Corrosion behaviour and galvanic coupling of titanium and welded titanium in LiBr solutions

Corrosion Science 49 (2007) 1000–1026 www.elsevier.com/locate/corsci

Corrosion behaviour and galvanic coupling of titanium and welded titanium in LiBr solutions E. Blasco-Tamarit, A. Igual-Mun˜oz, J. Garcı´a Anto´n *, D. Garcı´a-Garcı´a Departamento de Ingenierı´a Quı´mica y Nuclear, ETSI Industriales, Universidad Polite´cnica de Valencia, P.O. Box 22012, E-46071 Valencia, Spain Received 7 July 2005; accepted 7 July 2006 Available online 19 October 2006

Abstract Corrosion resistance and galvanic coupling of Grade 2 commercially pure titanium in its welded and non-welded condition were systematically analyzed in LiBr solutions. Galvanic corrosion was evaluated through two different methods: anodic polarization (according to the Mixed Potential Theory) and electrochemical noise (using a zero-resistance ammeter). Samples have been etched to study the microstructure. The action of lithium chromate as corrosion inhibitor has been evaluated. Titanium and welded titanium showed extremely low corrosion current densities and elevated pitting potential values (higher than 1 V). The results of both methods, anodic polarization and electrochemical noise, showed that the welded titanium was always the anodic element of the pair titanium–welded titanium, so that its corrosion resistance decreases due to the galvanic effect.  2006 Elsevier Ltd. All rights reserved. Keywords: A. Titanium; B. Polarization; C. Welding; C. Passivity

1. Introduction Refrigerants traditionally used in refrigerating compression systems belong to the chlorofluorocarbon (CFC) group, which are responsible for the depletion of the ozone layer.

*

Corresponding author. Tel.: +34 96 387 7630; fax: +34 96 387 7639. E-mail address: [email protected] (J. Garcı´a Anto´n).

0010-938X/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.07.007

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These CFCs are prohibited (Montreal Protocol, 1987) and their substitutes are submitted to severe legislations (Kyoto Protocol, 1997). Refrigeration absorption technology is a suitable alternative to the compression systems, since absorption units reduce the use of chlorofluorocarbon (CFC) refrigerants and eliminate concerns about lubricants in refrigerants [1]. Therefore, absorption heating and refrigerating systems are widely used in air conditioning units for centralized air conditioning purposes. Lithium bromide is one of the most widely used absorbents [2–4] because of its high hydration heat, good thermal stability, low viscosity, and lack of crystallization problems at usual working temperatures [5]. However, although LiBr possesses favorable thermophysical properties, it can cause serious corrosion problems on metallic components in refrigeration systems and on heat exchangers in absorption plants [1,5–8]. That is the reason why it is so important to carry out studies about the corrosion behaviour in LiBr media of the materials employed in the construction of LiBr absorption machines. With the advances in refrigeration technology new double effect LiBr absorption machines have been developed. In that respect, this work presents studies about the corrosion behaviour of titanium in LiBr solutions, since the use of titanium could be an alternative to the stainless steel, specifically in the parts of the absorption machines with the most aggressive conditions. The elevated corrosion resistance of titanium is a consequence of the stable, adherent, tenacious and permanent oxide film formed on its surface. Some authors [9] interpreted the electrochemical behaviour of titanium in terms of a two layer structure of the passive film. A barrier film, next to the metal, that inhibits the metal dissolution thanks to its low cationic conductivity and a porous outer layer. The exact nature of the passivation oxide has not been firmly established but it is generally agreed that this film displays the TiO2 stechiometry, accompanied by some Ti2O3 [9,10]. However, it is generally considered by several authors to be formed exclusively of TiO2 [11–15], since it is the stablest oxide within the stability limits of water [16]. This permanent oxide film forms equally on welds as on parent metal. This film is colourless and normally by anodising can produce a spectacular refractive colours [17]. This layer protects titanium and gives it its excellent resistance to corrosion in a wide range of aggressive media, such as halide solutions. That is why titanium could be employed in LiBr absorption refrigeration systems, since LiBr can cause serious corrosion problems on metallic components. Nevertheless, little information is available about titanium corrosion resistance in highly concentrated bromidecontaining solutions. Copper is one of the most common materials used in the construction of heat exchangers piping in LiBr absorption machines due to its high thermal conductivity. However, the structural elements are usually made of stainless steels or titanium, which present better corrosion resistance and mechanical properties. Thus, the alloys used to construct structural elements could cause important corrosion damage on copper by galvanic effect. The best solution to avoid galvanic corrosion is to make all the heat exchanger components with a unique material like titanium, which presents good thermal, mechanical and corrosion resistance properties [18]. Therefore, titanium seems to be a good technological alternative to construct heat exchangers in LiBr absorption machines. With regard to galvanic corrosion, due to its high corrosion resistance, titanium usually is the most noble metal and the cathode in a galvanic pair. Materials coupled to titanium are likely to experience accelerated corrosion and, in the process, titanium may pick up the hydrogen generated as the cathodic product of the corrosion reaction. It is very important

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to pay attention to this fact, since this hydrogen absorption can cause embrittlement, and the hydrogen evolution can produce the loss of the required vacuum in LiBr absorption machines. Failures due to corrosion have been observed in welds or heat affected zones in LiBr absorption machines, mainly due to galvanic corrosion between the base and the welded metal. Many studies have shown that both, mechanical and corrosion resistance, are dependent on the microstructure of the alloy, which is a function of the composition and heat-treatment used [19]. The welding procedure strongly alters the microstructure by heat-treatment, producing local variations in material composition and structures. These variations between the welded and base metal can cause galvanic corrosion [20]. For this reason this work includes the study of the galvanic corrosion between welded and non-welded titanium in LiBr solutions. The goal of this work was to study systematically the corrosion resistance of titanium in its welded and non-welded condition by means of electrochemical measurements. Galvanic corrosion was studied using two different electrochemical techniques: anodic polarization curves obtained imposing potentials and open circuit measurements (electrochemical noise). 2. Experimental procedure 2.1. Materials preparation and LiBr solutions The materials tested were Grade 2 commercially pure titanium (0.03%N, 0.1%C, 0.25%O, 0.3%Fe, 0.0125%H, Bal. Ti) used as base metal, the same Grade 2 titanium used as its corresponding filler metal, and the welded metal obtained using the Gas Tungsten Arc Welding (GTAW) procedure. Titanium electrodes were cylindrically shaped and covered with polytetrafluoroethylene (PTFE) coating, while the filler and welded metal were embedded in a non-conducting epoxi resin. The exposed areas of the electrodes were 0.5 cm2 for the base and the welded metal and 0.020 cm2 for the filler metal. To avoid the effect of the cathode/anode area ratio in the galvanic study, the titanium and welded titanium probes were identical (55 mm high and 8 mm in diameter with a 2 mm coating). All the electrodes were wet abraded from 500 SiC (silicon carbide) grit to 4000 SiC grit, and finally rinsed with distilled water. The welded samples were obtained from the titanium probes, which had a gap (2.5 mm wide and 3 mm deep) in the upper part of the bar; that gap was filled with the filler metal using GTAW procedure. Welding was applied using a tungsten electrode in an argon atmosphere and the welding conditions were maintained constant for all the welded samples (Table 1). The materials were tested in 400 g/l (4.61 M), 700 g/l (8.06 M) and 850 g/l (9.79 M), LiBr solutions, prepared from purissimum LiBr (98 wt%), from PANREAC. To evaluate the inhibitor effect of the additives, the tests were carried out in a commercial 850 g/l LiBr solution (9.79 M), from FMC Corporation Lithium Division (USA), with Li2CrO4 (4.3 g/l) as corrosion inhibitor and LiOH (0.08 g/l) as pH regulator. Tests were developed at 25 C. 2.2. Microstructural analysis The materials were etched in order to reveal their microstructure and to estimate possible microstructural variations produced during the GTAW welding procedure. The

