Corrosion behavior of iron-based alloys in the LiBr + ethylene glycol + H 2 O mixture E. Samiento-Bustos a , J.G. González-Rodriguez a,b, * , J. Uruchurtu a , V.M. Salinas-Bravo c a Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, CP 62210, Cuernavaca, Morelos, Mexico b CIMAV-Miguel de Cervantes 120, Chihuahua, Chih., Mexico c Instituto de Investigaciones Eléctricas, Gerencia de Materiales y Proceso Químicos, Av. Reforma 113, Col. Palmira, CP 62490, Cuernavaca, Morelos, Mexico article info Article history: Received 22 October 2008 Accepted 10 February 2009 Available online 20 February 2009 Keywords: A. Steels C. Pitting corrosion B. Electrochemical noise Impedance spectroscopy abstract The corrosion resistance of 1018 carbon steel, 304 and 316 type stainless steels in the LiBr (55 wt.%) + ethylene glycol + H 2 O mixture at 25, 50 and 80 °C has been studied using electrochemical techniques which included potentiodynamic polarization curves, electrochemical noise and electrochem- ical impedance spectroscopy techniques. Results showed that, at all tested temperature, the three steels exhibited an active–passive behavior. Carbon steel showed the highest corrosion rate, since both the pas- sive and corrosion current density values were between two and four orders of magnitude higher than those found for both stainless steels. Similarly, the most active pitting potential values was for 1018 car- bon steel. For 1018 carbon steel, the corrosion process was under a mixed diffusion and charge transfer at 25 °C, whereas at 50 and 80 °C a pure diffusion controlled process could be observed. For 316 type stain- less steel, at 25 and 50 °C a species adsorption controlled process was observed, whereas at 80 °C a dif- fusion controlled mechanism was present. Additionally, at 25 °C, the three steels were more susceptible to uniform type of corrosion, whereas at 50 and 80 °C they were very susceptible to localized type of corrosion. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Ethylene glycol is widely used as coolant in automotive heat exchangers, mixed with water, in a pH range between 7 and 8, due to its great heat absorption capacity [1–4]. However, lithium bromide (LiBr) heavy brines are one of the most widely used absor- bents [5–8] in absorption heat transformers because LiBr possesses favorable thermo physical properties. However, LiBr can cause seri- ous corrosion problems on metallic components of cooling systems and heat exchangers at absorption plants such as carbon steel, stainless steel, copper alloys and titanium [9]. An alternative way to reduce some of these disadvantages of the water/LiBr mixture is to add ethylene glycol to the system [8] because some thermo physical properties of the LiBr/water mixture, such as thermal con- ductivity, viscosity, maximum concentration etc. are improved [10,11]. The cheapest and first structural material candidate one can have is carbon steel [12], however, it is not precisely the most corrosion resistant for many environments [13,14]. For this reason, there exist a few works on the corrosion performance of carbon steel, but there are too many about the performance of stainless steels in LiBr–water mixtures [5–7,15–18]. An alternative way to reduce corrosion rate is by using inorganic inhibitors such as chro- mates, nitrates, molybdates, etc. just like the reported recently by Sarmiento-Bustos et al. [19]. Thus, the aim of the present work is to evaluate the corrosion performance of typical iron-based alloys commonly used in heat absorption systems such as 1018 carbon steel and 304 and 316 type stainless steels in the new proposed LiBr + ethylene glycol + H 2 O heat absorber mixture, and try to give some insights on the involved corrosion mechanisms. 2. Experimental procedure Materials tested included 1018 carbon steel, 304 and 316 type stainless steels, with chemical composition as given in Table 1, and encapsulated in a commercial polymeric resin. Cylindrical probes with 5.9 mm in diameter and an exposed area of 0.2728 cm 2 to the solution were used. All of them were abraded with papers 600 grade emery paper, and finally rinsed with dis- tilled water and ethanol (C 2 H 5 OH). Solution used was a normally aerated LiBr + ethyleneglycol + H 2 O mixture at room temperature, in a concentration of 614 and 217 g/l for LiBr and ethylene glycol, respectively. This concentration was chosen because, according to literature [10,11], the best thermo physical properties of the fluid are obtained at this concentration and re-crystallization of the lith- ium bromide is avoided. Polarization curves were obtained by polar- izing the specimens from 1000 to 1000 mV respect to the free corrosion potential value, E corr , at a scanning rate of 60 mV/min. 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.02.008 * Corresponding author. Address: Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, CP 62210, Cuernavaca, Morelos, Mexico. Tel./fax: +52 777 3 29 70 84. E-mail address: [email protected](J.G. González-Rodriguez). Corrosion Science 51 (2009) 1107–1114 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Transcript
