Corrosion Monitoring Using Electrochemical Noise

ISSN 1023-1935, Russian Journal of Electrochemistry, 2006, Vol. 42, No. 5, pp. 560565. MAIK Nauka /Interperiodica (Russia), 2006.Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 5, pp. 627632.

560

INTRODUCTION

Corrosion monitoring for industrial and processapplications is a key issue to achieve high standards ofreliable and safe operation. Different techniques formonitoring corrosion rate have been developed [1, 2].However, the choice of the right technique and theappropriate corrosion probe design is very importantfor a successful online corrosion monitoring program.

Linear polarization resistance (

LPR

) is one of thefirst techniques used for corrosion monitoring. Thistechnique is well known and documented [3, 4]. Lately,the electrochemical noise technique (

EN

) has beenincorporated for corrosion monitoring in differentindustries.

In the power industry, EN has been applied to mon-itor corrosion of materials performing in a high-tem-

perature gas environment with the presence of moltensalts characteristic of coal-fired boilers for monitoringfurnace wall corrosion [57] and superheater corrosion[8].

Molten deposits formed on furnace wall tubes ofcoal-fired boilers consist of a mixture of alkali-metalsulfate mixed with iron sulfide and pyrosulfates [9].However, in superheater tubes, composition of depositsformed are mainly alkali iron trisulfates [10].

Composition of molten deposits formed in fuel oilboilers is different from those formed in coal-fired boil-ers because of the nature of the fuel.

Residual fuel oil contains sodium, vanadium, andsulfur impurities, forming compounds like

Na

2

SO

4

,V

2

O

5

, and complex vanadates by reaction in the com-bustion system [11]. Complex molten vanadates are rel-atively low melting-point compounds acting as a flux

Corrosion Monitoring Using Electrochemical Noise and Linear Polarization Resistance

in Fuel Oil Combustion Gas Environment*

V. M. Salinas-Bravo,

a,z

J. Porcayo-Calderon

a

, and J. G. Gonzalez-Rodriguez

b

a

Instituto de Investigaciones Electricas, av. Reforma 113 Col Palmira, Cuernavaca, Morelos 62490, Mexico

b

CIICAp, UAEM, av. Universidad S/N, Cuernavaca, Morelos 62210, Mexico

Received July 4, 2005

Abstract

Corrosion monitoring of different steels is carried out online in a combustion rig firing 32 kg/h offuel oil. Two temperature-controlled probes are designed to allow control of the specimens temperature and theuse of electrochemical noise (EN) and linear polarization resistance (LPR) techniques for corrosion monitoring.Two probes are placed where the combustion gas reached a temperature of

850900

, and another one at thecombustion gas exit where the rig was at

200240

. Corrosion rates of an austenitic and a ferritic steel areobtained where the temperature of the combustion gas is

850900

, firing fuel oils with different content ofNaVS. Corrosion monitoring of mild steel is carried out in the test burning a fuel oil with the higher contentof NaVS by placing a corrosion probe in the low combustion gas temperature zone. The EN results show thatthis technique is able to assess the corrosion rate in an environment at high temperature where fuel oil ashesdeposited and at a temperature high enough where they start to melt and a corrosion process proceeds. Resultsshow that this technique is able to assess the corrosivity of fuel oil ashes originated from fuel oil containingdifferent amounts of sodium, vanadium, and sulfur as corrosion causing impurities. Results of the low-temper-ature probe show that EN and LPR are able to detect the onset of corrosion on mild steel as a result of sulfuricacid condensation on the probe. However, the corrosion rates are not the same, because localized corrosion istaking place as detected by the EN technique. It is demonstrated that the use of two techniques for corrosionmonitoring can give a better understanding of the corrosion process. Electrochemical techniques used to assessthe corrosion resistance of alloys at high and low temperatures prove to be a valuable tool for the purposes ofmaterials selection or controlling the main process variables that affect the corrosion resistance of materials inindustrial equipment.

DOI:

10.1134/S1023193506050156

Key words

: online corrosion monitoring, electrochemical noise, linear polarization, high-temperature corro-sion, fuel oil ashes

* The text was submitted by the authors in English.

z

Corresponding author, email: [email protected]

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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No. 5

2006

CORROSION MONITORING USING ELECTROCHEMICAL NOISE 561

removing protective oxide scales from superheatertubes, producing accelerated corrosion rates [12].

Corrosion monitoring using EN and electrochemi-cal impedance (EI) in gas environments where dewpoint corrosion may be the case has been done undercondenser nitric acid conditions [13] and in dual alkaliflue gas desulfurization system [14].

