Pitting and galvanic corrosion behavior of laser-welded stainless steels

Pitting and galvanic corrosion behavior of laser-welded stainless steels

Journal of Materials Processing Technology 176 (2006) 168–178 Pitting and galvanic corrosion behavior of laser-welded stainless steels C.T. Kwok a , ...

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Journal of Materials Processing Technology 176 (2006) 168–178

Pitting and galvanic corrosion behavior of laser-welded stainless steels C.T. Kwok a , S.L. Fong a , F.T. Cheng b,∗ , H.C. Man c a

Department of Electromechanical Engineering, University of Macau, Taipa, Macau, China Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China c Department of Industrial & Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China b

Received 18 October 2005; received in revised form 16 February 2006; accepted 21 March 2006

Abstract Autogenous welded specimens of austenitic (S30400 and S31603), duplex (S31803) and super duplex (S32760) stainless steels were fabricated by laser penetration welding (LPW) with a CW Nd:YAG laser in an argon atmosphere. The microstructure and the phases present in the resolidified zone of the laser-welded specimens were analyzed by optical microscopy and X-ray diffractometry, respectively. The pitting and galvanic corrosion behavior of the stainless steels in the laser-welded and unwelded conditions in 3.5% NaCl solution at 23 ◦ C was studied by means of electrochemical measurements. X-ray diffraction analysis showed that the phases present in the weld metal depended on the composition of the base metal. While the laser weld for S31603 retained the original austenitic structure, the laser weld of S30400 contained austenite as the major phase and ␦-ferrite as the minor phase. On the other hand, a slight change of ␦-ferrite to austenite ratio was found in both the laser welds of S31803 and S32760, with austenite present at the ␦-ferrite grain boundaries. The welds exhibited passivity but their pitting corrosion resistance was in general deteriorated as evidenced by a lower pitting potential and a higher corrosion current density compared with those of the unwelded specimens. The decrease in pitting corrosion resistance of the welds was attributed to microsegregation in the weld zone of S31603, and to the presence of ␦-ferrite in S30400. For the duplex grades S31803 and S32760, the disturbance of the ferrite/austenite phase balance in the weld zone could be the cause of the decrease in corrosion resistance. The initial free corrosion potentials of the unwelded specimens were considerably higher than those of the corresponding laser welds, indicating that the welds were more active and were expected to act as anodes in the weldment. The ranking of galvanic current densities (IG ) of the couples formed between the laser-welds (LW) and the as-received (AR) specimens with area ratio 1:1, in ascending order, is: AR S31603/LW S31603 < AR S31803/LW S31803 < AR S32760/LW S32760 < AR S30400/LW S30400. The recorded IG in all couples was low (in the range of nA/cm2 ). © 2006 Elsevier B.V. All rights reserved. Keywords: Laser welding; Stainless steels; Pitting corrosion; Galvanic corrosion; ␦-Ferrite

1. Introduction Owing to their excellent mechanical properties and corrosion resistance, stainless steels are extensively used in many industrial and medical applications. Commercial products such as razors, cigarette lighters, watch springs, motor and transformer lamination, hermetic seals, battery and pacemaker cans, and hybrid circuit packages require delicate welds with high quality and precision. A kilowatt laser beam can melt and vaporize the material, and the pressure of the vapor displaces the molten material so that a narrow and deep ‘keyhole’ is formed. The keyhole supports the transfer of the laser energy into the material and guides the laser beam deep into the material. Laser pene-



Corresponding author. Tel.: +852 2766 5691; fax: +852 2333 7629. E-mail address: [email protected] (F.T. Cheng).

0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.03.128

tration welding (LPW) can produce low-distortion and precise weldments with minimal heat-affected zones [1,2]. In LPW of stainless steels, phase transformation is common. The mechanical properties and corrosion resistance of laser-welded stainless steels may be deteriorated due to microsegregation, unfavorable phase content, presence of porosities, solidification cracking, micro-fissures and loss of materials by vaporization. Galvanic cell may also be set up between different parts of the weldment. Galvanic corrosion in weldments should not be overlooked because it can lead to accelerated deterioration of the anodic region especially in hostile environments. Pitting corrosion and galvanic corrosion have been investigated in the couples between dissimilar alloys such as 316L, Ti, Nb and Ta [3], CoCr and REX 734 [4], annealed and cold-worked 316L [5], and GTAW welded and unwelded N08031 [6]. The pitting corrosion resistance of several austenitic stainless steels welded by a CO2 laser has been investigated by Vilpas [7]. However, reports on the galvanic

