The effect of phosphorus addition on the corrosion behavior of ARC-MELTED Ni10TaP alloys in 12 M HCl

Corrosion Science, Vol. 38, No. 3, pp. 469485, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0010-938X/96 $lS.OO+O.OO

Pergamon

0010-938X(96)00144-1

THE EFFECT OF PHOSPHORUS ADDITION ON THE CORROSION BEHAVIOR OF ARC-MELTED Ni-lOTa-P ALLOYS IN 12 M HCI H.-J. LEE, E. AKIYAMA,

H. HABAZAKI, A. KAWASHIMA, K. HASHIMOTO

K. ASAMI and

Institute for Materials Research, Tohoku University, Sendai 980-77, Japan Abstract-The corrosion resistance of arc-melted Ni-lOTa-P alloys containing 0, 10 and 20at% phosphorus in 12 M HCl solution at 30°C was investigated. The alloys containing 0 and IOat% phosphorus suffer severe corrosion. The addition of 20 at% phosphorus to crystalline Ni-1OTa alloy results in a three-orders-of-magnitude decrease in the corrosion rate. The open circuit potentials of the Ni-1OTa alloys containing 0 and 10 at% phosphorus stay almost constant in the active region of nickel, while the open circuit potential of the Ni-lOTa-20P alloy increases almost linearly in the initial 2 h. The Ni-1OTa alloy consists of intermetallic N&Ta and immersion in 12 M HCl results in faceting dissolution. Ni-lOTa-IOP alloy is composed of major NisTa and N&P phases and minor NizTa and N&P phases. Immersion of Ni-lOTa-1OP alloy leads to preferential dissolution of the NisTa phase and to continuous thickening of the corrosion product film consisting mostly of tantalum as cations. NiIOTa-20P alloy consists of N&Ta, N&P, NizP and NiP phases. Immersion of Ni-lOTa-20P alloy gives rise to initial increase in elemental phosphorus on the surface as a result of selective dissolution of nickel and selective oxidation of tantalum. The formation of elemental phosphorus with a high cathodic activity is responsible for the initial ennoblement of the open circuit potential and for the formation of the passive film in which tantalum is highly concentrated. The higher corrosion resistance of Ni-lOTa-2OP alloy than Ni-lOTa-IOP alloy is attributable to the formation of the NisTa phase with a higher tantalum content than the N&Ta phase which is the readily corroded major intermetallic phase in the Ni-lOTa-1OP alloy. Keywords: A. alloy, B. SEM, B. XPS, B. X-ray diffraction, C. passive films.

INTRODUCTION The amorphous iron-phosphorus”2 and nickel-phosphorus alloys3-8 containing passivating elements such as chromium and tantalum possess an extremely high corrosion resistance in various environments. In particular, they are practically immune to pitting corrosion, even after anodic polarization in hydrochloric acids. The amorphous alloys consist of chemically homogeneous single phase solid solutions which do not contain crystalline defects acting as nucleation sites for corrosion. The chemically homogeneous single phase nature of the amorphous alloys provides the formation of a uniform passive film without weak points with respect to corrosion.’ On the other hand, amorphous alloys are in a thermodynamically metastable state and, hence, they are chemically more reactive than corresponding thermodynamically stable crystalline alloys.9”0 The high chemical reactivity provides rapid passivation of alloys containing passivating elements since the high reactivity leads to rapid selective dissolution of alloy constituents unnecessary for passive film formation and to rapid accumulation of passivating elements on the alloy surface. Manuscript received 26 July 1995. 469

