The stress corrosion cracking of welded austenitic stainless steels in MgCl2 solutions in the presence of NaI additions

The stress corrosion cracking of welded austenitic stainless steels in MgCl2 solutions in the presence of NaI additions

Corrosion Science, Vol. 23, No. 12. pp. 1285-1291, 1983 Printed in Great Britain. 0010-938X/83 $3.00 4- 0.00 Pergamon Press Ltd. THE STRESS CORROSIO...

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Corrosion Science, Vol. 23, No. 12. pp. 1285-1291, 1983 Printed in Great Britain.

0010-938X/83 $3.00 4- 0.00 Pergamon Press Ltd.

THE STRESS CORROSION CRACKING OF WELDED AUSTENITIC STAINLESS STEELS IN MgCl2 SOLUTIONS IN THE PRESENCE OF NaI ADDITIONS D. ITZHAK and D. ELIEZER Materials Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel Abstract---The influence of NaI additions on the stress corrosion cracking (SCC) of Type 304L and 316L welded stainless steels in 42 wt% aqueous MgCI~solution at 1M°C has been investigated. The results indicate clearly that addition of 1 N Nal to the boiling MgCI2solution prevents SCC of the welded steels. The I- can act as an effective cathodic inhibitor. The correlation between mechanical properties, fracture morphology and the mechanism of the inhibition behaviour of I- on SCC is discussed. INTRODUCTION STRESS corrosion cracking (SCC) of austenitic stainless steels weld metals in boiling MgCl~ solutions has been shown to occur exclusively along austenite-ferrite interphase boundaries in the weld metal substrates. 1,~ It has been suggested that this effect is a result of non-equilibrium segregation of chromium during weld metal solidification and cooling, s With regard to SCC it has previously been observed that the fusion zone in the welded stainless steel is less susceptible to SCC than single phase austenitic steels in chlorides since the 8 phase tended to inhibit crack growth. 2,a However, the heat affected zone (HAZ) is more susceptible to SCC due to Cr depletion along grain boundaries. Reports about the influence of alkali halide additions on SCC of stainless steels in boiling MgC12 solution are few. 4-7 In a previous paper 6 the influence of NaCI, NaBr and NaI additions on the SCC of Type 304 in 38 wt % aqueous MgCl., solution at 135°C was reported. It was suggested that the addition of NaI inhibits mainly the cathodic process and results in preventing SCC in this environment. To our knowledge no reports are available on the effects of NaI to SCC of welded stainless steels. The purpose of this investigation was to study the influence of NaI additions on the SCC in a susceptible system to SCC such as welded Type 304L and 316L stainless steels in MgCl2 boiling solution at 154°C. E X P E R I M E N T A L METHOD AISI Type 304L and 316L stainless steel sheets of 1.5 mm thickness have been welded using the gas tungsten arc welding process. Chemical composition of the weld metals are shown in Table 1 and the procedures employed in weld metal fabrication are described in Table 2. Weld metal specimens were machined from welded sheet materials such that the centre of the gauge section length was coincident with the centre of full penetration, single pass weldment length. This procedure minimized maerosegregation in the gauge section weld metal and provided in all weld metal gauge section with a consistent ferrite content and morphology. Manuscript received 9 May 1983. 1285

1286 TABLE I.

D. ITZHAKand D. ELIEZER CHEMICAL COMPOSITION OF TYPE

Cr 304L 316L

18.42 17.65 TABLE 2.

304L AND

3 1 6 L STAINLESS STEEL ELEMENT (wt~o)

Ni

Mn

Si

Co

Mo

Cu

C

8.71 11.10

1.42 1.80

0.64 0.52

0.16 0.32

0.07 2.08

0.06 0.21

0.025 0.026

PARAMETERS EMPLOYED IN THE FABRICATION OF TYPE 3 1 6 L WELD METALS

Welding process: Weldment type: Welding current: Arc voltage: Travel speed: Welding fixture:

304L XlqD

Gas tungsten arc Autogenous, full penetration 275 A 12.5 V, DC 0.66 cm s-~ Copper base and cover plates, full restraint

