THE EFFECT OF ANNEALING TEMPERATURES

Teknologi Indonesia © LIPI Press 2013

Teknologi Indonesia 36 (2) 2013: 97–104

THE EFFECT OF ANNEALING TEMPERATURES AFTER THERMOMECHANICAL PROCESS TO THE CORROSION BEHAVIOR OF Ni3(Si,Ti) IN CHLORIDE SOLUTION Gadang Priyotomo Research Center for Metallurgy Indonesian Institute of Sciences Kawasan PUSPIPTEK Gd.474 Setu Tangerang Selatan Banten E-mail: [email protected] Received:

Revised:

Accepted:

ABSTRACT The intermetallic compounds Ni3(Si,Ti) (L12 single phase) after thermomechanical (TMP) process could be used as high-temperatures structural parts or structural parts in chemical plant. The corrosion behavior of these compounds have been investigated using an immersion test, electrochemical method in 0.5 kmol/m3 HCl solution at 300C, where the morphology of corroded compounds was observed by scanning electron microscope. In addition, the corrosion behavior of austenitic stainless steel type 304 and C276 alloy was also studied under the same experimental conditions as references. It was found that the uniform corrosion was observed on as-cold rolled Ni3(Si,Ti), Ni3(Si,Ti) after annealing process at 6000C, and 7000C, while intergranular attack was found on Ni3(Si,Ti) after annealing process at 8000C, 9000C and 10000C. From the immersion test and polarization curves, all Ni3(Si,Ti) intermetallic compounds had moderate corrosion resistance in chloride solution, while C276 alloy and type 304 were the highest and the lowest corrosion resistance, respectively. The effect of annealing process of all Ni3(Si,Ti) compounds after TMP could not enhance their corrosion resistances more effectively. Keywords: Intermetallic compounds, immersion test, polarization curves, intergranular attack, uniform corrosion. ABSTRACT Logam intermetalik Ni3(Si,Ti) (fasa tunggal L12) setelah proces termomekanik digunakan sebagai material suhu tinggi atau material di industri kimia. Sifat korosi logam tersebut telah diinvestigasi melalui pengujian celup dan elektrokimia pada media larutan HCl 0,5 kmol/m3 pada suhu 300C, di mana pengamatan morfologi logam yang terkorosi dilakukan dengan peralatan scanning electron microscope. Sifat korosi baja tahan karat austenitik 304 dan paduan C276 juga diteliti sebagai material acuan. Korosi seragam diamati pada as-cold rolled Ni3(Si,Ti), Ni3(Si,Ti) setelah proses anil pada suhu 6000C, dan 7000C, di mana serangan batas butir telah ditemukan pada Ni3(Si,Ti) setelah proses anil pada suhu 8000C, 9000C dan 10000C. Melalui uji celup dan polarisasi, seluruh logam intermetalik Ni3(Si,Ti) mempunyai ketahanan korosi yang cukup dalam larutan klorrida, di mana paduan C276 dan tipe 304 mempunya ketahanan korosi yang tertinggi dan terendah. Pengaruh proses anil pada seluruh logam Ni3(Si,Ti) setelah proses termomekanik tidak signikan meningkatkan ketahanan korosi secara efektif. Kata kunci: Logam intermetalik, uji celup, kurva polarisasi, serangan batas butir, korosi seragam.

INTRODUCTION Ni3(Si,Ti) intermetallic compound of an L1 2 structure has particular ductility and strength properties; that is (1) an increase in strength with increasing operational temperature and (2) high ductility over a wide range of test temperature [1,2] . Furthermore, its strength value was higher

than that of other L12 intermetallic compounds which are being developed as advanced materials [3] . Ni3(Si,Ti) also has an excellent mechanical properties as low and high temperature structural materials compared to conventional alloys such as steel, stainless steel and nickel-base alloys. In addition, this compound also had a good oxidation resistance in air, at ambient and elevated