470

H.-J. Lee et al

Accordingly, the high corrosion resistance of amorphous alloys is attributed to chemical homogeneity as well as high passivating ability, which result in the rapid formation of the passive film with a high protective quality and uniformity.” The effect of phosphorus addition on the corrosion behavior of rapidly quenched amorphous and crystalline Ni-lOTa-P alloys in 12 M HCl at 30°C has been reported.12 When the phosphorus addition to Ni-lOTa-P alloys is insufficient, the corrosion-resistant amorphous alloys cannot be formed and the concentration of tantalum ions in the corrosion product film increases gradually to almost 100% of the cations. Similarly when the passive film is unstable because of low phosphorus contents or because of anodic polarization the passive film composed exclusively of tantalum as cations is formed but does not become stable and a relatively high current density is observed. A stable passive film consisting exclusively of tantalum cations can be formed on amorphous high tantalum alloys, such as Ni-30Ta-P alloys.13 However, even if the tantalum content is low, e.g. 10 at%, a sufficient addition of phosphorus gives rise to an amorphous single phase and to the formation of a stable and protective passive film, in which less than 80% of cations are tantalum, in aggressive 12 M HCl. In general, the addition of phosphorus to crystalline alloys exerts detrimental effects such as intergranular corrosion (IGC) and stress corrosion cracking (SCC), mostly due to segregation of phosphorus to grain boundaries. Zhang et a1.14 have previously reported the corrosion behavior of crystalline Cr-P alloys in hydrofluoric acid. Since phosphoruscontaining phases have high activities for both hydrogen evolution and oxygen reduction a small amount of phosphorus addition ennobles the open circuit potential slightly and enhances open circuit active dissolution of the phosphorus-free phase, while a sufficiently high addition of phosphorus, such as 0.8 at% or more, leads to spontaneous passivation of the alloy due to enhancement of passivation of bee Cr phase as a result of a significant ennoblement of the open circuit potential. Though the addition of phosphorus to amorphous Ni-Ta alloys is particularly effective the effect of the phosphorus addition to heterogeneous crystalline Ni-Ta-P alloys is not well known. In the present work the corrosion behavior of bulk crystalline Ni-lOTa-P alloys prepared by arc-melting was examined in 12 M HCl solution to clarify the dependence of the corrosion behavior on the amount of alloying phosphorus and to elucidate the mechanism of the corrosion resistance of the crystalline Ni-lOTa-P alloys in the concentrated HCl solution compared to that of amorphous Ni-lOTa-P alloys. EXPERIMENTAL

METHOD

Ni-lOTa-P alloys containing O-20 at% phosphorus were prepared by arc melting of laboratory-made nickel phosphide, nickel and tantalum under an argon atmosphere. Specimens with dimension of about 5 x 5 x 0.65 mm were cut from the arc-melted ingots for corrosion tests and polarization measurements. The structure and composition of the alloys were confirmed by X-ray diffraction (XRD) using Cu K, radiation and electron probe micro-analysis (EPMA). Prior to immersion and electrochemical measurements alloy specimens were polished mechanically with silicon carbide paper up to #lOOO in cyclohexane, degreased in acetone and dried in air. The electrolyte used was a reagent grade 12 M HCl solution open to air at 30°C. Corrosion rates were estimated from the weight loss after immersion for one week in 12 M HCl solution at 30°C. Polarization curves were measured potentiodynamically with a

Arc-melted Ni- 1OTa-P alloys

471

potential sweep rate of 50 mV/min. Potentiodynamic polarization for all arc-melted alloys used in this study was started separately toward anodic and cathodic directions after opencircuit immersion for 10min. The reference electrode used in this study was a saturated calomel electrode (SCE). The open circuit potential was also measured as a function of immersion time. X-ray photo-electron spectra were measured by means of a Shimadzu ESCA-850 electron spectrometer with Mg K, exitation (hv = 1253.6 eV) for the characterization of the surface film formed by immersion. Binding energies of photo-electrons were calibrated by the same method described elesewhere;‘5,‘6 the binding energies of the Au 4f7,2 and 4f512 electrons of gold metal and the Cu 2ps,z and 2pt,x electrons of copper metal were taken as 84.07, 87.74, 932.53 and 952.35eV, respectively, and the kinetic energy of the Cu L3M4,5M4,5 Auger electrons of copper metal was taken as 918.65 eV.” The composition and thickness of the surface film and the composition of the substrate alloy immediately under the surface film were quantitatively determined by the previously proposed method using integrated intensities of XPS spectra.“,i8 The quantitative determination was performed under the assumption of a three-layer model of the outermost contaminant hydrocarbon layer of uniform thickness, the surface film of uniform thickness and the underlying alloy surface of X-ray photo-electron spectroscopically infinite thickness, along with the assumption of a homogeneous distribution of constituents in each layer. The photo-ionization cross-sections for the Ta 4f, P 2p and C 1s electrons relative to the 0 1s electrons used were 2.617,19 0.78620 and 0.3509,21 respectively. Those for the Ni 2psj2 electrons in oxidized and metallic states were 2.315** and 7.468,” respectively. EXPERIMENTAL

