EG0800316 Effect of Heat Treatment and Stabilizing Elements on the Localized

Corrosion Behavior of Some Stainless Steels

K. El-Menshawy, M.M.A. Gad* and A.A. El-Sayed Metallurgy Department, Nuclear Research Center

Atomic Energy Authority, Cairo, Egypt *The Egyptian Styrenics Production Company

ABSTRACT

The sensitization resistance of both stabilized types 347 (Nb stabilized) and 321

(Ti stabilized) as well as the low carbon grade 304L stainless steels was evaluated utilizing electrochemical techniques. Stabilized alloys were solution treated at 1250oC followed by subsequent sensitization at 1050oC and 650oC. For the unstabilized 304L alloys, samples were solution treated at 1050oC then sensitized at 650oC.

Results of the present investigation revealed that the severity of crevice

corrosion in conjunction with the intergranular attack is slightly more prominent in alloy 321 than in alloy 347 in both heat treatment conditions. After heat treatment at 1050oC for one hour, 304L stainless steels showed a remarkably better crevice corrosion performance than both stabilized alloys. On the other hand both stabilized and unstabilized stainless steels suffered almost the same degree of attack after being sensitized at 650oC.

Surface investigation and EDXA results of tested specimens have revealed that

sensitization at 1050oC for both stabilized alloys resulted in the formation of Nb carbide (for alloys 347) and Ti carbide (for alloy 321) in the form of MC. On the other hand, M23 C6 type carbide have been formed in unstabilized 304L stainless steels sensitized at 650oC. Formation of carbides was verified through the clear relative enrichment of carbide formers at the sites, where these carbides were formed. Key Words: Stabilized stainless steels, sensitization, localized corrosion.

INTRODUCTION

Austenitic stainless steels are the main materials of construction for many critical applications

in chemical, petrochemical, power (conventional as well as nuclear) and oil refinery industries. This is because of their excellent corrosion resistance, adequate mechanical properties and ease of fabrication(1). Attention has been directed towards alloys which could combat localized type corrosion through either drastically reducing carbon levels in normal austenitic Cr-Ni stainless steels (possibly with Mo), or by using stabilized austenitic stainless steels with normal contents of carbon(2).

It is accepted that sensitivity to intergranular corrosion in austenitic stainless steels is caused

by the precipitation of Cr23C6 at grain boundaries(3). The formation of these carbides requires the

diffusion of chromium towards grain boundaries, thus leading to the creation of a Cr-depleted zone in the vicinity of the boundary(4).

Stainless steels contain minor additions of titanium, niobium or vanadium, all of which are

stronger carbide formers than chromium . This leads to less possibility for the creation of Cr-depleted zones, provided the concentration of these elements exceeds a certain limit and the alloys have received the proper heat treatment(3).

Some investigators(5 , 6) reported that Nb (in 347 stainless steels) is a better stabilizer than Ti

(in 321 stainless steels). However, others(7 , 8) have shown that the carbon content in stabilized stainless steels has a significant effect. Type 347 stainless steels containing 0.03%C, 0.6% Nb showed a better localized corrosion performance than a similar type containing 0.07%C and 0.75% Nb. The latter exhibited some type of corrosion cracking after 4000 hours service at 540oC, while the former type suffered no corrosion cracking. Some degree of sensitization was observed in an alloy containing 0.05%C and 0.65% Nb, but no stress corrosion cracking was reported(7) .

Austenitic stainless steels enjoy excellent uniform corrosion resistance but are highly

susceptible to crevice corrosion(9 , 10). Fabrication processes involving improper heat treatment, and continuous exposure of these materials to high temperature operating environments lead to drastic changes in their microstructure.

The present investigation aims at studying the effect of heat treatment and type of carbide

precipitation on the crevice corrosion of unstabilized 304L as well as stabilized 347 and 321 stainless steels in 0.5N NaCl solutions.

