EFFECT OF NICKEL AND TUNGSTEN ON DISSIMILAR MATERIAL CREVICE CORROSION RESISTANCE OF SUPER FERRITIC STAINLESS STEELS

摘要

The effects of nickel and tungsten on dissimilar material crevice (DMC) corrosion resistance of 30 percent chromium - containing ferritic steels were studied by electrochemical tests (polarization test, potentiostatic test, and galvanic corrosion test). In addition, X-ray photoelectron spectroscopy (XPS) was taken to analyze the passive film after electrochemical tests. The polarization test and ferritic chloride test shows that nickel effectively improved the corrosion resistance in the active zone and nickel is expected to effectively increase DMC corrosion resistance though nickel degraded the pitting corrosion resistance. In the mean while, tungsten partially substituted for molybdenum did not affected the active corrosion resistance or pitting corrosion resistance. The galvanic corrosion test shows that tungsten rather than nickel was more effective to DMC corrosion resistance indicating that the effects of alloy elements on the galvanic corrosion current seem to be inconsistent with those on polarization curve. The analysis of surface film with XPS provides the following solution for these inconsistencies" During long exposure at the active region, chromium and iron are preferentially dissolved and molybdenum oxide and tungsten oxide are enriched in the surface. Especially, the accumulation of tungsten oxide, which is well known as a stable species in an acidic solution, improved the DMC corrosion resistance.