Effect of Ce and Hf on the segregation of sulfur and the anodic polarization of nickel

Scripta METALLURGICA et M A T E R I A L I A

Vol.

28, pp. I 0 5 9 - I 0 6 3 , 1993 Printed in t h e U . S . A .

Pergamon Press Ltd. All rights reserved

EFFECT OF Ce AND Hf ON THE SEGREGATION OF SULFUR

AND THE ANODIC POLARIZATION OF NICKEL M.H. LEE AND Y.W. PARK

Department of Electronic Materials Science University of Suwon, Suwon P.O.B. 77, Korea and Department of Metallurgical Engineering SurgKyttr~wan University, Suwon, Korea (Received J u l y 20, 1 9 9 2 ) (Revised February 17, 1 9 9 3 ) Introduction It is ~nown that sulfur segregates to grain boundaries even at low bulk concentration and embrittles nickel and nickel-based alloys(1)-(lO) or accelerates intergranuler corrosion of this alloy(12)-(16). One way to reduce the sulfur segregation is to introduce strong sulfide forming elements into nickel such as Y(6), Ce, Cr(4) er~ Hf(4) which can tie up sulfur in the nickel ,mtrix. Another possible method would be to replace sulfur by segregating elements which help the cohesion of grain boundaries or improve grain boundary chemistry such as phosphorus(t0) or boron(20), (21). Mulford(4) reported that sulfur segregation was reduced significantly in Ni+2OCr and Ni+O.5Hf specimens. bbwever, cb_~omium sulfide or hafnium sttlfide precipitate at grain botn~daries ar~ they can deteriorate the corrosion properties of the alloy. Briant and Luthra(6) investigated Y-S alloy and reported that sulfur did not segregate at all in this alloy. Recently, other researchers(7) proposed that yittrium and hafnium have a beneficial effect on retarding sulfur segregation to the nickel surface. However, these same investigators have not examined the electrochemical properties of their alloys, which is very important for practical applications. Recently, Danielson and Baer(19) reported that sulfur, in a surface sulfate state, was found to increase the dissolution rate of nickel at active potential by a factor of 50. Danielson and Jones(14) claimed that nickel containing segregated phosphorus undergoes intergranular stress corrosion cracking whereas nickel containing segregated sulfur does not in I N HzS04 solution. It was also reported(17) that after passivation, sulfur present as an impurity in the 4fetel accumulates at the metal-pessive film interface and causes the breakdown of the passive film of nickel and nickel-based alloys. As mentioned above, sulfur plays various roles in nickel end nickel-based alloys. Therefore, in this article, we investigated the surface segregation of sulfur in nickel doped with Ce, and Mn by Auger electron spectroscopy, and the anodic polarization behavior of these alloys was examined compare to that of pure nickel.

Hf to

_Experimental Ni+5OOppm Ma, N i + 5 0 ~ Ce and N i + 4 7 0 ~ Hf specimens were prepared for Auger electron spectroscopy in the same way as described in my previous work(lO). Sulfur content of all specimens was analyzed to he about 10 pp~. The specimens were heated for 30 minutes at each temperature in the Auger vacuua cheaber by electron bombardment heater from 200~C to 900"C, and the Auger spectra were taken at room temperature. All the measurements were carried out in a UHV system and the surface segregation of sulfur was measured by normalized Auger peak height method. Anodic potentiodynamic polarization measurements were ~ e with scan rate of 100 mV/min, in O. I N H2S04 of PH I. 1 at 18~C. The same specimens were used for the Auger electron s ~ t r o ~ and e l e c t r o ~ c a l tests. The test solution was deaerated with a r ~ n gas. Saturated calomel electrode was used as a reference electrode.

0956-716X/93 Copyright (c) 1 9 9 3

1059 $6.00 + .00 Pergamon Press

Ltd.

1060

SEGREGATION

OF

S

IN

Ni

Vol.

28,

No.

