Synergistic Inhibition Between OP and NaCl on the Corrosion of Cold-rolled Steel in Phosphoric Acid

Materials Chemistry and Physics 90 (2005) 286297

Synergistic inhibition between OP and NaCl onthe corrosion of cold-rolled steel in phosphoric acid

Xueming Li a, Libin Tang a,b,a Department of Chemistry, Yunnan University, Kunming 650091, Peoples Republic of China

b Materials Research and Development Center, Kunming Institute of Physics,Kunming 650223, Peoples Republic of China

Received 28 February 2004; accepted 21 May 2004

Abstract

The effects of single OP and the mixture of various concentrations of OP and 0.1 M NaCl on the corrosion of cold-rolled steel in 1.03.5 Mphosphoric acid have been investigated by using weight loss method and polarization method. This study revealed that cold-rolled steelin phosphoric acid has been more efficiently inhibited by OP in the presence of NaCl than single OP, and inhibition efficiency increaseswith increasing concentration of OP at the same temperature, but decreases with increasing temperature studied. A synergistic effect existswhen OP and chloride ion are used together to prevent cold-rolled steel corrosion in 1.03.5 M phosphoric acid at every experimentaltemperature. The polarization curves showed that OP is a cathodic inhibitor, while the complex of OP and NaCl is a mixed-type inhibitorthat mainly inhibits the cathodic corrosion of the steel. The experimental results suggested that the presence of chloride ion in the solutionstabilized the adsorption of OP molecules on the metal surface and improved the inhibition efficiency of OP. The adsorption of single OPfollows the Langmuir adsorption isotherm, and the adsorption of the complex accords with the Langmuir adsorption isotherm too. Somethermodynamic parameters such as adsorption heat, adsorption entropy and adsorption free energy have been calculated by employingthermodynamic equations. Kinetic parameters such as apparent activation energy and pre-exponential factor have been calculated anddiscussed. In 1.53.5 M phosphoric acid, the concentration pre-exponential factor and acid concentration constant have been calculated,and the synergistic effect has been observed too. 2004 Published by Elsevier B.V.

Keywords: Corrosion; Synergistic inhibition; OP; Chloride ion; Cold-rolled steel; Phosphoric acid

1. Introduction

Acid solutions are widely used in industry. Thecorrosion of steel in acidic solution receives considerableconcern. Corrosion inhibitors are used to reduce the corro-sion rates of metallic materials in these acidic media. Usu-ally, organic compounds containing nitrogen [116], sulphur[5,11,13,17,18], oxygen [1924] and phosphorus [23,25,26]are used for inhibitors against the corrosion of steel in acidicmedia. Synergistic effects describe the increase in the effec-tiveness of a corrosion inhibitor in the presence of anothersubstance in the corrosive medium. Synergistic inhibitionis an effective means to improve the inhibitive force ofinhibitor, to decrease the amount of usage, to diversify theapplication of inhibitor in acidic media. It is necessary forcorrosion scientists to discover, explore and use synergism

Corresponding author. Tel.: +86 0871 5105360;fax: +86 0871 5152601.E-mail address: [email protected] (L. Tang).

in the complicated corrosive media. Actually, this phe-nomenon has been frequently used in practice [8,2629].The synergistic effects between organic inhibitors andhalide ions on metal corrosion in acidic solution have beenreported by many authors [8,19,27,30]. However, the reportabout synergism between surfactants and halide ions onmetal corrosion in phosphoric acid is rare. In this work,the synergistic inhibition between OP and chloride ion inphosphoric acid is investigated by means of weight lossand electrochemical methods, and the possible synergisticmechanism has been proposed.

2. Experimental

2.1. Materials

The experiments were performed with cold-rolled steelspecimens with the following composition: C 0.10%,Mn 0.50%, P 0.025%, S 0.025%, Fe remainder.

0254-0584/$ see front matter 2004 Published by Elsevier B.V.doi:10.1016/j.matchemphys.2004.05.012

X. Li, L. Tang / Materials Chemistry and Physics 90 (2005) 286297 287

2.2. Solutions

The aggressive solutions used were made of AR-gradephosphoric acid. Appropriate concentrations of acid wereprepared using distilled water. The concentration range ofOP employed was 0.570.0M in the absence and pres-ence of 0.1 M NaCl in various concentrations of phosphoricacid.

2.3. Gravimetric measurements

Prior to experiment, the cold-rolled steel sheets of30 mm 115 mm 00.5 mm were abraded with a se-ries of emery paper from 220 to 1000 grades. Then, thespecimens were washed several times with distilled waterthen with acetone and dried using a stream of air. Af-ter weighing accurately, the specimens were immersed in100 ml beaker, which contained 100 ml different concen-trations of phosphoric acid with and without addition ofdifferent concentrations of OP and 0.1 M NaCl. All theaggressive acid solutions were open to air. After 4 h, thespecimens were taken out, washed, dried and weighed ac-curately. Then the tests were repeated at different tempe-ratures.

2.4. Electrochemical measurements

For polarization studies, the steel specimen was em-bedded in PVC holder using epoxy resin with an exposedarea of 1.0 cm2 (0.155 in.2) as a working electrode. Aplatinum foil was used as an auxiliary electrode. Thereference electrode was a saturated calomel electrode(SCE) coupled to a Luggin capillary whose tip was lo-cated between the working electrode and the auxiliaryelectrode.

Before measurement, the electrode was immersed in testsolution at natural potential for ca. 2 h until a steady statewas reached. All polarization measurements were performedusing PARSTAT 2263 Potentiostat/Galvanostat (PrincetonApplied Research) at 25 C under aerated condition, and thescanning rate was 0.5 mV s1.

