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Extremely high corrosion resistance of chromium-containing amorphous iron alloys
Materials Science and Engineering, 23 ( 1 9 7 6 ) 285 - 288
285
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
Extremely High Corrosion Resistance of Chromium-Containing Amorphous Iron Alloys*
K. H A S H I M O T O a n d T. M A S U M O T O
The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai (Japan)
To evaluate the possibility of practical applications of high strength amorphous iron base alloys, the corrosion resistance of amorphous Fe-Cr-P-C and F e - C r - N i - P - C alloys was studied.
EXPERIMENTAL PROCEDURES
Amorphous FeCro-loNio.40Plo.17C(or B ) 3 - 1 0 alloys were prepared by centrifugal quenching. Subscripts of elements in the alloys denote at.%. The amorphous FeCrloPlsC7 alloy was crystallized to a single phase of b.c.c, structure with fine grains by annealing for 2 months at 350 °C. Solutions used for corrosion studies were 1 N NaCI, 1 M H2SO4, 1 M H2SO40.5 N NaC1, 0.01 - 1 N HC1 and 10% FeCI 3 • 6H20. Besides total immersion tests and potentiodynamic (sweep :ate of potential: 142 mV/min) and potentiostatic (polarization for 1 h at each potential) measurements, the passivation behavior was examined through current change with time after specimens were mechanically polished in solutions during anodic polarization at various potentials. The passive film formed on the amorphous alloy was studied by ESCA spectrum measurements which were performed with an AEI-ES200 electron spectrometer using MgKa 1.2-radiation (1253.6 eV).
talline Fe-Cr alloys and 18Cr-8Ni stainless steel. However, the corrosion rate of the amorphous FeCrP13C7 alloys decreased with increasing Cr content and the alloys containing ~>8 at.% Cr did not show any weight change detectable by microbalance after immersion for 168 h in 1 N NaC1 at 30 °C, 0.1 - 1 N HC1 at 30 °C and 10% FeC13 • 6H20 at 40 and 60 °C. Table 1 shows the results of the immersion test in 10% FeC13 • 6H20 which has frequently been employed for pitting corrosion tests of stainless steels. The amorphous FeCrsP13C7, FeCrloP13C7 and FeCrloNisP13C 7 alloys exhibited no weight change detectable by microbalance after immersion for 168 h. By contrast, crystalline 304 and 316L stainless steels suffered severe pitting and crevice corrosion. The corrosion potential of the amorphous FeCrP13C7 alloys with/>8 at.% Cr in 1 N NaC1 and 1 M H2SO4 solutions was about 0 V (SCE) which corresponds to the passive state of pure Cr. No anodic current higher than 10 -7 A/cm 2 was observed in the wide potential region from 0 to 800 mV through potentiostatic polariza, tion measurements. The addition of boron to the alloys instead of carbon did not influence their polarization curves. The anodic current TABLE 1 C o r r o s i o n r a t e s o f a m o r p h o u s alloys a n d crystalline
stainless steels in 10% FeCI 3 • 6 H 2 O RESULTS AND DISCUSSION Specimen
In the total immersion tests the amorphous
C o r r o s i o n rate, m m / y e a r
40°(3
60°C
17.75 -
120.0 29.24
FeCr0.2P13C7 alloys were severely corroded and polarization measurements showed significantly high reactivity as compared with crys*Paper p r e s e n t e d at t h e S e c o n d I n t e r n a t i o n a l Conference on Rapidly Quenched Metals, held at t h e Massachusetts Institute o f T e c h n o l o g y , Cambridge, Mass., N o v e m b e r 17 - 19, 1975.