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Teknologi Indonesia 36 (2) 2013

operational temperature [4]. Thus, this compound is selected to be a candidate of high-temperature material. Furthermore, the most preceding investigation regarding to this compound conducted the thermomechanical process (TMP) after homogenization process [4,5]. TMP is the most sophisticated combination of well-dened deformation process and heat treatment in single system [3]. In addition, microstructural control for grain size and texture is possible by TMP [5]. With regard to Ni3(Si,Ti) compound, there is little study on the corrosion behavior of Ni3(Si,Ti) after homogenization and TMP in aqueous solutions at ambient temperature, although Priyotomo and co-workers already investigated the corrosion behavior of as homogenized Ni3(Si,Ti) compound in acidic solution in detail [6,7,8]. Therefore, the objective of this work is to elucidate the corrosion behavior of Ni3(Si,Ti) intermetallic compounds after homogenization and TMP process in acidic solution.

MATERIALS AND METHODS The specimens Ni-11 at .% Si-9.5 at .% Ti compound with the addition of 50 wt. ppm of boron was prepared by using an arc melting method under an argon gas atmosphere. It was homogenized at 10500C for 48 h under an argon atmosphere and then cooled at a cooling rate of 100C /min in a vacuum furnace. In TMP process, homogenized ingot was treated with a warm rolling at 3000C in air until obtaining the desired thickness and then with a cold rolling until 1.2 mm of thickness in 75 % reduction. After obtaining its cold-rolled thinness, this sheet was nally annealed from 6000C to 1.0000C for 1 hour by using a vacuum furnace, where annealing process metal is the heating of metal to a specic temperature and then cooling it at a cooling rate of 100C /min. The identification of Ni 3(Si,Ti) intermetallic compound containing L12 single phase had been investigated by an author colleague and the author [9] at Osaka Prefecture University, where the author received that intermetallic compound for this research. Austenitic stainless steel type 304 and C276 alloy were as the reference for the

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experiments. The chemical composition of steel and the nominal compositions of those prepared materials are given in Table 1. Table 1. Composition of the materials investigated. Elements

Ni3(Si,Ti)

C-276

11.0 79.5 9.5 -

Type 304 At.% 0.027 0.68 0.947 0.047 0.006 7.6 19.34 71.4 -

C Si Mn P S Ni Cr Mo V Fe Ti W Co B

50

-

-

0.5 0.11 1.14 0.08 0.06 55.9 19.2 10.4 0.3 6.1 4.1 2.1

Pretreatment of the specimens and test solutions The specimens homogenized were cut into 1.2 mm x 9 mm x 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 solution, 0.5 kmol/m3 HCl solution was 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/m3 H2SO4 and 85 ml of methanol at a current density of 0.446 A/cm2 for 30 sec at a temperature of -300C.

Corrosion tests Immersion test The immersion test of the mechanically polished specimen was conducted to get a weight loss (ΔW); that is, 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 in 0.5 kmol/m3 HCl solution at 303 K under an open circuit condition. After the experiments, the morphology of the specimen

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surfaces was investigated by using scanning electron microscope (SEM). Electrochemical test The potential step method was used to measure polarization curves of the specimens in 0.5 kmol/m3 HCl solution open to air at 300C. The reference and counter electrodes used were Ag/ AgCl saturated with KCl and a platinum sheet, respectively. Polarization measurements were conducted in a potential range from -1073 mV to 1273 mV vs. Ag/AgCl where the potential was increased or decreased from a rest potential with a potential interval of 100 mV (partly 50 mV) and was held for 10 minutes at each potential.

