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The electrochemical corrosion of amorphous Ni36Fe32Cr14P12B6 alloy (Metglass 2826A)
Corrosion Science. Vol. 19, pp. 1001 to 1006 (~) Pergamon Press Ltd. 1979. Printed in Great Britain.
0010-938X/79/1201-I001 $02.00/0
THE ELECTROCHEMICAL CORROSION OF AMORPHOUS Nis.Fes2Cr14Pl~B. ALLOY (METGLAS 2826A)* T. KULIK, J. BASZKIEWlCZ, M. KAMINSKI, J. LATUSZKIEWlCZ and H. MATYJA Institute of Materials Science and Engineering, Warsaw Technical University, ul. Narbutta 85, 02-524 Warszawa, Poland Abstract--The resistance to electrochemical corrosion of alloy Metglas 2826A in the amorphous state and after crystallization was studied in non-aqueous and aqueous chloride solutions. The results were compared with the data obtained for alloy steels. Amorphous alloy Metglas 2826A was found to exhibit very high corrosion resistance, greatly exceeding that of high-alloy steels. After crystallization of this alloy, its corrosion resistance dropped to the level characteristic of alloy steels. The high stability of the passive film on the amorphous material was attributed to the homogeneity of alloy structure. INTRODUCTION THE CORROSIONresistance o f a m o r p h o u s alloys (metallic glasses) exposed to sulphuric acid and chlorides has been the object o f m a n y investigations. 1-4 Alloys containing more than 8 at.% c h r o m i u m and 13 at.% p h o s p h o r u s have been f o u n d to exhibit very high corrosion resistance. A n addition o f phosphorus to the chromiun~-containing alloy greatly enhances its corrosion resistance. The cause o f the very high corrosion resistance o f a m o r p h o u s alloys containing c h r o m i u m and phosphorus has not so far been fully elucidated. I n some papers ~"g the chemical composition o f the passive films on alloys containing Fe, Cr, P and C has been reported; this film was f o u n d to c o n t a i n - - a f t e r exposure in hydrochloride acid m e d i u m - - m a i n l y CrOx(OH)y, the p h o s p h o r u s content being only slight. Similar passive films occur on alloy steels. The high corrosion resistance o f a m o r p h o u s alloys o f this type has been attributed to the presence o f the passive film and to the structure o f the alloy, being free o f grain b o u n d a r i e s ) -~ The present study was designed to examine the behaviour o f the passive film on alloy NiaeFeaeCh4P~2Bs (Metglas 2826A) exposed to an aqueous and nonaqueous solution o f chloride ions, with the alloy in the a m o r p h o u s state and after its crystallization; the results were intended to be c o m p a r e d with the data obtained for alloy steels. EXPERIMENTAL METHOD Alloy Metglas 2826A was used in the form of a ribbon (3 mm wide and 0.055 mm thick), and its structure was examined by X-ray diffraction and TEM. s The uniformity of distribution of alloy components was determined on the cross-section and surface of the sample by means of an X-ray micro-analyzer. Samples were applied in amorphous state (a) and after two kinds of thermal treatment; (b) heating at a rate of 320 deg min-1 to 875 K; and (c) annealing for 9 h at 1100 K. Samples b and e were in crystalline state. As comparative materials ferritic steels were used: (S) containing 13 ~oCr, (T) containing 17 ~oCr and stabilized with titanium, and austerdtic steel (L) containing 17 %Cr, 12 ~oNi, 2 ~oMo and stabilized with titanium. *Manuscript received 31 January 1979. 1001
T. KULIK et al.
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Flo. 1.
Anodic polarization curves in aqueous NaCI solutions for alloy Ni86F.~.,Cr,PI=B6 in amorphous (a) and crystalline (b, c) state.