The following series of tests were conducted: 1. Stress corrosion tests were carried out in 42~ boiling MgCI2 (154°C) and with additions of 0.01 N NaI, 0.1 N NaI, and 1 N NaI. The tensile specimens having a gauge length of 50 mm and a cross-section of 6.5 crn~ tested in an Instron machine using a cross-head speed of 0.005 cm rain-1. Solution temperature and concentration were maintained constant (± 2°C) by an immersion heater reflux condenser system. MgCI~ test solutions were prepared from reagent grade hydrated MgCls crystals (MgClm'6H20) and distilled water according to ASTM standard G36-73. Prior to testing, solutions were boiled for at least 1 h. After fracture, the specimens were ultrasonically cleaned in water and acetone in order to remove traces of the salt. 2. In order to evaluate differences between the stress corrosion cracking resistance of the steel in the different environments U-bend stress corrosion tests were performed. 3. The fracture surfaces of all failed specimens were examined by scanning electron microscopy (SEM). E X P E R I M E N T A L RESULTS AND DISCUSSION Engineering stress-displacement curves obtained for Type 304L and 316L tested in the following boiling solutions: MgC12, MgCI~ + 0.01 N NaI, MgClz + 0.1 N NaI, MgCI~ + 1 N NaI, are shown respectively in Figs. 1 and 2. As can be seen additions of 1 N NaI resulted in a substantial increase in elongation to fracture and ultimate tensile stress (UTS) so that the mechanical properties are almost similar to those obtained in silicon oil at 154°C. The results show clearly that Type 304L welded austenitic stainless steels is more susceptible to SCC than Type 316L. Small additions of 0.01 N N a I to the 42 w t ~ MgCI~ resulted in a substantial increase in elongation to fracture comparing to pure MgCI~ solution. This behaviour is explained by the synergistic effect of the I - and the Mo so that rapid repassivation of the surface occurs. All the specimens failed in the HAZ. The time to failure obtained from U-bend test is presented in Table 3. It can be seen that for both steels addition of 1 N NaI to the boiling MgCI2 42 wt ~ solution prevents SCC. After exposure to corrosive atmosphere for 75 clays the U-Bend samples were not cracked. The influence of 1 N N a I addition on the SCC of Type 304 (not welded) was discussed previously and similar results were obtained, e During the U-Bend test it was observed that the solutions containing N a I turned to yellow-brown due to iodine evolution. Increasing N a ! concentration from 0.0! to 1 N results in higher intensity of the colouring solution. This behaviour indicates that the inhibition

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specimens tested in boiling solutions at 154°C. is more effective as the concentration of NaI increased. The I- ion acts as a reduction agent and reacts with the disssolved oxygen by the following reaction: 41- + 4H ÷ + 02 ~ 212 + 2H20 This reaction is most probably catalyzed by the metallic surface. Cathodic potentiodynamic measurements indicate that I - acts as a diffusion barrier against the proton or other cathodic reactants migration from the solution to the metal surface. As a result the cathodic reactants 02 and H + do not reach the cathodic regions. Ths inhibition effect of the NaI addition is explained by the following process: I - ion reacts slowly in a homogeneous reaction with 02 and H +. Due to this reaction the concentration of the dissolved oxygen decreased. During the exposure to the corrosive environment the I - ions are absorbed on the surface and acts as a diffusion barrier. In addition, rapid heterogenous reaction occurs between O2 and H + on the metal surface. As a result of the inhibition effect the probability that the film rupture model or the hydrogen embrittlement are not the main reasons to SCC. It can be concluded that I - acts as an effective cathodic inhibitor for SCC in boiling MgCI2 solutions. The main results are confirmed by fractographic observations. The fracture mode of all samples tested in the presence of silicone oil at 154°C was microvoid coalescence (MVC). Stress corrosion cracks in Type 304L and 316L weld metals obtained from the U-Bend test exhibit transgranular morphology (Fig. 3). The scanning electron microscope (SEM) fractograph illustrates the commonly observed river pattern SCC morphology. The cracks propagate in a purely transgranular mode. Parallel facets which are separated by steps are also observed (Fig. 3b).