RESULT AND DISCUSSIONS Weight loss Figure 1 shows the immersion time dependence of the weight losses for as-cold rolled Ni3(Si,Ti), Ni3 (Si,Ti) after annealing process at 600 0C, 7000C, 8000C, 9000C, and 1.0000C, type 304 and C276 alloy as a function of immersion time in 0.5 kmol/m3 HCl solution at 300C. The weight losses for these compounds tend to increase by

the increase of immersion time. In addition, out of the weight loss, it was found that the corrosion resistance was decreased in the order of C 276 > as-annealed Ni3(Si,Ti) at 1.0000C > as-annealed Ni3(Si,Ti) at 9000C > as-annealed Ni3(Si,Ti) at 7000C > as-cold rolled Ni3(Si,Ti) > as-annealed Ni3(Si,Ti) at 8000C > as-annealed Ni3(Si,Ti) at 6000C > type 304. On the other hand, as shown in Figure 1, Ni3(Si,Ti) of the intermetallic compounds have a moderate corrosion resistance, where the highest and the lowest are C276 alloy and type 304, respectively.

Polarization curves The polarization curves of as-cold rolled Ni3(Si,Ti), Ni 3(Si,Ti) after annealing process at 6000C, 7000C, 8000C, 9000C, and 1.0000C, type 304 and C276 alloy in 0.5 kmol/m3 HCl solution at 300C were shown in Figure 2, where those curves consist of anodic and cathodic polarization curves. The anodic polarization curve of type 304 showed an active region and quasi-passive region with a very narrow potential range, showing a very large anodic current density at the range potential from 400 mV to

Figure 1. The weight losses of ◆as-cold rolled Ni3(Si,Ti); Ni3(Si,Ti) after annealing process at ○6000C, ●7000C, ☐8000C, ■9000C, and △1.0000C; ◇type 304 and ▲C276 alloy as a function of immersion time in 0.5 kmol/m3 HCl solution at 303 K.

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Figure 2. The polarization curves of as-cold rolled Ni3(Si,Ti); Ni3(Si,Ti) after annealing process at 600°C, 700°C, 800°C, 900°C, and 1.000°C; type 304 and  C276 alloy in 0.5 kmol/m3 HCl solution at 30°C.

1.000 mV. On the other hand, that of C276 alloy showed no active region and passive region with a wide potential from 0 mV to 800 mV or up to oxygen potential range, showing a lowest anodic current density of passive region about the order of 10-6 A/cm2. Furthermore, the anodic curve of as-annealed Ni3(Si,Ti) after annealing process at 6000C, 7000C, 8000C, 9000C, and 1.0000C almost appeared to have the same active and quasi-passive regions within the anodic current density order of 10-3 and 10-2 A/cm2 but not that of as-cold rolled Ni3(Si,Ti). However, it would be difcult to say that the polarization curves of all Ni3(Si,Ti) intermetallic compounds have clearly the active and passive regions because of a very small difference in the magnitudes of anodic current densities between both regions of all Ni3(Si,Ti) compounds compared to those of type 304.

1.0000C, the Icorr of them were found to be order of as-cold rolled Ni3(Si,Ti) > as-annealed Ni3(Si,Ti) at 6000C > as-annealed Ni3(Si,Ti) at 8000C > as-annealed Ni3(Si,Ti) at 7000C > as-annealed Ni3(Si,Ti) at 9000C > as-annealed Ni3(Si,Ti) at 1.0000C. From the results obtained in Figure 1 (immersion test) and Figure 2 (polarization curves), the following things were concluded: 1) The highest and lowest of corrosion resistances are C276 alloy and type 304, respectively. 2) The weight losses and corrosion current densities for the compounds increased with the order of type 304 > Ni3(Si,Ti) intermetallic compounds > C276. 3) The different magnitude of weight losses and corrosion current densities among as-cold

The corrosion current densities (I corr) of materials in 0.5 kmol/m3 HCl solution at 300C are given in Table 2. The corrosion current densities clearly estimated at the open circuit potentials were found to be in the order of C276 < Ni3(Si,Ti) intermetallic compounds < type 304. However, by comparing the anodic current densities among as-cold rolled Ni3(Si,Ti), Ni3(Si,Ti) after annealing process at 6000C, 7000C, 8000C, 9000C, and

