Next Article in Journal
Characterization of the Micro-Arc Coatings Containing β-Tricalcium Phosphate Particles on Mg-0.8Ca Alloy
Previous Article in Journal
Mechanical Behavior of Two Ferrite–Martensite Dual-Phase Steels over a Broad Range of Strain Rates
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Metals
Volume 8
Issue 4
10.3390/met8040237
metals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Stress-Corrosion Cracking Behaviour of Lean-Duplex Stainless Steels in Chloride/Thiosulphate Environments

by
Federica Zanotto
1,2,*,
Vincenzo Grassi
1,
Andrea Balbo
1,
Cecilia Monticelli
1 and
Fabrizio Zucchi
1
1
“Aldo Daccò” Corrosion and Metallurgy Study Centre, University of Ferrara, Via G. Saragat 4A, 44122 Ferrara, Italy
2
Terra&AcquaTech Laboratory, University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy
*
Author to whom correspondence should be addressed.
Metals 2018, 8(4), 237; https://doi.org/10.3390/met8040237
Submission received: 1 March 2018 / Revised: 24 March 2018 / Accepted: 2 April 2018 / Published: 4 April 2018
Browse Figures
Versions Notes

Abstract

:
The stress-corrosion cracking (SCC) behaviour of two lean-duplex stainless steels (DSS 2304 and LDSS 2404) was studied by slow strain-rate tests (SSRT) in 20% NaCl solution at 80 °C (pH about 6) and in NACE TM-0177 solution at 25 °C (pH 2.7), both in the absence and in the presence of thiosulphate ions (S2O32−). The SCC susceptibility of the two alloys was compared to that of LDSS 2101 investigated in a previous study. LDSS 2404 was always immune to SCC, while DSS 2304 (and LDSS 2101) suffered this corrosion form at specific concentrations. The high SCC resistance of DSS 2404 in both environments was connected to its high Mo content, while the significant SCC susceptibility of LDSS 2101 in NACE TM-0177 solution was likely due to the high Mn content of the alloy.

1. Introduction

Duplex stainless steels (DSS) are very interesting materials due to the favourable combination of their mechanical properties, weldability and corrosion resistance in various environments. These characteristics support their use in various fields, from the paper to the petrochemical industries and from the construction sector to nuclear energy production [1]. Compared to conventional DSS (i.e., DSS 2205), lean-duplex stainless steels (LDSS) are grades with lower nickel (i.e., LDSS 2101, LDSS 2404) and/or molybdenum (i.e., DSS 2304) content [2,3]. Their employment is considered when they can lead to economical benefits in the design and maintenance of industrial plants or civil construction. Oil and gas equipment often operates in contact with chloride- and sulphide-rich media. The use of LDSS is proposed mostly on the basis of their pitting and stress-corrosion cracking (SCC) resistance in these environments [4]. Many papers deal with the SCC resistance of DSS 2205 [5,6,7,8,9] and superduplex stainless steels [10,11,12,13] in hydrogen sulphide (H2S) media. By contrast, few data are reported about the SCC behaviour of LDSS in these media. Johansson et al. [4] showed that no SCC was observed at H2S levels up to 0.15 bar for LDSS 2101 and 1 bar for DSS 2304 in NACE TM-0177. Ruel et al. [14] found that DSS 2304 had a higher SCC resistance in comparison to LDSS 2101 in 50 g/L NaCl, 5 g/L CH3COONa, at pH 2.8, with 50% H2S, at the temperature of 20 °C. They also suggested that Mn has a negative effect on SCC resistance, while N has a positive one. The SCC susceptibility of LDSS 2404 in environments containing H2S has not yet been investigated. Therefore, the study of the corrosion performances of LDSS in simulated oil and gas conditions is reputedly quite important. DSS 2304 was the first commercialized lean duplex. It was developed as a σ-free and low-cost grade, due to the Mo savings (Mo mass % around 0.3%) [1]. Among LDSS currently on the market, the 2101 grade is a suitable candidate for application in the oil and gas industry, in particular for mild well conditions and topside components [4], where it can satisfactorily substitute standard austenitic grades [15,16]. On the contrary, LDSS 2404 is a recently introduced Mo- and N-containing LDSS grade, designed as an economical alternative to the 2205 grade [17]. In our laboratory, we studied the corrosion resistance of LDSS 2101 extensively [18,19,20,21]. In particular, we focused on its SCC susceptibility in chloride solutions containing thiosulphate ions (S2O32−) [18,20]. The approach to add S2O32− instead of H2S gas in acid salt solutions in order to simulate the aggressiveness of sour well environments was suggested by Tsujikawa et al. [22], in order to study the pitting and SCC susceptibility of alloys in simulated well conditions, while minimizing the health hazards during laboratory tests and reducing the cost of equipment required to operate under safety conditions. In particular, they found that materials susceptible to SCC in H2S-containing solutions exhibited the same behaviour in the presence of S2O32− [23]. Since S2O32− is thermodynamically metastable, it may undergo disproportion and reduction reactions on the alloy surface to yield elemental S, which can be further reduced to H2S, depending on the solution pH and on the alloy corrosion potential [24]. As an example, H2S forms spontaneously on a carbon steel surface under free-corrosion conditions in acidic S2O32− solutions [25]. On the contrary, in the same solutions a continuous surface scratching is needed to detect H2S gas formation on a 316-type stainless steel [23], because scratching causes a shift of the alloy-corrosion potential, into the H2S stability domain.
Some research studies [26,27,28] helped to clarify the detrimental effect of S2O32− dissolved in aqueous solutions on the corrosion resistance of stainless steels. They showed that adsorbed sulphur monolayers can form on Fe, Ni, and Cr, due to S2O32− reduction on the metal surface, also under potential pH conditions in which usual Pourbaix diagrams predict no metal sulphide stability [26]. Moreover, they demonstrated that in neutral chloride solution, S2O32− exerts a deleterious effect on the pitting resistance of a Fe-17Cr alloy on which the S2O32− reduction occurs only in correspondence of bare alloy regions, leading to the formation of sulphide islands incorporated into the oxide layer and/or sulphur adsorbed at the metal/oxide interface [27]. Adsorbed sulphur is reported to inhibit repassivation in correspondence of passive film flaws formed by chloride ions, thereby affecting the corrosion resistance of stainless steels [26,29]. Depending on the conditions of the slow strain-rate tests (SSRT), in particular at sufficiently acidic pH, S2O32− addition in chloride solutions may produce both H2S and S, which can inhibit repassivation and catalyse the ingress of hydrogen in the alloys thus inducing sulphide-induced SCC (S-SCC) in stainless steels [28].
In our preceding work [18], SSRT were performed on LDSS 2101 during exposures to 20% NaCl solution at 80 °C (near neutral pH) and to standard NACE TM-0177 solution at 25 °C and pH 2.7, in the absence and in the presence of S2O32− at various concentrations. LDSS 2101 was susceptible to SCC in 20% NaCl solution at 80 °C in the presence of S2O32− at 10−3 M. However, the susceptibility decreased by increasing S2O32− concentration, due to a significant selective dissolution at the expense of the ferrite phase. The corrosion potential values recorded during the tests were at the limit of the H2S stability domain [23] and well within the stability domains of sulphur adsorbed on Fe, Ni and Cr [26]. Thus, at high S2O32−concentration, the selective corrosion of the ferrite phase, catalysed by adsorbed sulphur and stimulated by the galvanic coupling with the nobler austenite phase became competitive with SCC, reducing the SCC susceptibility of LDSS 2101, likely through crack tip blunting [30]. In NACE TM-0177 solution at 25 °C and pH 2.7, LDSS 2101 was susceptible to SCC even at 10−4 M S2O32− and the SCC susceptibility increased at increasing S2O32− concentration up to 10−2 M. Under these acidic conditions, the recorded corrosion potential values were significantly more negative than the equilibrium potentials of S2O32− reduction to both S and H2S [24], suggesting that S-SCC was the predominant failure mechanism, particularly at higher S2O32− concentrations. Our research studies [18] and other literature results [4] showed that, in NACE TM-0177 solution containing S2O32− or H2S, the selective dissolution of the ferrite phase became competitive with S-SCC only at higher pH and temperatures.
In this work, the SCC behaviour of LDSS 2404 and DSS 2304 was studied in 20% NaCl solution at 80 °C and in the standard NACE TM-0177 solution at pH 2.7 and temperature of 25 °C, both in the absence and presence of S2O32−, and was compared with LDSS 2101 behaviour under the same test conditions. The adopted S2O32− concentrations were selected on the basis of the results found for LDSS 2101 [18], so 10−2 and 10−1 M were used in 20% NaCl and 10−3 and 10−2 M were selected for NACE TM-0177 solution at pH 2.7. At the end of the tests, the samples were observed under an optical microscope (OM) to assess the morphology of the corrosive attack.

