. Salt spray test
As demonstrated by AESEC experiments, the neutral pH conditions
associated with a high chloride concentration result in a relatively
high dissolution rate of Fe from the Fe rich phase in the coating. This results
in a relatively rapid red rust formation. The Fe dissolution can also
result in the formation of cavities in place of the dissolved Fe–Si phase.
The Al rich phase should corrode by a pitting mechanism because of
the low solubility of Al oxide at close to neutral pH. The result would
be an acidification in the cavities and in the pits resulting in the formation
of Al2O3, identified by XRD in the corrosion products. In continuous
salt spray test the pH increase by cathodic oxygen reduction is limited
because the electrolyte film is rapidly renewed in the cathodic areas
but more difficult to remove from acidified pits. The Al-rich phase continues
to corrode by pitting mechanism forming aluminum oxide in the
vicinity of pits.