Corrosion Science 51 (2009) 1107–1114
Contents lists available at ScienceDirect
Corrosion Science
journal homepage: www.elsevier .com/locate /corsc i
Corrosion behavior of iron-based alloys in the LiBr + ethylene glycol + H2O mixture
E. Samiento-Bustos a, J.G. González-Rodriguez a,b,*, J. Uruchurtu a, V.M. Salinas-Bravo c
a Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa,CP 62210, Cuernavaca, Morelos, Mexicob CIMAV-Miguel de Cervantes 120, Chihuahua, Chih., Mexicoc Instituto de Investigaciones Eléctricas, Gerencia de Materiales y Proceso Químicos, Av. Reforma 113, Col. Palmira, CP 62490, Cuernavaca, Morelos, Mexico
a r t i c l e i n f o
Article history:Received 22 October 2008Accepted 10 February 2009Available online 20 February 2009
0010-938X/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.corsci.2009.02.008
* Corresponding author. Address: Centro de InvestigAplicadas, Universidad Autónoma del Estado de MorelChamilpa, CP 62210, Cuernavaca, Morelos, Mexico. Te
The corrosion resistance of 1018 carbon steel, 304 and 316 type stainless steels in the LiBr(55 wt.%) + ethylene glycol + H2O mixture at 25, 50 and 80 �C has been studied using electrochemicaltechniques which included potentiodynamic polarization curves, electrochemical noise and electrochem-ical impedance spectroscopy techniques. Results showed that, at all tested temperature, the three steelsexhibited an active–passive behavior. Carbon steel showed the highest corrosion rate, since both the pas-sive and corrosion current density values were between two and four orders of magnitude higher thanthose found for both stainless steels. Similarly, the most active pitting potential values was for 1018 car-bon steel. For 1018 carbon steel, the corrosion process was under a mixed diffusion and charge transfer at25 �C, whereas at 50 and 80 �C a pure diffusion controlled process could be observed. For 316 type stain-less steel, at 25 and 50 �C a species adsorption controlled process was observed, whereas at 80 �C a dif-fusion controlled mechanism was present. Additionally, at 25 �C, the three steels were more susceptibleto uniform type of corrosion, whereas at 50 and 80 �C they were very susceptible to localized type ofcorrosion.
� 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Ethylene glycol is widely used as coolant in automotive heatexchangers, mixed with water, in a pH range between 7 and 8,due to its great heat absorption capacity [1–4]. However, lithiumbromide (LiBr) heavy brines are one of the most widely used absor-bents [5–8] in absorption heat transformers because LiBr possessesfavorable thermo physical properties. However, LiBr can cause seri-ous corrosion problems on metallic components of cooling systemsand heat exchangers at absorption plants such as carbon steel,stainless steel, copper alloys and titanium [9]. An alternative wayto reduce some of these disadvantages of the water/LiBr mixtureis to add ethylene glycol to the system [8] because some thermophysical properties of the LiBr/water mixture, such as thermal con-ductivity, viscosity, maximum concentration etc. are improved[10,11]. The cheapest and first structural material candidate onecan have is carbon steel [12], however, it is not precisely the mostcorrosion resistant for many environments [13,14]. For this reason,there exist a few works on the corrosion performance of carbonsteel, but there are too many about the performance of stainlesssteels in LiBr–water mixtures [5–7,15–18]. An alternative way to
ll rights reserved.
ación en Ingeniería y Cienciasos, Av. Universidad 1001, Col.l./fax: +52 777 3 29 70 84.z-Rodriguez).
reduce corrosion rate is by using inorganic inhibitors such as chro-mates, nitrates, molybdates, etc. just like the reported recently bySarmiento-Bustos et al. [19]. Thus, the aim of the present work is toevaluate the corrosion performance of typical iron-based alloyscommonly used in heat absorption systems such as 1018 carbonsteel and 304 and 316 type stainless steels in the new proposedLiBr + ethylene glycol + H2O heat absorber mixture, and try to givesome insights on the involved corrosion mechanisms.
2. Experimental procedure
Materials tested included 1018 carbon steel, 304 and 316 typestainless steels, with chemical composition as given in Table 1,and encapsulated in a commercial polymeric resin. Cylindricalprobes with 5.9 mm in diameter and an exposed area of0.2728 cm2 to the solution were used. All of them were abradedwith papers 600 grade emery paper, and finally rinsed with dis-tilled water and ethanol (C2H5OH). Solution used was a normallyaerated LiBr + ethyleneglycol + H2O mixture at room temperature,in a concentration of 614 and 217 g/l for LiBr and ethylene glycol,respectively. This concentration was chosen because, according toliterature [10,11], the best thermo physical properties of the fluidare obtained at this concentration and re-crystallization of the lith-ium bromide is avoided. Polarization curves were obtained by polar-izing the specimens from �1000 to 1000 mV respect to the freecorrosion potential value, Ecorr, at a scanning rate of 60 mV/min.
