Detection of deleterious phases in duplex stainless steel by weak galvanostatic polarization in alkaline solution

Corrosion Science 48 (2006) 2560–2576 www.elsevier.com/locate/corsci

Detection of deleterious phases in duplex stainless steel by weak galvanostatic polarization in alkaline solution M.A. Domı´nguez-Aguilar

a,*

, R.C. Newman

b

a

b

Instituto Mexicano del Petro´leo, Programa de Ingenierı´a Molecular, Eje Central Norte La´zaro Ca´rdenas, No. 152, 07730 Me´xico D.F., Mexico University of Toronto, Department of Chemical Engineering and Applied Chemistry, 200 College Street, Toronto, Canada M5S 3E5 Received 13 December 2004; accepted 30 August 2005 Available online 17 November 2005

Abstract A novel electrochemical procedure has been developed to quantify the presence of secondary phases (nitride, v, r) in duplex stainless steel. This is based on anodic galvanostatic polarization in a weakly alkaline solution to detect the transpassive dissolution of Cr- and Mo-rich phases. By adjusting the current density, an almost linear relation can be obtained between the ‘‘time to reach a given potential’’ and impact toughness. The material used was UNS S32760 (Zeron 100), isothermally aged at 650, 750, 825 and 850 C. Distinctive features on the potential–time curves were correlated with the microstructure by examining reacted surfaces with backscattered imaging in the SEM.  2005 Elsevier Ltd. All rights reserved. Keywords: Alkaline solution; Super duplex stainless steel; r phase; Polarization; Potentiostatic; Galvanostatic; Transpassivity

1. Introduction High-chromium phases in stainless steel can be detected electrochemically, for instance by the easier reactivation from the passive state of their associated chromium-depleted *

Corresponding author. Tel.: +52 55 9175 6955; fax: +52 55 9175 8000. E-mail address: [email protected] (M.A. Domı´nguez-Aguilar).

0010-938X/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.08.017

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zones in acidic solutions—an approach used in the EPR (electrochemical potentiokinetic reactivation) test to detect sensitization induced by carbide precipitation. The EPR test has been modified by several authors to detect solute-depleted zones associated with intermetallic phases or nitrides in duplex stainless steels [1–5]. The approach is generally to experiment with minor modifications of the EPR solution (sulphuric acid + potassium thiocyanate) and the potential program. For 25Cr steels, HCl can be used instead of H2SO4, since high-alloy steels do not pit at ambient temperature; a promising approach would be to experiment with quite strong HCl solutions, up to 3 or 4 M. However, none of the EPR variants are simple to implement in the field, and anyway the problem of precipitation in duplex steels is more complex because the presence of r and related phases can result in a significant degradation of corrosion resistance and mechanical properties at quite low volume fractions that do not give planar-depleted zones. This may suggest the development of a new technique capable of detecting and distinguishing small amounts of r and other deleterious phases. In another paper [6] we discuss a halide-based procedure using neutral-pH solutions that also detects solute depletion and is much simpler, though less quantitative, than EPR. The present paper is concerned with the use of a solution that directly, but temporarily, attacks secondary phases. It is well known that anodic alkaline etching of phases in stainless steels is due to their preferential transpassive oxidation. Normally such etching uses a strong caustic solution that gives results in a few seconds to minutes. The electrochemistry of various phases in strong alkaline etch media has been studied [7], using potentiostatic and potentiodynamic techniques. The present work goes further in that it aims to develop a simple electrochemical test that uses neither strong acid or alkali, nor a potentiostat, yet can identify and quantify deleterious phases in a few seconds, and potentially can relate the electrochemical response to mechanical toughness as measured in a Charpy impact test at low temperature. Stainless steels and alloys containing chromium and molybdenum are susceptible to transpassive dissolution, because the oxides of these elements are unstable at high anodic potentials. Chromium dissolution can take place due to the oxidation of solid Cr III oxide to soluble Cr VI [8,9]; similarly, Mo IV oxide is oxidized to soluble Mo VI [10,11]. According to Pourbaix [12], the relevant equilibria are þ 2CrO 4 þ 10H þ 6e ¼ Cr2 O3 þ 5H2 O E0 ½V vs. SHE ¼ 1:311  0:0985 pH þ 0:0197 logðCrO 4Þ