Online corrosion rate measurements using electro-chemical noise have not been reported in environmentsof molten ashes resulting from burning fuel oil.

The objective of this work is to investigate theresponse of the EN technique to assess the corrosionrate of boiler tube materials exposed to combustion gasat high temperature firing two fuel oils with differentcontent of sodium, vanadium, and sulfur, and determinethe corrosion rate of mild steel by applying EN andLPR techniques simultaneously in the low-temperatureflue gas.

EXPERIMENTAL

Specially designed corrosion probes were used forcorrosion monitoring. Figure 1 shows a schematicdrawing of the probes used. The main body of the probeconsists of a 63.5 mm outside diameter stainless steeltube attached to a flange. Seven electrodes separated bya high-temperature resistant insulation material arepositioned at the end of the tube. The thickness of elec-trodes is 5.0 mm and they were fabricated from ASTMSA 213 T 22, ASTM SA 213 304, and carbon steel tubematerials. A type K thermocouple was spot-welded toelectrodes

1

and

7

on the inner diameter surface. AnNiCr wire was spot welded to the rest of electrodes inorder to allow electrical contact with the electrochemi-cal hardware. The NiCr wires were isolated by aceramic fiber sleeving to avoid electric contact with theprobe body. The temperature of the electrodes in eachprobe was controlled by means of a PID temperaturecontroller. Cooling media was provided by an instru-mented system that supplied filtered and dehumidifiedair. Air flow to each probe was controlled automaticallyby the set point in the temperature controller.

Electrochemical hardware used for corrosion moni-toring was an industrial type equipment. It has severalmodules for acquiring, applying, transmitting, and pro-cessing electrical signals derived from the electrochem-ical noise and linear polarization techniques. Acquir-ing, processing, and displaying data was done with aworkstation computer. Instantaneous and time-aver-aged corrosion rates were calculated and recordedalong with various other statistically relevant parame-ters for each probe such as noise resistance, standarddeviation of current and voltage, localization index,polarization resistance, solution resistance, etc. Thesystem is capable to monitor simultaneously fourprobes. Electrochemical potential and current noisewere collected once per second in 300 s blocks. 5-minaverage corrosion rate data were calculated and saved

using both ECN and LPR data, along with other rele-vant parameters of each technique. Probe temperaturedata as well was collected once per second in 300 sblocks and recorded every 5 min as an average.

The experimental work was carried out in a combus-tion rig burning 32 kg/h of residual fuel oil. The designof the combustion rig allows a combustion gas temper-ature gradient from exiting the furnace (

1100

) to thestack (

240

). The high-temperature corrosion moni-toring probes were inserted in the combustion gas ductwhere the gas temperature was in the range

850900

.The low-temperature corrosion monitoring probe waslocated in the stack duct where the combustion gaswere at

240

.Two online corrosion rate monitoring experiments

were carried out. In the first experiment, a lower con-tent of V, Na, and S fuel oil (designated as fuel oil A)was burned in the combustion rig, and, in the secondexperiment, a higher content of V, Na, and S fuel oil(designated as fuel oil B) was burned. Combustion con-ditions in both cases were the same (1% excess oxygenin flue gas). The table shows the composition of V, Na,and S in both fuels.

Two high-temperature probes were used in eachexperiment. One probe was fitted with electrodes madeof a ferritic steel (ASTM SA 213 T 22), and the other,with an austenitic steel (ASTM SA 213 304). In theexperiment with fuel oil B, a low-temperature corrosionmonitoring probe was positioned in the stack of thecombustion rig and was fitted with mild steel elec-trodes. Every test lasted 500 h. Initially, the temperatureof the two high-temperature corrosion probes was set to

1 2 73 4 5 6

Electrode Insulator

Air inlet

Fig. 1.

Schematic drawing of corrosion probes used for cor-rosion monitoring.

Composition of impurities in fuels

Vanadium, ppm

Sodium,ppm

Sulfur, wt %

Fuel oil A 226 29 3.4

Fuel oil B 280 59 3.8

Air outlet

562


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SALINAS-BRAVO et al.

400

for a period of 250 h. This allowed deposition offuel oil ashes on the probe surface. Afterwards, probetemperature in each test was increased by

20

stepsevery hour up to

590

. This temperature was fixed andcontrolled (

3

) for the rest of the test.Probe temperature in the low-temperature corrosion

monitoring experiment was set at a temperature of

148

. This temperature was above the dew point tem-perature of flue gas (

140

). After getting a base linewith probe temperature of

148

, the probe tempera-ture was set to

110115

. Experimental conditionslike fuel oil burning rate, probe temperature, and excessof oxygen in flue gas were kept constant through thetests.

