Microstructure characterization of electro- chemically activated aluminium Yingda Yu 1, Øystein Sævik 1, Jan Halvor Nordlien 2 and Kemal Nisancioglu 1 1 Department of Materials Technology, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway 2 Hydro Aluminum R&D Materials Technology, N-4256 Håvik, Norway Light Metal Surface Science

2 Background  In our Electrochemistry group, a long history to investigate the Al anodic activation phenomenon by high temperature annealing.  This curious anodic activation is found to be related with the segregation of the trace element lead (Pb) at the alloy surfaces.  This activation mechanism has not been fully explained, even though this phenomenon has long been exploited for practical purposes.  The objective of the present work is to provide further microstructure evidence for understanding this Al anodic activation phenomenon.

3 High temperature annealing leads to electrochemical activation as indicated by corrosion potential (1) and polarization measurements (2). (1) measured in acidified sea water after AA 3102 heat treatment at different temperature for 2h. (2) measured in 5% NaCl solution after AA 3102 heat treatment at 600ºC for different annealing time. JTB Gunderson PhD Thesis NTNU2002

4 The 50ppm Pb-Al binary model alloy after high temperature heat treatment in air is used in the present investigation for understanding the Pb segregation on Al surface. GD-OES depth profiles suggested the electrochemical activation is related to the enrichment of the trace element Pb at the oxide-metal interface. JTB Gunderson PhD Thesis NTNU2002 GD-OES depth profiles for AA3102 heat-treated for 60 min at 600°C

5 10 nm (111) (100) The results reported at the 2002 LMSS annual meeting (111) (100) Even though, no Pb particles were found in the surface region, TEM EDS analyses indicated the increasing Pb segregation at the metal-oxide interface with increasing annealing time.

6 Experimental Sample Detail 50ppm Pb-Al model system, 600ºC annealing for 1 hr, 2hr and 4hr TEM observation Philips CM30 operated at an accelerating voltage 300 kV TEM sample preparation Cross-sectional samples cut to slice  ground to 150  m  dimpled  to about 30  m  ion beam thinning at degree 3.5 kV (PIPS)

7 Where are the Pb-containing particles distributed? Pb-containing particles are segregated at the metal-oxide interface.

8 EDS micro-probe composition analysis Even though the particle size with only 5 nm in diameter, it can emit strong enough TEM EDS peaks, both for Pb L and Pb M.

9 Pb-containing particles are in metallic form  HRTEM investigation reveals that the fringe distances of the Pb- containing particles are around nm which close to Pb (111).  Moreover, by using the  -Al 2 O 3 as the internal reference, the particle lattice distance can be exactly determined as nm that suggests these particles are in metallic state.  -Al 2 O 3 Al Matrix nm nm

10 Lattice parameters of all possible Phases nm (106) nm (210) nm (111) PbAl 12 O 19 Hexagonal a = c = 0.219nm nm (111) nm (200) Pb FCC Cubic a = nm nm (111) nm (202) nm (133)  -Al 2 O 3 FCC Cubic a = nm Spinel nm (111) nm (200) Al FCC Cubic a = nm The lattice planes and distances related with the present HRTEM investigation ---

11 How to be related with surface activation?  The particle shapes are supposed not change too much under TEM beam since they are embedded inside the sample.  Even though the elliptic shaped particles were sometimes detached from the interface, their surfaces facing the metal interface were flat, indicating that these particles were originally located at metal-oxide interfaces. Epoxy Oxide Al Matrix Interface Al Matrix Interface Oxide

12  Particle size effect – inversely proportional to nano-particle size – lower the melting point Melting Point depression of the embedded nano-particles  The particle with no orientation relationship between Pb /Al interface is more unstable that that with semi-coherent interface under electron beam irradiation.  Understanding the interface state is important to reveal Pb activation behaviour Poor WettingGood Wetting first TEM investigation (Takagi J. Phys. Soc. Jap.1954)

13 Conclusions  Small Pb particles of the order 10 nm in size were detected and originally located at the metal-oxide interface.  The particles are metallic Pb based on lattice fringe measurement and thermodynamic considerations.  The initial HRTEM results reveal that these nano-particles are randomly oriented with incoherent interfaces with neighbouring alumina and aluminium.

14 Acknowledgments Light Metal Surface Science is funded by: The Norwegian Research Council DuPont Powder Coatings Electro Vacuum AS GSB Jotun Powder Coatings AS NORAL AS Norsk Industrilakkering AS Profillakkering AS An industry consortium consisting of: Hydro Aluminium