THE CHARACTERIZATION OF THE TARNISHING OF CU-15NI-8SN AND CU-5AL-5SN ALLOYS

The corrosion and layer formation of the alloys Cu-15Ni-8Sn and Cu-5Al -5Sn in 0.1 M NaOH solution upon exposure to air have been investigate d using X-ray Photo-electron Spectroscopy. For the alloy Cu-15Ni-8Sn a n enrichment of the nickel and tin ions is revealed if the sample is p repared electrochemically in a 0.1 M NaOH solution. Nickel acting as a barrier hinders the migration of the tin ions and as a result a thin and protective tarnishing film consisting of Cu(I) compounds is formed . In the 0.1 M NaOH solution a strong tin dissolution is observed for the alloy Cu-5Al-5Sn and a thick and porous oxide film consisting of C u(II) compounds is formed. An exposure to air results in tin enrichmen t. As a consequence, a higher corrosion rate is observed for the alloy Cu-5Al-5Sn in the 0.1 M NaOH solution and an improved corrosion behav iour under atmospheric conditions which is most probably due to the pr esence of tin.