The role of alloying elements in improving the corrosion resistance of amorphous iron base alloys

The role of alloying elements in improving the corrosion resistance of amorphous iron base alloys

Corrosion Science, Vol. 19, pp. 857 to 867 Pergamon Press Ltd. 1979. Printed in Great Britain THE ROLE OF A L L O Y I N G ELEMENTS IN I M P R O V I N...

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Corrosion Science, Vol. 19, pp. 857 to 867 Pergamon Press Ltd. 1979. Printed in Great Britain

THE ROLE OF A L L O Y I N G ELEMENTS IN I M P R O V I N G THE CORROSION RESISTANCE OF A M O R P H O U S I R O N BASE ALLOYS* K. HASmMOTO, K. ASAMI, M. NAKA and T. MASUMOrO The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai, Japan Abstract--X-ray photo-electron spectroscopy has been used to investigate the correlation between the compositions of a surface film and the underlying alloy and the beneficial effects of alloying elements. The addition of alloying elements less noble than iron increases the corrosion resistance in the active region by improving the protective quality of the corrosion product film in which the cations of alloying elements are significantly concentrated. Even if the passivity of alloying elements is not stable in the passive region of alloys, the alloying asists the formation of passive hydrated iron oxy-hydroxide film by decreasing the dissolution rate of alloys prior to the passive film formation. The improvement of corrosion resistance by alloying with the elements more noble than iron has been interpreted in terms of the decrease in the anodic activity of alloys. INTRODUCTION UNLESS extra metallic elements other than iron are added, amorphous iron-m~talloid alloys are chemically unstable and dissolve actively, not showing anodic passivation. 1 However, the alloys containing a certain amount of chromium passivate spontaneously even in 1N HCI which is open to the air. 1-4 (The amount of chromium necessary for spontaneous passivation is dependent upon base metal and metalloid.) These alloys have extremely high corrosion resistance and do not suffer pitting and crevice corrosion: -5 The addition of molybdenum or tungsten to amorphous F e - C 3 and F e - P - C 8 alloys is also effective in improving the corrosion resistance and leads to anodic passivation even in 1N HC1. Amorphous alloys are suited to the study of the effect of alloying elements on the corrosion behaviour of solid solution, because of the chemically homogeneous single phase nature of amorphous alloys. In previous work 7 the present authors have reported the effect of various metallic alloying elements on the corrosion behaviour of amorphous F e - P - C alloys in hydrochloric, sulphuric and nitric acids: The addition of titanium, zirconium, vanadium, niobium, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium or platinum decreases the open circuit corrosion rate and the current density in the active region with a consequent increase in the corrosion potential. The present work aims to clarify the role of metallic alloying elements in improving the corrosion resistance of amorphous F e - P - C alloys. X-ray photo-electron spectroscopy (XPS) has been applied to determine the composition and thickness of the surface film and the composition of the underlying alloy as a function of polarization potential. *Manuscript received 5 January 1979. 857

858

K. HASmMOTO,K. A s ~ L M. NAKAand T. MASUMOrO

EXPERIMENTAL METHOD A rotating wheel method was applied to the preparation of amorphous alloy ribbons of I ram in width and 30 ixm in thickness. The formation of the amorphous structure was confirmed by X-ray diffraction. The amorphous alloys used are summarized in Table I. Prior tolpolarizationthe specimens were polished with emery paper to No. 1500 in cyclohexane. TABLEI. NOMINALCOMPOSITIONOF AMORPHOUSALLOYS(at. Alloy Fe-IZr-P-C Fe--SV-P--C Fe-10V-P-C Fe-2Nb-P--C Fe- 10Ni-P-C Fe-10Pd-P-C Fe-5Pt-P-C

Fe

Zr

V

79 75 70 78 70 70 75

1 ----. .

.

Nb .

5 10 --. .

. .

Ni .

. . 2 -. .

Pd

. . .

P

C

---5

13 13 13 13 13 13 13

7 7 7 7 7 7 7

. . .

-10

Pt

. . --10

.

