Deposition of low secondary electron yield (SEY) carbon coatings by magnetron
sputtering onto the inner walls of the accelerator seems to be the most
promising solution for suppressing the electron cloud problem. However, these
coatings change their electron emission properties during long term exposure to
air. The ageing process of carbon coated samples with initial SEY of about 0.9
received from CERN is studied as a function of exposure to different
environments. It is shown that samples having the same initial SEY may age with
different rates. The SEY increase can be correlated with the surface
concentration of oxygen. Annealing of samples in air at 100-200 {\deg}C reduces
the ageing rate and even recovers previously degraded samples. The result of
annealing is reduction of the hydrogen content in the coatings by triggering
its surface segregation followed by desorption.Comment: 4 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop
on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba,
Italy; CERN Yellow Report CERN-2013-002, pp.149-15