RESULTS

Figure 1 shows scanning electron micrographs of as-polished Ni-lOTa-P alloys containing 10 and 20 at% phosphorus. Each alloy shows two different phases indicated by A and B. The quantitative analysis of the phases was carried out by electron probe microanalysis. The tantalum concentration of the phase indicated by A in the Ni-lOTa-1OP alloy is around 15 at% and that in the Ni-lOTa-20P alloy is around 36 at%. The phases indicated by A do not contain a detectable amount of phosphorus and, hence, are regarded as NisTa and Ni2Ta phases in Ni-lOTa-1OP and Ni-lOTa-20P, respectively. The change in the major intermetallic compound with increasing phosphorus content of the alloy is reasonable since the increase in the phosphorus content increases the amount of nickel phosphide. The phases indicated by B in both Ni-lOTa-1OP and Ni-lOTa-20P alloys are composed of nickel phosphides with a small amount of tantalum: about 2 at%. This fact suggests that the corrosion resistance of the nickel phosphide phase with tantalum is higher than that of the tantalum-free nickel phosphide formed in binary Ni-P alloys. Furthermore, the corrosion resistance of the Ni2Ta phase with a higher concentration of tantalum than the NisTa phase is higher than that of the NisTa phase. Consequently, the corrosion resistance of Ni-lOTa20P alloy should be higher than that of Ni-lOTa-1OP alloy. Figure 2 shows X-ray diffraction patterns of arc-melted Ni-lOTa-P alloys. For the phosphorus-free Ni-1OTa alloy a NisTa phase is observed. The alloy containing lOat% phosphorus is composed of the NisTa phase and a NisP phase with weak diffraction peaks. The arc-melted Ni-lOTa-20P alloy consists of nickel phosphides and NizTa phase. This is in agreement with the result of electron probe micro-analysis. Accordingly, it is evident that the nickel phosphides are easily formed by the addition of a large amount of phosphorus.

Scanning electron micrographs of as-polished Ni-lOTa-P alloys containing IO and 20 at% phosphorus

As-Polished

As-Polished

Fig. I,

Ni-lOTa-20P Alloy

Ni-lOTa-1OP Alloy

Arc-melted

Ni-lOTa-P

Arc-melted

20

30

40

50

2 B /Deg. Fig. 2.

X-ray diffraction

patterns

60

413

alloys

lngo

70

00

(Cu

W

of arc-melted

90

Ni-lOTa-P

alloys.

Figure 3 shows corrosion rates of arc-melted alloys in 12 M HCl solution at 30°C as a function of phosphorus content of alloys. The corrosion rate of nickel was estimated from the weight loss after immersion for 75 h while those for the Ni-1OTa alloys containing 0, 10 and 20 at% phosphorus and tantalum were obtained after immersion for 168 h. The corrosion rates of nickel and Ni-1OTa alloy after immersion in 12 M HCl were about 26 and 12 mm/year, respectively. In other words, the transformation from fee Ni to NisTa results in

20

Fig. 3.

Corrosion

rate of arc-melted

Ni-lOTa-P alloys measured at 30°C.

Ta

in 12 M HCI solution

open to air

Fig. 4.

The surface morphology

2OP

1OP

OP

pt.‘Pollshed

of the arc-melted

30 set

Ni-lOTa-P

1

min

alloys observed

5

by SEM after immersion

1 30min 1

in I2 M HCI solution

2h 4h

open to air at 30°C

Ni-lOTa-P Arc-melted Ingot (immersed at Ecorr)

