Corrosion Modeling, Prediction, Mitigation

Transcription

1 Corrosion Modeling, Prediction, Mitigation Siva Palani, Alan Rose, Keith Legg, USAF Corrosion Conference, June

2 Agenda How we do corrosion and galvanic corrosion today New tools for corrosion modeling, prediction, reduction Start with simple materials, interfaces Add in geometry Add in fluid layers Add in paints Add in local or service corrosion conditions Add in pitting Melding it all together Putting together the right set of tools for Corrosion Analysis 2

3 The Corrosion Problem 80% of structural damage issues stem from corrosion pits Annually Cost Readiness Safety Annual DoD cost $20.9bn Corrosion, fatigue, structural failure 25 days unavailable To make matters worse: In aircraft some of the worst corrosion is galvanic We are lightweighting with galvanically incompatible materials (CFCs, Al, Mg) We can no longer use the old but effective standbys (Cd, Chromates, etc.) So, we can no longer use the same outdated rule-ofthumb design methods G. Shoales, et al., Compilation of Damage Findings from Multiple Recent Teardown and Analysis Programs. Proc. of the 25th Symposium of the International Committee on Aeronautical Fatigue, held, May 27-29, 2009 (Stockholm, Sweden: ICAF, 2009). 3

4 Standard approach to galvanic corrosion Designers assess galvanic corrosion using tables or specs such as MIL-DTL or MIL-STD-889, based on galvanic potentials 4

5 Does galvanic corrosion assessment by ΔE work? SS Ti MIL-STD-889 says Ti is about the worst metal you can put next to Al stainless steel is much better. So, repair depots use stainless steel bushings to repair corroded bolt holes in Al spars Reality But look what happens when you corrosion test 2024 Al with 316 stainless and Ti6Al4V fasteners Ti SS Ti SS FEA Prediction 5

6 DoD approach to galvanic corrosion is changing Designers assess galvanic corrosion use tables, specs - MIL-STD-889 or MIL-DTL NAVAIR starting update to 889 to base it on galvanic current Electrochemical kinetics of material are not considered Most galvanic tables and charts are based on half-century old materials and data 6

7 This is the result F-18 wing and airframe corrosion We see this everywhere, especially wings. Water penetrates or just condenses; Cd lost from fastener; CFC wing panel galvanically corrodes Al airframe. Repair: remove corrosion and bush hole with PH stainless; now corrodes worse around bushing. Ti64 bushings usually a big improvement over stainless. More than 80% of military aircraft structural failures are initiated from corrosion pits NAVAIR Public Release SPR

8 A simple and accurate 1D approach for Predicting Galvanic Coupling Effect The Electrochemical Current-Voltage curve in 3.5% NaCl Ti/Al SS/Al Based on where polarization curves cross. Has been around for decades, but until now there has never been enough consistent data to use it: Corrdesa has developed the concept into Corrosion Djinn Software tool Critical component is the consistent database underlying it Corrosion Djinn has been verified & validated against validation test assembly measurements and FEA/CAE modeling 8

9 Djinn Database The critical aspect of Djinn is the database that underlies it Old galvanic tables are generic. New ONR and Djinn database is for specific materials, coatings, treatments, electrolytes Same database but slightly modified curves for Djinn, CorrosionMaster, CCM+ More data constantly added as needed by users, e.g. F-35 and MQ-9 CFCs, Ti6Al4V (Hi-Lok), Ti3Al2.5V (Hi-Lok), 1020 steel, Al 7050 and Al 7075-T6 for USAF, ONR Several trivalent Al treatments for Sikorsky Working on local version of database Proprietary or ITAR materials could then be added on local version 9

10 Original Database 10

11 Corrosion Djinn Demo 11

12 Current uses and limitations of the Djinn Current model Capabilities Relatively simple geometries Bushings, fasteners, butt joints, faying surfaces Fortunately, these are the most common problems Can handle any material, coating, electrolyte we can get data for Extract self-corrosion and thin-film curves by deconvolution Current Limitations Interfaces only geometry not included Not suitable for complex situations, e.g. several materials in close proximity Paints, insulating layers Not yet included Corrosion rate Corrosion pattern Self-corrosion Electrolyte thickness 12

13 Coming soon to a Djinn near you Corrosion rate (μm/yr) Area ratios modified to reflect CAE models Ability to add dimensions ΔE so people can compare corrosion current vs E Self-corrosion rate and Galvanic Severity Index (galvanic/self) Thin film and bulk electrolyte curves Graphics with standard interfaces (butt and lap joints, bushings, fasteners) with green/ yellow/red damage areas r a2 r i1 r h r i2 r c r a1 r c lnsulator Anode ` d a d i lns ` Cathode d c Anode ` d a d i ` Cathode ` dc t i 13

14 Capabilities of modeling methods Interface galvanic corrosion Selfcorrosion, Electrolyte thickness Geometry, CAD Fluid flow and thickness, Condensation, Cyclic corrosion, Temperature, chemistry Paint systems, Basing environment No satisfactory pitting models ΔE Djinn CorrosionMaster CCM+ 14

