In plasma spraying the preheating of low carbon steel substrates (X40) with a rough surface resulting from grit blasting, has received an increased interest because it promotes the adhesion and cohesion (A/C) of alumina coatings. Earlier works have reported the generation of a duplex Fe3O4/Fe2O3 layer after preheating by the d.c. plasma jet. Improved A/C values were obtained for an increase in Fe3O4 content in the oxide layer. Much improved adherence values were achieved with alumina spayed on Fel-xO layers. In this case, the Adhesion/cohesion values obtained on smooth substrate (Ra ~ 0.1 µm) were good (~ 40 MPa) and close to those obtained on rough ones (Ra ~ 5 µm). First the time evolution of the Fel-xO layer thickness and morphology produced in a controlled atmosphere furnace was studied. Then the influence of the plasma preheating of the wustite layers was studied by SEM, and XRD analyses. XRD analysis results indicated a slight variation in the oxide phases depending on the preheating cycles. Fe3O4 was found to increase when a decomposition of Fel-xO into Fe3O4 was observed.