Arc-melted

Ni-lOTa-P

alloys

475

the decrease in the corrosion rate to less than one half. The corrosion rate decreases about one order of magnitude by the addition of lOat% phosphorus. The arc-melted Ni-lOTa-20P alloy shows the excellent corrosion resistance. The corrosion rate is 1.8 x 10e2 mm/year, which is about three orders of magnitude lower than that of the Ni-1OTa alloy. Accordingly, phosphorus is a quite effective metalloid element in improving the corrosion resistance of not only amorphous Ni-Ta alloys’29’3 but also crystalline Ni-Ta alloys in concentrated hydrochloric acid. The scanning electron micrographs of the alloys immersed in 12 M HCl solution are shown in Fig. 4. Faceting dissolution can clearly be seen for the phosphorus-free alloy after immersion for 4 h. Selective corrosion of the NisTa phase occurs on the Ni-lOTa-IOP alloy. By contrast, the Ni-lOTa-20P alloy maintained mechanically polished traces after immersion in 12 M HCl solution for 4 h. This indicates that corrosion hardly occurs on the arc-melted Ni-lOTa-20P alloy. To clarify the mechanism of corrosion resistance of arc-melted Ni-lOTa-P alloys the structure of the arc-melted alloys before and after immersion for a prolonged time was investigated by X-ray diffraction. Figure 5 shows XRD patterns of arc-melted Ni-1OTa alloy before and after immersion in 12 M HCl solution for 24 h. A sharp decrease in 031 reflection can be seen after immersion for 24 h. This is in agreement with faceting dissolution shown in Fig. 4. The X-ray diffraction patterns of the Ni-lOTa-1OP alloy before and after immersion for 168 h are shown in Fig. 6. For the as-polished specimen intense diffraction peaks of the NisTa phase and weak diffraction peaks of the Ni3P phase are present. After immersion for 168 h reflections from the NiaTa phase almost disappear as a result of selective dissolution.

l”“V”“” l

Ni,Ta

Ni-IOTa Arc-melted lngo

30

40

50

2 e /Deg. Fig. 5.

X-ray diffraction

60

70

60

(Cu ka)

patterns of arc-melted Ni-1OTa alloy before and after immersion in 12 M HCl solution open to air at 30°C.

for 24 h

476

H.-J. Lee et al. 1’1’ l n

.

Ni,Ta Ni,Ta

I

’

I

’

1 ’

Ni-lOTa-1OP Arc-melted Ingo]

4 Ni,P v Ni,P

as-polished

. .

. A

.

20

30

40

50

2 o /Deg. Fig. 6.

X-ray diffraction

1 ’

60

70

60

90

(CU ka)

patterns of arc-melted Ni-lOTa-1OP alloy before and after immersion 168 h in 12 M HCI solution open to air at 30°C.

for

This is in agreement with the scanning electron micrographic observation shown in Fig. 4. Diffraction lines of N&Ta and Ni2P become detectable in addition to a significant increase in diffraction intensities for the NisP phase. The change in the diffraction pattern with immersion in 12 M HCl suggests a complicated solidification process. As can be clearly seen in Fig. 1 the NisTa phase is the primary crystal. After precipitation of the NisTa crystals the surrounding liquid becomes deficient in nickel and the NizTa phase begins to precipitate. When the liquid is cooled Ni2P crystals solidify in the nickel deficient liquid surrounding the intermetallic NisTa and the NisP phase solidifies in the bulk of the liquid. Accordingly, the selective dissolution of the NisTa phase results in the intensification of the diffraction patterns of the NizTa and Ni2P phases in addition to the NisP phase. These nickel phosphides containing a small amount of tantalum have better corrosion resistance in strong acids. The Ni2Ta phase with a higher concentration of tantalum is more corrosionresistant than the NisTa phase. Figure 7 shows X-ray diffraction patterns of the Ni-lOTa-20P alloy before and after immersion for 168 h. The X-ray diffraction patterns of this alloy are almost unchanged during prolonged immersion, indicating high corrosion resistance. The higher corrosion resistance of Ni-lOTa-20P alloy is mostly attributable to the change of intermetallic from NisTa to NilTa with a higher tantalum content, since nickel phosphides in both Ni-lOTa1OP and Ni-lOTa-20P alloys are corrosion-resistant, as can be seen from the fact that nickel phosphides remain after prolonged corrosion of Ni-lOTa-1OP alloy. For a better understanding of the corrosion behavior of these alloys electrochemical measurements and XPS analysis were performed. Figure 8 shows the change in the open circuit potential with time for arc-melted alloys in 12 M HCl solution at 30°C. The open

Arc-melted

n

Ni,Ta

l

Ni,P

Ni-lOTa-P

477

alloys

Ni- 1OTa-20P Arc-melted lngo

as-polished

immersed for 168h

d

I”“““““’ 20

30

40

50

2e/Deg. Fig. 7.