Fig. 3. Polarization curves for 1018 carbon, 304 and 316 stainless steels in 55%LiBr + ethylene glycol + H2O at 80 �C.
1108 E. Samiento-Bustos et al. / Corrosion Science 51 (2009) 1107–1114
Corrosion current density values, Icorr, were calculated by using Ta-fel extrapolation method. A saturated silver/silver chloride (Ag/AgCl) electrode was used as reference electrode whereas a plati-num wire was the auxiliary electrode. Curves were performed bytriplicate. Passive current density values, Ipass, were obtained asthe average value in the readings, when a more or less stable cur-rent was observed in the plots, and the pitting or breakdown po-tential, Epit, was taken as the potential value where an abruptincrease in the anodic current density was observed. Electrochem-ical Impedance Spectroscopy (EIS) measurements were done in thefrequency interval of 0.005 to 10,000 Hz with amplitude of ±10 mVat the free corrosion potential, since it is the expected value to befound on real circumstances, by using a PC4-300 Gamry potentio-stat. Parameters calculated to simulate the EIS data, among others,gave solution resistance values between 0.1 and 1 X cm2, giving,thus, solution conductivity values between 1 and 10 S/m. Just tocompare, sea water and tap water have conductivity values of 5and 0.05 S/m, respectively, whereas that for de-ionized water is5.5 � 10�6 S/m. Electrochemical noise measurements (EN) in bothcurrent and voltage were recorded. For this, an arrangement usingtwo identical working electrodes and a saturated Ag/AgCl electrodeas reference electrode was used. Electrochemical noise measure-ments were made recording simultaneously the potential and cur-rent fluctuations at a sampling rate of 1 point per second for aperiod of 1024 s using a zero resistance ammeter connected to apersonal computer. Removal of the DC trend from the raw noisedata was the first step in the noise analysis. To accomplish this, aleast square fitting method was used. Tests were performed at25, 50 and 80 �C by using a heating mantle controlled with a com-patible control thermostat. These temperatures were chosenbecause at higher temperatures the solution could be re-crystal-lized under the used working pressure, and most of the reporteddata are within this range of temperature, so, this way, we couldcompare our results with those reported in the literature.
3. Results and discussion
3.1. Polarization curves
Figs. 1–3 show the polarization curves at 25, 50 and 80 �C,respectively, for the different steels. At 25 �C, Fig. 1, 1018 carbonsteel had the most active Ecorr value, around �850 mV(Ag/AgCl)
whereas 304 and 316 type stainless steels had much nobler Ecorr
values, �300 and �200 mV(Ag/AgCl), respectively. The corrosioncurrent density values for carbon steel were close to 10�3 mA/cm2, two orders of magnitude higher than those obtained forboth stainless steels, which were practically the same, around10�5 mA/cm2. For 1018 carbon steel, the anodic current densityincreased with applied potential up to �600 mV(Ag/AgCl), wherethe current density decreased, but at �550 mV(Ag/AgCl) it startedto increase once again up to �550 mV(Ag/AgCl), where an anodicpeak was observed. After this anodic peak, a decrease in the cur-rent density was observed and then a further increase in thecurrent, indicating the presence of a very narrow passive region.Around �400 mV(Ag/AgCl) a second, very narrow passive regioncould be observed., Thus, it could be said that 1018 carbon steeldid not show a very stable passive region. On the other hand,both stainless steel did not show a passive region but only an
anodic limit current was observed which was more evident for316 type stainless steel with a breakdown or pitting potentialvalue, Epit, close to �125 mV(Ag/AgCl).
2 0 3 0 4 0 5 0 6 0 7 0 8 0 90
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
10 2
I cor
r (m
A/cm
2)
Temperature (°C)
1018
304316
Fig. 4. Effect of temperature on the change on the Icorr value for 1018 carbon, 304and 316 stainless steels in 55% LiBr + ethylene glycol + H2O.
20 30 40 50 60 70 80 9010-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
I pas
s (m
A/cm
2 )
Temperature (°C)
1018
304316
Fig. 5. Effect of temperature on the change on the Ipass value for 1018 carbon, 304and 316 stainless steels in 55% LiBr + ethylene glycol + H2O.