Results and Discussion I. Auger Electron Spectroscopy The surface segregation of sulfur on a pure nickel polycrystal and nickel-based alloys was examined by Auger electron spectroscopy. Auger spectra of Ni+500ppm Mn, Ni+5OOppm Ce and Hi+470ppm Hf were taken from room temperature to 900"C and they were compared to that of pure nickel polycrystal in Fig. I. In pure nickel specimen, sulfur starts to segregate significantly from 550"C and it gives maximum HAPH(Normalized Auger Peak Height) of 0.9. The addition of Mn into nickel did not suppress the sulfur segregation significantly, but only retarded it in some degree. This can be explained by the fact that Mn has a relatively weak sulfide forming capability compared with Ce or Hf. Ce addition was found to be more effective than Mn addition in suppressing the segregation of sulfur due to its stronger sulfide forming ability. Cerium reduced the maximum NAPH of S/NI down to O. 65, which is about 70 percent of that of pure nickel. At higher temperatures above 700"C, the segregated sulfur seems to be stable, which is probably due to the high stability of cerium sulfide. In Ni+470ppm Hf specimen, sulfur did not segregate noticeably up to 700"C, and sulfur segregated with a maximum NAPH of 0.3, which is about 30 percent of that of pure nickel. The segregation of sulfur decreases rapidly with temperature increase from 800"C to 900"C in this Ni+Hf specimen. It was also noticed that the segregation of sulfur was reversible above 800"C. Mulford(4) also reported a very low sulfur segregation in his Hi÷O. 5Hf alloy compared with Ni-W and Ni-Cr alloys. Our results seam to indicate that hafnium is the most powerful alloying element to suppress sulfur segregation in nickel and nickel-based alloys. However, our Ni-Hf alloy showed pitting corrosion, which will be discussed later. From Fig. I, it is also noticed that the temperature at which sulfur starts to segregate significantly (the maximum slope of NAPH/TE~) has increased from 550"C in pure nickel to 600"C in Ni-Mn, 650"C in Ni-Ce and 750"C in Ni-Hf specimen. This implies that hafnium is the most powerful element which can tie up sulfur by forming hafnium sulfide in the nickel and, therefore, it can retard the sulfur segregation the most. Interestingly enough, the sequence of this temperature is in the same order with that of the Gibbs free energy of sulfide formation(AG°sfs>AG°c.s>AGoMns). This implies that the degree of retardation of sulfur segregation on a nickel surface is proportional to the sulfide forming ability of each alloying element. Another noticeable phenomenon occurred in Ni-Hf alloy. That is, surface segregation of sulfur in this specimen diminished above 800"C. However, any other elements including hafnium did not segreagata, to replace sulfur, on the surface of this specimen. 2. Electrochemical Study It is known that there are definite correlations between segregated impurities and intergranular corrosion or I G ~ as reviewed by Was(11) and Lantardsion(12). Jones and Beer(13) reported that sulfur accelerated corrosion rate in the passive region and the additior of phosphorus to nickel resulted in an accelerated corrosion rate in the transpessive region in O. 1 N H2S04 solution. Recently, Danielson and Beer(19) reported that sulfur increases corrosion in the active region greatly, whereas it decreases the passive currant density in Ni-S alloys. More recently, Danimlson and Jones(14) proposed the model for that nickel containing segregated phosphorus undergoes intergranular stress corrosion cracking whereas nickel containing s ~ e g a t e d sulfur does not. In this study, small amounts of Mn, Ce and Hf were added into pure nickel to examine their effects on the anodic polarization behavior and to correlate them to the results obtained from the sulfur segregation study. The anodic polarization curves of Ni+5OOppm Mn, Ni+5OOppe Ce and Ni+470ppm Hf in a O. 1 N H2S04 solution were compared with that of pure nickel at room temperature in Fig. 2. In the test solution, all alloy specimens exhibited the same open circuit potential of +0.05 VH. The critical currant densities of all alloy specimens were by one order of magnitude lower than that of pure nickel. This can be explained by the fact that the presence of small amounts of alloying elements, such as Mn, Ce and Hf, which form sulfides on the surface facilitate the passivation of the nickel. Similar results were reported that )4o decreased the active dissolution of Hi-Mo alloy by reducing the surface concentration of sulfur in this alioy(15). On the contrary the currents in the passive region of potentials were higher for all alloys than p u r e nickel. This implies that t h e alloy specimens form less protective film in the passive range. This experimental result can be explained by the Marcus' suggnstion(17) that impurities are not necessarily dissolved together with metal ions, and it can be accumulated at the metal-passive film. This intepretation by Marcus can be strongly supported by o u r experimental results, and furthermore, it can be speculated that the passive film of Hi-Ce specimen might be more protective than that of Ni-Mn specimen. This speculation is based on the fact that the Gibbs free energy of CeS formation is much larger than the Gibbs free energy of M ~ formation.

9

Vol.

28,

No.

