Experimental and Computational Studies of IG-SCC of AA7050-T6

Transcription

1 Topical Day on Numerical Simulation of Localised Corrosion, SCK CEN, Belgium, October 16, 2002 Experimental and Computational Studies of IG-SCC of AA7050-T6 K. R. Cooper, R. G. Kelly Center for Electrochemical Science & Engineering Dept. of Materials Science and Engineering University of Virginia Charlottesville, VA USA Acknowledgements: Alcoa Foundation, J. Moran, E. Colvin, J. Staley R. Gangloff, J. Scully, L.Young, G. Young

2 Al-Zn-Mg-(Cu) Alloys Exhibit E-Dependent IG EAC Strong Potential Dependence Limited at high potentials by free surface pitting Mechanisms HE and AD Crack environment unknown Potential (E) X +/- ph Speidel (1975)

3 EAC Mechanisms & the Crack Environment Anodic Dissolution Echem knife da dt = M nfρ i Tip Hydrogen Embrittlement da = f θ, C D H, Eff dt ( ) 3e - Al H H Al3+ H 3+ H H H + H 2 H 2 O H e - Al Potential 3e - A - Al Al e - Al 3+ + H 2 O AlOH 2+ + H + H + + e - H H 2 O + e - H + OH - [X +/- ] Distance from Crack Tip

4 Objectives Characterize crack environment as f (da/dt) Local electrode potential (E Crack ) Chemistry ph, [X +/- ] Modeling Crack tip opening displacement (CTOD) Conductivity (κ) Tip corrosion height i Tip dissolution-based crack advance Mechanistic interpretation of EAC process Relative contribution of AD and HE

5 Material & Properties Alloy Zn Mg Cu Si Fe Cr Ti Zr Al < bal. YS * MPa AA 7050-T6 TS * MPa K Ic ** MPa m * S-orientation ** S-L S 200 µ m T L (RD)

6 Experimental Approach Crack growth rate measured using fracture mechanics Bulk Environment: 0.5 M CrO M Cl -, ph 9.2 Reservoir Reference electrode Micro electrodes Crack Environment In situ mini-re, ph, Cl - Crack Chemistry post test extraction and analysis of solution Pump

7 Take Away Points Crack tip chemistry controls IG crack growth kinetics (da/dt) in AA7050-T6 Acidity is main driver Crack tip potential can decouple from E app Major Influences on E(x) and [Al 3+ ](x): Crack tip opening Presence of a resistive crack tip film Minor: crack wall current EPFM does not predict crack tip for IGSCC Crack tips are much tighter

8 Phenomenology At E APP = V SCE, slow da/dt (5.9x10-7 mm/s) Spontaneous transition to fast da/dt (1.5x10-4 mm/s) after incubation V SCE, high da/dt (2.3x10-5 mm/s) maintained Crack Length, mm o C (#50-108) 0.5 M Na2CrO M NaCl, ph 9.2 da/dt in mm/s 5.9x10-7 E Trans -0.5 V SCE 1.5x x V SCE L. Young Time, days

9 Phenomenology T6 0.5 M Na 2 CrO M NaCl K = MPa m 1 mm / 5 min Hysteresis in Rate Incubation t from 4-50 h Transition to High- Rate Lowering of E does not return rate to incubation levels Log (da/dt), mm/s Crack Growth Rate (mm/sec) L. Young, 99 (#1) (#2) (#3) (#4) (#5) (#6) 1 mm / hour 1 mm / day 1 mm / week 1 mm / month 1 mm / year (V vs. SCE) Potential, V SCE

10 In-Situ Crack Measurements a vs. t ca. 40 h incubation Coincident with da/dt increase: E vs. t Large IR drop to tip ph, [Cl - ] vs. t Drop in ph, rise in [Cl - ] Crack Length, mm Potential, V vs. SCE ph Crack Length vs. Time 7050 # hour at 163 o C 0.5 M Na 2 CrO M NaCl, ph 9.2 Change E APP 6x10-7 mm/s 2x10-5 Probe Hole Applied and Crack Potential vs. Time Applied E Crack E ph and [Cl - ] vs. Time Cl - ph Bulk ph Bulk Cl [Cl-], M Time, days

11 Crack Length, mm Control of Cracking Rate by 121 o C (50-110) 0.5 M Na 2 CrO M NaCl, ph 9.2 Start injection 2.3x10-6 mm/s Crack Tip Chemistry V SCE Stop 1.5x10-4 Hole Time, days E APP < E Trans (-0.5 V SCE ) 65x increase in da/dt Effect is rapid ( 10 min) Injection Solution: 0.2 M AlCl M Cr 6+ as CrO 3 + Na 2 CrO 4, ph 3.1