E. Samiento-Bustos et al. / Corrosion Science 51 (2009) 1107–1114 1109
At 50 �C, Fig. 2, the Ecorr value for 1018 carbon steel practically didnot change, since it was made more active for only 50 mV(Ag/AgCl). TheEcorr values for 304 and 316 type stainless steels, however, were mademore active, reaching values close to �800 and �700 mV(Ag/AgCl),respectively, 500 mV(Ag/AgCl) more negative than the correspondingvalues obtained at 25 �C. The Icorr values for all the steels increasedtwo orders of magnitude, reaching values of 1 � 10�1 mA/cm2 for1018 carbon steel and 316 type stainless steel, whereas the valuefor 304 steel was close to 8 � 10�3 mA/cm2. The passive current den-sity values were close to 1 � 10�1 mA/cm2 for 1018 carbon steel and1 � 10�2 mA/cm2 for both stainless steels. The lowest pitting poten-tial value was for 1018 carbon steel, around �600 mV(Ag/AgCl),whereas the highest value, �50 mV(Ag/AgCl), was for 316 type stain-less steel. Compared with the values obtained at 25 �C, the Epit weremade more active at 50 �C. The passive current density values forboth stainless steels showed some erratic readings, which mightbe related to unstable passive properties, especially for 316 typestainless steel.
When the temperature increased up to 80 �C, the Ecorr value forall steels were made nobler, Fig. 3, and the corrosion current den-sity values increased for at least two orders of magnitude, exceptfor 316 type stainless steel, which remained more or less constant.The Icorr value for 1018 carbon steel was close to 1 mA/cm2, whereas304 type stainless steel reached a value close to 10�1 mA/cm2. Thepassive region for 1018 carbon steel was unstable, similar to thatobserved at 25 �C, with a pitting potential close to 900 mV(Ag/AgCl),very close to the values obtained for both stainless steels. This va-lue was obtained at the onset of the increase in the anodic currentdensity, and no further decrease in the current density value couldbe observed. It must be said, however, that at 80 �C there is lessdissolved oxygen in solution, raising the E
corrvalue, explaining the
high values exhibited on Fig. 3. Additionally, the water oxidationpotential value is around 935 mV(Ag/AgCl), explaining why all thesteels exhibited similar pitting or breakdown potential values.The passive regions showed by 304 and 316 type stainless steelswere less stable than that exhibited at 50 �C, since the passive cur-rent density did not reach a stable value. There was an anodic peakaround 750 mV(Ag/AgCl), present in the three steels, but it was moreevident for 1018 carbon steel. The lowest passive current densityvalue was for 316 type stainless steel, whereas the highest valuewas for 1018 carbon steel.
Hu [12] studied the corrosion behavior of 1018 carbon steel incommercial 55% LiBr + 0.07 mol/l LiOH. The corrosion current den-sity values increased from 20 to 60 lA/cm2 when the temperatureincreased from 145 to 240 �C. In our case, the Icorr value increasedfrom 1 � 101 to 1 � 104 lA/cm2 when the temperature increasedfrom 25 to 80 �C. On the other hand, Igual-Muñoz et al. [7] evalu-ated the corrosion resistance of three stainless steels in commer-cial LiBr (850 g/l) at 25, 50, 75 and 85 �C. Those stainless steelshad higher contents of Cr, Ni and Mo (23, 11, and 2.8, respectively)than those for commercial 304 and 316 stainless steels used in thiswork. The corrosion rates were within 0.2 and 0.6 lA/cm2 at 25 �Cand increased up to 0.9 lA/cm2 when the temperature increasedup to 85 �C. Our results for 304 and 316 type stainless steels,according to Fig. 4, gave Icorr values around 10�2 lA/cm2 at 25 �Cand increased up to10 lA/cm2, at 80 �C. Thus, in all cases, corrosionrates in the LiBr + ethylene glycol mixture were higher than thoseexhibited in LiBr aqueous solution in similar conditions.
By adding ethylene glycol to LiBr, not only the thermo physicproperties are improved, but the crystallization of lithium bromideis avoided too. Thus, by increasing the temperature, an increase inthe corrosion rate is expected since the oxygen solubility and dif-fusivity increases, bringing an increase in the passive current den-sity. Additionally, the passive film properties are improved as canbe seen on Figs. 1–3, were it can be seen that, at 25 �C, carbon steelshowed two very narrow passive regions. As the temperature was
increased, the passive region was widened. This might be due tothe fact that ethylene glycol forms a protective film as the temper-ature increases or seals the pores present in the incipient passivefilm at 25 �C. The improvement of the passive film properties asthe temperature increases, together with an increase in both oxy-gen solubility and diffusivity could explain the change in the Icorr
values. This improvement in the passive film properties, especiallythe widening in the passive zone, explains why the breakdown orpitting potential shifts towards nobler values as the temperatureincreased, reaching a high value at 80 �C.
