Assessment of the Electrolyte Composition in the Degree of Sensitization in AISI 347H Stainless Steel

Assessment of the Electrolyte Composition in the Degree of Sensitization in AISI

347H Stainless Steel

V. L. Cruz-Hernández

_{, M. A. García-Rentería}

_{, R. García-Hernández}

_{and V. H. López-Morelos}

2_{Facultad de Metalurgia, Universidad Autónoma de Coahuila, Carretera 57 km. 5 CP 25720, Monclova, Coahuila, México}

This study evaluated the suitability of the depassivator in revealing low degrees of sensitization (DOS) in samples of wrought AISI 347H austenitic stainless steel in the as-received condition and solution heat treated (SHT) at 1050 C. Assessment of the electrolyte composition was performed by the double loop electrochemical potentiokinetic reactivation (DL-EPR) test at room temperature. The electrolyte 1.0M H2SO4 +

0.50M HCl was found to be adequate for detecting low DOS. Microstructural characterization of the as-received material revealed the presence of Cr-rich carbides. These carbides were responsible of the susceptibility to IGC. [doi:10.2320/matertrans.M2016272]

(Received July 28, 2016; Accepted November 15, 2016; Published December 16, 2016)

Keywords:  sensitization, austenitic stainless steel, AISI 347H, depassivator, double loop electrochemical potentiokinetic reactivation test

1.  Introduction

Austenitic stainless steels (ASS) are the most commonly alloys in industries where aggressive environments prevail1)_.

The AISI 347 is an ASS constituted by a matrix of austenite with the presence of ferrite in small percentages and it is sta-bilized with Nb. Typical applications refer to industries as oil extraction, refineries, thermo-electric and nuclear power plants due to its high corrosion resistance at high tempera-ture2)_{. The principal characteristics of this alloy are the}

com-bination of good mechanical property and corrosion resis-tance in comparison to conventional ASS due to the presence of Nb which reduces the sensitization phenomenon and in-creases creep resistance as a result of the formation of NbC3)_.

Thermodynamically, the formation of niobium carbides are more favored and takes place in a broader range of tempera-tures4) _{as compared to chromium carbides (450–850 C)}5,6)_.

The precipitation of NbC occurs in grain boundaries, non-co-herent twin boundaries, dislocations and stacking faults5,7,8)_.

A requisite for nucleation and growth of niobium carbides is met when there is at least a Nb:C mass ratio of 10:13)_{. In this}

alloy, the highest affinity of C for Nb than Cr enhances the resistance to intergranular corrosion (IGC) by reducing the susceptibility to localized attack in virtue of the minimization of chromium depleted zones9)_{. Nevertheless, when there is C}

available in solid solution, it is possible the precipitation and growth of chromium-rich carbides8) _{and the IGC resistance of}

this alloy may be reduced when it is subjected to thermome-chanical processes as hot deformation10)_{. In this regard, Murr}

and Advani11) _{suggested a SHT in the range of temperatures}

between 1000–1100 C to dissolve these chromium-rich car-bides after hot deformation and improving the resistance to IGC.

The double loop electrochemical potentiokinetic reactiva-tion (DL-EPR) test has been used as an effective technique for estimating the degree of sensitization (DOS) in ASS. Modifications to the conventional DL-EPR test have been made in the electrolyte depending on the type of alloy, but the

most common depassivator or activator in the dissolution of metals is potassium thiocyanate. This compound is used due to its catalytic effect in diluted solutions containing sulfuric acid, because potassium thiocyanate promotes the dissolution of the passive layer in chromium depleted zones, increasing the current density during the anodic dissolution in the activa-tion process and subsequently along the grain boundaries during the reactivation loop12)_{. It is established that a}

dispro-portionate reaction of the decomposition of potassium thiocy-anate takes place producing the adsorption of S in the surface of the sample, which is stable in the potential range of anodic dissolution, where S compounds are reduced according to the reaction

SCN−_→CN−_+S

ads (1)

Based on this reaction, it was observed that an increase above 0.02 M in the concentration of potassium thiocyanate led to general corrosion and reduced the possibility of the passiva-tion of the material even at high potentials13)_{. In highly}

corro-sion resistant Ni-based alloys the substantial increase of po-tassium thiocyanate can act as inhibitor due to the high con-centration of S adsorbed on the surface of the material. To overcome this inconvenient, hydrochloric acid was used as an activator in a greater concentration than sulfuric acid along with potassium thiocyanate in concentration up to 0.001 M14)_,

with good reproducibility15)_.