Materials Icorr (A/cm2) As-cold rolled Ni3(Si,Ti) 9.10-6 As-annealed 6000C Ni3(Si,Ti) 7.10-6 0 As-annealed 700 C Ni3(Si,Ti) 4.10-6 0 As-annealed 800 C Ni3(Si,Ti) 5.10-6 As-annealed 9000C Ni3(Si,Ti) 2.10-6 0 As-annealed 1000 C Ni3(Si,Ti) 1.5.10-6 Type 304 6.10-5 C-276 5.10-8

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Table 2. The corrosion current densities (Icorr) of materials in 0.5 kmol/m3 HCl solution at 300C.


rolled Ni3(Si,Ti) and Ni3(Si,Ti) after annealing process at 6000C,7000C, 8000C, 9000C, and 1.0000C are almost the same compared to C276 alloy and type 304. 4) The corrosion resistance of Ni3(Si,Ti) after annealing process at 10000C are the highest compared to other Ni3(Si,Ti) intermetallic compounds.

(a)

(a)

(c)

(c)

5) The results of the immersion test on all prepared materials mostly had a good correspondence with those of the polarization test, but partly small part such as as-cold rolled Ni3(Si,Ti) had no correspondence with both tests. Therefore, further experiments will be carried out to clarify this phenomenon.

(b)

(b)

(d)

(d)

(e)

(f)

(e)

(f)

Figure. 3. Microstructures of as-cold rolled Ni3(Si,Ti) (a), as-annealed Ni3(Si,Ti) at 6000C (b), 7000C (c), 8000C (d), 9000C (e) and 10000C (f) after immersion times 24 h in 0.5 kmol/m3 HCl solution at 300C.

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Surface morphology Figure 3 shows the surface morphology of as-cold rolled Ni3(Si,Ti) (a), as-annealed Ni3(Si,Ti) at 6000C (b), 7000C (c), 8000C (d), 9000C (e) and 10000C (f) after immersion times 24 h in 0.5 kmol/m 3 HCl solution at 300C. It was found that the surfaces of as cold rolled Ni 3(Si,Ti), as-annealed Ni 3(Si,Ti) at 6000 C, and 7000 C had showed predominantly uniform corrosion, while intergranular attack was also found on the surfaces of as-annealed Ni3(Si,Ti) at 8000C, 9000C and 1.0000C.

The effect of annealing temperatures to corrosion properties of Ni3(Si,Ti) From the previous sections, with regard to as-cold rolled Ni3(Si,Ti) and as-annealed Ni3(Si,Ti) at 6000C (b), 7000C (c), 8000C (d), 9000C (e) and 10000C, it was found that their magnitude of weight losses and corrosion current densities tended to be slightly different. Furthermore, by determining the standard deviation of data points of weight losses particularly from the following equation 1, the standard deviation value of their weight losses at immersion time of 24 h and 96 h are 1.7.10-7 and 2.84.10-8, respectively. On the other hand, the standard deviation of their corrosion current densities at potential of 0 mV and +100mV are 0.0107 and 0.004633279, respectively. Therefore, their standard deviation values means that their data points of the results of weight losses and corrosion current densities are no different significantly. However, in particularly, as seen in Figure 1, the magnitude of weight loss for as-annealed Ni3(Si,Ti) at 1.0000C are slightly higher than that for other Ni3(Si,Ti) compounds in the same order of 10-9 A/cm2.

to enhance their corrosion resistances effectively. 2) The corrosion resistance of as-annealed Ni3(Si,Ti) at 10000C is little higher than that of other as-annealed Ni3(Si,Ti) intermetallic compounds. 3) The corrosion susceptibility of as-annealed Ni3(Si,Ti) at 8000C tends to rise. From conclusion point 1 aforementioned, we consider that the effect of those compounds is no effective to enhance their corrosion resistances, whereas Priyotomo [10,11] found that the corrosion resistance of austenitic stainless steel increased by a solution annealing process at 10000C as well as that of medium carbon steel [12]. On the other hand, from the conclusion point 2, it can be elucidated two following things: (a) the dense slip bands of Ni3(Si,Ti) after cold rolling was observed in grains and (b) there was the recrystallisation of grains at Ni3(Si,Ti) compounds after annealing process. For point (a), Kaneno and co-worker found that the dense slip bands of Ni3(Si,Ti) after cold rolling was observed in grains with the increase of hardness, indicating a work hardening [2] as well as the present result in Figure 4. Due to work hardening, the weaker passive layer took place on the surface of austenitic stainless steel [13]. On the other hand, from point (b), it is well-known that the recrystallisation of grains at Ni3Al intermetallic compound took place the nucleation and the growth of new crystallites, thus, the recrystallisation of them could reduce