Electrochemical parameters such as corrosion and passive cur-rent density values, Icorr, and Ipass together with the pitting orbreakdown potential, Epit obtained from polarization curves aresummarized in Figs. 4–6. Fig. 4 shows that, except at 25 �C, 1018carbon steel had the highest corrosion current density values foralmost two orders of magnitude than that for both stainless steels.Fig. 5 shows also that the highest passive current density value wasfor 1018 carbon steel, in some cases for up to four orders of mag-nitude, whereas both stainless steels had very similar Ipass values.The fact that both parameters, Icorr and Ipass increased in a linearfashion for carbon steel but not for the stainless steels, shows thatthe formed surface protective film is much more stable in the laterthan in the former. On the other hand, Fig. 6 shows the effect oftemperature on the Epit value for the different steels. One method
-800
-600
-400
-200
0
200
400
600
800
1000
E pit
(mV
vs A
g/Ag
Cl)
Temperature (°C)
1018
304
316
20 30 40 50 60 70 80
Fig. 6. Effect of temperature on the change on the breakdown or pitting potentialvalue, Epit, for 1018 carbon, 304 and 316 stainless steels in 55% LiBr + ethyleneglycol + H2O.
0
1000
2000
3000
4000
5000
6000
7000
0 2 4 6 8 10 12
0
1
2
3
4
Z im
(Ohm
s.cm
2 )
Z re (Ohms.cm 2)
1018
0 100 200 300
0
100
200
300
Z im
(Ohm
s.cm
2 )
Zre (Ohms.cm 2)
1018304
Zim
(Ohm
s.cm
2 )
Zre (Ohms.cm2 )
316
250C
304
0 1000 2000 3000 4000 5000 6000 7000
Fig. 7. Nyquist diagram for 1018 carbon, 304 and 316 stainless steels in 55%LiBr + ethylene glycol + H2O at 25 �C at Ecorr (at the end of the tests the Ecorr valueswere �865, �313 and �220 mVAg/AgCl, respectively).
0 10000 20000 30000 40000
0
10000
20000
30000
40000
0 100 200 300 400 500 600
0
100
200
300
400
500
600
Z im(O
hms.
cm2)
Zre (Ohms.cm2)
1018
Zim
(Ohm
s.cm
2 )
Zre (Ohms.cm2)
304
316
500C
Fig. 8. Nyquist diagram for 1018 carbon, 304 and 316 stainless steels in 55%LiBr + ethylene glycol + H2O at 50 �C at 25 �C at Ecorr (at the end of the tests the Ecorr
values were �920, �715 and �810 mVAg/AgCl, respectively).
1110 E. Samiento-Bustos et al. / Corrosion Science 51 (2009) 1107–1114
to evaluate the resistance of alloys to pit initiation is to comparethe Epit values [15–18], parameter which reflects the kinetics forpit initiation. It is generally accepted that the nobler the Epit valuethe more resistant is the alloy for pit initiation and the longer thetime required for pit initiation (induction time) at potentials belowEpit. This parameter decreased towards more active values with anincrease in the temperature from 25 to 50 �C, but, with a further in-crease in the temperature, the Epit value increased towards noblervalues. The lowest value was obtained for 1018 carbon steel, espe-cially at 25 and 50 �C, but at 80 �C the three materials had verysimilar Epit values. All these values are summarized in Table 2.
3.2. EIS measurements
Impedance data in the Nyquist format for the experiments at25 �C are shown on Fig. 7 which shows two depressed, capaci-tive-like semicircles with their centers in the real axis, one at highfrequencies and another one at lower frequencies. The first semi-circle indicates the charge transfer from the metal to the electro-lyte, whereas the second one emerges due to adsorption effectsand the formation of a porous, non-protective layer. The presenceof this porous layer could be the reason why the passive layershown on Fig. 1 was unstable. The semicircle diameter is equiva-lent to the linear polarization resistance, Rp, or charge transferresistance, Rct, which is inversely proportional to the corrosion cur-rent density value, Icorr. Thus, the highest Rct value, and thus, thelowest Icorr value was for 316 type stainless steel, whereas the low-est Rct value was for 1018 carbon steel. For 1018 carbon steel Hu[12] reported a value for Rp close to 188 X cm2 at 145 �C, four timeshigher than the value shown on Fig. 6 for this steel.
The equivalent electrical circuit for a two step reaction wherean intermediate adsorbate or porous layer is formed is a parallelof two RC parallel network circuit elements. Thus, the effects ofthe adsorbed intermediate or porous layer, is reflected in the sec-
Table 2Effect of temperature on the electrochemical parameters for the different steels obtained frocurrent densities were within ±3% of the reported value.
ond parallel RC combination. The effect of this combination inthe total impedance is to generate a semicircle, depending on therelative value of the two time constants associated to each semicir-cle. The two depressed semicircles observed in the obtained re-sults, could be described by a complex distribution of therelaxation times or time constants associated.