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corrosion behavior in laser weldments of stainless steels are scarcely reported. The aim of the present study is to investigate the pitting corrosion behavior of various stainless steels in the laser-welded and unwelded conditions in 3.5% NaCl solution, and the galvanic effect in the laser weldments. The change in the pitting corrosion behavior of the laser-welded specimens and the presence of galvanic effect in the laser-weldments were explained in terms of the change in microstructure and chemical compositions. It must be pointed out that no attempt of optimization of processing parameters was made. The present study thus would serve only as a preliminary baseline investigation for reference in future studies on laser welding of stainless steels. Moreover, detailed mechanical properties will be reported elsewhere. 2. Experimental details Austenitic (UNS S30400 and S31603), duplex (UNS S31803) and super duplex (S32760) stainless steels with different chemical compositions (Table 1) were selected in the present study. The as-received (AR) stainless steels were in the form of plates with a thickness of 1 mm. LPW was carried out using a highpower CW Nd:YAG laser with a power of 0.9 kW and a beam size of 0.5 mm in diameter (6.1 × 105 W/cm2 ). The laser beam was transmitted by an optical fibre and focused onto the specimen by a BK-7 lens with a focal length of 80 mm. The flexible optical fibre delivery was controlled by an X–Y table. Argon flowing at 20 l/min was used as the shielding gas. In order to reduce thermal distortion of the workpiece, it was held in place by a clamping device. Beam scanning speed of 35 mm/s was used. Such laser parameters were chosen for achieving full penetration and minimal thermal distortion. The bead-on-plate seam welds were made on the plates by line scanning of the focused laser beam. The laser-welded specimens were sectioned, polished and etched. The microstructure and phases in the resolidified zones were analyzed by optical microscopy (OM) and X-ray diffractometry (XRD), respectively. The radiation source of the X-ray diffractometer was Cu K␣ with nickel filter and generated at 1.2 kW and the scan rate was 0.25◦ s−1 , with the scanning direction along the weld. The respective volume fraction of ␦-ferrite present in the stainless steels was evaluated by the direct comparison method [8]. Due to rapid solidification, the specimens exhibited a preferred orientation to some extent. The integrated intensities of the (1 1 1), (2 0 0) and (2 2 0) diffraction peaks for ␥-austenite and the (1 1 0) and (2 0 0) peaks for ␦-ferrite were taken into account. The volume fraction of ␦-ferrite (C␦ ) was calculated using the following expression according to the ASTM Standard E974 [9]:



C␦ = 1 +



= 1+

(I␥(1 1 1) /R␥(1 1 1) ) + (I␥(2 0 0) /R␥(2 0 0) ) + (I␥(2 2 0) /R␥(2 2 0) ) 1.5((I␦(1 1 0) /R␦(1 1 0) ) + R(I␦(2 0 0) /R␦(2 0 0) )) (I␥(1 1 1) /182.8) + (I␥(2 0 0) /81.6) + (I␥(2 2 0) /44.4) 1.5((I␦(1 1 0) /233.8) + (I␦(2 0 0) /31.9))

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the hardness of the laser-welded specimens was determined using a Vickers microhardness tester. The load applied was 200 g and the loading time was 15 s. Cyclic potentiodynamic polarization scans were carried out using an EG&G PARC 273 corrosion system according to ASTM Standard G61-94 [11] for investigating the pitting corrosion behavior. Free corrosion potential (Ec ) measurement and galvanic corrosion test were conducted using the same corrosion system conforming to ASTM Standard G71-81 [12]. The base stainless steels and their welds for corrosion studies were cut into 6 mm × 10 mm plates. Prior to corrosion tests, the surface of all specimens was freshly ground and then polished with 1 ␮m-diamond paste in order to keep the surface roughness consistent. The specimens were embedded in epoxy resin with an exposed area of 4 mm2 of the weld track or unwelded region. The specimens were cleaned, degreased and dried before the polarization test in 3.5% NaCl solution, which was kept at a constant temperature of 23 ◦ C and open to air. A saturated calomel electrode (SCE) was used as the reference electrode and two parallel graphite rods served as the counter electrode for current measurement. For the cyclic potentiodynamic polarization test, all data were recorded after an initial delay of 30 min for the specimen to stabilize. The potential was increased from 200 mV below the corrosion potential in the anodic direction at a scan rate of 5 mV s−1 . The scan was then reversed when an anodic current density of 5 mA cm−2 was reached and continued until the loop closed at the protection potential. Galvanic corrosion test was performed using the built-in zero-resistance ammeter (ZRA) in the potentiostat. The galvanic current density (IG ) and galvanic potential (EG ) in the couples formed by the as-received specimen and the laser weld were continuously monitored for 24 h. The exposed area ratio of anode to cathode of all galvanic couples was 1:1.

