study of molten salt corrosion of hk-40m alloy applying linear polarization resistance and...

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Corrosion Science 46 (2004) 2663–2679

Study of molten salt corrosion of HK-40malloy applying linear polarization resistanceand conventional weight loss techniques

C. Cuevas-Arteaga a,*, J. Uruchurtu-Chavar�ın a,J. Porcayo-Calderon b, G. Izquierdo-Montalvo b, J. Gonzalez a

a Facultad de Ciencias Quimicas e Ingenieria, Universidad Autonoma del Estado de Morelos,

Av. Universidad 1001, Col. Chamilpa, C.P. 62210 Cuernavaca, Mor., Mexicob Instituto de Investigaciones Electricas Palmira 113, C.P. 62440 Temixco, Mor., Mexico

Received 13 March 2003; accepted 5 March 2004

Available online 17 April 2004

Abstract

Corrosion rates from electro-chemical polarization resistance technique (LPR) and weight

loss method (WL) of HK-40m alloy exposed to 80 mol% V2O5–20Na2SO4 at 600 and 700 �Cwere obtained at a maximum time of 10 days. Results were supported by X-ray diffraction and

electron microscopy analysis. A comparison of corrosion rates from both techniques indicated

that corrosion rates from LPR were higher than that from WL, being the values more or less

in the same order of magnitude. At 600 �C corrosion rates values were twofold; whereas at 700

�C threefold. The difference in results from both techniques was mainly explained by the fact

that V2O5 behaves as a semiconductor oxide, and even though Na2SO4 is totally ionic, the

corrosion mechanism with this mixture may not display a purely electro-chemical process.

Some qualitative characteristics were observed for both techniques.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: High temperature corrosion; Hot corrosion; Polarization; Weight loss

1. Introduction

Metallic materials may suffer accelerated corrosion at high temperatureswhen their surfaces are in contact with a thin film of molten salts in an oxidizing

* Corresponding author. Tel./fax: +77-73-29-70-84.

E-mail address: [email protected] (C. Cuevas-Arteaga).

0010-938X/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2004.03.002

mail to: [email protected]

hp

Resaltado

2664 C. Cuevas-Arteaga et al. / Corrosion Science 46 (2004) 2663–2679

environment. This phenomenon can be seen in the gas side of power station boilers

when the fuel is a residual oil. This type of corrosion is called high temperature

corrosion, occurring on the heating surfaces of furnaces super-heaters or re-heaters,

and it is due to the formation of ashes during combustion processes. Ashes have a

high concentration of compounds formed by vanadium, sodium and sulphur, mainly

as Na2SO4–V2O5 complex and sodium–vanadates mixtures [1–4]. Some mixtures ofthese compounds have low melting points ranged from 480 to 510 �C; which are

smaller than metallic surfaces temperatures, turning into a liquid state and increasing

the corrosion rates.

Electro-chemical techniques have been used to study high temperature corrosion

phenomena; differences were found between corrosion rates from weight loss and

from LPR, attributed to an excessive chemical cleaning made to specimens from

weight loss method [5–7]. Other studies performed [8–10] using polarization curves,

showed electronic characteristics of corrosion mixture, which were related to themetal elements and oxides dissolved in molten salt. Simulating furnace wall type

corrosion encountered in boilers, experimental polarization curves were obtained

showing ohmic type resistive characteristics and no indication of polarization.

Electro-chemical impedance technique was also applied, showing that the Nyquist

diagram indicated purely electronic characteristics, resulting from the electronic

properties of the oxide layer, and no indication of capacitive (ionic) components

were observed. It was also mentioned, that the estimation of corrosion rates using

electro-chemical data, when significant electronic conductivity exists in the scales areincorrect [11].

In this paper, the corrosion resistance performance of a modified HK-40m alloy

exposed by immersion for 10 days in deep crucible melts of 80 mol% V2O5–

20Na2SO4 at 600 and 700 �C was evaluated. Two electro-chemical techniques such as

LPR and polarizations curves were used and compared to the weight loss method.

