Materials Science and Engineering A203 (1995) 324-331
The effect of heat treatment on corrosion behavior of laser surface melted 304L stainless steel O. V e d a t A k g f i n a, M u s t a f a
U r g e n b, Ali F u a t ~ a k i r b
alstanbul University, Department of Metallurgical Engineering, Avcdar, ]stanbul, Turkey blstanbul Technical University, Department of Metallurgical Engineering, 80626 Ayaza~,a, ]stanbul, Turkey
(Received 27 October 1994; in revised form 27 February 1995)
Laser surface melting (LSM) improves the passivation and pitting behavior of 304L stainless steel. This improvement was attributed to microstructural modification, mainly to increased 6-ferrite content, and elimination and/or redistribution of sulfur based inclusions. In this study, in order to clarify the role of microstructural changes that took place during LSM on electrochemical behavior, laser surface melted 304L grade stainless steel was heat treated at 1050 °C. A detailed analysis of the electrochemical behavior of as-received, laser surface melted and laser surface melted + heat treated 304L in sulfuric acid and sodium chloride solutions was conducted. Effects of laser surface melting and post heat treatment on pit morphology was also investigated. Keywords: Heat treatment; Corrosion behaviour; Stainless steel
The 300 series austenitic stainless steels are mostly recognized for their excellent corrosion resistance that emanates from a protective, passive oxide layer on the surface . Even though this passive layer is an inexpensive means of corrosion protection, depending on the environment, it sometimes breaks down, causing severe localized corrosion attack, such as pitting, crevice, and stress corrosion cracking, leading to catastrophic failures. Microstructure plays an important role in the improvement of corrosion resistance of austenitic stainless steels . Constituents of the microstructure such as grain size, phases, precipitates and inclusions are strongly influenced by solidification rate or heat treatment. Furthermore, under rapid solidification conditions, 6-ferrite, which is retained as a minor phase in austenite matrix, also has some effect on corrosion resistance of austenitic stainless steels. This effect has not been understood well and has been a source of controversy in the literature. Both beneficial  and detrimental  effects have been reported.
The formation of 6-ferrite in austenitic stainless steels can be explained in terms of the solidification process. Depending upon compositions and cooling rates, austenitic stainless steels exhibit four solidification modes; primary austenite, primary 3-ferrite, primary austenite followed by eutectic ferrite and primary b-ferrite followed by austenite [5,6]. For example, type 304 stainless steel, having a composition in the chromium rich side of the eutectic triangle, can have two solidification modes as cooling rate changes. At cooling rates less t h a n 10 6 K s -1, the solidification mode is primary 6-ferrite followed by austenite. However, when cooling rates exceed 10 6 K s - l, type 304 stainless steel solidifies entirely in the primary austenite mode, called partitionless solidification, and 6-ferrite is not observed . The presence of some 6-ferrite in the microstructure is, however, desirable for welding applications since 3-ferrite traps impurity elements, such as S and P, and prevents hot cracking in austenitic stainless steels [8,9]. Another constituent of the microstructure which has a detrimental effect on corrosion resistance is inclusions. Among these, manganese sulfide (MnS) type inclusions are well characterized as being the most 0921-5093/95/$09.50 © 1995 -- Elsevier ScienceS.A. All rights reserved SSD1 0921-5093(95)09807-0
O.V. Akgiin et al. / Materials' Science and Engineering A203 (1995) 324 331
Table 1 Chemical composition (wt.%) of type 304L stainless steel C
Nb + Ta
favorable sites for pit initiation in austenitic stainless steels , Laser surface melting (LSM) is a surface modification technique that improves corrosion resistance properties of type 304 and 304L stainless steels [11-14]. The improved properties in these alloys are attributed to removal and/or redistribution of sulfur based inclusions as well as microstructural changes that come about as a result of rapid cooling [11,14]. The observed microstructural changes are reduction in subgrain size and introduction of (5-ferrite as a minor phase. The solubility of sulfur in 6-ferrite is known to be higher than that of austenite [15,16]. It is suggested that during primary d-ferrite solidification, sulfur which is trapped in ferrite phase becomes less available for the formation of inclusions in austenite phase. In the present study, the mechanisms that improved the pitting and passivation behavior of laser surface melted 304L stainless steel were investigated. A particular consideration was given to understanding how microstructural changes affects this behavior. As-received and LSM samples were tested for passivation behavior in 1 N H 2 S O 4 and pitting corrosion resistance in 3.5 wt.% NaCI solution. Then, LSM samples were heat treated at 1050 °C for 40 rain to dissolve the minor /~-ferrite phase. Corrosion resistance of heat treated samples was also tested in the same solutions.