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Table 1 Gas tungsten arc welding parameters Welding process Welding wire Shielding gas Polarity Welding voltage Welding current Gas flow rate

GTAW (manual) Grade 2 titanium Argon DC electrode negative 15 V 65 A 10 l/min

influence of microstructure on the corrosion behaviour of the welded metal was also evaluated. The microstructures of titanium as base metal, titanium as filler metal and welded titanium were revealed by etching the samples with Kroll’s reagent (5% nitric acid and 1% hydrofluoric acid in water) [21] for 30 s. 2.3. Anodic polarization tests Potentiodynamic anodic polarization curves were determined according to ASTM G-5 [22] using a Solartron 1287 potentiostat. The tests were carried out in a three-electrode glass cell. The potentials of the working electrode were measured vs a silver–silver chloride (Ag–AgCl) reference electrode with 3 M potassium chloride (KCl) solution. The auxiliary electrode was a platinum (Pt) wire. Polarization curves were recorded in LiBr solution deaerated for 10 min by bubbling nitrogen prior to immersion. A nitrogen atmosphere over the liquid surface was maintained during the whole test. Before each polarization, the sample was immersed in the test solution for 1 h at the Open Circuit Potential (OCP). The average value of the potentials recorded during the last 300 s was finally adopted as the OCP value. After the OCP test, the specimen potential was reduced progressively to 1000 mV during 60 s in order to create reproducible initial conditions. The electrode potential was then scanned from 1000 mVAg/AgCl to 3000 mVAg/AgCl at 0.1667 mV/s. Corrosion potentials (Ecorr) and corrosion current densities (icorr) were obtained from polarization curves using the Tafel analysis. Temperature was maintained at 25 C during the tests. Pitting potentials (potential at which the current density reaches 100 lA/cm2 [23]) and passivating current densities were determined from the polarization curves in order to define the behaviour of the materials in the corrosive media. Inhibitor efficiency (IE) was also calculated in order to evaluate the action of the lithium chromate as corrosion inhibitor. The galvanic corrosion between the welded and the base material was evaluated from the anodic polarization curves by superimposing the potentiodynamic curves of both alloys. The galvanic current density and mixed potential of the pairs were estimated from the intersection point of the curves, according to the Mixed Potential Theory [24]. 2.4. Electrochemical noise tests The electrochemical noise measurements have been performed using a zero-resistance ammeter (ZRA). Titanium and welded titanium were connected to a Solartron 1287

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potentiostat, which was used as a ZRA. The galvanic current and potential established between the pairs were measured every 0.5 s during 8 h. The reference electrode was silver/silver chloride (Ag/AgCl) 3 M potassium chloride (KCl). The level of the instrumentation noise depended on the experiments, although it always was far below the level of the signal at any frequency. The current sign was positive when the direction of the electrons was from Working Electrode 1 (WE1) to WE2, thus WE1 was corroding. Current values were negative when the electrons flowed in the opposite direction, that is, the WE2 was corroding. The assays were designed with titanium as WE1 and welded titanium as WE2. So that, it was possible a sign reversal of the current, depending on which material was preferentially corroding. 2.5. On line visualization system The experimental device [7,25] consists of two elements: the electrochemical unit and the image acquisition section. This method allows the visualization of the corrosion phenomena on the materials in real-time simultaneously to the electrochemical data acquisition without disturbing the electrochemical system. The electrochemical system is composed of the data acquisition equipment, which registers the electrical signal (current and potential) obtained from the corrosion processes taking place inside a horizontal electrochemical cell [26,27]. On the other hand, the image acquisition unit is formed by a triocular microscope–stereoscope (NIKON SMZ-U) zoom 1:10 and a color video camera (SONY SSC-C370P), assembled to the optical device. 3. Experimental results 3.1. Materials examination Grade 2 commercially pure titanium is an alpha alloy [28]. Fig. 1 shows base titanium (a) and filler titanium (b) microstructure after etching. This is a single-phase alpha microstructure with equiaxed grains [21,29]. These equiaxed grains are formed when the alpha alloys are worked and annealed in the alpha phase field [28]. The 0.25 wt% oxygen, 0.10 wt% carbon and 0.03 wt% nitrogen content of Grade 2 titanium stabilizes the alpha phase [30]. Fig. 1(c) shows images of the welded titanium after etching with Kroll’s reagent. The microstructure of welded titanium exhibit Widmansta¨tten plates of alpha phase. 3.2. Electrochemical results 3.2.1. Open circuit potentials (OCP) The OCP values of the materials studied in the LiBr solutions are summarized in Table 2. It can be observed that the OCP values increased with the LiBr concentration in all the free-inhibitor samples. During the total time of immersion, 1 h, the potential recorded increases with time, stabilizing after 10 min. Titanium presented much higher OCP values than those obtained by the filler and the welded titanium. So that, titanium remained immune at higher potentials than the filler and the welded titanium. In the solutions with no additives the difference in the OCP values between titanium as base metal and in the other conditions increased with the LiBr

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Fig. 1. Microstructure of Grade 2 titanium as base metal (a), filler metal (b) and welded metal (c) (100·).

concentration from more than 700 mV in the 400 g/l to 1200 mV in the 850 g/l LiBr solution.

