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ZE41 Mg metal-matrix composite in 0.5 M NaNO3
Corrosion Science, Vol. 34, No. 11, pp. 1761-1772, 1993 Printed in Great Britain.
0010-938X/93 $6.00 + 0.00 © 1993 Pergamon Press Ltd
THE GALVANIC CORROSION O F SiC M O N O F I L A M E N T / ZE41 Mg METAL-MATRIX C O M P O S I T E I N 0.5 M N a N O 3 L. H. HIHARAand P. K. KONDEI'UDI Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, U.S.A.
Abstract The effect of composite constituents and dissolved 0 2 on the corrosion behavior of a SiC monofilament/ZE41 Mg metal-matrix composite (MMC) exposed to 0.5 M NaNO 3 was studied. The SiC monofilament (MF) (which consisted of a carbon core, a layer of chemical-vapor deposited SiC, and an outer carbon-rich surface) supported cathodic current densities comparable to that of graphite. The corrosion of pure Mg and a ZE41A Mg alloy was not affected by dissolved 0 2 . Corrosion rates of the MMC, however, increased with dissolved 0 2 since the MFs were effective 02-reduction sites, which enchanced galvanic corrosion.
INTRODUCTION
MAGNESIUM metal-matrix composites (MMCs) are structural materials that are strong, stiff and light weight, and therefore, have potential application in the aerospace I and automotive industries. 2 Magnesium, however, is the most active structural metal in the electromotive force s e r i e s , 3 and has a strong tendency to corrode. Dissolved 02 in solution does not significantly affect the corrosion of Ms. 3 Corrosion rates are highly dependent on metallic purity. 3 Noble impurity elements that have low hydrogen overvoltages (e.g. Fe, Ni, Co, and Cu) 4 serve as efficient cathodic sites, which accelerate the corrosion rate of Ms. In sea-water, pure Mg (purified by vacuum sublimation) corrodes at the rate of 0.25 m m y - l , but commercial Mg corrodes at about 100-500 times faster due to impurities. 3 Since the corrosion rate of Mg is highly dependent on the presence of cathodic sites, the incorporation of noble fibers and particles into Mg MMCs can result in severe galvanic corrosion. Czyrklis5 and Trzaskoma6 have shown that galvanic corrosion is significant in graphite/Ms (Gr/Mg) MMCs. In de-aerated 50 ppm chloride-containing solutions at 25°C, Gr/Mg MMCs corrode about 40 times faster than the matrix alloy. 5 Timonova et al. 7 have shown that the galvanic-corrosion rate also depends on the type of reinforcement constituent. They reported that the galvanic-corrosion rate of an Mg alloy could be about 1000 times higher when coupled to carbon filaments than when coupled to as-manufactured, tungsten-core, boron filaments in 0.005 N NaCI [it was not specified if the solution was de-aerated or aerated). In this study, the corrosion behavior of an experimental SiC monofilament/ZE41 Mg MMC was investigated using potentiodynamic polarization and the zeroresistance ammeter (ZRA) techniques. The SiC was in the form of a monofilament (MF) having a carbon core and carbon-rich surface. The objectives of the study were to determine the role of each MMC constituent and dissolved O2 on MMC corrosion behavior. Experiments were conducted in de-aerated and oxygenated 0.5 M NaNO3 Manuscript received 15 September 1992; in amended form 3 April 1993. 1761
1762
L . H . HIHARAand P. K. KONDEPUDI
a t 30°C. S o d i u m n i t r a t e w a s s e l e c t e d as t h e e l e c t r o l y t e s i n c e it is a n e u t r a l , n o n oxidizing salt which does not significantly affect the corrosion of Mg. 4 Galvaniccorrosion studies for NaCI, and Na2SO 4 have also been conducted. 8 Monolithic ZE41A Mg, pure (99.95%) Mg, SiC MF, hot-pressed SiC, pitch-based graphite fiber, and the SiC MF/ZE41 Mg MMC were examined. The corrosion behavior of Z E 4 1 A M g w a s c o m p a r e d t o t h a t o f p u r e M g t o d e t e r m i n e if t h e a l l o y i n g e l e m e n t s have a significant effect on corrosion behavior. Hot-pressed SiC and graphite fibers w e r e a l s o s t u d i e d t o d e t e r m i n e if t h e S i C M F s ( w h i c h a r e a c t u a l l y c a r b o n / S i C composites) are electrochemically similar to SiC, graphite or neither. EXPERIMENTAL METHOD Silicon carbide MF/ZE41 Mg M M C electrodes The SiC MF/ZE41 Mg MMCs were produced by Textron Specialty Materials (Lowell, MA) for experimental purposes. The MMCs were in rod form having diameters equal to about 0.43 cm. The SiC MFs were aligned uniaxially along the length of the rod with a volume fraction of about 0.49. The MMCs were aged at 329°C for 2 h to achieve the T5 temper. To make planar electrodes, the MMC rod was sectioned into disks. One surface of the disks was coated with silver paint to make electrical contact with a copper wire lead. The sides and silver-coated surface of the disks were then sealed with an epoxy adhesive (Epoxy-patch of Dexter Corp., Seabrook, NH). The front planar surface was left bare and served as the electrode surface. The SiC MFs were oriented perpendicularly to the electrode surface. The area fraction of exposed SiC MF was approximately 0.49. Pure Mg and ZE41A Mg electrodes Planar pure Mg (i.e. 99.95% metallic purity) and ZE41A Mg electrodes were fabricated from rods using the same method described above for the MMC. The nominal composition of the ZE41A Mg alloy is 4.2 wt% Zn, 0.7 wt% Zr, 1.2 wt% rare-earth elements, and a balance of Mg. The alloy was heat treated under conditions identical to that of the MMC. The surface area of the ZE41A Mg electrodes was 0.81 cm 2. Two sizes of pure Mg electrodes were used: a 1.27-cm2 electrode was used for the polarization experiments and a 0.023-cm2 electrode was used for the Z R A experiments to measure galvanic currents between pure Mg and SiC MFs (cross-sections exposed). It was necessary to use a small 0.023-cm2 Mg electrode in order to maintain a 0.5 area fraction of Mg due to the small size of the SiC MF electrodes (cross-section exposed) (see description of SiC MF electrodes below). Graphite electrodes Planar graphite electrodes were fabricated with Thornel P-100 graphite fibers (Union Carbide Co., Danbury, CT), which are unidirectional, continuous, about 10/~m in diameter, and pitch-based. Fifteen tows of the fiber (about 2000 fibers/tow) were aligned unidirectionally and infiltrated with an epoxy resin (Epon 828 resin of Miller-Stephenson Chemical Co. Inc., Danbury, CT). The resulting product, a graphite/epoxy composite rod, was cut into disks by sectioning the rod perpendicularly to the axis of the fibers. The disks were then made into electrodes by silver painting the back side to make electrical contact with a copper-wire lead. The silver paint and copper wire were then coated with Epoxy-patch. The total cross-sectional surface area of the graphite fibers was about 0.024 cm 2 Silicon carbide M F electrodes Planar SiC MF electrodes were fabricated with Textron SiC MF, which are unidirectional, continuous, and about 140/zm in diameter. The SiC MF is comprised of a 33-/~m diameter, pitch-based carbon fiber core surrounded by chemical-vapor-deposited SiC with an outer, carbon-rich surface. A total of 147 SiC MFs were bundled and then processed in an identical manner to the graphite-fiber planar electrodes described above. The total cross-sectional surface area of the SiC MFs was about 0.023 cm2 . Note that in these planar electrodes, only the cross-sectional filament surface is exposed. To study the electrochemical properties of the carbon-rich surface of the SiC MFs, electrodes baring only the carbon-rich circumferential surface were prepared. Silicon carbide MFs were cut into lengths of about 3 cm. Only the circumferential surface near each end of the MFs was silver painted onto a copperwire lead to make electrical contact. The electrical-contact area and the copper-wire lead were then sealed
Magnesium metal-matrix composite
1763
in Epoxy-patch so that only the circumferential surface of the MFs was exposed. A sufficient number of SiC MFs was used so that the total circumferential surface area was about 0.79 cm 2.