MoO 4

ð1Þ

þ

þ 4H þ 2e ¼ MoO2 þ 2H2 O

E0 ½V vs. SHE ¼ 0:606  0:1182 pH þ 0:0295 logðMoO 4Þ

ð2Þ

In principle, then, transpassivity should be easier in alkaline than in acidic or neutral media with oxygen as the oxidant, because the equilibria quoted are more steeply pH dependent than the oxygen electrode. It can also be observed that Mo transpassivity may occur at a significant lower potential than that of Cr. However, the weight percent of Mo in stainless steels is not usually high enough to make a significant contribution to transpassive behaviour. This may not be true of some secondary phases such as v. It is doubtful whether stainless steels can undergo stable transpassive dissolution in mild alkaline solution, since Fe III forms a stable oxide or oxyhydroxide under these conditions [13]. This is both a drawback and an advantage of using such a solution. The drawback is that physical etching of secondary phases for visual correlation with the electrochemical

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response is very slow; the advantage is that there is a defined quantity of charge that flows during transpassive dissolution of Cr and Mo before the establishment of an Fe-rich oxide (which may or may not have a higher residual passive current than the stainless steel had in its stable passivity region). Such a defined quantity of charge lends itself to methods of quantification that do not rely upon elaborate potentiodynamic measurements. 2. Experimental The investigated material consisted of a super duplex stainless steel of the type 25Cr– 7Ni–3Mo (UNS S327601) with the composition given in Table 1. The alloy was obtained in the form of a 16 mm diameter round bar, which was previously solution annealed at 1100 C. It had a duplex microstructure free of precipitates and containing approximately 42% of ferrite as determined by quantitative metallography. Samples cut from the rod were machined into standard Charpy V samples 55 mm in length with a 10 · 10 mm cross section. A set of six samples was isothermally aged in air at each specific time and temperature. The main program was carried out on material isothermally aged at 675 and 825 C for 100, 300, and 1000 s. In addition, other heat treatments were completed at 750 C for 6, 12, 18, 24, 30 and 60 min, as well as 800 and 850 C for 12, 18 and 24 min, to favour v phase formation. Other samples were heat treated at 850 C for 1, 72, 113 and 120 h, and at 825 C for 113 h, in order to enlarge secondary phase precipitates. A summary of the heat treatments applied to this material is shown in Fig. 1, together with TTT data of Nilsson [14] for the similar 25Cr steel UNS S32750 (SAF 2507). Strutt and Lorimer [15] determined the TTT diagram for r phase only in Zeron 100 (UNS S32760); the highest transformation rate under isothermal ageing is located at 900 C and 5 min, so that the diagram shown is quite similar regarding to the r phase transformation. The TTT data are indicative only—any discrepancies between this diagram and the electrochemical or mechanical test results should not be taken to indicate that the diagram is wrong. Samples were water quenched after ageing in order to retain r and related phases (v, nitride). Those samples that required an isothermal treatment in excess of 1 h were aged in a furnace with an argon atmosphere to reduce oxidation. The temperature was monitored directly on the samples during heat treatment, using a K-type thermocouple connected to a digital thermometer. Charpy impact toughness testing according to ASTM A 370 [16] was performed at 50 C in triplicate for the as-received material and for most of the aged materials. Samples were tested with the notch oriented in the long transverse direction. Test temperature was automatically controlled by immersing the Charpy samples in a recirculating bath containing a solution of ice and acetone. Light optical metallography was used to quantify the ferrite content in the as-received material and to display r phase in some of the aged samples. The samples were ground and diamond-polished to a 0.25 lm finish prior to electrolytic etching in oxalic acid (10%) and KOH (10 N). The etching was done at 2.5 V for 10 s in oxalic acid and at 1.5 V for 10 s in KOH using a platinum cathode. Scanning electron microscopy using back-scattered electrons (SEM-BSE) was used to quantify r, and occasionally v phase and nitride, on unetched samples (very light etching was used on the as-received material

1

Trademark ZERON 100.

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Table 1 Chemical composition (wt%) of UNS S32760 Element

C

Si

Mn

P

S

Cr

Mo

Ni

Cu

W

N

V

Fe

Content

0.017

0.24

0.54

0.021

0.001

24.97

3.58

6.97

0.52

0.54

0.22

0.1

Bal.

