The Corrosion Behavior of Ni 3 Al / Ni 3 V Two-Phase Intermetallic Compounds in Various Acidic Solutions

The corrosion behavior of the Ni3Al/Ni3V two-phase intermetallic compounds with and without minor elements (Nb, Co, and Cr) to be composed of L12 phase (Ni3Al) and a mixed phase of L12 (Ni3Al) and D022 (Ni3V) has been investigated by using an immersion test in 0.5 kmol/m3 HCl, H2SO4, and HNO3 solutions. The surface morphology was observed before and after the immersion test by scanning electron microscope (SEM). The results were compared to those of the L12 single-phase Ni3(Si,Ti) and austenitic stainless steel type 304. In all acidic solutions, preferential dissolution of (L12 + D022) phase was found in Ni3Al/Ni3V, but no intergranular attack, whereas the attacks took place on Ni3(Si,Ti). The Ni3Al/Ni3V showed a higher corrosion resistance in HCl solution and a lower resistance in HNO3 solution than Ni3(Si,Ti) and type 304. The addition of the minor elements enhanced corrosion resistance in HNO3 solution, but not clearly in HCl and H2SO4 solutions. In HCl and H2SO4, their weight losses during the immersion test were almost the same.

Furthermore, Ni base intermetallic compounds such as single-phase Ni 3 (Si,Ti) have attractive properties for hightemperature structural material (e.g., high phase stability and strength at high temperature), but the intermetallic compounds have been shown to suffer from (1) poor ductility at room temperature and low strength at high temperature, and (2) to have the decrease in strength at high temperature.However, multiphase intermetallic compounds such as Ni 3 Al/Ni 3 V two-phase intermetallic compounds were found to improve these weakness points [13].
At present, with regard to the Ni 3 Al/Ni 3 V two-phase intermetallic compounds, there are few studies on their corrosion behavior in aqueous solutions at ambient temperature.Therefore, the purpose of this work is to elucidate the corrosion behavior of the Ni 3 Al/Ni 3 V two-phase intermetallic compounds with and without minor elements of Nb, Co, and Cr in HCl, H 2 SO 4 , and HNO 3 solutions and to compare with that of L1 2 single-phase Ni 3 (Si,Ti) intermetallic compound and austenitic stainless steel type 304.

Experimental
2.1.The Specimens.The Ni 3 Al/Ni 3 V two-phase intermetallic compounds with and without minor elements used in the present study were fabricated from the raw materials of 99.9 wt% Ni, 99.9 wt% Al, 99.9 wt% V, 99.9 wt% Nb, 99.9 wt% Co, and 99.9 wt% Cr.These compounds were prepared as buttons by using an arc melting method under an argon gas atmosphere using a nonconsumable tungsten electrode on a copper hearth.A small amount of boron was added to suppress an intergranular fracture in environments such as air, water, and hydrogen gas [1][2][3][4][5][6][7][8][9][10][11][12][13].Then the button ingot was homogenized at 1553 K for 3 h in vacuum and then furnace cooled.The plate specimens with a 1 mm thickness were prepared by cutting perpendicular to the basal plane of the homogenized button, and then test specimens were cut to the geometry of 1.0 mm × 10.0 mm × 20.0 mm by using an electrodischarge machine.The nominal chemical compositions of two types of Ni 3 Al/Ni 3 V twophase intermetallic compounds (hereafter referred to as a type 1 without minor elements and type 2 with minor elements) were shown in Table 1.Here, major alloy elements are Ni, Al, and V and minor elements such as Nb, Co, and Cr are added at the levels of several atomic percents.The addition of Co and Cr was reported to improve oxidation property, while that of Nb to improve high temperature strength and wear properties [8].

Pretreatment of the Specimens and Test Solutions.
The specimens homogenized were cut into 1.2 mm × 9 mm × 15 mm.Then they were polished to 1.0 micrometer alumina paste, degreased by acetone in an ultrasonic cleaner and washed with distilled water.The test solutions, 0.5 kmol/m 3 HCl, H 2 SO 4 , and HNO 3 , were prepared by reagent grade chemicals and distilled water.For microstructure observation, galvanostatic etching of the mechanically polished specimens was conducted in a solution consisting of 15 mL of 17.8 kmol/m 3 H 2 SO 4 and 85 mL of methanol at a current density of 0.446 A/cm 2 for 30 sec at a temperature of 243 K.