2. Materials and Methods

The tests were performed on LDSS 2404 and DSS 2304 stainless steels (supplied by Outokumpu S.p.A., Sheffield, United Kingdom, under annealed conditions), having the nominal chemical compositions (in mass %) reported in Table 1 [31], where also the composition of the reference LDSS 2101 is reported. The alloy Pitting Resistance Equivalent Number (PREN) values are also reported. This index, specifically developed to predict the resistance to pitting corrosion of Stainless Steels (SS) [32], is now considered a simplified way to compare SS corrosion performances against the dependence of their chemical composition and was recently applied also to DSS [33]. Usually, it is calculated on the basis of the Cr, Mo and N contents of the SS, from the formula reported in Table 1. The PREN values predict that LDSS 2404 is the most corrosion-resistant alloy because of its high Cr, Mo and N content, while DSS 2304 and LDSS 2101 are expected to present comparable resistance to localized corrosion attack.
The microstructure and the corrosion attack morphologies were documented by observations of the long transversal sections of tensile samples under an optical microscope (OM), after surface polishing with diamond pastes (up to 1 µm) and etching with Beraha’s reagent.
The susceptibility to SCC was evaluated by slow strain-rate tests (SSRT), with a strain rate of 1 × 10−6 s−1 [34]. Tensile samples were machined from 1.5 mm-thick steel sheets, with their longitudinal direction parallel to the sheet lamination direction. They had an overall length of 230 cm and a gauge portion of 20 × 5 × 1.5 mm. Their surface was ground (always parallel to the stress direction) down to 800 grit emery papers, rinsed with deionized water, and degreased with acetone. Finally, they were screened by a two-component epoxy varnish, thereby leaving only the gauge portion exposed to the solution.
For SSRT, the tensile samples were inserted in an electrochemical cell filled by the following nitrogen-deaerated and thermostated solutions:
  • 20% NaCl in the absence and in the presence of sodium thiosulphate (Na2S2O3), at concentrations of 10−2 and 10−1 M, T = 80 °C (measured pH 6); or
  • 5% NaCl + 0.5% CH3COOH (the basic standard solution NACE TM-0177 [35], without H2S gas) in the absence and in the presence of Na2S2O3 at concentrations of 10−3 and 10−2 M, T = 25 °C (measured pH 2.7).
SSRT were also performed in demineralized water at 80 °C and in air at 25 °C, as reference conditions. During each test, the stress–strain curve was recorded. Stress (in MPa) is the ratio of the applied load to cross-sectional area of the gauge portion (5 mm × 1.5 mm) in tensile samples, while strain is the ratio of the sample elongation to its original length, as evaluated from the relative movement of the tensile-machine crosshead. The obtained strain values are unitless, but they were expressed in percentage, as they were multiplied by 100. In parallel to stress–strain curve recording, the open circuit potential (EOCP, versus saturated calomel electrode (SCE)) values were measured. Each test was performed in triplicate.
The SCC susceptibility was evaluated by the ratio (R) between the percentage strain to fracture (εf %) in the test solution and that in air (for 25 °C tests) or water (for 80 °C tests). R values equal to or higher than 0.8 were considered an index of immunity to SCC [36].
At the end of the tests, the gauge length section of the samples were observed with an optical stereomicroscope and side surfaces were examined by an OM, after polishing and etching with Beraha’s reagent, with the purpose of analysing crack initiation and morphology.
On unstressed samples with an exposed surface area of 72 mm2, EOCP measurements were performed during 42 h of immersion in the previously described aggressive solutions. After about 24 h of immersion, on still passive electrodes a scratch was performed by using a glass tip to verify their repassivation capability. At the end of the 42 h immersion, anodic polarization curves were recorded, starting from EOCP, with a scan rate of 0.2 mV/s.

3. Results

3.1. Microstructures of the Studied Alloys

Figure 1 shows the microstructures of DSS 2304 and LDSS 2404 in their long transversal sections, characterized by elongated austenitic grains (lighter phase) embedded in a ferritic matrix (darker phase). These microstructures are quite similar to those of LSDD 2101 [18] and are typical of rolled-duplex stainless steels. On the three alloys, the volume fraction of ferrite was always quite close to that of austenite (51 ± 1% ferrite and balance austenite), which corresponds to the correct phase balance for these products [37].

