CORROSION AND OXIDATION BEHAVIOR OF POLYMER DERIVED CERAMIC COATINGS WITH PASSIVE GLASS FILLERS ON AISI 441 STAINLESS STEEL

Polymer derived ceramic (PDC) polysilazane-based double layer composite coatings on steel substrates consisting of a PDC bond-coat, and a PDC-based top-coat containing ceramic passive (ZrO2) and active (ZrSi2) fillers, as well as a specially tailored passive glass filler with high melting temperature were developed. The corrosion and oxidation behaviour of the ferritic stainless steel AISI 441 with and without the PDC coating system was evaluated. SEM examination revealed the composite coatings were not dense but contained small closed pores with a diameter up to 100 nm. Cracks and occasionally also delamination were detected. The static corrosion tests under hydrothermal conditions at the temperature of 200 degrees C resulted in partial oxidation of the uncoated steel substrate, whereas the coated sample was protected by the PDC coating. No oxidation of the top coat was detected after the static corrosion test. The high temperature oxidation behaviour in a flow-through oxygen atmosphere was investigated at the temperatures of 900 degrees C, 1000 degrees C and 1100 degrees C. X-ray diffraction confirmed extensive corrosion of the uncoated AISI 441 stainless steel accompanied by formation of a Cr2O3, TiO2 and a (Mn,Cr)(3)O-4 spinel containing layer of corrosion products. Beneficial effect of the PDC coating was observed at the temperature of 900 degrees C, demonstrated by marked reduction of the weight gain of coated steel after 196 h of exposure to flowing oxygen. At higher temperatures the protective action of the PDC coat was not observed, and a thick layer of corrosion products was formed at the steel/coating interface.