Fig. 1. Solution annealing and isothermal ageing heat treatments applied to UNS S32760, together with schematic TTT data originally developed for the related alloy UNS S32750 [14]. The dots represent the heat treatments applied to the experimental material.

to contrast austenite and ferrite). The chemical compositions of secondary phases were estimated on unetched surfaces by EDX analysis with standard machine settings when particles displayed a suitable size (>1 lm); these compositions were evaluated in at least two different samples with the same ageing treatment; at least three measurements were recorded on each sample. When secondary phase particles were small (<1 lm) and/or elongated, the analysis procedure was adjusted to diminish the substrate effect; the acceleration potential was decreased (<20 kV) and the tilt angle increased (>30). The latter procedure was applied to intermetallics (r, v) developed after short ageing times, and also a few enlarged nitride particles formed by long ageing. The compositions of secondary phases were reported as the average of the measurements per element together with the associated standard errors, which were used to decide the number of significant figures to display. For the lower ageing temperatures the results are indicative only, but the procedure was successful within its limitations, as shown by comparing the analyses for short and long ageing times (i.e. smaller and larger particles) at the same temperature. The presence of nitrogen in the particles identified as nitrides was confirmed, for one large particle, by EPMA analysis. The analysis also showed carbon, but this varied with time, indicating that it was deposited from the vacuum system. We shall assume that these particles are nitrides, even though this analysis was not satisfactory and we cannot rule out the presence of some carbon. The backscattered contrast of these particles indicated a low mean atomic number. Ferrite and secondary phases were quantified in the as-received and aged materials, respectively, by using the manual point count method in accordance with ASTM E 562 [17].

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The total number of points used was at least 4000 distributed as 100 points in a minimum of 40 separate fields of view. The spacing between fields was uniform and followed a systematic array (matrix type) to cover most of the area of 1 cm2 used. Depending on the size of particular particles, a suitable magnification (250–800·) was selected; as an increase in magnification decreased the field of examined area and enhanced the field to field variability, so the number of fields to be observed was increased as convenient to obtain a volume fraction measurement with a minimum relative accuracy of 10%, which allowed volume fractions of approximately 0.2% or more to be quantified. Samples for electrochemical testing were cut from the ends of fractured impact specimens, avoiding deformed material. The face cut and exposed to the testing medium was therefore transverse to the original long axis of the Charpy sample. All samples were mounted in epoxy resin, hole drilled in the back of the mount, and electrical connection achieved by spot-welding to a nichrome wire. The area of the working electrode was approximately 1 cm2; electrode preparation consisted of grinding, polishing to 1/0.25 lm diamond finish and edge-sealing to minimize crevices, prior to exposure to the test solution. Anodic polarization curves were measured at room temperature in 0.1 M Na2CO3 solution, using an EG&G-PAR 273 A potentiostat controlled by a computer through the CorrWare 2 software. The potential scan rate was 0.25 mV/s. A three-electrode system was employed, which consisted of an auxiliary platinum counter electrode, a saturated calomel reference electrode (SCE), and the test material as the working electrode. The volume of the cell was 250 ml and no deaeration was provided (this is unnecessary for studies that are mainly concerned with the transpassive region, where the oxygen reduction current is many orders of magnitude lower than even the passive current). In this weakly alkaline solution (0.1 M Na2CO3) the polarization curves for the aged samples showed diverse peaks at the onset of transpassivity. For the most part, these peaks were temporary phenomena—with time, the current would decrease nearly to zero. These peaks were present within a region extending from approximately 0.2 to 0.7 V (SCE) and from 106 to 105 A/cm2; their detection could be enhanced by using a two-step procedure consisting of a potentiostatic hold (to give a reproducible starting condition) followed by weak galvanostatic anodic polarization using a current in the range seen on the polarization curves (or a little higher, since the low scan rate had already allowed the current at each peak to decay substantially). One candidate standard procedure consisted of applying a pre-passivation potential of 0.25 V (SCE) for 1 min followed by a current density of 10 lA/cm2 for at least 10 min. The electrode potential was recorded as a function of time and secondary phase detection was associated with multiple arrests or inflections in the potential–time curve. In the absence of deleterious phases, the potential would rise almost linearly to a value where the alloy as a whole was evolving oxygen. With deleterious phases present, the potential would arrest at particular values depending on the particular phases present. The effect of test current density on electrode potential evolution was evaluated in order to maximize discrimination amongst material conditions. The parameters for the final ÔstandardÕ technique were optimized after testing several potential–current density combinations. Potentiostatic etching tests were performed on selected samples; etching at particular current peaks was done for long times (60 h). These tests were aided by optical and scanning electron microscopy in order to detect preferential etching on a particular phase; back-scattered images were taken before and after testing. High purity metallic iron