Hot-pressed silicon carbide electrodes Planar electrodes were fabricated from bars of SiC that were produced by Ceradyne, Inc., Costa Mesa, CA. The SiC was hot-pressed to near theoretical densities (>98%) without sintering aids or binders. The SiC bars were cut into 9 x 9 ram-square wafers about 1 mm thick. The entire back side of the SiC wafers was silver painted to make electrical contact with a copper-wire lead. This procedure was followed to ensure that the ohmic drop through the SiC wafer would be uniform over the electrode face during polarization experiments. The resistance through the thickness of the SiC wafers was about 1 kl), and thus, ohmic losses were calculated to be less than about 0.1 V for current densities (CDs) less than 10 -4 A cm 2. The sides and silver-coated surface of the wafers were then sealed with Epoxy-patch. The front planar surface was left bare and served as the electrode surface.
Aqueous solutions Near neutral 0.5 M NaNO 3 solutions were prepared from 18 Mfl cm water and analytical grade NaNO 3. The solutions were kept at 30 __+ 0.1°C, and de-aerated with high-purity N2 (99.9%) or oxygenated with high-purity 0 2 (99.6%). Gas pressure was 1 atm.
Electrochemical experiments Potentiodynamic polarization experiments were conducted with a Model 273 PAR potentiostat/ galvanostat (EG&G, Princeton, N J). The electrodes were stabilized at their corrosion potential (Ecorr) prior to polarization. Stabilization times typically ranged from 30 to 90 min for all electrodes except the SiC MF/ZE41 Mg MMCs. The MMCs were stabilized for less than 15 rain to prevent excessive corrosion prior to polarization. Potentials were measured against a saturated calomel electrode. All electrodes were polarized at a rate of 0.1 mV s- 1 except the SiC MF/ZE41 Mg MMC electrodes, which were polarized at a rate of 1 mV s i. For the Mg electrodes, sweep-rate effects were not observed for rates less than I mVs- 1. To be conservative, 0. ! mV s - 1sweep rates were used. This sweep rate was also used for the graphite, SiC, and SiC MF electrodes to maintain consistency. The sweep rate for the MMC electrode was set at 1 mV s- 1 to avoid excessive corrosion of the matrix. At low sweep-rates, matrix corrosion becomes excessive, leaving the SiC MFs in relief, which increases the SiC-MF area ratio. Each polarization experiment was performed three times: twice on one electrode and once on another to verify reproductibility. The surface of all planar electrodes was polished to a 0.05/~m finish with gamma alumina powder, kept wet, and rinsed with 18 MI) cm water prior to each experiment. To generate polarization diagrams, the logarithm of the CD was averaged over three experiments and plotted as a function of potential. The galvanic current (Igalv) was measured between pure Mg and SiC MF (cross-section exposed) electrodes of equal surface area (i.e. 0.023 cm2) with a ZA 100 zero-shunt ammeter (Intertech Systems, Inc., San Jose, CA). The 0.50 area fraction of SiC MF was used so that the results could be compared to that of the actual MMC which had a 0.49 area fraction of exposed SiC MF. The galvanic current was normalized with respect to the Mg area to obtain the galvanic-corrosion CD (ig~lv). The value of/gaJv was plotted as a function time for each experiment.
Corrosion morphology Scanning electron microscopy (SEM) was used to characterize the corrosion morphology of the MMC. The corrosion product on the MMC was removed using a boiling solution of 15% CrO3 and 1% AgNO3 . The solution does not attack Mg, but removes the corrosion product by dissolving Mg(OH)2. EXPERIMENTAL RESULTS The anodic and cathodic polarization diagrams of pure Mg and ZE41A Mg are p l o t t e d t o g e t h e r in F i g . 1 f o r b o t h d e - a e r a t e d a n d o x y g e n a t e d 0.5 M N a N O 3 a t 30°C. D a t a f r o m F i g . 1 w e r e u s e d t o o b t a i n t h e n o r m a l c o r r o s i o n C D (icorr) f o r p u r e M g a n d ZE41A Mg by Tafel extrapolation. I n F i g . 2, t h e a n o d i c p o l a r i z a t i o n d i a g r a m s o f p u r e M g a n d Z E 4 1 A M g a r e p l o t t e d with the c a t h o d i c p o l a r i z a t i o n d i a g r a m s of SiC M F (cross-section e x p o s e d and fiber surface exposed), hot-pressed SiC, and P-100 graphite fiber (cross section exposed)
1764 .1
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log I Wcm =] FIG. 1.
Polarization diagrams of pure Mg and ZE41A Mg exposed to de-aerated and oxygenated 0.5 M NaNO3 at 30°C. Scan rate = 0.1 mV s -1.
for de-aerated 0.5 M NaNO3 at 30°C. Figure 3 corresponds to oxygenated solutions, but is otherwise similar to Fig. 2. The data from Figs 2 and 3 were used to predict iGALV of Mg coupled to equal areas of other materials. In Fig. 4, anodic and cathodic polarization diagrams of the SiC MF/ZE41 Mg MMC are plotted for both de-aerated and oxygenated 0.5 M NaNO3 at 30°C. Data from Fig. 4 were used to estimate the corrosion CD of the matrix (icorr,mat~ix)" Figure 5 shows iGALV measured as a function of time using the Z R A technique for pure Mg coupled to SiC MF (cross-section exposed and fiber surface exposed), hot-
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FIG. 2. Anodic polarization diagrams of pure Mg and ZE41A Mg plotted with the cathodic polarization diagrams of hot-pressed SiC, SiC MF, and P-100 graphite exposed to de-aerated 0.5 M NaNO 3 at 30°C. Scan rate = 0.1 mV s -1.
1765
M a g n e s i u m metal-matrix composite
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Fro. 3. Anodic polarization diagrams of pure Mg and Z E 4 1 A Mg plotted with the cathodic polarization diagrams of hot-pressed SiC, SiC MF, and P-100 graphite exposed to oxygenated 0.5 M NaNO3 at 30°C. Scan rate = 0.1 m V s -1.
pressed SiC, and P-100 graphite fiber (cross-section exposed) for de-aerated 0.5 M NaNO3 at 30°C. Figure 6 shows igalv for oxygenated solutions. Each curve corresponds to a single experiment. An SiC MF/ZE41 Mg MMC was exposed to oxygenated 0.5 M NaNO3 for 5 days in the open-circuit condition. Corrosion of the MMC was extensive as shown in the SEM micrograph in Fig. 7. The extent of corrosion was clearly revealed by removing the corrosion product (Fig. 8).