15 Impact of electrolyte thickness on corrosion galvanic corrosion currents are very different although ΔE is the same Under bulk conditions CFC cathodic reaction increase due to higher O 2 reduction rate with thinner electrolytes Under thin films - Corrosion Djinn also captures the electrochemical kinetics increase of material due to change in electrolyte thickness from submersion to condensed films. - Thin electrolytes (condensates) increase corrosion by at least ~10x - Note: Al corrosion rate is controlled by electrolyte thickness on the CFC 15

16 Effect of electrolyte film thickness and polarization curve Current density along bisecting line Current low and broad Current high and narrow Current high and very narrow Top, bulk polarization curve. CAE of 1,000µm film thickness Middle, 20µm polarization. But CAE 1,000µm film thickness, so we just see impact of kinetics for the thin film Bottom, 20µm polarization and CAE 20µm film thickness, so we see impact of kinetics for the thin film and IR drop of thin film

17 Update to MIL-STD-889B Galvanic corrosion ONR SBA s MIL-STD-889 Working Group has begun work on updating the standard to accurately base it on corrosion current not galvanic potential difference (ΔE) Change will take place over the next couple of years Galvanic Current table for common finishes Will involve measuring electrochemical polarization curves for 60+ modern materials/coatings/finishes, perhaps 2 or 3 electrolytes Curve Crossing calculation for less common and proprietary finishes Full CAE/FEA modeling for complex assemblies 17

18 When complexity demands CAE Leading Edge Flap Actuation System 18

19 Galvanic corrosion prediction workflow Define the problem Acquire the assembly geometry - CAD Al Define the assembly materials/finishes PH SS Define environment & electrochemistry Build the galvanic FEA model Evaluate design options 19

20 Compare the options We can model the entire assembly. All we care about here is the bushing Results shown in order of reduced corrosion current. Note: The total corrosion current, and therefore relative order of the coating and treatment options is the same for curve crossing analysis and CAE. CAE also gives the corrosion pattern and therefore the corrosion current density at the interface 20

21 Galvanic prediction validation Loss, 7d, mm bare 7050 anodized Measured CAE/FEA Corrosion Djinn Note: Total corrosion is accurate but corrosion depth and exact location is not, because there is no way to account for pitting, which is highly stochastic 21

22 Front Add in CFD for fluid flow, condensation, evaporation Back ~40 Am -2 CFC hole Al Bare ~4 Am -2 Electrolyte builds up at ledge and spills over. Thick down sides. Fasteners The Barge with wheelhouse Modeling electrochemistry and fluid simultaneously lets us model condensation, evaporation, diurnal cycles, mission parameters Anodized Effect of fluid thickness distribution on current density 22

23 Add in paints, insulators, anodize Insulator Insulating layer (e.g. glass ply) used to stand Al off from CFC Must be long distance to reduce corrosion significantly Paints absorb water and act as resistive layers on surface EIS measures impedance Anodize layers are insulators, but are penetrated in thin areas or weak spots, creating deep pits 23

24 Crack Depth (mm) Relative Humidity (%) Cyclic corrosion Diurnal changes tied to geographic location Doug Dudis (2016 ASETSDefense Wshop): Corrosion from condensation Advanced Environmental Severity Index AA7075-T651 V-Notched Sample (k = 14.4 MPa-m 1/2 ) Jim Dante: Al Galvanic Corrosion current peaks and SCC cracks propagate during drying cycle Crack Depth (mm) Relative Humidity (%) Time (hr)

25 What are we missing? Pitting There are pitting models but none developed for modeling Pitting is statistical so cannot be predicted except as probability distributions Fundamental limitation of any model of course Material degradation Doing this via polarization and EIS measurements Polarization curves at t>0 to be added to Database Will eventually need some kind of degradation modeling 25

26 Database Holistic approach to corrosion control Two tools, same database Use Corrosion Djinn Tool for simple geometries and scoping during/after design Use FEA for the difficult and critical areas Corrosion Djinn Find hot spots Fix hot spots FEA/CAE Analyze/fix high-risk areas 26

27 Where are we going with modeling? Anyone can use Djinn very quickly Quick and easy way for most common problems Does not cover complex systems or show what happens over time FEA and CAE needed for complex and multi-material systems Currently these methods require specialists Can take a long time to set up CAD and model FEA such as CorrosionMaster requires training and experience CAE (CCM+) is more complex but allows full process modeling, not just electrochemistry. Currently requires specialist However, Siemens CCM+ has Programmable Simulation Assistants which we can set up to make analysis possible for most non-specialist engineers 27

28 Acknowledgements Elements of this work have been sponsored by; The USAF Air Force Materiel Command, Walter Juzukonis, under Contract FA C-5088 The Office of Naval Research (ONR), William Nickerson, under Contract N M-0075 and The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the US Navy, Air Force Materiel Command, USAF or the US Government.