Rolling Direction

(1) Slip bands

S = N= Xi = X=

Standard deviation The number of items in the sample An observed sample The mean

Thus, it is concluded in three following things: 1) The effect of annealing process of Ni3(Si,Ti) intermetallic compounds is no signicance

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Figure 4. A microstructure of as-cold rolled Ni3(Si,Ti) alloy before corrosion test, showing formation of slip bands.


the value of free energy and improve corrosion resistance [14] as well as the present results of Ni3(Si,Ti) intermetallic compounds could. Furthermore, in the previous paragraph, the corrosion susceptibility of as-annealed Ni3(Si,Ti) at 8000C tends to rise. We consider the same phenomena which took place on Ni3Al intermetalllic compound in chloride solution [13]. The corrosion current density of Ni3Al within the temperature ranging from 7000C – 8000C could be related to the incomplete progress of the heatactivated processes beginning in places of highest concentration of stresses and, in consequence, creating the local electrochemical micro-cells [13] . In addition, it may also take place with alloy with heterogeneities such as grains of different sizes [15] as well as those of shown in Figure 3(d). Therefore, this idea can be applied to consider the present result. However, further investigation will be conducted to clarify this idea in detail.

The comparison of the corrosion property of Ni3(Si,Ti) with type 304 & C276 alloy Priyotomo and co-worker already found that the corrosion resistance of the homogenized Ni3(Si,Ti) was much higher than that of type 304 [7,8] . In the HCl solution which is very aggressive, it is known that the austenitic stainless steel such as types 304 has heavily corrosion as shown in Figure 1. However, it was found that Ni3(Si,Ti) intermetallic compounds after thermomecanical process evidently had the higher corrosion resistance than that of type 304. This means that Ni3(Si,Ti) was more resistant to chloride ion attack than type 304. On homogenized Ni3(Si,Ti), although a stable lm such as a passive lm cannot be formed in the HCl solution for the materials, a precipitated lm was observed to be formed for the materials after the immersion test and was easily removed [7,8] as well as the present results. It would be reasonable to consider that the precipitated lm would inhibit a direct dissolution from metal surface and inuence the corrosion resistance of materials. In addition, it was also found that C276 alloy had the highest corrosion resistance than Ni3(Si,Ti) and type 304. As seen in Figure 2, the anodic curve of C276

alloy showed no active region and only the passive region, due to spontaneous passivation with a wide passive range as well as the other result [16,17] . This means that C276 alloy was remarkable for the resistance of chloride ion attack than the other tested materials.

CONCLUSION The effect of annealing process of Ni3(Si,Ti) after TMP could not enhance their corrosion resistances more effectively. The corrosion resistance of Ni3(Si,Ti) after annealing process at 10000C is little higher than that of the other Ni3(Si,Ti) intermetallic compounds. The corrosion resistance of Ni3(Si,Ti) after annealing process at 8000C decreases slightly due to the local electrochemical micro-cells. Furthermore, Ni3(Si,Ti) intermetallic compounds after TMP had moderate corrosion resistance in chloride solution, while C276 alloy and type 304 were the highest and the lowest corrosion resistance. On the other hand, the uniform corrosion was observed on as-cold rolled Ni3(Si,Ti), Ni3(Si,Ti) after annealing process at 6000C, and 7000C, while intergranular attack had been found on Ni3(Si,Ti) after annealing process at 8000C, 9000C and 1.0000C.

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