X-ray diffraction analysis and micro-structural observations by scanning electronic

microscopy of the corrosion products of the exposed specimens, helped to determine

qualitatively the internal resistance of the alloy and help to the understanding ofcorrosion mechanism under high vanadium molten salts.

2. Experimental procedure

The corrosion mixture was made from analytical grade reagents: 80 mol% V2O5–

20Na2SO4, and the test temperatures were 600 and 700 �C. The amount of molten

salt used for the three techniques in each experiment was 500 mg/cm2, which was

placed in a 20 ml silica crucible. For the three techniques, the atmosphere above the

melt was static air. The specimens were prepared as rectangular parallelepipeds sized

10 · 5 · 3 mm for linear polarization resistance technique and weight loss method,

and 5 · 4 · 2 mm for polarization curves. These specimens for electro-chemicaltechniques were the working electrodes, to which one 80 wt.% Cr–20Ni wire was spot

welded. This wire was used as electrical connection between the working electrode

and the potentiostat. The specimens were ground to 600 grade emery paper, washed

Fig. 1. Potential measurements between two platinum reference electrodes exposed to 80 mol% V2O5–

20Na2SO4 at 700 �C.

C. Cuevas-Arteaga et al. / Corrosion Science 46 (2004) 2663–2679 2665

in distilled water, degreased with acetone and dried under an air stream. For electro-

chemical techniques, the reference electrode and counter electrode were a 5 mmdiameter and 150 mm long platinum wire. In previous research, the platinum elec-

trode was tested to assess its stability as a reference electrode under the experimental

conditions considered in the present work, such as mentioned in some reported

studies [12,13], see Fig. 1.

It is important to point out that in high temperature corrosion phenomenon,

alloys are covered with a thin molten salt layer and with a corrosive gas atmosphere,

whereas the experimental procedure carried out in this work were deep melt tests;

therefore the experimental conditions and corrosion rates obtained are not repre-sentative of fireside corrosion conditions. However, bulk molten salt tests are a

viable proposition for material corrosion evaluation.

2.1. Procedure for electro-chemical tests

The 20 ml silica crucible with the corrosion mixture was placed inside an electrical

furnace. Before exposing, the samples were measured in order to determine the area,

which had to be in contact with molten salt. When the test temperature was stabi-

lized, the three electrodes cell was introduced inside the molten salt; the connectionbetween the electrodes with the potentiostat was made immediately. Polarization

curves were obtained 40 min after exposing the specimen to the corrosion mixture

when the corrosion potential reached a stable condition. The instantaneous linear

polarization resistance data were taken every 30 min during a maximum period of 10

days.

Electro-chemical test were made using an ACM Instruments Auto DC poten-

tiostat, controlled by a personal computer. The polarization of the working electrode

for LPR and polarization curves was accomplished potentiodynamically at scan rateof 1 mV/s. Previously some polarization curves were obtained at different scan rates

to observe the effect of the sweep rate on the corrosion conditions, see Fig. 2, which

will be explained later. For obtaining LPR data, the specimen was polarized at

-2000

-1500

-1000

-500

0

500

1000

1500

2000

1 10 100 1000

Current Density (mA/cm2)

Pote

ntia

l (m

V) P

t Ref

eren

ce E

lect

rode

. 1 mV/s

0.5 mV/s

0.2 mV/s

Fig. 2. Polarization curves obtained from the exposure in 80 mol% V2O5–20Na2SO4 mixture at 700 �C,applying different potential sweep rates.


±10 mV with respect to the free corrosion potential Ecorr; whereas for obtaining

polarization curves the specimens were polarized at ±150 mV respect to the same

potential.