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Polarization diagrams of SiC MF/ZE41 Mg M M C exposed to 0.5 M N a N O 3 at 30°C. Scan rate = 1 m V s -1.
1766
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Galvanic-corrosion CD vs time for pure Mg coupled to hot-pressed SiC, SiC MF, and P-100 graphite exposed to de-aerated 0.5 M NaNO3 at 30°C.
DISCUSSION
The effects of MMC constituents and dissolved 02 on corrosion behavior of SiC MF/ZE41 Mg MMC were deduced from polarization and Z RA data. The corrosion and electrochemical properties of the MMC constituents are discussed first to develop an understanding of how the constituents behave individually. Then, the corrosion behavior of the MMC and the galvanic interaction between the constituents are discussed.
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Galvanic-corrosion CD vs time for pure Mg coupled to hot-pressed SiC, SiC MF, and P-100 graphite exposed to oxygenated 0.5 M NaNO3 at 30°C.
FIG. 7.
SEM micrograph of an SiC MF/ZE41 Mg M M C that was exposed to oxygenated 0.5 M N a N O 3 at 30°C for 5 days.
FIG. 8. SEM micrograph of M M C shown in Fig. 7 after corrosion product was removed in a boiling solution of 15% CrO 3 and 1% AgNO3, which dissolves corrosion products but not Mg metal. 1767
Magnesiummetal-matrixcomposite
1769
Electrochemical properties of the constituents The corrosion of Mg is not affected significantly by the presence of 02. 3 That is exemplified by the polarization diagrams of pure Mg and ZE41A Mg immersed in 0.5 M NaNO3 (see Fig. 1), which are virtually unaffected by solution oxygenation since the proton reduction kinetics is significantly greater than that of 02 reduction. For pure Mg immersed in either de-aerated or oxygenated 0.5 M NaNO3, icorr obtained by Tafel extrapolation was about 3 × 10 -4 A cm -2. The average values of icorr for ZE41A Mg were approximately 4 x 10 -4 A cm-2 and 3 × 10 -4 A cm-2 in de-aerated and oxygenated 0.5 M NaNO3, respectively. The slightly higher values corresponding to the de-aerated solution was attributed to normal variation in data and not to a different corrosion mechanism since the shape of the polarization curves in the deaerated and oxygenated solutions was similar. The alloying elements in ZE41A Mg had the effect of increasing cathodic CDs (in comparison to pure Mg). The alloying elements did not have significant effects on anodic polarization behavior, and thus, pure Mg and ZE41A Mg are likely to have very similar galvanic-corrosion rates when coupled to cathodic reinforcements (see Figs 2 and 3). The SiC MF is comprised of a carbon-filament core surrounded by chemicalvapor-deposited SiC with an outer, carbon-rich surface. Since the SiC MF is actually a composite comprised of SiC and carbon, it was of particular interest to determine if the SiC MFs are electrochemically similar to SiC, graphite, or neither. The SiC MFs were compared to hot-pressed SiC and P-100 graphite fiber (cross-section exposed). The cross-sectional surface baring the carbon core of the SiC MFs and the carbonrich outer surface were examined independently. In de-aerated 0.5 M NaNO3 (see Fig. 2), the cathodic polarization curve of the cross-sectional SiC-MF surface was very similar in shape and in CD to that of P-100 graphite fiber, indicating that both materials have similar H ÷ reduction kinetics. The cathodic polarization curve of the carbon-rich SiC MF surface was also similar in shape to that of P-100 graphite, but CDs were about 1 to 2 decades less at a given potential. The cathodic curve of hotpressed SiC was different in shape from the other materials, and had lowest CDs when potentials were less than about -1.2 V(SCE). In Fig. 2, at potentials noble to approximately - 0 . 9 V(SCE), cathodic currents are most likely caused by diffusionlimited 02 reduction due to traces of dissolved 02. In oxygenated 0.5 M NaNO3 (see Fig. 3), the reduction of 02 caused cathodic CDs of all materials to increase. The polarization curves show an initial Tafel regime followed by a diffusion-limited, O2-reduction regime. The highest cathodic CDs were observed for the P-100 graphite fibers. The CDs for P-100 graphite and the cross-sectional and carbon-rich surfaces of the SiC MFs were greater than 10 -3 A cm -2 in the diffusion-limited, O2-reduction regime; however, that of hot-pressed SiC was about a decade less. In both de-aerated and oxygenated solutions, the carbon core and carbon-rich surface of the SiC MFs apparently caused the cathodic curves of the SiC MFs to show stronger resemblance to that of P-100 graphite than that of hot-pressed SiC.
MMC corrosion The corrosion mechanisms of the MMC were studied using polarization data. The MMC corrosion rate was determined using polarization data of the MMC, polarization data of the constituents, and ZRA data. The SiC MF/ZE41 Mg MMC corroded at slightly higher rates (as determined by
1770
U H. HIHARAand P. K. KONDEPUDI
Tafel extrapolation) in oxygenated solutions (see Fig. 4) compared to de-aerated solutions. The corrosion CD of the matrix i corr,matrixin oxygenated 0.5 M NaNO3 was about 1.6 x 10 -3 A cm -2, which is approximately three times the value for the de-aerated solution. The value of icorr,matrix is twice the corrosion CD of the MMC since icorr,matrix is normalized with respect to the matrix area, which is one-half of MMC area due to the 50 vol. % content of SiC MFs. The increase in MMC corrosion rate in oxygenated solutions could not be attributed to 02 reduction occurring on the matrix since the cathodic CDs of monolithic ZE41A Mg did not increase in the presence of dissolved 02 (see Fig. 1). The increase in MMC corrosion rate could be attribute to 02 reduction occurring on the SiC MFs since dissolved 02 significantly increased the cathodic CDs of the SiC MFs (compare Figs 2 and 3). Galvanic corrosion between the ZE41 Mg matrix and SiC MFs was, therefore, a primary mechanism in MMC corrosion in oxygenated solutions. The total corrosion of the MMC matrix can also be approximated from polarization and Z R A data of the MMC constituents. The total corrosion of an anode in a galvanic couple results from galvanic corrosion between the anode and cathode plus additional simultaneous corrosion of the anode, which is called local corrosion, caused by cathodic reactions that occur on the anode. Therefore, the total corrosion CD of the MMC matrix icorr,matrix is equal to the local component /local plus the galvanic component igalvIt was assumed that the local component/local could be estimated by setting it equal to icorr of uncoupled, monolithic ZE41A Mg, implying that the difference effect9 is very slight. This estimation was made because the potential of ZE41A Mg should shift only slightly from its open-circuit potential if the alloy is galvanically coupled to SiC MF (see Figs 2 and 3). A very small shift in potential should not significantly affect cathodic-reaction kinetics of ZE41A Mg, and therefore, /local should not be significantly less than ico~r. Accordingly, the values of /local were estimated to be about 3 x 10 -4 A cm -2 in the oxygenated solution and about 4 x 10 -4 A cm -2 in the de-aerated solution. The galvanic component igalv was determined