3. Results and discussion 3.1. Microstructural and metallographic analysis Full penetration was achieved in all specimens in the LPW and the widths of welds were about 0.8 mm. Typical crosssectional view of laser-welded (LW) S31603 is shown in Fig. 1.

−1

−1

(1)

where I␥(h k l) and I␦(h k l) are the integrated intensities of a given crystallographic plane (h k l) from the ␥ and ␦ phases, respectively, and the values of R␥(h k l) and R␦(h k l) of ␥ and ␦ for various planes were obtained from Jatczak et al. [10]. The chemical compositions of the resolidified microstructure after LPW were analyzed by energy dispersion X-ray spectrometry (EDS). In addition,

Fig. 1. Cross-sectional view of laser-welded S31603.

Table 1 Nominal compositions (wt.%) of various stainless steels

S30400 S31603 S31803 S32760 a

Fe

Cr

Ni

Mo

Mn

W

Cu

C

Si

P

S

N

Creq /Nieq a

Balance Balance Balance Balance

18.4 17.6 22.5 25.6

8.7 11.2 5.6 7.2

– 2.5 2.9 4.0

1.6 1.4 1.5 0.6

– – 0.2 0.8

2.1 1.4 1.6 0.7

0.08 0.03 0.03 0.03

0.3 0.4 0.4 0.3

0.1 – – –

0.1 – – –

– – – 0.2

1.70 1.62 3.59 2.09

Creq = [Cr] + [Mo] + 1.5[Si] + 0.5[Cb]. Nieq = [Ni] + 0.5[Mn] + 30[C] + 30[N].

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Fig. 2. XRD patterns of as-received stainless steels and laser welds: (a) S30400, (b) S31603, (c) S31803 and (d) S32760.

X-ray diffraction analysis showed that the phases present in the weld metals depended on the composition of the base metal. According to the XRD patterns in Fig. 2, the laser weld for S31603 retained the original austenitic structure, while the laser weld for S30400 contained austenite as the major phase and ␦-

ferrite as the minor phase. On the other hand, both S31803 and S32760 were mainly composed of ␦-ferrite, with austenite as the minor phase. The microstructures of the stainless steels before and after laser welding are shown in Fig. 3. In LW S30400, the skeletal network of residual ␦-ferrite is present in the austenitic

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matrix (␥) as shown in Fig. 3(e). Austenitic dendrites in different orientations are observed in LW S31603 as shown in Fig. 3(f). After laser welding, the grain size of LW S30400 and LWS31603, which were predominantly austenitic, was refined due to rapid solidification. On the contrary, the grain size in LW

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S31803 and LW S32760 was increased and their microstructures are shown in Fig. 3(g and h), respectively. The Widmanstatten structure of semi-continuous dendritic austenite was present in the columnar grain boundaries of ferrite. When the melt pool of the weld zone of the duplex grade stainless steels solidifies,

Fig. 3. Microstructure of various stainless steels in unwelded (as-received, AR) condition: (a) AR S30400, (b) AR S31603, (c) AR S31803 and (d) AR S32760; and in laser-welded (LW) condition: (e) LW S30400, (f) LW S31603, (g) LW S31803 and (h) LW S32760.

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Fig. 3. (Continued ).