12 Injecting Aggressive Solution at T6 Crack Tip Initiates High-Rate da/dt Crack Growth Rate (mm/sec) 10-3 T6 (L. Young '99) E Trans Applied Potential (V vs. SCE) E APP < E Trans (-0.5 V SCE ) Incubation is related to development of critical crack tip environment, not time to get to grain boundary or critical E APP Competitive processes: Al 3+ Production (corr.) + Hydrolysis + Migration (Cl - ) Diffusion Injection Solution: 0.2 M AlCl M Cr 6+ as CrO 3 + Na 2 CrO 4, ph 3.1

13 Aggressive Tip Chemistry Required for High-Rate da/dt Injecting corrosion inhibiting solution decreases da/dt o C (50-111) 0.5 M Na 2 CrO M NaCl, ph V SCE E Trans ~ -0.5 V SCE Crack Length, mm x10-5 Inj. #1 #2 1.9x x M Na 2 CrO 4, ph 9.2 Inj #1 = 390 µl Injection Solution: Inj #2 = 180 µl 1x Time, hours

14 Aggressive Tip Chemistry Required for High-Rate da/dt Crack Growth Rate (mm/sec) E Trans LY T6 S-free LY T6 S-rich 121 o C, S-rich (50-109) 121 o C, S-rich (50-110) 121 o C, S-rich (50-111) Inj #2 Inj #1 Injection Solution: 0.5 M Na 2 CrO 4 ph Applied Potential (V vs. SCE) Inj #1 - Transition out of incubation Can fully suppress transition Inj #2 - Fully established, highrate da/dt Did not fully suppress transition

15 Crack Growth Rate = f (Crack Tip [Al 3+ ], ph) Critical chemistry ph < 4 Al 3+ > 0.2 M Aggressiveness of crack tip solution Dissolution / repassivation kinetics Hydrogen production / absorption Crack Growth Rate, mm/s CGR vs. Crack Tip ph 0.5 M Na 2 CrO M NaCl, ph 9.2 K = 10 to 15 MPa m 1/2, OCP = -950 to -850 Potentials in mv vs. SCE OCP AA7075-T6, 1 M NaCl (Nguyen '82) AA7050-T651 AA7050 Near T o C) AA7050 Slightly OA 163 o C) Bulk Incubation Crack Tip ph

16 Probing the Potential Dependence: In Situ Crack Potential Measurements Low TIP Potential Net anodic current mini-reference probe hole 12.7 mm Crack growth direction Crack Potential, V vs. SCE Applied Potential Distance to Crack Tip, mm

17 In Situ Crack Potential Measurements Crack wake is fully polarized Local potential is complex f (E APP and Distance to tip) 12.7 mm micro-reference probe hole Crack growth direction Potential, V vs. SCE Distance to crack tip = 10.5 mm o C OCP Dist. to crack tip = 1.0 mm Time, minutes

18 In-Situ Crack Potential Measurements Crack wake fully polarized Define wake as > 3 mm from tip Crack Potential, mv SCE Distance from crack tip at time of measurement in mm No IR Drop Crack Wake Applied Potential, mv SCE

19 Crack tip E independent or weakly dependent on E APP If E TIP f (E APP ), as E APP, E [X +/- ] at tip Crack Potential, mvsce Distance from crack tip at time of measurement in mm No IR Drop Applied Potential, mvsce As crack grows, probe is further in wake

20 Modeling Crack Potential Steady State Assumptions Constant κ Equal D for all species V j = I j R j I j = ITip + 2 iwall, x AWall, x x= 0 da dt diss M ITip = nfρ B w j Tip R j = r A j j 1 κ j B i Wall,j Tip corr. height, w Tip CMOD θ I Tip L r j CTOD x z y

21 How Wide are Crack Tips? In Situ Bolt-loaded & Measured CSOD TG Fatigue Blunt tip IG EAC Sharp tip K = 13 MPa m Fatigue EAC 203 nm 2.8 µm B. Gable D. Hass

22 Crack Tip Shapes Modeled 5 Crack Opening Displacement, µm Dean and Hutchinson Blunt Sharp, δ Tip = 250 nm Sharp, δ Tip = 50 nm Distance from crack tip, µm

23 Fit of Crack Shapes to Models Air Fatigue reasonable match to blunt crack per EPFM Corrosion Fatigue matches trapezoid with 2 µm tip IGSCC crack is very, very tight Crack Opening Displacement (δ), µm Crack Opening Displacement (δ), µm Exptl. Air Fatigue δ Exptl. Air Fatigure δ - Fit (r 2 = 0.92) Exptl. Corrosion Fatigue δ Exptl. Corrosion Fatigue δ - Fit (r 2 = 0.92) Exptl. EAC δ Exptl. EAC δ - Fit (r 2 = 0.999) Calc. δ for quais-static crack (Dean et. al.) Calc. δ for static crack (Anderson) Distance from Crack Tip, µm Distance from Crack Tip, µm