Crystalline stainless steel 18Cr-8Ni 17Cr-14Ni-2.5Mo A m o r p h o u s alloy
FeCrsP13C7 FeCrloPlaC 7 FeCrloNisP13C 7
0.0000 0.0000
0.0000 0.0000 0.0000
286
=~.. sM
%so,
soln. IM H~S04 • 0:SNNaCl
300mY
300mY
C"
lO f8-8 STEEL
F
o
~ 0
5
Time
(a)
I0 (sec)
15
MORPHOUS
Iosi 0 Time
I0 (sec)
15
(b)
Fig. 1. Current density us. time curves for the amorphous FeCrloP13C 7 alloy and 18Cr-8Ni stainless steel measured after abrading the specimens during anodic polarization at 300 mV (SCE) in 1 M H2SO 4 (a) and in 1 M H2SO4-0.5 N NaCI (b).
of the amorphous FeCr10P13C7 alloy in the passive region of the potentiodynamic polarization curve was 2 orders of magnitude lower than that of the crystalline FeCrl0Pz3C7 alloy consisting of a single phase of b.c.c, structure with fine grains. After the immersion test, the crystalline alloy showed uneven, severe general corrosion. Consequently, the high corrosion resistance of the amorphous alloys appears to result at least partly from the fact that the alloys consist of a uniform single phase which contains no defects such as grain boundaries, dislocations, compositional fluctuations, etc. which act as nucleation sites for corrosion. The change in current density with time was measured after the surface film on the specimen was removed b y mechanical polishing in the solution during anodic polarization. Examples of the results are shown in Fig. 1. The amorphous alloy is characterized b y a higher current density at the initial stage and by a subsequent decrease to lower current density in the steady state as compared with 18Cr-8Ni stainless steel. This behavior of the amorphous alloy was not affected b y C1- addition to the solution. However, 18Cr-8Ni stainless steel showed a high steady current density in H2SO4NaC1 solutions due to pitting corrosion. The high reactivity of the bare surface of the amor-
s, ln.
I N H~SO, 300mY
#0
E
15
AMORPmOS t. D
~o
5
0
18-8 SI"EE
5
I0 Time
15
(sec)
Fig. 2. Change in the amount of charge passed with time as obtained by integration of the current density-time curve measured in 1 M H2SO4 at 300 mV (SCE). phous alloy w i t h o u t surface film is consistent with the fact that the amorphous FeCro.sPlsC7 alloys, on which a stable passive film was n o t formed, showed high corrosion rates in immer-
287
sion tests. The integration of the current density vs. time curves gave the change in the a m o u n t of charge with time shown in Fig. 2. Assuming that all charges passed were employed for film formation, a four times thicker film is formed on the amorphous alloy during the initial stage of one second than on 304 stainless steel. Consequently, the high corrosion resistance of the amorphous alloys can be attributed partly to the rapid formation of a thick corrosion-resistant film due to the high reactivity of the alloys themselves. The passive film formed on the amorphous FeCrl0PlsC7 alloy by immersion for 168 h in 1 N HC1, during which no weight change was detected, was compared with the air-formed film on the alloy through ESCA spectrum measurements. Figure 3 shows Cr 2ps/2 spectra. The signal of the metallic Cr in the substrate, which should appear at a b o u t 574 eV, is n o t observed inasmuch as a thicker film was formed as compared with ordinary stainless steels. The peak at a b o u t 577 eV results from the oxidized state of Cr. Figure 4 shows Fe 2p3/2 electron spectra where signals at 706.3 and 710 eV correspond to the metallic and oxidized states of Fe, respectively. After immersion in HC1, oxidized iron in the air-formed film is almost removed. Figure 5 shows O ls electron spectra from the specimens. The peaks due to oxygen in C r - O and F e - O bonds
~ Ii0 cps
t~
o
7;,
7;2
Before immersion
I
577 576 575 574 573 Binding Energy eV
7,',
I
7,o
Binding
I
I
7o9 Energy
I
707
706
eV
Fig. 4. Fe 2P3/2 electron spectra of amorphous FeCrlo P13C7 alloy before and after immersion in 1 N HCI.
T~o~p~
/
cr-o
irnrnersio
532 Binding
53l Energy
Fe-O
530
529
eV
Fig. 5. O ls electron spectra of amorphous FeCr10P13 C7 alloy before and after immersion in 1 N HCI. Arrows indicate signals of oxygen in Cr-O and Fe-O bonds.