EXPERIMENTAL

Material The test materials were austenitic stainless steels of types, AISI 304L, AISI 321 and AISI 347.

The chemical composition of these materials is given in table 1.

Table 1: Chemical composition of the tested alloys

Material Cr Mo C Ni Fe Mn Ti Nb+Ta Others 304L 18.09 0.26 0.015 9.68 69.23 1.78 <0.01 <0.01 P,S,Si 321 17.32 0.39 0.045 10.23 68.73 1.59 0.5 0.00 N,Co,Cu347 17.38 0.20 0.039 9.65 70.06 1.38 0.00 0.55 N,Si,Co

Heat Treatment The stabilized steels (347 and 321) were solution heat treated for 2 hours at 1250oC followed

by water quenching. Part of the specimens was then sensitized at 1050oC, the other part was sensitized for the same duration of time at 650oC. The low carbon grade 304L steel was solution treated at 1050oC for one hour, and then water quenched. It was then subsequently sensitized at 650oC. All sensitization heat treatments were conducted by soaking specimens at the prescribed temperature for one hour followed by water quenching.

Prior to conducting corrosion runs specimens were etched using a solution composed of 50 ml. HCl+25 ml. saturated CuSO4 solution. Metallographic examination of the etched specimens was carried out using an optical microscope as well as a Scanning Electron Microscope, SEM (Jeol, JSM 5400) equipped with Energy Dispersive X-ray Analysis (EDXA) unit.

Crevice Corrosion Tests

A conventional multi-necked 1-litre glass corrosion cell was used to conduct crevice corrosion runs. A Saturated Calomel Electrode, (SCE), was used as a reference electrode and a graphite electrode as a counter electrode. The working electrode was the stainless steel test specimen, fixed in a special holder specially made to create a crevice between the test specimen and a PTFE crevice former. The test solution was 0.5N NaCl solution prepared from analytical grade NaCl and double distilled water. An advanced electrochemical corrosion testing machine (Applied Research Model 6310 Electrochemical Impedance Analyzer) was used for electrochemical corrosion evaluation.

Surface Morphology

Surface examination of the specimens was conducted after completing the crevice corrosion runs using the scanning electron microscope.

RESULT AND DISCUSSION Effect of heat treatment on microstructural changes Solution heat treatment

The microstructure of the tested alloys at different heat treatment conditions is shown in figure 1. Etched stabilized alloys (347 and 321) revealed that the microstructure consists of grains of typical FCC morphology with profuse annealing twins and the second phase particles had dissolved at the solution treatment temperature of 1250oC (figure 1-a and b). The microstructure of the unstabilized 304L steel shows almost equiaxed grains, no annealing twins were observed (figure 1-g).

The EDX analysis of some areas selected from the microstructure SEM of the tested alloys is

summarized in table 2. Table 2: EDXA results of etched solution heat treated specimens

Temperature element

1250oC 321

1250oC 347

1050oC 304L

Cr 17.84 18.22 18.13 Mn 1.39 1.79 2.51 Fe Bal Bal Bal Ni 10.17 9.16 10.26

Nb ----- 0.53 ----- Ti 0.40 ----- ----- Mo ----- 0.23 -----

It is clear that EDX analysis obtained for the stabilized alloys (347 and 321) at 1250oC, as well

as for alloy 304L at 1050oC, is almost the same as the original chemical composition (table 1). Sensitization heat treatment:

The microstructure of stabilized alloys (347 and 321) sensitized at 1050oC is displayed in figure 1-c and d. It is clear that for both alloys the microstructure shows some carbide precipitation along the grain boundaries. The tendency towards carbide formation is confirmed by EDXA around the grain boundaries. Some of the EDXA findings are summarized in table 3.

