镁合金,LPSO

Figure8|Thesurfacepittingmorphologiesaftercyclicpolarizationtest(10min),(a)theMDZ-Calloy;(b)theMDZ-545alloy.

Figure7|CyclicpolarizationcurvesoftheMDZ-CandMDZ-545alloysatascanrateis2.5mV/sin0.9wt.%NaClsolutionafterimmersingfor10min,2hand4h,respectively.

EISmeasurement.Fig.9ashowstheNyquistspots,measuredatcorrosionpotential,ontheMDZ-Calloyimmersedin0.9wt.%NaClaqueoussolutionsfordifferenttimes.AllEISdiagramsexhi-bitthesameconfigurations.TheBodespots(Fig.9b)demonstratesthatthesamplepresentsahighfrequencyresistivebehaviorfollowedbyahigh/medium(100–10Hz)capacitiveresponse.Additionally,adistortedinductanceloopisobservedatmedium/lowfrequencies(,1–0.1Hz).Thediameterofcapacitiveloopisreducedastheimmersiontimeisincreased.Bycomparison,itisseenfromtheNyquistspots(Fig.9c)oftheMDZ-545alloythattheEIScurvesof1hand4harecomposedoftwoloopsandtheothercurvescontainmerelyaloop.IncontrasttotheMDZ-Csamples,twodifferentcha-racteristicsareobserved,whereinthediameterofcapacitiveloopisincreasedwithincreasingimmersingtimeandtheinductanceloopishardlydetected.

TheSEMgraphsafterEIStestsareshowninFig.10,whereitrevealsthatbothoxidationfilmsarerupturedduringtheEIStest(Fig.10a-1and10b-1).However,underhighmagnificationsoftheMDZ-C(Fig.10a-2and10a-3),thenakedmicrostructureisiden-tifiedinthebottomofcracks.Conversely,anewhoneycomboxida-tionfilmisobservedintheMDZ-545sample,suggestingthattheabilitytoformanoxidationfilmoftheMDZ-545alloyisstrongerthanthatoftheMDZ-Coneaftertheruptureofoxidationfilm.Corrosionproducts.TheXRDresults(Fig.11a)revealthatthephasesintheun-immersedMDZ-CalloyscontainMgmatrixand18R-LPSOphase,whichisconsistentwiththeaboveSEMandTEMresults.ThelowvolumefractionandlimitationofXRDtechniqueisresponsiblefortheabsenceofeutecticphase.Somenewpeaksareobserved,whicharewellassignedtothefollowingcrystallinephase:Mg(OH)2,Dy(OH)3andMgCl2compounds.IntheMDZ-545alloy(Fig.11b),thepeaksareidentifiedasMgmatrixand14H-LPSOphase,whicharepresentinboththeun-immersedsampleandintheoneimmersedfor12h.Nevertheless,comparedwiththeMDZ-Csample,theMgCl2peaksarehardlydetectedintheMDZ-545sample,whileMg(OH)2andDy(OH)3compoundsarestilldetectable.

ThedetailedresultsaresummarizedinTable1.Basically,twoEcorrvaluesshiftforwardpositivelywithincreasingimmersiontime.FortheMDZ-Calloy,theEPPshowsthesametrendasEcorr.However,aconstantEPPof21303mVisobservedintheMDZ-545alloy.TheIcorroftheMDZ-Calloyisincreasedwithextendingimmersiontime,resultinginalargeACRvalue(24mmy21).However,theoppositetrendisachievedintheMDZ-545alloy.Thoughthebackwardscanbreakstheoxidationfilmlayerdown,therelativelystableIcorrandACRvaluesareobserved.TherepresentativeSEMmicrographsoftheMDZ-CandMDZ-545alloysaftercyclicpolarization(10min)areshowninFig.8.InregardstotheMDZ-Calloy,aporousbandalongthegrainboundariesisobserved,whichindicatesthattheoxidationfilmisabsolutelybrokendownduringtheEIStesting(alsoconfirmedinFig.10).Conversely,onlysomeisolatedpittingspotsaredetectedintheMDZ-545alloyaftercyclicpolarizationtest.

Discussion

Inpresentstudy,theprecipitatesaremostlydistributedingrainboundariesintheMDZ-Calloy,whichiscomposedofa-Mgand18R-LPSOphase.18R-LPSOphasedissolvesgraduallyand

14H-

Table1|Electrochemicalparametersofthesamplesderivedfrompolarizationtestsin0.9wt.%NaCl

SampleMDZ-CMDZ-545

Immersedtime10min2h4h10min2h4h

Ecorr(mV)216002154621507216052157121516

Icorr(mAcm22)0.10110.13081.04210.09510.08620.0461

EPP(mV)214532131021213213032130321303

ACR(mmy21)2.33443.020224.0622.19511.99041.0645