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(99.99%), chromium (99.70%) and molybdenum (99.95%) as well as alloy Nickel-200 (>99% Ni), obtained as annealed bars of different diameters, were also characterized by polarization curves. Their electrochemical behaviour was taken as a reference for preferential transpassive dissolution of elements when compared with the as-received and aged materials. Sample preparation, instruments and equipment were the same as those indicated above; the electrode areas varied according to material availability. 3. Results and discussion 3.1. Identification of phases Fig. 2 shows examples of the conventionally etched microstructures observed after some of the isothermal ageing treatments. The compositions of all the samples aged at 825 and 850 C (Table 2) determined by microanalysis showed the characteristic composition of v [18], r [19] and chromium nitride (except for nitrogen) [20]. The composition of the duplex steel after microanalysis of ferrite and austenite was calculated by multiplying the compositions of d/c times the volume fraction ratio of 4:6 and then comparing to the original steel composition to verify microanalysis accuracy. Although standard errors based on repeatability were sometimes in the range of 0.03%, we recognize that there are

Fig. 2. Light microscopy of UNS S32760: (a) as-received condition; (b) aged at 675 C—1000 s; (c) 825 C— 300 s; (d) 825 C—1000 s after electrolytic etching in oxalic acid (10%) followed by KOH (10 N).

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Table 2 Compositions (wt%) of phases in the as-received and aged condition Fe

Cr

Ni

Mo

W

Cu

Mn

Si

V

Steel as-received d c (d + c)

62.6 61.7 63.9 63.1

25.0 27.6 24.3 25.6

7.0 5.0 7.7 6.6

3.6 4.0 2.4 3.0

0.5 0.6 0.4 0.5

0.5 0.3 0.6 0.5

0.5 0.6 0.5 0.5

0.2 0.2 0.2 0.2

0.1 – – –

825 C—100 s v r

54.6 57.0

28.3 31.0

3.8 4.2

10.7 6.2

1.5 0.6

0.1 0.2

0.6 0.5

0.4 0.3

– –

825 C—300 s v r

53.1 58.1

27.2 31.5

3.8 4.3

12.6 4.5

2.0 0.5

0.2 0.2

0.7 0.7

0.4 0.2

– –

825 C—1000 s v r

51.8 57.0

27.0 32.0

3.5 4.0

14.3 5.3

2.2 0.6

0.1 0.2

0.6 0.7

0.5 0.2

– –

825 C—113 h v r Nitride

51.4 55.2 19.7

27.9 32.8 72.6

3.2 3.4 1.3

14.4 6.7 3.0

2.0 0.8 –

0.1 0.1 –

0.6 0.6 –

0.4 0.4 0.3

– – 3.1

larger systematic errors than that in the EDX technique, so we have adopted the following procedure: for major alloying elements, where the standard error was less than 0.1%, we have quoted the result to 3 significant figures and given no error; where the error is greater than 0.1%, we have quoted an appropriate number of significant figures. 3.2. Alkaline testing As a baseline, Fig. 3 shows the anodic polarization behaviour of UNS S32760 treated at different temperatures and times in the alkaline solution. The precipitation of Cr and Mo rich phases generates electrochemical activity in the range of 0.25–0.7 V (SCE) associated with transpassive dissolution of the Cr and Mo components. The purpose of the galvanostatic test (Fig. 4) is to make a horizontal cut through these features (whose heights are time-dependent) and if possible to detect them one by one, i.e. to discriminate the different deleterious phases. This figure clearly shows the presence of multiple arrests in potential depending on the particular phases present; the lowest arrest is hypothesized to be due to Cr2N, the next to v and the highest (which sometimes splits into two features) to r. This reasoning is based on the lack of iron in the nitride, which should thus dissolve at or near the transpassivity potential of pure Cr, and the high Mo content in v, which should lower its transpassivity potential compared with r. The TTP diagram for secondary phases in a duplex steel of the type 25Cr–7Ni–4Mo (Fig. 1) suggests that phases precipitate in this order in UNS S32760 at 825 C at short times >3 min: chromium nitride, v and r phases. However at 675 C, the presence of r phase is not expected until about 1000 s [15]. Fig. 5 shows the potentiodynamic curves for high purity electrodes of Fe, Cr, Ni and Mo, compared with annealed and aged samples of the duplex stainless steel. Mo shows a very low transpassivity potential, but even in v phase there is not enough Mo to sustain such dissolution and its transpassivity is shifted to a much higher potential where both Cr