3.2. Slow Strain-Rate Test (SSRT) Results in 20% NaCl Solution at 80 °C without/with S2O32−

Figure 2 depicts the stress–strain curves of DSS 2304 and LDSS 2404 acquired during SSRT performed at 80 °C in water and in 20% NaCl solution, in the presence and in the absence of S2O32− at 10−2 and 10−1 M concentrations.
The εf % obtained in 20% NaCl solution for both alloys were very close to those measured in water and the samples presented a ductile type fracture like that shown in Figure 3a. Thus, the alloys were not susceptible to SCC in this environment. With the addition of 10−2 M S2O32−, εf % of DSS 2304 decreased from 47 (obtained in water at 80 °C) and 45 (obtained in chloride solution without S2O32−) to about 37%. The sample presented a fracture of a more brittle type (Figure 3b), in comparison to that shown in Figure 3a, with black corrosion products deposited on the gauge length near to the fracture surface. As expected [9,25,38], iron sulphides were detected in that region by scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM–EDS) analyses.
The micrograph of the long transversal section (parallel to the load direction in SSRT) presented in Figure 4a reveals the presence of selective corrosion of the ferrite phase, with a tendency to form crack-like attacks in some points along the sample gauge length.
When S2O32− concentration was increased to 10−1 M, εf % increased to a value close to that obtained with only chloride solution. The sample presented a moderately ductile type fracture, with black corrosion products close to the fracture region. In this case, only a selective attack of the ferrite phase can be observed by OM (Figure 4b). As already observed for LDSS 2101 [18], the increase in S2O32− concentration induced a significant dissolution of the ferrite phase and a reduction of attack localization and crack propagation.
In the presence of S2O32−, the εf % values of LDSS 2404 slightly decreased (Figure 2b) in comparison to that obtained in the absence of S2O32−, but no significant differences could be detected with the variation of S2O32− concentration from 10−2 to 10−1 M.
All 2404 samples tested in the presence of S2O32− showed a diffused corrosion attack with dark corrosion product formation (Figure 5a). The micrograph of the long transversal section of the sample tested in the presence of S2O32− at 10−2 M, reported in Figure 5b, only evidences a scarcely penetrating selective attack of the ferrite phase. This corrosion form characterized also the sample tested in the presence of 10−1 M S2O32−.
Table 2 shows the average εf % values from all SSRT, including the εf % standard deviations.
Figure 6 presents the EOCP-strain trends acquired during SSRT on DSS 2304 and LDSS 2404. In the absence of S2O32−, the EOCP of DSS 2304 tested around −0.33 VSCE, while that of LDSS 2404 showed slightly more positive values (around −0.25 VSCE at the end of the test). These EOCP values are compatible with the formation of a protective oxide layer [28]. In the presence of 10−2 M S2O32−, the EOCP of DSS 2304 shifted towards more negative values, reaching a plateau at −0.47 VSCE, with a further dropping to about −0.5 VSCE at the end of the test. Conversely, in this environment, the EOCP of LDSS 2404 was initially similar to that measured in the only chloride solution (red dashed curve), but, after a few hours of the test, it dropped to values more negative than −0.4 VSCE, then reaching about −0.45 VSCE. When S2O32− concentration was increased to 10−1 M, both EOCP of DSS 2304 and that of LDSS 2404 stabilized at around −0.43 VSCE. As already noticed for LDSS 2101 [18], all EOCP values recorded in deaerated 20% NaCl solution at 80 °C (measured pH 6) in the presence of S2O32− were located at the limit of the stability domain of H2S [23] and fell well within the domain of adsorbed sulphur on the metal surfaces [26,28].
Figure 7 shows the EOCP-time trends recorded on DSS 2304 and LDSS 2404 electrodes during static immersions of 42 h in 20% NaCl solution at 80 °C, in the absence and in the presence of S2O32− at 10−2 and 10−1 M concentrations. In the absence of S2O32−, both alloys exhibited fast oscillations of EOCP around −0.1/−0.15 VSCE values and a scratch on the electrode surface after 24 h of immersion produced only a spike to about −0.2 VSCE, followed by fast repassivation (Figure 7a,b). This suggests that the alloys are able to maintain the integrity of the air-formed passive film. When 10−2 M S2O32− was added to the 20% NaCl solution, the EOCP values of DSS 2304 became more strongly negative from the beginning of the test, stabilizing at values close to −0.45 VSCE, in agreement with those measured during the SSRT in the same solution. This indicates that the air-formed passive film of DSS 2304 is no more compact and protective in this environment.
Conversely, EOCP of LDSS 2404 was about −0.15 VSCE in this solution until a scratch was produced, which determined a significant drop of EOCP to values lower than −0.40 VSCE (in agreement with trends observed during SSRT). These values were then maintained until the end of the test. This suggests that on this alloy the passive film is more stable than that formed on DSS 2304, but its repassivation at large flaws (scratches) is hindered by the likely conversion of S2O32− to sulphur and scarcely protective sulphides on the bare metal surface [27]. In the presence of 10−1 M S2O32−, both alloys exhibited EOCP values in the −0.45 ÷ −0.5 VSCE range, indicating active conditions. It is reasonable that the higher the S2O32− concentration, the higher the adsorbed sulphur coverage, which tends to inhibit the repassivation of film flaws and stimulate the corrosion attack, particularly on the less noble ferrite phase. Galvanic coupling with austenite may favour this selective attack [30,39,40,41,42].
Figure 8 presents the anodic polarization curves recorded in 20% NaCl solution without or with 10−2 M S2O32− on both alloys, after 42 h of immersion. In the absence of S2O32−, both alloys were passive and in spite of the scratch they exhibited an anodic curve with a wide passive-potential range spanning from EOCP up to a potential value where a rapid current increase was detected, due to pitting corrosion (Epit). LDSS 2404 showed a higher Epit and a lower passive-current density in comparison to DSS 2304, evidence of a superior passive-layer protectiveness in this environment. In a solution containing 10−2 M S2O32−, DSS 2304 was not scratched, because it was in an active state since the beginning of the test, with EOCP values around −0.45 VSCE. Its final anodic polarization curve shows a relatively low anodic slope, confirming that in this environment the passive film is markedly damaged. Conversely, LDSS 2404 which exhibited a noble EOCP during the immersion and underwent a 250 mV decrease after scratching, displayed a polarization curve typical of pseudopassive corrosion behaviour, evidenced by quite a high anodic slope of the polarization curve and relatively high passive currents (about 3 × 10−5 A cm−2). These are likely connected to the presence of the artificial defect in the oxide film. The steep anodic slope testifies to the high intrinsic corrosion resistance of the alloy in spite of the defect. In the presence of 10−1 M S2O32− (curves not shown), both alloys resulted active just after the immersion, due to the significant environmental aggressiveness, and polarization curves similar to that exhibited with DSS 2304 in 10−2 M S2O32− were obtained.
These tests suggest that in 20% NaCl solution the alloys are scarcely affected by the presence of flaws in the passive film, such as those artificially produced or those caused by tensile straining during SSRT, because fast spontaneous healing of the passive film occurs. This may hinder both SCC and general corrosion attack of the alloys in this environment.
S2O32− addition affects the alloy oxide stability of DSS 2304 under dynamic (Figure 6) and also under static (Figure 7 and Figure 8) conditions, at both S2O32− concentrations. On this alloy, active EOCP values are detected in conjunction with selective ferrite corrosion (static and SSRT) and SCC attack (SSRT at 10−2 M S2O32− concentrations). S2O32− is also aggressive towards LDSS 2404 (Figure 6 and Figure 7). In 10−2 M S2O32−, the alloy is passive under static conditions, but undergoes a 250 mV shift of EOCP towards the negative direction if the film is damaged by tensile strain (Figure 6) or by scratching (Figure 6, Figure 7 and Figure 8), due to the onset of ferrite corrosion. The histogram of Figure 9 compares the R index values calculated for DSS 2304 and LDSS 2404 to those obtained for LDSS 2101 [18]. An R value of 0.79 indicates a slight susceptibility to SCC of DSS 2304 in 20% NaCl solution at 80 °C in the presence of the lowest tested S2O32− concentration (10−2 M). The same behaviour was observed for LDSS 2101 [18]. No SCC susceptibility was detected for LDSS 2404.
The different behaviour of the studied alloys can be predicted from their PREN values (Table 1) and is in strong correlation with their Mo content. In particular, LDSS 2404 with its Mo percentage about 5 times higher than that of DSS 2304 and LDSS 2101 ensures the best corrosion behaviour. Literature results [43,44] show that alloyed Mo reduces SCC and pitting-corrosion susceptibility of Ni–Cr–Mo–Fe alloy in a H2S-Cl– environment, due to the formation of stable molybdenum sulphide in the outer layer of the surface film which, being cation-selective, protects the inner chromium oxide layer from Cl− attack. Moreover, Mo containing SS and DSS grades also exhibit a higher pitting-corrosion resistance in Cl−/S2O32− environments in comparison to low-Mo grades [45].