2.2. Procedure for weight loss tests

Standard weight loss tests were also carried out under the same experimental

conditions than that for LPR and polarization curves tests, following the procedure

according to ASTM G31 [14]. In these experiments, two different specimens for each

day were used, except at the 10th day where three specimens were used. The samples

were weighed before exposure by means of an analytical and a digital balance with a

precision of 0.0001 g. Twenty-one specimens were totally packed in corrosive mix-

tures contained in independent silica crucibles, and then they were introduced in anelectric furnace in a static air atmosphere. Each day two specimens were removed

from the furnace until period completion. The corrosion products were removed

from the set of two corroded alloys before the final weighing by mechanical and

chemical cleaning according to ASTM G1 standard [15]; except the third sample of

the 10th day, which was kept to be analyzed by SEM.

The mechanical procedure consisted in cleaning the surfaces of specimens in a

repetitively way until the surfaces were free of corrosion products, being the mass

loss determined after each cleaning by weighing the specimens. The specimens werescrapped with at least two different sharp metallic tools, trying to get out the solid

scales, which were yellow and brown in color. These characteristic colors helped a lot

not to remove any base metal. As it was explained before, the exposure was made by


duplicate, and when the differences in weight loss at the final weighing was more than

10%, the test was repeated. In this way, the results precision was controlled. Besides,

at the last two or three weighing, the specimens were observed by means of an optical

microscope, which was useful to determine the remains of corrosion products. In this

way, the probability to affect the real weight loss measurement was tried to keep it

low. The specimens free of corrosion products were washed with acetone, and thenweighed using an analytical balance. The evaluation of the corrosion rate was

determined as weight loss by means of calculating the difference between the initial

and final weight divided by the initial area of specimens for each day.

X-ray studies on corrosion products were performed, in order to determine the

kind of metal oxides the alloy developed, only the inner film of the scale was ana-

lyzed. In such a way the oxidation valences of metal oxides was determined. These

analyses were made by means of a Siemens D-500 Diffractometer, operating with Cu

Ka radiation, whereas the interpretation of results was made taking into account thepowder diffraction data file reference [16]. Also, one of the three specimens exposed

for 10 days at each temperature was mounted (without de-scaling) in bakelite,

metallographically polished and the cross section analyzed by scanning electron

microscopy to investigate the morphology and distribution of reaction products.

X-ray mapping and microprobe analysis were carried out using a Microspec WDX-

3PC system connected to a Zeiss DSM960 scanning electron microscope.

3. Results

3.1. Microstructural observations

Fig. 3 shows a micrograph of a cross section of the corroded specimen exposed to

the melt at 600 �C during 10 days after applying the weight loss method. X-ray

mappings of chromium, nickel, iron, vanadium and sulphur are also presented.

According to the X-ray diffraction results (Fig. 4), at the beginning of exposure HK-

40m alloy developed a Cr2O3 protective oxide and Fe2O3, which have been dissolved

by molten salts along the time of the exposure (see mappings of Cr and Fe in Fig. 3).

The chromium oxide is one of the most protective compounds when molten salts are

present. Wong and Marchan Salgado [4], have reported that the protection of theoxide scale is dependent upon the formation of a dense and coherent layer, which

must be free of cracks, and can limit the transport of the gas and corrosion species to

the metal surface. Just in the metallic surface-corrosion products interface, there is a

chromium-depleted irregular zone, also indicating the dissolution of Cr2O3. With

respect to the nickel mapping, it can be noticed the presence of a continuous film

with a certain concentration of nickel in the metal–oxide scale interface, taking the

place of the initially formed Cr2O3. It can be assumed that nickel has diffused from

the substrate to the scale, to form a 30 lm thickness film, which after 10 days ofexposure appears as a protective layer, although the results from XRD did not

showed this oxide (see Figs. 4 and 5). This is maybe because there was a very low

concentration of nickel oxide. After 10 days of exposure, a nickel protective scale

Fig. 3. Electron image of the metal–scale interface and X-ray mappings of Cr, Ni, Fe, V and S of HK-40m

after exposing 10 days to 80 mol% V2O5–20Na2SO4 at 600 �C.


was formed, providing a good corrosion resistance when molten salts have a high

concentration of vanadium and low concentration of sodium, according to Wilson

[17]. Under such conditions, the alloy is now lower in Ni and Cr, the last one due to

the accelerated oxidation and subsequent dissolution of its oxide. With regard to the

Cr2O3 formed at the beginning of exposure, it is possible that this oxide had been a

good protective compound in such conditions of high vanadium, and together with

Fig. 4. X-ray diffraction results of corrosion products of HK-40m exposed one day to 80 mol% V2O5–

20Na2SO4 at 600 �C.