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10

-3

As-received material Aged at 675°C-1000 s Aged at 750°C-1000 s Aged at 825°C-1000 s Aged at 850°C-120 h

-4

I (A/cm2)

10

2567

σ χ 10

-5

Nitride

10

-6

0

0.2

0.4 0.6 E (V vs SCE)

0.8

1

Fig. 3. Polarization curves for UNS S32760 in naturally aerated 0.1 M Na2CO3 solution at room temperature, showing an increase in electrochemical activity in the transpassive region with ageing. Labels show the phases that are hypothesized to be responsible for particular features (scan rate 0.25 mV/s).

0.75

E (V vs SCE)

0.65

0.55

0.45

σ

σ

χ

As-received material Aged at 675°C-1000 s Aged at 750°C-1000 s Aged at 825°C-1000 s Aged at 850°C-120 h

Chromium nitride 0.35

0.25 0

1200

2400

3600

4800

6000

Time (s) Fig. 4. Electrode potential of UNS S32760 during anodic galvanostatic polarization at 10 lA/cm2 in 0.1 M Na2CO3 solution at room temperature, following pre-passivation at 0.25 V (SCE) for 1 min. Labels show the phases hypothesized to be responsible for particular features in the potential–time curves.

and Mo can dissolve. On the other hand, the transpassivity potential of Cr almost coincides with the first peak for the aged steel that is hypothesized to be due to chromium nitride; in the absence of iron, Cr can dissolve freely from the nitride. The highest peak

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10

-3

10

-4

I (A/cm2)

10

χ -5

10

σ

Nitride

-6

10

As-received material Aged at 850°C-120 h Fe-99.99% Ni-99.00% Cr-99.70% Mo-99.95%

-7

10

-8

10

-9

10

-0.6

-0.4

-0.2

0

0.2 0.4 E (V vs SCE)

0.6

0.8

1

Fig. 5. Potentiodynamic scans on polished samples of iron, chromium, nickel, molybdenum, and annealed and aged UNS S32760, in 0.1 M Na2CO3 solution at room temperature at a scan rate of 0.25 mV/s. Preliminary peak identifications are indicated for the aged steel.

for the aged steel, thought to be due to r phase, appears at a potential where pure Cr would be dissolving at its maximum rate; there is little assistance from Mo in establishing this potential. The best chance to obtain a clear correlation of phases with current peaks or potential arrests was to age a set of samples of UNS S32760 at 825/850 C for a long period—113/ 120 h —and then etch potentiostatically for long times to reveal the otherwise very subtle attack on the secondary phases (for the galvanostatic tests, this amounted to less than 1 mC/cm2, referred to the whole surface; thus for a phase with a volume fraction of say 1%, the attack is less than 100 mC/cm2, or about 20–40 nm of metal oxidized, depending on the oxidation state). This treatment allowed the precipitation of large particles of nitride and v phase in a matrix of r and c2 (Fig. 6). A potentiostatic etching test was then performed on these samples; first at 0.28 V (SCE), where the first characteristic peak was observed, for 2.5 days. BSE-imaging after testing (Fig. 7) clearly revealed a preferential etching of nitride particles. The apparent matrix dissolution around some of these particles is probably an artifact due to the presence of an electron-transparent metal layer over the undercut surface at the vacant particle site. The complete loss of nitrides would be consistent with the lack of iron or nickel that could provide a passive film after selective dissolution of Cr and Mo; V is present but should also dissolve transpassively. Samples aged at 825 C—1000 s were etched at 0.27 V (SCE), a potential where nitride dissolution was suggested by the potentiodynamic curves obtained at 825 C for longer ageing times. BSE-imaging before testing on samples aged at 825 C—1000 s (Fig. 8) displayed two types of nitride precipitation; a small round nitride mainly formed at the d/c grain boundaries, and a fine nitride dispersion at the d/d and d/c grain boundaries. Images after testing (Fig. 9) revealed a preferential etching/dissolution of round nitride particles

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Fig. 6. Back-scattered electron image of UNS S32760 aged at 825 C—113 h, before potentiostatic etching. The apparent cavities are nitride particles.