3.3. SSRT Results in NACE TM-0177 Solution at 25 °C without/with S2O32−

Figure 10 depicts the stress–strain curves of DSS 2304 (a) and LDSS 2404 (b) acquired during SSRT performed in air at 25 °C and in NACE TM-0177 solution at 25 °C, in the absence and in the presence of 10−3 and 10−2 M S2O32−.
In the absence of S2O32−, both alloys present stress–strain curves quite similar to those recorded in air at the same temperature, suggesting that they do not suffer SCC in this environment. At the lowest tested S2O32− concentration (10−3 M), DSS 2304 showed a stress–strain curve similar to that recorded in air (Figure 10) and a ductile-type fracture, without secondary cracks. At the highest tested S2O32− concentration (10−2 M), this alloy underwent a significant εf % reduction and several secondary cracks, likely developed from pits [30] (Figure 11), were detected in the gauge length portion of tensile specimens, characterized by a brittle fracture type. These secondary cracks propagated within the α phase and followed both γ/γ and α/γ grain boundaries, but an intense selective attack of the ferrite phase was not observed. Like DSS 2304, LDSS 2101 [18] also exhibited decreasing εf % at increasing S2O32− content in the NACE TM-0177 solution.
By contrast, LDSS 2404 maintained high ductility, comparable to that obtained in air at 25 °C, also in the presence of S2O32− at both tested concentrations, as shown in Figure 10. The ductile-type fracture obtained on this alloy in air and in the presence of 10−2 M S2O32− is shown in Figure 12.
The EOCP-strain curves for DSS 2304 and LDSS 2404 during SSRT in NACE TM-0177 solution containing or not containing S2O32− are presented in Figure 13. Without S2O32−, the EOCP of DSS 2304 set around −0.3 VSCE and that of LDSS 2404 to slightly more positive values (about −0.24 VSCE, towards the end of the test), at which a stable passive layer forms on both alloys [28]. When S2O32− was added, all EOCP values shifted towards more positive values with the exception of that recorded on DSS 2304, in the presence of the highest S2O32− concentration (10−2 M). In this case, the EOCP values decreased down to −0.5 VSCE, likely due to the synergistic effect of Cl− and S2O32− [24], favouring the development of pits from which a crack propagated. Under these acidic pH conditions, such a negative potential value permits the reduction of S2O32− to S and H2S [22,23] on the crack’s walls and favours H penetration and embrittlement of the alloy, inducing S-SCC. In this environment, a concentration of 10−4 M S2O32− was sufficient to induce S-SCC on LDSS 2101 and SCC susceptibility increased at increasing S2O32− concentration, because more H2S formed on the active sites of the sample surface [18].
Actually, at this low pH, the nobler EOCP values of −0.14/−0.3 VSCE achieved by LDSS 2404 in the presence of S2O32− and by DSS 2304 in diluted S2O32− solutions are also compatible with the reduction of S2O32− to adsorbed sulphur and then to H2S [26,28]. However, it is likely that, at these nobler EOCP values, only small amounts of S and H2S can be produced that may be insufficient to stabilize metastable pits from which SCC develops [21].
During static immersions in NACE TM-0177 solution containing 10−2 M S2O32−, EOCP-time trends (Figure 14) were recorded for 42 h on DSS 2304 and LDSS 2404 electrodes.
In spite of the absence of applied stress, the results were similar to those obtained during SSRT and shown in Figure 13. In fact, also under static conditions, the EOCP of DSS 2304 shifted rather quickly in the negative direction, from an initial value of −0.25 VSCE down to −0.5 VSCE. In this test, the electrode was not scratched. Conversely, LDSS 2404 exhibited stable noble EOCP values close to −0.1 VSCE, indicating the presence of passive conditions, capable of recovering quickly after scratching (at about 24 h of immersion, Figure 14). In Figure 15, the final anodic polarization curves are shown. They confirm that DSS 2304 is under active conditions, while LDSS 2404 exhibit a passive behaviour, with passive currents around 10−5 A cm−2 or lower, in spite of the presence of the scratch.
The histogram of Figure 16 compares the R index values calculated for DSS 2304 and LDSS 2404, to those obtained for LDSS 2101 [18] in NACE TM-0177 solution at 25 °C, in the absence and in the presence of S2O32−. DSS 2404 was never susceptible to SCC independent of S2O32− concentration, as R values were always very close to 1. By contrast, DSS 2304 (and LDSS 2101) showed an increase in SCC susceptibility at increasing S2O32− content. A 10−2 M concentration of S2O32− was necessary to induce SCC failure on DSS 2304, while a 10−4 M S2O32− addition was sufficient for LDSS 2101 [18]. Moreover, the latter alloy was significantly more susceptible to SCC in comparison to DSS 2304 at concentrations of 10−3 and 10−2 M.
In NACE TM-0177 solution, LDSS 2101 results significantly more susceptible to SCC in comparison to DSS 2304 (Figure 16), and a S2O32− concentration of 10−4 M is high enough to induce a slight SCC susceptibility on this alloy [18]. This behaviour may be connected to the presence of a relevant content of Mn, which has a detrimental effect on pitting-corrosion resistance [14,46]. A PREN formula different from that reported in Table 1 has been proposed [47,48], in order to rate alloys regarding their pitting-corrosion resistance and including the influence of Mn content. According to this formula (PRENMn = %Cr + 3.3%Mo + 30%N – 1%Mn), LDSS 2101 is less resistant (PRENMn = 24), than DSS 2304 ((PRENMn = 27) and LDSS 2404 (PRENMn = 34). Considering that in Cl−/H2S solutions SCC cracks are reputed to develop from pits [30], a high Mn content may contribute to affect pitting and thus SCC resistance in SSRT. In LDSS 2404, the detrimental effect of Mn is counterbalanced by the high content of Mo, so ensuring high SCC resistance [31,46].
In S2O32 containing 20% NaCl solution at 80 °C, the detrimental influence of Mn is likely less relevant to explain differences in SCC behaviour of the studied alloys, because pitting is not detected. Instead, the selective corrosion of the ferrite phase appears competitive with SCC. Therefore, LDSS 2101 and DSS 2304 show almost the same behaviour, while LDSS 2404 maintains high performance because of its high Mo content.