Polarization was carried out in 0.1N H2SO, which is prepared by using reagent grade chemicals and de-ionized water. The solution was open to the air. Measurements of anodic polarization curves were made by potentiodynamic method with a potential sweep rate of 2.37 × 10-3 V's-1, starting from the corrosion potential at room temperature. Surface films were formed by immersion or potentiostatic polarization. After removal from the electrolyte, the specimens were rinsed with de-ionized water and dried by air blasting. X-ray photoelectron spectra were measured by AEI-ES 200 electron spectrometer with Mg Kay,2 excitation (1253.6 eV). Binding energies of electrons were determined by a calibration method described elsewhere.3,DThe quantitative determination of the composition and thickness of the surface film and the composition of the underlying alloy by XPS was performed by the same method as described in previous papers?°. ~ Integrated intensities of photoelectrons were obtained for Fe 2P8/I, V 2ps/~, Ni 2pal2, Zr 3dal2.s/2, Nb 3d812.5/~, Pd 3ds/2,sj2, Pt 4fs/2m2, P 2p, S 2p, O ls and C Is spectra.The ratios of photo-ionization cross-sections reported by Scofield~ and by some of the present authorsS.°.~°.~8 were used for the quantitative determination. E X P E R I M E N T A L RESULTS Figures 1 a n d 2 show anodic polarization curves of a m o r p h o u s F e - 1 3 P - 7 C alloys c o n t a i n i n g various metallic elements less a n d more n o b l e t h a n iron, respectively. The addition of all metallic elements lead to a decrease in anodic current density with a c o n s e q u e n t e n n o b l e m e n t of corrosion potential. W h e n certain a m o u n t s of these elements are added, passivation takes place by anodic polarization. I t has been f o u n d t h a t the corrosion rates o f the alloys in various acids decrease with the increase in the c o n t e n t s of these elements. 7 X-ray photo-electron spectra were measured for the specimens immersed or potentiostatically polarized. The Fe 2ps/2 a n d P 2p spectra consisted of two peaks corresponding to the oxidized state in the surface film and the metallic state* i n the underlying alloy. The Fe 2pal2 spectrum for the oxidized state was further c o m p o s e d of spectra for ferrous and ferric states.14, is The V 2ps/2 spectrum from F e - 1 0 V - 1 3 P - 7 C alloy immersed or polarized in the active region consisted o f three peaks corresponding to the higher (ca. 515.4 eV) and lower (ca. 513.1 eV) states o f oxidation a n d the metallic state (512.8 eV). The F e - 1 0 V - 1 3 P - 7 C alloy polarized in the passive region did n o t show the detectable V 2ps/2 spectrum. F e - 5 V - 1 3 P - 7 C , F e - I Z r - 1 3 P - 7 C a n d *The binding energy of the low binding energy peak of P 2p electrons coincided with that for iron phosphide. It is, therefore, unknown whether the low binding energy peak of P 2p electrons arises from the metallic state or the anionic state. In the present work this state of pho: phorus is conveniently written as the metallic state.

Role of alloying elements in improving corrosion resistance

104

I f Fe-(3P-TC

10a

Jl / ~

-

-

Fe-2Nb-13P-TC ' I Fe-lir'I3P-Tc

•'•I02

//

~. I01 .

~

"I

--e-IOV-13P-TC I0 o

O.1NH2504

~lO-t l 0-~ -0.5

0.5 1.0 POTENTIAL

0

1.5 2.0 V (SCE)

2.5

FIG, 1. Anodic polarization curves of amorphous Fe-13P-7C alloys containing metallic elements less noble than iron measured in 0.1N H~SO4.

I°<| lo~-

I..i-I

!

Fe-13P-TC i

I

L

-~,.I

loOl _

': i

i o.IN.:so,

!

~

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_

.

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Fro. 2.

-0.5

0

0.5 1.0 1.5 2.0 POTENTIAL V ~CE)

2.5

Anodie polarization curves of amorphous Fe-13P-TC alloys containing metallic elements more noble than iron measured in 0.1N H~SOa.