At 50 �C, the impedance data for the three steels showed a sim-ilar behavior to that shown at 25 �C, Fig. 8, i.e. one semicircle athigh frequencies and another one at lower frequencies, but thediameter of the second semicircle was larger than that shown at25 �C, so large that the data seems to describe a straight line, indi-
m the polarization curves. Standard deviations in potentials were ±10 mV, whereas in
E. Samiento-Bustos et al. / Corrosion Science 51 (2009) 1107–1114 1111
cating a corrosion process controlled by both charge transfer anddiffusion of reactants through the passive film formed on the steel.The impedance values were greater than those found at 25 �C, indi-cating a much adherent, less porous protective layer, whichemerges on the polarization curves shown on Fig. 2 as a muchmore stable passive region. Finally at 80 �C, Fig. 9, impedance datafor the three steels described a small semicircle at high frequenciesfollowed by a straight line at lower frequencies, indicating a corro-sion process under a mixed charge transfer and diffusion control,diffusion through this protective, passive layer.
The created equivalent circuit to model the EIS data is shown inFig. 10 whereas the calculated parameters are shown in Table 3. OnFig. 10, Rs represents the solution resistance, Rf and Cf are related toa porous film resistance and capacitance respectively, Rct and Cdl
represent the charge transfer resistance and double layer capaci-
0 5000 10000 15000 20000
0
5000
10000
15000
20000
0 2 00 400 600 800 1000 1200 1400 16000
200
400
600
800
1000
1200
1400
1600
Z re(Ohms.cm2 )
Z im
(Ohm
.cm
2 )
304
0 5 10 15 200
5
10
15
20
Z im
(Ohm
s.cm
2 )
Zre (Ohms.cm2)
1018
Zim
(Ohm
s.cm
2)
Zre (Ohms.cm2)
316
304
800C
Fig. 9. Nyquist diagram for 1018 carbon, 304 and 316 stainless steels in 55%LiBr + ethylene glycol + H2O at 80 �C at Ecorr (at the end of the tests the Ecorr valueswere 490, �10 and �5 mVAg/AgCl, respectively).
Fig. 10. Electrical equivalent circuits used to simulate the EIS data.
Table 3Parameters used to simulate the EIS data.
Steel T (�C) Rs (X cm2) Rct (X cm2) Cdl (F/cm2)
1018 25 3.38 3550 3.9 � 10�4
50 3.26 762 1.7 � 10�3
80 0.263 7.51 1.9 � 10�3
304 25 0.211 5230 7.8 � 10�4
50 1.82 1860 5.9 � 10�4
80 0.94 284 3.3 � 10�4
316 25 2.22 4220 5.4 � 10�5
50 1.97 3770 5.5 � 10�4
80 1.31 690 3.1 � 10�4
tance, and, finally, W represents the Warburg impedance with amagnitude expressed by
½Zw� ¼ r½2k2=D�1=2 ð1Þ
where r is the Warburg coefficient given by
r ¼ b= nFCsð2DÞ1=2h i
ð2Þ
where k is the thickness of the diffusion layer, D is the diffusioncoefficient of the species involved, [Zw] is the diffusional resistanceor Rw, b the cathodic Taffel slope, n the number of involved elec-trons, F the Faraday constant and Cs the surface oxygen concentra-tion. The obtained Nyquist diagrams present two capacitive loops athigh frequency and a low frequency straight line or what appears tobe an incomplete inductive loop. This corresponds to a speciesadsorption process and afterwards a diffusion process (45� straightline), corresponding to pure diffusion with an infinite diffusionlayer. The straight line becoming an incomplete third loop foldingtowards the X axis corresponds to a diffusion adsorption processof species with a limited diffusion layer as suggested [20,21]. Thisbehavior is associated to mass transport control of the corrosionprocess. Thus, for 1018 carbon steel, at 25 �C, it seems that the cor-rosion process is under a mixed finite diffusion at high frequenciesand charge transfer from the metal to the solution at lower frequen-cies. However, at both 50 and 80 �C, where a 45� straight line ap-pears, a pure diffusion controlled process could be observed. Onthe other hand, for 316 type stainless steel, two capacitive loopsat high and low frequencies could be seen at 25 and 50 �C, corre-sponding to a species adsorption controlled process, whereas at80 �C a diffusion controlled mechanism is present now. From Table3 we can see that Rp decreased as the temperature increased; a sim-ilar behavior was observed on the polarization curves (Figs. 1–3),however, both the film resistance, Rf, and diffusion layer resistance,Rw. increased when the temperature increased from 25 to 50 �C, butthey decreased with a further increase in the temperature up to80 �C, but will discuss this below.