X-ray diffraction

70

60

00

90

(Cuk~)

patterns of arc-melted Ni-lOTa-20P alloy before and after immersion 168 h in 12 M HCI solution open to air at 30°C.

for

circuit potentials of Ni-1OTa alloys containing 0 and 10 at% phosphorus stay almost constant in the active region of nickel. In all immersion times the open circuit potential of Ni-lOTa-1OP alloy is slightly higher than that of the phosphorus-free Ni-1OTa alloy. By contrast, the open circuit potential of Ni-lOTa-20P alloy increases almost linearly with immersion time and then becomes almost constant. Accordingly, the addition of a large 0.1

, ., ., ., ., ., , , ., ,

.

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z_

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'. 2

'a 4

"1.

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a

Time / h Fig. 8.

Change

in the open circuit potential with immersion time for arc-melted immersed in 12 M HCl solution open to air at 30°C.

Ni-lOTa-P

alloys

478

H.-J. Lee et al.

Potential Fig. 9.

Cathodic

/

V vs SE

and anodic polarization curves of arc-melted Ni-IOTa-P HCl solution open to air at 30°C.

alloys measured

in 12 M

amount of phosphorus in arc-melted Ni-Ta alloy leads to an ennoblement of the open circuit potential similarly to the amorphous Ni-lOTa-P alloys.12 Figure 9 shows the cathodic and anodic polarization curves of arc-melted Ni-lOTa-P alloys measured in 12 M HCl solution at 30°C. The polarization curves of nickel and tantalum are also shown for comparison. The potentiodynamic polarization curves of the phosphorus-containing alloys are changed with alloy phosphorus contents similarly to phosphorus-containing amorphous Ni-Ta alloys. The addition of phosphorus enhances the cathodic reaction. The active region of the alloys becomes narrower and the current density decreases with the increase in the phosphorus content. Accordingly, the phosphorus addition ennobles the open circuit potential. The active peak current density of the phosphorus-free Ni-1OTa alloy is as high as 1.8 x lo3 A/m2, while that of the Ni-lOTa-20P alloy is three orders of magnitude lower than that of phosphorus-free Ni-1OTa alloy, coinciding with the corrosion rate in Fig. 3. It is, therefore, evident that the anodic dissolution is suppressed by a large amount of phosphorus addition, even in crystalline alloys. Figure 10 shows the cathodic and anodic polarization curves of the arc-melted NilOTa-20P alloy after immersion for different times in 12 M HCl solution. The open circuit potential of the Ni-lOTa-20P alloy is ennobled by immersion for 8 h and the anodic current is depressed by the spontaneous formation of a passive film on which the cathodic reaction for oxygen reduction is considerably active. Accordingly, because the phosphide phases are quite active for both the oxygen reduction and the hydrogen evolution and because of the suppression of anodic dissolution prolonged immersion leads to spontaneous passivation. Figure 11 shows the change in the thickness of the surface film formed on the arc-melted Ni-lOTa-P alloys in 12 M HCl solution at 30°C as a function of immersion time. The thickness of the air-formed film on these alloys is about 2 nm regardless of the phosphorus content. The thickness of the film formed on the arc-melted Ni-1OTa alloy increases initially but then decreases to an almost constant thickness of 3 nm. The thickness of the film on the Ni-lOTa-1OP alloy increases continuously with immersion time. The film formed on the Ni-lOTa-20P alloy is thin and is not thickened with immersion time. The alloy maintained

Arc-melted

Ni-lOTa-P

I

alloys

I

479

I

I

Ni- 1OTa-20P Arc-melted

Ingot

12 M HCI, 30°C

53

k\ I:%after

_

10"

:;

I

1 oq,

\I

-0.5

0

Potential Fig. 10.

immersion for8h

::