Cr 2p~/2
578
Fe 2p3/2
J
533 Fe CrwP13C7
FeCr~o~sC~
572
Fig. 3. Cr 2Ps/2 electron spectra of amorphous FeCrlo P13C7 alloy before and after immersion in 1 N HCI.
are indicated by arrows. The signal from F e - O bonds is absent after immersion in HC1. The broad signal in the high energy region, which arises from oxygen in the m e t a l - O H bond and b o u n d water, is decreased b y immersion. Oxidized phosphorus was also found in the film. Therefore, both the passive film and the airformed film are composed of CrOx(OH)y, FeOx,(OH)y,, b o u n d water and oxidized phosphorus. By immersion for 168 h in 1 N HC1, almost all of FeO~,(OH)¢ and some b o u n d water were removed from the air-formed film and the passive film formed is c o m p o s e d mainly of CrO~(OH)y. The constituents of the passive film are similar to those of the film on
288ordinary stainless steels and the passive film itself is highly corrosion-resistant. CONCLUSIONS The corrosion resistance of amorphous FeCr-P-C and Fe-Cr-Ni-P-C alloys was investigated by immersiontests and electrochemical and ESCA measurements.The followingconclusions can be drawn: (1) When the amorphous FeCrPlaC7 and FeCrNiPlsC7 alloys are immersedin 1 N NaCI, 0.01 - 1 N HCI and 10% FeCIs" 6H20 solutions, no pitting corrosion occurred and no weight change was detected by a microbalanceon the amorphous alloys containing i>8 at.% Cr after immersion for 168 h. (2) The extremely high corrosion resistance
of the amorphous iron alloys can be ascribed to the following characteristics: a. The corrosion potential of the amorphous alloys in neutral and acidic solutions is in the passive region of pure Cr. b. The amorphous alloys are in a uniform, homogeneous single phase in the amorphous state and nucleation sites for corrosion such as grain boundaries, dislocations, compositional fluctuations etc. do not exist. Hence, uniform passive film without defects can be formed on the amorphous alloys. c. Because of the high reactivity of the amorphous alloys themselves, formation of the passive film and recovery of ruptured sites of the passive film are rapid. d. The passive film, which consists mainly of CrOx (OH)y, is highly corrosion-resistant.
285
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
Extremely High Corrosion Resistance of Chromium-Containing Amorphous Iron Alloys*
K. H A S H I M O T O a n d T. M A S U M O T O
The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai (Japan)
To evaluate the possibility of practical applications of high strength amorphous iron base alloys, the corrosion resistance of amorphous Fe-Cr-P-C and F e - C r - N i - P - C alloys was studied.
EXPERIMENTAL PROCEDURES
Amorphous FeCro-loNio.40Plo.17C(or B ) 3 - 1 0 alloys were prepared by centrifugal quenching. Subscripts of elements in the alloys denote at.%. The amorphous FeCrloPlsC7 alloy was crystallized to a single phase of b.c.c, structure with fine grains by annealing for 2 months at 350 °C. Solutions used for corrosion studies were 1 N NaCI, 1 M H2SO4, 1 M H2SO40.5 N NaC1, 0.01 - 1 N HC1 and 10% FeCI 3 • 6H20. Besides total immersion tests and potentiodynamic (sweep :ate of potential: 142 mV/min) and potentiostatic (polarization for 1 h at each potential) measurements, the passivation behavior was examined through current change with time after specimens were mechanically polished in solutions during anodic polarization at various potentials. The passive film formed on the amorphous alloy was studied by ESCA spectrum measurements which were performed with an AEI-ES200 electron spectrometer using MgKa 1.2-radiation (1253.6 eV).
