Relationship between electrochemical reaction processes and environment-assisted crack growth under static and dynamic atmospheric conditions

Transcription

1 Engineering Conferences International ECI Digital Archives International Workshop on the Environmental Damage in Structural Materials Under Static Load/ Cyclic Loads at Ambient Temperatures Proceedings Relationship between electrochemical reaction processes and environment-assisted crack growth under static and dynamic atmospheric conditions Fritz Friedersdorf Luna Innovations, Carlos Hangarter Excet Inc. Nate Brown Luna Innovations Steve Policastro Naval Research Laboratory Matt Merrill Luna Innovations Follow this and additional works at: Part of the Engineering Commons Recommended Citation Fritz Friedersdorf, Carlos Hangarter, Nate Brown, Steve Policastro, and Matt Merrill, "Relationship between electrochemical reaction processes and environment-assisted crack growth under static and dynamic atmospheric conditions" in "International Workshop on the Environmental Damage in Structural Materials Under Static Load/Cyclic Loads at Ambient Temperatures", A.K. Vasudevan, Office of Naval Research (retired), USA Ronald Latanision, Exponent, Inc., USA Henry Holroyd, Luxfer, Inc. (retired) Neville Moody, Sandia National Laboratories, USA Eds, ECI Symposium Series, (2016). This Abstract and Presentation is brought to you for free and open access by the Proceedings at ECI Digital Archives. It has been accepted for inclusion in International Workshop on the Environmental Damage in Structural Materials Under Static Load/Cyclic Loads at Ambient Temperatures by an authorized administrator of ECI Digital Archives. For more information, please contact

2 Relationship Between Electrochemical Processes and Environment-assisted Crack Growth Under Static and Dynamic Atmospheric Conditions Patrick Kramer, Carlos Hangarter, Fritz Friedersdorf, Steve Policastro, Nate Brown, and Matt Merrill Luna Innovations, Naval Research Laboratory, and Excet Inc. Luna gratefully acknowledges that this material is based upon work supported by the United States Navy under Contract No N C Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the United States Navy. International Workshop on the Environmental Damage in Structural Materials Under Static Load/Cyclic Loads at Ambient Temperatures June, 2, 2016

3 Objective Advance atmospheric corrosion measurements to support the Sea-based Aviation (SBA) corrosion theory and degradation modeling activities AA6061 A286 Initial focus on galvanic coupling and corrosion of boldly exposed surfaces FEA Models SS304 Curtesy of Beasy Corrosion data library (PDS) Longer term goal to extend modeling to localized corrosion and lifetime damage accumulation CL Corrosion cracking and crevice corrosion Service life prediction Our efforts are to use measurements to inform and verify multi-physics finite element analysis models This presentation focuses on relating electrochemical data for occluded cells to cracking processes Model and experimental results for CFRP and AA7050 butt joint (Air Force FA M-5062) 2

4 Environmental Conditions Potential (V vs. Ag/Ag/Cl) O 2 Concentration (M) Environmental conditions relevant to atmospheric corrosion Air and surface temperature Ag/AgCl Cl Sensor RE Relative humidity Contaminant loading and composition These environmental parameters determine the properties and spatial distribution of thin film electrolytes Salt concentration and film thickness Conductivity Oxygen concentration and diffusion rates Conductivity (ohm-cm) Conductivity (at 25 C) O2 Solubility at 1 atm 1.2E E E E E E-04 4 [NaCl] (mol/l) F Custom Relative Humidity (%) Calibration Curve Electrolyte NaCl Concentration (M) 0.0E y = x R 2 = Shedd M. E., Master of Science Thesis, University of Virginia, ln([nacl]) 3

5 Measurements for Model Development and Verification Model development requires the use of appropriate material and environmental inputs Solution properties (salt loading, film thickness, and conductivity) Material performance data (potentiodynamic scans) Present work focuses on AA7075-T6 corrosion in sodium chloride solutions Reactions that are of primary importance to atmospheric corrosion, crevice corrosion, and cracking in chloride solutions: O H O 4e 4OH Oxygen reduction Alloy dissolution Hydrolysis Hydrogen reduction 2 M M 2 M n ne n ( nz) zh 2 O M ( OH ) z zh H e H ads 4