The relative magnitude of the charge transfer resistance, doublelayer capacitance and the Warburg coefficient associated to thestraight line, determine the extent at which the charge transfersemicircle is distorted by the Warburg impedance (mass transfereffects). Adsorption effects can result in a second semicircleappearing at low frequencies. If the effects of diffusion of two spe-cies are important, a Warburg element appears. As the results pre-sented in our case, after one or two semicircles, almost a straightline related to diffusion is sometimes present at lower frequencies,where the effect of diffusion and/or adsorption of oxygen or otherspecies on the electrode reaction is considered.
A 45� straight line represents a mass transfer process involvingpurely ionic diffusion effect. It is considered in this case a diffusionboundary layer to be infinite, i.e., the region of solution near thesurface where the concentration of species are different from thebulk concentration, could be of infinite thickness. The change of
1112 E. Samiento-Bustos et al. / Corrosion Science 51 (2009) 1107–1114
the angle degree of the Warburg impedance section, folding to-wards the real axis will mean that at some distance from the sur-face, the concentration of solution species are constant, due, forinstance, to convection in the solution; and this distance is theboundary layer thickness. From the impedance response obtainedfor the different materials and temperature conditions, these dif-ferent conditions were observed.
3.3. EN measurements
In order to evaluate the steels tendency towards localized typeof corrosion, noise measurements in both current and potentialwere performed. As an example, electrochemical potential and cur-rent noise time records for 1018 carbon steel after a trend removalare shown on Fig. 11. Removal of the DC trend from the raw noisedata was the first step in the noise analysis. To accomplish this, aleast square fitting method was used. The general appearance orstructure of electrochemical potential and current time records ap-peared to be similar for the two time series conditions, showingtransients of high intensity and low frequency, with an abrupt in-crease in their value and a slow decay, typical of localized type ofcorrosion such as pitting. The PSD (Power spectral density) in cur-rent for 1018 and 304 steels at 25 �C are shown on Fig. 12. It can beseen that the transients for 304 type stainless steel had higher
0 200 400 600 800 1000
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Time (s)
I (m
A/cm
2)
1018
-25
-20
-15
-10
-5
0
5
10
E (m
V vs
Ag/
AgC
l)
Fig. 11. Typical voltage and current time series obtained for 1018 carbon steel at25 �C.
10-4
10-3
10-2
10-1
100
10-5
10-4
10-3
10-2
10-1
Pow
er S
pect
ral D
ensi
y (A
2 / Hz)
Frecuency (Hz)
250C
304
1018 carbon steel
Fig. 12. Power spectral density in current for 1018 and 304 steels at 25 �C.
intensity than those for 1018 carbon steel, indicating a higher sus-ceptibility towards a film breakdown and the nucleation of local-ized type of corrosion.
The ratio between the potential noise and the current noisegave the noise resistance time series (Rn). The results are presentedin Figs. 13–15 for the different steels. We can see that the lowestaverage noise resistance value was obtained at 25 �C, and it in-creased at 50 �C; a further increase in the temperature up to80 �C brought a slight decrease in the average Rn value. If we com-pare this change of Rn with temperature with that exhibited by Rf
and Rw, Table 3, we can see that it is the same trend that both Rf
and Rw showed, but different to that exhibited by Rct. The Rn valueappears to be proportional to Rf rather than to the Rct value formany systems. Recently [22] it has been reported that the Rn valuewas a measure of the diffusion layer resistance, Rw, rather than toRct, when the metal is covered with a defected, porous film and theprocess is under ions diffusion control along the metal/film inter-face. This could be the reason why the Rn values increased withthe temperature, but Rct did not.
At 25 �C, some transients of high frequency could be observed,and they disappeared as the temperature increased. A decreasein the Rn value is associated to the rupture of the protective passivefilm, whereas an increase in its value corresponds to a repassiva-tion or healing of the passive film. Thus, the transients observed
0 200 400 600 800 100010-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
104
105
106
Rn
(O
hms.
cm2)
Time (s)
304
80°C
50°C
25°C
Fig. 14. Effect of temperature on the change of the noise resistance value, Rn, for304 type stainless steel in 55% LiBr + ethylene glycol + H2O.
101
103
105
Rn (
Ohm
s.cm
2)
Time (s)
25°C
50°C
80°C
1018
0 200 400 600 800 1000
Fig. 13. Effect of temperature on the change of the noise resistance value, Rn, for1018 carbon steel in 55% LiBr + ethylene glycol + H2O.
0 200 400 600 800 100010-2
10-1
100
101
102
103
104
105
106
107
Rn
(Ohm
s.cm
2 )
Time (s)
316
250C
800C
500C
Fig. 15. Effect of temperature on the change of the noise resistance value, Rn, for316 type stainless steel in 55% LiBr + ethylene glycol + H2O.
