates the good

inhibitive properties of the studied inhibitor

At constant concentration, mild steel corrosion rate

increases with increasing solution temperature

obeying Arrhenius relationship

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Am. J. Applied Sci., 8 (12): 1353-1362, 2011

Fig. 9:

Micrographs for mild steel surface in 1.5M

H2SO4 in absence (A) and presence of 1% (B)

and 10% (C) of CU at 30 C

The present results are in good agreement with

those obtained previously by (Noor, 2004) when CU

had been studied as corrosion inhibitor for mild steel in

HCl solution at different temperatures. On the other

hand, The data in Table 2 was interpreted as below:

As was observed the reaction constants (B) have

negative sign, indicating that the mild steel

corrosion rate is inversely proportional to the

concentration of CU. However, the absolute value

of constant B increases with increasing temperature

up to 50 C and then no change in B value was

observed with further increase in temperature. This

result indicates that CU becomes more effective as

corrosion inhibitor with increasing temperature and

at relatively high temperatures no appreciable

change in the inhibition efficiency was observed

Fig. 10: Micrographs for mild steel surface in 1.5M

H2SO4 in absence (A) and presence of 1% (B)

and 10% (C) of CU at 70 C

The obtained correlation coefficients

(0.94 r2 0.99) indicate that the corrosion rates of

mild steel in the presence of different

concentrations of CU fit well Eq. 3. Additional

evidence of the quality of fit is presented in Fig.

11 in which predicted values of r are plotted

against the corresponding experimental values of

different concentrations of CU. Reasonable

agreements between experimental and predicted

results are obtained

The inhibitor action could be explained by

Fe(Inh)ads reaction intermediate as follows Eq. 10

(Dubey and Singh, 2007):

n +

Fe

+ Inh U Fe(Inh) ads U Fe + Inh + n e (10)

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Am. J. Applied Sci., 8 (12): 1353-1362, 2011

Fig. 11:Experimental values against predicted values of

mild steel corrosion rate in 1M H2SO4 solution

containing different concentrations of CU at

different temperatures

The adsorbed layer combats the action of sulfuric

acid solution and enhances protection of the metal

surface (Quraishi et al., 2000). When there is

insufficient Fe(Inh)ads to cover the metal surface (if the

inhibitor concentration was low or the adsorption rate

was slow), metal dissolution would take place at sites

on the mild steel surface which are free of Fe(Inh)ads.

With high inhibitor concentration a compact and

coherent inhibitor layer forms on mild steel surface,

reducing the attack on the metal surface. Hence, the

inhibition efficiency is then directly proportional to the

fraction of the surface covered with adsorbed inhibitor.

Figure 2 implies that most of the corrosion

inhibition is achieved between 1% and 6% of CU with

only small improvements at 10% or higher. In general,

the inhibitor efficiency was observed to be increasing

with increasing both CU concentration and solution

temperature. These results can be discussed as follows:

The increase in IE% with increasing CU

concentration is attributed to the interaction

between the inhibitor species and mild steel surface

leading to adsorb the former on the latter. The

adsorbed quantity increases with inhibitor

concentration and accordingly more active

corrosion centers were reduced (Shetty et al., 2006;

Achary et al., 2008). On the other hand, the limited

change in IE% at relatively higher concentrations

of CU may be related to surface saturation with

inhibitor species (Noor, 2009)

The increase in IE% with increasing temperature

was interpreted in the literature by different ways.

Amar and El Khorafi (1973), related this to specific

interactions between the metal surface and the

inhibitor molecules. Considered that with increase

in temperature some chemical changes occur in the

inhibitor molecules leading to an increase in the

electron densities at the adsorption centers of the

molecule causing improvement in inhibitor

efficiency finally. Considered that the increase of

IE% with increasing temperature is a result of

change in the nature of adsorption mode; the

inhibitor species are being physically adsorbed at

lower temperatures while chemisorption is

favoured as temperature increases

To prove the chemisorption process for CU species

on mild steel surface, some thermodynamic

considerations for both inhibitor adsorption and

corrosion activation must be evaluated.

Thermodynamic-adsorption considerations:

Obviously, Fig. 4 shows the dependence of DGads on T,

indicating a good correlation among the

thermodynamic parameters. The negative values of

DGads (Table 3) emphasize the spontaneity of the

adsorption process and the stability of the adsorbed

layer on the steel surface. As was observed the values

of DGads become more negative with increasing

temperature, indicating that the adsorption power of

CU increases with the increase of temperature. On the

other hand, the high positive value of DHads (Table 3)

ensures that CU species adsorbed chemically on mild

steel surface, while the accompanied large, positive

value of DSads (Table 3) indicates that an increase in

disordering takes place in going from reactants to the

metal-adsorbed species reaction complex. Similar

results were reported in recent works (Bentiss et al.,

2005; Noor, 2007).

Thermodynamic-activation considerations: The

obtained data in Table 4 can be interpreted as below.

