SURFACE TREATMENT METHOD FOR IMPROVING CREVICE CORROSION RESISTANCE OF TUNGSTEN CONTAINED TWO PHASE STAINLESS STEEL

摘要

PURPOSE: A surface treatment method for improving crevice corrosion resistance of tungsten contained two phase stainless steel by reinforcing passivity layer of the two phase stainless steel is provided. CONSTITUTION: In a two phase stainless steel consisted of ferrite phase and austenite phase, the surface treatment method for improving crevice corrosion resistance of the two phase stainless steel comprises a step of dipping two phase stainless steel into a 3.5% sodium solution for one week; a step of dipping the two phase stainless steel into a 3.5% sodium aqueous solution having pH of less than 2 or greater than 10 between a region (Eo=1.228-0.059pH) where water is decomposed to generate oxygen and a region (Eo=0.000-0.0059pH) where water is decomposed to generate hydrogen; a step of maintaining the two phase stainless steel in the electric potential by increasing an electric potential of strip to WO3=1 stable region (Eo=-0.029-0.059pH) or more if the aqueous solution has pH of less than 2 and increasing an electric potential of the strip to a region (Eo=0.049-0.078pH+0.00981log(WO4¬2-)) where WO4¬2- is stable or more if the aqueous solution has pH of greater than 10 so that tungsten alloyed in the two phase stainless steel is changed into WO3 or WO4¬2-; and dipping the two phase stainless steel into the aqueous solution again for one week, wherein the two phase stainless steel comprises 0.03 wt.% or less of C, 1.0 wt.% or less of Si, 2.0 wt.% or less of Mn, 0.04 wt.% or less of P, 0.004 wt.% or less of S, 2.0 wt.% or less of Cu, 5.0 to 8.0 wt.% of Ni, 22 to 27 wt.% of Cr, 1.0 to 2.0 wt.% of Mo, 3.0 to 5.0 wt.% of W, 0.13 to 0.27 wt.% of N, and one or more of elements selected from the group consisting of 0.03 wt.% or less of Ca, 0.1 wt.% or less of Ca, 0.005 wt.% or less of B and 0.5 wt.% or less of Ti, a ratio (Croq/Nieq) of chromium equivalent to nickel equivalent is 2.2 to 3.0% when the nickel equivalent (Nieq) is defined as Nieq=xNi+30xxC+0.5xxMn+0.33xxCu+30x(xN-0.045), and the chromium equivalent (Croq) is defined as Croq=xCr+xMo+1.5xxSi+0.73xxM, and a weight ratio (W/Mo) of tungsten to molybdenum is 2.6 to 3.4.