Electrochemical Behaviour and Surface Analytical of Welded

© 2012 ISIJ 2266

ISIJ International, Vol. 52 (2012), No. 12, pp. 2266–2272

Electrochemical Behaviour and Surface Analytical of Welded Stainless Steel in the Room Temperature Simulated PWR Water

Hong LUO,1) Xiaogang LI,1)* Chaofang DONG,1) Yanling HOU,2) Kui XIAO1) and Xuequn CHENG1)

1) Corrosion and Protection Centre, University of Science and Technology Beijing, Beijing, 100083 China.2) Yangzhou University, Yangzhou, 225009 China.

(Received on July 21, 2012; accepted on August 22, 2012)

In this study, the electrochemical behaviour of different parts of welded AISI 304L stainless steel in thesimulated Pressurized Water Reactor (PWR) water at room temperature (25 deg.C) was studied by the dif-ferent electrochemical techniques such as the potentiodynamic polarization measurements and electro-chemical impedance spectroscopy, and the compositions of the passive film formed on the surface wereinvestigated by XPS. In addition, the surface images of samples after electrochemical tests were also car-ried out by SEM. The results indicated that the heat affected zone has the worst corrosion behaviour. Theheat affected zone has more pits than the other parts, which may lead to reducing corrosion resistance.In addition, the passive film of different zones of the welded stainless steel has the different chemicalcompositions. The Cr element on the fusion zone and heat affected zone are mainly the Cr(met), Cr2O3, andCr (OH)3.while on the base metal surface is the Cr(met), Cr2O3,Cr (OH)3 and CrO3.

KEY WORDS: stainless steel; welding; surface analysis; XPS.

1. Introduction

Austenitic stainless steels, such as type 304L and theirwelded forms are used in various plants including nuclearpower systems because of their excellent corrosion resis-tance, good strength at high temperature and fracture tough-ness at low temperature.1–3) They are susceptible to thelocalized corrosive attacks, such as pitting corrosion, inter-granular corrosion and stress corrosion cracking, especiallyin chloride containing environments. Furthermore, weldingoften makes this situation worse, owing to the metallurgicalchanges and residual stresses introduced.4,5)

There are many austenitic stainless steel and their weldedcomponents in contact with primary water in pressurizedwater reactors (PWR). Stress corrosion cracking (SCC) ofstainless steel is an important degradation phenomenon inPWR.6,7) The pitting corrosion of stainless steel and weldedforms in specific conditions always evolve as the source ofstress corrosion crack. Sensitization in the weld heat affect-ed zone (HAZ) is an important material condition whichmakes stainless steel susceptible to SCC. The oxide filmformed on the stainless steels in PWR is also reported toplay a key role in the initial formation of cracks in the pro-cess of SCC.8,9) The oxide film properties depend on thenature and composition of the alloy as well as the aggressiveenvironment, because the film is in constant exchange ofspecies with the electrolyte and its thickness and composi-tion is consequently altered by the environment.10,11) There-fore, to further understand the composition and properties of

passive film formed on stainless steels could probably helpus to solve the corrosion failures occurring in the most PWRinternals.

Weldments are indispensable in the manufacture of mostcomponents. Austenitic stainless steels can be readily weld-ed through various arc welding processes.12,13) According tothe microstructures, a welded area is roughly divided intothree distinct zones, the fusion zone (FZ), heat affected zone(HAZ) and base zone (BZ). The corrosion problems com-monly associated with welding of austenitic stainless steelsare related to both precipitation effects and chemical segre-gation.4,14,15) It is important to investigate the corrosionbehaviour related to the segregation of weld metal in orderto consider its effects on the practical welding of stainlesssteels. A great number of studies have been carried out relat-ed to pitting corrosion in the HAZ of stainless steel weld-ments. Tovar16) using different microscopy, potentiodynamicpolarization and EIS to study the microstructure and pittingcorrosion of different zones of AISI 316L stainless steelwelds in the LiBr solutions, showing that HAZ have theworst corrosion behaviour. Lu17) has investigated the pro-cesses of gas tungsten arc welding (GTAW) and laser-beamwelding (LBW) on the pitting and stress corrosion crackingbehaviours of 304L stainless steel, the results show that thegas tungsten arc welding process made the weld metal andheat affected zone more sensitive to pitting corrosion thanbase metal, but the laser-beam welding process improvedthe pitting resistance of the weld metal. Garcia18) reports thatthe HAZ was the most critical zone for pitting corrosion for304 and 316 stainless steel.

