corrosion modeling, prediction, mitigation · update to mil-std-889b galvanic corrosion •onr...

Corrosion Modeling, Prediction, Mitigation

Siva Palani, [email protected] Rose, [email protected] Legg, [email protected]

USAF Corrosion Conference, June 20171Corrdesa Copyright 2017

mailto:[email protected]



Corrdesa Copyright 2017

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


Cost• Annual DoD

cost $20.9bn

Safety

• Corrosion, fatigue, structural failure

Readiness• 25 days

unavailable

Annually

3

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➢ So, we can no longer use the same outdated rule-of-thumb design methods

The Corrosion Problem – 80% of structural damage issues stem from corrosion pits

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).

Standard approach to galvanic corrosion

4

Designers assess galvanic corrosion using tables or specs such as MIL-DTL-14072 or MIL-STD-889, based on galvanic potentials


Does galvanic corrosion assessment by ΔE work?

TiSS

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

But look what happens when you corrosion test 2024 Al with 316 stainless and Ti6Al4V fasteners

Reality

FEA Prediction 5

Ti SS Ti SS


Most galvanic tables and charts are based on half-century old materials and data6

DoD approach to galvanic corrosion is changing

• Designers assess galvanic corrosion use tables, specs - MIL-STD-889 or MIL-DTL-14072

• NAVAIR starting update to 889 to base it on galvanic current

Electrochemical kinetics of material are not considered

This is the result –F-18 wing and airframe corrosion

NAVAIR Public Release SPR-2012-982

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 pits7

Corrdesa Copyright 2017 8

A simple and accurate 1D approach for Predicting Galvanic Coupling Effect

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

Ti/Al SS/Al

The Electrochemical Current-Voltage curve in 3.5% NaCl

Corrosion Djinn™ has been verified & validated against validation test assembly measurements and FEA/CAE modeling


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

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Original Database

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Corrdesa Copyright 2017 11

Corrosion Djinn Demo

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

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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

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`

lnsulator

``

AnodeCathode

ditidcda

rh

ra2

rcra1

rc

ri2

ri1

` `

Anode Cathode

di dcda

lns

Capabilities of modeling methods

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Interface galvanic corrosion

Self-corrosion, Electrolyte thickness

Geometry, CAD

Fluid flow and thickness, Condensation, Cyclic corrosion, Temperature, chemistry

Paint systems, Basing environment

DjinnCorrosionMaster

CCM+

No satisfactory

pitting models

ΔE


Impact of electrolyte thickness on corrosion

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Under bulk conditions 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

galvanic corrosion currents are very different although ΔE is the same

CFC cathodic reaction increase due to higher O2

reduction rate with thinner electrolytes

Current density along bisecting line

Effect of electrolyte film thickness and polarization curve

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

Current low and broad

Current high and narrow

Current high and very narrow


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

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Leading Edge Flap Actuation

System

When complexity demands CAE

18Corrdesa Copyright 2017

Galvanic corrosion prediction workflow

Define the problem

Acquire the assembly geometry - CAD

Define the assembly materials/finishes

Define environment & electrochemistry

Build the galvanic FEA model

Evaluate design options

Al

PH SS


Compare the options

We can model the entire assembly. All we care about here is the bushingResults 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


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Galvanic prediction validation

Loss, 7d, mm3 7050 bare 7050 anodized

Measured 0.84 1.3

CAE/FEA 1.25 1.16

Corrosion Djinn 1.25 1.1

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

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Add in CFD for fluid flow, condensation, evaporation

Front Back

Bare

Anodized

~4 Am-2

~40 Am-2

Electrolyte builds up at ledge and spills over. Thick down sides.

The “Barge” with wheelhouse

Al

CFC

Fasteners

hole

Effect of fluid thickness distribution on current densityModeling electrochemistry and

fluid simultaneously lets us model condensation, evaporation, diurnal cycles, mission parameters

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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

Cyclic corrosion – Diurnal changes tied to geographic location

35

45

55

65

75

85

95

Re

lati

ve H

um

idit

y (

%)

156 180 204 228 2520

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8AA7075-T651 V-Notched Sample (k = 14.4 MPa-m

1/2)

Crac

k D

ep

th (

mm

)

Time (hr)

Crack Depth (mm)

Relative Humidity (%)

Jim Dante:Al Galvanic Corrosion current peaks and SCC cracks propagate during drying cycle

Doug Dudis (2016 ASETSDefense Wshop): Corrosion from condensationAdvanced Environmental Severity Index


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

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Holistic approach to corrosion control

Corrosion Djinn

FEA/CAEAnalyze/fix high-risk areas

✓ Two tools, same database

✓ Use Corrosion Djinn Tool for simple geometries and scoping during/after design

✓ Use FEA for the difficult and critical areas

Find hot spots

Fix hot spots

Dat

abas

e

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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

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Acknowledgements

Elements of this work have been sponsored by;

• The USAF Air Force Materiel Command, Walter Juzukonis, under

Contract FA8650-15-C-5088

• The Office of Naval Research (ONR), William Nickerson, under

Contract N00014-12-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.

corrosion modeling, prediction, mitigation · update to mil-std-889b galvanic corrosion •onr...

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