/

I

I

0.5

1

1.5

V vs SCE

Cathodic and anodic polarization curves of arc-melted Ni-lOTa-2OP alloy measured immersion for different times in 12 M HCI solution open to air at 30°C.

after

the metallic luster after immersion for 20 h. Consequently, the film formed on the arc-melted Ni-lOTa-20P alloy is highly protective even in this aggressive acid, although the corrosion resistance of the alloy is lower than that of the amorphous Ni-lOTa-20P alloy.‘* Figure 12 shows the cationic fractions in the surface films formed on arc-melted alloys immersed in 12 M HCI solution as a function of immersion time. The concentration of TaS+ ion is remarkably high in comparison with the alloy composition. The tantalum content in the films formed on these alloys after immersion for 30 s is high when the phosphorus content of the alloy is high. The tantalum content in the film on the phosphorus-free NiIOTa alloy increases up to 30 min and then decreases by prolonged immersion. This change is similar to the change in the film thickness as shown in Fig. 11. Although oxidized tantalum is more stable in the form of solid than oxidized nickel in the strong acid, tantalum is not

A

&A

4 2OP

1 I

Pollshed

.I

.I

I

0

.I

2

.I

3

.,

4

1 5

Time / h Fig.

11. The thickness of the surface film on arc-melted solution

Ni-lOTa-P open to air at 30°C.

alloys immersed

in 12 M HCI

H.-J. Lee et al.

480

0.6 ‘;= cl ? LL

0.4

AsPOliShed

0

1

2

3

4

5

Time / h Fig. 12.

Change

in cationic fraction in the surface films formed on the arc-melted Ni-lOTa-P with immersion time in 12 M HCI solution open to air at 30°C.

alloys

highly concentrated in the corrosion product film on the Ni-1OTa alloy. The cationic fraction of tantalum in the film on the Ni-lOTa-20P alloy, whose film thickness is not largely changed with time of immersion, becomes more than 0.7 within 30 s and a steady value of less than 0.9 after about 30 min. The cationic fraction of tantalum in the film on the Ni-lOTa-1OP alloy, from which selective dissolution of the NisTa phase occurs, is of the order of 0.6 within 30 min. However, the cationic fraction of tantalum in the film analysed after immersion for 2 h exceeds 0.8 and the fraction after 4 h is about 0.95. It is reasonable that both film thickness and cationic fraction for the corrosion-resistant alloy are not largely changed by immersion while they are changed significantly for the less corrosion-resistant alloy. Accordingly, the large amount of the phosphorus addition is quite effective for the formation of a highly protective passive film not only on amorphous alloys but also on arcmelted crystalline alloys. Figure 13 shows the atomic fraction in the alloy surface immediately under the surface film after immersion in 12 M HCl at 30°C. The compositions of the alloy surface just below the surface film for the Ni-1OTa and Ni-lOTa-2OP alloys are almost the same as those of bulk alloy. By contrast, phosphorus is concentrated in the surface of the Ni-lOTa-1OP alloy by immersion. Figure 14 shows the concentration of phosphorus in the surface film. It has been known that phosphorus present in the surface is readily oxidized during transfer of the specimen through air from the electrolytic cell to the X-ray photo-electron spectrometer.23 Accordingly, oxidized phosphorus detected by XPS always includes that formed by oxidation of elemental phosphorus after the specimen is taken out from the electrolyte. It has also been known that phosphorus forms an elemental phosphorus layer on the alloy surface as a result of selective dissolution of nickel when nickel-phosphorus alloys are immersed in acids.24 Accordingly, as can be seen in Fig. I3 for the Ni-lOTa-IOP alloy, when the alloy surface is covered with the thick corrosion product film, elemental phosphorus accumulated before thickening of the corrosion products is covered with the corrosion products and can be detected as elemental phosphorus by XPS analysis. From the above

Arc-melted

Ni-1 OTa-P alloys

481

Ni-1OTa

Arc-melted ingot 12M HCI, 30 C E = Ecorr

-

LJ

%y-----p------a

&iLd

Ta

0

c

lmhersion*

Time ‘/

4

lmher.sion*

Time ‘/

&fL---ff~

A.P0liSh~d0

P Ta 4

ImAersion* of nickel, Ni-lOTa-P

Time “/

6

h

____O

Fig. 13. Atomic fractions surface film for the arc-melt

6

h

Fw%k0

:

1

4

6

h

tantalum and phosphorus in the substrate alloy under the alloys after immersion in 12 M HCI solution open to air at 30°C.

consideration it can phosphorus shown accumulated on the preferential oxidation

be estimated that the initial increase in the amount of oxidized in Fig. 14 results from air oxidation of elemental phosphorus alloy surface due to preferential dissolution of nickel and due to of tantalum. For Ni-lOTa--IOP alloy the elemental phosphorus layer

482

H.-J. Lee

.

