Localized Corrosion of a 7075 Aluminum Alloy Exposed to KCl · Aluminium alloys are the second most widely used engineering metallic alloys after steels. 7075 is heavily used within

Localized Corrosion of a 7075 Aluminum Alloy Exposed to KCl

Monday, 20 April 2015 1

Christopher F. Mallinson

Department of Mechanical Engineering Sciences

The Surface Analysis Laboratory

Introduction

-Aluminium alloy 7075 and pitting corrosion

Experimental

-Procedure

-Marking intermetallics for repeat analysis

Results from three intermetallics

-Auger point spectra

-Energy dispersive x-ray point spectra and phase analysis

-Scanning Auger maps

-Energy dispersive x-ray maps

Confirmation of galvanic activity

-Magnesium deposition

Conclusions


Outline


Auger is not dead!

Scanning Auger Microscope Microlab 350

C.G. Littlejohns, M. Nedeljkovic, C. F. Mallinson, J. F. Watts, G. Z. Mashanovich, G. T. Reed and F. Y. Gardes, Scientific Reports, 5, 8288, (2015)

Point #8 Point #7 Point #6 Point #5 Point #4 Point #3 Point #2Point

8.0µm

Aluminium alloys are the second most widely used engineering metallic alloys

after steels. 7075 is heavily used within the aerospace industry because of its

high specific strength to weight ratio and excellent corrosion resistance because

of the protective aluminium oxide layer.

The protective oxide means the alloy is more likely to undergo localised pitting

corrosion at intermetallic particles.

Pits form at these sites when the alloy is exposed to an aqueous corrosive

media. Pitting is more dangerous than uniform corrosion as a small but deep pit

can lead to catastrophic failure of a component.


Why are we interested in corrosion of aluminium?

Introduction

Alloy Uniform corrosion Pitting corrosion


Experimental

• Aluminium was polished to a 1 μm finish.

• Intermetallics marked using Vickers microhardness indentations.

• EDX was performed on each and three were selected for further study.

• Sample exposed to KCl 3.5 wt.%, pH 7, for time periods of 0, 15, 45 min and 2, 4, 8,

16 hours.

• Point AES and EDX spectra as well as SAM and EDX maps and SEM micrographs

were collected after each exposure.

• After 16 hours exposure the three intermetallics were cross sectioned using FIB and

SEM micrographs collected.


Marking Intermetallics and their composition

Intermetallics marked using Vickers micro-hardness indents to allow repeated analysis in the same geometry

10 intermetallics marked on the surface of the alloy

Wt.% Mg Al Si Cr Fe Ni Cu Zn

Matrix 2.6 88.8 0.2 0.2 0.3 ND 1.3 6.6

Ten intermetallics 1.1 68.4 2.8 0.8 14.8 0.8 5.7 3.9

EDX quantification


AES point spectra

Intermetallic #1

SEM micrograph t=0

Native surface

Ar+ ion sputtered


Intermetallic #1 Phase Mapping and Composition

Wt.% Mg Al Si Cr Fe Cu Zn

Intermetallic ND 58.3 3.3 5.9 27.9 3.1 1.5

Matrix 1.9 88.6 ND 0.2 ND 1.9 7.4

EDX quantification

Matrix Intermetallic EDX point spectra


Scanning Auger Maps from Intermetallic #1

SEM

Al KLL

O KLL

Fe LMM

Cu LMM

Cr LMM

Mg KLL

Si KLL


EDX Maps Intermetallic #1

Al Kα

O Kα

Fe Kα

Cu Kα

Cr Kα

Zn Kα

Si Kα

Mg Kα

SEM

Cl Kα


AES Point Spectra Intermetallic #1 with increasing Exposure Time in KCl


Post Exposure Phase Analysis Intermetallic #1

Wt.% Mg Al Si Cr Fe Ni Cu Zn Cl O

A 1.6 88.5 ND 0.3 0.2 ND 1.8 7.3 0.3 Omitted

B 0.7 57.4 1.0 0.4 0.8 0.1 1.0 2.1 5.0 44.5

C 11.6 28.5 24.1 0.2 1.5 0.1 0.7 1.5 1.7 42.7

D 0.6 65.4 0.3 4.4 15.5 0.8 8.7 4.0 0.3 Omitted

EDX phase quantification

SEM micrograph t=0 SEM micrograph t=16h


AES point spectra

Intermetallic #2

Native surface

Ar+ ion sputtered



Wt.% Mg Al Si Cr Fe Cu Zn

Intermetallic ND 58.3 3.3 5.9 27.9 3.1 1.5

Matrix 1.9 88.6 ND 0.2 ND 1.9 7.4

EDX quantification

Matrix Intermetallic EDX point spectra



SEM

Al KLL

O KLL

Fe LMM

Cr LMM

Si KLL


EDX maps intermetallic #2

Al Kα

O Kα

Fe Kα

Cu Kα

Cr Kα

Zn Kα

Si Kα

Mg Kα

SEM




AES point spectra

Intermetallic #3

Native surface

Ar+ ion sputtered

SEM micrograph t=0



Wt.% Mg Al Si Cr Fe Ni Cu Zn

Intermetallic 0.4 53.2 0.1 ND 11.6 1.5 33.2 ND

Matrix 1.9 89.9 ND 0.2 ND ND 1.7 7.3

EDX quantification

Matrix Intermetallic

EDX point spectra



SEM

Al KLL

O KLL

Fe LMM

Cu LMM

Si KLL

Zn LMM


EDX Maps Intermetallic #3




Post Exposure Phase Analysis Intermetallic #3

Wt.% Mg Al Si Cr Fe Ni Cu Zn Cl O

A 1.9 84.0 ND 0.3 0.1 ND 2.0 8.2 ND 3.0

B 1.3 76.0 0.2 0.5 0.4 0.1 2.9 9.9 ND 8.3

C 0.3 45.7 1.5 0.1 8.8 1.0 28.2 6.4 0.1 8.0

D 0.6 51.3 3.3 0.3 5.3 0.4 17.5 6.9 0.1 13.8

SEM micrograph t=0 SEM micrograph t=16h

EDX phase quantification


Focussed Ion Beam milling

Intermetallic #1

Intermetallic #2 Intermetallic #3


Diagnosis of Galvanic Activity by Cation Precipitation

To investigate if the intermetallics

were cathodically active the alloy

was exposed to 0.1 M MgCl2

solution for 15 minutes.

When the magnesium chloride is

dissolved the Mg2+ ions are attracted

to the negatively charged cathodic

regions on the sample surface.

The Mg2+ ions react with OH-

hydroxyl ions formed from the

reduction of water at the cathode.

Mg(OH)2 is formed and is extremely

insoluble and so immediately

deposited onto the cathodic surface

where it is formed.

• The first two iron rich Al-Fe-Cu/Si-Cr intermetallics were not involved in micro-

galvanic corrosion. However, crevice corrosion was observed at the

matrix/intermetallic interface.

• The third copper rich (Al7Cu2Fe) intermetallic was found to act as a pitting

initiation site and it has been shown to act as a cathode to the surrounding

alloy and the pitting attack is concentrated at the matrix adjacent to the

intermetallic.

• The corrosion products Al(OH)3, SiO2 and Zn(OH)2 were observed on the

surface of the copper rich intermetallic.


Conclusions and Future Work


Thank you for listening

Any questions?

Localized Corrosion of a 7075 Aluminum Alloy Exposed to KCl · Aluminium alloys are the second most widely used engineering metallic alloys after steels. 7075 is heavily used within

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