Although there have been many studies on electrochemi-* Corresponding author: E-mail: [email protected]: http://dx.doi.org/10.2355/isijinternational.52.2266

ISIJ International, Vol. 52 (2012), No. 12

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cal or corrosion behaviours of stainless steel and their weld-ments in the chloride environments, However, little isknown about the influence of chloride on the corrosionbehaviour of AISI 304L stainless steel weldments in simu-lated PWR water, especially in the electrochemical behav-iours and passive film of the welds. The aim of the presentstudy is to investigate the corrosion behaviour of three dis-tinct zones (fusion zone, heat affected zone and base zone)of AISI 304L SS in the simulated PWR primary water bymeans of potentiodynamic polarization, EIS and XPS. Fur-thermore, the surface images and corrosion resistance of thedifferent zones was also investigated.

2. Experimental

2.1. Materials and SolutionsAll the specimens used in this work were fabricated from

a sheet of AISI 304L stainless steel with the thickness of 2cm. The compositions of the stainless steel in weight percentwere 0.01% C, 18.02% Cr, 9.20% Ni, 1.0% Mn and 0.27%Si. The weldments were prepared using an automatic metalinert gas (MIG) procedure with argon as the shield gas, noedge preparation and AISI 304L type as the filler material.The microstructures of the welded 304L SS was shown inthe Fig. 1. As is shown in the Fig. 1(a), three major areaswere differentiated: the fusion zone (FZ), heat affected zone(HAZ) and base zone (BZ). In the heat affected zone no pre-cipitates are observed, neither carbides nor intermetallics.The weld fusion zone (Fig. 1(b)) shows delta ferrite areas inaustenitic stainless steels.

The electrochemical test specimens were cut into 1 cm ×3 cm, the working surface was polished mechanically usingsuccessive grade emery papers up to 2 000 grit. Then, thesurface was polished with 0.1 μ m alumina polishing pow-der, degreased with alcohol, cleaned in water, and finallydried in air. Furthermore, an insulating lacquer coating wasused to protect untested area and only the part of area con-taining a welded zone of interest was exposed to the corro-sive medium during the measurements.

The test solutions (H3BO3 + LiOH + NaCl) containing Bions 1 000 ppm, Li ions 2.5 ppm and Cl anions 2 ppm wereprepared to simulate PWR primary water. All solutions wereprepared from analytical grade and chemically purereagents.

2.2. Electrochemical MeasurementsA conventional cell with three electrodes was utilized, in

which the testing specimen was used as the working elec-trode, a platinum plate was used as the counter electrode andsaturated calomel electrode (SCE) was connected to the cellthrough Luggin capillary as reference electrode. All poten-tials reported in this work are referred to SCE.

Before the tests, the solution was purged with nitrogengas for 30 min, and the gas flow was maintained through thewhole test. Prior to anodic polarization and EIS measure-ments, the working electrodes were initially reduced poten-tiostatically at –0.6 V (vs. OCP ) for 20 min to remove air-formed oxides.

The polarization curves were recorded potentiodynami-cally using a scan rate of 0.5 mVs–1 starting from –0.8 V (vs.OCP) to the anodic direction until the current densityreached to 1 mA cm–2, where the potential scan wasreversed. Each type of electrochemical measurement wasrepeated at least three times. The various electrochemicalparameters were obtained from the polarization curves, i.e.,corrosion potential (Ecorr), corrosion current density (icorr),passive current density (ip) pitting potential (Ep) and repas-sivation potential (Erp).