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Table 2 OCP values of the materials in the LiBr solutions Materials

Titanium Filler metal Welded metal

OCP (mVAg/AgCl) 400 g/l

700 g/l

850 g/l

Commercial

177 560 571

244 511 541

702 482 507

349 516 556

The presence of chromate in the commercial LiBr solution did not shift the OCP to more positive values, so that the anodic inhibitor character of chromates [31] was not observed in any condition of titanium. 3.2.2. Potentiodynamic polarization curves 3.2.2.1. Titanium. Fig. 2(a) shows the potentiodynamic curves of titanium. Due to the protecting oxide film formed on the titanium surface current density levels were extremely low, lower than 1 lA/cm2 in a wide range of potentials (from 1000 mV to 1000 mV). Anodic polarization of titanium exhibited essentially passive behaviour at potentials from the corrosion potential to 1 V approximately. The sudden increase in the current density shifted towards nobler potentials as the LiBr concentration decreased; however the current densities registered increased as the LiBr concentration decreased. It was observed the corrosive character of 400 g/l LiBr solution since it registered the highest current densities, three orders of magnitude higher than in the 700 g/l, 850 g/l and commercial LiBr solutions. However the current density increase began at very positive potentials (1.75 V). The potentiodynamic curves of titanium in the 700 g/l, 850 g/l and commercial LiBr solution presented two anodic peaks, which according to several authors [1,32] coincide with the oxidation of TiO, Ti2O3 and TiO2. These peaks were distinguished at the same potential in the solutions without chromates. The first peak was registered at 1.55 V and the second one at 2.15 V. In the commercial solution these peaks appeared less resolved and shifted to nobler potentials, 1.7 V and 2.3 V respectively. They were well defined at high bromide concentrations, with or without additives, but they were not present in the less concentrated solution. The figure inset (Fig. 2(a)) shows a particular situation, since in the 400 g/l LiBr solution the polarization curve of titanium crosses in three points the abscises axis corresponding to the current density measured zero. The first crossing point corresponds to an active state (activity) and the last one to a passive state (stable passivity). In the second crossing point there is a competition between the active and the passive state (unstable passivity) [33–35]. In the 400 g/l LiBr solution the current density constantly increased from 1.75 V to 2.55 V, potential at which current oscillations appeared (Fig. 2). Fig. 3 shows images corresponding to the titanium at different moments of the potentiodynamic test in the 400 g/l LiBr solution. At the pitting potential (1474 mV) the electrode surface does not show visible signs of degradation (Fig. 3(a)). The first signs of corrosion appeared simultaneously to the abrupt increase in current density at 1750 mV (Fig. 3(b)). In Fig. 3(c) oxygen bubbles can be observed on the damaged surface, which correspond to the oxidation of water. At these high potentials, the oxygen current evolution adds to that attributable

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Fig. 2. Potentiodynamic curves of titanium as (a) base metal, (b) filler metal and (c) welded metal in the LiBr solutions.

to the metal oxidation; the bubbles formed also might facilitate pit nucleation [1]. The corrosion phenomenon begins at localized sites of the surface of the electrode. The first signs of corrosion appeared on the perimeter of the electrode and progressed to the centre (Fig. 3(d)). Finally, all the surface was covered with corrosion product, as Fig. 3(e) shows.

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Fig. 3. Images of titanium in 400 g/l LiBr solution at different moments of the potentiodynamic test: (a) 1474 mVAg/AgCl (Ep) (7.5·), (b) 1750 mVAg/AgCl (7.5·), (c) 1787 mVAg/AgCl (7.5·), (d) 2146 mVAg/AgCl (7.5·), (e) 2550 mVAg/AgCl (7.5·), (f) 2650 mVAg/AgCl (7.5·), (g) 3000 mVAg/AgCl (7.5·), (h) after the test (7.5·), (i) after the test (20·).

This image, acquired at 2550 mV, corresponds to both the beginning of the current oscillations and the removal of the film of corrosion products. The current oscillations (with a mean value around 500 mA/cm2 and an amplitude of 150 mA/cm2) and film rupture continue (Fig. 3(f)) to the end of the test. At 3000 mV (Fig. 3(g)) the film of the corrosion products disappeared, but the oxygen evolution continued. The rupture of the corrosion product film was not uniform, which can be deduced from the current oscillation produced at the highest potentials. Fig. 3(h) and (i) shows images of the electrode surface after the test, which appears completely degraded. The equiaxed grains of a-titanium were revealed during the test due to the high potentials reached (Fig. 3(i)). This high positive potential that is applied during anodic polarization produces a pseudo-general type of attack on the surface which is similar to etching.

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After the test, in the 700 g/l, 850 g/l and commercial LiBr solutions the electrode surface was not damaged. It was possible to distinguish a multicoloured surface (unnoticeable at the acquired images), as a consequence of the formation of the oxide film [36], which remains stable up to the end of the test. 3.2.2.2. Filler metal. Fig. 2(b) shows the potentiodynamic polarization curves of the filler metal in the LiBr solutions. The filler metal also presented passive behaviour in the studied solutions, and registered low current density values up to 1 V approximately. In the LiBr solutions without additives the filler metal registered a continuous increase in current density up to a maximum, from which it decreased defining an anodic peak. This anodic peak was observed around 1.5 V in the 400, 700 and 850 g/l solutions, which also coincides with the first anodic peak registered in the base titanium (Fig. 2(a)). The sharp increase of the current density shifted to nobler potentials as the LiBr concentration decreased, while the maximum current density value of the peak increased with the LiBr concentration. In the commercial LiBr solution the anodic peak at 1.4 V is not well defined. From this peak, a continuous increase of the current density with the potential was registered, registering 735 lA/cm2 at the end of the test. This value was lower than that obtained in 850 g/l but higher than that obtained in the less concentrated solutions. It was notable the high current density levels reached in the more concentrated solutions, (850 g/l and commercial), one order of magnitude higher than those registered in 400 g/l and 700 g/l. The filler metal surface remained undamaged at the end of the test. As it happened during the polarization curves of the base metal, it was possible to distinguish a multicoloured surface due to the formation of the protective oxide film. 3.2.2.3. Welded metal. Fig. 2(c) shows the potentiodynamic curves of the welded titanium in 400 g/l, 700 g/l, 850 g/l and commercial LiBr solutions. Welded titanium shows a passive beahaviour in the studied LiBr solutions, analogously to that observed in the base and filler metal. Current density levels were extremely low (around 1 lA/cm2) over a wide range of potentials. From 1 V approximately current density increased abruptly. This increase shifted to nobler potentials as the LiBr concentration diminished. Welded titanium presented a similar behaviour in the LiBr solutions without corrosion inhibitors. In all the LiBr solutions the anodic polarization generated an anodic peak, which diminished as the LiBr concentration decreased, analogously to that observed in the filler metal. The anodic peaks registered in the 700 g/l and 850 g/l solutions correspond to those defined in the titanium as base and filler metal at 1.5 V in the same solutions. The behaviour of welded titanium in the commercial solution is different to that observed in the solutions without additives. In this case the current density registered a continuous increase from the anodic peak registered at 1.4 V, similarly to the situation observed in the filler metal. In fact, the behaviour of welded titanium was more similar to filler titanium than to base titanium. The surface electrode was examined after each test and no sign of damage was observed. It was also possible to distinguish the multicoloured surface due to the formation of the protective oxide film. 3.2.3. Corrosion potentials and corrosion current densities Corrosion potentials and corrosion current densities obtained from the Tafel analysis in the different LiBr solutions for all tested materials are summarized in Table 3. Even at very