”

I

Immersion Fig. 14. crystalline

et al

Ni- 1OTa-xP

-

Arc-me/ted ingot 72 M I%/, 30 c E = Ecorr

-

Tinie /

h

The concentration of phosphorus expressed as P/(Ni +Ta) in the surface films formed on Ni-I OTa-P alloys immersed for various periods of time in 12 M HCl solution open to air at 30°C.

is initially formed, but because the corrosion product film grows the content of oxidized phosphorus in the film is decreased by thickening of the corrosion products. On the other hand, a large amount of accumulation of elemental phosphorus on the surface of the NilOTa-20P alloy ennobles the open circuit potential because of the high activity of elemental phosphorus for the cathodic oxygen reduction.25 After ennoblement of the open circuit potential phosphorus is oxidized at high potentials and the phosphorus content of the stable, thin passive film becomes almost constant since the oxidized phosphorus is dissolved.

DISCUSSION The phosphorus-free Ni-1OTa alloy consists of NisTa, while Ni-1OTa alloys containing phosphorus are composed of intermetallic NisTa or Ni?Ta and nickel phosphides as shown in Figs 1,2,6 and 7. The arc-melted Ni-lOTa-20P alloy shows excellent corrosion resistance in comparison with the Ni-1OTa and Ni-lOTa-1OP alloys though its corrosion resistance is lower than that of amorphous Ni-lOTa-20P alloy.‘* The addition of a large amount of phosphorus results in the formation of the NilTa phase with a higher concentration of tantalum than the NisTa phase in addition to the formation of the corrosion-resistant nickel phosphide phases containing a small amount of tantalum. The Ni-lOTa-1OP alloy after immersion for 168 h is composed of NizTa and Ni2P in addition to the NisP as a result of selective dissolution of NisTa. Accordingly, the higher corrosion resistance of Ni-lOTa-20P alloy than Ni-lOTa-1OP alloy is mostly attributable to the change of the intermetallic from NisTa to Ni,Ta with a higher tantalum content, since nickel phosphides are corrosionresistant as can be seen from the fact that nickel phosphides remain after prolonged corrosion of Ni-lOTa-1OP alloy. In summarizing these considerations we can generally say the following: Immersion of the Ni-lOTa-1OP alloy in 12M HCl results in selective dissolution of the N&Ta phase leaving nickel phosphide phases on the surface. After prolonged immersion the open circuit potential of the Ni-lOTa-1OP alloy is only slightly higher than that of the phosphorus-free Ni-1OTa alloy. This fact indicates that, although the nickel phosphide phases with a small amount of tantalum are more corrosion-resistant than the NisTa phase, spontaneous

