Validation of a Galvanic Corrosion Computer Model …...CORROSION MODELING FOR LIFE PREDICTION April...

CORROSION MODELING FOR LIFE PREDICTION

April 2010, Rome

Validation of a Galvanic Corrosion Computer Model for AA2024 and CFRP

with localised damaged coatings

Andres PERATTA1, Theo HACK2, Robert ADEY3, Siva PALANI4, John BAYNHAM5, Hubertus LOHNER6

(1) CM BEASY, UK, [email protected]

(2) EADS, Germany, [email protected]


(4) EADS, Germany, [email protected]


(6) AIRBUS, Germany, [email protected]

CORROSION MODELING FOR LIFE PREDICTION - ROME 2010

OUTLINE

• Objectives

• Conceptual model and methodology

• Governing equations

• Computational model

• Case Studies– Bare Samples

– Local damage in protective layer

• Conclusions


OBJECTIVES

• The aim of this work is to develop and validate a

computational model for galvanic corrosion (GC) in

macroscopic scale for typical case scenarios appearing in an aircraft environment

• The present work is based on the study of a planar bi-

material GC model composed of AA2024 and CFRP


VALIDATION APPROACH

Validation experiment� Measurement of polarisation

curves of the electrodes involved

� Measurement of potential field in the

electrolyte by scanning reference

electrode

� Measurement of total current

between anode and cathode

Numerical modelling� Geometry definition (3D CAD)

� Definition of physical/electrochemical

properties

� Mesh generation

� Numerical calculation (BEM, bottom-up

approach)

� Post-processing & results interpretation

• Results comparison

• Predictive and sensitivity analysis

Co-planar

Bi-material

GC model


ADVANTAGES OF BEM FOR GC MODELLING

• BEM is based on the solution of the leading PDE, i.e. the exact solution of the Laplacian operator is used.

• The mesh discretisation is required on surfaces only (i.e. volumetric meshes are avoided), thus allowing the method to dealmore efficiently with complicated geometrical situations in the pre-processing stage.

• Potential and gradients are treated as independent DOF and are both involved in the formulation. In this way, the current density and electric field vectors are not numerically differentiated from a potential field, but directly introduced in the modelling as new DOF. This feature introduces an additional bonus in terms of numerical accuracy.

• DOF are associated with physical quantities on surfaces where most of the interesting physical processes occur, rather than in the bulk of the electrolyte, where the numerical solution is usually known.

BEM = Boundary Element Method; PDE = Partial Differential Equation; DOF = Degree of freedom;


COMPUTATIONAL MODEL

Electrolyte: 30 x 14 x He cm

Mesh

~2000 elements

~5000 Nodes

AA2024GAP

CFRP

Variable

Electrolyte Height Paint


COMPUTATIONAL MODELLING

• Polarisation curves

• Electrolyte

conductivity

• Model geometry

• Electrical circuit

defined between

electrodes

• Electric currents and

potential on the sample

and in the electrolyte

• Metal voltages

• Electrode potentials

• Total currents flowing

through the wires

INPUT DATA OUTPUT DATA


EXPERIMENTAL WORK

• Measurement of polarisation curves (input data)

• Measurement of electric potential in the electrolyte and total current in the

bi-material coplanar galvanic corrosion cell (validation)

Increasing chloride content

Increasing chloride content


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50 60 70 80

Time [h]

Cu

rren

t d

en

sit

y [

A/m

²]

SCANNING ELECTRODE

ANODE CATHODE

V

ELECTROLYTE

SCANNING ELECTRODE

CFRP ground AA2024 milled

It

Typical measurement of total current

Experimental setup

Bare material


SIMULATION RESULTS

Electrolyte Potential

z


CASE STUDY 1 – BARE SAMPLES

• Anode: AA2024 Unclad + milled• Cathode: CFRP ground• Electrolyte (NaCl)

Jn

CFRP AA2024GAP


SIMULATION RESULTS

CFRP AA2024


MODEL vs EXPERIMENT

TOTAL CURRENT

7.01 mA6.94 mA

EXPERIMENTMODEL

t1 – t0 = 1h


CASE STUDY 2: COATING LOCALLY DAMAGED

ANODE (AA2024)

Coated area (blue)

Pinhole

0.5 cm

variation of cathode area

Exposed cathode(CFRP)

Masking

LaquerTape


COMPUTATIONAL MODEL

CFRP

INSULATING

SURFACE

COATING DAMAGE

(EXPOSED AA2024)


OBSERVATIONS

Varying C/A ratio

CA=100 CA=150 CA=200


CASE 01

CASE 02

EXPERIMENTAL & SIMULATION RESULTS

Cathodic region Anodic region

Cathodic region Anodic region

Variation

of

distance


COMPARISON OF TOTAL CURRENT

0.0E+00

5.0E-02

1.0E-01

1.5E-01

2.0E-01

2.5E-01

3.0E-01

3.5E-01

1 2 3 4 5 6 7 8 9 10

SAMPLE

TO

TA

L C

UR

RE

NT

[m

A]

EXPERIMENTAL

MODELLING

C/A = const

σ1 = const

Xt = const

σ2 = const

C/A

Xt = const

σ1 = const

Xt

C/A


SUMMARY

• Computer models have been developed, tested and compared against experimental results based on a co-planar bi-metallic arrangement

• The tests involved bare and partially coated samples of CFRP and AA2024

• The observables used for the comparison between experimental and numerical results were:– Total electric current flowing between electrodes

– Electric potential in a number of different points in the electrolyte


CONCLUSIONS

• Good agreement of simulation results with the experimental measurements

• For the cases considered (medium conductivity, cm scale)– Variations of the distance between pinhole and

cathode within a length scale of few centimetres did not produce a substantial change of total current.

– The cathode to anode surface ratio is a dominating parameter in the GC process

• The model forms the basis of a tool for materials testing and corrosion modelling in aerospace structures

Validation of a Galvanic Corrosion Computer Model …...CORROSION MODELING FOR LIFE PREDICTION April...

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