RESULTS

Figures 2 and 3 show the corrosion rates obtained inthe high-temperature probes fitted with electrodesmade of ASTM SA 213 T 22 type ferritic stainless steelburning fuel oil A and B in the combustion rig. Figures

4 and 5 show the corrosion rate obtained in the high-temperature probes fitted with electrodes made ofASTM SA 213 304 type austenitic stainless steel burn-ing fuel oil A and B in the combustion rig. FromFigs. 25, it can be seen that both ferritic and austeniticstainless steels corrode at a faster rate in the experimentwhere the fuel oil with the highest content in Na, V, Swas burnt in the combustion rig.

Figure 6 shows the corrosion rate monitoring usingboth electrochemical techniques (EN, LPR) simulta-neously in the low-temperature probe fitted with mildsteel electrodes. This experiment was carried out burn-ing fuel oil type B in the combustion rig.

The sensitivity of the EN technique to a change inthe electrode temperature was investigated in the testburning fuel oil type B. Figure 7 shows that a change inthe probe temperature resulted in a change in corrosionrate. In this case, the corrosion rate reached a maximumvalue, then decreased down to a fairly constant value.This value will correspond to the corrosion rate for thetemperature set in the probe.

0 5 10 15 20

Time, h

0

2

4

6

8

25

400

500

600

700

300

Tem

pera

ture

,

C

1

2

Fig. 2.

Corrosion rate monitoring of the probe made of fer-ritic steel burning fuel oil type A.

0 5 10 15 20

Time, h

0

2

4

6

8

25

400

500

600

700

300

Tem

pera

ture

,

C

1

2

Fig. 3.

Corrosion rate monitoring of probe made of ferriticsteel burning fuel oil type B.

0 5 10 15 20

Time, h

0

2

4

6

8

25

400

500

600

700

300

Tem

pera

ture

, C

1

2

Fig. 4.

Corrosion rate monitoring of the probe made of aus-tenitic steel burning fuel oil type A.

0 5 10 15 20

Time, h

0

2

4

6

8

25

400

500

600

700

300

Tem

pera

ture

, C

1

2

Fig. 5.

Corrosion rate monitoring of the probe made of aus-tenitic steel burning fuel oil type B.

Cor

rosi

on r

ate,

mm

/yea

r

Cor

rosi

on r

ate,

mm

/yea

r

Cor

rosi

on r

ate,

mm

/yea

r

Cor

rosi

on r

ate,

mm

/yea

r


Vol. 42

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2006


Results of the low-temperature probe showed that,while the probe temperature is kept above the acid dewpoint temperature of the flue gas (

140

), the responseof both techniques is minimal. As soon as the tempera-ture of the electrodes is set to a temperature below theacid dew point temperature of flue gas, the response ofboth techniques starts to show.

Localization index monitoring was obtained fromthe EN data in the low-temperature probe (see Fig. 8).The response to a change in combustion conditions(%

2

in flue gas) was also investigated. Figure 9shows the response of the probe in terms of the changeof the corrosion rate with the oxygen content in the fluegas, when the probe temperature was set just below theacid dew point temperature. It can be seen that anincrease in the oxygen contents increases the corrosionrate about 0.2 mm/year when the percentage in oxygenincreases by 1%.

DISCUSSION

The results obtained using EN for corrosion moni-toring in a high-temperature gas environment whereflue oil ashes are deposited have demonstrated that thistechnique can be used in this environment. This tech-nique was able to assess the corrosion rate of materialsexposed to fuel oil ashes derived from fuel oil havingdifferent amounts of Na, V. Fuel oil ash corrosivity hasbeen documented taking into account the content of Na,V, S in ashes [13, 14] or Na, V in the fuel oil [17

19].In this case, higher corrosion rates were obtained forboth steels tested with fuel oil B, which is the fuel oilwith higher amounts of Na, V and sulfur. The EN sen-sitivity to a change in the electrode temperature wasconfirmed. The response of the technique to a change inprobe temperature was very good as can be observed inFig. 7. This is important for assessing the corrosion rateof materials at different temperatures in a corrosiveenvironment.

0 10 20 30 40

Time, h

0

0.2

0.4

0.6

0.8

110

120

130

140

Tem

pera

ture

, C

1

2

1.0 150

3

0 5 10 15 20

Time, h

0

2

4

6

400

500

600

Tem

pera

ture

, C

1

2

8 700

Fig. 6.

Corrosion monitoring in the low-temperature probemade of mild steel.

0 20 40

Time, h

0

0.4

0.8

1.2

110

130

150

Tem

pera

ture

, C

1

2

120

140

3010

Fig. 8.