Ecorr was obtained as the potential at which the net currentdensity was equal to zero. icorr were determined by theextrapolation of the cathodic apparent Tafel slopes to thepoint that yields the corrosion potential. Extrapolation start-ed over about 50 mV away from Ecorr and the same range ofpotential values was used throughout the tests. The mostprecise determination of the icorr values by Tafel extrapola-tion is when both the anodic and cathodic branches showlinearity. Ep marks the end of the passive potential regionand the transition from passive to transpassive behaviour,when dissolution by pitting is the dominating reaction. ip

characterizes the dissolution behaviour of the metal in thepassive potential region. Erp refers to the limit below whichthe metal remains passive, and it was defined as the poten-tial where the forward and reverse scans cross.

The electrochemical impedance spectroscopy (EIS) mea-surement was conducted through the Ametek of PAR 2 273electrochemical system to analysis of the electrochemicalbehaviour of the oxide film formed on the specimen surface.The measurement frequency ranged from 1 mHz up to 104

Hz, with applied AC amplitude of 10 mV. Fitting to themeasured EIS data was made using Zview 2.70 software.Fig. 1. The microstructure of the different zones of the welded

304L SS.

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2.3. XPS Surface AnalysisThe specimens were subsequently exposed in the simu-

lated solutions for 200 h at ambient temperature (25 deg.C),then the chemical composition of the surface films wasinvestigated by Kratos AXIS Ultra DLD Type X-ray photo-electron spectrometer (XPS) with a monochromatic Al Kαradiation source. The source was operated at 15 kV and 20mA with the analysis angle was 45°. Binding energies werecalibrated against the surface carbon contamination at 284.6eV. The curve fitting was performed using the commercialsoftware Xpspeak version 4.1, which contains the Shirleybackground subtraction and Gaussian-Lorentzian tail func-tion, to achieve better spectra fitting.

2.4. Surface Morphology ObservationThe microstructure of the different parts of the welded

stainless steel was observed by using the optical microscopeand the surface morphology of different parts of the stainlesssteel after electrochemical tests were also carried out by thescanning electron microscope of FEI Type Quanta 250.

3. Results and Discussion

3.1. Potentiodynamic Polarization MeasurementsFigure 2 shows the potentiodynamic polarization cyclic

curves for the different zones (BZ, HAZ and FZ) of welded304L SS in the in solution. As can be seen from the polar-ization curves of different zones, the corrosion processesand passivation behaviour of the stainless steel in the solu-tions are the same. As is shown in Fig. 2, the curves showthat all the electrodes exhibited a similar passive behaviour.The BZ and FZ characterized by an extreme low anodic cur-rent density and wide range of passive regions (–600 mVSCE

to 550 mV), while the HAZ has the lower passive regions.The electrochemical parameters calculated from the

curves are listed in Table 1. As is shown in the Table 1, theHAZ has the highest icorr and the BZ is the lowest. Since theicorr is a parameter related to the corrosion rate, higher valuesinvolve greater corrosion rates. Thus, the HAZ of 304L SSpossesses the worst corrosion resistance in terms of corro-sion rate values in the simulated PWR environments.

Pitting potential (Ep) determines the pitting corrosion sus-ceptibility of the different zone of the welded stainless steelin the ithiated and borated solution with the presence of

chloride ions, and the chlorides are very aggressive ions,which promoting passive film breakdown of stainlesssteels.19,20) According to the parameters in Table 1, the low-to-high order of the pitting potentials in solutions is Ep

(HAZ) < Ep (FZ) < Ep (BZ), indicating the order of pittingresistance. In fact, Luo21) observed that the coarse grains inHAZ have a negative influence on corrosion resistance. Thebase zone could be related to the better Cr2O3 protectivefilm formed on the metal surface due to its homogeneousaustenitic microstructure. According to Lee,22) a fully auste-nitic structure has the lowest susceptibility to pitting corro-sion attack since the passive film on such homogenousmicrostructure is more resistant to pitting corrosion.