Fig. 5. X-ray diffraction results of corrosion products of HK-40m exposed 10 days to 80 mol% V2O5–

20Na2SO4 at 600 �C.


the NiO scale developed under the Cr2O3 film, protected the alloy, since the HK-40m

presented low corrosion rates and no internal degradation. The iron is dispersed into

the corrosion products layer, but in a lesser extent than chromium, which implies

that it has been oxidized and dissolved by vanadium compound. The mapping of

vanadium presents an intermediate concentration of this element in the corrosion

products layer, showing the chemical interaction with chromium and iron, and to a


lesser extent with nickel. The mapping of sulphur shows the low concentration of this

compound in the high vanadium mixture, indicating no effects in the corrosion

mechanisms and no evidence of internal degradation by sulphidation.

A SEM micrograph for HK-40m alloy and its respective mappings of the prin-

cipal elements and sulphur are presented in Fig. 6. The micrograph shows the metal–

Fig. 6. Electron image of the metal–scale interface and X-ray mappings of Cr, Ni, Fe, Si and S of HK-40m

after exposing 10 days to 80 mol% V2O5–20Na2SO4 at 700 �C.


corrosion products interface of the corroded HK-40m alloy after 10 days of exposure

during weight loss tests to 80 mol% V2O5–20Na2SO4 at 700 �C. Approximately, 150

lm thickness deep into the substrate, the chromium has been depleted by the effect of

oxidation. In the chromium depleted zone into the metal, there is an important

concentration of nickel and iron, seeming that nickel did not react in a significant

way during the corrosion process, while the iron has taken part in a more importantway. Also, it can be observed a chromium-rich region in the corrosion products

layer, where the nickel and iron counting is low, being the chromium surely enough

related with corrosion compounds as an effect of dissolution by molten salts. The

mapping of silicon shows an irregular distribution in the metal–scale interface,

meaning that the alloy attempted to protect itself developing a silicon oxide, which

in alloys containing chromium improves the corrosion resistance under conditions

of residual oil combustion due to the low ionic diffusion rates through its oxides

[3].Just on the metal surface where it is expected that the Cr2O3 scale was initially

formed, which is consistent with the information obtained from XRD after one day

of exposure (see Fig. 7); there is a low concentration of sulphide phase in the metal–

scale interface which had not penetrated into the metal substrate. This sulphide zone

is associated on the right side (in a short area), mainly with chromium and to a lesser

extent with silicon along the sulphide region, forming external sulphides. This sul-

phide phase can be easily identified in the mappings for S, Si and Cr.

In contrast to the behavior at 600 �C where there was no evidence of sulphi-dation, at 700 �C, even thought the concentration of sulphur in the corrosion

mixture is low, it has been noticed the participation of sulphur in the corrosion

mechanism, showing the effect of temperature in the corrosion process. Oxidation

and sulphidation processes increased with temperature, as evidenced from the

Fig. 7. X-ray diffraction results of corrosion products of HK-40m exposed one day to 80 mol% V2O5–

20Na2SO4 at 700 �C.


corrosion rates obtained from LPR and weight loss method, which were higher

than the obtained at 600 �C.

3.2. X-ray diffraction results

X-ray diffraction spectra results for the corrosion products obtained for HK-40m

alloy exposed to 80 mol% V2O5–20Na2SO4 at one and 10 days of exposure, are

presented in Figs. 4 and 5 for 600 �C and 7 and 8 for 700 �C. With respect to these

results, it can be seen that corrosion products were NiO, Cr2O3, Fe2O3 and the

FeCr2O4 spinel. Other important phases such as sodium vanadyl vanadates were

obtained: 1.1.5 Na2O ÆV2O4 Æ 5V2O5 (m.p. 625 �C), 5.1.11 5Na2O ÆV2O4 Æ 11V2O5

(m.p. 535 �C), Na8V24O63, and NaV2O8. The presence of 1.1.5 and 5.1.11 compoundshas been reported as specially corrosive, due to V4þ and V5þ couple facilitate oxygen

transport through the melt, and these are in good agreement with respect to those

compounds identified in this work [18–21]. Furthermore, vanadates are able to

dissolve or destroy the normally protective oxide layer, exposing metallic surface for

further oxidation [21].