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Materials

Titanium Filler metal Welded metal

400 g/l

700 g/l 2

850 g/l 2

Commercial 2

Ecorr (mV)

icorr (lA/cm )

Ecorr (mV)

icorr (lA/cm )

Ecorr (mV)

icorr (lA/cm )

Ecorr (mV)

icorr (lA/cm2)

167 671 629

0.02 0.36 0.20

380 668 723

0.04 0.02 0.14

280 428 746

0.07 0.54 0.19

253 879 1093

0.06 0.35 0.03

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Table 3 Corrosion potentials (Ecorr(mVAg/Ag/Cl)) and corrosion current densities (icorr (lA/cm2)) in LiBr solutions

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negative corrosion potentials the current corrosion density was maintained at very low levels (<0.6 lA/cm2 ) in Grade 2 titanium as base, filler and welded metal. In the LiBr solutions without additives the base metal showed generally the lowest corrosion current densities and the noblest corrosion potentials. In solutions without additives, the corrosion current density of the base metal increased with the LiBr concentration, but the filler and welded metal registered the lowest values in 700 g/l. In general, with respect to the corrosion potential, the materials registered the noblest values in 400 g/l. The presence of chromate as inhibitor in commercial LiBr solution only shifted the corrosion potential of the base metal towards less negative values. Chromates allowed all the materials to register lower corrosion current densities than in the 850 g/l LiBr solution. The efficiencies of lithium chromate as corrosion inhibitor in 850 g/l LiBr solution related to commercial LiBr solution were calculated. The evaluation of the capacity of an inhibitor to reduce the corrosion rate of a material immersed in a solution was calculated by means of the inhibition efficiency (IE): IE ð%Þ ¼

i0corr  iIcorr  100 i0corr

ð1Þ

where i0corr is the corrosion current density in the medium without the inhibitor and iIcorr is the corrosion current density in the medium with the inhibitor (Table 3). Chromates present in the commercial solution registered the highest efficient inhibitor capacity on the welded metal (83.77%), followed by the filler metal (34.08%) and finally by the base titanium (13.10%). 3.2.4. Pitting potential and passive current densities Table 4 shows the pitting potential values of the studied materials in terms of the LiBr concentration. This table shows the excellent pitting corrosion resistance of titanium, according to the high Ep obtained (>1 V). Table 4 does not show the pitting potentials of welded titanium in the 400 g/l, 700 g/l and commercial LiBr solutions, neither the Ep of the filler metal in the 400 g/l solution because these materials did not reach 100 lA/ cm2 during the scan in these LiBr solutions. From the potentiodynamic curves shown in Fig. 2(b) and (c) we can suggest that the pitting resistance of filler and welded titanium increases following the sequence: 400 g/l > 700 g/l > commercial > 850 g/l. With regard to base titanium, Table 4 shows that the highest pitting potential was also registered in the 400 g/l solution. The aggressiveness of the LiBr solution to pitting increases with the concentration of bromides. The chromates present in the commercial solution shifted the pitting potentials of the filler and welded titanium to more positive values, as compared to those obtained in the 850 g/l uninhibited solution, but this effect was not observed in the base titanium. Table 4 Pitting potentials of the materials in the LiBr solutions Materials

Ep (mVAg/AgCl) 400 g/l

700 g/l

850 g/l

Commercial

Titanium Filler metal Welded metal

1474 – –

1209 1678 –

1268 959 1309

1231 1287 –

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1.50

8

ip (μA/cm2)

1.25

7 6

1.00

5 0.75

4 3

0.50

2 0.25 0.00 300

ip (μA/cm2) Filler Titanium

1012

1 400

500

600

700

800

0 900

[LiBr] (g/l) Titanium

Welded Titanium

Filler Titanium

Commercial LiBr

Fig. 4. Passivation current densities of the materials in the LiBr solutions.

Fig. 4 shows a clear difference between the passive current density of each form of titanium, following the sequence: filler titanium > welded titanium > base titanium. Comparing the values obtained in the inhibited and uninhibited 850 g/l LiBr solution, it was observed that chromates favoured the passivity of titanium as filler and welded metal, but they made the passivity of base titanium difficult. 3.2.5. Galvanic corrosion Galvanic corrosion between the base and welded metal was studied using two different electrochemical techniques: anodic polarization curves obtained by imposed potentials, and open circuit measurements (electrochemical noise measurements, ENM). 3.2.5.1. Imposed potential measurements (anodic polarization curves). Fig. 5 shows the potentiodynamic curves of titanium and welded titanium in semilogarithmic representation in the studied LiBr solutions. It can be observed that the welded titanium was the anodic member of the pair in all the LiBr solutions. Titanium determined the mixed potential (EM) of the pair, which was established close to its corrosion potential (Ecorr). The coupling of titanium to welded titanium polarized the corrosion potential of welded titanium to more anodic values. Table 5 summarizes the corrosion potential (Ecorr) of the uncoupled titanium and welded titanium, as well as the mixed potential (EM) established between the pair in the LiBr solutions and the differences between the corrosion potential of the cathodic and the anodic member of the pair (EC  EA). In the same way, Table 5 shows the corrosion current density (icorr) of the uncoupled metals, the galvanic current density generated due to the coupling of the pair (iG) and the ratio iG/icorr. Letters ‘‘A’’ (anodic) and ‘‘C’’ (cathodic) indicate the character of each member in the pair. Table 5 shows that the worst galvanic effect occurred in the commercial solution, in which the galvanic current density was more than twenty times higher than the corrosion

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Fig. 5. Potentiodynamic curves of titanium and welded titanium in the LiBr solutions.