C.T. Kwok et al. / Journal of Materials Processing Technology 176 (2006) 168–178 Table 2 Volume fraction of ␦-ferrite and microhardness in as-received (AR) and laserwelded (LW) regions Stainless steels

C␦ (AR) (%)

C␦ (LW) (%)

Difference in C␦ (%)

Hv0.2 (AR)

Hv0.2 (LW)

S30400 S31603 S31803 S32760

0 0 60 54

14 0 62 50

+14 0 +2 −4

176 179 268 290

301 194 316 314

the possible phase transformation sequence upon cooling may be represented as: Liquid → liquid + ferrite → ferrite → ferrite + austenite The degree of completion of the transformation and hence the final phase structure of the weld metal depend on the composition of the base metal and the welding parameters [13]. Based on the XRD spectra, the volume fractions of ␦-ferrite (C␦ ) of various laser welds were calculated using Eq. (1) and shown in Table 2. ␦-ferrite did not exist in LW S31603, whereas 14%, 62% and 50% of ␦-ferrite were present in LW S30400, LW S31803 and LW S32760, respectively. The effect of laser welding on the microstructural change of various stainless steels

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under the same laser processing conditions was different because of their difference in chemical compositions. The ratios of the chromium equivalent (Creq ) and the nickel equivalent (Nieq ) of the stainless steels are shown in Table 1. As the Creq /Nieq ratio increased, the ferrite-forming tendency of the stainless steels increased. The highest volume fraction of ␦-ferrite was observed in S31803, which had the highest Creq /Nieq ratio. In addition, the solid-state transformation of ␦-ferrite to austenite is considered to be diffusional. Thus, the high solidification rate typical in laser processing would also suppress the ferrite-to-austenite transformation, resulting in a high fraction of ␦-ferrite. The increase in volume fraction of ␦-ferrite in LW S31803 in the present study was also reported by others [14,15] in the autogenous laser welding of duplex stainless steels. The disturbance of the ferrite/austenite phase balance in the weld metal might be remedied via the use of welding consumables having a ‘more austenitic’ composition, and/or the use of a shielding gas containing an appropriate amount of N2 , which is an austenite stabilizer [16]. However, there is a higher probability of forming intermetallic precipitates and nitrides in the weld metal, both of which would decrease the corrosion resistance [2,17]. The hardness profiles along the depth and across the crosssection of the weld zones of various specimens are shown in Fig. 4, and the results are summarized in Table 2. The hardness

Fig. 4. Hardness profiles of various laser-welded specimens: (a) along the depth of the cross-section and (b) across the cross-section.

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Fig. 5. Ec vs. time of the as-received stainless steels and their corresponding weldments: (a) S30400, (b) S31603, (c) S31803 and (d) S32760 in 3.5% NaCl solution.

values in the welds were nearly constant and higher than those of the base metals (hardness outside the welds) as shown in Fig. 4(b). The hardness of the weld zone for S31603, S31803 and S32760 was higher by 8–18% as compared with that of the base metals. The most significant change in hardness is observed in LW S30400, with an increase of about 70%. The increase in hardness could be attributed to the refinement of grains and also to the presence of hard ␦-ferrite. 3.2. Free corrosion potential From measurements of the free corrosion potentials (Ec ) of the base stainless steels and their corresponding laser welds (Fig. 5), information about the dynamic behavior of the passive oxide film might be obtained. The Ec of the unwelded specimens increased towards more noble values and became steady at the end of the 2-h test. This reflects that the growth of passive oxide was almost complete. The steady values of Ec after 2 h are shown in Table 3. For unwelded S32760, the oxide film was the most stable because it was highly alloyed with the elements Cr, Mo and N, all of which could enhance passivity. It can be also observed that the Ec of all unwelded stainless steels are considerably higher than those of their corresponding welds. For instance, the Ec of as-received S32760 (3 mV SCE) is higher than that of its laser weld (−327 mV SCE).

3.3. Pitting corrosion behavior Cyclic potentiodynamic polarization curves of various unwelded stainless steels and their laser welds in 3.5% NaCl solution at 23 ◦ C are shown in Fig. 6. The corrosion parameters, including pitting potential (Epit ), protection potential (Eprot ) and corrosion current density (icorr ), are summarized in Table 3. All unwelded stainless steels and their laser welds exhibited passivity in 3.5% NaCl solution. However, there was a general and substantial shift of the polarization curves towards higher current densities, indicating deterioration in corrosion resistance.

Table 3 Corrosion parameters of unwelded (AR) and laser-welded (LW) stainless steels in 3.5% NaCl solution at 23 ◦ C, open to air Specimens

Ec (mV)

Epit (mV)

Eprot (mV)

icorr (␮A/cm2 )

AR S30400 LW S30400 AR S31603 LW S31603 AR S31803 LW S31803 AR S32760 LW S32760

−270 −310 −256 −343 −301 −348 −3 −327

330 85 423 195 1170 671 1040 1040

−39 −11 76 39 1179 −111 980 954

0.416 3.475 0.252 6.456 0.405 9.429 0.138 0.518

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Fig. 6. Potentiodynamic polarization curves of the as-received stainless steels and their corresponding weldments: (a) S30400, (b) S31603, (c) S31803 and (d) S32760 in 3.5% NaCl solution.