2. Experimental procedure The composition of type 304L Stainless Steel (SS) used in this study is given in Table 1. An as-received bar of 304L SS was cut to a size of 5 x 3 x 1.2 cm plates. They were polished with 600 grit SiC paper and cleaned with methanol in an ultrasonic cleaner before laser treatment. LSM was performed with a continuos wave CO2 laser which provides a maximum output of 5 kW. Specimens were translated under laser beam at a speed of 7 m m s ~. Nitrogen was used as shielding gas during processing. Sample surface area of 15 cm 2 was melted with a 50% overlap. The standard potentiodynamic technique was used in electrochemical corrosion experiments. The system consisted of a potentiostat ( E G & G M273) controlled by an Apple IIe computer The reaction vessel was a 1 1 6-neck Greene flask. The reference electrode was a saturated calomel elec-
trode (SCE) and was kept outside the vessel in a different compartment. Potentials were measured via a L u g g i n - H a b e r capillary. Two cylindrical glassy carbon rods were used as counter electrodes. The specimen surfaces were prepared by mechanical grinding. Final grinding was conducted with 600 grit SiC paper. Samples were ultrasonically degreased in 50% a l c o h o l + 50% ether solution for 5 rain before masking. Surfaces that were not to be exposed to the test solution were masked with resin saturated beeswax. Exposed surface areas were approximately 4 cm 2. Samples were tapered on one end and were mounted on Stern-Macrides type electrode holder. All experiments were carried out in nitrogen deaerated 1 N sulfuric acid and 3.5 wtY, NaC1 solutions. Experiments were started 30 rain after the electrode was introduced into the deaerated solution. The scan rate used was 20 mV min ~. All potentials in this work are given with respect to SCE at room temperature. Heat treatment of the LSM samples was carried out at a temperature of 1050 °C to dissolve retained g-ferrite phase in austenite matrix. At this temperature, austenite is the stable phase for the 304L composition . Holding the samples at this temperature for 40 min was assumed to dissolve the c~-ferrite phase. Specimens were heated at 1050 °C for 40 rain in argon atmosphere and then quenched in water. Hereafter, as-received, laser surface melted and laser surface melted then heat treated samples are named as AR 304L, LSM 304L and L S M - H T 304L, respectively. The surfaces of the as-received, LSM and heat treated 304L samples were observed in an SEM (Jeol T330) following electrochemical experiments that were conducted in a 3.5 wtY/,~ NaC1 solution. To reveal the inner morphological aspects of the pits, specimens were also examined following ultrasonic cleaning in 10% HNO3 solution for 5 min, to remove the corrosion products.
3. Results 3.1. Cathodic and anodic behavior in 1 N H 2 S O 4 solution
Representative curves of the cathodic and anodic polarization experiments conducted in 1 N H2SO4 solutions with AR304L, LSM 304L and LSM-HT 304L samples are given in Fig. 1. In Figs. 2 and 3, active and
O.V. Akgiin et al. / Materials Science and Engineering A203 (1995) 324 331
" " "'-'"
" " ""1
• " ""'"1
r "! .... I I
uV j -120360;
...,....I . . 0 1
...... ._1 ...... J ...,...
/ / / ! 1
t I I
AR 304L LSM304L LSM-HT 304L
uJ oO3 1 2 8 0
i (laA/cm 2)
i (laA/cm 2 )
Fig. I. Cathodic and anodic polarization behaviors of AR 304L, LSM 304L and LSM-HT 304L samples in 1 N HzSO 4 solution.
Fig. 3. Transpassive regions of polarization curves of AR 304L, LSM 304L and LSM-HT 304L samples in 1 N H2SO 4 solution.
transpassive portions of the same representative polarization curves are given in detail. Electrochemical parameters derived from two experiments are listed in Table 2. Although the parameters obtained from the two experiments showed some differences, the overall features observed in cathodic, anodic and transpassive regions of the polarization curves were consistent with
each other. The polarization curves given in Figs. 1, 2 and 3 belong to experiments specified as Expt. 1 in Table 2. The following sections of the polarization curves are evaluated individually: (a) cathodic region, (b) active region, (c) active-passive transition region, (d) passive region, (e) transpassive region.