current density of the uncoupled welded titanium, as reflected the ratio iG/icorr (23). However, the highest galvanic current density was registered in the less concentrated solution, 400 g/l. It can be concluded that chromates aggravate the galvanic effect, since the highest ratio iG/icorr and the highest difference EC  EA were registered in the commercial solution. 3.2.5.2. Open circuit measurements (electrochemical noise). The electrochemical noise measurements (ENM) were performed using a zero-resistance ammeter (ZRA). 3.2.5.2.1. Galvanic current density and galvanic potential profiles examination. Electrochemical noise is the measurement of the naturally occurring fluctuations in the corrosion potential and galvanic current of corroding electrodes. One of the principal advantages of the electrochemical noise technique is that it can be used without disturbing the system under investigation. Fig. 6 shows the galvanic current density and the galvanic potential profiles of the titanium–welded titanium pair during 8 h immersed in the studied LiBr solutions. The negative values of current density registered mean that welded titanium was the anodic member of the pair, so that it was corroding. There was a general tendency for the galvanic current density to diminish during the first hour as the metal passivates by oxide film growth [37], and to stabilize from the second one. The continuous register of current is occasionally interrupted by brief transients, indicating pit nucleation and localized corrosion [37]. The most remarkable characteristic of the passive systems is to present potential and current signals with quite low amplitudes [38], like those registered by the pair titanium–welded titanium (Fig. 7). This figure shows an example of the general amplitude of the signals. Mansfeld and Sun [39], obtained this kind of profiles assaying a Ti alloy in Ringer’s solution, and they also associated this situation with passive behaviour.

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Materials

400 g/l

700 g/l

Welded metal Ecorr (mVAg/AgCl) EM (mVAg/AgCl) EC  EA (mVAg/AgCl) icorr (lA/cm2) iG (lA/cm2) iG/icorr

629

Titanium

Welded metal

Titanium

Welded metal

Titanium

167

723

380

746

280

1093

253

444 343 0.02C

1.46 7

Commercial

Welded metal

356 462 0.20A

850 g/l

Titanium

0.14A

361 466 0.04C

0.87 6

0.19A

140 1346 0.07C

0.66 3

0.03A

0.06C 0.72 23

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Table 5 Mixed potentials (EM) and galvanic current densities (iG)

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Fig. 6. Galvanic current density and galvanic potential profiles of the titanium–welded titanium pair.

120

i

-0.035

116

Potential (mVAg/AgCl)

Current Density (μ A/cm2)

-0.030

E -0.040 21600

22200

22800

23400

24000

24600

112 25200

TIME(s) Fig. 7. Galvanic current and potential of the titanium–welded titanium pair in the 700 g/l LiBr solution during 1000 s in the seventh hour.

The magnitude of the galvanic current density experienced very low oscillations, lower than 0.3 lA/cm2, during all the test, indicating that welded titanium was in the passive state [40]. Typical times for metastable pitting events are of an order of less than 10 s [41]. In the 850 g/l LiBr solution the current transients were more noticeable than in the rest of LiBr solutions, anyway they did not exceed an amplitude of 0.4 lA/cm2 and a duration of 1 s. So that, they indicated metastable pitting. From Fig. 6 it can be deduced that current transient activity (both number and amplitude of transients) depends on the bromide concentration, since metastable pitting increases with the LiBr concentration, as also reported [42].

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In the 700 g/l, 850 g/l and commercial solutions, no signs of corrosion attack during the exposure period were observed on the electrodes surface, indicating the formation of an excellent protective layer on the surface of the specimens (welded titanium was spontaneously passive). The lack of damage on the electrodes surface was in agreement with the profiles obtained in these LiBr solutions, since although they presented current transients, both their amplitude and their duration were low. These kind of transients characterized by a duration inferior to 1 s are associated with nucleation of metastable pits, with a low charge involved, wherewith no pits were produced [41,42]. On the contrary, at the end of the test in 400 g/l, welded titanium presented a completely polished surface broken in localized points in the form of irregular pits, due to localized corrosion (Fig. 8). This fact agrees with the profile obtained in the 400 g/l solution, since two relevant events were registered at the third and fifth hours respectively. The pits were associated with the increases in current density, demonstrating rapid localised metal oxidation [37]. The pits were able to repassivate, since potential and current density values were recovered. However, although they did not continue growing to develop stable pits, they were capable of developing into metastably growing pits [37] and welded titanium was visibly damaged at the end of the test. 3.2.5.2.2. Examination of individual events. It is possible to observe in Fig. 6 that all the individual metastable events presented the same shape. The potential decreased sharply down to a minimum value associated with the anodic reaction. From this value the potential presented a slow recovery to the initial value with a practically exponential form associated with a cathodic reaction which restored the electric equilibrium of the system [38]. With regard to the current events, they registered a sharp increase followed by a practically exponential drop, indicating repassivation of the pit. The local minimum in every potential fluctuation corresponds to the point of repassivation [41]. Fig. 9 shows an example of the typical shape of these individual events. Particularly these events were registered in the 400 g/l LiBr solution and they correspond to metastably growing pits [37]. Although they finally repassivated, they presented a lifetime longer than the mean duration of the nucleation of metastable pits (1 s); in addition the current reached a relatively high level. Concretely, the first event presented an amplitude of 0.5 lA/cm2, a potential drop of 50 mV and a duration of 850 s, while the second one registered an amplitude of 0.2 lA/ cm2, a potential drop of 25 mV and a duration of 750 s. The charge (integration of individual current transients) involved during these individual events was calculated, obtaining 255 lC and 125 lC respectively. These charges are higher than those registered in the rest of the solutions, which is associated with the pits present in the welded titanium surface after the test in the 400 g/l LiBr solution. 3.2.5.2.3. Statisticial analysis in the temporal domain. Fig. 10 shows the mean value of the potential (a) and current density (b) registered between the couple titanium–welded titanium during each hour of the test. The error bars denote the standard deviation of the measured values. It can be observed that, in all the LiBr solutions, the potential shifted to nobler values with time, which meant an ennoblement of the metal. Welded titanium presented an anodic behaviour during the test, which is reflected by the negative current density value. The current density values registered diminished with time. With regard to the influence of the LiBr concentration, there were slight differences between the potentials registered at the end of the test in 400 g/l and in 700 g/l (Fig. 10(a)), showing the noblest values the most concentrate solutions. The anodic inhibitor effect of chromates was not evident, since the potentials registered in the commercial

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Fig. 8. Images of the welded titanium surface at the beginning (a) and end (b,c) of the test in 400 g/l LiBr solution.

solution almost coincided with those obtained in the 850 g/l uninhibited solution. The high values of the potential standard deviation (rE) at the first hour were due to the initial increase of the galvanic potential during this time. The highest current densities were registered in 400 g/l solution (Fig. 10(b)), while values obtained in 700 g/l and 850 g/l were very similar. Chromates were also no effective, since the current density registered in the commercial solution was higher than in the 850 g/l. The first hour also registered

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Fig. 9. Galvanic current and potential of the titanium–welded titanium pair in the 400 g/l LiBr solution.