3.3. Electro-chemical measurements

As it has been mentioned in Section 2, in order to check if the platinum reference

electrode was stable under the experimental conditions considered, another platinum

electrode was immersed in the working salt and the potential of the first electrode

Fig. 8. X-ray diffraction results of corrosion products of HK-40m exposed 10 days to 80 mol% V2O5–

20Na2SO4 at 700 �C.


was monitored with time once the temperature was stabilized. Accordingly with Fig.

1, the potential at the beginning was 55 mV nobler than the second platinum elec-

trode, but after 30–40 min this difference was very stable, having a fluctuation of

about 3 mV, as seen in Fig. 1.

The Tafel slopes polarization curves were obtained for the experimental condi-

tions tested. To obtain information about the sweep rate, Fig. 2 presents polarizationcurves obtained at different sweep rates. Similar curves were obtained, except for a

small difference obtained for the low sweep rate at high cathodic over-potential (1000

mV). The anodic branch remained the same for the three different sweep rates,

showing a passive region at high anodic over-potentials.

The polarization curves in the Tafel region which is not very well defined, ob-

tained experimentally for the two temperatures are presented in Fig. 9. Through

these curves, the Tafel slopes were determined, being: ba ¼ 74 mV/decade and

bc ¼ 80 mV/decade for 600 �C, and ba ¼ 78 mV/decade and bc ¼ 76 mV/decade at700 �C.

Fig. 10 shows the experimental linear polarization resistance in time obtained

from LPR technique for HK-40m alloy exposed to 80 mol% V2O5–20Na2SO4 at 600

and 700 �C. As it can be seen, in general the polarization resistance at 700 �C is

smaller that that at 600 �C, indicating that the corrosion rates can be expected higher

at the higher temperature. After almost 8 days, the polarization resistance at 600 �Cis increasing until the end of the experiment; this behavior suggests the material is

achieving a passive state. To convert the linear polarization resistance (ohms cm2) tocorrosion current density Icorr (mA/cm2), it was necessary to apply the Stern–Geary

equation (Eq. (1)) being a function of the Tafel slopes [22]. The Tafel slopes were

used in Eq. (2) to calculate the Tafel constant B. Subsequently, the Faraday law (Eq.

(3)) was used to obtain the mass loss through the use of Icorr data.

Icorr ¼BLpr

ð1Þ

Fig. 9. Polarization curves for HK-40m exposed to 80 mol% V2O5–20Na2SO4.

Fig. 10. Linear polarization resistance for HK-40m exposed to 80 mol% V2O5–20Na2SO4.


being B a relationship between the Tafel slopes:

B ¼ babc2:303ðba þ bcÞ

ð2Þ

Applying the Faraday Law [22], where the mass loss M is given in g/cm2 min:

M ¼ KIcorrðEWÞ ð3Þ
EW is the equivalent weight given by
EW ¼ 1P nifi

AWi

ð4Þ

where ni is the number of transferred electrons during oxidation process for eachelement i of the alloy (2 for Ni, and 3 for Fe and Cr, according to the X-ray dif-

fraction results), fi is the weight fraction of element i and AWi is the molecular weight

of element i. To calculate the overall mass loss at the end of each day, it was nec-

essary to integrate all data records. This was done making an integration of each

datum multiplied by the interval time at which the linear polarization resistances

were taken. The integration equation (5) was used next to obtain the mass loss in

g/cm2.

MLj ¼X48j

i¼1

MDt ð5Þ

being i the number of data obtained experimentally and j the days of exposure.