Table 3: AEDX results of etched sensitized specimens

1050oC 650oC Temperature

elements

321 347 321 347 304L

Cr 18.81 18.39 18.34 18.38 21.62 Mn 1.79 1.86 2.29 1.96 0.79 Fe Bal Bal Bal Bal Bal Ni 9.98 9.08 10.84 9.84 10.39 Nb ----- 1.64 ----- 0.57 ----- Ti 1.53 ----- 0.44 ----- -----

The microstructure of both stabilized alloys (347 and 321) and unstabilized 304L sensitized at

650oC is displayed in figure 1-e, f and h, respectively. It is clear that sensitization of alloy 347 at 650oC resulted in chromium carbide precipitation along the grain boundaries. This is confirmed by the absence of any change in niobium concentration if compared to original chemical composition (table 1 and 2). This suggests that the heat treatment of the stabilized alloy 347 at 650oC, can not simply induce the precipitation of niobium carbide. This is confirmed by the EDXA results given in table (3). The same concept also applies for the stabilized alloy 321. As for alloy 304L, chromium is the only carbide former at this temperature. Electrochemical corrosion Heat treatment at 1250oC and at 1050oC

Stabilized alloys solution heat treated at 1250oC and subsequently sensitized at 1050oC were electrochemically tested by applying the cyclic potentiodynamic polarization technique. Figure 2 shows a series of cyclic potentiodynamic polarization runs for alloy 321 solution treated at 1250oC (curve 1) and sensitized at 1050oC (curve 2). The same technique was applied to evaluate the crevice corrosion susceptibility of alloy 347 receiving a similar heat treatment and also for alloy 304L solution treated at 1050oC. The electrochemical data derived from polarization curves are summarized in table 4.

Figure (2) Cyclic potentiodynamic polarization curves for 321 alloy tested In 3% NaCl solution

Table 4: Electrochemical corrosion parameters calculated from cyclic potentiodynamic

tests (at different heat treatment temperatures)

Alloy 321 Alloy 347 Alloy 304L

Alloy Parameter 1250oC 1050oC 1250oC 1050oC 1050oC Ecorr (mV) - 680 -800 -620 -750 -550 I at vertex (A/cm2) 2 x 10-2 3 x 10-1 1x 10-2 2 x 10-1 2 x 10-2 I at 300mV (A/cm2) 2 x 10-3 1 x 10-1 1x 10-3 4x 10-2 2 x 10-4 I at 100mV (A/cm2) 4x 10-3 2x 10-1 1x 10-3 1x 10-1 1x 10-3

Both fig.2 and table 4 indicate that solution heat treated stabilized alloys enjoy a higher corrosion resistance compared to the sensitized condition. Table 4 also indicates, that there is no significant difference between the crevice corrosion resistance of the two stabilized alloys in terms of the dissolution current values at the vertex potential, where the current density of solution treated 347 is only half the value of the corresponding current density for alloys 321. However in the sensitized condition the vertex crevice corrosion current density corresponding to alloy 347 is 15 times less than that corresponding to alloy 321. The dissolution current values at –300mV also confirm that alloy 347 possesses a relatively higher corrosion resistance compared to alloy 321. It could also be noted (table 4) that values of the free corrosion potential Ecorr. (which indicates the thermodynamic tendency to corrosion) are consistently more noble (60 and 50mV) for alloy 347 than for alloy 321, in both the solution treated and sensitized conditions.

On the other hand, at 1050oC which corresponds to the solution heat treatment temperature for

ally 304L and to the sensitization temperature for both stabilized alloys, 304L alloy displays a higher resistance to crevice corrosion if compared to the other two stabilized alloys heat treated at the same temperature. Though dissolution current values at the vertex potential were almost the same for the three alloys, a remarkable difference is found (2 to 3 orders of magnitude) at -300mV and at 100 mV/SCE.