859

860

K. HASHIMOTO, K. ASAMI, M. NAKA and T. MASUMOTO

Fe-2Nb-13P-7C alloys were polarized only in the active region. The V 2pal2 spectrum from the Fe-SV-13P-7C alloy did not show the signal for the metallic state but only those for the oxidized states. Similarly, the Zr 3d5/2 and Nb6/~ spectra revealed single peaks for the oxidized state at c a . 182.4 and 207.3 eV, respectively. The Ni 2palz spectrum exhibited an intense signal for the metallic state. The signal for the oxidized nickel appears only from the alloy immersed or polarized in the active region. The Pd 3d5/2 and Pt 4f~/~. spectra showed only single peaks for the metallic state at c a . 335.9 and 71.8 eV, respectively. The S 2p spectrum revealed a peak at c a . 169 eV. In the O ls spectrum, a low binding energy peak assigned to oxygen in metal-O-metal bond (OM oxygen) was superimposed upon a high binding energy peak for oxygen in metal-OH and -OHm. bonds (OH oxygen). 9-11,14 In OH oxygen, oxygen in phosphate was also included. ~ After the integrated intensities were separately obtained for oxidized and metallic states and for OH and OM oxygen, the quantitative determination of the compositions of surface film and underlying alloy was performed. The analytical results are shown in Figs. 3-7. Figure 3 exhibits the fraction of cations in the surface films formed on Fe-ML-13P-7C alioys containing metallic elements, Mr, less noble than iron. The passive film formed on Fe-10V-13P-7C alloy does not contain vanadium ion and consists exclusively of ferrous and l~erric ions as cations. A remarkable enrichment of cations of the alloying elements, however, takes place in the surface films formed on all four alloys in the active region including the open circuit potential.

I

¢.o

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~0.8 i

2 0.6

o --V--o

..'~-_

r

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• V

on Am. Fe - 5 V-13P-7C

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~ t

~

~

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I

!

.~

t

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I

i t

0 0.2 0.4 POTENTIAL

I 0

0.6 0.8 V (5CE)

I O.

1.0

1.2

1.4

1.6

FIG. 3. Cationic fractions in surface films formed on amorphous Fe-13P-TC alloys containing V, Nb or Zr by immersion or polarization in 0.1N HjSO4 and by emerypolishing in cyclohexane. Included for comparison in the figure are the atomic ratios of metallic elements to the sum of iron and metallic elements in bulk alloys.

Numbers of various anions bonded t o a cation in the surface film formed on Fe-10V-13P-7C alloy are given in Fig. 4 as a function of polarization potential. The surface film consists of oxy-hydroxide, phosphate, sulphate and bound water.

Role of alloying elements in improving corrosion resistance I.~

,

FILM I [ on Fe-IOV-13P-TC

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0

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I

/

~ ,0.4

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~ 0.2 o EMERY -0.4 -0.2 0 0.2 0.4 0.6 0.8 POL 1SHED PO TEN TIAL V (5CE)

1.0

1.2

FIG. 4. Numbers of anions bonded to a cation in surface film formed on amorphous Fe-10V-13P-7C alloy by emery-polishing in cyclohexane and by immersion or

polai'ization in 0.1N H=SO4. From the above analytical results average compositions of surface films formed in the active and passive regions can be expressed as: (Fe u 0 . 2 6 Fe m 0 . 2 7 VlV0 . 4 7 / ~ (PO4)o . 10(SO4)0. 0401.00(OH)0.67

"

0.06H20 at

--

0.1

V(SCE)

and (Fello.asFetlIo.65) (PO4)0.26 (SO4)0.0300.66 (OH)o.62 " 0.49H20 at 1.0 V(SCE).

The estimation can be done for the average compositions of the surface films formed on other amorphous Fe-ML-13P-7C alloys in the active region at -- 0.1 V(SCE):

(l::,~II l::etll vlV "~'

0 . 2 1 J"