Fig. 7. Back-scattered electron image of UNS S32760 aged at 825 C—113 h, after potentiostatic etching at 0.28 V (SCE) for 2.5 days in 0.1 M Na2CO3 solution at room temperature.

together with the enhanced visibility (and presumed dissolution) of the fine nitride dispersions. Based on this information, namely the preferential dissolution of nitride particles under both ageing conditions (825 C—1000 s, 825 C—113 h), it can be concluded that the feature around 0.27–0.28 V (SCE) in the potentiodynamic curves can be associated with the presence of chromium nitride. By varying the current used in the galvanostatic test, this feature could be emphasized (though shifted to higher potential at higher currents)—for example, at 35 lA/cm2 (Fig. 10) we can clearly display the coincidence of the first potential

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Fig. 8. Back-scattered electron image of UNS S32760 aged at 825 C—1000 s, before potentiostatic etching.

Fig. 9. Back-scattered electron image of UNS S32760 aged at 825 C—1000 s, after potentiostatic etching at 0.27 V for 2.5 days in 0.1 M Na2CO3 solution at room temperature.

arrest in samples aged at 675 and 825 C. The reason that the nitride feature tends to show a distinct arrest, rather than merely an inflection, is that the dissolution from nitrides occurs at a more constant rate than the other phases, which gradually passivate with time due to iron enrichment in the surface film. Potentiostatic etching at 0.44 V (SCE) was also performed on samples aged at 825 C for 1000 s, a potential where the reaction of v phase was hypothesized. The image before testing (Fig. 11) shows bright v phase particles generated by this heat treatment, while after testing (Fig. 12) it reveals a complete dissolution of nitride particles together with

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0.80

E (V vs SCE)

0.70

0.60

0.50 As-received material Aged at 675°C-1000 s Aged at 825°C-1000 s Aged at 825°C-113 h

0.40

0.30 0

120

240

360

480

60

Time (s) Fig. 10. Galvanostatic response of UNS S32760 at 35 lA/cm2 in 0.1 M Na2CO3 solution at room temperature, following pre-passivation at 0.25 V (SCE) for 1 min.

Fig. 11. Back-scattered electron image of UNS S32760 aged at 825 C—1000 s, before potentiostatic etching.

partial dissolution of v phase, but an intact r phase. Additionally, a potentiostatic etching test was undertaken at 0.60 V (SCE) on samples aged at 825 C for 1000 s, a potential where the third peak for r phase was hypothesized. The image before testing (Fig. 13) displayed nitride, v and r phases, while after testing (Fig. 14) a complete dissolution of nitride and significant attack of v occurred, while r phase at grain boundaries and within ferrite showed evidence of partial dissolution. Fig. 15 shows this effect at 0.60 V (SCE)

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Fig. 12. Back-scattered electron image of UNS S32760 aged at 825 C—1000 s, after potentiostatic etching at 0.44 V (SCE) for 2.5 days in 0.1 M Na2CO3 solution at room temperature.

Fig. 13. Back-scattered electron image of UNS S32760 aged at 825 C—1000 s, before potentiostatic etching.

after an even longer (four-day) test, where the propagation r of phase attack across some particles was evident. There is some evidence in Fig. 3 that the features ascribed to v and r shift in potential with ageing time or temperature (note that a similar analysis is not possible for the galvanostatic tests because there is inherently such a shift—the current density on each phase varies with volume fraction and polarization time). It is difficult to discern a clear trend in phase composition in Table 2 that might explain the effect of time, even though the last treatment (113 h) is such that the entire microstructure is transformed into a new

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Fig. 14. Back-scattered electron image of UNS S32760 aged at 825 C—1000 s, after potentiostatic etching at 0.60 V (SCE) for 2.5 days in 0.1 M Na2CO3 solution at room temperature.

