Localized Corrosion of a 7075 Aluminum Alloy Exposed to KCl Christopher F. Mallinson Department of Mechanical Engineering Sciences The Surface Analysis Laboratory Monday, 20 April 2015 1
Outline 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
Scanning Auger Microscope Microlab 350 Auger is not dead! Point #8 Point #7 Point #6 Point #5 Point #4 Point #3 Point #2 Point 8.0µm 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) Monday, 20 April 2015 3
Introduction Why are we interested in corrosion of aluminium? 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. Alloy Uniform corrosion Pitting corrosion Monday, 20 April 2015 4
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 5
Marking Intermetallics and their composition 10 intermetallics marked on the surface of the alloy Intermetallics marked using Vickers micro-hardness indents to allow repeated analysis in the same geometry EDX quantification 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 Monday, 20 April 2015 6
Intermetallic #1 AES point spectra Native surface SEM micrograph t=0 Ar + ion sputtered Monday, 20 April 2015 7
Intermetallic #1 Phase Mapping and Composition Matrix Intermetallic EDX point spectra EDX quantification 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 Monday, 20 April 2015 8
Scanning Auger Maps from Intermetallic #1 SEM Al KLL O KLL Fe LMM Cu LMM Mg KLL Cr LMM Si KLL Monday, 20 April 2015 9
EDX Maps Intermetallic #1 SEM Al Kα O Kα Fe Kα Cu Kα Cr Kα Zn Kα Si Kα Mg Kα Cl Kα Monday, 20 April 2015 10
AES Point Spectra Intermetallic #1 with increasing Exposure Time in KCl Monday, 20 April 2015 11
Post Exposure Phase Analysis Intermetallic #1 EDX phase quantification 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 SEM micrograph t=0 SEM micrograph t=16h Monday, 20 April 2015 12
Intermetallic #2 AES point spectra Native surface Ar + ion sputtered Monday, 20 April 2015 13
Intermetallic #2 Phase Mapping and Composition Matrix Intermetallic EDX point spectra EDX quantification 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 Monday, 20 April 2015 14
Scanning Auger Maps from Intermetallic #2 SEM Al KLL O KLL Fe LMM Cr LMM Si KLL Monday, 20 April 2015 15
EDX maps intermetallic #2 SEM Al Kα O Kα Fe Kα Cu Kα Cr Kα Zn Kα Si Kα Mg Kα Monday, 20 April 2015 16
AES Point Spectra Intermetallic #2 with increasing Exposure Time in KCl Monday, 20 April 2015 17
Intermetallic #3 AES point spectra Native surface SEM micrograph t=0 Ar + ion sputtered Monday, 20 April 2015 18
Intermetallic #3 Phase Mapping and Composition Matrix Intermetallic EDX point spectra EDX quantification 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 Monday, 20 April 2015 19
Scanning Auger Maps from Intermetallic #3 SEM Al KLL O KLL Cu LMM Fe LMM Zn LMM Si KLL Monday, 20 April 2015 20
EDX Maps Intermetallic #3 Monday, 20 April 2015 21
AES Point Spectra Intermetallic #3 with increasing Exposure Time in KCl Monday, 20 April 2015 22
Post Exposure Phase Analysis Intermetallic #3 EDX phase quantification 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 Monday, 20 April 2015 23
Focussed Ion Beam milling Intermetallic #1 Intermetallic #2 Intermetallic #3 Monday, 20 April 2015 24
Diagnosis of Galvanic Activity by Cation Precipitation To investigate if the intermetallics were cathodically active the alloy was exposed to 0.1 M MgCl 2 solution for 15 minutes. When the magnesium chloride is dissolved the Mg 2+ ions are attracted to the negatively charged cathodic regions on the sample surface. The Mg 2+ 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. Monday, 20 April 2015 25
Conclusions and Future Work The first two iron rich Al-Fe-Cu/Si-Cr intermetallics were not involved in microgalvanic corrosion. However, crevice corrosion was observed at the matrix/intermetallic interface. The third copper rich (Al 7 Cu 2 Fe) 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, SiO 2 and Zn(OH) 2 were observed on the surface of the copper rich intermetallic. Monday, 20 April 2015 26
Thank you for listening Any questions? Monday, 20 April 2015 27