The value between the corrosion potential and pittingpotential, Ep-Ecorr, is used to evaluate the corrosion risk. Itis generally admitted that the higher the difference betweenEp and Ecorr, the lower the probability to pitting corrosion.So the ability of pitting resistance of the welded zone isHAZ < FZ < BZ. In addition, the imperfect or unstable pas-sivity region (Ep-Erp) where pits can’t initiate but existingpits can propagate also defines the hysteresis loop. The nar-rower the hysteresis loop, the easier it becomes to repassi-vate the pit. Furthermore, the reasons why the hysteresisloop narrows must be considered since it is associated witha shift of the Erp to more positive values as indicative ofgood repassivation properties.

To compare, a similar solution (H3BO3 + LiOH) with noCl anions is prepared. Figure 3 shows the potentiodynamicpolarization cyclic curves for the different zones (BZ, HAZand FZ) of welded 304L SS in the in solution with no Clanions. As can be seen from the Fig. 3, the BZ and FZ char-acterized by an extreme low anodic current density and widerange of passive regions, while the HAZ has the lower pas-sive regions. Compared to the curves containing with the Cl

Fig. 2. Cyclic polarization curves of the different zones of thewelded 304L SS in the solutions with no Cl anions.

Table 1. Parameters of the welded materials obtained from thecyclic polarization curves in the solution with Cl anions.

Zones Ecorr(mVSCE)

icorr(μA/cm2)

Ep(mVSCE)

ip(μA/cm2)

Erp(mVSCE)

BZ –544 ±12 0.013 ± 0.02 498 ± 8 1.46 ± 0.3 109 ± 5

HAZ –568 ± 10 0.038 ± 0.04 182 ± 10 2.52 ± 0.2 40 ± 12

FZ –540 ± 8 0.022 ± 0.03 431 ± 6 1.51 ± 0.1 88 ± 10

Fig. 3. Cyclic polarization curves of the different zones of thewelded 304L SS in the solutions with 2 ppm Cl anions.


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anions, the chlorides can affect the icorr, Ecorr, Ep, and also thestability of the passive film. The presence of chloride ionsadsorbed on the surface or incorporated in the passive filmcan be detrimental to film’s stability and lead to the occur-rence of pitting.20) Ecorr is lower with the addition of the Clanions, and this tendency is noticeable especially at higherchloride concentrate. The value of icorr goes higher with theaddition of the Cl anions. Moreover, The low to high orderof the pitting potentials in solutions is Ep (HAZ) < Ep

(FZ) < Ep (BZ), indicating the order of pitting resistance,and the Ep of the three welded zones is all higher than thethat in the solutions with the 2 ppm Cl anions.

3.2. EIS MeasurementsTo complete the characterization of the stainless steel/

electrolyte interface, EIS measurements are made in a widefrequency range. Impedance spectra, which are normallydisplayed either in the form of a Nyquist diagram or of aBode plot, has proved capable of accessing relaxation phe-nomena whose relaxation time varies over orders of magni-tude and permits single averaging within an experiment toobtain high precision levels.

Figure 4 shows examples of the Nyquist plots for the dif-ferent zone of stainless steel in the solution with Cl anions.In Fig. 4, the EIS results of different zone (BZ, HAZ andFZ) in simulated circulated water solution shows distinctdifference in the impedance spectra revealing that the pas-sive film stability behaviour is different. The increasing inthe semi-circle arc indicated an increasing in the film stabil-ity, while decreasing in semi-circle radius indicates adecreasing in the passive film resistance.