3. Experimental results and discussion

The molecular formula of OP used as inhibitor is shownas follows:

Obviously, OP contains many oxygen atoms withlone-pair electrons. Thus, the substance meets the require-ments as a good inhibitor.

Fig. 1. Relationship between weight loss and concentration of OP atdifferent temperatures.

3.1. Effect of single OP on the corrosion ofcold-rolled steel

3.1.1. Effect of concentration of single OP on the weightloss and corrosion rate of cold-rolled steel

The weight loss of cold-rolled steel in 1.0 M phosphoricacid in the presence of various concentrations of OP wasdetermined. The weight loss as a function of concentrationof OP is plotted in Fig. 1 at different experimental tempera-tures. In all cases, the weight loss decreases with increasingthe concentration of OP. At concentrations above 50.0M,increasing inhibitor concentration does not affect the valuesof weight loss obviously. Fig. 1 also shows that weight lossincreasing with an increase in temperature at each concen-tration of OP, while, at concentrations above 50.0M, theweight loss of specimen keeps steady at every experimentaltemperature basically.

The value of corrosion rate was calculated from the fol-lowing equation [1,31]:

v = m1 m2St

(1)

where m1 is the mass of the specimen before corrosion, m2the mass of the specimen after corrosion, S the total area ofthe specimen, t the corrosion time and v the corrosion rate.

Fig. 2 provides the influence of inhibitor concentration onthe corrosion rate of cold-rolled steel in 1.0 M phosphoricacid at different temperatures. It can be seen that the changeof corrosion rate with the concentration of OP is similar tothat of weight loss.

3.1.2. Effect of concentration of single OP on theinhibition efficiency

With the calculated corrosion rate, the inhibition effi-ciency of inhibitor for the corrosion of steel was obtainedby using the following equation [32]:

E (%) = v0 vv0

100 (2)

288 X. Li, L. Tang / Materials Chemistry and Physics 90 (2005) 286297

Fig. 2. Relationship between corrosion rate and concentration of OP atdifferent temperatures in 1.0 M phosphoric acid.

where v0 and v are the corrosion rates of the specimen in1.0 M phosphoric acid without and with the addition of in-hibitor, respectively.

The variation in inhibition efficiency with concentrationof OP for cold-rolled steel in 1.0 M phosphoric acid at dif-ferent temperatures is shown in Fig. 3. The results obtainedshow that inhibition efficiency increases with increasing in-hibitor concentration at all the temperatures studied. At rel-ative lower concentration of OP (0.530.0M), inhibitionefficiency increases with increasing concentration of OP.Fig. 3 also shows that at concentrations above 50.0M, thechanges of concentration and temperature do not affect theinhibition efficiency markedly.

3.1.3. Adsorption isotherm for single OPAssuming the corrosion inhibition was caused by the ad-

sorption of OP, and the degree of surface coverage () fordifferent concentrations of OP in 1.0 M phosphoric acid wasevaluated from weight loss measurement using the Sekineand Hirakawas method [25]:

= v0 vv0 vm (3)

where vm is the smallest corrosion rate.

Fig. 3. Relationship between inhibition efficiency and concentration ofOP at different temperatures.

Table 1The values of coverage obtained from the weight loss for different con-centrations of OP in 1.0 M phosphoric acid at different temperatures

OP (M) Coverage30 C 35 C 40 C 45 C

0.5 0.422 0.187 0.160 0.0371.0 0.639 0.411 0.325 0.1832.0 0.815 0.682 0.529 0.3593.0 0.872 0.776 0.697 0.4665.0 0.910 0.845 0.780 0.577

10.0 0.937 0.922 0.862 0.70330.0 0.987 1.000 0.986 0.90750.0 0.923 0.977 1.000 0.99870.0 1.000 0.967 0.971 1.000

The values of coverage have been calculated and listedin Table 1. Apparently, the value of coverage increases withan increase in concentration of OP at each experimentaltemperature basically.

Assuming that the adsorption of OP on steel surface fol-lows the simplest adsorption isotherm, i.e., the Langmuiradsorption isotherm, then the Langmuir adsorption isothermwas applied to investigate the adsorption mechanism by thefollowing equation [1,3335]:c

= 1

k+ c (4)

where k is the equilibrium constant of adsorption processand c the inhibitor concentration.

The regression between c/coverage and c has been doneusing computer, and the corresponding parameters have beenlisted in Table 2. Obviously, the linear correlation coeffi-cients and slopes are close to 1.000, which means that theassumption is correct, i.e., the adsorption of OP on steel sur-face obeys the Langmuir adsorption isotherm.

Fig. 4 also clearly shows that the good linear relationshipbetween c/coverage and c at 35 C.

3.1.4. Thermodynamic parameters for single OPThermodynamic model can be used to explain the adsorp-

tion phenomenon of inhibitor molecule. Using the obtainedadsorption coefficients, the adsorption free energy, adsorp-tion heat and adsorption entropy can be calculated.

The adsorption heat was calculated according to the VantHoff equation [1,34]:

ln k = HadsRT

+ constant (5)

Table 2Some parameters of the linear regression between c/ and c

Temperature (C) k Slope Linear correlationcoefficient

30 1.754 106 1.016 0.99935 9.593 105 1.008 1.00040 5.867 105 0.988 1.00045 1.732 105 0.905 0.992


Fig. 4. Dependence of c/coverage on concentration of OP at 35 C.

To obtain the adsorption heat, the regression between ln kand 1/T was dealt with. Clearly, the adsorption heat can beobtained by using the slope of the regression. Under the ex-perimental conditions, the adsorption heat can be approxi-mately regarded as the standard adsorption heat (H0).

To obtain the standard adsorption free energy (G0), thefollowing equation was employed [1,36]:

k = 155.5

exp[G

0

RT

](6)

According to the thermodynamic basic equation G0 =H0T S0, the standard adsorption entropy, S0 can becalculated.