3.4. Corrosion rates

After using Eqs. (1)–(5), and obtaining the cumulative mass loss for each day, it

was possible to make a comparison between results obtained from weight loss and

Fig. 11. Comparison of mass loss obtained from weight loss method and LPR techniques for HK-40m

exposed to 80 mol% V2O5–20Na2SO4.


linear polarization resistance techniques, which is presented in Fig. 11. As can be

observed, data from LPR at 700 �C were obtained up to 8 days only; this was due to

the specimen was totally dissolved before the planned time of exposure. It is evident

that the LPR mass loss obtained daily for both temperatures is higher than that

obtained from the weight loss method; being the values more or less in the same

order of magnitude. For both temperatures the trend in the corrosion behavior is the

same. At 600 �C the difference is twofold, whereas at 700 �C is threefold. The effect oftemperature is evidenced by the increase in mass loss obtained with both techniques.

Fig. 12. Comparison of corrosion rates from weight loss method and LPR data for HK-4m exposed to 80

mol% V2O5–20Na2SO4.


There is a possibility that underestimation of the polarization resistance (and hence

over-estimation of the mass loss), can occur by using a higher sweep rate and thus

measuring in non-steady state system conditions.

To determine a daily average of corrosion rate, each datum from Fig. 11 was

divided by its corresponding time of exposure, obtaining the plots presented in Fig.

12. At 600 �C the qualitative behavior is similar for both techniques as expected, andthe corrosion rate values decreased from 0.0096 to 0.0035 g/cm2 day for the weight

loss method and from 0.035 to 0.0092 g/cm2 day for LPR technique. At the higher

temperature the corrosion rate from LPR is always decreasing until the end of

exposure, whereas the corrosion rate from weight loss method had a similar behavior

than that from LPR during the first 5 days, increasing slightly afterwards.

4. Discussion

The electron probe microanalysis of corroded HK-40m alloy exposed to the

corrosion mixture at 600 and 700 �C during 10 days showed the dissolution of

chromium at both temperatures, especially at 700 �C, indicating that oxidation anddissolution increases with temperature. It could be observed that at the beginning of

exposure, the alloy developed a Cr2O3 protective oxide. The mapping of nickel shows

almost no dissolution, and only in the case at 600 �C it formed an intermediate

concentration film of NiO in the metal–scale interface. At 700 �C, it was noticed the

formation of an irregular and porous silicon film, giving some protection to the

alloy. An important feature of the corrosion mechanism at 700 �C was a low degree

of external sulphidation associated with Cr and Si, although the concentration of

sulphur in the corrosion mixture is low.At 700 �C the corrosion rate for both techniques was higher than at 600 �C, and

total dissolution was observed at this higher temperature. The corrosion activity

increased with the temperature; this was mainly due to a decrease in the viscosity of

the molten salt and an increase in the electric conductivity of the mixture, thus

favoring the diffusion of chemical active species. It is expected that the greater dif-

fusion of active vanadate species increases the electro-chemical cathodic reaction,

having a concentration depolarization [6,7,23]. As can also be observed in the results

obtained, the linear polarization resistance at 700 �C was very low, producing highcorrosion rates. In conclusion, it most be considered that the total dissolution

phenomenon originated by the presence of a thin film of molten salts mixtures are

very depolarizing, generating high dissolution rates at 700 �C.The conventional weight loss method, even though with some limitations, is a

reliable technique, which is capable to evaluate the performance of metallic materials

used for residual oil boilers. The application of electro-chemical techniques repre-

sents a powerful tool to obtain some aspects not possible to determine by weight loss

technique, for example: instantaneous corrosion rates, type of corrosion (generalizedor localized), and corrosion mechanism and process-controlling step. In this work,

the differences in corrosion rates from both techniques are maybe due to melts high

in vanadium pentoxide retain essentially semiconducting characteristics [17,24,25],


so it is expected that transport processes in the ionic melts occur by ion movement

