Localized Corrosion of a 7075 Aluminum Alloy Exposed to KCl

Transcription

1 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

2 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

3 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

4 Introduction Why are we interested in corrosion of aluminium? Aluminium alloys are the second most widely used engineering metallic alloys after steels 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

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

6 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 ND Ten intermetallics Monday, 20 April

7 Intermetallic #1 AES point spectra Native surface SEM micrograph t=0 Ar + ion sputtered Monday, 20 April

8 Intermetallic #1 Phase Mapping and Composition Matrix Intermetallic EDX point spectra EDX quantification Wt.% Mg Al Si Cr Fe Cu Zn Intermetallic ND Matrix ND 0.2 ND Monday, 20 April

9 Scanning Auger Maps from Intermetallic #1 SEM Al KLL O KLL Fe LMM Cu LMM Mg KLL Cr LMM Si KLL Monday, 20 April

10 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

11 AES Point Spectra Intermetallic #1 with increasing Exposure Time in KCl Monday, 20 April

12 Post Exposure Phase Analysis Intermetallic #1 EDX phase quantification Wt.% Mg Al Si Cr Fe Ni Cu Zn Cl O A ND ND Omitted B C D Omitted SEM micrograph t=0 SEM micrograph t=16h Monday, 20 April

13 Intermetallic #2 AES point spectra Native surface Ar + ion sputtered Monday, 20 April

14 Intermetallic #2 Phase Mapping and Composition Matrix Intermetallic EDX point spectra EDX quantification Wt.% Mg Al Si Cr Fe Cu Zn Intermetallic ND Matrix ND 0.2 ND Monday, 20 April

15 Scanning Auger Maps from Intermetallic #2 SEM Al KLL O KLL Fe LMM Cr LMM Si KLL Monday, 20 April

16 EDX maps intermetallic #2 SEM Al Kα O Kα Fe Kα Cu Kα Cr Kα Zn Kα Si Kα Mg Kα Monday, 20 April

17 AES Point Spectra Intermetallic #2 with increasing Exposure Time in KCl Monday, 20 April

18 Intermetallic #3 AES point spectra Native surface SEM micrograph t=0 Ar + ion sputtered Monday, 20 April

19 Intermetallic #3 Phase Mapping and Composition Matrix Intermetallic EDX point spectra EDX quantification Wt.% Mg Al Si Cr Fe Ni Cu Zn Intermetallic ND ND Matrix ND 0.2 ND ND Monday, 20 April

20 Scanning Auger Maps from Intermetallic #3 SEM Al KLL O KLL Cu LMM Fe LMM Zn LMM Si KLL Monday, 20 April

21 EDX Maps Intermetallic #3 Monday, 20 April

22 AES Point Spectra Intermetallic #3 with increasing Exposure Time in KCl Monday, 20 April

23 Post Exposure Phase Analysis Intermetallic #3 EDX phase quantification Wt.% Mg Al Si Cr Fe Ni Cu Zn Cl O A ND ND ND 3.0 B ND 8.3 C D SEM micrograph t=0 SEM micrograph t=16h Monday, 20 April

24 Focussed Ion Beam milling Intermetallic #1 Intermetallic #2 Intermetallic #3 Monday, 20 April

25 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

26 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

27 Thank you for listening Any questions? Monday, 20 April