4. Conclusions

  • During SSRT, the studied LDSS grades did not exhibit SCC in 20% NaCl solution at 80 °C and in NACE TM-0177 solution at 25 °C and pH 2.7, in the absence of S2O32−.
  • The addition of S2O32− in 20% NaCl solution at 80 °C produced selective corrosion of the ferrite phase in all alloys.
  • In 20% NaCl solution containing S2O32−, a selective corrosion attack on the ferrite phase was in competition with SCC. The latter corrosion form prevailed at the lowest tested S2O32− concentration (10−2 M) on both DSS 2304 and LDSS 2101. At 10−1 M, ferrite dissolution produced crack blunting and SCC was not observed. LDSS 2404 was immune to SCC at both S2O32− concentrations.
  • In NACE TM-0177 solution at 25 °C and pH 2.7 containing S2O32−, no selective ferrite corrosion was observed. On LDSS 2101 and DSS 2304, the higher the S2O32− concentration, the stronger the alloy susceptibility to S-SCC. LDSS 2404 was immune to SCC.
  • The SCC behaviour of the three alloys studied strongly depended on their composition. In particular, the presence of a significant amount of Mo increased the SCC resistance of LDSS 2404 in both environments, while the high Mn content in LDSS 2101 negatively affected the alloy SCC resistance in NACE TM-0177 solution containing S2O32−, where pits play an important role as crack initiators.

Acknowledgments

This work was developed within the framework of the 2007–2013 Emilia-Romagna Regional Operational Programme of the European Regional Development Fund (ERDF ROP)–priority 1 Industrial research and technological transfer: “Creation of Technopoles for industrial research and technology transfer”. The material was kindly supplied by Outokumpu S.p.A.