Fig. 15. Back-scattered electron image of UNS S32760 aged at 825 C—1000 s, after potentiostatic etching at 0.60 V (SCE) for 4 days in 0.1 M Na2CO3 at room temperature.

arrangement of c2 and r. A complicating factor is that the EDX analyses are less reliable at shorter times, where the particles are smaller and some contribution of the substrate to the X-ray signal is possible, even for particles several microns across (see for example the slight but consistent increase in the apparent Mo content of v phase with ageing time). The double peak sometimes seen in the region ascribed to r is another complicating factor. Probably there is a real increase in the peak potentials for v and r with increasing ageing temperature, due to a slight reduction in the equilibrium Mo

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and Cr contents of these phases with increasing temperature. The split peak for r may not be due to the presence of two distinct chemical compositions—there could be a twostage transpassivity mechanism, as seen for stainless steels in acid. It is tempting though to suggest that r can grow with two slightly different compositions depending on the neighbouring phase. This could be investigated in further work. It is not particularly relevant to the validity of the galvanostatic test. 3.3. Correlation with mechanical and corrosion properties The correlation between Charpy impact toughness and the volume fraction of r phase is given in Fig. 16. The presence of minor amounts of r phase reduces impact toughness. It is important to note that for samples aged at 825 C, impact toughness fell below 27 J after only 5 min of isothermal ageing; SEM-EDX confirmed that r phase was the main intermetallic precipitated at this temperature (Fig. 2). Charpy impact toughness was the parameter used to correlate the volume fraction of r phase with the galvanostatic electrochemical response. The electrode potential after a particular time, or the time to reach a particular potential (the latter is more logical but not necessarily more useful) can be correlated with toughness, and variations in current density can be used to tune the test to produce either a threshold type of response or a quasilinear response as shown in Figs. 17 and 18. In the alkaline solution, Mo and N are particularly reactive, and there is a concern that the test might overstate the importance of phases rich in these elements, at the expense of the r phase (at least for mechanical properties—not for corrosion properties which are severely degraded by Mo depletion due to v phase). However the nitride phase can be substantially pre-dissolved during the pre-passivation stage to avoid interfering with the most deleterious phase (r).

Impact toughness (J/cm2) at -50° C

300 250 200 150 100 50 0

0

2

4

6

8

10

% σ-phase Fig. 16. Correlation of Charpy impact toughness with volume fraction of phase in UNS S32760 aged at 675, 750, 825 and 850 C for different times.

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0.85

Potential (V vs SCE)

0.80 0.75 0.70 10μΑ/cm² 15μΑ/cm²

0.65

25μΑ/cm² 35μΑ/cm²

0.60

50μΑ/cm²

0.55 0.50

0

50

100 150 200 Charpy impact toughness (J/cm2) at -50˚C

250

300

Fig. 17. Correlation of alkaline galvanostatic test results (potential after 500 s) with Charpy impact toughness for UNS S32760, showing the effect of test current density.

1000 25μΑ/cm²

800

35μΑ/cm²

Time (s)

50μΑ/cm²

600

400

200

0

0

50

100 150 200 Charpy impact toughness (J/cm2) at -50˚C

250

300

Fig. 18. Correlation of alkaline galvanostatic test results (time to reach 0.75 V vs. SCE) with Charpy impact toughness for UNS S32760, showing the effect of test current density.

4. Conclusions The alkaline galvanostatic test has the ability to detect and distinguish minor contents of the most deleterious phases (chromium nitride, v, r) by the transpassive dissolution of Cr and/or Mo in these phases. In principle, Cr nitrides can be pre-reacted in the alkaline

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test by a prior polarization treatment, so that they do not interfere with the quantification of the other damaging phases. The current-peak potentials of 0.27, 0.44 and 0.60 V (SCE), determined after anodic polarization in the alkaline solution, are related to the presence of chromium nitride and well-formed particles of v and r phases, respectively. The stabilizing effect of Fe and Ni on the transpassive dissolution of Mo and Cr in intermetallics delays but does not prevent their detection; in fact, the differing passivation tendency of Fe in these phases greatly aids their discrimination. There are promising correlations between the volume fraction of secondary phases and the mechanical and corrosion properties of duplex stainless steel. Acknowledgements The authors would like to thank Materials Engineering Ltd. of Aberdeen, who kindly performed the mechanical testing. We would also thank to Weir Materials and Foundries, Manchester, for supply of material. Acknowledgments are also due to the Instituto Mexicano del Petro´leo for sponsorship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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