E (mVAg/AgCl )

a

250 200 150 100

400 g/l 700 g/l 850 g/l Commercial

50 0 0

1

2

3

4

5

6

7

8

9

Time (h)

i (μA/cm2)

b

0.000

-0.200

400 g/l 700 g/l 850 g/l Commercial

-0.400

-0.600 0

1

2

3

4

5

6

7

8

9

Time (h) Fig. 10. Mean value of the potential (a) and current density (b) registered between the couple titanium–welded titanium during each hour of the test. Error bars denote the standard deviation of the measured values.

the highest current density standard deviation (ri) values in all the LiBr solutions, since at the beginning of the test the current density presented the highest value and sharply dimin-

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ished during the first hour. In general, the high standard deviation values coincide with individual events in the galvanic potential or the galvanic current density profiles (Fig. 6). 4. Discussion 4.1. Effect of the welding procedure Fig. 1 shows the microstructure of titanium, which changes from equiaxed alpha grains in the base and filler titanium to Widmansta¨tten plates of alpha phase, which is the usual microstructure obtained when titanium is air-cooled after the welding [28,43]. According to the electrochemical results obtained it can be suggested that these variation in titanium microstructure causes a deterioration of its general corrosion behaviour. Concretely, both OCP (Table 2) and corrosion potential (Table 3) values of welded titanium were more negative than those obtained by the base metal. Furthermore, welded titanium registered higher corrosion current densities than base titanium. These parameters were more similar to those obtained in the filler than in the base metal. The welding of titanium does not deteriorate its pitting corrosion resistance, since the welded metal registered a better pitting behaviour than the base metal (Table 4). 4.2. Effect of the bromide concentration on corrosion resistance In spite of titanium is more susceptible to pitting corrosion in bromide medium than in other halides [13], its passive capacity allows it to remain intact in a wide range of potentials in the studied bromide solutions. Whatever the nature of the passive film be, the increase of the OCP with exposure time indicate the growth of that film on the electrode surface [44]. Some authors [45] associate this shift of the potential to more positive values to the formation and repair of the passive film, which was greatly damaged during the surface preparation. In addition, an elevated passive current density implies that titanium dissolves to form the oxide film, so that this film is thickening. This is observed in filler and welded titanium (Fig. 4), which present higher passivation current densities than the base metal. Probably, due to this thickening of the passive film, the filler and welded titanium present higher Ep than the base titanium, even when they do not reach 100 lA/cm2 in the 400 g/l solution (Table 4). The passive current density of the base and welded titanium (Fig. 4) slightly changed with the LiBr concentration. However filler titanium registered an increase of the passive current density with the bromide concentration that, according to studies developed by Virtanen and Curty [42] on titanium in halide media, is due to very small activation/ repassivation events in the passive film. Once Ep is reached stable pits are formed and the initiation rate of metastable pits decreases sharply [46]. From the pitting potentials shown in Table 4 and the potentiodynamic curves shown in Fig. 2 it can be concluded that pitting corrosion susceptibility increases as the LiBr concentration increases, as also reported other authors [11,42] who studied pitting corrosion of titanium in halide media. One of the reasons proposed [12] for the local destruction of the passive film is that halides such as Br, may penetrate into the film due to their size and high deformability. Hence, communication canals are created, which allow the access of the aggressive media to the unprotected metal [34]. Several authors have reported that pitting occurs with Ti

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dissolving in the Ti+4 state. Galvele’s model [11,47] for pitting involves the localized acidification at the corrosion site through the equation Ti + 2H2 O ! TiO2 + 4Hþ + 4e

ð2Þ

In Galvele’s model, halide acts to accelerate the metal dissolution reaction, and Ep is the overpotential needed to drive the metal dissolution reaction such that sufficient hydrolysis occurs and the local pH drop results. Thus, there may be a combined effect of local acidification coupled with halide migration and adsorption at the pit embryo. This model explains the results shown in Table 4, since the most concentrated solution (850 g/l) presented the most active pitting potential. In fact, the filler and welded metal do not reach 100 lA/cm2 in the less concentrated solutions. 4.3. Effect of the corrosion inhibitor Although chromate is usually an anodic inhibitor, occasionally it can act as cathodic inhibitor [31]. Despite the fact that the base and filler titanium present the same composition (Grade 2 commercially pure titanium), the effect of chromates is different on each one. Chromates act as anodic inhibitor on the titanium as base metal, shifting the corrosion potential to nobler potentials as compared to the 850 g/l solution (Table 3). However, chromates act as cathodic inhibitor on the filler metal, shifting the corrosion potential to the active direction. This fact can be attributed to the different machining procedure employed in each material, since base titanium was in bar form while filler titanium was in wire form. In fact, the grain size decreases (Fig. 1) from the base metal to the filler metal. Chromates also act as cathodic inhibitor on the welded titanium, indicating the similarities between filler and welded metal corrosion behaviour. Regarding current corrosion densities, chromates allowed all the materials to register lower values in the commercial solution than in the 850 g/l LiBr solution. From the pitting potentials shown in Table 4 and the potentiodynamic curves shown in Fig. 2, it was concluded that the presence of chromates in the commercial solution increased the pitting corrosion resistance of filler and welded titanium, but they slightly affected the pitting behaviour of the base metal. According to Cheng and Luo [46] and Uhlig [48], this increase of pitting resistance in the commercial solution can be due to the fact that CrO2 4 is an oxidizing agent and it can be reduced to Cr2O3 and adsorbed on the electrode surface forming an insoluble film that can passivate the alloy. With regard to the passive current densities (Fig. 4), chromates favoured the passivation of filler and welded titanium but they made base titanium passivation difficult. Again the behaviour of the filler and the welded metal is similar, and different to the base metal, as explained above. Regarding to the galvanic effect, the highest ratio iG/icorr and the highest difference EC  EA were registered in the commercial solution (Table 5). Thus, it can be suggested that chromates aggravated the galvanic effect, since the most relevant galvanic effect (EC  EA > 130 mV) [34] and the less compatibility between titanium and welded titanium (iG/icorr > 5) [40] occurred in the commercial solution. This is probably due to the different role of the chromates on the base (chromates act as anodic inhibitor shifting the corrosion potential to nobler values) and on the welded titanium (chromates act as cathodic inhibitor shifting the corrosion potential to more active values), which increases the pair dissimilarity.