Additionally, free corrosion potential-time runs have been carried out for all the tested alloys

in both the solution treatment and sensitization conditions. The test duration was 3000 seconds and the change in free corrosion potential value was recorded every 400 seconds. For alloy 321, the measured free corrosion potential values at zero time (i.e. immediately at the start up of the run) were –700 and -760 mV/SCE for 1250oC and 1050oC, respectively. There is a difference of about 60mV between both heat treatment temperatures. These findings were in agreement with those found through applying cyclic potentiodynamic technique, where the difference is about 120mV/SCE. Table 5 shows that there is a sensible shift in the free corrosion potential in the active direction in the first 1000 seconds then the values remained practically unchanged for the remaining duration of the test. Moreover, the difference between the corrosion potential values at both heat treatment temperatures remains almost the same all through test duration. The same trend was found for alloy 347, however, the measured values were different (table 5). On the other hand, for alloy 304L the immediate measured corrosion potential values were almost the same at both heat treatment temperatures (1050oC and 650oC) and of about –760mV/SCE. However, the difference began to increase drastically all through the test duration to reach 50mV/SCE at the end of run (the measured values were –800 and 850mV/SCE for specimens

heat treated at 1050oC and 650oC, respectively). Table 5 summarizes the Ecorr-time values for the tested alloys at different heat treatment temperatures.

Table 5: Ecorr-time (mV, seconds) values for the tested alloys at different heat treatment temperatures

Alloy 321 347 304L

Time 1250oC 1050oC 1250oC 1050oC 1050oC 650oC 0.00 -700 -760 -756 -768 -760 -760 200 -730 -765 -790 -776 -765 -780 600 -750 -780 -762 -782 -767 -798 1000 -755 -785 -762 -782 -770 -810 1400 -760 -790 -765 -782 -780 -830 1800 -765 -790 -768 -782 -789 -840 2200 -770 -790 -770 -782 -789 -840 2600 -770 -790 -772 -782 -789 -840 3000 -770 -790 -772 -782 -789 -840

Surface examination of corroded specimens

Corroded surfaces of the stabilized alloys heat treated at 1250oC and 1050oC and tested in 0.5N NaCl solution and unstabilized alloy 304L heat treated at 1050oC and tested in the same test solution were investigated by SEM. Figure 3 shows the SEM micrographs of these tested alloys. It is clear that for stabilized alloys at 1250oC, alloy 347 shows a higher resistance to crevice corrosion compared to alloy 321 (figure 3, a and c). This is obvious from the morphology of the corrosion sites (sites under the crevice former) where the extent of attack is more pronounced in alloy 321 than in alloy 347. Although at the sensitization temperature of 1050oC, both alloys suffered severe crevice corrosion, alloy 347 is still more resistant than alloy 321 (figure 3-b and d). It is also clear from fig 3-e that at 1050oC the severity of crevice corrosion attack on alloy 304L is far less than that incurred by both stabilized alloys, since this temperature corresponds to the solution heat treatment for alloy 304L, while at this temperature the stabilized alloys undergo sensitization. Results of the EDXA of some corroded areas under the crevice former are summarized in table 6.

Figure (3) SEM micrographs of corroded surfaces for the tested alloys.

Table 6: EDXA results of specimens heat treated at different temperatures (after conducting crevice corrosion tests)

321 347 304L Alloy

Element%

1250oC 1050oC 1250oC 1050oC 1050oC

Cr 23.98 17.54 28.56 18.18 19.64 Ni 8.4 6.48 3.22 9.89 9.87 Mn 1.7 1.32 5.01 1.63 1.76 Fe Bal. Bal. Bal. Bal. Bal. Ti 1.79 9.35 ----- ----- ----- Nb ----- ----- 1.48 9.48 -----

It is clear from table 6, that for both stabilized alloys heat treated at 1250oC, there is a

remarkable change in chromium concentration compared to that of the original alloy composition (table 1). Also, at 1050oC there is a remarkable increase in the concentration of stabilizing elements Nb (alloy 347) and Ti (alloy 321) compared to the original values. Niobium is known to be a strong carbide former, and it promotes the precipitation of MC-type carbide rather than M23C6-type intergranular carbide (3,11). There is also evidence in the literature that TiC and other phases of titanium carbide could be identified through metallography and XRD in sensitized type 321 stainless steel(7,12).On the other hand, for alloy 304L solution heat treated at 1050oC, the concentration of the main alloying elements is almost the same as that of the original concentration.