Corrosion Test. The corrosion test of the mechanically polished specimen was conducted by immersion in the three acidic solutions at 303 K under an open circuit condition.
A corrosion weight loss (ΔW) was estimated from the difference in weights of the specimens before and after the immersion test at various immersion times up to a maximum time of 96 hours.After the experiments, the morphology of the specimen surfaces was observed by using scanning electron microscope (SEM).

Microstructure of Ni 3 Al/Ni 3 V Two-Phase Intermetallic
Compounds.The Ni 3 Al/Ni 3 V two-phase intermetallic compound exhibited the two phases composed of the cuboidal L1 2 precipitate and (L1 2 + D0 22 ) phase [1,2] as shown in the magnified photos in Figure 1.Furthermore, Figure 1 shows the microstructure of the Ni 3 Al/Ni 3 V two-phase intermetallic compound after the galvanostatic etching.The intermetallic compound consists of two regions of an L1 2 phase of Ni 3 Al (dark areas) and a mixed phase of L1 2 of Ni 3 Al and D0 22 of Ni 3 V (white areas).In addition, Figure 1 clearly shows that the size of the cuboidal L1 2 precipitate decreases with decreasing the Al content: in other words, a volume fraction of the cuboidal L1 2 precipitate increases with the increase in the Al content in the compounds.The observed microstructures were consistent with those reported in the previous studies [1][2][3][4][5][6][7][8][9][10][11][12][13].HCl solution at 303 K, where that for the L1 2 single-phase Ni 3 (Si,Ti) and austenitic stainless steel type 304 was shown for comparison [19].The weight losses of both compounds were found to be significantly lower than those of Ni 3 (Si,Ti) and type 304.In addition, in the figure magnified in Figure 2, the weight loss for type 1 was almost the same as that for type 2 over the whole immersion time.

Weight Loss.
Figure 3 shows the change in weight loss with immersion time for type 1 and type 2 in 0.5 kmol/m 3 H 2 SO 4 solution at 303 K, where that for Ni 3 (Si,Ti) and type 304 was also shown for comparison [19].The weight losses of Ni 3 (Si,Ti) and type 304 were much smaller than those in Figure 2, whereas those of type 1 and type 2 became slightly larger than those in Figure 2. Furthermore, from the figure magnified in Figure 3, it was found that the weight losses of both type 1 and type 2 were larger than those of Ni 3 (Si,Ti) and type 304, and the weight loss of type 2 was larger than that of type 1.
Figure 4 shows the change in weight loss with immersion time for type 1 and type 2 in 0.5 kmol/m 3 HNO 3 solution at 303 K, where that for Ni 3 (Si,Ti) and type 304 was also shown for comparison [19].In this solution, the weight losses of both type 1 and type 2 were larger than those of Ni 3 (Si,Ti) and type 304.This result was found to be clearly different from that in the HCl solution.In addition, the weight loss of type 1 was larger than that of type 2.
From the results obtained, it was found that the magnitude of weight losses for type 1 and type 2 in acidic solutions increased in order of HNO 3 > H 2 SO 4 > HCl, while that for Ni 3 (Si,Ti) and type 304 increased in order of HCl > H 2 SO 4 > HNO 3 solutions.surface morphologies of type 1 and type 2 at the immersion times of 24 hours and 96 hours in the HCl solution.In the HCl solution, a preferential dissolution of the mixed phase of L1 2 and D0 22 (Ni 3 Al and Ni 3 V) was slightly observed at the initial stage (24 hours).At the immersion time of 96 hours, its preferential dissolution became significant.Furthermore, it was found that an intergranular attack was not observed on both compounds, although the attack was observed at Ni 3 (Si,Ti) in HCl solution in previous work [19].