Author Contributions

Fabrizio Zucchi and Cecilia Monticelli conceived and designed the experiments; Federica Zanotto and Vincenzo Grassi performed the experiments; Federica Zanotto and Andrea Balbo analyzed the data; Federica Zanotto, Cecilia Monticelli and Andrea Balbo wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Charles, J.; Chemelle, P. The history of duplex developments, nowadays DSS properties and duplex market future trends. In Proceedings of the 8th Duplex Stainless Steels conference, Beaune, France, 13–15 October 2010. [Google Scholar]
  2. Bhattacharya, A.; Singh, P.M. Role of microstructure on the corrosion susceptibility of UNS S32101 duplex stainless steel. Corrosion 2008, 64, 532–540. [Google Scholar] [CrossRef]
  3. Jiang, Y.; Tan, H.; Wang, Z.; Hong, J.; Jiang, L.; Li, J. Influence of Creq/Nieq on pitting corrosion resistance and mechanical properties of UNS S32304 duplex stainless steel welded joints. Corros. Sci. 2013, 70, 252–259. [Google Scholar] [CrossRef]
  4. Johansson, E.; Pettersson, R. Lean Duplex Stainless Steel within the Oil and Gas Industry. In European Corrosion Congress; Gubkin Russian State University of Oil and Gas: Moscow, Russia, 2010; Volume 4, pp. 2869–2878. [Google Scholar]
  5. Kudo, T.; Tsuge, H.; Moroishi, T. Stress corrosion cracking resistance of 22 duplex stainless steel in simulated sour environments. Corrosion 1989, 45, 831–838. [Google Scholar] [CrossRef]
  6. El-Yazgi, A.A.; Hardie, D. Stress corrosion cracking of duplex and super duplex stainless steels in sour environments. Corros. Sci. 1998, 40, 909–930. [Google Scholar] [CrossRef]
  7. De Moraes, F.D.; Bastian, F.L.; Ponciano, J.A. Influence of dynamic straining on hydrogen embrittlement of UNS-G41300 and UNS-S31803 steels in a low H2S concentration environment. Corros. Sci. 2005, 47, 1325–1335. [Google Scholar] [CrossRef]
  8. Sozanska, M.; Kłyk-Spyra, K. Investigation of hydrogen induced cracking in 2205 duplex stainless steel in wet H2S environments after isothermal treatment at 675, 750 and 900 °C. Mater. Charact. 2006, 56, 399–404. [Google Scholar] [CrossRef]
  9. Liu, Z.Y.; Dong, C.F.; Li, X.G.; Zhi, Q.; Cheng, Y.F. Stress corrosion cracking of 2205 duplex stainless steel in H2S–CO2 environment. J. Mater. Sci. 2009, 44, 4228–4234. [Google Scholar] [CrossRef]
  10. Mukai, S.; Okamoto, H.; Kudo, T.; Ikeda, A. Corrosion behaviour of 25 Pct Cr duplex stainless steel in CO2–H2S–Cl– environments. J. Mater. Energy Syst. 1983, 5, 59–66. [Google Scholar] [CrossRef]
  11. Rhodes, P.R.; Welch, G.A.; Abrego, L. Stress corrosion cracking susceptibility of duplex stainless steels in sour gas environments. J. Mater. Energy Syst. 1983, 5, 3–18. [Google Scholar] [CrossRef]
  12. Schofield, M.J.; Bradshaw, R.; Cottis, R.A. Stress corrosion cracking of duplex stainless steel weldments in sour conditions. Mater. Perform. 1996, 35, 65–70. [Google Scholar]
  13. Cao, L.; Anderko, A.; Gui, F.; Sridhar, N. Localized corrosion of corrosion resistant alloys in H2S-Containing environments. Corrosion 2016, 72, 636–654. [Google Scholar] [CrossRef]
  14. Ruel, F.; Saedlou, S.; Mendibide, C.; Manchet, S.L.; Wolski, K. Effect of the substitution of Ni by N and Mn in Lean Duplex Stainless steels on Stress corrosion Cracking assisted by H2S. In NACE–International Corrosion Conference Series; National Association of Corrosion Engineers: Houston, TX, USA, 2017; pp. 3376–3387. [Google Scholar]
  15. Olsson, J.; Snis, M. Duplex—A new generation of stainless steels for desalination plants. Desalination 2007, 205, 104–113. [Google Scholar] [CrossRef]
  16. Snis, M.; Olsson, J. Reduce costs for storage and distribution of desalted water—Use duplex stainless steel. Desalination 2008, 223, 476–486. [Google Scholar] [CrossRef]
  17. Yang, Y.; Tan, H.; Zhang, Z.; Wang, Z.; Jiang, Y.; Jiang, L.; Li, J. Effect of annealing temperature on the pitting corrosion behavior of UNS S82441 duplex stainless steel. Corrosion 2013, 69, 167–173. [Google Scholar] [CrossRef]
  18. Zanotto, F.; Grassi, V.; Balbo, A.; Monticelli, C.; Zucchi, F. Stress corrosion cracking of LDX 2101® duplex stainless steel in chloride solutions in the presence of thiosulphate. Corros. Sci. 2014, 80, 205–212. [Google Scholar] [CrossRef]
  19. Zanotto, F.; Grassi, V.; Merlin, M.; Balbo, A.; Zucchi, F. Effect of brief heat treatments performed between 650 and 850 °C on corrosion behaviour of a lean duplex stainless steel. Corros. Sci. 2015, 94, 38–47. [Google Scholar] [CrossRef]
  20. Zanotto, F.; Grassi, V.; Zucchi, F.; Merlin, M.; Balbo, A.; Monticelli, C. Effect of sensitization on stress corrosion cracking behaviour of LDX 2101 stainless steel. La Metallurgia Italiana 2016, 108, 24–33. [Google Scholar]
  21. Zanotto, F.; Grassi, V.; Balbo, A.; Monticelli, C.; Melandri, C.; Zucchi, F. Effect of brief thermal aging on stress corrosion cracking susceptibility of LDSS 2101 in the presence of chloride and thiosulphate ions. Corros. Sci. 2018, 130, 22–30. [Google Scholar] [CrossRef]
  22. Tsujikawa, S.; Miyasaka, A.; Ueda, M.; Ando, S.; Shibata, T.; Haruna, T.; Katahira, M.; Yamane, Y.; Aoki, T.; Yamada, T. Alternative for evaluating sour gas resistance of low-alloy steels and corrosion-resistant alloys. Corrosion 1993, 49, 409–419. [Google Scholar] [CrossRef]
  23. Tsujikawa, J. An alternative test method for evaluating the sour gas resistance of low-alloy steels and CRAs. In Proc. Corrosion ’92; NACE International: Huston, TX, USA, 1992; p. 45. [Google Scholar]
  24. Choudhary, L.; Macdonald, D.D.; Alfantazi, A. Role of thiosulfate in the corrosion of steels: A review. Corrosion 2015, 71, 1147–1168. [Google Scholar] [CrossRef]
  25. Kappes, M.; Frankel, G.S.; Sridhar, N.; Carranza, R.M. Reaction Paths of Thiosulphate during Corrosion of Carbon Steel in Acidified Brines. J. Electrochem. Soc. 2012, 159, C195–C204. [Google Scholar] [CrossRef]
  26. Marcus, P.; Protopopoff, E. Thermodynamics of thiosulphate reduction on surfaces of iron, nickel and chromium in water at 25 and 300 °C. Corros. Sci. 1997, 39, 1741–1752. [Google Scholar] [CrossRef]
  27. Duret-Thual, C.; Costa, D.; Yang, W.P.; Marcus, P. The role of thiosulphates in the pitting corrosion of Fe–17Cr alloys in neutral chloride solution: electrochemical and XPS study. Corros. Sci. 1997, 39, 913–933. [Google Scholar] [CrossRef]