Sensitization at 650oC

Specimens of the three investigated alloys where given additional heat treatment

(sensitization) at 650oC after the solution heat treatment to clarify the type of corrosion predominant in the alloys sensitized at this temperature. A series of cyclic potentiodynamic runs have been carried out for all test specimens, and the electrochemical parameters are summarized in table 7.

Table 7: Electrochemical corrosion parameters calculated from cyclic potentiodynamic tests for specimens sensitized at 650oC

Alloy

Parameter

321 347 304L

Ecorr (mV) -650 -580 -720 I at vertex (A/cm2) 2 x 10-2 2 x 10-2 2 x 10-2 I at –300mV (A/cm2) 3 x 10-3 3 x 10-4 4 x 10-3

I at 100mV (A/cm2) 5 x 10-3 5 x 10-3 7 x 10-3

Table 7 showed that for stabilized alloys, alloy 347 still has a less tendency to crevice corrosion compared to alloy 321. This is obvious from the corrosion potential values (-580 for 347 and –650 for 321). Moreover, the dissolution current at a particular potential positive to the corrosion potential (-300 mV. SCE) is less by an order of magnitude for alloy 347 than alloy 321, although the current values at the vertex potential and at 100mV are the same. For the unstabilized alloy 304L, a more active corrosion potential is measured compared to the stabilized alloys.

SEM investigation of the corroded surfaces (figure 4) showed that at lower magnification, the

three tested alloys suffered the same degree of crevice corrosion. At higher magnification, the mode of attack showed a tendency towards intergranular corrosion of different features. The EDXA results for some selected corroded areas are summarized in table 8.

Figure (4) SEM micrographs of corroded surfaces for the tested alloys after sensitization at 650 Cْ

347

(a)

347

(b)

321

(c)

321

(d)

304 L 304 L (f)

Table 8: EDXA results of specimens heat treated at 650oC (after conducting crevice corrosion tests)

321 347 304L Alloy

Element%

W.A* B.A.+ W.A* B.A.+ W.A* B.A.+

Cr 23.78 29.75 26.43 24.5 24.09 27.54 Ni 10.6 8.79 7.5 7.01 6.28 8.72 Mn 1.81 1.04 3.6 3.0 3.5 2.82 Fe Bal. Bal. Bal. Bal. Bal. Bal. Ti 2.2 1.5 ----- ----- ----- ----- Nb ----- ----- 1.98 1.18 ----- -----

W.A.* white areas B.A.+ black areas

Table 8 shows that the concentration of chromium in all the three alloys exposed to crevice

corrosion after being sensitized at 650oC is remarkably higher than the original concentration, while the concentration of the stabilizing elements (Nb, Ti), though slightly higher than the original concentration, remains by far below the levels identified in specimens exposed to crevice corrosion after being sensitized at 1050oC (table 6). This clearly indicates that Cr is the active carbide forming element at 650oC for all the three alloys. Both Nb and Ti play little or no role in the sensitization process at this temperature for alloys 321 and 347.

It should however be stated that more detailed and deeper investigation (both electrochemical and surface analysis) is still needed to help understand the modes of corrosion occurring under crevices in sensitized stabilized stainless steels and the features of intergranular corrosion types that might be prevailing.

CONCLUSIONS 1. Stabilized alloys sensitized at 1050oC exhibited a tendency towards carbide

formation. 2. Stabilized alloy 347 possesses a somewhat better corrosion resistance than alloy 321. 3. Chromium is the major carbide former in stabilized alloys heat treated at 650oC little or no

carbide formation is attributed to stabilizing elements. 4. Intergranular corrosion takes place during crevice corrosion of sensitized specimens.

REFERENCES

(e)

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