An alloy sample (3 x 8 ram) was mounted in epoxy resin so that its free side was exposed to the corrosive medium. After stabilization of corrosion potential (0.5 h) the samples were polarized anodically at a rate of 1000 mV h -1, and changes in anodic current were registered with a recorder. A saturated calomel electrode (SCE) was used as a reference electrode. The corrosive medium consisteA of 0.1 ~o or 3~o aqueous solutions of sodium chloride or else of a 5.3?/0 glycol solution of sodium chloride. In aqueous medium measurements were taken at room temperature, and in the non-aqueous medium at 323 K (50°C). EXPERIMENTAL
RESULTS
Results obtained for the aqueous NaCl solution are presented in Fig. 1. In the amorphous state (a) the investigated alloy was fully resistant to electrochemical corrosion in aqueous chloride solutions, whereas in the crystalline state (b and c) it corroded uniformly. Figure 2 shows the anodic curves obtained with glycol containing 5.3 ~oNaCl, for alloy Metglas 2826A and for alloy steels S, T and L. Corrosive attack is exemplified in Figs. 3-5. Metglas 2826A annealed isothermally (c) was dissolved uniformly in all solutions, this manifesting itself by the appearance of the surface structure (Fig. 3). The alloy heated continuously (b) was also uniformly destroyed in all solutions (Fig. 4), with two stages of dissolution. At the first stage the outer layer of metal dissolved (its remainder is visible in Fig. 4), whereupon the next layer dissolved somewhat more slowly. The chemical compositions of both layers were analysed comparatively with the use of an X-ray micro-analyser. The outer layer of metal contained less chromium, more nickel, more iron and less phosphorus, as compared with the less soluble matrix. The decrease in chromium and phosphorus contents is accounted for by somewhat lower corrosion resistance of the surface as compared with the matrix. The observed phenomenon is difficult to interpret. One of the possible explanations
FIG. 3. Alloy Metglas 2826A in crystalline state (T = 1100 K, t = 9 h) after the occurrence of electrochemical corrosion in a saturated NaC1 solution in glycol. FI6.4. Alloy Metglas 2826A in crystalline state (T = 825 K, dT/dt = 320 K rain -1) after the occurrence of electrochemical corrosion in a saturated NaCI solution in glycol (corrosion current density 10 mA cm-~).
FIo. 5. Alloy Metglas 2826A in amorphous state after the occurrence of electrochemical corrosion in a saturated NaC1 solution in glycol [current density: (a) 6.25 mA cm -2, (b) 12.5 mA cm-2].
Electrochemical corrosion of amorphous Nia6Fes~CrI4P~zB6ahoy ~
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FxG. 2. Anodic polarization curves in a saturated NaC1 solution in glycol for alloy Ni.Fes~Cr14P]2B6 in amorphous (a) and crystalline (b, c) state, and for alloy steels (S, T, L). can be found in Ref. 6; it involves selective dissolution of iron and nickel in the case of alloy steels in passive state. The investigated alloy in the amorphous state (a) corroded only in a non-aqueous glycol solution of sodium chloride (Fig. 5); however, it exhibited a very high breakdown potential. The observed course of corrosive attack could be divided into two stages. At the initial stage corrosion was o f a local nature which was reflected by the formation of a few shallow and symmetrical holes, mainly localized on scratches on the ribbon surface. However, these holes were formed also at the sites free of scratches. The holes did not grow any deeper but their number steadily increased. As a result after a long time corrosive attack was nearly uniform. CONCLUSION Alloy Metglas 2826A in the amorphous state exhibits a high corrosion resistance exceeding that of high-alloy steels; after crystallization, its corrosion resistance approaches that of alloy steels. The resistance o f the passive film formed on the surface of the amorphous alloy decreases in the non-aqueous medium, as has been found for alloy steels.7, s This is confirmed by findings concerning the beneficial effect of water on formation of passive films?-4,s The high stability of passive films on amorphous materials can be attributed to the uniformity of the alloy structure. Acknowledgement--This work was supported by Ministry of Science,High Education and Technology
(Grant No. MR.I-21). REFERENCES 1. M. NAKA,K. HASHIMOTOand T. MASUMOTO,Japan Inst. Metals 38, 835 (1974). 2. K. HASHIMOTOand T. MASUMOTO,Mater. Sei. Engng 23, 285 (1976).
1006 3. 4. 5. 6.
T. KULIK et al.
K. ASAMI,K. HASHIMOTO,T. MASUMOTOand S. SHIMODAIRE,Corros. Sci. 16, 909 (1976). M. NAKA, K. HASHIMOTOand T. MASUMOTO,Sci. Rep. ~itu A, 26, 283 (1977). T. KUUK, J. LATUSZKIEWlCZand H. MATYJA,Archwm Hutn. 24, 103 (1979). M. PRA~AK, n . KUCHYNKAand T. SEN, 6th European Congress on Metallic Corrosion, p. 287. Chemical Society, London (1977). 7. A. SZUMMER,J. BASZKIEWICZ,M. KAmNSKI, D. KRUPA, Ochr. przed Koroz]a. 2, 29 (1977). 8. K. O. SATO, Corros. Sci. 8, 809 (1968).