Different models had been proposed for interpretationimpedance spectra on a passive metal surface. According tosome references,23,24) the equivalent circuit is presented inFig. 5. This model assumes that the passive film does nottotally recover the metal and cannot be considered as a homo-geneous layer but rather as a defective layer. In fact, neither

real surfaces of solids in the active range nor passive films onmetallic substrates can be considered to be ideally homoge-neous. The equivalent circuit consists of Rs(CPE // Rp) ele-ments, where Rs is the solution resistance; CPE is the con-stant phase in parallel connection with Rp which is thepolarization resistance.

For the description of a frequency independent phase shiftbetween alternating potential and its current response, a con-stant phase element (CPE) is used. Recently, CPE have beenused for modeling frequency dispersion behaviour corre-sponding to different physical phenomena such as surfaceheterogeneity which results from surface roughness, impuri-ties, dislocations, grain boundaries, fractality, distribution ofthe active sites, adsorption of inhibitors, formation of porouslayers.25,26) CPE is defined in impedance representation as:

........................... (1)

where Z0 is the CPE constant, ω is the angular frequency (inrad/s), i2 = –1 is the imaginary number and n is the CPEexponent. The factor n, defined as a CPE power, is anadjustable parameter that always lies from 0 to 1. Table 2depicts the best fitting parameters (based on circuit depictedin Fig. 5). The CPE increased in the HAZ, indicating anincrease of defects on the surface film.

Polarization resistance, Rp, is commonly used as a mea-sure of the resistance of a metal to the corrosion damage.Figure 6 shows the Rp obtained from the fitting parameters(using the circuit depicted in Fig. 5) for the films formed insolutions. The relatively high Rp values are observed for theBZ, which imply a better corrosion protective ability. Theobtained results allows to conclude that the HAZ is not onlythe zone that shows the worse anodic behaviour but also theone presenting the worse repassivation. On the contrary, theBZ has the best localized corrosion behaviour. This could berelated to the fact that there are differentiated localized cor-rosion phenomena according to the different weldments

Fig. 4. Nyquist plots of different zones of welded 304L SS in thesolution with Cl anions.

Fig. 5. Equivalent circuit for the analysis of impedance spectra inthe solution with Cl anions.

Table 2. Equivalent circuit parameters for impedance spectra in thesolution with Cl anions.

Zones Rs(Ω cm2)

CPE(Ω–1 cm–2 s–n) n Rp

(Ω cm2)

BZ 1.2345 1.378 × 10–4 0.8658 156 852.3

HAZ 1.1298 4.892 × 10–4 0.7923 89 817.8

FZ 1.3357 1.699 × 10–4 0.7875 127 355.6

Fig. 6. Rp values of the different zone of the weld 304L SS in thesolution with Cl anions.

Z Z n( ) ( )ω ω= ⋅ −0 i

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zones. There is dendritic corrosion in the weld metal andintergranular corrosion in the HAZ. Both could be the mainmechanisms for the nucleation and growth of the corre-sponding pits.

3.3. The Passive Film in the SolutionFigure 7 shows XPS results in the whole range of binding

energy after three zones of 304L SS in the solution with the

Cl anions. As is shown in the Fig. 7, both the metallic andoxidized states of Cr 2p3/2, Fe 2p3/2 and Ni 2p3/2 are present-ed. According to the peak strength, Cr, Fe and Ni dominatethe components of the film. The signals of the oxidizedstates of WZ passive film have greater intensities than thatof the oxidized states of the BZ and HAZ. Oxygen species,O2– and OH–, in passive film play the role of connectingmetal ions. The O 1s spectra may also be split into threecomponents O2– (530.2 eV), OH– (531.8 eV) and H2O (533eV). The O2– is the primary constituent of the passive film.