All the thermodynamic parameters were obtained andlisted in Table 3.

The negative value of adsorption free energy means thatthe adsorption of OP on steel surface is a spontaneous pro-cess. Table 3 also shows that the absolute value of adsorp-tion free energy decreases with increasing the experimentaltemperature. According to Eq. (6), it can be concluded thata decrease of the absolute value of adsorption free energyleads to the reduction in equilibrium constant of the adsorp-tion process. That is to say, with increasing temperature moreand more inhibitor molecules depart from the steel surface.

The negative value of adsorption heat indicates that theadsorption is an exothermic process, the magnitude of ad-sorption heat is larger than the common physical adsorptionheat, reaching the common chemical adsorption heat, prob-ably meaning that the chemical adsorption takes place.

Table 3The thermodynamic parameters of adsorption of different concentrationsof OP on the steel surface at different temperatures in 1.0 M phosphoricacid

Temperature(C)

G0(kJ mol1)

H(kJ mol1)

S0 (J mol1 K1)

30 46.359 118.923 239.36635 45.578 118.923 238.01740 45.037 118.923 235.94445 42.530 118.923 240.116

The negative values of S0 might be explained in thefollowing way: before the adsorption of inhibitors onto thesteel surface, inhibitor molecules might freely move in thebulk solution (the inhibitor molecules were chaotic), butwith the progress in the adsorption, inhibitor molecules wereorderly adsorbed onto the steel surface, as a result, a decreasein entropy [1].

3.1.5. Kinetic parameters for single OPThe adsorption phenomenon has been successfully ex-

plained by using thermodynamic parameter, to further eluci-date the inhibition properties of single OP, the kinetic modelwas employed.

The natural logarithm of the corrosion rate (v) can berepresented as a linear function of 1/T, i.e. [1]:

ln v = EaRT

+ lnA (7)

where Ea represents the apparent activation energy, R theuniversal gas constant, T the absolute temperature, and Athe pre-exponential factor. The regression between ln v and1/T was completed using the computer, and it is found thatall the regression coefficients are close to 1, which meansthat the linear relationship between ln v and 1/T is good. Theapparent activation energy and pre-exponential factor canbe calculated from the slope and intercept of the regression,respectively. All the kinetic parameters were calculated andlisted in Table 4.

Table 4 and Fig. 5 suggest that the apparent activation en-ergy, at relative lower concentration (0.52.0M), increaseswith an increase in concentration of OP, while at a rangefrom 2.0 to 50.0M, decreases with increasing concentra-tion of OP as a whole. That is to say, in this system there isa maximum for the apparent activation energy. The result isthe same as some studies [1].

It is worth noting that the variance in the pre-exponentialfactor is similar to the variance in the apparent activationenergy.

The dependence of the apparent activation energy on theconcentration of OP is shown in Fig. 5.

Table 4Some parameters of the linear regression between ln v (corrosion rate)and 1/T

OP (M) Pre-exponentialfactor (g m2 h1)

Ea (kJ mol1) Linear regressioncoefficient

0.5 3.303 1015 83.842 0.9811.0 1.131 1017 93.496 0.9872.0 4.134 1018 103.530 0.9963.0 2.810 1018 102.980 0.9905.0 1.156 1018 101.050 0.988

10.0 1.464 1017 96.153 0.98530.0 5.726 1014 82.634 0.97650.0 3.917 1010 57.882 0.96970.0 6.002 1012 70.954 0.992


Fig. 5. Dependence of apparent activation energy on the concentration ofOP.

3.2. Effect of OP and NaCl on the corrosion of cold-rolledsteel

3.2.1. Effect of concentration of OP on the weight loss andcorrosion rate of steel in 1.0 M phosphoric acid in thepresence of 0.1 M NaCl

Fig. 6 shows the plot of weight loss against the concen-tration of OP for cold-rolled steel in 1.0 M phosphoric acidcontaining 0.1 M NaCl at different temperatures. It is clearthat at a certain experimental temperature, weight loss ofspecimen decreases with an increase of concentration of OPin the presence of 0.1 M NaCl, and at concentrations above10M, with increasing concentration of OP the value ofweight loss does not change remarkably. At a certain con-centration of OP, the weight loss of specimen increases witha rise in the temperature. At relative higher concentrationof OP (above 10M), the weight loss of specimen keepssteady at every experimental temperature. Thus, comparedwith single OP, the effective concentration of OP used as acorrosion inhibitor is reduced drastically.

Fig. 7 gives the influence of concentration of OP in thepresence of 0.1 M NaCl on the corrosion rate of cold-rolledsteel in 1.0 M phosphoric acid at different temperatures. Itcan be seen that the change of corrosion rate with concen-tration of OP is similar to that of weight loss.

Fig. 6. Relationship between weight loss and concentration of OP in thepresence of 0.1 M NaCl.

Fig. 7. Relationship between corrosion rate and concentration of OP inthe presence of 0.1 M NaCl.

3.2.2. Effect of concentration of OP on the inhibitionefficiency in 1.0 M phosphoric acid in the presence of 0.1 MNaCl

Fig. 8 shows the plot of inhibition efficiency against theconcentration of OP for cold-rolled steel in 1.0 M phospho-ric acid containing 0.1 M NaCl at different temperatures. Itcan be seen from Fig. 8 that with increasing concentrationof OP, there is an increase in inhibition efficiency at all ex-perimental temperatures. Furthermore, Fig. 8 shows that in-creasing concentration of OP and experimental temperaturedo not affect inhibition efficiency obviously at concentrationof OP above 10M in 1.0 M phosphoric acid containing0.1 M NaCl, and the value of inhibition efficiency remainsabove 90%. Compared with Fig. 3, the inhibition efficiencyof OP has been greatly enhanced by adding 0.1 M NaCl, andthe effect of temperature on the inhibition efficiency is nolonger apparent.