9

SEGREGATION

OF

S

IN

Ni

1061

In the Ni+470ppm Hf specimen, pitting corrosion occurred. In this specimen, sulfur may be remained at the metal-passive film interfaces due to its very large Sibbs free energy of hafnium sulfide formation and the concentration of sulfur increases with increasir~ dissolution and, therefore, the passive film loses its adherence. Marcus(17) also reported that sulfur is resposible for the breakdown of the passive film and subsequent pitting in Ni-Fe-S alloys. Judging from these experimental results, Ce might be the most eligible alloying element to suppress the sulfur segregation in nickel and nickel-based alloys without having pitting corrosion.

Conclusions i. Among the tested elements, Hf was found to be the most effective element in suppressing the sulfur surface segregation in nickel. However, Ni+470ppm Hf specimen showed pitting corrosion in 0. i N H2S04 solution. 2. All the elements(Mn,Ce and Hf) added to nickel at the concentration of about 500ppm improved the ability to passivate the nickel but deteriorated the properties of the passive film. 3. Ce addition into nickel might be the best choice among the examined elements to reduce the sulfur segregation in nickel-based alloys without deteriorating the corrosion characteristics. References I) 2) 3) 4) 5) 6) 7) 8) 9) (i0) (11) (12) (13) (14) (15) (16) (17) (i8) (19) (20) (21)

B.Ladna and H.K. Birnbaum, Acta Metall., 36, 745(1988) C.L.Briant and A.I.Taub, Acta Metall., 36, 2761(1988) C.L.White and W.Losch, Scripta Met., 19, 665(1985) R.A. Mulford, Met.Trans.A, 14A, 865(1983) T.Miyahara,K.Stolt,D.A.Reed and H.K.Birnbaum, Scripta Met., 19, 117(1985) C.L.Briant and K.L.Luthra, Met. Trans. A, 19A, 2099(1988) C.L.White,R.A.Padgett, C.T.Liu, Scripts Met., 18, 1417(1984) T.C. Lee,l.M. Robertson and H.K.Birnbaum, Acta Metall., 37, 407(1989) F.Ferhat,D.Roptin and S.Saindrenan, Scripta Met., 22, 223(1988) M.H.Lee and C.O.Park, Scripta Met., 25, 591(1991) S.S.Was, Corrosion, 46 319(1990) R.M. Lantanision, J. Mater. Eng., I0, 143(1988) R.H. Jones,M.J. Danielson and P.R.Baer, J.Materials for Energy System, 8, 185(1986) M.J.Danielson,C.A.Oster and R.H. Jones, Corrosion Science, 32, i(1991) P. Marcus and M. Moscatelli, J. Electrochem.Soc., 136, 1634(1989) Y.J. Kim and R.A.Oriani, Corrosion Science, 44, 360(1988) P.Marcus and H. Talah, Corrosion Science, 29, 455(1989) D. Macdonald, M. Ben-Haim and J. Pallix, J. Electrochem. Soc., 136, 3269 (1989 ) M.J.Danielson and D.R.Baer, Corrosion Science, 29, 1265(1989) R.M.Kruger,S.F.Claeys and G.S.Was, Corrosion, 41, No.9, 504(1985) J.Flis and D.J.Duguette, Corrosion, 41, No.12, 700(1985)

1062

SEGREGATION OF S IN Ni

I

I

I

I

I

Vol.

I

I

I

28, No.

,

1.0

Heating

ffl

PureNickel Nickel 4. 500ppm Mn Nickel +500ppmCe Nickel Hf

Q n

Time : 30 Min

0.6

sh ~" S/Hi/' / ;

m w ~ Q ~

S ~

"

/ -

,,

~ [] /

N

-.

o

//

@

2% m

-

m

E

~ o.2

z

0

|

0

m

loo-

I

200

300

400 500 Tempe ra tu re, °C

600

700

800

Fig. i Normalized Auger peak height of sulfur vs. temperature showing effects of Mn, Ce and Hf additions on sulfur segregation.

900

9

Vol. 28, No. 9

SEGREGATION OF S IN Ni

I

I

W 'I'II

I

I

!

2100

0.1 N H2SO4

1700

100 m V / m i n 25oC

ilz

IIIII

I

I

•

I

IIIII

I

I

'I

I l

IIII

i

' ''''w1

• Pure Nickel z~ Ni-5OO ppmCe • Ni-500 ppmMn Ni-470ppm Hf

,t s ...o.o.~o°.

i er ~

o°~

oq~o''°

500

J

I

1063

---

L

I

I I ilill

..-'"

i

I I illill

I

I

I I illll

I

I

I I liill

w Current Density, A/cm 2

Fig. ~°Anodic polarization curve of nickel-based alloys showing effects of Mn, Ce and Hf additions.

I

|

I

I III

10