The use of Nb, Ti, Zr and V as stabilizing elements delays the onset of sensitization due to the formation of carbides with these elements at higher temperature than chromium carbides with much less C available in solid solution. The outcome of the precipitation of carbides other than chromium carbides is an increase in IGC resistance6,16,17)_{. In spite of the}

use of stabilizing elements, Hong et al.18) _{found the}

precipita-tion of chromium carbides in 347 ASS after hot deformaprecipita-tion. Precipitation of chromium rich carbides, M23C6, is feasible

considering the kinetics of this process. Dissolution of niobi-um carbides and Nb free regions enable carbon diffusion into Cr-rich zones and thereby precipitation of chromium carbides making the steel susceptible to IGC3,19)_{. In this context, the}

thermo-metallurgical history of these alloys is crucial in their

*

performance. A SHT has proved to be advisable for restoring the resistance to IGC as reported by Kina et al.20) _{in samples}

with different metallurgical conditions. Assessment of the susceptibility to localized attack by these authors was per-formed in a 0.5M sulfuric acid + 0.01M potassium thiocya-nate solution.

This study was undertaken in order to assess the capability of an electrolyte to disclose light DOS using the DL-EPR test at room temperature in AISI 347 ASS by modifying the acti-vator agent with additions of Cl− _{ions with and without} KSCN.

2.  Experimental Details

Plates, 7 mm thick, of wrought AISI-347H ASS with the chemical composition given in Table 1 were used. Samples of 10 ×  20 ×  7 mm were taken from the as-received ASS sheet and subjected to a SHT at 1050 C and held at temperature for 30, 60 and 90 minutes followed by water quenching. Dissolu-tion of chromium carbides occurs at this temperature along with grain growth of the austenitic matrix11,20)_{. For}

micro-structural characterization, the samples were mirror like pol-ished following standard metallographic preparation and etched by immersion-stirring in a solution containing 8.43 mL hydrochloric acid + 2.80 mL nitric acid + 3.75 mL ethylic al-cohol, rinsed with a stream of water and dried. Grain size was measured, with software facilities, in samples with and with-out SHT from digital images captured in the optical micro-scope (OM) and scanning electron micromicro-scope (SEM) equipped with an energy dispersive X-ray detector (EDX).

In order to evaluate the DOS by the DL-EPR test, samples were embedded in epoxy resin and a copper wire was at-tached in the rear for electric connection and used as working electrode in a conventional three electrodes electrochemical cell using a graphite bar as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. Samples were previously ground with emery paper (1200 grit), rinsed with distilled water and degreased with ethylic alcohol. Be-fore the DL-EPR tests, samples were stabilized at open cir-cuit potential (Eoc) during 20 minutes. Cyclic potentiokinetic

polarization was conducted from Eoc and open to air at a

scan-ning rate of 1 mV/s to an overpotential of 200 mV vs. SCE at ambient temperature (25 C) using a potentiostat Solartron 1280B.

The composition of the electrolytes used is shown in Ta-ble 2. Tests were made three times for reproducibility using samples subjected to identical surface preparation and fresh electrolyte in every electrochemical test. The DOS was deter-mined with the Ir/Ia ratio, where Ir and Ia are the maximum peaks of the current density during anodic reactivation and activation, respectively. After the DL-EPR test, samples were taken into the SEM to observe the degree of damage and mor-phology of the corrosion features generated with every elec-trolyte. Correlation between DL-EPR curves and visual

ob-servations of the samples in the SEM pointed to the solution with better reproducibility for detecting light DOS due to the presence of M23C6 or Cr-rich phases in terms of quantitative

and qualitative results.

3.  Results

3.1  Microstructural characterization

Figure 1 shows the typical microstructure of the AISI 347H ASS in the as-received condition. The microstructure corre-sponds to an austenitic matrix with twins and niobium car-bides allocated in the grain boundaries and within the grains. A mean grain size of 13 µm was measured for the ASS in the as-received condition. The microstructures of the SHT sam-ples are shown in Fig. 2. It can be seen grain growth with time when holding at temperature, namely; 16.5, 26.5 and 60.6 µm for holding times of 30, 60 and 90 minutes, respectively. In the microstructure of the heat-treated samples the presence of twins is also observed. SHT of the AISI-347H ASS also

re-Table 1 Chemical composition of the AISI 347H ASS (mass%).

C Mn S P Si Cr Ni Nb Co Cu Mo N Ti Al

0.04 1.5 0.001 0.03 0.36 17.3 9.3 0.64 0.28 0.45 0.41 0.04 0.005 0.004

Table 2 Composition of the electrolytes essayed.