E. Samiento-Bustos et al. / Corrosion Science 51 (2009) 1107–1114 1113
at 25 �C could be associated to the unstability of this film. Theoccurrence of current transients is known to be caused by the ini-tiation, temporary growth and repassivation of individual metasta-ble pits, or breakdown and subsequent repassivation of passivefilms and is caused by the absence of a stable film. Increase inporosity could be one of the main changes in film properties withan increase in the temperature which might originate current, volt-age or resistance transients effects as those observed on Figs. 12–14. Similar results were obtained by Russell and Newman [23,24]when tested iron in sulfuric acid and in other sulfate-containingsolutions. The proposed model by these authors was a continuouscycling between active and passive states, including the formationof a porous film of sulfate, which was considered the be precursorof the passivation phenomenon. Transient pulses such as the onesobserved in Fig. 11, are similar to the ones observed by them. In aLiBr + ethylene glycol solution, such a porous film could be a mix-ture of iron bromides (a mixture of FeBr2 and Fe2Br3) and ironhydroxides, for 1018 carbon steel, and a double film of chro-mium(III) oxide with iron bromides [7]. It is generally acceptedthat for alloys containing big amounts of chromium, Cr2O3 basedproducts form layers and they are responsible for the superior cor-rosion resistance of stainless steels [25]. However, the chromiumcontents in carbon steel is so low as to form a continuous Cr2O3
layer, so the passive behavior of this steel in Figs. 1–3 can not beexplained on the existence of this chromium oxide. Instead, theexistence of a double layer of iron bromides and iron hydroxides,due to the presence of the OH group in the ethylene glycol, canbe accounted for the passive behavior of 1018 carbon steel.
Wang et al. [26] have reported an effect of increased tempera-ture on the protective properties of the passive films, resulting ina decreased resistance to breakdown. They attributed this to tworeasons. One, the porosity of the passive films increases with tem-perature, as it is often assumed. Two, the passive film undergoes anintrinsic modification of its chemical composition and/or physicalstructure resulting, for example, in a variation in the density ofvacancies or voids in the passive film. Thus, from these observa-tions, it can be said that the protective properties of the passivefilms decrease as the temperature of the electrolyte increases, ascan be seen from Figs. 4–6, from the evolution of the corrosion,passive current density and pitting potential values. So, pit initia-tion appears related in all cases to a loose in the protective charac-teristic of the passive layer and to the presence of aggressivebromide ions in the electrolyte. However, if a sufficient amountof aggressive ions are present and the environment is oxidizing en-
ough, such as the heavy bromide solutions studied here, localizedcorrosion sites have been observed to initiate anywhere.
All results have shown that the best passive properties wereexhibited by 316 type stainless steel, whereas the worst passivefilm was the formed on 1018 carbon steel. Carbon steel does notcontain chromium, so the passive film must contain adsorbed ionsfrom the solution and its reaction with steel. For 304 and 316 typestainless steels, the main passive film must contain chromiumoxide, Cr2O3, however, the difference among them is that 316 con-tains between 2% and 3% molybdenum whereas 304 stainless steeldoes not contain it. The enhanced passive properties in presence ofMo was reported to be caused by the adsorption of molybdates orthe formation of Mo-rich precipitates which are incorporated intothe passive film and decreased the anodic metal dissolution [27]. Ithas been proposed that during the passive current density mea-surements, molybdenum appreciably reduces the number ofrepassivation events, as evidenced by transients shown on Figs.13 and 14. Polarization curves also showed the beneficial effectsof Mo in reducing active dissolution and in increasing the pittingpotential value, Epit; but in such a concentrated bromide solution,chromium plays an important role, as evidenced in the polarizationcurves.
4. Conclusions
The corrosion behavior of 1018 carbon steel and 304 and 316type stainless steels has been evaluated in a 55% LiBr + ethyleneglycol + H2O at 25, 50 and 80 �C. The main results can be summa-rized as follows:
(a) At all tested temperatures, the three steels showed anactive–passive behavior, where the passive and corrosion currentdensity values for 1018 carbon steel were between two and fourorders of magnitude higher than those found for both stainlesssteels. The pitting potential value was higher for 316 followed by304 type stainless steel, whereas the most active values were for1018 carbon steel.
(b) For 1018 carbon steel, it seems that the corrosion processwas under a mixed diffusion and charge transfer from the metalto the solution at 25 �C. However, at both 50 and 80 �C a pure dif-fusion controlled process could be observed. On the other hand, for316 type stainless steel, at 25 and 50 �C a species adsorption con-trolled process was observed, whereas at 80 �C a diffusion con-trolled mechanism was present.
(c) At 25 �C, the three steels were more susceptible to uniformtype of corrosion, whereas at 50 and 80 �C they were very suscep-tible to a localized type of corrosion such as pitting. The Rn valuesfollowed the same tendency with the temperature than that exhib-ited by Rf, but the Rct values followed a different behavior, becausethe metal was covered with a defected, porous film.
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