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For the oxidation and high temperature corrosion resistance of 9-12% Cr steels, the rapid formation of a dense protective Cr-rich oxide scale is important. The initial oxidation processes were investigated on a 9%CrMo-steel and a 12%CrMoV-steel. Auger spectroscopic studies in the first hours showed a non-uniform oxidation and effects of surface, segregation and microstructure. Grain boundaries and martensite laths provide sufficient Cr to the surface for chromia formation, in other areas formation of Si0 2 may prevent Fe-oxide growth. For up to 100 hours the oxidation kinetics is largely controlled by diffusion of the selectively oxidized elements Cr, Mn and Si in the steel, only gradually the transition to control by diffusion in the oxide occurs. Thickness and composition of the scale, and also the diffusion profiles in the steel were determined after 10 and 100 hours oxidation by SNMS, revealing important effects of surface treatments, such as electropolishing, polishing, grinding or sandblasting. Strong surface deformation favours the formation of Cr-rich scales and of flat Cr-depletion minima. Also the effects of different oxidation atmospheres, H2-H20, N2-H2-H20 and N 2-0 2 were investigated and described. The kinetics of the oxide growth and the possibilities to obtain protective scales by pre-oxidation are discussed.

The corrosion inhibition effect of the following organic additives: 2-butin-1,4-diol (BD), N,N 0 -dimethylthiourea (DMTU), N,N 0 -diethylthiourea (DETU) and N,N 0 -diisopropylthiourea (DITU), in steel pickling baths (sulphuric acid solutions) was studied by means of weight loss and electrochemical measurements like open circuit potential (OCP), polarization curves and impedance spectroscopy (EIS). A significant decrease in the corrosion rate was observed in presence of the investigated compounds depending on the nature and the concentration of the additives. The weight loss experiments show an inhibitory efficiency that reaches values beyond 98 %. The protective effect increases with the enlargement of the lateral chain size in the case of the thioureas. The electrochemical experiments show also a protective effect of the organic additives against corrosion. Their adsorptive behaviour is found to follow the Langmuir model.