0.35 v

0.44/

~ (PO4)om (SO4)0.03Oia0(OH)0.70 " 0.33H~.O on Fe-5V-13P-7C,

Fe II0 . 3 3 Fem 0 . 4 4 N t'v '-' 0 . 2 2 /a (PO4)o . lo (SO4)o.ol Oo.00 (OH)1.o0 " 1.26H20 on F e - 2 N b - I 3P-7C, F,,II0 . 2 g ~r~tn 0 . 4 g Zr TM0 . 3 6 /~ (PO4)o.Io (SO4)o . 00400. 70 (OH)l.~s " 0.44H20 on F e - l Z r - 1 3 P - 7 C . In contrast to amorphous Fe-ML-13P-7C alloys amorphous Fe-MM-13P-7C alloys containing metallic elements, MM, more noble than iron do not show an enrichment of those alloying elements in the surface film formed in the active region, as can be seen in Fig. 5. The concentration of nickel ion in the surface film formed on Fe-10Ni-13P-7C alloy in the active region is nearly the same as the atomic ratio of nickel to the sum of nickel and iron in the bulk alloy. The nickel ion is not found in the passive film. Palladium and platinum are not included in the surface films formed on Fe-10Pd-13P-7C and Fe-5Pt-13P-7C alloys in both the active and passive regions.

862

K. HASHIMOTO,K. ASAMI,M. NAKAand T. MASUMOTO

1.0 ~0.8

~'o.6

_ _

I

I

I

o

Ni

on

Fe-IONi-13P-TC

I

I



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on

Fe-IOPd-13P-TC

A

Pt

ol~ F e - 5 P t - 1 3 P - ? C

[ I

~0.4

I

O'INH250'~

~0.2

L I A

0

-0.4 -0.2

0 0.2 0.4 POTENTIAL

0.6 0.8 V ('SCEI

o

I

i

BULK ALLOY

l.O 1.2 1.4

FIG. 5. Cationic fractions in surface films formed on amorphous Fe-13P-7C alloys containing Ni, Pd or Pt in 0.IN HaSO,. Included for comparison in the figure are the atomic ratios of noble metals to the sum of iron and noble metals in bulk alloys. Accordingly the surface films formed on the amorphous Fe-MM-13P-7C alloys in both the active and passive regions consist exclusively of ferrous and ferric ions as cations. As can be seen in Fig. 6 the phosphate content of the passive films is very

~1o,.,.~ 0 " 8 t~ tu

o

0.6~o0-4-

tFe-IONi-13P-TC I I I 1

~ .

Fo-lOp~-I3P.TC

--~

Fe-5Pt-I3P-TC

O ~N HeSO~-

.

:~ ~ 0 . 2 13

i1~ 0

BULK ALLOY

-0.4-0.2

0

0.2 0.4

POTENTIAL

0.6 0.8

1.0

1.2 1.4

V (5CE)

ion

FIG. 6. Number of phosphate bonded to a cation in surface films formed on amorphous Fe-13P-7C alloys containing Ni, Pd or Pt in 0.IN HjSO4. low. The average compositions of the surface films formed on the Fe-10Ni-13P-7C alloy in the active and passive regions can be expressed as: Fe n 0 . 3 9 1L~~%' I I I 0 . 5 5 hx ,' LI ; H

~ (PO4)o . t~ (SO4)o.o200s~ (OHio.04 " 1.03H=O at

0.061

0.1 V(SCE)

and (FeIIo.uFeII[o.~) (PO4)o.os (SO~)o.0zOx.u (OH)o.0a • 0.67H=O at 1.0 V(SCE). The latter composition is almost the same as the composition of the passive film formed on the Fe-10Pd-13P-7C alloy: (Feno.ssFenIo.s~) (PO~)o.os (SO4)o.o401.o4 (OHio.so " 0.44HsO at 1.0 V(SCE).

Role of alloying elements in improving corrosion resistance

863

The phosphate content increases in the surface film on the Fe-10V-13P-7C alloy by passivation but rather decreases on the Fe-MM-13P-7C alloys by passivation. Accordingly, the increase in the phosphate content seems unnecessary for the passivation of the amorphous Fe-M-13P-7C alloys. It can be said that the passi'vation takes place by the formation of hydrated iron oxy-hydroxide film. Vanadium was deficient in the surface of Fe-10V-13P-7C alloy immediately under the surface film. The alloying elements in underlying alloys were not found for other Fe-ML-13P-7C alloys by XPS. In contrast to this fact, alloying elements more noble than iron are concentrated in the alloy surfaces immediately under the surface films on the Fe-Mm--13P-7C alloys, as can be seen in Fig. 7. 0.5

01N. so,

I [~}d

~"

0.3

I ~Q

I

.~------'e--

~.'~ Pt ~Fe-5Pt-13P-?C ~f

o.