0010-938X/79/1201-I001 $02.00/0
THE ELECTROCHEMICAL CORROSION OF AMORPHOUS Nis.Fes2Cr14Pl~B. ALLOY (METGLAS 2826A)* T. KULIK, J. BASZKIEWlCZ, M. KAMINSKI, J. LATUSZKIEWlCZ and H. MATYJA Institute of Materials Science and Engineering, Warsaw Technical University, ul. Narbutta 85, 02-524 Warszawa, Poland Abstract--The resistance to electrochemical corrosion of alloy Metglas 2826A in the amorphous state and after crystallization was studied in non-aqueous and aqueous chloride solutions. The results were compared with the data obtained for alloy steels. Amorphous alloy Metglas 2826A was found to exhibit very high corrosion resistance, greatly exceeding that of high-alloy steels. After crystallization of this alloy, its corrosion resistance dropped to the level characteristic of alloy steels. The high stability of the passive film on the amorphous material was attributed to the homogeneity of alloy structure. INTRODUCTION THE CORROSIONresistance o f a m o r p h o u s alloys (metallic glasses) exposed to sulphuric acid and chlorides has been the object o f m a n y investigations. 1-4 Alloys containing more than 8 at.% c h r o m i u m and 13 at.% p h o s p h o r u s have been f o u n d to exhibit very high corrosion resistance. A n addition o f phosphorus to the chromiun~-containing alloy greatly enhances its corrosion resistance. The cause o f the very high corrosion resistance o f a m o r p h o u s alloys containing c h r o m i u m and phosphorus has not so far been fully elucidated. I n some papers ~"g the chemical composition o f the passive films on alloys containing Fe, Cr, P and C has been reported; this film was f o u n d to c o n t a i n - - a f t e r exposure in hydrochloride acid m e d i u m - - m a i n l y CrOx(OH)y, the p h o s p h o r u s content being only slight. Similar passive films occur on alloy steels. The high corrosion resistance o f a m o r p h o u s alloys o f this type has been attributed to the presence o f the passive film and to the structure o f the alloy, being free o f grain b o u n d a r i e s ) -~ The present study was designed to examine the behaviour o f the passive film on alloy NiaeFeaeCh4P~2Bs (Metglas 2826A) exposed to an aqueous and nonaqueous solution o f chloride ions, with the alloy in the a m o r p h o u s state and after its crystallization; the results were intended to be c o m p a r e d with the data obtained for alloy steels. EXPERIMENTAL METHOD Alloy Metglas 2826A was used in the form of a ribbon (3 mm wide and 0.055 mm thick), and its structure was examined by X-ray diffraction and TEM. s The uniformity of distribution of alloy components was determined on the cross-section and surface of the sample by means of an X-ray micro-analyzer. Samples were applied in amorphous state (a) and after two kinds of thermal treatment; (b) heating at a rate of 320 deg min-1 to 875 K; and (c) annealing for 9 h at 1100 K. Samples b and e were in crystalline state. As comparative materials ferritic steels were used: (S) containing 13 ~oCr, (T) containing 17 ~oCr and stabilized with titanium, and austerdtic steel (L) containing 17 %Cr, 12 ~oNi, 2 ~oMo and stabilized with titanium. *Manuscript received 31 January 1979. 1001
T. KULIK et al.
1002
....
0.1% NsCI+H10 3 . 0 % N a C I + H~O
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2"
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/'/ I
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j
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4~o
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1
8;o
~2oo CORROSION POTENTIAL, mV
Flo. 1.
Anodic polarization curves in aqueous NaCI solutions for alloy Ni86F.~.,Cr,PI=B6 in amorphous (a) and crystalline (b, c) state.