1288

D. ITZHAKand D. ELIEZER

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Ft~. 2. Engineering stress-displacement curves obtained for Type 316L weld specimens tested in boiling solutions at 154°(2. TABLE 3.

304L AND 316L WELDED STAINLESS STEELAT 154°C

U-BEND TEST RESULTS FOR TYPE

Time to fracture (h) 304L

316L

Environment

4 20 190 Were not cracked after 75 days

42 wt % MgCI2 42 wt% MgCI~ + 0.01 N NaI 42 wt% MgCls + 0.1 NNaI

22 48 288 Were not cracked after 75 days

42 wt % MgCI, 42 wt% MgCI~ + 0.01 N Nal 42 wt% MgCI~ + 0.1 N Nal

42 wt% MgCI~ ÷ 1 N Nal

42 wt % MgCI.., + I N NaI

Typical stress corrosion fracture surfaces obtained from the slow strain rate test in 154°C boiling MgCI~ solution are presented in Fig. 4. Stress corrosion cracks in Type 316L weld metal specimens exhibit a mixed transgranular intergranular mode with the intergranular type predominantly (Fig. 4a). It is of interest to note that transgranular stress corrosion cracking in the weld metal appears morphologically similar to transgranular stress corrosion cracking in the wholly austenitic base metal. Figure 4(b) shows that addition of N a I to the MgCI~ solution changes the mode of fracture from brittle in the MgCI2 solution to ductile in the presence of 0.01 N N a I in the Type 316L weld steel and in the presence of 1 N N a I in weld Type 304 stainless

FIG. 3. SEM fractographs from slow crack regions obtained from U-Bend test. (a) 304L in boiling 42 wt% MgCl~ solution with 0.1 N Nal (190 h). (b) 316L in boiling 42 wt% MgCI~ solution with 0,1 N Nat (288 h).

FIG. 4. Scanning electron microscope fractographs of welded specimens after slow strain rate test at 154°C. (a) 316L welded in 42 wt ~ MgCI2. (b) 316L welded in 42 wt MgCI2 + 0.01 N Nal. (c) 304L welded ila42 w t ~ MgCl~ ÷ l N NaI. (d) 316L welded in 42 wt ~ MgCl~ -- l N Nal.

Stainless steels in MgCI2 solutions

1291

steel (Fig. 4c). In the presence of 1 N NaI the failure mode of both Type 304L and 316L welded steels was ductile (Figs. 4c and d). CONCLUSIONS

The following conclusions can be drawn from the results obtained: 1. Addition of IN NaI to 42 w t ~ boiling MgCI2 solution at 154°C prevents SCC of welded Type 304L and 316L stainless steel. 2. I- acts as an effective cathodic inhibitor for SCC in boiling MgCI., solutions. 3. Small additions of 0.01 N Nal improves substantially the elongation of Type 316L due to the synergistic effect of the I- and the molybdenum presence that causes rapid repassivation. 4. Fractographic observations indicate that additions of 0.01 N Nal for the welded Type 316L and 0.1 N for the Type 304L change the mode of fracture from the mixed mode of intergranular and transgranular to ductile mode of fracture. REFERENCES 1. D. H. SHERMAN,D. J. DUQUETTEand W. F. SAVAOE,Corrosion 31,736 (1975). 2. F. STALDERand D. J. DUQUETTE,Corrosion 33, 67 (1977). 3. J. W. FLOWERS,F. H. BECKand M. G. FO~qTANA,Corrosion 19, 186 (1963). 4. H, H. UHLIGand E. W. COOK,Jr., J. electrochem. Soc. 11, 173 (1969). 5. N. D. GRr~,~ and F. M^zzA, 2rid European Symposium on Corrosion lnhibitors, p. 401, University of Ferrara, Italy (1966). 6. P. PINKUS,D. ELIr.ZERand D. ITZHAK,Corros. Sci. 21, 417 (1981). 7. D. ELIEZER, P. PINKUS and D. ITZrlAK, Environmental Degradation of Engineering Materials. Vol. 2, p. 193 (1981).