24 Modeling Crack E - CTOD and i wall CTOD contols E Crack Blunt CTOD does not give match to E(x) Sharp ( nm) CTOD produces steep E i wall = 5 µa/cm 2 increases IR-drop ~ 10% Crack Potential, V vs. SCE i Wall = 0 Model CTOD 50 nm 250 nm E-P Analysis (~2 mm) CTOD = 250 nm ETip, V SCE I wall = 0 I wall = 5 ma/cm Distance from Crack Tip, mm

25 2-D Simulation of Near Tip For CTOD of interest, centerline position can be approximated by 1-D CTOD = 50 nm Potential, V vs. SCE CTOD = 5 mm Potential, V vs. SCE Distance from crack tip, mm Distance from crack tip, mm Distance from centerline, mm Distance from centerline, mm

26 Validation of 1-D Model for Centerline Potential, V vs. SCE CTOD (d) = 50 nm 200 nm 1 mm 5 mm D Model 2-D Model at Centerline Distance from crack tip, mm

27 Modeling Crack Potential Film Effects -0.4 Low κ < /Ω-cm) Must be thin (10 s nm) Salt film (AlCl 3 -like) Moderate κ s µm Al-Cl-OH H 2 bubbles (Pickering 84) Crack Potential, V vs. SCE Constant κ = /Ω-cm 20 nm film κ = /Ω-cm µm film κ = /Ω-cm 1 µm film κ = /Ω-cm 10 µm film κ = /Ω-cm µm film κ = /Ω-cm 100 µm film κ = /Ω-cm 100 µm film κ = /Ω-cm Distance from Crack Tip, mm

28 Modeling Crack Potential Film Effects Crack Potential at 1 mm from Tip, V vs. SCE Constant κ = /Ω-cm 20 nm film κ = /Ω-cm 1 µm film κ = /Ω-cm 1 µm film κ = /Ω-cm 10 µm film κ = /Ω-cm 10 µm film κ = /Ω-cm 100 µm film κ = /Ω-cm 100 µm film κ = /Ω-cm Range of Experimental Observations -0.75

29 Modeling Crack Potential Film Effects Crack Potential, V vs. SCE Distance from Crack Tip, mm 7050-T651, E App = V SCE 7050, 12 hr/163 C, , 12 hr/163 C, , 12 hr/163 C, , 6 hr/163 C, Constant κ = /Ω-cm First 2 mm κ = /Ω-cm First 2 mm κ = /Ω-cm

30 Crack Tip k < Bulk k implies salt film/h 2 bubbles at tip Higher κ leads to ppt of AlCl 3 for over 1 cm from tip Due to higher I needed for E Potential, V vs. SCE Potential, [Al 3+ V ], vs. M SCE T651, , 2x , 12 hr/163 o C, , 6x , 12 hr/163 o C, , 9x , 12 hr/163 o C, , 5x , 6 hr/163 o C, , 4x D Model, κ = 0.1 1/Ω-cm 1-D Model, κ = /Ω-cm 1-D Model, κ = /Ω-cm Distance from Crack Tip, mm [Al 3+ ] solubility in 1 M Cr(VI) κ = 0.1 1/Ω-cm κ = /Ω-cm κ = /Ω-cm δ Tip = 200 nm i Wall = 2x10-7 A/cm 2 E Tip = V SCE Distance from Crack Tip, mm

31 So What Crack tip must be small in order to achieve the sharp E-gradient near the tip Predicted CTOD ~ 250 nm, EPFM ~ 5 µm Measured CSOD ~ 200 nm The crack tip must be saturated over some distance << 1 mm Not anhydrous AlCl 3 conductivity too low Saturated Al-Cl - OH-Cr(VI) = upper bound on κ

32 Implications - Crack Growth by AD Calculated da/dt due to tip dissolution Tip corrosion height < 100 nm (Newcomb et. al., 2000) Effective crack κ - H 2 (Pickering, 1984) Fraction of observed da/dt due to tip dissolution Effective Crack Conductivity, 1/W-cm Tip Corrosion Height, nm >> 10 > 10 > Observed da/dt = 5x10-5 mm/s κ BULK = /Ω-cm Model parameters: CTOD = 200 nm, CMOD = 110 µm, L = 17 mm, i wall = 1 µa/cm 2, E = 0.4 V

33 Summary of AA7050-T6 EAC Tip Conditions Crack tip likely has gelatinous Al-OH-Cl-Cr(VI) film Active crack tip potential is approximately -0.9 V SCE independent of the external potential majority of IR drop confined to the first 3 mm from the crack tip indicative of net anodic current in the near-tip region E support [X +/- ] at tip The CTOD for IG EAC is approximately 200 nm 10x smaller than for fatigue cracking Poorly characterized by EPFM analysis of a moving crack Quantifying dissolution at tip needs two key unknowns: effective crack conductivity tip corrosion height