talline Fe-Cr alloys and 18Cr-8Ni stainless steel. However, the corrosion rate of the amorphous FeCrP13C7 alloys decreased with increasing Cr content and the alloys containing ~>8 at.% Cr did not show any weight change detectable by microbalance after immersion for 168 h in 1 N NaC1 at 30 °C, 0.1 - 1 N HC1 at 30 °C and 10% FeC13 • 6H20 at 40 and 60 °C. Table 1 shows the results of the immersion test in 10% FeC13 • 6H20 which has frequently been employed for pitting corrosion tests of stainless steels. The amorphous FeCrsP13C7, FeCrloP13C7 and FeCrloNisP13C 7 alloys exhibited no weight change detectable by microbalance after immersion for 168 h. By contrast, crystalline 304 and 316L stainless steels suffered severe pitting and crevice corrosion. The corrosion potential of the amorphous FeCrP13C7 alloys with/>8 at.% Cr in 1 N NaC1 and 1 M H2SO4 solutions was about 0 V (SCE) which corresponds to the passive state of pure Cr. No anodic current higher than 10 -7 A/cm 2 was observed in the wide potential region from 0 to 800 mV through potentiostatic polariza, tion measurements. The addition of boron to the alloys instead of carbon did not influence their polarization curves. The anodic current TABLE 1 C o r r o s i o n r a t e s o f a m o r p h o u s alloys a n d crystalline
stainless steels in 10% FeCI 3 • 6 H 2 O RESULTS AND DISCUSSION Specimen
In the total immersion tests the amorphous
C o r r o s i o n rate, m m / y e a r
40°(3
60°C
17.75 -
120.0 29.24
FeCr0.2P13C7 alloys were severely corroded and polarization measurements showed significantly high reactivity as compared with crys*Paper p r e s e n t e d at t h e S e c o n d I n t e r n a t i o n a l Conference on Rapidly Quenched Metals, held at t h e Massachusetts Institute o f T e c h n o l o g y , Cambridge, Mass., N o v e m b e r 17 - 19, 1975.
Crystalline stainless steel 18Cr-8Ni 17Cr-14Ni-2.5Mo A m o r p h o u s alloy
FeCrsP13C7 FeCrloPlaC 7 FeCrloNisP13C 7
0.0000 0.0000
0.0000 0.0000 0.0000
286
=~.. sM
%so,
soln. IM H~S04 • 0:SNNaCl
300mY
300mY
C"
lO f8-8 STEEL
F
o
~ 0
5
Time
(a)
I0 (sec)
15
MORPHOUS
Iosi 0 Time
I0 (sec)
15
(b)
Fig. 1. Current density us. time curves for the amorphous FeCrloP13C 7 alloy and 18Cr-8Ni stainless steel measured after abrading the specimens during anodic polarization at 300 mV (SCE) in 1 M H2SO 4 (a) and in 1 M H2SO4-0.5 N NaCI (b).
of the amorphous FeCr10P13C7 alloy in the passive region of the potentiodynamic polarization curve was 2 orders of magnitude lower than that of the crystalline FeCrl0Pz3C7 alloy consisting of a single phase of b.c.c, structure with fine grains. After the immersion test, the crystalline alloy showed uneven, severe general corrosion. Consequently, the high corrosion resistance of the amorphous alloys appears to result at least partly from the fact that the alloys consist of a uniform single phase which contains no defects such as grain boundaries, dislocations, compositional fluctuations, etc. which act as nucleation sites for corrosion. The change in current density with time was measured after the surface film on the specimen was removed b y mechanical polishing in the solution during anodic polarization. Examples of the results are shown in Fig. 1. The amorphous alloy is characterized b y a higher current density at the initial stage and by a subsequent decrease to lower current density in the steady state as compared with 18Cr-8Ni stainless steel. This behavior of the amorphous alloy was not affected b y C1- addition to the solution. However, 18Cr-8Ni stainless steel showed a high steady current density in H2SO4NaC1 solutions due to pitting corrosion. The high reactivity of the bare surface of the amor-
s, ln.