(a) Cathodic region -350
AR 304L and LSM 304L showed different cathodic polarization behavior (Fig. 2). AR 304L gave the expected cathodic polarization curve with a - 1 1 0 mV per decade cathodic Tafel slope . For LSM 304L, two different regions were observed in, the cathodic polarization curve with slopes of - 127 and - 86 mV per decade. After heat treatment of the LSM 304L, the cathodic behaviour with two slopes became less evident; the region with lower slope had almost disappeared.
. -560 . . .
\ \ \''\ ' ,
- kSM 3041_
~ -. . ' ,
: - - LSM-HT 304L
. . . , .... I
(b) Active region
. . . , .... I 1
10 i (i.zNcnl2)
. ..,.~. 4
Fig. 2. Cathodic and active anodic regions of polarization curves of AR 304L, LSM 304L and LSM-HT 304L samples in 1 N H2SO 4
solution. (5~ + 7~ region represents the cathodic contribution from both 7 and &phases and 6c + 7a region represents the anodic contribution from/~ and cathodic contribution from ;, phases in LSM 304L alloy).
For the comparison of samples in the active region, corrosion potentials (Ecorr), and corrosion current densities (i.... ) were considered. Corrosion current densities of the samples were calculated by extrapolating the linear part (E-log/scale) of cathodic polarization curve to Ecorr because the alloys investigated did not exhibit a clear linear active (anodic) region due to fast activepassive transition. A similar approach was also used by Troselius . The corrosion potential of the alloy shifted to more positive values (about 30 mV) following laser melting. The corrosion potential of LSM 304L did not show any significant change following heat treatment at 1050 °C.
O.V. Akgiin et al. Materials' Science and Engineering A203 (1995) 324 331
Table 2 Cathodic and anodic polarization parameters of AR304L, LSM 304L and L S M - H T 304L in 1 N H2SO 4 solution Electrochemical parameters
b~(mV per decade)
E~,~,rlmV vs. SCE) i~o,.r(/~a cm 2) G.(laa cm ~) Eu.(mV vs. SCE)
Expt. Expt. Expt. Expt. Expt. Expt. Expt. Expt. Expt. Expt.
1 2 I 2 1 2 1 2 1 2
A R 304L
L S M - H T 304L
- 110 - 107 -418 -428 82 73 165 175 906 906
- 127 and - 8 6 - 135 and - 7 5 -392 -388
- 113 100 386 -378 30 25 45 40 906 906
The calculation of corrosion current density by Tafel extrapolation for LSM 304L will not give correct results owing to the presence of g-ferrite in the alloy. The anodic contribution of the g-ferrite to the cathodic polarization behavior of the alloy makes this type of calculation incorrect (see discussion). Thus, only corrosion currents calculated for single phase alloys, namely AR 304L and LSM-HT 304L, were considered . The corrosion current density of the LSM-HT 304L was lower than that of AR 304L.
(c) Active-passive transition Among the three types of samples the highest icr was observed for the AR 304L alloy. Laser surface melting resulted in lowered i~r values. These results showed that laser melting of the surface of the alloy improved its passivation behavior as previously observed [12,14]. Restoration of the austenitic structure by heat treatment did not have an adverse effect, on this improvement achieved by LSM. (d) Passive region The passive behaviors of all samples in the passive region were not appreciably different (Fig. 1). (e) Transpassive region Transpassive behaviors of the samples are given in Fig. 3. In all the alloys investigated, transpassivity started at 906 mV. In the AR 304L sample, after the onset of transpassivity, current density increased steeply up to 4000 ~A c m - 2 , leveled off at around 1320 mV and then exhibited a minimum at 1480 mV. This behavior is typical of AISI 304 SS . However, LSM changed the transpassive behavior of 304L significantly. The leveling of the current took place at slightly higher potentials (1350 mV) and at a higher current density (25000 ILA cm 2 ) and the minimum at 1480 mV was not observed. In the case of LSM-HT 304L the transpassive behavior was similar to that of AR 304L except the minimum was at 1480 mV.