I

Fe-I OPd-I3P-TC

°0.4

/ : ~.._

.0--- 0

- - Ni

Fe-IONi.13P-?C-

~. ~t~ o.

BULK ALLOY

-0.4 -0.2 0 0.2 0.4 0.6 0.8 POTENTIAL V (5CE)

1.0

FIG. 7. Atomic ratios of noble metals to the sum of iron and noble metals in alloy surfaces immediately under the surface films. The amorphous Fe-13P-7C alloys containing Ni, Pd or Pt are immersed or potentiostatically polarized in 0.1N H2SO4. DISCUSSION

The addition of all elements examined is effective in increasing the corrosion resistance of amorphous Fe-13P-7C alloy and results in a decrease in the open circuit corrosion rate and anodic current density with a consequent rise in the corrosion potential. In addition, the alloying leads to the passivation of alloys by anodic polarization. Cations of the alloying elements less noble than iron are significantly concentrated in the surface films formed in the active region but are deficient in the underlying alloys. Contrary to this, alloying elements more noble than irort are not concentrated or not found in the surface films but are concentrated in the underlying alloys. Accordingly, the role of alloying elements in improving the corrosion resistance of alloys must be considered separately depending upon their electronegativities relative to that of base metal of the alloys.

Improvement of corrosion resistance by alloying with elements less noble than base metal of alloy It is well known that the addition of molybdenum to stainless steels improves the corrosion resistance, particularly to pitting. Stainless steels must be used in the passive state. However, in solutions such as 1N HCI where stainless steels do not passivate

864

K. HASI-IIMOTO,K. ASAM1,M. NAKAand T. MASUMOTO

spontaneously, the addition of molybdenum to the steels decreases the open circuit corrosion rate and the current density in the active region.", 17 The same phenomenon has been found for amorphous Fe-I 3P-7C alloys containing molybdenum or tungsten.6 The amorphous Fe-13P-7C alloy dissolves actively, not passivating in 1N HCI. However, the addition of molybdentlm or tungsten to the alloy leads to a decrease in the corrosion rate and anodic current density and to the passivation by anodic polarization. XPS studies have revealed the significant enrichment of molybdenum ions in the surface films formed in the active region on the stainless steel 'e and amorphous Fe-Mo-P-C alloys, t8 In particular, the molybdenum content in the surface films is higher when the active dissolution rate is higher. For amorphous iron x~ and cobalt 2° base alloys containing chromium, the open circuit corrosion rate in the active region is lower when the chromium content in the surface films is higher. The present work reveals the fact that significant enrichment of cations of alloying elements less noble than base metal of alloys occurs in the surface films formed in the active region. In general, many metals less noble than iron are in the passive state in the active region of iron-base alloys. Accordingly, these elements are apt to enrich in the corrosion product film on the iron base alloys. In particular, a high rate of active dissolution facilitates the enrichment of these elements in the film. The corrosion product film appears to act as a diffusion barrier to the dissolution of alloys. Such a protective quality of the corrosion product film would largely depend upon the passivating capability of the alloying element and its content in the film. It can, therefore, be said that the alloying with elements less noble than the base metal of an alloy improves the protective quality of the corrosion-product film and hence the corrosion resistance of the alloy in the active region. The alloying elements less noble than base metal are further divided into two groups with respect to their role in the passivation of alloy. For chromium and titanium, where the alloying element can form a stable passive film in the passive region of alloy and has a higher passivating capability than base metal, the alloy passivates by the formation of the passive film of the alloying element. For instance, amorphous Fe-3Cr-2Mo-13P-7C alloy passivates in 1N HC1 by the formation of hydrated chromium oxy-hydroxide film,x9despite the fact that the alloy contains only 3 at. % chromium. The formation of passive hydrated chromium oxy-hydroxide film has been found on various chromium bearing alloys, such as Fe-Cr alloysflt stainless steels,t3,tsA7 Inconel 6002~, Incoloy22 and amorphous Fe -~8, Co -4 and Ni -~ base alloys. The role of these alloying elements in the passivation of an alloy is not greatly different from the role in improving the corrosion resistance in the active region. An increase in the chromium content of alloy leads to a continuous increase in the corrosion potential and chromium content in the surface film and to a continuous decrease in the anodic current density.4 When the chromium content of alloy attains to a certain amount the spontaneous passivation takes place. Other alloying elements such as vanadium, niobium, molybdenum and tungsten are unstable in the passive region of iron-base alloys. Alloying with these elements facilitates the passivation of iron-base alloys by forming a passive iron oxy-hydroxide film, even in 1N HCI.~a Figure 8 shows a schematic of the passivation process aided by molybdenum, which is representative of these alloying elements. In general, even if