I N H~SO, 300mY
#0
E
15
AMORPmOS t. D
~o
5
0
18-8 SI"EE
5
I0 Time
15
(sec)
Fig. 2. Change in the amount of charge passed with time as obtained by integration of the current density-time curve measured in 1 M H2SO4 at 300 mV (SCE). phous alloy w i t h o u t surface film is consistent with the fact that the amorphous FeCro.sPlsC7 alloys, on which a stable passive film was n o t formed, showed high corrosion rates in immer-
287
sion tests. The integration of the current density vs. time curves gave the change in the a m o u n t of charge with time shown in Fig. 2. Assuming that all charges passed were employed for film formation, a four times thicker film is formed on the amorphous alloy during the initial stage of one second than on 304 stainless steel. Consequently, the high corrosion resistance of the amorphous alloys can be attributed partly to the rapid formation of a thick corrosion-resistant film due to the high reactivity of the alloys themselves. The passive film formed on the amorphous FeCrl0PlsC7 alloy by immersion for 168 h in 1 N HC1, during which no weight change was detected, was compared with the air-formed film on the alloy through ESCA spectrum measurements. Figure 3 shows Cr 2ps/2 spectra. The signal of the metallic Cr in the substrate, which should appear at a b o u t 574 eV, is n o t observed inasmuch as a thicker film was formed as compared with ordinary stainless steels. The peak at a b o u t 577 eV results from the oxidized state of Cr. Figure 4 shows Fe 2p3/2 electron spectra where signals at 706.3 and 710 eV correspond to the metallic and oxidized states of Fe, respectively. After immersion in HC1, oxidized iron in the air-formed film is almost removed. Figure 5 shows O ls electron spectra from the specimens. The peaks due to oxygen in C r - O and F e - O bonds
~ Ii0 cps
t~
o
7;,
7;2
Before immersion
I
577 576 575 574 573 Binding Energy eV
7,',
I
7,o
Binding
I
I
7o9 Energy
I
707
706
eV
Fig. 4. Fe 2P3/2 electron spectra of amorphous FeCrlo P13C7 alloy before and after immersion in 1 N HCI.
T~o~p~
/
cr-o
irnrnersio
532 Binding
53l Energy
Fe-O
530
529
eV
Fig. 5. O ls electron spectra of amorphous FeCr10P13 C7 alloy before and after immersion in 1 N HCI. Arrows indicate signals of oxygen in Cr-O and Fe-O bonds.
Cr 2p~/2
578
Fe 2p3/2
J
533 Fe CrwP13C7
FeCr~o~sC~
572
Fig. 3. Cr 2Ps/2 electron spectra of amorphous FeCrlo P13C7 alloy before and after immersion in 1 N HCI.
are indicated by arrows. The signal from F e - O bonds is absent after immersion in HC1. The broad signal in the high energy region, which arises from oxygen in the m e t a l - O H bond and b o u n d water, is decreased b y immersion. Oxidized phosphorus was also found in the film. Therefore, both the passive film and the airformed film are composed of CrOx(OH)y, FeOx,(OH)y,, b o u n d water and oxidized phosphorus. By immersion for 168 h in 1 N HC1, almost all of FeO~,(OH)¢ and some b o u n d water were removed from the air-formed film and the passive film formed is c o m p o s e d mainly of CrO~(OH)y. The constituents of the passive film are similar to those of the film on
288ordinary stainless steels and the passive film itself is highly corrosion-resistant. CONCLUSIONS The corrosion resistance of amorphous FeCr-P-C and Fe-Cr-Ni-P-C alloys was investigated by immersiontests and electrochemical and ESCA measurements.The followingconclusions can be drawn: (1) When the amorphous FeCrPlaC7 and FeCrNiPlsC7 alloys are immersedin 1 N NaCI, 0.01 - 1 N HCI and 10% FeCIs" 6H20 solutions, no pitting corrosion occurred and no weight change was detected by a microbalanceon the amorphous alloys containing i>8 at.% Cr after immersion for 168 h. (2) The extremely high corrosion resistance
of the amorphous iron alloys can be ascribed to the following characteristics: a. The corrosion potential of the amorphous alloys in neutral and acidic solutions is in the passive region of pure Cr. b. The amorphous alloys are in a uniform, homogeneous single phase in the amorphous state and nucleation sites for corrosion such as grain boundaries, dislocations, compositional fluctuations etc. do not exist. Hence, uniform passive film without defects can be formed on the amorphous alloys. c. Because of the high reactivity of the amorphous alloys themselves, formation of the passive film and recovery of ruptured sites of the passive film are rapid. d. The passive film, which consists mainly of CrOx (OH)y, is highly corrosion-resistant.