from polarization diagrams using the mixed-potential theory (Figs 2 and 3) and from Z R A experiments (Figs 5 and 6). Since the anodic polarization curves of pure Mg and ZE41A Mg were very similar (indicating that galvanic-corrosion behavior will also be similar), the Z R A results which were obtained using pure Mg were also used to predict galvanic-corrosion rates of ZE41A Mg. In Figs 2 and 3, the anodic polarization diagrams of pure Mg and ZE41A Mg are plotted with the cathodic polarization diagram of SiC MF (crosssection exposed). The CD at the intersection of the anodic and cathodic curves is the value of igalv for a galvanic couple having equal anode and cathode areas. The value of igal~ obtained using this procedure corresponds to an MMC having a 0.5 area fraction of SiC MFs with their cross-sections exposed. For oxygenated 0.5 M NaNO3 (Fig. 3), the mixed-potential theory predicts that igalvfor ZE41A Mg coupled to SiC MF (cross-section exposed) will be equal to about 3.2 × 10 -3 A cm -2, which is in fairly good agreement to the steady-state value of 1.3 × 10 -3 A cm -2 obtained from the Z R A experiment for pure Mg coupled to SiC MF (cross-section exposed). For de-aerated 0.5 M NaNO3, the mixed-potential theory predicts that igalvwill be equal to about 6 × 10 -4 A cm -2, which is closely bracketed by two steady-state values obtained from the Z R A experiments (see Fig. 5). Values of icorr,matrix that were obtained from (1) polarization data of the MMC,
M a g n e s i u m metal-matrix composite
1771
(2) polarization data of the constituents and (3) ZRA data are tabulated in Table 1 for comparison. The.penetration rate of the matrix was calculated in cm y-1 from icorr,matrix values using Faraday's law (Table 1). Although there is some variation in values based on the three methods, penetration rates in the oxygenated solutions are consistently greater than those in the de-aerated solutions. In oxygenated 0.5 M NaNO3, penetrations rates varied from about 3.7 to 8.0 cm y - l , which are about five to 11 times that of uncoupled, monolithic ZE41A Mg. In de-aerated 0.5 M NaNO3, penetrations rates varied from about 1.4 to 2.7 cm y-l, which are about two to three times that of uncoupled, monolithic ZE41A Mg. The very high penetration rates indicate that these materials should not be used unprotected in moist environments. An SiC MF/ZE41 Mg MMC corroded severely in the open-circuit condition during a 5-day exposure period to oxygenated 0.5 M NaNO3 at 30°C. The surface of the MMC was encrusted with corrosion product (Fig. 7). Removal of the corrosion product revealed significant loss of metal (Fig. 8). Galvanic corrosion might be reduced by thoughtful materials selection. Galvanic corrosion in Mg MMCs is cathodically controlled, and thus, galvanic-corrosion rates could be reduced by selecting reinforcement constituents that sustain low cathodic CDs. In oxygenated solutions, tga~vcould be limited to about 10 -4 A cm -2 with hot-pressed SiC; however, it could be as high as 3.2 x 10 -3 A cm -2 with P100 graphite or SiC MFs (see Fig. 3). In de-aerated 0.5 M NaNO3, igalv could be limited to
TABLE 1.
CORROSIONC D s
A N D PENETRATION RATES IN SODIUM NITRATE SOLUTIONS
AT
30°C
Technique
/local* (A cm -2)
igalvt (A cm -2)
icorr.rnatrix t (A cm 2)
Penetration rate of matrix (cm y - i )
--
--
1.6 × 10 3
3.7
3 X 10 -4
3.2 × 10 3
3.5 x 10 -3
8.0
3 x 10 4
1.3 × 10 -3
1.6 x 10 -3
3.7
--
--
6 × 10 -4
1.4
4 × 10 -4
6 × 10 -4
1.0 × 10 -3
2.3
4 x 10 -4
8 x 10-4:[:
1.2 x 10 -3
2.7
Oxygenated solutions Tafel extrapolation of M M C polarization diagram Mixed-potential Theory ZRA De-aerated solutions Tafel extrapolation of M M C polarization diagram Mixed-potential Theory ZRA
*All values in this column were obtained from polarization diagrams of Z E 4 1 A Mg using Tafel extrapolation. tigalv and /corr,matrix correspond to galvanic couples and M M C s with 50% area fraction of SiC MF (cross-sections exposed). ~average of two values. Note: the penetration rates of monolithic Z E 4 1 A Mg in de-aerated and oxygenated 0.5 M N a N O 3 at 30°C were 0.9 and 0.7 cm y - i , respectively.
1772
L . H . HIHARAand P. K. KONDEPUDI
about 10 -5 A c m -2 (a fraction of icorr of Mg) with hot-pressed SiC (see Fig. 2). Galvanic-corrosion CDs would be about 100 times higher if P100 graphite or SiC MFs were used. These igalvvalues were in fairly good agreement with those measured from actual galvanic couples using the Z R A technique (see Figs 5 and 6). Thus, fabricating Mg MMCs with SiC of electrochemical properties similar to that of hotpressed SiC could result in SiC/Mg MMCs that possess corrosion resistance approaching that of the matrix alloy. Ideally, reinforcement materials should impart beneficial properties such as strength and stiffness to the composite without sacrificing corrosion resistance. CONCLUSIONS
The cathodic CDs of the SiC MFs were generally much greater than that of hotpressed SiC and closer to that of P-100 graphite fiber. The carbon core and carbonrich surface of the SiC MFs apparently caused the MFs to behave more like graphite than SiC. The SiC MF/ZE41 Mg MMCs corroded at higher rates in oxygenated 0.5 M NaNO3 compared to de-aerated 0.5 M NaNO3. That is uncharacteristic of the corrosion behavior of Mg and its alloys which is normally unaffected by the presence of dissolved O2. The SiC MFs, which are inert electrodes upon which H ÷ and 02 reduction may occur, induced galvanic corrosion in the MMC. The reduction of 02 on SiC MFs caused MMC corrosion rates to be higher in oxygenated solutions compared to de-aerated solutions, where galvanic corrosion was driven by H + reduction. The fabrication of Mg MMCs from SiC that is electrochemically similar to hot-pressed SiC could significantly reduce galvanic-corrosion rates in these materials since hot-pressed SiC sustained significantly lower cathodic CDs compared to the SiC MFs. Acknowledgements--The financial support of the National Science Foundation (NSF) (grant # DMR9057264) is gratefully acknowledged. The authors are particularly grateful to Dr B. A. MacDonald of NSF. The contributions of MMC materials from Mr M. A. Mittnick of Textron Specialty Materials, and Mg alloys from Mr R. H. Emerson of Magparts are gratefully acknowledged. REFERENCES 1. W.C. HARRIGAN,JR., MetalMatrix Composites: Processing and Interfaces (eds R. K. EVERETTand R. J. ARSENAULT), p. 1. Academic Press, San Diego, CA (1991). 2. S. R. LAMPMAN,Adv. Mat. Processes 139, (5), 17 (1991). 3. H. H. UHLIG and R. W. REVIE, Corrosion and Corrosion Control, 3rd Edn. John Wiley and Sons, New York (1985). 4. G. BUTLER and H. C. K. ISON, Corrosion and its Prevention in Waters, p. 91. Robert E. Krieger, Huntington, New York (1978). 5. W. F. CZYRKLIS, Corrosion~85, Paper No. 196. National Association of Corrosion Engineers, Houston, Texas (1985). 6. P. P. TRZASKOMA. Corrosion 42,609 (1986). 7. M. A. TIMONOVA G. I. SPIRYAKINA,V. F. STROGANOVA,and L. A. ZOLOTAREVA,Metallovedenie i Termicheskaya Obrabotka Metallov 11, 33 (1980). 8. L. n . HIHARA and P. K. KONDEPUDI, Corros. Sci. (1993) (submitted). 9. W. A. WESLEY and R. H. BROWN, Corrosion Handbook (ed. H. H. UHLIG), p. 481. John Wiley and Sons, New York (1948).
