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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
Monday, 20 April 2015 2
Outline
Monday, 20 April 2015 3
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.
Monday, 20 April 2015 4
Why are we interested in corrosion of aluminium?
Introduction
Alloy Uniform corrosion Pitting corrosion
Monday, 20 April 2015 5
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.
Monday, 20 April 2015 6
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
Monday, 20 April 2015 7
AES point spectra
Intermetallic #1
SEM micrograph t=0
Native surface
Ar+ ion sputtered
Monday, 20 April 2015 8
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
Monday, 20 April 2015 9
Scanning Auger Maps from Intermetallic #1
SEM
Al KLL
O KLL
Fe LMM
Cu LMM
Cr LMM
Mg KLL
Si KLL
Monday, 20 April 2015 10
EDX Maps Intermetallic #1
Al Kα
O Kα
Fe Kα
Cu Kα
Cr Kα
Zn Kα
Si Kα
Mg Kα
SEM
Cl Kα
Monday, 20 April 2015 11
AES Point Spectra Intermetallic #1 with increasing Exposure Time in KCl
Monday, 20 April 2015 12
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
Monday, 20 April 2015 13
AES point spectra
Intermetallic #2
Native surface
Ar+ ion sputtered
Monday, 20 April 2015 14
Intermetallic #2 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
Monday, 20 April 2015 15
Scanning Auger Maps from Intermetallic #2
SEM
Al KLL
O KLL
Fe LMM
Cr LMM
Si KLL
Monday, 20 April 2015 16
EDX maps intermetallic #2
Al Kα
O Kα
Fe Kα
Cu Kα
Cr Kα
Zn Kα
Si Kα
Mg Kα
SEM
Monday, 20 April 2015 17
AES Point Spectra Intermetallic #2 with increasing Exposure Time in KCl
Monday, 20 April 2015 18
AES point spectra
Intermetallic #3
Native surface
Ar+ ion sputtered
SEM micrograph t=0
Monday, 20 April 2015 19
Intermetallic #3 Phase Mapping and Composition
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
Monday, 20 April 2015 20
Scanning Auger Maps from Intermetallic #3
SEM
Al KLL
O KLL
Fe LMM
Cu LMM
Si KLL
Zn LMM
Monday, 20 April 2015 21
EDX Maps Intermetallic #3
Monday, 20 April 2015 22
AES Point Spectra Intermetallic #3 with increasing Exposure Time in KCl
Monday, 20 April 2015 23
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
Monday, 20 April 2015 24
Focussed Ion Beam milling
Intermetallic #1
Intermetallic #2 Intermetallic #3
Monday, 20 April 2015 25
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.
Monday, 20 April 2015 26
Conclusions and Future Work
Monday, 20 April 2015 27
Thank you for listening
Any questions?