(for example Na2SO4, which is totally ionic), while in the semiconducting melts will

also involve electron transfer processes. Also, the corrosion activity of a mixture

depends on its electric conductivity. As it is expected, as the temperature is increased,

the mobility of the ions of the salt is increased and in that way also the conductivity

[13].On the other hand, it is known that 80 mol% V2O5–20Na2SO4 is very acidic,

extremely corrosive, potentially oxidant, and it has a great capability for absorbing

oxygen when it is in a molten state [17,18,24,26]; hence, it is expected that dissolution

of metallic oxides or metallic surface has an important influence upon the changes of

melts, with respect to transforming from an ionic to an electronic molten salt by the

interaction of metal elements dissolved in the melt. With regards to this change,

Farrel et al. [27] have pointed out that when molten phases change from ionic to

electronic conductor, electro-chemical monitoring may have some problems. Thiscan be also, one of the main reasons for the differences observed in the results ob-

tained from both techniques.

According to Scully [22], there are some complications related to the polariza-

tion resistance technique; one of them that maybe applied in this work, is the

presence of variable valences as part of the electro-chemical processes of the cor-

rosion of metals, and the electro-chemical oxidation of some other electro-active

species besides that from the corroding metal (for example species of vanadium or

sulphur). Although a XRD analysis was performed of the corrosion products,being this analysis not so extensive (only the inner film of corrosion products was

analyzed), it could not be possible to detect some other species, and since the

oxidation valences of metal oxides considered for calculations did not lead to

corrosion rates similar to that from weight loss method. Also, there is the possi-

bility that the main corrosion products are not the soluble ones. A formation of a

secondary corrosion product layer adherent to the surface will violate the condi-

tions needed for the application of the simple Stern–Geary equation for the esti-

mation of the corrosion rate. Indeed, Figs. 11 and 12, between 2 and 5 h for thehigher temperature condition, the plateau observed in the weight loss measure-

ments plot suggests the existence of a protective corrosion product layer over the

surface.

Some other complications mentioned by Scully are the following: the use of a

large over-potential, invalidating the assumption of a linear relationship between the

current density and potential, and the possibility of anodic and cathodic reactions

are not charge transfer controlled processes, as required for the derivation of Eq. (1).

In fact, the polarization curves did not show a clear Tafel region, the curves wereplotted in a linear–linear scale, obtaining an almost linear plot, suggesting an ohmic

behavior and hence electron transfer reactions proceeding in parallel to the charge

transfer corrosion process.

On the other hand, results from electro-chemical impedance spectroscopy (EIS)

were additionally obtained under the same conditions than that for LPR, indicating

that the rate controlling step in the corrosion process was by charge transfer at both

temperatures, the results of which will be published later [28,29].


5. Conclusions

A comparison of corrosion rates from electro-chemical polarization resistance

technique and weight loss method have been determined for the HK-40m alloy ex-

posed at a maximum time of 10 days to 80 mol% V2O5–20Na2SO4 at 600 and 700 �C.This comparison indicated that corrosion rates from LPR were higher than thatfrom the weight loss method. The difference in results from both techniques was

mainly attributed to the fact that vanadium pentoxide behaves as a semiconductor

oxide, and even though Na2SO4 is totally ionic, the corrosion mechanism with this

mixture may not display a purely electro-chemical process. Another reason was the

possible presence of variable valences as part of the electro-chemical processes of the

metal corrosion and/or the oxidation of some other electro-active species besides that

from the corroding metal.

It is evident that LPR technique maybe a powerful tool for monitoring hot cor-rosion processes when the controlling-step is by charge transfer, but such technique

induce some errors when the molten salt behaves partially as a semiconductor

compound.

Acknowledgements

The authors gratefully acknowledge the support of this research by the Chemical

Science and Engineering Faculty of Morelos University and the Electrical Research

Institute, both in Mexico. The authors also acknowledge the technical support ofRene Guardian from the Physics Research Center-UNAM, and Carlos Limon from

the Electrical Research Institute.

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study of molten salt corrosion of hk-40m alloy applying linear polarization resistance and...

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