Investigation of glass-like sol-gel coatings for corrosion protection of stainless steel against liquid and gaseous attack

Glass-like sol-gel coatings have been investigated as corrosion protective coatings on stainless steel. Magnesium- and borosilicate coatings with thic kness of about 100-700 nm and methyl-modified SiO2 coatings with a thicknes s of about 2 mu m were deposited on stainless steel plates by dip-coating. The coatings were densified between 400 degrees C and 500 degrees C in diff erent atmospheres (N-2, air) for Ih. The corrosion protection against gaseo us attack was investigated by accelerated corrosion tests at 800 degrees C in air for 1 h. A corrosion protection factor was calculated from the relat ion Fe/Fe2O3, determined by XRD on the surface of coated and uncoated sampl es. Methyl-modified SiO2 coatings showed a protection factor, which was 2 o rders of magnitude higher than for the other coatings. Electrochemical inve stigations were performed on samples submerged in a NaCl solution for 200 h . The corrosion propagation, polarization resistance and impedance vector w ere measured. For accelerated corrosion tests, polarization intensity curve s were determined for high potentials of up to 1 V. Again excellent results were obtained for the methyl-modified SiO2 coatings, which remained passiv e for 200 h. Results of the salt spray corrosion test, however, showed no c orrosion protection by the sol-gel coatings. After 2000 h in the salt spray chamber the steel was corroded and the coatings peeled off. It is conclude d that for the further development of these coatings an improved interfacia l passivation will be required.