In addition, the pitting corrosion resistance and the repassivation capability of the laser welds were lower than those of the unwelded specimens as reflected by lower values in both Epit and Eprot . The ranking of the pitting corrosion resistance of the specimens is as follows: ARS30400 < ARS31603 < ARS32760 < ARS31803 and LWS30400 < LWS31603 < LWS31803 < LWS32760. Decrease in Epit is observed in the laser welds for S30400 (from 330 mV to 85 mV SCE), S31603 (from 423 mV to 195 mV SCE) and S31803 (from 1170 mV to 671 mV SCE). On the other hand, there is no significant change in Epit in the laser weld for S32760. In addition, the corrosion current densities of all welds increased. While the results in the present study indicate a decrease in corrosion resistance for stainless steels due to laser welding, laser cladding (LC) of stainless steel on mild steel followed by laser remelting resulted in increase of corrosion resistance as reported by Li et al. [18,19]. This is not unexpected as in laser surfacing the processing condition is chosen to yield a homogeneous surface layer while in welding the primary aim is to achieve joining. Some improvement in pitting resistance of

laser-surface melted S30400 [20–23] and S31603 [20,24] was reported by several authors due to the removal of MnS inclusions (for S30400 and S31603) and the trapping of sulfur in the ␦-ferrite (for S30400). For S32760, there is no significant change in Epit and Eprot resulting from laser surface melting [20], consistent with the present results. The change in corrosion behavior due to laser welding could arise from different causes depending on the base stainless steel. Pan et al. attributed the deleterious effect of ␦-ferrite on the pitting corrosion resistance of S32100 to the galvanic effect existing between ␥-austenite and ␦-ferrite [25]. In fusion welding, solidification from the melt pool in general results in local compositional variations, which would in turn result in less stable passive film and hence lower corrosion resistance [26]. The compositional heterogeneity in the weld metal could arise from three main causes: microsegregation during weld metal solidification, element partition in solid-state transformation from ferrite to austenite, and precipitation of intermetallic phases, carbides and nitrides, leading to the formation of Cr-depleted regions. The microstructure in a weld is a complex function of the solidification parameters (solidification rate and temperature gradient at the solid/liquid interface), which in turn are determined by the processing parameters and the alloy composition [27,28]. Though microsegregation is generally small in

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Table 4 EDS composition analysis of different regions in laser welds Specimen

Region

Cr (wt.%)

Ni (wt.%)

Mo (wt.%)

LW S30400

␥ ␦

16.4 19.9

7.8 8.2

LW S31603

Dendrite core (␥) Interdendritic (␥)

16.1 18.2

9.6 12.3

1.5 2.1

LW S31803

␥ ␦

20 24

6.3 5.4

1.8 2.6

LW S32760

␥ ␦

26.7 26.1

7.1 7.2

3.2 3.8

– –

laser welding in comparison with conventional welding because of a higher solidification rate in the former, some degree of microsegregation still occurred in the laser welds of various stainless steels in the present study. EDS compositional analysis of the different phases or regions in the laser welds is shown in Table 4. For LW S31603, the difference in composition between the dendritic cores and the interdendritic regions reveals microsegregation during solidification, with a subsequent Cr and Mo enrichment of the liquid phase when the solidification front grew from the core to the boundary of the dendrites while Ni segregated in the opposite direction, similar to that reported in the literature [29]. For LW S30400, which exhibited primary austenitic solidification, a small amount of ferrite was formed from the melt between dendrites as result of microsegregation. For LW S31803, which was predominantly ferritic, difference in the composition between austenite and ferrite existed, similar to the case of LW S30400. The Cr, Ni and Mo contents in the austenite and ferrite for LW S32760 were very close in each phase, indicating minimal microsegregation. The impairment in the pitting corrosion resistance of the laser welds might be attributed to microsegregation, and also to unfavorable ferrite/austenite phase content [30], in addition to the presence of defects arising from welding. The corrosion behavior of LW S32760 was relatively close to that of the base metal because microsegregation was minimal. 3.4. Galvanic corrosion behavior Plots of galvanic potentials (EG ) and galvanic current densities (IG ) of the galvanic couples between the base stainless steels and their corresponding laser welds as a function of time are shown in Fig. 7. Since the values of the Ec for all base stainless steels were higher than those of their welds (Fig. 5), the welds were more active and are expected to act as the anode when coupled to the corresponding base metals. From Fig. 7, it can be observed that the current densities of all the couples were changing with time in the initial stage and then reached different steady-state values at the end of the 24-h test. The values of EG and IG for different galvanic couples attained after 24 h are shown in Table 5. The ranking of IG in the galvanic couples in ascending order is as follows: ARS31603/LWS31603 < ARS31803/LWS31803 < ARS32760/LWS32760 < ARS30400/LWS30400

