Stable manganochromite spinel on stainless steel surface

Abstract

The present invention is a method to treat an external layer on a steel or stainless steel substrate. More particularly the disclosure provides a method to increase the amount of manganochromite spinel (Cr2MnO4) in the outer most surface of a steel or a stainless steel. The present disclosure seeks to provide a process to prepare a treatment of an external surface on a steel or stainless steel substrate by subjecting the surface to an atmosphere of steam and air or synthetic air (a combination of oxygen and other inert gases such as nitrogen or argon) while subjecting the substrate to a static electrical charge from +7.0 to +14.0 kV. The present disclosure also seeks to provide the coated substrate.

1 . A method to enhance a magnochromite content of a surface of a stainless steel substrate, wherein a mixed metal oxide is formed on the surface of the stainless steel substrate by applying a +7.0 to a +14.0 kV static charge to the substrate while exposing the surface to a treating atmosphere comprising 50 to 80 wt % steam and 20 to 50 wt % air at a temperature from 200° C. to 750° C., forming a treated substrate. 
     
     
         2 . The method according to  claim 1 , wherein the components of the treating atmosphere are dosed in an amounts 0.05 to 0.10 g·m −2 ·s −1  air; 0.5 to 1.0 g·m −2 ·s −1  steam; and an overall flow rate from 0.55 to 1.10 g·m −2 ·s −1 . 
     
     
         3 . The method according to  claim 1 , wherein the substrate is selected from a carbon steel or wrought stainless steel, austentic stainless steel and HP, HT, HU, HW and HX stainless steel, heat resistant steel, and nickel based alloys provided the minimum content of chromium in the substrate is not less than 15 wt %. 
     
     
         4 . The method according to  claim 3 , wherein after treatment the surface of the treated substrate has a thickness not less than 2 μm. 
     
     
         5 . The method according to  claim 4 , wherein the surface of the treated substrate comprises from 9.8 to 20.0 wt % of a compound of the formula Cr 2 O 3 , from 10.4 to 43.3 wt % of a compound of the formula Cr 2 MnO 4 , and from 0 to 22.3 wt % of a compound of the formula Cr 1.7 Fe 0.3 O 3 . 
     
     
         6 . The method according to  claim 5 , wherein the positive static charge on the substrate is from +9.0 to +10.0 kV. 
     
     
         7 . The method according to  claim 6 , wherein the treated surface on the treated substrate covers not less than 70% of the treated substrate. 
     
     
         8 . The method according to  claim 7 , wherein the treatment is at a temperature from 700° C. to 750° C. 
     
     
         9 . The method according to  claim 8 , wherein the treated surface of the treated substrate comprises from 9.0 to 11.0 wt % of a compound of the formula Cr 2 O 3 , from 40.0 to 44.0 wt % of a compound of the formula Cr 2 MnO 4 , and from 20.0 to 22.5 wt % of a compound of the formula Cr 1.7 Fe 0.3 O 3 , the sum of the components adding up to 100 wt %. 
     
     
         10 . The method according to  claim 9 , wherein the positive static charge on the substrates is from +9.0 to +10.0 kV. 
     
     
         11 . The method according to  claim 10 , wherein the thickness of the treated surface of the treated substrate is from 2 μm to 5 μm. 
     
     
         12 . The method according to  claim 11 , wherein the substrate comprises from 13 to 50 wt % of Cr, from 20 to 50 wt % of Ni, and the balance is substantially Fe. 
     
     
         13 . The method according to  claim 12 , wherein the substrate further comprises at least 0.2 wt % up to 3 wt % of Mn; from 0.3 to 2 wt % of Si; less than 3 wt % of Ti; less than 2.0 wt % of Nb and all other trace metals; and C in an amount of less than 2.0 wt %. 
     
     
         14 . The method according to  claim 11 , wherein the substrate comprises from about 50 to 70 wt % of Ni; from about 10 to 20 wt % of Cr; from about 10 to 20 wt % of Co; and from about 5 to 9 wt % of Fe and the balance comprising one or more of the trace elements to bring the composition up to 100 wt %. 
     
     
         15 . The method according to  claim 14 , wherein the substrate further comprises at least 0.2 wt % up to 3 wt % of Mn; from 0.3 to 2 wt % of Si; less than 3 wt % of Ti; less than 2.0 wt % of Nb and all other trace metals; and C in an amount of less than 2.0 wt %. 
     
     
         16 . The method according to  claim 11 , wherein the substrate comprises from 40 to 65 wt % of Co; from 15 to 20 wt % of Cr; from 13 to 20 wt % of Ni; less than 4 wt % of Fe; up to 20 wt % of W; and the balance comprising one or more trace elements to bring the composition up to 100 wt %. 
     
     
         17 . The method according to  claim 16 , wherein the substrate further comprises at least 0.2 wt % up to 3 wt % of Mn; from 0.3 to 2 wt % of Si; less than 3 wt % of Ti; less than 2.0 wt % of Nb and all other trace metals; and C in an amount of less than 2.0 wt %. 
     
     
         18 . A stainless steel substrate having a treated surface having a thickness of not less than 2 μm comprising from 26.1 to 69.6 wt % of a compound of the formula Cr 0.10 Fe 0.65 Ni 0.25 , from 9.8 to 20.0 wt % of a compound of the formula Cr 2 O 3 , from 10.4 to 43.3 wt % of a compound of the formula Cr 2 MnO 4 , and from 0 to 22.3 wt % of a compound of the formula Cr 1.7 Fe 0.3 O 3 , the sum of the components adding up to 100 wt %. 
     
     
         19 . The substrate according to  claim 18 , wherein the thickness of the treated surface of the substrate is from 2 μm to 5 μm. 
     
     
         20 . The substrate according to  claim 19 , wherein the substrate comprises from 13 to 50 wt % of Cr, from 20 to 50, preferably from 25 to 50 wt % of Ni, and the balance is substantially iron. 
     
     
         21 . The substrate according to  claim 20 , wherein the substrate further comprises at least 0.2 wt % up to 3 wt % of Mn; from 0.3 to 2 wt % of Si; less than 3 wt % of Ti, less than 2.0 wt % of Nb and all other trace metals; and C in an amount of less than 2.0 wt %. 
     
     
         22 . The substrate according to  claim 19 , wherein the substrate comprises from about 50 to 70 wt % of Ni; from about 10 to 20 wt % of Cr; from about 10 to 20 wt % of Co; and from about 5 to 9 wt % of Fe; and the balance comprising one or more trace elements to bring the composition up to 100 wt %. 
     
     
         23 . The substrate according to  claim 19 , wherein the substrate further comprises at least 0.2 wt % up to 3 wt % of Mn; from 0.3 to 2 wt % of Si; less than 3 wt % of Ti; less than 2.0 wt % of Nb and all other trace metals; and C in an amount of less than 2.0 wt %. 
     
     
         24 . The substrate according to  claim 19 , wherein the substrate comprises from 40 to 65 wt % of Co; from 15 to 20 wt % of Cr; from 13 to 20 wt % of Ni; less than 4 wt % of Fe; up to 20 wt % of W; and the balance one or more trace elements to bring the composition up to 100 wt %. 
     
     
         25 . The substrate according to  claim 24 , wherein the substrate further comprises at least 0.2 wt % up to 3 wt % of Mn; from 0.3 to 2 wt % of Si; less than 3 wt % of Ti; less than 2.0 wt % of Nb and all other trace metals; and C in an amount of less than 2.0 wt %.