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The presence of chromates, which can be adsorbed on the electrode surface forming a stable and protective film [46], seems to be responsible for the reduction of metastable pits in the commercial solution. Cheng and Luo [46] suggested that more chromate ions in the solution will promote the formation of stable passive film and lead to a lower active– passive transition current at peaks. That was in agreement with the observations obtained from both the potentiodynamic curves (Fig. 2) and the current density profiles (Fig. 6), since the current transients (suggested to be metastable events) observed in the 850 g/l solution were more noticeable than those observed in the commercial solution. 4.4. Electrochemical noise measurements Mansfeld [39] suggested that it was doubtful that a single index derived by statistical methods could identify a certain corrosion mechanism. This author proposed that localisation index (LI) should be considered as a measure of the deviation from the assumed identical behaviour of the two test electrodes (index to measure system asymmetry) and not as an indicator of corrosion mechanisms. However, in this study LI has not been employed as a decisive indicator of the corrosion mechanism, but to confirm the conclusions obtained after the examination of the profiles. The examination of the profiles and individual events of the ENM (Fig. 6), suggests that the electrodes were passive in all the LiBr solutions, being able to present localized corrosion processes. To confirm this observation the LI (Eq. (3)) has been used, since according to [38] this parameter allows discriminating between the uniform and localized corrosion processes. LI (0 6 LI 6 1) was calculated as ri LI ¼ ð3Þ irms where ri is the current density standard deviation and irms is the root square mean of the current density. In general, the localized corrosion processes happen on passive electrodes, which register low current densities. As a result, current fluctuations are relatively elevated in comparison with the mean value of the current density registered. Therefore the ri, and consequently the LI, are high. However, uniform corrosion processes generate high current density values in comparison with the current fluctuations. As a result, their LI are lower than those calculated for localized corrosion processes. Several authors [38] indicated that LI values close to 1 are associated with localized corrosion, while LI values close to 0 are associated with uniform corrosion. Table 6 shows the LI values of the pair titanium–welded titanium calculated from the values obtained in the tests in the studied LiBr solutions. The LI values were closer to 1 than to 0, indicating passive electrodes initially which could present localized corrosion, verifying the results obtained after the profiles examination. 4.5. Galvanic corrosion The worst general corrosion behaviour presented by the welded metal causes that it was the anode of the pair titanium–welded titanium in all the LiBr solution, corroding while titanium remained immune. The galvanic couple of an anode with a little area and a cathode with a large area produces high anodic dissolution rates due to the high anodic current density reached. So that, it is important that the ratio anode area/cathode

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Table 6 Localisation index of the pair titanium–welded titanium in the LiBr solutions Localisation index

400 g/l

700 g/l

850 g/l

Commercial

LI

0.62

0.90

0.88

0.67

area be favourable to the anode. Titanium welds constitute a little area with respect to the rest of titanium in the LiBr absorption machines, which is the more adverse situation. This fact evidences the vulnerability of welds to corrosion in LiBr absorption machines and the necessity of paying special attention to these zones during maintenance. Regarding to the imposed potential measurements, minimal differences of 100–130 mV between the corrosion potential of the cathodic and the anodic member of the pair (EC  EA) are necessary to considerer the galvanic effect relevant [34]. It is possible to notice in Table 5 that the galvanic effect between titanium and its weldment is important in all the studied LiBr solutions, with EC  EA values higher than 300 mV. The highest EC  EA difference was registered in the commercial solution (1346 mV), in spite of the fact that the noblest mixed potential was established in this solution with additives (140 mV). However, due to the passive behaviour of titanium, this potential difference does not involve very high galvanic current densities, as it will be shown below. The corrosion resistance of welded titanium decreased due to the galvanic effect produced by coupling with titanium, since the corrosion current density of the welded metal is accelerated in all the LiBr solutions studied (Table 5). According to Mansfeld and Kendel [49], the relative increase in the corrosion rate of the anodic member of the pair could be expressed by the ratio iG/icorr, where iG is the galvanic current density and icorr is the corrosion current density of the uncoupled anodic member. The magnitude of this ratio may be used as a guide that reflects the severity of the galvanic effect in a couple, and it was suggested that a value less than 5 implies compatibility of the members in the couple [40]. The open circuit measurements show that generally (except in the 400 g/l LiBr solution) the welded titanium surface was practically undamaged, as a consequence of the passive film formed on the electrode. According to Birch and Burleigh [16], this oxide film is generally formed as soon as the electrode is exposed to the electrolyte. Metastable pitting events were observed in the galvanic potential and current density profiles (Fig. 6), being more noticeable in the 850 g/l LiBr solution than in the rest of the solutions. This fact was due to the increase of nucleation of metastable pits as the aggressive anion (bromide) concentration increases [23,50–52]. These events, the duration of which did not exceed 1 s, correspond to the initiation, temporary growth, and repassivation of individual metastable pits [53–55]. In spite of the fact that the galvanic potential and galvanic current profiles (Fig. 6) registered in 700 g/l, 850 g/l and commercial LiBr solutions presented current transients, the electrode surface appeared undamaged after the test. However, some pits were detected after the assay in the 400 g/l solution. These results are related to the highest charge involved in the events registered in 400 g/l LiBr solution, and they are agree with Wharton and Wood [41], who suggested that as the lifetime of the event (pit) becomes longer, the charge involved is correspondingly higher and thus the size of the pit will be greater. Thus, there will be a higher probability for a metastable pit not to repassivate and continue growing as a stable pit. Conversely, the shorter lifetime, the less likely it will be to a metastable pit to progress to stable pit growth.

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4.6. Mixed potential theory vs open circuit measurements Fig. 11 shows the mixed potential (EM) and galvanic current density (iG) obtained according to the mixed potential theory and the mean values of the galvanic potential and current density recorded during 8 h at open circuit using a ZRA (EZRA, iZRA), in the LiBr solutions. The galvanic current density values (iG) have been represented without the negative sign for a better comparison with the iZRA values. Results obtained from both techniques indicated that the anodic member of the pair titanium–welded titanium was always the welded titanium, which corroded while titanium remained protected.

a

300

E (mVAg/AgCl)

200 100 0 -100 -200 -300 -400 -500 300

400

500

600

700

800

900

800

900

[LiBr] (g/l) EM

EZRA

Commercial LiBr solution

b

2

i (μA/cm2)

1.5

1

0.5

0 300

400

500

600

700

[LiBr] (g/l) ig iZRA Commercial LiBr solution

Fig. 11. Mixed potential (EM) and galvanic current density (iG) obtained according to the mixed potential theory and galvanic potential and current density obtained using ZRA (EZRA, iZRA), in the LiBr solutions.