Surface Morphology of Ni
Figure 6 shows the surface morphologies of type 1 and type 2 at the immersion times of 24 hours and 96 hours in the H 2 SO 4 solution.The results were almost similar to those in Figure 5. Furthermore, Figure 7 shows the surface morphologies of type 1 and type 2 at the immersion times of 24 hours and 96 hours in the HNO 3 solution.Although the morphologies were similar to those in the HCl and H 2 SO 4 solutions, especially for type 2, the degree of corrosion was larger than that in the HCl and H 2 SO 4 solutions, in a sense that a dissolution of the L1 2 phase appeared to occur clearly in the HNO 3 solution as well as the development of the preferential dissolution for the mixed phase of L1 2 and D0 22 (Ni 3 Al and Ni 3 V), especially compared to those in the HCl solution.3) type 2 containing the minor elements showed a higher corrosion resistance than type 1 without them in HNO 3 solution.These three points would be explained as follows.

The Effect of Vanadium on Corrosion Resistance of
Schlain et al. [20] reported that a high purity vanadium (99.9%) was highly resistive to corrosion in H 3 PO 4 , H 2 SO 4 , and HCl solutions at 35 • C, but corroded rapidly in HNO 3 solution.The corrosion of vanadium would explain the above points.Since the corrosion behavior of type 1 and type 2 including vanadium should be reflected by that of the bulk vanadium in the present solutions (point 1), vanadium was present only in the mixed phase (point 2), and the amount of vanadium in type 2 was smaller than that in type 1 (point 3).
For the effect of the minor elements on corrosion resistance, we tried to observe the distribution of the minor elements using electron probe microanalyzer (EPMA).It was found that the distribution appeared to be uniform, showing little enrichment of the minor elements in either the L1 2 phase or the mixed phase.This result would come from the fact that as shown in Figure 1, their microstructures were very fine with several hundreds nm diameter [21], and thus it was difficult to determine the enrichment site.Therefore, the difference in the corrosion resistance between type 1 and type 2 could not be explained only by the influence of minor elements.In addition, type 1 and type 2 showed the higher corrosion resistance in the HCl solution compared to Ni 3 (Si,Ti) and type 304.This suggests that the addition of vanadium would inhibit the corrosion of type 1 and type 2 in the HCl solution and, however, enhance the corrosion in the H 2 SO 4 and HNO 3 solutions.However, to clarify the effect of vanadium, further experiments must be conducted.

Conclusion
The corrosion behavior of the Ni 3 Al/Ni 3 V two-phase intermetallic compounds without the minor elements (type 1) and with them (type 2) in HCl, H 2 SO 4 , and HNO 3 solutions with 0.5 kmol/m 3 has been studied and compared with that of the L1 2 single-phase Ni 3 (Si,Ti) and austenitic stainless steel type 304.The following conclusions were obtained.
(1) The type 1 and type 2 showed the higher corrosion resistance in HCl solution and the lower corrosion resistance in HNO 3 solution than Ni 3 (Si,Ti) and type 304 in HCl and H 2 SO 4 solutions.
(2) Both type 1 and type 2 had the preferential dissolution of the mixed phase of L1 2 and D0 22 took place in all acidic solutions.
(3) It was found that the corrosions of type 1 and type 2 were enhanced by the addition of vanadium in HNO 3 solution, while they were inhibited by vanadium in HCl solution.
Figure 2  shows the change in weight loss with immersion time for type 1 and type 2 in 0.5 kmol/m3

3
Al/Ni 3 V Two-Phase Intermetallic Compounds after Immersion Test.

Figure 5
shows the

Figure 5 :Figure 6 :
Figure 5: SEM microstructures of Ni 3 Al/Ni 3 V two-phase intermetallic compounds: (a) type 1 and (b) type 2 after the immersion times of 24 hours and 96 hours in 0.5 kmol/m 3 HCl solution at 303 K.

Figure 7 :
Figure 7: SEM microstructures of Ni 3 Al/Ni 3 V two-phase intermetallic compounds: (a) type 1 and (b) type 2 after the immersion times of 24 hours and 96 hours in 0.5 kmol/m 3 HNO 3 solution at 303 K.
Ni 3 Al/Ni 3 V Two-Phase Intermetallic Compounds.From the above results on type 1 and type 2, it was found that (1) the weight loss increased in order of HNO 3 > H 2 SO 4 > HCl solutions, (2) the preferential dissolution of the mixed phase of L1 2 and D0 22 (Ni 3 Al and Ni 3 V) took place, and (

Table 1 :
Chemical compositions of the materials investigated.