60 65 906 906
3.2. Pitting behavior in 3.5 wt.% NaCl solution The pitting potentials of the samples tested in 3.5% NaC1 are given in Table 3. LSM increased the pitting potential of the 304L alloy. The positive shift in the pitting potential was 145 inV. These results were consistent with previous studies [11-14,20]. After the heat treatment conducted to restore austenite phase, the pitting potential of the LSMHT 304L further increased another 35 mV compared to LSM304L (Table 3). The morphological investigation of the pits indicated that the pit morphology of the AR 304L was changed by LSM treatment. The morphology of cleaned pits formed on the AR 304L and LSM 304L is given in Figs. 4 and 5(a,b), respectively. The pits on AR 304L were partially covered and irregular; the size of the orifice was 1/2 to 2/3 of the whole diameter of the pit. Cleaning treatment removed both the cover from the surface and the corrosion products from inside the pits. The interior of the pits can be described as rough and non-uniform (Fig. 4). The pits formed in LSM samples were bigger but shallower (Fig. 5(a)). Inside the pits, a semi-continuos /5-ferrite network structure was observed (Fig. 5(b)). Cells in the network were approximately 2 - 5 /zm in diameter. The walls of the cells, which are 6-ferrite, did not dissolve as much as the inside of the cells, which resulted in hill and valley type morphological appearance. Below the ferrite network, a faceted structure was observed. White precipitates, approximately 1 /tin in size, were also present (Fig. 5(b)). The EDS analysis of these precipitates showed that they were composed of Mn-Si. Similar formations of M n - S i precipitates were also observed in other studies [14,21]. Though heat treatment slightly increased the pitting potential (Table 3), the most significant effect of heat treatment of LSM 304L was on the pit morphology of the samples. After heat treatment of LSM 304L the general appearance of pits reverted back to that of
O.V. Akgiin et al. / Materials Science and Engineering A203 (1995) 324 331
Table 3 Pitting potentials of the AR 304L, LSM 304L and LSM-HT 304L Samples in 3.5 wt.% NaCI Samples
Pitting potential (mV vs. SCE)
186 202 170
331 300 325
365 352 344
Average +_ S.D.
186 + 13.06
318.6 _+ 13.42
353.6 + 8.65
those encountered on AR 304L. Investigation of the interior of pits revealed that the white precipitates (Mn-Si) present in LSM 304L samples were not affected by heat treatment (Fig. 6).
The electrochemical test results of the AR 304L, LSM 304L, and LSM-HT 304L alloys will be discussed by considering the behavior of these alloys in two different type of solutions; namely, sulfuric acid (cathodic, active, passive and transpassive behavior) and chloride solutions (pitting behavior). The test results obtained in sulfuric acid solutions showed that both LSM and heat treatment after LSM had an effect on the polarization curves to some extent. The first noticeable effect was in the cathodic region of the LSM 304L sample tested in sulfuric acid solution. There were two linear regions, with different slopes, present in the cathodic polarization curve of LSM 304L (Fig. 2). However, after heat treatment of LSM 304L the cathodic behavior with two slopes became less evident and the lower slope had almost disappeared. Hence the observed two slope exhibiting behavior of LSM 304L alloy was most probably related to the presence of d-ferrite phase in the microstructure
of LSM 304L. The still remaining traces of lower slope behavior in heat treated alloy (LSM-HT 304L) can be attributed to the presence of retained d-ferrite that was not totally eliminated by heat treatment. In LSM 304L there are two phases with different chemical compositions; austenite and 6-ferrite. d-Ferrite is the phase with higher chromium but lower nickel compared to austenite. It is known that in steels having
Fig. 4. Pits formed on AR 304L in 3.5% NaCl solution (after cleaning in 10% HNO3).
Fig. 5. (a) General view of pit formed on LSM 304L in 3.5% NaCI solution (after cleaning in 10% HNO3). (b) Details of the inner morphology of the same pit formed on LSM 304L in 3.5% NaC1 (after cleaning in 10% HNO3).