Arc-melted Ni-IOTa-P alloys

483

passivation requires the presence of the major passivating phase with a higher tantalum content such as the NizTa phase. The Ni-lOTa-20P alloy consists of the NizTa phase as the primary crystal and nickel phosphides. Because the NizTa phase is more corrosion-resistant the initial formation of the elemental phosphorus layer (Fig. 14) as a result of selective dissolution of nickel from nickel phosphides leads to the initial ennoblement of the open circuit potential (Fig. 8) and to the passivation of the NizTa phase. In conclusion, the addition of a large amount of phosphorus to the Ni-1OTa alloy results in the formation of the NizTa phase with a high tantalum content and the beneficial effect of a high concentration of phosphorus leads to an ennoblement of the open circuit potential and to spontaneous passivation due to passivation of the major N&Ta phase. The corrosion product film on the Ni-lOTa-1OP alloy is thickened with time of immersion and the cationic fraction of tantalum in the surface film increases. The formation of the thick corrosion product film with a high tantalum content is reasonable because the solubility of tantalum ions is lower than that of nickel ions, but this fact does not correspond to an increase in the corrosion resistance, because the corrosion product film is not protective. The surface film formed on the Ni-lOTa-20P alloy is not thickened with time of immersion, and the alloy maintains the metallic luster after prolonged immersion. The cationic fraction of tantalum in the surface film formed on the alloy becomes more than 0.7 within 30 s and a steady value of less than 0.9 after about 30 min as a result of the formation of a highly protective passive film. Consequently, the sufficiently high addition of phosphorus is responsible for the formation of the highly protective passive film. Zhang et al. l4 studied the effect of phosphorus on the corrosion resistance of Cr-P alloys containing O-5 at% phosphorus in 47% HF solution. They rtported that the Cr-P alloys containing less than 0.8 at% phosphorus have a low corrosion potential and dissolve actively in HF solution. However, when the phosphorus content increases to 0.8 at% or more, all the Cr-P alloys are spontaneously passive. The phosphides, Cr3P, precipitated continuously at the grain boundaries by the addition of a large amount of phosphorus in the arc-melted Cr-P alloy, can effectively prevent intergranular corrosion and enhance passivation of the bee Cr phase consisting of crystalline grains because Cr3P has high activities for both hydrogen evolution and oxygen reduction. Habazaki et a1.24found that phosphorus forms an elemental phosphorus layer on the alloy surface as a result of selective dissolution of nickel when nickel-phosphorus alloys are immersed in acids. In the present study the initial increase in the amount of the oxidized phosphorus in the surface film formed on the Ni-lOTa-P alloys is attributable to the results from air oxidation of elemental phosphorus accumulated on the alloy surface due to preferential dissolution of nickel and due to preferential oxidation of tantalum as shown in Fig. 14. The content of oxidized phosphorus in the film on the alloy containing 10 at% phosphorus is decreased by thickening the corrosion products with immersion time. By contrast, the open circuit potential of the Ni-lOTa-20P alloy is ennobled by the high activity of elemental phosphorus for the cathodic oxygen reduction. The ennoblement of the open circuit potential results in passivation of the primary NizTa phase by the formation of the tantalum-enriched passive film. Once passivation occurs, because of decrease in the anodic dissolution current, the open circuit potential is further increased without further acceleration of cathodic reaction, that is, further accumulation of phosphorus. On the contrary, phosphorus is dissolved at high potentials as phosphate. Accordingly, the phosphorus content becomes almost constant due to the formation of the stable and thin passive film when the immersion time is increased.

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CONCLUSIONS The effect of phosphorus addition on the corrosion behavior of arc-melted crystalline Ni-lOTa-P alloys containing 0, 10 and 20 at% phosphorus was investigated in 12 M HCl solution open to air at 30°C. The following conclusions were drawn. The phosphorus-free Ni-1OTa alloy consists of NisTa, while Ni-1OTa alloys containing 10 and 20 at% phosphorus are composed of intermetallic NisTa and Ni*Ta, respectively, in addition to nickel phosphides. The corrosion rate of the arc-melted Ni-lOTa-20P alloy is 1.8 x 10e2 mm/year, which is about three orders of magnitude lower than that of the Ni1OTa alloy. The Ni-lOTa-20P alloy is spontaneously passive due to the formation of the highly protective passive film, which leads to an ennoblement of the open circuit potential. XPS analysis reveals that the fraction of tantalum ions in the film formed on the NilOTa-20P alloy, whose film thickness is not largely changed with time of immersion, becomes more than 0.7 within 30 s and a steady value of less than 0.9 after about 30 min. The concentration of tantalum ions in the film formed on the phosphorus-free Ni-1OTa alloy increases up to 30min and decreases by prolonged immersion and that on the NilOTa-1OP alloy increases continuously with immersion time. A large amount of the initial accumulation of elemental phosphorus on the Ni-lOTa20P alloy leads to an ennoblement ofthe open circuit potential, because of the high activity of elemental phosphorus for the cathodic oxygen reduction and the spontaneous passivation due to passivation of the major Ni2Ta phase.

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