For an oxide whose presence in a film can be signed by measuring the bandgap, the conduction type (chromia n and p in fig.1), but also the lateral distribution (hematite (Fe,Cr)2O3 in fig.2 ), or the adherence level (TiO2 rutile in fig.3) are all information that can be provided by photoelectrochemical techniques and help us to better understand the mechanisms conditioning the durability of in-service alloys.
fig.1: photocurrent vs. potential curves of chromia Cr2O3 fthermally grown on pure chromium, underlining the n-type (resp. p-type) SC of the internal (resp. external) part of the scale.
fig.2: photocurent images at 2.41 eV et 2.73 eV of an oxide film thermally grown on a ferrtic steel, underlining the spatial distribution of haematite Fe2O3 and of the rhomboedric solid solution (Fe,Cr)2O3.
This kind of information allows to better understand the development of the catastrophic oxidation of steels, linked with the formation of iron-rich oxides instead of a passive chromia layer Cr2O3.
fig.3: interface Cohesion Image (ICI) and Structural Qualitu Image (SQI) obtained from photocurrent recorded at differents applied potetials on a thermal oxide thin film of TiO2 rutile, underlining heterogeneities of dechohesion and growth stresses.
These heterogeneities are linked to the presence of pores, cavities, micro disbondings which can lead to the oxide film spallation and consequantly to the rapid degradation of the material.