Role of alloying elements in improving corrosion resistance

'IDISSOLUTION RATE

I

F/////

~Me SPECIES

865

I

LLOY//~/I

//~I

I

l Me SPECIES l P////A "PAssIvE I/ALLOY / . , I F/////I

~

I/ -

Y///

ALLOY/ /I //I

FIG. 8. Schematic of passivation of iron-base alloy with the aid of molybdenum added to the alloy.

the potential is set in the passive region, an increase in dissolved metals in the vicinity of the electrode surface, that is, active dissolution is a necessary precursor for passive film formation.24 Initial active dissolution in the passive region is very rapid because of the high polarization potential. As in the active region, rapid active dissolution provides rapid enrichment of cations of the alloying element in the vicinity, of the alloy surface and hence results in the rapid formation of corrosion product film which acts as an effective diffusion barrier to further dissolution of the alloy. For instance, when surface films on 30Cr and 30Cr-2Mo ferritic stainless steels are removed mechanically during potentiostatic polarization in the passive region, the sharp decrease in current density appears on the 30Cr-2Mo steel after the instantaneous initial active dissolution, in spite of the fact that the composition of passive films formed on both steels is the same. 16 In this manner, the rapidly formed corrosion product film in which the cations of alloying elements are significantly concentrated serves to slow down the rate of dissolution of alloy and facilitates the formation of a passive film at the interface of the corrosion product film and the alloy. The corrosion product film is formed as a result of active dissolution and is not stable at a high potential in the passive region. Accordingly, when the condition for the rapid supply of alloying element to form the corrosion product film fails by a lowering of the dissolution rate through passive film formation, the corrosion product film dissolves in the solution. Consequently, alloying elements such as vanadium, niobium, molybdenum or tungsten are not found in passive films formed on iron base alloys after prolonged polarization, but they definitely assist the passivation of such alloys. The improvement of pitting resistance by the addition of molybdenum to stainless steels can be interpreted similarly in terms of the beneficial effect of molybdenum on the repassivation of actively dissolving micro-pores in the passive hydrated chromium oxy-hydroxide film.16 As mentioned above, the role of alloying elements such as vanadium, niobium and tungsten in facilitating the passivation of alloys is the same as that of molybdenum. This fact suggests that alloying with these elements improves the pitting resistance of stainless steels if the elements exist in the steel matrix.

866

K. HASmMCrrO,K. ASAMI,M. N ~

and T. MASUMOTO

Improvement o f corrosion resistance by alloying with elements more noble than base metal o f alloy