E1 0.5M H2SO4 + 0.01M KSCN

E2 1.0M H2SO4 + 0.01M KSCN

E3 1.0M H2SO4 + 0.01M KSCN + 0.5M NaCl

E4 1.0M H2SO4 + 0.50M HCl

sulted in the modification of the niobium carbides. Typically, solution annealing of ASS`s is performed in the temperature range of 1020 to 1150 C5)_{. The SHT of 1050 C is close to the}

temperature of dissolution of niobium carbides. Thus, solu-tion annealing of the samples at this temperature gave rise to a slow dissolution-reprecipitation process. For heat-treating times of 30 and 60 minutes, slight coarsening of the niobium carbides is appreciated. Particle size measurements from SEM images of the NbC particles reveals a very small in-crease from 1.01 ±  0.75 µm for the as-received stainless steel to 1.103 ±  0.59 and 1.205 ±  0.68 µm for holding times of 30 and 60 minutes, respectively, and a decrease to 0.7 ±  0.36 µm for holding after 90 minutes. This behavior is explained by the dissolution-reprecipitation mechanism activated at this temperature where for the longest holding time, besides the dissolution of chromium carbides (this made more C avail-able for precipitation of NbC), a very large fraction of the initial NbC particles dissolved and reprecipitated into very fine NbC particles. In addition, the sample held at tempera-ture for 30 minutes exhibits an even distribution of NbC with little clustering while the sample held for 60 minutes presents a number of austenitic grains with fewer NbC and other grains with clustering of niobium carbides as pointed by the

arrows in Fig. 2(b). Holding for 90 minutes at 1050 C, Fig. 2(c), resulted in finer NbC evenly distributed in the aus-tenitic matrix and the presence of colonies of large niobium carbides as indicated by the circle in Fig. 2(c).

3.2 Assessment of the electrolyte composition

Figures 3(a) and (b) show the DL-EPR curves of the as-re-ceived AISI-347H ASS essayed with the distinct electrolytes and the Ir/Ia ratios, respectively. It can be seen from Fig. 3(a) that increasing the concentration from 0.5 (E1) to 1 M (E2) of sulfuric acid only produces a little increase in the activation current (approximately 0.01 A/cm2_{). For these electrolytes}

according to Fig. 3(b) the Ir/Ia ratios indicate that the as-re-ceived alloy is not susceptible to IGC as the DOS is negligi-ble. The curve of the composition E3 shows a very high in-crease in Ia of approximately 0.035 A/cm2 _{with respect to}

composition E1. However, the reactivation current, Ir, peaks also at a high value. This suggests that for the composition of electrolyte E3 generalized corrosion instead of localized cor-rosion is taking place. Regarding solution E4, the plot in Fig. 3(b) shows that little DOS may be revealed with this composition. In addition, a significant reduction in Ia is ob-served (approximately 10 times with respect to solution E1), this effect is ascribed to the elimination of the excess of S that

Fig. 2 Microstructures of the AISI-347H ASS as observed in the SEM after SHT at 1050 C for (a) 30, (b) 60 and (c) 90 minutes.

can be adsorbed on the surface of the sample by substituting potassium thiocyanate by hydrochloric acid.

The above observation can be related to the fact that with the concentration of potassium thiocyanate in solution E1 and E2 it is not possible to detect little DOS induced by the ther-momechanical process experienced by the as-received mate-rial. However, substitution of the depassivator with another containing Cl− _{ions (sodium chloride or hydrochloric acid)} promotes the activation and the dissolution of chromium de-pleted zones restricting thus adsorption of SO42− from the

reaction

H2SO4→SO42−+H2 (2)

This mechanism enables reaction of the Cl− _{ions with the} me-tallic matrix without increasing the surface area of elemental S adsorbed. Nevertheless, in presence of potassium thiocya-nate there is an increase in the adsorption of S promoting gen-eralized corrosion due to the significant concentration of SO42− + Cl−1 and the formation of S2O32− ions resulting too

much aggressive for the AISI-347H alloy. Examination of the surface of the samples in the SEM after DL-EPR tests, as shown in Fig. 4(a), confirms generalized corrosion. Con-versely, when hydrochloric acid substitutes potassium thiocy-anate, adsorption of elemental S on the metallic surface is prevented during anodic dissolution. This effect leads to a significant reduction in the Ia and Ir peak values because only SO42− + Cl−1 ions are involved in the oxidation of the metal

matrix during the DL-EPR test. Figure 4(b) shows the fea-tures of the surface after exposure to electrolyte E4 during the DL-EPR test. In this instance, it is only observed localized corrosion in grain boundaries as well as in the vicinities of niobium carbides.