Inhibition efficiencies of OP for cold-rolled steel in 1.0 Mphosphoric acid in the absence and presence of 0.1 M NaClat different temperatures are given in Table 5. The inhibitionefficiency of OP increases with a rise in concentration at ev-ery temperature studied, both in the absence and presence of0.1 M NaCl. It is found that single OP shows good inhibitionefficiency at relative higher concentration (above 50.0M),however, the complex of OP and 0.1 M NaCl has a better

Fig. 8. The relationship between inhibition efficiency and concentrationof OP in the presence of 0.1 M NaCl.


Table 5Inhibition efficiency obtained from the weight loss method for differentconcentrations of OP without and with the addition of 0.1 M NaCl in1.0 M phosphoric acid at different temperatures

OP (M) NaCl (M) Inhibition efficiency (%)30 C 35 C 40 C 45 C

0.5 0 33.361 15.115 12.585 2.8331.0 0 50.466 33.335 25.647 14.0522.0 0 64.407 55.248 41.682 27.6113.0 0 68.872 62.912 54.904 35.8565.0 0 71.895 68.451 61.481 44.395

10.0 0 74.027 74.715 67.922 54.11130.0 0 77.938 81.046 77.675 69.81050.0 0 72.917 79.146 78.800 76.75770.0 0 78.982 78.389 76.542 76.947

0 0.1 43.016 37.870 33.018 31.8560.5 0.1 72.233 68.385 64.043 52.2571.0 0.1 80.106 80.493 82.909 69.7112.0 0.1 84.132 86.048 86.041 85.0003.0 0.1 86.161 87.690 86.150 85.4615.0 0.1 87.435 88.922 89.643 87.092

10.0 0.1 90.859 90.807 91.203 90.97130.0 0.1 90.912 93.329 92.748 92.65250.0 0.1 92.399 94.014 92.629 93.13870.0 0.1 92.652 92.175 94.175 93.224

inhibition efficiency (IE) at lower concentration (10.0M),that is to say, the addition of NaCl further increases theinhibition efficiency values. For example, at 40 C 1.0MOP has 25.647% IE and 0.1 M NaCl has 33.018% IE. Theircomplex has 82.909% IE, which is bigger than the summa-tion of the first one and the second one. This suggests thatthere is a synergistic effect between OP and NaCl. It can beseen from Table 5 that at low concentration of OP (below10.0M), the higher the temperature, the more obvious thesynergistic effect.

3.2.3. Adsorption isotherm for the complex of OP andchloride ion

Assuming the corrosion inhibition was caused by the ad-sorption of OP and NaCl, the degree of surface coverage ()for different concentrations of OP in the presence of 0.1 MNaCl in 1.0 M phosphoric acid was evaluated from weightloss measurement using the Sekine and Hirakawas method[25].

The values of coverage are obtained and listed in Table 6.From Table 6, it can be seen that the surface coverage de-gree increases with increasing the concentration of OP at alltemperatures studied. On the other hand, the values of cov-erage do not be affected markedly by increasing temperatureat a certain concentration of OP.

With the obtained values of coverage, the regression be-tween c/coverage and c at different temperatures has beencompleted using computer. Fig. 9 is the relationship betweenc/coverage and c at 45 C. The straight line means that thereis a good linear relationship between c/coverage and c, sug-gesting that the experimental data can be explained by theLangmuir adsorption isotherm.

Table 6The values of coverage obtained from the weight loss for different con-centrations of OP with the addition of 0.1 M NaCl in 1.0 M phosphoricacid at different temperatures

OP (M) NaCl (M) Coverage30 C 35 C 40 C 45 C

0.5 0.1 0.650 0.727 0.558 0.3231.0 0.1 0.785 0.856 0.835 0.6122.0 0.1 0.854 0.915 0.881 0.8643.0 0.1 0.889 0.933 0.882 0.8725.0 0.1 0.911 0.946 0.934 0.899

10.0 0.1 0.969 0.966 0.956 0.96330.0 0.1 0.970 0.993 0.979 0.99150.0 0.1 0.996 1.000 0.977 0.99970.0 0.1 1.000 0.980 1.000 1.000

Fig. 9. Dependence of c/coverage on concentration of OP in the presenceof 0.1 M NaCl at 45 C.

The obtained adsorption coefficients, slopes, and lin-ear correlation coefficients from the regressions betweenc/coverage and c are listed in Table 7. It is shown that allthe linear correlation coefficients are equal to 1.000 andall the slopes are close to 1, the good linear relationshipbetween c/coverage and c also indicates that the adsorptionof inhibitor onto steel surface accords with the Langmuiradsorption isotherm.

3.2.4. Thermodynamic parameters for the complex of OPand NaCl

To obtain the adsorption heat, the regression between ln kand 1/T was dealt with. Clearly, the adsorption heat canbe obtained by using the slope of the regression, and therelationship between ln k and 1/T is shown in Fig. 10.

Table 7Some parameters of the linear regression between c/ and c for OP inthe presence of 0.1 M NaCl

Temperature (C) k Slope Linear correlationcoefficient

30 2.432 106 0.997 1.00035 4.002 106 0.984 1.00040 2.582 106 1.002 1.00045 1.672 106 0.990 1.000


Fig. 10. The relationship between ln k and 1/T.

Fig. 10 indicates the approximate linear relationshipbetween ln k and 1/T. The adsorption heat was calculatedaccording to the Vant Hoff equation. Under the experimen-tal conditions, the adsorption heat can be approximatelyregarded as the standard adsorption heat (H0).

