Local corrosion behaviour of free machining titanium alloys investigated by SKPFM and in-situ AFM

F. Depentori, J. Laukart, S. Benfer, C. Siemers, W. Fürbeth

Proceedings EUROCORR 2011


The corrosion properties of new, free machining titanium alloys were investigated. These alloys are characterized by an improved machinability due to the addition of Lanthanum, which forms second phase particles. Before technical use, these
alloys have to be investigated for their corrosion properties. To achieve this goal, different integral methods like electrochemical impedance spectroscopy (EIS), po
tentiodynamic polarization and exposure tests are used. The materials’ stability is investigated in solutions of sodium chloride, fluoride containing solutions and diluted acids.
The characterization of local corrosion properties via AFM is in the focus of this work. Scanning Kelvin Probe Force Microscopy (SKPFM) and in-situ AFM in a fluid cell
with and without external polarization (EC-AFM) are used to characterize local corrosion properties. The material surface is investigated before and after corrosi
on experiments with Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA).
Results show that all alloys contain precipitates, mainly consisting of Lanthanum and Copper, which determine the early stage corrosion stability of the alloys. These precipitates undergo selective dissolution, which can be monitored by in-situ AFM and usually happens during the first 24 h of exposure, depending on the testing media. In SKPFM, the precipitates show a negative Volta potential difference to the matrix, which indicates a low precipitate stability and preferential dissolution. Different heat treatments like hot for ging or annealing can have beneficial as well as detrimental effects on the stability of the alloys.

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