#

The values of both Eapp and DH# in absence and

presence of different concentrations of CU are

positive, indicating that the corrosion process is

endothermic

#

The lower values of Eapp in the inhibited solutions

as compared to that of the uninhibited solution

suggest chemisorption mechanism for the CU

species on mild steel in the studied medium

(Popova et al., 2003). This result is in good

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Am. J. Applied Sci., 8 (12): 1353-1362, 2011

agreement with the obtained thermodynamic data

of adsorption (Table 3)

#

The decrease in Eapp with CU concentration (Fig. 7)

supports the idea of chemisorption mechanism. This

was attributed by (Hoar and Holliday, 1953) to a

slow rate of inhibitor adsorption with a resultant

closer approach to equilibrium during the

experiments at the higher temperature. Furthermore,

(Riggs and Hurd, 1967) explained that the decrease

in activation energy of corrosion at higher levels of

inhibition arises from a shift of the net corrosion

reaction from that on the uncovered part on the

metal surface to the covered one

#

Eapp -Cinh relation (Fig. 7) shows a plateau in the

concentration range from 2-10% which may be

attributed to that with increasing inhibitor

concentration, the covered area with inhibitor

species increases and the metal surface becomes

close to be saturated, leading to limited change in

the apparent activation energy. While at 14% of

CU concentration a drop in E# value was

app

observed which indicates that the metal surface

may be completely blocked with chemically

adsorbed inhibitor species leading to further

#

decrease in the Eapp

As expected DH# values have the same trend as

#

that for Eapp , noticing that the latter is larger than

the former. Noor (2007) attributed this result to

the gaseous reaction (hydrogen evolution)

associated with the corrosion process which may

lead to a decrease in the total volume of the

corrosion system. So, according to the basis of

thermodynamics the inequality E# >DH# is true.

app

Large and negative values of DS# imply that the

activated complex in the rate determining step

represents an association rather than a dissociation

step, meaning that a decrease in disordering takes

place on going from reactants to the activated

complex (March, 1992). However, the value of DS#

decreases with increasing CU concentrations

Surface morphological studies: Inspection Fig. 8

through A to C indicates that the amount of corrosion

products as well as the size of pits on mild steel surface

are proportional to the solution temperature, meaning

that mild steel surface attacked severley by raising the

temperature from 30-70 C. Figure 9 and 10 show

interesting behaviour with the addition of 1% and 10%

CU at low and high temperatures. This is that mild

steel surface in the presence of CU is modified and

becomes smooth not only by increasing CU

concentration but also by increasing solution

temperature, emphasizing the chemisorption

mechanism suggested previously.

Inhibitor constituents and adsorption mechanism:

Table 5 illustrates the main constituents of CU as given,

while Fig. 12 represents the molecular structure of the

main organic constituents of CU and their IUPAC

names. Inspection of CU constituents, reveals that the

organic components can be classified as nitrogenous

organic compounds. N-containing organic compounds

were reported in the literature as effective corrosion

inhibitors for mild steel in acid solutions (Shetty et al.,

2006; Popova et al., 2003; Muralidharan et al., 1995;

Ebenso et al., 1999; Noor, 2005).

Chemisorption process involves charge sharing or

charge transfer from the inhibitor molecules to the

metal surface. This is possible in case of positive as

well as negative charges on this surface. The presence

of inhibitor molecules having relatively loosely bound

electrons or hetero atoms (nitrogen in the present work)

with lone-pair electrons, with a transition metal having

vacant, low-energy orbital facilitates the

chemisorptions mechanism (Bentiss et al., 2005).

Figure 13 shows the suggested chemisorption

mechanism between the vacant d-orbital of Fe atoms

in mild steel surface and the nitrogen atoms of CU

organic constituents. It is impossible to say which

one of these organic constituents is responsible for

CU inhibitive action. So CU can be treated as a

package of inhibitors which may act synergistically.

Table 5: The average concentration level of the main constituents of

Camel s urine

The constituent Urea Uric acid Creatinine Chloride Phosphate Sulphate

The concentration 0.195 6.041 0.052 0.45 0.171 7.76

(g L-1)

Fig. 12: The molecular structure and the IUPAC name

of the main organic constituents of CU

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Am. J. Applied Sci., 8 (12): 1353-1362, 2011

Fig. 13: The suggested chemisorption mechanism

between the vacant d-orbital of Fe atoms in

mild steel surface and the nitrogen atoms of the

organic constituents of CU

CONCLUSION

CU acts as a good inhibitor for the corrosion of

mild steel in 1.5 M H2SO4. The inhibition

efficiency values increase with the inhibitor

concentration and the solution temperature

The adsorption of CU on the mild steel surface

obeys the Modified Langmuir adsorption isotherm

suggestion monolayer adsorption of CU species

o

The negative values of ( DGads ) emphasize the

spontaneity of the adsorption process and the

stability of the adsorbed layer on the steel surface.

o

DGads values become more negative with

increasing temperature, indicating that the

adsorption power of CU increases with the increase

of temperature

o

The estimated high, positive value of DHads ensures

that CU species is adsorbed chemically on mild

steel surface

All values of E*

app. for mild steel corrosion in

inhibited solutions were lower than that for the

uninhibited solution indicating the occurrence of

chemisorption mechanism for the CU species on

mild steel in the studied medium

The surface morphology of mild steel in absence

and presence of inhibitor revealed that with

increasing both CU concentration and solution

temperature, mild steel surface is modified and

looks smooth

Good correlation between the inhibitor constituents

and its inhibitory action was obtained

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