O.V. Akgiin et al. / Materials Science and Engineering A203 (1995) 324-331
Fig. 6. Details of the inner morphology of the pit formed on LSM-HT 304L in 3,5% NaCI (after cleaning in 10% HNO~).
equivalent composition to 304L, the Cr and Ni contents of ferrite are in the range 25.9-26.2% and 4-6%, respectively . Semiquantitative EDS analysis of the cS-ferrite phase in LSM 304L alloy for Cr and Ni contents gave comparable results; the chromium and nickel content of ,~-ferrite was in the range 22-24% and 5-6%, respectively. The difference in the chemical composition of austenite and cS-ferrite phases results in different electrochemical behavior ; the increase in the chromium and decrease in the nickel contents of the alloy shifts the active, active-passive transition curve of the alloy to more negative potentials [23,24]. In the anodic polarization curves of two phase (duplex) stainless steels (ferrite content around 50%), the anodic contribution of both phases, which reveal themselves as two humps, can be differentiated . The magnitude of the anodic hump of the ferrite phase depends on the critical current density and on the amount of this phase. The corrosion potential differences estimated from the polarization curves of duplex stainless steels showed that the difference between the corrosion potentials of these two phases is approximately 80 mV . However, in the study conducted on F e - C r alloys by Yau and Streicher , the corrosion potential difference between 12% and 25% Cr-containing stainless steels was only 25 mV. In the duplex stainless steels, decrease in nickel content and the enrichment of other elements such as sulfur in ferrite might lead to higher potential differences between c~-ferrite and austenite. In this study, the LSM alloy contained about 5-10% g-ferrite, which was low when compared to duplex stainless steels. The anodic contribution of ferrite phase was not expected to be very significant and it might not overcome the cathodic reaction occurring on austenite. Hence it can show itself by changing the slope of the cathodic polarization curve. The two sloped regions in
the cathodic polarization curve of LSM 304L can be defined as follows: the region with higher slope represents the sum of cathodic reactions occurring on both austenite and cS-ferrite (Yc + ~5c region in Fig. 2) and the region with lower slope represents the cathodic reaction on austenite and anodic reaction on c~-ferrite (7~ + g~ region in Fig. 2). The SEM investigation of the LSM 304L specimen held at cathodic region ()'c + (Sa region) revealed that c~-ferrite phase dissolved actively . Since the cathodic region neighbouring the corrosion potential of the LSM 304L is actually a compound curve, which also includes the anodic region of" c~-ferrite phase, the calculation of corrosion current by Tafel extrapolation for LSM 304L would not give correct results. Active-passive transition behavior of the alloy was also affected by LSM, which decreased i~. The positive changes observed in active and active-passive transition regions following LSM were not eliminated by heat treatment. Hence the beneficial effect achieved by LSM was not directly related to the presence of ~5-ferrite but to another effect. This effect could be the change of chemical composition that takes place during LSM. It is well known that compositional changes do take place during LSM of the stainless steels [14,26] especially in terms of minor constituents such as Mn [14,26], S , and C . The amount of these elements in the alloy decreases following laser treatment. Among these, sulfur is the most critical element; it affects the active corrosion rate and the critical current density of the alloy [27-29]. Since the solubility of sulfur is very low in the iron alloys, it forms inclusions in the microstructure. The most detrimental inclusion type for corrosion resistance properties is MnS. In aqueous corrosive environments MnS dissolves very easily, giving rise to several sulfur anions and they promote active dissolution of the alloy . The Mn content of the LSM samples is reduced, about 25%, by laser treatment and some of the remaining Mn forms Mn Si compounds [14,26]. Additionally this study showed that the M n - S i compounds were stable in acidic solutions, since they were not effected by the highly acidic chloride solutions in pits (Fig. 5(b)). Thus, the decrease in Mn content by LSM and the formation of M n - S i compounds would result in decreasing the amount of Mn available for the formation of MnS inclusions in the alloy. In sulfuric acid solution the transpassive behavior of laser surface melted samples showed differences compared to AR 304L. Following LSM, both the leveling in current starting at 1350 mV and the minimum at 1480 mV had disappeared and the transpassive current increased (Fig. 3). It is possible to relate the increase of the current at the potential range between 1350 and 1500 mV to the presence of c~-ferrite phase in the microstructure. ~5-Ferrite phase, being rich in Cr, has a
O, V. Akgiin et al. ,' Materials Science and Engineering A203 (1995) 324.-331