Fig. 7. Time dependence of (a) EG and (b) IG for various galvanic couples in 3.5% NaCl solution.

The galvanic corrosion rates (i.e. IG ) in all these couples were low, only in the range of nA/cm2 . The galvanic corrosion rate in a galvanic couple depends on the difference in corrosion potentials (the driving force) of the members in the galvanic couple and on their polarization characteristics (the resistance), both of which in turn depend on the compositions and microstructures of the members [31]. IG for AR S30400/LW S30400 was the highest due to a large difference in volume fraction of ␦-ferrite for members in this couple (+14%). The amount of ␦-ferrite in the austenite matrix determines the Ec , and hence the IG in the couple. The values of IG in AR S32760/LW S32760 and AR S31803/LW S31803 were much smaller since the difference in volume fraction of ␦Table 5 Steady-state EG and IG of galvanic couples between the unwelded stainless steels and their weldments Galvanic couples

EG (mV)

IG (nA/cm2 )

AR S30400/LW S30400 AR S31603/LW S31603 AR S31803/LW S31803 AR S32760/LW S32760

−311 −330 −304 −347

78.6 −26 8.6 35

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ferrite was relatively small. IG was the lowest in AR S31603/LW S31603 because there was no change in phase structure (100% austenite) in this case, and galvanic corrosion was attributable to the minor changes in microstructure. Nevertheless, the galvanic couples formed between the base stainless steels and their welds exhibited negligible galvanic effect as evidenced by the low values of IG , only of the order of nA/cm2 . There is no risk of triggering the phenomenon of pitting corrosion, because the Epit of the welds for various stainless steels were in the range of 85–1040 mV versus SCE and much higher than the EG of various couples (ranging from −347 mV to −304 mV SCE). In addition, the welded surface of different stainless steels after the galvanic corrosion tests did not reveal any surface damage such as pits or discoloration. 4. Conclusions Autogenous laser welding of two austenitic and two duplex stainless steels in Ar atmosphere was attempted and the pitting and galvanic corrosion behavior of the weldments were studied. The following conclusions can be drawn: 1. The laser weld of S30400 was essentially austenitic with the presence of a small amount of ferrite while the laser weld of S31603 remained purely austenitic. On the other hand, the ferrite/austenite phase balance for the two duplex stainless steels was slightly disturbed. 2. The microhardness of the laser welds for various stainless steels generally increased, possibly due to the increase in the volume fraction of ferrite, or to grain refinement. 3. All laser welds for stainless steels exhibited passivity in 3.5% NaCl solution but their pitting resistance deteriorated as evidenced by lower pitting potentials and higher corrosion current densities compared with those of the base metals. It is attributed to microsegregation (for all stainless steels studied), to the presence of ␦-ferrite (S30400) or to incorrect phase balance (S31803 and S32760). 4. Galvanic current densities in the couples formed between the base stainless steels and their welds were very low (in range of nA/cm2 ), indicating very small galvanic effect. Acknowledgments The authors wish to acknowledge the support from the infrastructure of the University of Macau and the Hong Kong Polytechnic University. References [1] J. Mazumder, Laser beam welding, ASM Handbook, vol. 6, 10th ed., Welding, Brazing, and Soldering, ASM International, Materials Park, OH, USA, 1990, pp. 263–268. [2] D. Schuocker, Welding with lasers, in: High Power Lasers in Production Engineering, Imperial College Press, UK, 1999, pp. 337–370. [3] J. Gluszek, J. Masalski, Galvanic coupling of 316L steel with titanium, niobium, and tantalum in Ringer’s solution, Br. Corros. J. 27 (1992) 135–138.

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