0010-938X/93 $6.00 + 0.00 © 1993 Pergamon Press Ltd
THE GALVANIC CORROSION O F SiC M O N O F I L A M E N T / ZE41 Mg METAL-MATRIX C O M P O S I T E I N 0.5 M N a N O 3 L. H. HIHARAand P. K. KONDEI'UDI Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, U.S.A.
Abstract The effect of composite constituents and dissolved 0 2 on the corrosion behavior of a SiC monofilament/ZE41 Mg metal-matrix composite (MMC) exposed to 0.5 M NaNO 3 was studied. The SiC monofilament (MF) (which consisted of a carbon core, a layer of chemical-vapor deposited SiC, and an outer carbon-rich surface) supported cathodic current densities comparable to that of graphite. The corrosion of pure Mg and a ZE41A Mg alloy was not affected by dissolved 0 2 . Corrosion rates of the MMC, however, increased with dissolved 0 2 since the MFs were effective 02-reduction sites, which enchanced galvanic corrosion.
INTRODUCTION
MAGNESIUM metal-matrix composites (MMCs) are structural materials that are strong, stiff and light weight, and therefore, have potential application in the aerospace I and automotive industries. 2 Magnesium, however, is the most active structural metal in the electromotive force s e r i e s , 3 and has a strong tendency to corrode. Dissolved 02 in solution does not significantly affect the corrosion of Ms. 3 Corrosion rates are highly dependent on metallic purity. 3 Noble impurity elements that have low hydrogen overvoltages (e.g. Fe, Ni, Co, and Cu) 4 serve as efficient cathodic sites, which accelerate the corrosion rate of Ms. In sea-water, pure Mg (purified by vacuum sublimation) corrodes at the rate of 0.25 m m y - l , but commercial Mg corrodes at about 100-500 times faster due to impurities. 3 Since the corrosion rate of Mg is highly dependent on the presence of cathodic sites, the incorporation of noble fibers and particles into Mg MMCs can result in severe galvanic corrosion. Czyrklis5 and Trzaskoma6 have shown that galvanic corrosion is significant in graphite/Ms (Gr/Mg) MMCs. In de-aerated 50 ppm chloride-containing solutions at 25°C, Gr/Mg MMCs corrode about 40 times faster than the matrix alloy. 5 Timonova et al. 7 have shown that the galvanic-corrosion rate also depends on the type of reinforcement constituent. They reported that the galvanic-corrosion rate of an Mg alloy could be about 1000 times higher when coupled to carbon filaments than when coupled to as-manufactured, tungsten-core, boron filaments in 0.005 N NaCI [it was not specified if the solution was de-aerated or aerated). In this study, the corrosion behavior of an experimental SiC monofilament/ZE41 Mg MMC was investigated using potentiodynamic polarization and the zeroresistance ammeter (ZRA) techniques. The SiC was in the form of a monofilament (MF) having a carbon core and carbon-rich surface. The objectives of the study were to determine the role of each MMC constituent and dissolved O2 on MMC corrosion behavior. Experiments were conducted in de-aerated and oxygenated 0.5 M NaNO3 Manuscript received 15 September 1992; in amended form 3 April 1993. 1761
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L . H . HIHARAand P. K. KONDEPUDI
a t 30°C. S o d i u m n i t r a t e w a s s e l e c t e d as t h e e l e c t r o l y t e s i n c e it is a n e u t r a l , n o n oxidizing salt which does not significantly affect the corrosion of Mg. 4 Galvaniccorrosion studies for NaCI, and Na2SO 4 have also been conducted. 8 Monolithic ZE41A Mg, pure (99.95%) Mg, SiC MF, hot-pressed SiC, pitch-based graphite fiber, and the SiC MF/ZE41 Mg MMC were examined. The corrosion behavior of Z E 4 1 A M g w a s c o m p a r e d t o t h a t o f p u r e M g t o d e t e r m i n e if t h e a l l o y i n g e l e m e n t s have a significant effect on corrosion behavior. Hot-pressed SiC and graphite fibers w e r e a l s o s t u d i e d t o d e t e r m i n e if t h e S i C M F s ( w h i c h a r e a c t u a l l y c a r b o n / S i C composites) are electrochemically similar to SiC, graphite or neither. EXPERIMENTAL METHOD Silicon carbide MF/ZE41 Mg M M C electrodes The SiC MF/ZE41 Mg MMCs were produced by Textron Specialty Materials (Lowell, MA) for experimental purposes. The MMCs were in rod form having diameters equal to about 0.43 cm. The SiC MFs were aligned uniaxially along the length of the rod with a volume fraction of about 0.49. The MMCs were aged at 329°C for 2 h to achieve the T5 temper. To make planar electrodes, the MMC rod was sectioned into disks. One surface of the disks was coated with silver paint to make electrical contact with a copper wire lead. The sides and silver-coated surface of the disks were then sealed with an epoxy adhesive (Epoxy-patch of Dexter Corp., Seabrook, NH). The front planar surface was left bare and served as the electrode surface. The SiC MFs were oriented perpendicularly to the electrode surface. The area fraction of exposed SiC MF was approximately 0.49. Pure Mg and ZE41A Mg electrodes Planar pure Mg (i.e. 99.95% metallic purity) and ZE41A Mg electrodes were fabricated from rods using the same method described above for the MMC. The nominal composition of the ZE41A Mg alloy is 4.2 wt% Zn, 0.7 wt% Zr, 1.2 wt% rare-earth elements, and a balance of Mg. The alloy was heat treated under conditions identical to that of the MMC. The surface area of the ZE41A Mg electrodes was 0.81 cm 2. Two sizes of pure Mg electrodes were used: a 1.27-cm2 electrode was used for the polarization experiments and a 0.023-cm2 electrode was used for the Z R A experiments to measure galvanic currents between pure Mg and SiC MFs (cross-sections exposed). It was necessary to use a small 0.023-cm2 Mg electrode in order to maintain a 0.5 area fraction of Mg due to the small size of the SiC MF electrodes (cross-section exposed) (see description of SiC MF electrodes below). Graphite electrodes Planar graphite electrodes were fabricated with Thornel P-100 graphite fibers (Union Carbide Co., Danbury, CT), which are unidirectional, continuous, about 10/~m in diameter, and pitch-based. Fifteen tows of the fiber (about 2000 fibers/tow) were aligned unidirectionally and infiltrated with an epoxy resin (Epon 828 resin of Miller-Stephenson Chemical Co. Inc., Danbury, CT). The resulting product, a graphite/epoxy composite rod, was cut into disks by sectioning the rod perpendicularly to the axis of the fibers. The disks were then made into electrodes by silver painting the back side to make electrical contact with a copper-wire lead. The silver paint and copper wire were then coated with Epoxy-patch. The total cross-sectional surface area of the graphite fibers was about 0.024 cm 2 Silicon carbide M F electrodes Planar SiC MF electrodes were fabricated with Textron SiC MF, which are unidirectional, continuous, and about 140/zm in diameter. The SiC MF is comprised of a 33-/~m diameter, pitch-based carbon fiber core surrounded by chemical-vapor-deposited SiC with an outer, carbon-rich surface. A total of 147 SiC MFs were bundled and then processed in an identical manner to the graphite-fiber planar electrodes described above. The total cross-sectional surface area of the SiC MFs was about 0.023 cm2 . Note that in these planar electrodes, only the cross-sectional filament surface is exposed. To study the electrochemical properties of the carbon-rich surface of the SiC MFs, electrodes baring only the carbon-rich circumferential surface were prepared. Silicon carbide MFs were cut into lengths of about 3 cm. Only the circumferential surface near each end of the MFs was silver painted onto a copperwire lead to make electrical contact. The electrical-contact area and the copper-wire lead were then sealed
Magnesium metal-matrix composite
1763
in Epoxy-patch so that only the circumferential surface of the MFs was exposed. A sufficient number of SiC MFs was used so that the total circumferential surface area was about 0.79 cm 2.
Hot-pressed silicon carbide electrodes Planar electrodes were fabricated from bars of SiC that were produced by Ceradyne, Inc., Costa Mesa, CA. The SiC was hot-pressed to near theoretical densities (>98%) without sintering aids or binders. The SiC bars were cut into 9 x 9 ram-square wafers about 1 mm thick. The entire back side of the SiC wafers was silver painted to make electrical contact with a copper-wire lead. This procedure was followed to ensure that the ohmic drop through the SiC wafer would be uniform over the electrode face during polarization experiments. The resistance through the thickness of the SiC wafers was about 1 kl), and thus, ohmic losses were calculated to be less than about 0.1 V for current densities (CDs) less than 10 -4 A cm 2. The sides and silver-coated surface of the wafers were then sealed with Epoxy-patch. The front planar surface was left bare and served as the electrode surface.
Aqueous solutions Near neutral 0.5 M NaNO 3 solutions were prepared from 18 Mfl cm water and analytical grade NaNO 3. The solutions were kept at 30 __+ 0.1°C, and de-aerated with high-purity N2 (99.9%) or oxygenated with high-purity 0 2 (99.6%). Gas pressure was 1 atm.
Electrochemical experiments Potentiodynamic polarization experiments were conducted with a Model 273 PAR potentiostat/ galvanostat (EG&G, Princeton, N J). The electrodes were stabilized at their corrosion potential (Ecorr) prior to polarization. Stabilization times typically ranged from 30 to 90 min for all electrodes except the SiC MF/ZE41 Mg MMCs. The MMCs were stabilized for less than 15 rain to prevent excessive corrosion prior to polarization. Potentials were measured against a saturated calomel electrode. All electrodes were polarized at a rate of 0.1 mV s- 1 except the SiC MF/ZE41 Mg MMC electrodes, which were polarized at a rate of 1 mV s i. For the Mg electrodes, sweep-rate effects were not observed for rates less than I mVs- 1. To be conservative, 0. ! mV s - 1sweep rates were used. This sweep rate was also used for the graphite, SiC, and SiC MF electrodes to maintain consistency. The sweep rate for the MMC electrode was set at 1 mV s- 1 to avoid excessive corrosion of the matrix. At low sweep-rates, matrix corrosion becomes excessive, leaving the SiC MFs in relief, which increases the SiC-MF area ratio. Each polarization experiment was performed three times: twice on one electrode and once on another to verify reproductibility. The surface of all planar electrodes was polished to a 0.05/~m finish with gamma alumina powder, kept wet, and rinsed with 18 MI) cm water prior to each experiment. To generate polarization diagrams, the logarithm of the CD was averaged over three experiments and plotted as a function of potential. The galvanic current (Igalv) was measured between pure Mg and SiC MF (cross-section exposed) electrodes of equal surface area (i.e. 0.023 cm2) with a ZA 100 zero-shunt ammeter (Intertech Systems, Inc., San Jose, CA). The 0.50 area fraction of SiC MF was used so that the results could be compared to that of the actual MMC which had a 0.49 area fraction of exposed SiC MF. The galvanic current was normalized with respect to the Mg area to obtain the galvanic-corrosion CD (ig~lv). The value of/gaJv was plotted as a function time for each experiment.
Corrosion morphology Scanning electron microscopy (SEM) was used to characterize the corrosion morphology of the MMC. The corrosion product on the MMC was removed using a boiling solution of 15% CrO3 and 1% AgNO3 . The solution does not attack Mg, but removes the corrosion product by dissolving Mg(OH)2. EXPERIMENTAL RESULTS The anodic and cathodic polarization diagrams of pure Mg and ZE41A Mg are p l o t t e d t o g e t h e r in F i g . 1 f o r b o t h d e - a e r a t e d a n d o x y g e n a t e d 0.5 M N a N O 3 a t 30°C. D a t a f r o m F i g . 1 w e r e u s e d t o o b t a i n t h e n o r m a l c o r r o s i o n C D (icorr) f o r p u r e M g a n d ZE41A Mg by Tafel extrapolation. I n F i g . 2, t h e a n o d i c p o l a r i z a t i o n d i a g r a m s o f p u r e M g a n d Z E 4 1 A M g a r e p l o t t e d with the c a t h o d i c p o l a r i z a t i o n d i a g r a m s of SiC M F (cross-section e x p o s e d and fiber surface exposed), hot-pressed SiC, and P-100 graphite fiber (cross section exposed)
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for de-aerated 0.5 M NaNO3 at 30°C. Figure 3 corresponds to oxygenated solutions, but is otherwise similar to Fig. 2. The data from Figs 2 and 3 were used to predict iGALV of Mg coupled to equal areas of other materials. In Fig. 4, anodic and cathodic polarization diagrams of the SiC MF/ZE41 Mg MMC are plotted for both de-aerated and oxygenated 0.5 M NaNO3 at 30°C. Data from Fig. 4 were used to estimate the corrosion CD of the matrix (icorr,mat~ix)" Figure 5 shows iGALV measured as a function of time using the Z R A technique for pure Mg coupled to SiC MF (cross-section exposed and fiber surface exposed), hot-
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1765
M a g n e s i u m metal-matrix composite
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pressed SiC, and P-100 graphite fiber (cross-section exposed) for de-aerated 0.5 M NaNO3 at 30°C. Figure 6 shows igalv for oxygenated solutions. Each curve corresponds to a single experiment. An SiC MF/ZE41 Mg MMC was exposed to oxygenated 0.5 M NaNO3 for 5 days in the open-circuit condition. Corrosion of the MMC was extensive as shown in the SEM micrograph in Fig. 7. The extent of corrosion was clearly revealed by removing the corrosion product (Fig. 8).
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1766
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Galvanic-corrosion CD vs time for pure Mg coupled to hot-pressed SiC, SiC MF, and P-100 graphite exposed to de-aerated 0.5 M NaNO3 at 30°C.
DISCUSSION
The effects of MMC constituents and dissolved 02 on corrosion behavior of SiC MF/ZE41 Mg MMC were deduced from polarization and Z RA data. The corrosion and electrochemical properties of the MMC constituents are discussed first to develop an understanding of how the constituents behave individually. Then, the corrosion behavior of the MMC and the galvanic interaction between the constituents are discussed.
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Galvanic-corrosion CD vs time for pure Mg coupled to hot-pressed SiC, SiC MF, and P-100 graphite exposed to oxygenated 0.5 M NaNO3 at 30°C.
FIG. 7.
SEM micrograph of an SiC MF/ZE41 Mg M M C that was exposed to oxygenated 0.5 M N a N O 3 at 30°C for 5 days.
FIG. 8. SEM micrograph of M M C shown in Fig. 7 after corrosion product was removed in a boiling solution of 15% CrO 3 and 1% AgNO3, which dissolves corrosion products but not Mg metal. 1767
Magnesiummetal-matrixcomposite
1769
Electrochemical properties of the constituents The corrosion of Mg is not affected significantly by the presence of 02. 3 That is exemplified by the polarization diagrams of pure Mg and ZE41A Mg immersed in 0.5 M NaNO3 (see Fig. 1), which are virtually unaffected by solution oxygenation since the proton reduction kinetics is significantly greater than that of 02 reduction. For pure Mg immersed in either de-aerated or oxygenated 0.5 M NaNO3, icorr obtained by Tafel extrapolation was about 3 × 10 -4 A cm -2. The average values of icorr for ZE41A Mg were approximately 4 x 10 -4 A cm-2 and 3 × 10 -4 A cm-2 in de-aerated and oxygenated 0.5 M NaNO3, respectively. The slightly higher values corresponding to the de-aerated solution was attributed to normal variation in data and not to a different corrosion mechanism since the shape of the polarization curves in the deaerated and oxygenated solutions was similar. The alloying elements in ZE41A Mg had the effect of increasing cathodic CDs (in comparison to pure Mg). The alloying elements did not have significant effects on anodic polarization behavior, and thus, pure Mg and ZE41A Mg are likely to have very similar galvanic-corrosion rates when coupled to cathodic reinforcements (see Figs 2 and 3). The SiC MF is comprised of a carbon-filament core surrounded by chemicalvapor-deposited SiC with an outer, carbon-rich surface. Since the SiC MF is actually a composite comprised of SiC and carbon, it was of particular interest to determine if the SiC MFs are electrochemically similar to SiC, graphite, or neither. The SiC MFs were compared to hot-pressed SiC and P-100 graphite fiber (cross-section exposed). The cross-sectional surface baring the carbon core of the SiC MFs and the carbonrich outer surface were examined independently. In de-aerated 0.5 M NaNO3 (see Fig. 2), the cathodic polarization curve of the cross-sectional SiC-MF surface was very similar in shape and in CD to that of P-100 graphite fiber, indicating that both materials have similar H ÷ reduction kinetics. The cathodic polarization curve of the carbon-rich SiC MF surface was also similar in shape to that of P-100 graphite, but CDs were about 1 to 2 decades less at a given potential. The cathodic curve of hotpressed SiC was different in shape from the other materials, and had lowest CDs when potentials were less than about -1.2 V(SCE). In Fig. 2, at potentials noble to approximately - 0 . 9 V(SCE), cathodic currents are most likely caused by diffusionlimited 02 reduction due to traces of dissolved 02. In oxygenated 0.5 M NaNO3 (see Fig. 3), the reduction of 02 caused cathodic CDs of all materials to increase. The polarization curves show an initial Tafel regime followed by a diffusion-limited, O2-reduction regime. The highest cathodic CDs were observed for the P-100 graphite fibers. The CDs for P-100 graphite and the cross-sectional and carbon-rich surfaces of the SiC MFs were greater than 10 -3 A cm -2 in the diffusion-limited, O2-reduction regime; however, that of hot-pressed SiC was about a decade less. In both de-aerated and oxygenated solutions, the carbon core and carbon-rich surface of the SiC MFs apparently caused the cathodic curves of the SiC MFs to show stronger resemblance to that of P-100 graphite than that of hot-pressed SiC.
MMC corrosion The corrosion mechanisms of the MMC were studied using polarization data. The MMC corrosion rate was determined using polarization data of the MMC, polarization data of the constituents, and ZRA data. The SiC MF/ZE41 Mg MMC corroded at slightly higher rates (as determined by
1770
U H. HIHARAand P. K. KONDEPUDI
Tafel extrapolation) in oxygenated solutions (see Fig. 4) compared to de-aerated solutions. The corrosion CD of the matrix i corr,matrixin oxygenated 0.5 M NaNO3 was about 1.6 x 10 -3 A cm -2, which is approximately three times the value for the de-aerated solution. The value of icorr,matrix is twice the corrosion CD of the MMC since icorr,matrix is normalized with respect to the matrix area, which is one-half of MMC area due to the 50 vol. % content of SiC MFs. The increase in MMC corrosion rate in oxygenated solutions could not be attributed to 02 reduction occurring on the matrix since the cathodic CDs of monolithic ZE41A Mg did not increase in the presence of dissolved 02 (see Fig. 1). The increase in MMC corrosion rate could be attribute to 02 reduction occurring on the SiC MFs since dissolved 02 significantly increased the cathodic CDs of the SiC MFs (compare Figs 2 and 3). Galvanic corrosion between the ZE41 Mg matrix and SiC MFs was, therefore, a primary mechanism in MMC corrosion in oxygenated solutions. The total corrosion of the MMC matrix can also be approximated from polarization and Z R A data of the MMC constituents. The total corrosion of an anode in a galvanic couple results from galvanic corrosion between the anode and cathode plus additional simultaneous corrosion of the anode, which is called local corrosion, caused by cathodic reactions that occur on the anode. Therefore, the total corrosion CD of the MMC matrix icorr,matrix is equal to the local component /local plus the galvanic component igalvIt was assumed that the local component/local could be estimated by setting it equal to icorr of uncoupled, monolithic ZE41A Mg, implying that the difference effect9 is very slight. This estimation was made because the potential of ZE41A Mg should shift only slightly from its open-circuit potential if the alloy is galvanically coupled to SiC MF (see Figs 2 and 3). A very small shift in potential should not significantly affect cathodic-reaction kinetics of ZE41A Mg, and therefore, /local should not be significantly less than ico~r. Accordingly, the values of /local were estimated to be about 3 x 10 -4 A cm -2 in the oxygenated solution and about 4 x 10 -4 A cm -2 in the de-aerated solution. The galvanic component igalv was determined from polarization diagrams using the mixed-potential theory (Figs 2 and 3) and from Z R A experiments (Figs 5 and 6). Since the anodic polarization curves of pure Mg and ZE41A Mg were very similar (indicating that galvanic-corrosion behavior will also be similar), the Z R A results which were obtained using pure Mg were also used to predict galvanic-corrosion rates of ZE41A Mg. In Figs 2 and 3, the anodic polarization diagrams of pure Mg and ZE41A Mg are plotted with the cathodic polarization diagram of SiC MF (crosssection exposed). The CD at the intersection of the anodic and cathodic curves is the value of igalv for a galvanic couple having equal anode and cathode areas. The value of igal~ obtained using this procedure corresponds to an MMC having a 0.5 area fraction of SiC MFs with their cross-sections exposed. For oxygenated 0.5 M NaNO3 (Fig. 3), the mixed-potential theory predicts that igalvfor ZE41A Mg coupled to SiC MF (cross-section exposed) will be equal to about 3.2 × 10 -3 A cm -2, which is in fairly good agreement to the steady-state value of 1.3 × 10 -3 A cm -2 obtained from the Z R A experiment for pure Mg coupled to SiC MF (cross-section exposed). For de-aerated 0.5 M NaNO3, the mixed-potential theory predicts that igalvwill be equal to about 6 × 10 -4 A cm -2, which is closely bracketed by two steady-state values obtained from the Z R A experiments (see Fig. 5). Values of icorr,matrix that were obtained from (1) polarization data of the MMC,
M a g n e s i u m metal-matrix composite
1771
(2) polarization data of the constituents and (3) ZRA data are tabulated in Table 1 for comparison. The.penetration rate of the matrix was calculated in cm y-1 from icorr,matrix values using Faraday's law (Table 1). Although there is some variation in values based on the three methods, penetration rates in the oxygenated solutions are consistently greater than those in the de-aerated solutions. In oxygenated 0.5 M NaNO3, penetrations rates varied from about 3.7 to 8.0 cm y - l , which are about five to 11 times that of uncoupled, monolithic ZE41A Mg. In de-aerated 0.5 M NaNO3, penetrations rates varied from about 1.4 to 2.7 cm y-l, which are about two to three times that of uncoupled, monolithic ZE41A Mg. The very high penetration rates indicate that these materials should not be used unprotected in moist environments. An SiC MF/ZE41 Mg MMC corroded severely in the open-circuit condition during a 5-day exposure period to oxygenated 0.5 M NaNO3 at 30°C. The surface of the MMC was encrusted with corrosion product (Fig. 7). Removal of the corrosion product revealed significant loss of metal (Fig. 8). Galvanic corrosion might be reduced by thoughtful materials selection. Galvanic corrosion in Mg MMCs is cathodically controlled, and thus, galvanic-corrosion rates could be reduced by selecting reinforcement constituents that sustain low cathodic CDs. In oxygenated solutions, tga~vcould be limited to about 10 -4 A cm -2 with hot-pressed SiC; however, it could be as high as 3.2 x 10 -3 A cm -2 with P100 graphite or SiC MFs (see Fig. 3). In de-aerated 0.5 M NaNO3, igalv could be limited to
TABLE 1.
CORROSIONC D s
A N D PENETRATION RATES IN SODIUM NITRATE SOLUTIONS
AT
30°C
Technique
/local* (A cm -2)
igalvt (A cm -2)
icorr.rnatrix t (A cm 2)
Penetration rate of matrix (cm y - i )
--
--
1.6 × 10 3
3.7
3 X 10 -4
3.2 × 10 3
3.5 x 10 -3
8.0
3 x 10 4
1.3 × 10 -3
1.6 x 10 -3
3.7
--
--
6 × 10 -4
1.4
4 × 10 -4
6 × 10 -4
1.0 × 10 -3
2.3
4 x 10 -4
8 x 10-4:[:
1.2 x 10 -3
2.7
Oxygenated solutions Tafel extrapolation of M M C polarization diagram Mixed-potential Theory ZRA De-aerated solutions Tafel extrapolation of M M C polarization diagram Mixed-potential Theory ZRA
*All values in this column were obtained from polarization diagrams of Z E 4 1 A Mg using Tafel extrapolation. tigalv and /corr,matrix correspond to galvanic couples and M M C s with 50% area fraction of SiC MF (cross-sections exposed). ~average of two values. Note: the penetration rates of monolithic Z E 4 1 A Mg in de-aerated and oxygenated 0.5 M N a N O 3 at 30°C were 0.9 and 0.7 cm y - i , respectively.
1772
L . H . HIHARAand P. K. KONDEPUDI
about 10 -5 A c m -2 (a fraction of icorr of Mg) with hot-pressed SiC (see Fig. 2). Galvanic-corrosion CDs would be about 100 times higher if P100 graphite or SiC MFs were used. These igalvvalues were in fairly good agreement with those measured from actual galvanic couples using the Z R A technique (see Figs 5 and 6). Thus, fabricating Mg MMCs with SiC of electrochemical properties similar to that of hotpressed SiC could result in SiC/Mg MMCs that possess corrosion resistance approaching that of the matrix alloy. Ideally, reinforcement materials should impart beneficial properties such as strength and stiffness to the composite without sacrificing corrosion resistance. CONCLUSIONS
The cathodic CDs of the SiC MFs were generally much greater than that of hotpressed SiC and closer to that of P-100 graphite fiber. The carbon core and carbonrich surface of the SiC MFs apparently caused the MFs to behave more like graphite than SiC. The SiC MF/ZE41 Mg MMCs corroded at higher rates in oxygenated 0.5 M NaNO3 compared to de-aerated 0.5 M NaNO3. That is uncharacteristic of the corrosion behavior of Mg and its alloys which is normally unaffected by the presence of dissolved O2. The SiC MFs, which are inert electrodes upon which H ÷ and 02 reduction may occur, induced galvanic corrosion in the MMC. The reduction of 02 on SiC MFs caused MMC corrosion rates to be higher in oxygenated solutions compared to de-aerated solutions, where galvanic corrosion was driven by H + reduction. The fabrication of Mg MMCs from SiC that is electrochemically similar to hot-pressed SiC could significantly reduce galvanic-corrosion rates in these materials since hot-pressed SiC sustained significantly lower cathodic CDs compared to the SiC MFs. Acknowledgements--The financial support of the National Science Foundation (NSF) (grant # DMR9057264) is gratefully acknowledged. The authors are particularly grateful to Dr B. A. MacDonald of NSF. The contributions of MMC materials from Mr M. A. Mittnick of Textron Specialty Materials, and Mg alloys from Mr R. H. Emerson of Magparts are gratefully acknowledged. REFERENCES 1. W.C. HARRIGAN,JR., MetalMatrix Composites: Processing and Interfaces (eds R. K. EVERETTand R. J. ARSENAULT), p. 1. Academic Press, San Diego, CA (1991). 2. S. R. LAMPMAN,Adv. Mat. Processes 139, (5), 17 (1991). 3. H. H. UHLIG and R. W. REVIE, Corrosion and Corrosion Control, 3rd Edn. John Wiley and Sons, New York (1985). 4. G. BUTLER and H. C. K. ISON, Corrosion and its Prevention in Waters, p. 91. Robert E. Krieger, Huntington, New York (1978). 5. W. F. CZYRKLIS, Corrosion~85, Paper No. 196. National Association of Corrosion Engineers, Houston, Texas (1985). 6. P. P. TRZASKOMA. Corrosion 42,609 (1986). 7. M. A. TIMONOVA G. I. SPIRYAKINA,V. F. STROGANOVA,and L. A. ZOLOTAREVA,Metallovedenie i Termicheskaya Obrabotka Metallov 11, 33 (1980). 8. L. n . HIHARA and P. K. KONDEPUDI, Corros. Sci. (1993) (submitted). 9. W. A. WESLEY and R. H. BROWN, Corrosion Handbook (ed. H. H. UHLIG), p. 481. John Wiley and Sons, New York (1948).