Because of rapid quenching, amorphous alloys do not contain precipitates, segregates, second phases etc. and are regarded as chemically homogeneous solid solutions. Accordingly, noble metal~ added to amorphous alloys do not form cathodic inclusions etc. and simply ennoble the alloy. In other words, alloying with noble metals decreases the anodic activity of alloys and hence increases the corrosion resistance. Because of their high chemical stability the cations of these metals are not found in the surface film on the alloys. During immersion or anodic polarization these elements remain selectively without dissolving and are concentrated in the alloy surface immediately under the surface film. The decrease in the anodic activity o f alloy by the enrichment of noble metals further decreases the dissolution rate and facilitates the formation of passive hydrated iron oxy-hydroxide film. CONCLUSION Alloying with various metallic elements improves the corrosion resistance of amorphous F e - P - C alloys. The beneficial effects of alloying elements less noble than iron can be interpreted as follows: active dissolution of alloy leads to the formation of corrosion product film in which the cations of these elements are significantly concentrated. The passivafing capability of alloying elements in the active region of an alloy and their presence in the corrosion product film mainly determine the protective quality of the corrosion product film which acts as a diffusion barrier to the dissolution of alloy. The elements with a higher passivating capability than iron produce their own passive films. However, even if the passivity of the elements is not stable in the passive region of alloy, alloying with these elements facilitates the formation of the passive hydrated iron oxy-hydroxide film in the same way as that the elements improve the corrosion resistance in the active region. The role of alloying elements more noble than iron in increasing the corrosion resistance of alloy can be illustrated in terms of the decrease in the anodic activity of alloy. REFERENCES 1. M. NAKA,K. HASHIMOTOand T. MASUMOTO,Corrosion 32, 146 (1976). 2. K. HASmMOTO,M. KA~¥^, K. ASAMIand T. MASOMOTO,Boshoku Gijutsu 26, 445 (1977). 3. M. NAKA,K. HASmMOTO,A. INOUEand T. MASUMOTO,J. Non-Cryst. Solids 31, 347 (1979). 4. K. HASmMoTO,K. ~ I , M. N^KAand T. MASta~OTO,Boshoku Gijutsu 28, 271 (1979). 5. R. B. DIEGI.~, Corrosion 35, 250 (1979). 6. M. NAKA,K. HASmMOTOand T. MASUMOTO,Y. Non-Cryst. Solids 29, 61 (1978). 7. M. NAKA,K. HASHZMOTOand T. MASUMO'rO,J. Non-Cryst. Solids 31, 355 (1979). 8. K. ASAMI, J. Electron Spectrosc. 9, 469 (1976). 9. K. ~ and K. HASmMOTO,Corros. Sci. 17, 559 (1977). I0. K. As~vn, K. HASmMOTOand S. SHIMODAIRA,Corros. ScL 17, 713 (1977). 11. K. ASAMI,K. I-IASmMOTOand S. SHIMODAXP~,Corros. Sci. 18, 151 (1978). 12. J. H. SCOFIELD,Y. Electron Spectrosc. 8, 129 (1976). 13. K. TeRAMOTO,K. ASAMIand K. HASmMOTO,Boshoku Gijutsu 27, 57 (1978). 14. K. ~ I , K. HASmMOTOand S. SmMODAInA,Corros. Sci. 16, 35 (1976). 15. K. ~ , K. HASmMOTOand S. SHIMODAXRA,Corros. Sci. 16, 387 (1976). 16. K. I-IT,stnMOTO,K. ASAMIand K. TEnT,MOTO, Corros. Sci. 19, 3 (1979). 17. K. HASmMO'rOand K. As.~, Corros. Sci. 19, 260 (1979). 18. K. HASW_MOTO,M. NAKA,K. ASAMIand T. MASUMOTO,Corros. Sci. 19, 165 (1979).

Role of alloying elements in improving corrosion resistance

867

19. K. HASHIMOTO,M. NAKA, J. NOGUCHI,K. ASAMXand T. MASUMOTO,Passivity of metals, Prec. 4th Intern. Syrup. on Passivity, Electrochem. Soc. (1977), p. 156. 20. M. NAKA,K. ASAMI,K. HASHIMOTOand T. MASUMOTO,Prec. 3rd Int. Conf. on Rapidly Quenched Metals, Metal Soc. (1978), p. 449. 21. K. ASAMX,K. HASHIMOTOand S. SHIMODAIRA,Cortes. Sci. lg, 151 (1978). 22. K. HASHIMOTOand K. ASAMI,Cortes. Sci. 19, 427 (1979). 23. K. ASAMI,K. HASHIMOTO,T. MASUMOTOand S. SHIMODAIRX,Cortes. Sci. 16, 909 (1976). 24. N. SATOand G. OKAMOTO,Trans. Japan Inst. Metals 2, 113 (1961).