higher transpassive dissolution rate compared to austenite. An increase in Cr content of stainless steels promotes the transpassive dissolution . The presence of a phase which has a higher Cr content can be effective in the increased transpassive dissolution rate. SEM examination of LSM 304L samples that are held at transpassive potentials had shown that 3-ferrite network dissolved with a higher rate . When this phase was eliminated by heat treatment, the lower dissolution rate of the as-received sample was restored except for the minimum at 1480 mV. Since the contribution arising from the presence of 6-ferrite phase in the microstructure was eliminated by heat treatment, the relation between the chemical composition and elimination of the minimum at 1480 mV can be better understood by comparing the transpassive behavior of AR 304L and L S M - H T 304L samples. The observed minimum at 1480 mV in the transpassive curve of AR 304L can be attributed to the presence of Mn which is contributing to the transpassive behavior of 304 stainless steel (Fig. 3). It is known that high manganese (18%) containing stainless steels exhibit a very pronounced minimum at the transpassive range . Disappearance of this minimum could be related to the decrease of the amount of Mn contributing to the transpassive behavior of AR 304L (both by elimination during LSM and by formation of Mn Si). The behavior of the LSM samples in chloride solutions revealed that LSM has a beneficial effect on the pitting resistance of the alloy, consisting in shifting of the pitting potential to more positive values and changing the pit morphology. The positive shift observed in pitting potential following LSM could be explained in terms of MnS inclusion redistribution or elimination as discussed above. It is clear that observed shallow pit morphology in LSM samples was due to the 6-ferrite network structure. As seen from Fig. 5(b), (5-ferrite phase formed an almost continuous network around the austenite phase. In acidic chloride solutions in the pit, 3-ferrite phase did not dissolve as much as austenite owing to its higher Cr content phase. Based on similar observations for Mg A1 alloys it was suggested that the presence of "a network with higher corrosion resistance acts as corrosion barrier against pit propagation" which resulted in shallow pits . The effect of heat treatment on pitting potentials of LSM samples was not as significant as the changes observed in terms of morphology. The observed increase in pitting potential was about 35 mV more positive than that of LSM sample. This increase in pitting potential can be explained by the decrease of ,~-ferrite-austenite phase boundaries, which may act as pit initiation sites, following heat treatment.
The positive effects of LSM on corrosion behavior of stainless steels is attributed to elimination/redistribution of sulfur in the alloy [11,14]. Our results indicated an additional mechanism for the beneficial effects of LSM on corrosion of 304L SS, based on the behavior of Mn during LSM treatment. During LSM treatment some of the manganese was evaporated and some of it formed M n - S i compounds. Formation of MnS-based inclusions depends on the activity of both Mn and S in the alloy. If the activity of Mn is low than the amount of Mn in the sulfide inclusions decreases and the amount of Cr + Fe increases . These inclusions are not as detrimental as Mn-rich sulfide inclusions because of their lower solubility in aqueous corrosive solutions. It is well known that austenitic stainless steels with lower Mn content are more resistant to corrosion .
The following conclusions can be drawn from the results of the present study: 1. It is possible to evaluate the microstructural and compositional changes that took place during laser surface melting of stainless steels by investigating the electrochemical behaviors of AR 304L, LSM 304L and LSM + H T 304L samples in sulfuric acid and sodium chloride solutions. .
LSM affected the cathodic polarization, active-passive transition, transpassive and pitting behavior of 304L alloy. By heat treatment at 1050 °C only the cathodic behavior reverted back to the original behavior of as-received 304L.
. Chemical compositional rather than microstructural changes that took place during LSM had a greater effect on the corrosion behavior of 304L stainless steel. 4.
Examination of the transpassive behavior of both LSM 304L and LSM + H T 304L samples showed: (i) a decrease of available Mn, by removal or formation of M n - S i compounds during LSM treatment; (ii) the increase of the transpassive dissolution rate of the LSM 304L alloy was due to the higher dissolution rate of the ferrite phase.
Decreasing the available Mn for the formation of MnS inclusions by LSM could be effective in enhancing the corrosion resistance of 304L stainless steels.
The presence of 6-ferrite network in LSM 304L restricted the vertical growth of pits.
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Acknowledgements The authors acknowledge the valuable comments and contribution of Prof. Dr. O.T. Inal during preparation of the manuscript.
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