An alloy sample (3 x 8 ram) was mounted in epoxy resin so that its free side was exposed to the corrosive medium. After stabilization of corrosion potential (0.5 h) the samples were polarized anodically at a rate of 1000 mV h -1, and changes in anodic current were registered with a recorder. A saturated calomel electrode (SCE) was used as a reference electrode. The corrosive medium consisteA of 0.1 ~o or 3~o aqueous solutions of sodium chloride or else of a 5.3?/0 glycol solution of sodium chloride. In aqueous medium measurements were taken at room temperature, and in the non-aqueous medium at 323 K (50°C). EXPERIMENTAL
RESULTS
Results obtained for the aqueous NaCl solution are presented in Fig. 1. In the amorphous state (a) the investigated alloy was fully resistant to electrochemical corrosion in aqueous chloride solutions, whereas in the crystalline state (b and c) it corroded uniformly. Figure 2 shows the anodic curves obtained with glycol containing 5.3 ~oNaCl, for alloy Metglas 2826A and for alloy steels S, T and L. Corrosive attack is exemplified in Figs. 3-5. Metglas 2826A annealed isothermally (c) was dissolved uniformly in all solutions, this manifesting itself by the appearance of the surface structure (Fig. 3). The alloy heated continuously (b) was also uniformly destroyed in all solutions (Fig. 4), with two stages of dissolution. At the first stage the outer layer of metal dissolved (its remainder is visible in Fig. 4), whereupon the next layer dissolved somewhat more slowly. The chemical compositions of both layers were analysed comparatively with the use of an X-ray micro-analyser. The outer layer of metal contained less chromium, more nickel, more iron and less phosphorus, as compared with the less soluble matrix. The decrease in chromium and phosphorus contents is accounted for by somewhat lower corrosion resistance of the surface as compared with the matrix. The observed phenomenon is difficult to interpret. One of the possible explanations
FIG. 3. Alloy Metglas 2826A in crystalline state (T = 1100 K, t = 9 h) after the occurrence of electrochemical corrosion in a saturated NaC1 solution in glycol. FI6.4. Alloy Metglas 2826A in crystalline state (T = 825 K, dT/dt = 320 K rain -1) after the occurrence of electrochemical corrosion in a saturated NaCI solution in glycol (corrosion current density 10 mA cm-~).
FIo. 5. Alloy Metglas 2826A in amorphous state after the occurrence of electrochemical corrosion in a saturated NaC1 solution in glycol [current density: (a) 6.25 mA cm -2, (b) 12.5 mA cm-2].
Electrochemical corrosion of amorphous Nia6Fes~CrI4P~zB6ahoy ~
.
C
1005
b
3
°i
S T
L
!
/
/
j •
f-~
i
0
o
L
i
400
8(~0
200 CORROSION POTI=NTIAL, mV
FxG. 2. Anodic polarization curves in a saturated NaC1 solution in glycol for alloy Ni.Fes~Cr14P]2B6 in amorphous (a) and crystalline (b, c) state, and for alloy steels (S, T, L). can be found in Ref. 6; it involves selective dissolution of iron and nickel in the case of alloy steels in passive state. The investigated alloy in the amorphous state (a) corroded only in a non-aqueous glycol solution of sodium chloride (Fig. 5); however, it exhibited a very high breakdown potential. The observed course of corrosive attack could be divided into two stages. At the initial stage corrosion was o f a local nature which was reflected by the formation of a few shallow and symmetrical holes, mainly localized on scratches on the ribbon surface. However, these holes were formed also at the sites free of scratches. The holes did not grow any deeper but their number steadily increased. As a result after a long time corrosive attack was nearly uniform. CONCLUSION Alloy Metglas 2826A in the amorphous state exhibits a high corrosion resistance exceeding that of high-alloy steels; after crystallization, its corrosion resistance approaches that of alloy steels. The resistance o f the passive film formed on the surface of the amorphous alloy decreases in the non-aqueous medium, as has been found for alloy steels.7, s This is confirmed by findings concerning the beneficial effect of water on formation of passive films?-4,s The high stability of passive films on amorphous materials can be attributed to the uniformity of the alloy structure. Acknowledgement--This work was supported by Ministry of Science,High Education and Technology
(Grant No. MR.I-21). REFERENCES 1. M. NAKA,K. HASHIMOTOand T. MASUMOTO,Japan Inst. Metals 38, 835 (1974). 2. K. HASHIMOTOand T. MASUMOTO,Mater. Sei. Engng 23, 285 (1976).
1006 3. 4. 5. 6.
T. KULIK et al.
K. ASAMI,K. HASHIMOTO,T. MASUMOTOand S. SHIMODAIRE,Corros. Sci. 16, 909 (1976). M. NAKA, K. HASHIMOTOand T. MASUMOTO,Sci. Rep. ~itu A, 26, 283 (1977). T. KUUK, J. LATUSZKIEWlCZand H. MATYJA,Archwm Hutn. 24, 103 (1979). M. PRA~AK, n . KUCHYNKAand T. SEN, 6th European Congress on Metallic Corrosion, p. 287. Chemical Society, London (1977). 7. A. SZUMMER,J. BASZKIEWICZ,M. KAmNSKI, D. KRUPA, Ochr. przed Koroz]a. 2, 29 (1977). 8. K. O. SATO, Corros. Sci. 8, 809 (1968).