Figure 5 shows complementary characterization, in detail, by SEM-EDX of the as-received sample exposed to

electro-lyte 4. Figure 5(a) reveals the presence of precipitates in the grain boundaries. The EDX spectrum of the angular precipi-tate pointed by the arrow in the micrograph of Fig. 5(a) along with its elemental quantification given in Table 3 indicate that these precipitates correspond to Cr-rich carbides, likely of the M23C6 type. A number of precipitates with different

morphol-ogies are observed in the micrograph of Fig. 5(c). In this in-stance, the experimental evidence gathered by EDX elemen-tal microanalysis showed that these phases correspond to ni-obium carbides. Thus, according to these results it is feasible to use the solution E4 at ambient temperature for detecting small DOS when niobium and chromium carbides coexist.

3.3  Effect of the SHT on IGC resistance

Once that the use of hydrochloric acid as depassivator was found to be more adequate as compared to potassium thiocy-anate or sodium chloride, DL-EPR testing of the SHT sam-ples was carried out in order to evaluate the effect of the heat treatment in terms of susceptibility of the samples to IGC. Figure 6 shows characteristic DL-EPR curves and the Ir/Ia ratios as a function of the metallurgical condition of the AISI 347H stainless steel. From the plot shown in Fig. 6(b), it is evident that the Ir/Ia ratio of the SHT samples significantly decreased with respect to the value of the as-received materi-al. As a matter of fact, this behaviour was expected. Micro-structural characterization of the as-received AISI 347H stainless steel disclosed the presence of both NbC and Cr-rich carbides. Thus, heating of the samples up to 1050 C not only coarsened the grain structure and changed the features of the niobium carbides, it also dissolved the chromium carbides

Fig. 4 Surface damage of the as-received samples as observed in the SEM after DL-EPR test in electrolyte; (a) E3 and (b) E4.

Fig. 5 Details of the microstructure of the as-received samples as observed in the SEM after DL-EPR test in electrolyte E4.

Table 3 Elemental quantification of the EDX spectra shown in Fig. 5.

C Si Cr Fe Mo Mn Nb Ni

Spectra b mass% 5.97 ---- 5.26 17.05 ---- 0.57 67.92 3.14 at.% 29.27 ---- 5.95 17.95 ---- 0.61 42.99 3.20 Spectra d mass% 2.06 0.45 25.84 70.58 1.04 ----

----formed during hot rolling of the stainless steel. Rapid cooling of the samples by water quenching prevented any re-precipi-tation of chromium carbides with any C available after hold-ing at temperature for some time. The Ir/Ia ratio of the SHT samples drew an approximate value of 0.01 meaning that, virtually, AISI 347H stainless steel in these metallurgical conditions is not susceptible to experience IGC. This assump-tion is further supported by the features observed in the SEM of these samples after the DL-EPR test as shown in Fig. 7. The images, captured in secondary electron mode, do not ex-hibit localized corrosion of the surfaces. Only very few isolat-ed pits were observisolat-ed.

The findings of this study indicate that care must be taken in the use of Nb stabilized stainless steel, because thermome-chanical processing may induce susceptibility to IGC by the precipitation of Cr-rich carbides. This metallurgical condition is likely to be worsened during subsequent fusion welding of components. Thermal cycles experienced in the heat affected zone of the base material induce dissolution of NbC and be-cause of the rapid cooling complete reprecipitation of NbC will not occur, leaving C available for nucleation of new and further growth of pre-existant chromium carbides.

4.  Conclusions

(1) The presence of niobium carbides, as it is well known, in the microstructure of the stabilized austenitic stain-less steel exhibited a positive effect in reducing the DOS. However, analysis of the as-received plate re-vealed some susceptibility to IGC as a result of its ther-momechanical history. The engineers must be aware that this problem may be worsened by a subsequent fu-sion welding step of the AISI-347H alloy.

(2) It is possible to detect small DOS in the AISI-347 stain-less steel with the use of hydrochloric acid as depassiv-ator in substitution of potassium thiocyanate in DL-EPR test.

(3) The as-received AISI-347H stainless steel presented the highest DOS due to the precipitation of chromium car-bides during its thermomechanical processing.

(4) A SHT at 1050 C for 30 minutes is effective in dissolv-ing chromium carbides so that IGC resistance of wrought AISI-347H stainless steel is restored.

Fig. 6 Evaluation of the effect of SHT on the IGC resistance. (a) DL-EPR

Acknowledgement

VLCH thanks CONACyT for the scholarship provided.

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