  28. Marcus, P.; Oudar, J. Corrosion Mechanism in Theory and Practice, 2nd ed.; Marcel Dekker Inc.: New York, NY, USA, 1995; pp. 240–248. ISBN 0-8247-0666-8. [Google Scholar]
  29. Newman, R.C.; Isaacs, H.S.; Alman, B. Effect of sulfur compounds on the pitting behavior of type 304 stainless steel in near-neutral chloride solutions. Corrosion 1982, 38, 261–264. [Google Scholar] [CrossRef]
  30. Marcus, P.; Oudar, J. Corrosion Mechanism in Theory and Practice, 2nd ed.; Marcel Dekker Inc.: New York, NY, USA, 1995; pp. 342–346. ISBN 0-8247-0666-8. [Google Scholar]
  31. Canderyd, C.; Pettersson, R. UNS S82441—A new duplex stainless steel grade for the process industries. In NACE–International Corrosion Conference Series; National Association of Corrosion Engineers: Houston, TX, USA, 2012; Volume 5, pp. 4081–4090. [Google Scholar]
  32. Lo, K.H.; Shek, C.H.; Lai, J.K.L. Recent developments in stainless steels. Mater. Sci. Eng. R 2009, 65, 39–104. [Google Scholar] [CrossRef]
  33. Garfias-Mesias, L.F.; Sykes, J.M.; Tuck, C.D.S. The effect of phase compositions on the pitting corrosion of 25 Cr duplex stainless steel in chloride solutions. Corros. Sci. 1996, 38, 1319–1330. [Google Scholar] [CrossRef]
  34. Jones, R.H.; Ricker, R.E. Stress-Corrosion Cracking. In Corrosion, Metals Handbook, 9th ed.; ASM International: Geauga County, OH, USA, 1988; Volume 13, p. 262. [Google Scholar]
  35. NACE standard TM-0177-90 Standard Test Method Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking in H2S Environments; NACE International: Huston, TX, USA, 1990.
  36. Zucchi, F.; Trabanelli, G.; Grassi, V. Stress corrosion cracking of 13% Cr martensitic steels in sodium chloride solutions in the presence of thiosulphate. Mater. Corros. 2000, 51, 207–212. [Google Scholar] [CrossRef]
  37. “Duplex, Lean Duplex and Super Duplex Stainless Steels—Twice as Strong as Austenitics”. Available online: http://www.outokumpu.com/en/products-properties/more-stainless/stainless-steel-types/duplex/Pages/default.aspx (accessed on 24 March 2018).
  38. Ezuber, H.M. Influence of temperature and thiosulfate on the corrosion behaviour of steel in chloride solutions saturated in CO2. Mater. Des. 2009, 30, 3420–3427. [Google Scholar] [CrossRef]
  39. Bellezze, T.; Giuliani, G.; Viceré, A.; Roventi, G. Study of stainless steels corrosion in a strong acid mixture. Part 2: anodic selective dissolution, weight loss and electrochemical impedance spectroscopy tests. Corros. Sci. 2018, 130, 12–21. [Google Scholar] [CrossRef]
  40. Tsai, W.T.; Chen, J.R. Galvanic corrosion between the constituent phases in duplex stainless steel. Corros. Sci. 2007, 49, 3659–3668. [Google Scholar] [CrossRef]
  41. Lo, I.H.; Tsai, W.T. Effect of selective dissolution on fatigue crack initiation in 2205 duplex stainless steel. Corros. Sci. 2007, 49, 1847–1861. [Google Scholar]
  42. Femenia, M.; Pan, J.; Leygraf, G.; Luukkonen, P. In situ study of selective dissolution, of duplex stainless steel 2205 by electrochemical scanning tunnelling microscopy. Corros. Sci. 2001, 43, 1939–1951. [Google Scholar] [CrossRef]
  43. Marcus, P.; Moscatelli, M. The role of alloyed Molybdenum in the dissolution and passivation of nickel-molybdenum alloys in the presence of adsorbed sulfur. J. Electrochem. Soc. 1989, 136, 1634–1637. [Google Scholar] [CrossRef]
  44. Tomio, A.; Sagara, M.; Doi, T.; Amaya, H.; Otsuka, N.; Kudo, T. Role of alloyed molybdenum on corrosion resistance of austenitic Ni–Cr–Mo–Fe alloys in H2S–Cl–environments. Corros. Sci. 2015, 98, 391–398. [Google Scholar] [CrossRef]
  45. Wang, Y.; Singh, P.M. Corrosion Behaviour of Austenitic and Duplex Stainless Steels in Thiosulfate- and Chloride-Containing Environments. Corrosion 2015, 71, 937–944. [Google Scholar] [CrossRef]
  46. An, L.C.; Cao, J.; Wu, L.C.; Mao, H.H.; Yang, Y.T. Effects of Mo and Mn on Pitting Behavior of Duplex Stainless Steel. J. Iron Steel Res. Int. 2016, 23, 1333–1341. [Google Scholar] [CrossRef]
  47. Rondelli, G.; Vicentini, B.; Cigada, A. Influence of nitrogen and manganese on localized corrosion behaviour of stainless steels in chloride environments. Mater. Corros. 1995, 46, 628–632. [Google Scholar] [CrossRef]
  48. Zhang, W.; Jiang, L.Z.; Hu, J.C.; Song, H.M. Study of mechanical and corrosion properties of a Fe-21.4Cr-6Mn-1.5Ni-0.24N-0.6Mo duplex stainless steel. Mat. Sci. Eng. A 2008, 497, 501–504. [Google Scholar]
Metals 08 00237 g001 550
Figure 1. Microstructure of the long transversal section (parallel to load direction in slow strain-rate tests (SSRT)) of DSS 2304 (a) and LDSS 2404 (b), after etching with Beraha’s reagent.
Figure 1. Microstructure of the long transversal section (parallel to load direction in slow strain-rate tests (SSRT)) of DSS 2304 (a) and LDSS 2404 (b), after etching with Beraha’s reagent.
Metals 08 00237 g001
Metals 08 00237 g002 550
Figure 2. Stress–strain curves obtained during SSRT performed on DSS 2304 (a) and LDSS 2404 (b) at 80 °C, in water and in 20% NaCl solution, in the absence and in the presence of 10−2 and 10−1 M S2O32−.
Figure 2. Stress–strain curves obtained during SSRT performed on DSS 2304 (a) and LDSS 2404 (b) at 80 °C, in water and in 20% NaCl solution, in the absence and in the presence of 10−2 and 10−1 M S2O32−.
Metals 08 00237 g002
Metals 08 00237 g003 550
Figure 3. Macrograph acquired on the gauge length of DSS 2304 sample after SSRT in 20% NaCl solution at 80 °C, in the absence (a) and in the presence of 10−2 M S2O32− (b).
Figure 3. Macrograph acquired on the gauge length of DSS 2304 sample after SSRT in 20% NaCl solution at 80 °C, in the absence (a) and in the presence of 10−2 M S2O32− (b).
Metals 08 00237 g003
Metals 08 00237 g004 550
Figure 4. Micrographs of the long transversal section (parallel to the load direction) of DSS 2304 after SSRT in 20% NaCl solution at 80 °C, in the presence of (a) 10−2 and (b) 10−1 M S2O32−.
Figure 4. Micrographs of the long transversal section (parallel to the load direction) of DSS 2304 after SSRT in 20% NaCl solution at 80 °C, in the presence of (a) 10−2 and (b) 10−1 M S2O32−.
Metals 08 00237 g004
Metals 08 00237 g005 550
Figure 5. Macrograph (a) and micrograph of the long transversal section (parallel to the load direction) (b) acquired on the gauge length of the LDSS 2404 sample after SSRT in 20% NaCl solution at 80 °C, in the presence of 10−2 M S2O32−.