For the OP in the presence of 0.1 M NaCl, all the obtainedthermodynamic parameters are summarized in Table 8. Thenegative values of G0 indicates the spontaneous adsorp-tion of inhibitor on the surface of cold-rolled steel. The neg-ative values of G0 also show the strong interaction of theinhibitor molecules on the cold steel surface [37,38]. Com-pared with Table 3, the values in Table 8 show that in thesolution containing 0.1 M NaCl the absolute value of adsorp-tion free energy at each experimental temperature is bigger.The result leads to an increase in the equilibrium constant ofadsorption in solution containing OP and NaCl, which meansthat more inhibitor molecules adsorb onto the steel surfacecompared with single OP at the same experimental temper-ature. Therefore, it can be seen that inhibition effect of OPhas been strengthened by a long way due to existing of NaCl.

The negative values of adsorption heat suggest that theadsorption of inhibitor molecule onto the steel surface inphosphoric acid is an exothermic process. The driving forcefor the adsorption is the decrease in enthalpy rather thanthe increase in entropy. The magnitude of adsorption heatis between heat of liquefaction and chemical reaction heat,which probably means that the existence of both physicaladsorption and chemical adsorption.

The negative values of adsorption entropy indicate thatthe adsorption is a process accompanied by a reduction inentropy.

Table 8The thermodynamic parameters of adsorption of different concentrationsof OP with the addition of 0.1 M NaCl on the steel surface at differenttemperatures

Temperature(C)

G0(kJ mol1)

H0(kJ mol1)

S0(J mo1 K1)

30 47.183 24.571 74.59235 49.237 24.571 80.04740 48.896 24.571 77.67845 48.526 24.571 75.297

Table 9Some parameters of the linear regression between ln v (corrosion rate)and 1/T in the presence of 0.1 M NaCl

OP (M) Pre-exponentialfactor (g m2 h1)

Ea (kJ mol1) Linear regressioncoefficient

0.5 5.239 1016 93.213 0.9881.0 5.960 1014 82.956 0.9312.0 1.416 1011 62.357 0.9813.0 1.958 1012 69.334 0.9925.0 3.386 1011 65.197 0.959

10.0 1.554 1011 63.848 0.98030.0 6.457 109 56.104 0.97850.0 1.036 1011 63.487 0.99570.0 6.846 109 56.482 0.919

3.2.5. Kinetic parameters for the complex of OP and NaClThe regression between ln v and 1/T was completed using

computer, and it is found that all the regression coefficientsare close to 1, which means that the linear relationship be-tween ln v and 1/T is good. The apparent activation energyand pre-exponential factor can be calculated from the slopeand intercept of the regression, respectively. All the kineticparameters were calculated and listed in Table 9.

Table 9 shows that the apparent activation energy actsas a variable of the concentration of OP. The relationshipbetween the apparent activation energy and concentration ofOP is shown in Fig. 11.

3.3. Polarization studies

3.3.1. Polarization curve of single OPThe polarization behavior of cold-rolled steel in 1.0 M

phosphoric acid in the absence and presence of differentconcentrations of OP is shown in Fig. 12. Electrochemicalparameters such as corrosion current density (icorr), corro-sion potential (Ecorr) and Tafel slopes are given in Table 10.

Fig. 12 shows that the addition of OP to 1.0 M phospho-ric acid shifts the polarization curves to more negative val-ues. The corrosion potential moves in the negative directionobviously with the addition of OP. This result suggests thatthe hydrogen evolution reaction is retarded by the additionof OP. In the aerated acid solutions, cathodic currents are

Fig. 11. The relationship between Ea and concentration of OP in thepresence of 0.1 M NaCl.


Fig. 12. Polarization curves for various concentration of OP in 1.0 Mphosphoric acid tested at 25 C.

attributed to the sum of two partial cathodic currents: thehydrogen evolution and the reduction of dissolved oxygenin the solution [39,40], the later does not depend on the con-centration of inhibitor [40]. From results given in Table 10,an obvious shift of corrosion potential (Ecorr) towards morenegative values compared with that of 1.0 M phosphoric acidis observed. These results indicate that OP primarily behavesas a cathodic inhibitor.

3.3.2. The polarization curve for synergistic inhibitionThe inhibition efficiency may be calculated by corrosion

current density using the following formula [37]:

E (%) = 100(icorr icorr)

icorr(8)

where icorr and icorr are the corrosion current densities in theabsence and presence of inhibitor, respectively. The valuesof inhibition efficiency are listed in Table 11.

Fig. 13 clearly indicates that the cathodic reaction is in-hibited by the presence of 0.1 M NaCl. In the presence ofthe mixture of OP and NaCl, however, both anodic and ca-

Table 10Electrochemical parameters of OP obtained from polarization measure-ments in 1.0 M phosphoric acid at 25 C

OP(M)

Ecorr (mV vs.SCE)

icorr(A cm2)

c (mV dec1) a (mV dec1)

0 465.131 1.406 102 91.208 20.5370.5 484.420 2.831 102 109.592 33.3662.0 491.043 2.338 102 115.350 40.473

10.0 487.334 2.103 102 119.271 38.500

Fig. 13. Polarization curves for various kinds of inhibitor in 1.0 M phos-phoric acid tested at 25 C.

thodic reactions are dramatically inhibited. The corrosionpotential for the mixture of OP and NaCl is slightly shiftedto cathodic direction, meaning that the mixture behaves asa mixed-type inhibitor that mainly inhibits the cathodic cor-rosion of the steel in 1.0 M phosphoric acid.

Fig. 13 suggests that with the addition of NaCl, OP acts asan excellent inhibitor for steel corrosion in phosphoric acid,showing that there is a strong synergistic inhibition betweenOP and NaCl, the result is in agreement with that obtainedfrom the weight loss technique.

The electrochemical parameters have been calculated andlisted in Table 11. Apparently, the inhibition efficiency calcu-lated from corrosion current density for the mixture reachesa considerable value (88.620%). There is a small differencein the inhibition efficiencies calculated from the corrosioncurrent density and the weight loss method (corrosion rate).This may be the result of the difference in the two methods.The data obtained from weight loss method are average val-ues, while the data obtained from electrochemical techniquesare instantaneous values. Another possible reason may bethe difference in the temperature. The electrochemical ex-periment was carried out at 25 C, while the weight loss ex-


Table 11Electrochemical parameters of the mixture of OP and NaCl obtained from polarization measurements in 1.0 M phosphoric acid at 25 C

NaCl (M) OP (M) Ecorr (mV vs. SCE) icorr (A cm2) c (mV dec1) a (mV dec1) IE (%)0 0 465.131 1.406 102 91.208 20.537 00.1 0 458.943 5.630 101 114.244 13.751 59.9570.1 0.5 473.616 2.803 101 96.798 28.698 80.0640.1 2.0 469.847 1.600 101 93.012 27.265 88.6200.1 10.0 465.218 1.805 101 94.519 23.988 87.162

periment was carried out at temperatures ranging from 30to 45 C.

3.4. Effect of 2 3M OP and 0.1 M NaCl on the corrosionof steel in 1.533.5 M phosphoric acid

Generally speaking, both experimental temperature andacid concentration can affect the corrosion rate of metal.The relationship [v = A exp(Ea/RT)] between corrosionrate (v) and experimental temperature (T) has been widelyused by the corrosion scientists [6,19,2225], as for the rela-tionship between corrosion rate (v) and acid concentration;however, only Mathurs [41] empirical equation has beendiffusely used. Many studies [42,43] show that there is anexponential relationship between the corrosion rate and theacid concentration.

In order to investigate the effect of phosphoric acid con-centration on the corrosion of steel, an empirical equationhas been employed, i.e.

v = N exp(Bc) (9)where v is the corrosion rate, N is defined as the concentra-tion pre-exponential factor, B is defined as the acid concen-tration constant, and c is the acid concentration.

Eq. (9) can be written asln(v) = ln(N)+ Bc (10)

Fig. 14 clearly shows that the regression between ln(v)and c gives a straight line with the intercept of ln(N) andslope of B. The calculated concentration pre-exponential fac-tor (N), acid concentration constant (B) and regression co-efficient (r) are listed in Table 12.

The regression coefficients listed in Table 12 are close to1, which indicates that there is a good linear relationshipbetween the natural logarithm of corrosion rate (v) and acidconcentration (c), the good linear relationship between ln(v)and acid concentration is also proved by Fig. 14. Eq. (9)indicates that the concentration pre-exponential factor (N)is directly proportional to corrosion rate (v), meaning thatthe higher the value of N, the bigger the value of corrosionrate. Table 12 obviously suggests that in the presence of in-hibitors, there is a decrease in concentration pre-exponentialfactor (N), leading to the decrease in corrosion rate. In thepresence of single 0.1 M NaCl and single 2M OP, theconcentration pre-exponential factors (N) are 20.016 and

17.670 g mm2 h1 at 40 C, respectively. With the addi-tion of the mixture of 0.1 M NaCl and 2M OP, however,the concentration pre-exponential factor (N) drastically de-creases to 3.224 g m2 h1. This may be the indication ofthe presence of synergistic inhibition. At other experimentaltemperatures, the change of N is similar to that at 40 C.

Eq. (9) also indicates that there is an exponential relation-ship between the acid concentration constant (B) and corro-sion rate (v), which means that a small variation in acid con-centration constant (B) may cause an obvious change in cor-rosion rate (v). For 1.53.5 M phosphoric acid at 40 C, theblank acid concentration constant equals 0.273 M1, whilefor 1.53.5 M phosphoric acid in the presence of 0.1 M NaCl,the value of B is equal to 0.188, which is smaller than thatof the blank (0.273 M1). The change of the value of Bleads to the decrease in corrosion rate. At other experimentaltemperatures, the same change has been observed. There-fore, for 1.53.5 M phosphoric acid in the presence of 0.1 MNaCl, both the decreases in B and N can cause the reductionin corrosion rate. As far as the systems of 1.53.5 M phos-phoric acid in the presence of 2M OP with and withoutaddition of 0.1 M NaCl are concerned, the values of B arebigger than those of the blanks at all experimental tempera-tures. This indicates that in these systems, the reduction ofcorrosion rate is mainly decided by the decrease in N ratherthan the decrease in B.

Fig. 14. Relationship between ln v and concentration of phosphoric acidin the absence and presence of various kinds of inhibitors at 40 C.


Table 12Some parameters of the linear regression between ln(v) and c

T (C) Solution N (g m2 h1) B (M1) r30 1.53.5 M phosphoric acid 12.217 0.307 0.994

1.53.5 M phosphoric acid + 0.1 M NaCl 8.514 0.218 0.9911.53.5 M phosphoric acid + 2M OP 4.112 0.577 0.9901.53.5 M phosphoric acid + 2M OP + 0.1 M NaCl 1.899 0.335 0.966

35 1.53.5 M phosphoric acid 20.141 0.280 0.9981.53.5 M phosphoric acid + 0.1 M NaCl 15.568 0.198 0.9951.53.5 M phosphoric acid + 2M OP 12.103 0.431 0.9921.53.5 M phosphoric acid + 2M OP + 0.1 M NaCl 2.482 0.527 0.988



3.5. Effect of phosphoric acid concentration on corrosionrate of steel

Fig. 15 indicates that corrosion rate increases with the in-crease in concentration of phosphoric acid in the absenceand presence of inhibitor. For the acid solution with the ad-dition of single OP, the acid concentration affects the corro-sion rate prominently, while for the acid solution containingsingle NaCl the corrosion rate does not change too muchwith increasing the acid concentration. In 1.53.5 M phos-phoric acid, the corrosion rate for the solution with the addi-tion of single OP is bigger than that for the solution with the

Fig. 15. Dependence of corrosion rate on concentration of phosphoricacid at 40 C.

addition of single NaCl. Furthermore, the corrosion rate forthe solution in the presence of single OP is close to that ofthe blank, which indicates that 2M OP has weak protec-tion against steel corrosion with an increase in acid concen-tration. Combined with NaCl, however, the corrosion rateof OP is the smallest compared with the corrosion rates ofthe blank, single OP and single NaCl, which suggests thatthe presence of NaCl can strengthen the protection of OPagainst the steel corrosion.

3.6. Effect of phosphoric acid concentration on inhibitionefficiency

The relationship between inhibition efficiency and phos-phoric acid concentration is shown in Fig. 16. It is obviousthat inhibition efficiency for single 0.1 M NaCl increaseswith an increase in acid concentration, however, inhibitionefficiency for single 2M OP rapidly decreases with an in-crease in acid concentration. It can be deduced that in 4.0 Mphosphoric acid, 2M OP may lose its protection totally. InFig. 16, represents the summation of the inhibition ef-ficiency for single 0.1 M NaCl and single 2M OP. Clearly,the summation of inhibition efficiency is smaller than thatof the mixture of 0.1 M NaCl and 2M OP ( representsthe inhibition efficiency of the mixture), this is the result ofthe synergistic inhibition between OP and NaCl.

3.7. Explanation for synergism

It is well known that OP is a kind of nonionic surfac-tant, which contains many oxygen atoms with the two setsof lone-pair electrons. In neutral aqueous solution, it existsin the form of neutral molecule, in acidic solution, however,OP may be protonated, leading to the positive charge in themolecule. It is also well known that steel surface charges pos-


Fig. 16. Dependence of inhibition efficiency on concentration of phos-phoric acid at 40 C.

itive charge because of EcorrEq=0(zerochargepotential) >0, thus, it is difficult for the positively charged OP to ap-proach the positively charged steel surface due to the elec-trostatic repulsion, this is why single OP cannot act as anexcellent inhibitor for steel corrosion in acidic solution with-out containing halide ions. With the presence of halide ions,the specific adsorption of halide ions causes the negativelycharged surface of steel, thus, by means of electrostaticattraction (physical adsorption), protonated OP can easilyreaches steel surface, followed by the transference of elec-tron from oxygen atom to the d-orbital of iron atom (chem-ical adsorption) at the steel/solution interface. This is thereason that the chemical adsorption occurs in the presenceof single OP, and with the addition of NaCl, both physicaland chemical adsorptions occur, which is in good agreementwith the experimental data.

It is worth noting that polycentric adsorption may takeplace as the result of the existence of many oxygen atomsin one OP molecule.

Another possible reason for the synergism is that the addi-tion of chloride ion in aqueous solution reduces the amountof the free water molecule, leading to the increase in theactive concentration of OP in solution.

4. Conclusions

From the weight loss and electrochemical studies, it canbe easily concluded that as follows.

OP acts as a cathodic inhibitor for steel corrosion in 1.0 Mphosphoric acid, the inhibition efficiency increases with in-creasing concentration, but decreases with an increase in

experimental temperature. The adsorption of OP is a spon-taneous process, and the process is an exothermic processaccompanying a decrease in entropy.

In 1.0 M phosphoric acid, the adsorption of OP on steelsurface follows the Langmuir adsorption isotherm, onlychemical adsorption occurs. With the addition of 0.1 MNaCl in 1.0 M phosphoric acid, the adsorption of OP alsoaccords with the Langmuir adsorption isotherm, both phys-ical and chemical adsorptions occur.

With the addition of 0.1 M NaCl in 1.53.5 M phosphoricacid, the inhibition efficiency increases with an increase inconcentration of phosphoric acid. The synergistic effect be-tween OP and NaCl is observed. The mixture can be used asan excellent inhibitor for steel corrosion in phosphoric acidover a wide temperature range.

Compared with single OP, the complex of OP and NaClexhibits a stronger tendency to adsorb onto steel surface,since the absolute values of adsorption free energy for thecomplex are bigger than those for single OP.

Single OP acts as a cathodic inhibitor against steel corro-sion in phosphoric acid, the mixture of OP and NaCl behavesas a mixed-type inhibitor that mainly inhibits the cathodiccorrosion of the steel. The presence of NaCl can drasticallyimprove the inhibition efficiency of OP. This may be theresult that the presence of NaCl stabilize the adsorption ofOP, indicating there is a synergistic inhibition for the steelcorrosion in phosphoric acid.

The synergistic inhibition between OP and NaCl existsover a wide concentration range of phosphoric acid.

The experimental data can satisfactorily explained by theempirical concentration equation, which has been put for-ward by the authors.

References

[1] L.B. Tang, G.N. Mu, G.H. Liu, Corros. Sci. 45 (2003) 2251.[2] M.A. Quraishi, H.K. Sharma, Mater. Chem. Phys. 78 (2002) 18.[3] E.E. Foad, E.I. Sherbini, Mater. Chem. Phys. 61 (1999) 223.[4] F. Bentiss, M. Traisnel, M. Lagrenee, Corros. Sci. 42 (2002) 127.[5] M.A. Quraishi, D. Jamal, Corrosion 56 (2000) 983.[6] S. Martinez, I. Stern, Appl. Surf. Sci. 199 (2002) 83.[7] T.B. Du, J.J. Chen, D.Z. Cao, J. Mater. Sci. 36 (2001) 3903.[8] E.E. Ebenso, Mater. Chem. Phys. 79 (2003) 58.[9] M. Hosseini, S.F.L. Mertens, M. Ghorbani, M.R. Arshadi, Mater.