6 Environmental Conditions The surface electrochemistry over a component varies based on atmospheric parameters and geometry Conditions expected for atmospheric corrosion: Salt concentrations varying from 2.6 M to 4.6 M NaCl Boldly exposed surface and crevice mouth - oxygenated at near neutral ph Bulk electrolyte and thin films Occluded cells - deoxygenated at low ph (ph 2 4) High humidity crevice, ORR at boldly exposed surfaces IR drop down crevice low humidity (drying) crevice, ORR at mouth IR drop down crevice Bold Mouth Tip Bold Mouth Tip 5

7 Potentiodynamic Scans

8 Bulk Electrolyte and Thin Film PDS Testing PDS tests were performed in a flat cell (bulk solution) and in a thin film test cell The three electrode thin film cell uses a porous, hydrophilic membrane (fiberglass fabric) to support the electrolyte over a surface with thicknesses in the range of μm A specific volume (4 μl/cm 2 ) of salt solution is applied to the membrane The solution equilibrates based on the relative humidity Working electrode one inch square sample of AA7075-T6 Counter electrode carbon cloth counter electrode with hydrophobic surface treatment Reference electrode Ag/AgCl reference electrode, Luggin probe with glass frit Test type Flat cell Atm cell RH (%) (40-70) (80) (90) 80% 90% Salt Conc [M NaCl] Temperature (C) OCP hold 30 to 60 minutes Scan Rate 0.2 mv/second Ref Electrode SCE Ag/AgCl 7

9 AA7075-T6 Surface Treatment Polishing creates thin Zn/Mg enriched layer at surface that alters polarization/corrosion behavior of AA7075-T6 Chemical etching with NaOH was used to remove layer to obtain more representative corrosion properties 8

10 PDS Results PDS results can be used to estimated galvanic (concentration cell) currents using mixed potential theory PDS data are used as FEA model inputs that provide area dependent responses for given electrolyte conditions (conductivity / IR drop in the thin film electrolytes) Aluminum alloy pits at E corr for oxygenated bulk and thin film electrolyte AA7075-T6 is essentially a non-polarizable electrode Extremely high ORR and free corrosion rates (i corr ) are measured with thin film electrolytes Deaerated low ph solutions decrease E corr of A7075-T6 9

11 Coupled Electrode Testing

12 Coupled Electrodes Hole Geometry Coupled electrodes with a hole geometry were used to measure galvanic currents between hole wall and bottom surface Hole diameter 1 mm Two electrodes separated by 4 mm thick cast epoxy Tape used to mask boldly exposed surface AA7075-T6 coupon thickness 2 mm Occluded cell aspect ratio of 6:1 Insulating Tape AA7075 Hole electrode Tape Surface Well 1 mm 2 mm 4 mm Cast epoxy insulator ZRA 11

13 Coupled Electrodes Hole Geometry Environmental testing with coupled electrode and occluded cell geometries Hole was filled with saturated NaCl solution (6 M NaCl) RH was cycled using steps of 80, 70, 60, 40, 60, 70, 80, 90% Salt precipitates were visible at and below 70% RH Deliquescence RH approximately 76% for NaCl 90% RH 80% RH 70% RH 60% RH 40% RH 12

14 Current (na) Current (A/cm 2 Cathode ) Current (A/cm 2 Anode ) Hole Electrode Current Galvanic current increased with RH Average current for the last hour of each RH step was used to estimate galvanic current Hole bottom was anodic to the hole wall RH: i cell ([Cl], O 2 activity, Volume, IR) Currents are very low, but consistent with oxygen concentration cell O 2 OH - Cl - O2 M(OH) M + Cl - M + H + Cathode e - Anode Area Cathode = 2.44 Area Anode 80% 70% 60% % 60% Time (ksec) Relative Humidity (%) 70% 80% 90%

15 Crevice Cell Measurements Segmented electrodes are used to measure spatial distribution of current Sectioned aluminum alloy sheets bonded with GRP prepreg to form multi-electrode array Eight electrodes connected via multichannel ZRA Tests were done using only three or two electrodes Occluded cell aspect ratio of 12.7 : 0.08 (160) Segmented electrode Segmented electrode and crevice former 14

16 Relative Humidity (%) Segmented Electrode Testing Tests were done in a programmable RH and temperature cabinet The segmented electrodes were exposed to saturated NaCl solution ( 6 M NaCl) prior to testing Three-electrode: membrane fully saturated Two-electrode: crevice filled solution The segmented electrodes were then subjected to cyclic humidity testing Two hour hold at each RH, three cycles, constant temperature of 35 C Deliquescence RH DRH Efflorescence RH ERH Time (hours) 15