Change in the localization index monitoring usingEN data in the low-temperature probe as a function of time.

0 20 40

Time, h

0.2

0.6

0.8

1.2

40

Tem

pera

ture

, C

1

2

80

120

0

0.4

600

B

Fig. 9.

Corrosion rate response of the low-temperatureprobe in terms of the oxygen percentage in the flue gas.

Cor

rosi

on r

ate,

mm

/yea

r

Cor

rosi

on r

ate,

mm

/yea

r

Cor

rosi

on r

ate,

mm

/yea

r

Loc

aliz

atio

n in

dex

Fig. 7.

Effect of increasing temperature in the electrodesmade of ASTM SA 213 304 type stainless steel, burningfuel oil type B in the combustion rig.

564


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SALINAS-BRAVO et al.

An important issue that has to be taken into accountfor corrosion monitoring using high-temperatureprobes in flue gas is the probe conditioning. Before cor-rosion monitoring starts, it is necessary to allow depo-sition of fuel oil ashes on the probe electrodes. This isachieved by setting the probe temperature between

100150

lower than the temperature where corro-sion is expected to start. Another important aspect tobear in mind for corrosion monitoring in high-temper-ature gas environments is the fact that two processes aretaking place at the same time on the probe: the deposi-tion of fuel oil ashes and the consumption of corrosivespecies deposited because of the corrosion process.Figure 7 shows how an increase in probe temperatureresults in an increase in corrosion rate up to a maximumvalue, then decreases and reaches a steady value. Thissuggests that, after increasing the probe temperature,there are enough corrosive species deposited and thisallows higher corrosion rates. As the corrosion processcontinues, aggressive species are consumed and corro-sion rate values start to decrease until a constant valueis obtained. A constant value is obtained when equilib-rium exists between ash deposition rate and corrosionrate.

Corrosion monitoring in the low-temperature probeusing both EN and LPR techniques provided interestingresults. After the probe temperature was set to

115

,this is below the acid dew point temperature of flue gas(

140

), corrosion rate obtained by LPR techniquestarted to increase, and EN corrosion rate values seemto change little with respect to the base line obtainedwith the probe temperature of

148

(Fig. 7). The dif-ference in corrosion rates between the two techniquescan be explained because of the type of corrosion thatis taking place on the probe electrodes.

Localization index monitoring recorded at the sametime that EN data was taken is shown in Fig. 8. Fromthis figure, it can be observed that, after the probe tem-perature has set to

115

, some localization indexevents start to appear for a period of approximately12 h. After this time, localization index values start toincrease reaching values near 1.0. This is an indicationthat localized corrosion is taking place on the elec-trodes. Therefore, corrosion rates obtained with ENtechnique in this particular environment have to be ana-lyzed carefully. The advantage of using two electro-chemical techniques simultaneously allows to investi-gate the kind of corrosion process that is taking place.

In corrosion monitoring, it is important that achange in a process variable that affects corrosion rateof materials can be detected by the corrosion probeemployed. A change in the O

2

percentage in flue gaswas made while corrosion monitoring was taking place(Fig. 9). An increase in the oxygen percentage in fluegas from 1 to 2% resulted in an increase in corrosionrate while keeping probe temperature constant. After

changing oxygen concentration in flue gas back to 1%,the corrosion rate of the mild steel electrodes went backto values close to those obtained before the increase. Ahigher oxygen concentration in flue gas increases theamount of SO

3

and water vapor; therefore, the amountof sulfuric acid condensing is higher. This explain ahigher corrosion rate in these conditions. Decreasingoxygen concentration in flue gas brings the opposite.

Proper design of a corrosion probe in corrosionmonitoring is a key issue for a successful monitoringprogram. In this case, the probe design employed forhigh- and low-temperature corrosion monitoring in fluegas from burning fuel oil gave good results in terms ofcorrosion rate and response to process variables.

CONCLUSIONS

Electrochemical noise technique was successfullyapplied to online monitor corrosion rate in a flue gasenvironment resulting from burning fuel oil. This tech-nique was able to detect the change in corrosion rate ofmaterials exposed to high-temperature flue gas derivedfrom two fuel oils having different contents of NaVS. Probe conditioning is an important issue to be takeninto account in high-temperature corrosion monitoring.Otherwise results may be uncertain. Low-temperaturecorrosion monitoring in flue gas environment using ENand LPR techniques simultaneously prove to be a valu-able approach in order to understand the corrosion pro-cess that is taking place in such environment. Finally,probe design and probe response to a change in a pro-cess condition it is very important to carry on a success-ful corrosion rate monitoring program in industry.

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Corrosion Monitoring Using Electrochemical Noise

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