Chem. Phys. 78 (2003) 800.[10] S.N. Banerjee, S. Misra, Corrosion 45 (1989) 780.[11] M.A. Ameer, E. Khamis, G. Al-Senani, J. Appl. Electrochem. 32

(2002) 149.[12] P. Li, T.C. Tan, J.Y. Lee, Corrosion 53 (1997) 186.[13] S.T. Arab, E.A. Noor, Corrosion 49 (1993) 122.[14] G. Moretti, G. Quartarone, A. Tassan, A. Zingales, Electrochim. Acta

41 (1996) 1971.[15] S.S. Abd El Rehim, M.A.M. Ibrahim, K.F. Khalid, Mater. Chem.

Phys. 70 (2001) 268.[16] M. El Azhar, B. Mernari, M. Traisnel, F. Bentiss, M. Lagrene,

Corros. Sci. 43 (2001) 2229.[17] M.H. Wahdan, Mater. Chem. Phys. 49 (1997) 135.[18] G. Schmitt, Br. Corros. J. 19 (1984) 165.[19] Y. Feng, K.S. Siow, W.K. Teo, A.K. Hsieh, Corros. Sci. 41 (1999)

829.


[20] M. Gojic, Corros. Sci. 43 (2001) 919.[21] M.M. Osman, S.S. Abad El Rehim, Mater. Chem. Phys. 53 (1998)

34.[22] S. Martinez, I. tern, J. Appl. Electrochem. 31 (2001) 973.[23] X.H. To, N. Pebere, N. Pelaprat, B. Boutevin, Y. Hervaud, Corros.

Sci. 39 (1997) 1925.[24] M. Sahin, S. Bilgi, Appl. Surf. Sci. 147 (1999) 27.[25] I. Sekine, Y. Hirakawa, Corrosion 42 (1986) 272.[26] E. Khamis, E.S.H. El-Ashry, A.K. Ibrahim, Br. Corros. J. 35 (2000)

150.[27] F. Bentiss, M. Bouanis, B. Mernari, M. Traisnel, M. Lagrenee, J.

Appl. Electrochem. 32 (2002) 671.[28] M. Hosseini, S.F.L. Mertens, M.R. Arshadi, Corros. Sci. 45 (2003)

1473.[29] S. Rajendran, B.V. Apparao, N. Palaniswamy, Electrochim. Acta 44

(1998) 533.[30] T. Du, J. Chen, D. Cao, Br. Corros. J. 35 (2000) 229.

[31] L.B. Tang, L. Li, J. Zhang, G.H. Liu, G.N. Mu, J. Yunnan Univ.(Chinese) 23 (2001) 398.

[32] P. Manjula, S. Manonmani, P. Jayaram, S. Rajendran, Anti-CorrosionMethods, Materials 48 (2001) 319.

[33] G.N. Mu, T.P. Zhao, M. Liu, T. Gu, Corrosion 52 (1996) 853.[34] T.P. Zhao, G.N. Mu, Corros. Sci. 41 (1999) 1937.[35] X.M. Li, L.B. Tang, J. Zhang, G.N. Mu, G.H. Liu, J. Yunnan Univ.

(Chinese) 24 (2002) 15.[36] E. Khamis, Corrosion 46 (1990) 476.[37] M. Elachouri, M.S. Hajji, M. Salem, S. Kertit, J. Aride, R. Coudert,

E. Essassi, Corrosion 52 (1996) 103.[38] B.V. Savitri, S. Mayanna, Ind. J. Chem. Technol. 3 (1996) 256.[39] W.J. Lorenz, F. Mansfeld, Corros. Sci. 21 (1981) 647.[40] J.OM. Bockris, B. Yang, Br. J. Electrochem. Soc. 138 (1991) 2237.[41] P.B. Mathur, T. Vasudevan, Corrosion 38 (1982) 171.[42] L. Wang, G.J. Yin, J.G. Yin, Corros. Sci. 43 (2001) 1197.[43] L. Wang, Corros. Sci. 43 (2001) 2281.

Synergistic inhibition between OP and NaCl on the corrosion of cold-rolled steel in phosphoric acidIntroductionExperimentalMaterialsSolutionsGravimetric measurementsElectrochemical measurements

Experimental results and discussionEffect of single OP on the corrosion of cold-rolled steelEffect of concentration of single OP on the weight loss and corrosion rate of cold-rolled steelEffect of concentration of single OP on the inhibition efficiencyAdsorption isotherm for single OPThermodynamic parameters for single OPKinetic parameters for single OP

Effect of OP and NaCl on the corrosion of cold-rolled steelEffect of concentration of OP on the weight loss and corrosion rate of steel in 1.0M phosphoric acid in the presence of 0.1M NaClEffect of concentration of OP on the inhibition efficiency in 1.0M phosphoric acid in the presence of 0.1M NaClAdsorption isotherm for the complex of OP and chloride ionThermodynamic parameters for the complex of OP and NaClKinetic parameters for the complex of OP and NaCl

Polarization studiesPolarization curve of single OPThe polarization curve for synergistic inhibition

Effect of 2 3M OP and 0.1M NaCl on the corrosion of steel in 1.5-33.5M phosphoric acidEffect of phosphoric acid concentration on corrosion rate of steelEffect of phosphoric acid concentration on inhibition efficiencyExplanation for synergism

ConclusionsReferences

Synergistic Inhibition Between OP and NaCl on the Corrosion of Cold-rolled Steel in Phosphoric Acid

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