17 Current (µa) RH (%) Crevice Measurements Three Electrodes Cathodic currents at the crevice tip peak during: wetting above the DRH drying between the DRH and ERH The largest cathodic currents at the crevice tip occurred during drying Three electrode crevice testing Bold Mouth Tip Time (hr)

18 Current (µa) RH (%) Crevice Measurements - Two Electrodes The two-electrode response to RH variation also had high crevice tip cathodic current densities during drying when RH decreased from 70% to 60% Gamry Interface 1000 was used to make two-electrode ZRA measurements Run 1 Run Time (hr) 17

19 Environment Assisted Cracking

20 Environmental Cracking An instrumented four point bend test sample is used to measure crack growth during atmospheric corrosion tests Four point bend test produces S-L cracks in thin plate/sheet materials Don t need thick plate material to obtain Mode I loading for S-L cracking Useful only for alloys with anisotropic SCC susceptibility Four Point Bend Test Sample 100 x 12.7 X 19 mm Rolling Direction Crack Orientation 19

21 Beam Fracture Sample Loading of the beam sample results in mixed mode loading (Mode I and II) Crack initiates at relief notches Relief notches to concentrate stress Gage Section Four point bending promotes balanced cracking from each notch Stress intensity decreases with crack length Estimate K ISCC Under constant load, beam deflection can be used to estimate crack length T S L Crack initiation sites (4 L x 0.5 H x 0 Beam deflection as a function of crack depth Mode I and II loading as a function of crack depth (A) (C) (B) 20

22 Atmospheric Corrosion Bend Tests S-L cracking along the sample mid-plane has been demonstrated using AA5083 and AA7075 alloys Cracking lengths of 10 to 15 mm achieved without ductile overload Inductive displacement transducer can be used to measure crack length in situ SCC bend fixture following GM9540P salt spray exposure S-L cracks in thin plate 5083-H116 (DoS 40) 7075-T651 21

23 Initial Crack Test Results Preliminary testing using the instrumented bend test was done with AA7075-T651 Spring loaded to KI = 10 MPa-m0.5 ASTM G85-A5 for 300 hours for salt loading and initiation Measurements then performed in cyclic humidity (RH 30 90%) at 35 C Peak velocities during drying, around 40% RH Instrumented Bend Test (AA7075) 22

24 Crack Length (mm) RH (%) Average crack velocity (nm/s) RH (%) Environment Assisted Cracking Tests Bend tests were done using AA7075-T6 samples exposed to the same humidity cycles used for the crevice testing Samples were pre-cracked using cathodic polarization in salt solution Bend test load frames with samples were salted and then placed in humidity cabinet Sample displacement was continuously monitored to estimate crack length as a function of time Crack velocity was calculated as average da/dt for the two tests Crack length as a function of time Time (hr) Crack velocity as a function of time Time (hr)

25 Current (µa) Crack velocity (nm/s) Cracking and Crevice Electrode Data Maxima in the crack velocities corresponded to cathodic current peaks at the crevice tip during wetting and drying transitions For wetting, peak above the DRH For drying, peak below DRH and above ERH Drying Wetting Time (hr) 0 24

26 Discussion For high humidity and contaminant (salt loading): Boldly exposed surfaces with thin electrolytes have high corrosion currents Aluminum alloy acts as a non-polarizable electrode and can support high cathodic current densities under thin film electrolyte conditions Crevices with IR drop may not be strongly coupled to boldly exposed surfaces Minimal driving force for aluminum dissolution and acidification within crevices and cracks For intermediate humidities Boldly exposed surface may be dry, have poor connectivity to mouth, with high IR drop over surface Crevice mouth supports both ORR and anodic dissolution, with some coupling to the crevice tip Anodic dissolution within the crevice, near the crack mouth, supports acidification of the occluded cell Occluded cell geometry may restrict diffusion / ionic mobility Important difference between hole electrode results versus crevice data Crevice tip may become acidified, deoxygenated, and support both anodic and cathodic reactions Acidification makes hydrogen reduction more favorable Hydrogen reduction at crevice tip may increase ph 25

27 Discussion Cracks or tight crevices may have three zones of activity ORR at mouth Anodic dissolution near the ORR mouth area O H O e M M n ne OH Hydrolysis lowers ph and promotes hydrogen ion reduction At the crack tip hydrogen ion reduction may be favored Proton reduction may increase ph at crack tip Results consistent with known importance of hydrogen in SCC of high strength aluminum alloys M H n ( nz) zh2 O M( OH) z e H ads zh OH - M(OH) O 2 M + e - M + Cl - H + H Cl - + Cl - H e - 26

28 End