The chromium in the oxide layer plays important roles inthe composition, electronic properties and corrosion resistanceof passive films. The XPS results on 304L SS surface for Crregion (Fig. 8) indicate the occurrence of oxides CrxOy in eachcase, with varying amount of free chromium in the differentzones of the stainless steel. At analyzing the XPS chromiumregion for binding energy equaling from 580 eV to 570 eV,there exist three constituent peaks representing metallic stateCr(met) (574.1 eV), Cr2O3 (576.3 eV), Cr (OH)3 (577.1 eV) andCrO3 (578.1 eV) .The oxidized species are the primary con-stituents of the passive film. As can be seen from Fig. 9, thecomposition of passive film formed on the FZ and HAZ aremainly the Cr(met), Cr2O3 and Cr(OH)3; While, on the BZ sur-face is the Cr(met), Cr2O3, Cr (OH)3 and CrO3.

Fe2p3/2 spectra (Fig. 10) can be separated into several

Fig. 7. XPS results for different zones of weld 304L SS surfaces inthe solution with Cl anions.

Fig. 8. XPS results for Cr region of different zones of weld 304LSS surfaces in the solution with Cl anions.

Fig. 9. XPS spectra for Cr2p ionisation for films formed on stainlesssteel in the solution with Cl anions (a)FZ; (b)HAZ; (c)BZ.

Fig. 10. XPS results for Fe region of different zones of weld 304LSS surfaces in the solution with Cl anions.


2271 © 2012 ISIJ

constituent peaks representing the metallic state (Fe(met))(707.7 eV), the bivalent species ((Fe2+) (709.4 eV)) andtrivalent species ((Fe3+)(711.0 eV)). The relative peakheights of FeO (709.4 eV) and Fe2O3 (710.9 eV) indicatethat Fe2+ and Fe3+ are the primary iron oxidized species inthe passive film of the three different zone of the 304L SS.As shown in the Fig. 11, the Fe in the passive film of thedifferent zones has the different composition. The elementin the BZ are mainly Fe(met), FeO, Fe3O4; in the HAZ aremainly Fe(met), Fe2O3 and FeOOH; in the FZ are mainlyFe(met), FeO and FeOOH.

3.4. Surface Morphology ObservationThe SEM images of different parts of the welded stainless

steel after polarization tests are shown in the Figs. 12(a),12(b) and 12(c). It is seen that the exposure in the solutionswith Cl anions, the difference between the morphology onthe BZ, HAZ and FZ are relatively big. As can be seen fromthe Fig. 12(b), there are many more pits observed on the sur-face of the HAZ, suggesting the lowest pitting resistance.And the size of the pits on the HAZ is much lager than thaton the BZ and FZ.

4. Conclusions

The corrosion behaviour of different zones of the weldedAISI 304L SS in the simulated PWR water were studied bypotentiodynamic measurements and EIS. The compositionsof the passive films formed on different zones at ambienttemperature (25 deg.C) were investigated by XPS. The sur-face images after polarization tests were also carried out bySEM. The main conclusions obtained from this work arepresented below:

(1) The polarization results revealed BZ presents thebest overall corrosion behaviour compared to the otherwelded zones. However, HAZ shows the highest corrosion,passivation and repassivation current densities, i.e. greatercorrosion rates and a worse ability to repassivation. The Clanions can affect the pitting corrosion of the different partsof the welded stainless steel.

(2) The passive film formed on the stainless steel sur-face is different, The Cr element on the FZ and HAZ aremainly the Cr(met), Cr2O3 and Cr(OH)3; while on the BZ sur-face is the Cr(met), Cr2O3 , Cr (OH)3 and CrO3.

(3) After the polarization electrochemical test, the HAZsurface has many more pits, which may lead to reducing thecorrosion resistance of the stainless steel.

AcknowledgementThis work is supported by the Fundamental Research

Funds for the Central Universities (No. FRF-TP-11-006B)and the National Natural Science Foundation of China(No.51131005).

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Fig. 11. XPS spectra for Fe2p ionisation for films formed on stainlesssteel in the solution with Cl anions (a)FZ; (b)HAZ; (c)BZ.

Fig. 12. Typical morphologies of the surface after polarization testsin the solution with Cl anions (a) FZ; (b) HAZ; (c) BZ.

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Electrochemical Behaviour and Surface Analytical of Welded