Figure 5. Macrograph (a) and micrograph of the long transversal section (parallel to the load direction) (b) acquired on the gauge length of the LDSS 2404 sample after SSRT in 20% NaCl solution at 80 °C, in the presence of 10−2 M S2O32−.
Metals 08 00237 g005
Metals 08 00237 g006 550
Figure 6. EOCP-strain trends of DSS 2304 (a) and LDSS 2404 (b) during SSRT in 20% NaCl solution at 80 °C, in the absence and presence of 10−2 and 10−1 M S2O32−.
Figure 6. EOCP-strain trends of DSS 2304 (a) and LDSS 2404 (b) during SSRT in 20% NaCl solution at 80 °C, in the absence and presence of 10−2 and 10−1 M S2O32−.
Metals 08 00237 g006
Metals 08 00237 g007 550
Figure 7. EOCP-strain trends of DSS 2304 (a) and LDSS 2404 (b) during immersion in 20% NaCl solution at 80 °C, in the absence and presence of 10−2 and 10−1 M S2O32−.
Figure 7. EOCP-strain trends of DSS 2304 (a) and LDSS 2404 (b) during immersion in 20% NaCl solution at 80 °C, in the absence and presence of 10−2 and 10−1 M S2O32−.
Metals 08 00237 g007
Metals 08 00237 g008 550
Figure 8. Polarization curves recorded on DSS 2304 and LDSS 2404 after 42 h immersion in 20% NaCl solution at 80 °C, in the absence and in the presence of 10−2 M S2O32−.
Figure 8. Polarization curves recorded on DSS 2304 and LDSS 2404 after 42 h immersion in 20% NaCl solution at 80 °C, in the absence and in the presence of 10−2 M S2O32−.
Metals 08 00237 g008
Metals 08 00237 g009 550
Figure 9. R index of the studied LDSS in 20% NaCl solution at 80 °C, in the absence and in the presence of 10−2 and 10−1 M S2O32−.
Figure 9. R index of the studied LDSS in 20% NaCl solution at 80 °C, in the absence and in the presence of 10−2 and 10−1 M S2O32−.
Metals 08 00237 g009
Metals 08 00237 g010 550
Figure 10. Stress–strain curves obtained during SSRT performed in air at 25 °C and in NACE TM-0177 solution at 25 °C and pH 2.7, in the absence and in the presence of 10−3 or 10−2 M S2O32− on DSS 2304 (a) and on LDSS 2404 (b).
Figure 10. Stress–strain curves obtained during SSRT performed in air at 25 °C and in NACE TM-0177 solution at 25 °C and pH 2.7, in the absence and in the presence of 10−3 or 10−2 M S2O32− on DSS 2304 (a) and on LDSS 2404 (b).
Metals 08 00237 g010
Metals 08 00237 g011 550
Figure 11. Macrograph (a) and micrograph (b) of the long transversal section of DSS 2304 sample after SSRT in NACE TM-0177 solution at 25 °C and pH 2.7, in the presence of 10−2 M S2O32−.
Figure 11. Macrograph (a) and micrograph (b) of the long transversal section of DSS 2304 sample after SSRT in NACE TM-0177 solution at 25 °C and pH 2.7, in the presence of 10−2 M S2O32−.
Metals 08 00237 g011
Metals 08 00237 g012 550
Figure 12. Macrograph of LDSS 2404 sample after SSRT in air at 25 °C (a) and in NACE TM-0177 solution at 25 °C and pH 2.7, in the presence of 10−2 M S2O32− (b).
Figure 12. Macrograph of LDSS 2404 sample after SSRT in air at 25 °C (a) and in NACE TM-0177 solution at 25 °C and pH 2.7, in the presence of 10−2 M S2O32− (b).
Metals 08 00237 g012
Metals 08 00237 g013 550
Figure 13. EOCP-strain trends of DSS 2304 (a) and LDSS 2404 (b) during SSRT in NACE TM-0177 solution at 25 °C and pH 2.7, in the absence and presence of 10−3 and 10−2 M S2O32−.
Figure 13. EOCP-strain trends of DSS 2304 (a) and LDSS 2404 (b) during SSRT in NACE TM-0177 solution at 25 °C and pH 2.7, in the absence and presence of 10−3 and 10−2 M S2O32−.
Metals 08 00237 g013
Metals 08 00237 g014 550
Figure 14. EOCP-time trends of DSS 2304 and LDSS 2404 during immersion in NACE TM-0177 solution at 25 °C and pH 2.7, in the presence of 10−2 M S2O32−.
Figure 14. EOCP-time trends of DSS 2304 and LDSS 2404 during immersion in NACE TM-0177 solution at 25 °C and pH 2.7, in the presence of 10−2 M S2O32−.
Metals 08 00237 g014
Metals 08 00237 g015 550
Figure 15. Polarization curves recorded on DSS 2304 and LDSS 2404 after 42 h immersion in NACE TM-0177 solution at 25 °C and pH 2.7, in the presence of 10−2M S2O32−.
Figure 15. Polarization curves recorded on DSS 2304 and LDSS 2404 after 42 h immersion in NACE TM-0177 solution at 25 °C and pH 2.7, in the presence of 10−2M S2O32−.
Metals 08 00237 g015
Metals 08 00237 g016 550
Figure 16. R index of the studied LDSS in NACE TM-0177 solution at 25 °C and pH 2.7, in the absence and presence of 10−3 and 10−2 M S2O32−.
Figure 16. R index of the studied LDSS in NACE TM-0177 solution at 25 °C and pH 2.7, in the absence and presence of 10−3 and 10−2 M S2O32−.
Metals 08 00237 g016
Table
Table 1. Nominal chemical composition (mass %) and PREN values (PREN = %Cr + 3.3 × %Mo + 16 × %N [32]) of the studied duplex stainless steels (DSS) and the reference lean-duplex stainless steel (LDSS) 2101.
Table 1. Nominal chemical composition (mass %) and PREN values (PREN = %Cr + 3.3 × %Mo + 16 × %N [32]) of the studied duplex stainless steels (DSS) and the reference lean-duplex stainless steel (LDSS) 2101.
DSSCMnCrNiMoNFePREN
LDSS 24040.023.0243.61.60.27bal.34
DSS 23040.02-234.80.30.10bal.26
LDSS 21010.035.0211.50.30.22bal.26
Table
Table 2. Average values and standard deviations of εf % for DSS 2304 and LDSS 2404 obtained from SSRT at 80 °C in water and in 20% NaCl solution, in the absence and in the presence of 10−2 and 10−1 M S2O32−.
Table 2. Average values and standard deviations of εf % for DSS 2304 and LDSS 2404 obtained from SSRT at 80 °C in water and in 20% NaCl solution, in the absence and in the presence of 10−2 and 10−1 M S2O32−.
Test Solutionεf %
DSS 2304LDSS 2404
water 80 °C47 ± 145 ± 1
20% NaCl45 ± 145 ± 2
20% NaCl + 10−2 M S2O32−37 ± 239 ± 3
20% NaCl + 10−1 M S2O32−42 ± 339 ± 3

Share and Cite

MDPI and ACS Style

Zanotto, F.; Grassi, V.; Balbo, A.; Monticelli, C.; Zucchi, F. Stress-Corrosion Cracking Behaviour of Lean-Duplex Stainless Steels in Chloride/Thiosulphate Environments. Metals 2018, 8, 237. https://doi.org/10.3390/met8040237

AMA Style

Zanotto F, Grassi V, Balbo A, Monticelli C, Zucchi F. Stress-Corrosion Cracking Behaviour of Lean-Duplex Stainless Steels in Chloride/Thiosulphate Environments. Metals. 2018; 8(4):237. https://doi.org/10.3390/met8040237

Chicago/Turabian Style

Zanotto, Federica, Vincenzo Grassi, Andrea Balbo, Cecilia Monticelli, and Fabrizio Zucchi. 2018. "Stress-Corrosion Cracking Behaviour of Lean-Duplex Stainless Steels in Chloride/Thiosulphate Environments" Metals 8, no. 4: 237. https://doi.org/10.3390/met8040237

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop