corrosion behavior of magnesium and its composites.pdf

8/13/2019 corrosion behavior of magnesium and its composites.pdf

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aper No.441 CORROSIONC. 7

CORROSION BEHAVIOR OF MAGNESIUM ALLOY AZ91 AND ITSMMC fN NaCl SOLUTION

W.M.Chmr*, F.T.Cheng*, L.K,Leung*, R.J.Horylev*, T.M.Yue+*Department of Applied Physics and Materials Research CentreDepartment of Manufacturing EngineeringThe Hong Kong Polytechnic UniversityHunghomHong Kong

ABSTRACTThe corrosion behaviors of magnesium alloy AZ9 lC and its Al*O~(shotttibre)-reinforced metal matrix composite(MMC) in alkaline solutions (PH 10.5) contsiniig chloride were compared rising potentiodymnnic polarization measurements,electrochemical impedance spectroscopy (EIS) and immersion tests. Dependence of polarization characteristics (corrosionpotential and corrosion current density) on chloride concentration was similar for both materials. The corrosion potential E..~decrensed linearly with the logarithm of chloride concentration, while the corrosioncurrent density i,.mincreased rapidly atchloride concentrations larger than 0.1A,with the i for the MMC consistentIyhigber than that for AZ91C, reaching a factorof about 3 in 3.5Asodinm chloride solution. EIS data indicated that the protective film on the MMC was inferior to that on

the matrix alloy. W]tb stirring and at frequencies higher than 5 Hz, the elecfrode/electrolyte interface might be described bythe simplified Randles model for both materials. Immersion tests showed that the corrosion rate of the MMC, in terms ofweight loss, was about 7 times that of tbe matrix alloy. Thus the present investigation indicated that in alkaline solutionscontaining chloride ions, the presence of Al~03 shofl tibres in the alloy did not drastically alter the corrosion behavior, butsignificantly reduced its corrosinn resistance.

Kewvords:magnesiumalloy Al O~-reinforcedetsl mabix composite, effect of chloride concentration, polarization,cormison potential, corroison rate, electrochemical impedance spectroscopy.

INTRODUCTIONThe development of metal nmhix composites (MMCS) has been one of the major innovations in materials science

and technology in the past 20 years. MMCShave been designed primarily for improvement in mechanical properties such ashigher specific stiffrress and specific stiengtb. They may also possess other desirable properties compared with their matrixmetals, such as a very low coefficient of thermal expsnsion and a h@r thermal deformation resistance. 12Thus, MMCS areespecially attractive in applications where it is advantageous to employ low density strncturd materials, such as in aerospace,automotive, medical and sports industries. On the other hand, owing to the presence of an addhiofml phase, the corrosionresistance of a metal ma iixcomposite may be lowered by an amonnt dependent on the composite system and the fabricationroute. Among the low density metals, A1-b8sed MMCS are well developed and their properties, inclndmg the corrosionbehavinrs, quite extensively stndied. 3,4A firrtber and more ambitious step in weight savings is to replace Al-based materials

Copyright01997 by NACE International. Requests for permission to pubfish this manuscript in any form, in p?rf or in whole must be made in writing to NACEInternational, Conferences Division, PO. BOX218340, Houston, Texas 7721 8-834Q. The material presented and the views exPres ed in thispaper are solely those of tha author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.


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magnitude at correspondhg potentials. Thus itmay be concluded that the protective film was easily broken down by chlorideions even at a favorable pH of 10.5. Values of the corrosion potential Ewmand the corrosion current density i Omextmctedfrom the polarization cnrves, were plotted against chloride concenbation and shown in Fig. 2 and Fig. 3 respectively.Variations of E- with chloride concentration were almost identical for both materials. In both cases, the variations obeyed asimple logarithm relationship,E.OII=a-blog [Cl] , where [Cl- ] was the chloride concentration in W%with a= -1,48 for AZ91C= -1.52 for MMCand b = -0.12 for both AZ91C and MMC,

The shitl of E... towards more active values as [Cl-] incrensed can be attributed to the adsorption of chloride ions onthe electrode surface, thus changing the corrosion potential.At low chloride concentitions (below 0.1%), the corrosion cnrrent density was small, but at higher concentrationsi~. incre~ed rapidly, bOtbfOrthe ~C md the matrix alloy. This can be attributed to the adsorption of chloride ions on thespecimen surface, leading to film breakdown and formation of active sites. The cnrrosion current density of the MMC wasconsistently higher Omnthat of the matrix alloy, and more apparently so at higher chloride concentrations. At 3.50/,, the imfor the MMC was about three times that for the ma rix alloy. Thus it nmy be concluded that in the case of the MMC, theprotective film hss greater diff]ctdty in repairing itselt

AC Impedance MeasurementsNyquist plots of AZ91C nnd A1203(shorttibre)/AZ91C in alkaline solutions with 3.5% NaCl are shown in Fig. 4.Absence of the Warburg impedance indicated that the corrosion process was not controlled by difision through corrosionproducts on the specimen surface, while the contribution of ditlision of the reacting species through the solution was

obviously insignificant, as stirring was present. Low-frequency (fs 5Hz) inductive loops were present in the Nyquist plots ofAZ91C and its MMC. This might nrise from the adsorption and rapid desmption of reaction intermediate. The discussionsabove suggest that for~> 5Hz, the electrode/electrolyte interface can be modeled by a resistor ~ ( the polarization resistance)and a capacitor Cd ( the double-layer capacitance) in pnrallel. By using plots of& versus -c&, where ~. and -Zti are thereal and imagimmypans of the AC impedance, ~ and C.Jwere determined. Values nf 1 and Cd for AZ91C and its MMC areshown in Table 1. The relative values of ~ are consistent with the relative vnlues of i m obtained horn polarizationmeasurements, both benring a ratio in the range 3-4. The value of Cd for the MMC was larger than that for the matrix. Thedifference in i and Cd between the AZ91C and its MMC might arise tlom two causes. Fust, the protective film on theMMC might be less protective than that on the matrix alloy*, Secnndly, the wetted area might be larger in the case of theMMC due to the presence of the A120Jrnatrix interfaces.

Weight Loss MeasurementsIn 3.5% NaCl solutions at pH 10.5 and 20C, the corrosion rate, obtained tiom a 3-day immersion test, were-4g/ml/d for AZ91C and -30g/m2/d for the MMC, Tbus the corrosion rate of the MMC was about 7.5 times that of the

matrix alloy. To compare WISratio with the ratio of the i,.mfrom pokuizatinn measurements, conversion is required. one hasto take into consideration of the fact tba~ (i) the conducting nrea on the MMC specimen was less than the 1 cmz nominalelectrode area due to the presence of the non-conducting AIZ03 ; (ii) weight Inss due to fall out of Al~03 when the matrixcorroded was not reflected in the corrosion current; and (iii) the MMC and the matrix had a density ratio of 1.26, Assumingthe area ratio and the volnme ratio of A1103in the matrix to be 0.2,

ratio of calculated weight loss= ratio of icmm ((1+0.2)/(1-0.2)) X 1.26= 1.89 x ratin of ic r=5.86,which is comparable to the measured weight loss ratio,

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CONCLUSIONS1, For Mg alloy AZ91C and its Al~03 (short fibre)-reinforced MMC, variation of the polarization characteristics with

chloride concentration followed the same trend.2. Thecomosion potentials for botimaterials decreased linemly wititie logtiti oftiechloride concentition.3. Atchloride concen@atiom smaller thm O.l%, tiecomosion rate, asrepresented bythecorrosion current density, wasnegligibly small. Above that concentmdion,thcorrosionrate increased rapidly, with tbe MMCcorroding atahigber ratetimtienon-rehforced matiixmeWl. In3.5%NaCl solution atpH 10,5 and 20C, tbei,.m forthe MMCwasaboutthree

times that for the matrix alloy, indicating inferior corrosion resistance of the MMC.4. ~eEISdati at6equencies M@ertim 5Hzshowed thattie elec@ode/elec&ol~e htetiace for botimaterials mi@tbecharacterized by the simplified Randles model.5. Values oftiedouble-layer capacitmce C~lmdtie comosion cmentdensi& i..~inticated tiat(i) tieprotective fib onthe MMC was inferior to that on the matrix alloy and (ii) the wetted area on the MMC was larger due to the presence ofthe A1,O,/matrix interfaces.6. lntemsofwei@t loss, tiecomosion rate oftie MMCwasabout seven times thatoftie ma~ixalloy.7. Thepresence of AIZO,shofifibres htiealloy tidnot&astically alter tiecomosion behavior, butsi~ificmtly reduced

its corrosion resistance by a factor of about six to seven,

REFERENCES1. W. A. Femmdo, JomalofMaterials Engtieetig,l 1,4(1989) p.299-3l3.2. R. G. Wendt, W. C. MosMer, B. Shaw, P. Miller, mdD. L. Oslon, Comosion, 50, l1(l994):p.8l9-8263. K. A.Lucas, H. Clarke, Corrosion of Alwntilum-based Metal Matrix Composites, (New York, John Wiley and Sons,1993)4. H. K. Mak, F. T. Cheng, L. K. Lemg, T. M. Yue, ''Comosion Behavior of~8090 mdiSSiC MMC'', Comosion'96Paper 515, (Nate International, Houston Texas 1996)5. A. F. Chmmasco, T. K. Aune, D. Hawke, W. Unswmth,J.Hills,ASMHandbook,V13,C orrosion, 10th ed. (Ohio, ASMInternational, 1990) p. 740-754.6. M. G. FontiAComosion Engheerhg,3rd ed. ~ew York, McGraw-Hill, 1987), p.239.7. C, Gabrielli, Identification of Electrochemical Process by Frequency Response Analysis, (Solution Group, 1980); Ref(5),ASTM Standard G106.8. M. Dattilo, Materials Perfommnce,25, ll(1986):p. l8-22.

TABLE 1VALUES OF ~ AND Cd EXTRACTED FROM AC IMPEDANCE MESAUREMENTS3,5% NaCl solution &/( Qcm2) Cdll fl Icd

AZ91C 226 11.9MMC 51 13,6

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.1.0.L5(MC1 I. 0 % NaCl-12 AZ91C --:~

Current Density (/+/cm*)Fig. d. Polwlzatlon C.WW of A291C and lb Al,O,-relnfom,6d MMCin r.lkdlne solutions (PH10.5) at various chlorlde Concentration.

x .AZ91 c3.1+ - MMC

I.u

. .1 x AZ91C

.1.2 . WAC

.,.3.

.1,4.1.,..,., - \.-17,~ ,0$

Chloride Concentration inWY.Fig. 2. Varhtlon of corro$ lo. rmtentlal In alkaline solutions (DHwith chloride c.mcentratlon

c (a /.W.84nlH\

: 3 ,& I ~,;.4 10 ?0Chloride concentration in wt% Z,, ( Q cmz )

ig 3. Variation of .mrroslon mmunt dmeny in alkaline solutions Fig, 4. Nyqulst plots of &?91C and its A120,-rein forced MMC in(pH 10.6) with chloride concentration. 3,5 NaCl s.alti.a at PH 10.5.

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