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With regard to the current density, the highest values were registered in the 400 g/l LiBr solution using both techniques. Furthermore, the differences between iG and iZRA diminished as the LiBr concentration increased. The chromates did not act as corrosion inhibitors, since independently of the electrochemical technique employed, higher galvanic current densities were registered in the commercial solution than in the uninhibited 850 g/l solution. In the case of the galvanic potential, the same general tendency was observed in the results obtained from both techniques. The galvanic potential diminished as the LiBr concentration increased from 400 g/l to 700 g/l, and then shifted to nobler values as the LiBr increased to 850 g/l. In the LiBr solutions without additives the differences between EM and EZRA were around 500 mV. The pair titanium–welded titanium was under anodic control, since the anodic branch of the anodic member of the couple (welded titanium) determined the galvanic current density. The wide range of potentials in which welded titanium remained passive allowed not only increasing its corrosion resistance, but also diminishing the galvanic effect when coupled to titanium. The passive film formed on the electrode surface made the current flow through the electrode difficult, as a consequence the galvanic current density established was very low. Otherwise, titanium determined the mixed potential (EM) of the pair, which was established close to its corrosion potential (Ecorr) (Table 5). In the same way, the galvanic potential values (EZRA) (Fig. 11) obtained by means of electrochemical noise measurements (ENM) were similar to the OCP values of the uncoupled titanium (Table 2). The EM values obtained using the Mixed Potential Theory were lower than those obtained by the electrochemical noise measurement (EZRA); likewise, the Ecorrof titanium was lower than its OCP. This phenomenon is justified, because during the open circuit measurements (both OCP test of an uncoupled metal and ENM of a galvanic pair) a protective oxide film, predominantly constituted by TiO2, was formed on the titanium surface, shifting the potential to nobler values. However, at the beginning of the imposed potential tests the potential was diminished to 1 V. At this negative potential titanium surface was modified, and the corresponding Ecorr calculated from the potentiodynamic curve was more active. With regard to the galvanic current density, the values obtained by the imposed potential techniques (iG) were somewhat higher than those obtained using open circuit measurements (iZRA). This was explained by the formation of a passive film during the open circuit test, which allows registering low current density values, while this passive film is modified at the beginning of the imposed potential test. Anyway, the maximum difference between iG and iZRA was very low, around 1 lA/cm2. Finally, open circuit measurements provide measurements more accurate than those obtained using the imposed potential technique, and it is the best technique to know the real behaviour of a galvanic pair, as well as to monitorize the corrosion of a system. However, although the imposed potential technique provides galvanic potential and galvanic current density values less accurate, it provides useful information about the response of the system at different potentials. So that, these two techniques are complementary and both are necessary to an accurate study of the system. 5. Conclusions Titanium, filler titanium and welded titanium spontaneously passivated in all the LiBr solutions, since their OCP values were situated in the passivation zone of the polarization curves.

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The pitting corrosion susceptibility of the studied materials increased as the LiBr concentration increased. The welding procedure deteriorates the general corrosion behaviour of titanium, so that the welded titanium was the anodic member of the pair titanium–welded titanium in all the LiBr solutions. However, welding does not deteriorate its pitting corrosion resistance. According to the potential and current density profiles obtained by ENM, it can be concluded that the welded metal remained passive during the tests in the LiBr solutions under study. It was possible to distinguish some individual events associated with metastable pitting corrosion. This current transient activity increases with the LiBr concentration, and the presence of chromates caused a reduction of metastable pits. Chromates aggravate the galvanic effect, since the highest ratio iG/icorr and the highest difference EC  EA were registered in the commercial solution. Nobler values of galvanic potential and lower values of galvanic current density were obtained using open circuit measurements than imposed potential measurements. Open circuit measurements provide more accurate galvanic potential and galvanic current density values than those obtained using the imposed potential technique. Acknowledgement We wish to express our gratitude to MCYT (PPQ2002-04445-C02-01), and to Dr. M. Asuncio´n Jaime for her translation assistance. References [1] A. Igual-Mun˜oz, J. Garcı´a-Anto´n, J.L. Guin˜o´n, V. Pe´rez-Herranz, Corrosion 59 (2003) 606–615. [2] J.W. Furlong, The Air Pollution Consultant, Elsevier, Amsterdam, 1994. [3] A.D. Althouse, in: A.D. Althaus, C.H. Turnquist, A.F. Bracciano (Eds.), Modern Refrigeration Air and Air Conditioning, Goodheart-Willcox publishers, 1988. [4] K. Gilchrist, R. Lorton, R.J. Green, Applied Thermal Engineering 22 (2002) 847–854. [5] J.L. Guin˜o´n, J. Garcı´a-Anto´n, V. Pe´rez-Herranz, G. Lacoste, Corrosion 50 (1994) 240–246. [6] A. Igual-Mun˜oz, J. Garcı´a-Anto´n, J.L. Guin˜o´n, V. Pe´rez-Herranz, Corrosion 58 (2002) 995–1003. [7] J. Garcı´a-Anto´n, A. Igual-Mun˜oz, J.L. Guin˜o´n, V. Pe´rez-Herranz, J. Pertusa-Grau, Corrosion 59 (2003) 172–180. [8] J. Garcı´a-Anto´n, V. Pe´rez-Herranz, J.L. Guin˜o´n, G. Lacoste, Corrosion 50 (1994) 91–97. [9] F. Contu, B. Elsener, H. Bo¨hni, Corrosion Science 46 (2004) 2241–2254. [10] M.V. Popa, E. Vasilescu, P. Drob, I. Mirza-Rosca, A. Santana-Lopez, M. Anghel, C. Vasilescu, in: EuroCorr’ 01, 2001. [11] C.S. Brossia, G.A. Cragnolino, Corrosion Science 46 (2004) 1693–1711. [12] J.W. Schultze, M.M. Lohrengel, Electrochimica Acta 45 (2000) 2499–2513. [13] K. Abou-Zeid, D. Ellerbrock, T. Haruna, D. Macdonald, E. Sikora, M. Urquidi-Macdonald, A. Wuensche, L. Zhang, Fundamental Studies of Passivity and Passivity Breakdown, PennState (ed.), 1997. [14] S.M. Wilhelm, Corrosion 48 (1992) 691–703. [15] Y. Shterenberg, H. Straze, D. Itzhak, Materials Science Forum 126–128 (1993) 753–756. [16] J.R. Birch, T.D. Burleigh, Corrosion 56 (2000) 1233–1241. [17] Titanium Information Group, Titanium for Architectural Applications, 1999. [18] Titanium Information Group, Titanium for Offshore and Marine Applications, 1999. [19] A.V. Benedetti, P.L. Cabot, J.A. Garrido, A.H. Moreira, Electrochimica Acta 45 (2000) 2187–2195. [20] T. Hemmingsten, H. Hovdan, P. Sanni, N.O. Aagotnes, Electrochimica Acta 47 (2002) 3949–3955. [21] S. Lathabai, B.L. Jarvis, K.J. Barton, Materials Science and Engineering A 299 (2001) 81–93. [22] ASTM G-5, Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements, 1994.

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