CA3037315A1

CA3037315A1 - Stable manganochromite spinel on stainless steel surface

Info

Publication number: CA3037315A1
Application number: CA3037315A
Authority: CA
Inventors: Evan MAH; Allen REULE; Krista STANG
Original assignee: Nova Chemicals Corp
Current assignee: Nova Chemicals Corp
Priority date: 2019-03-20
Filing date: 2019-03-20
Publication date: 2020-09-20
Also published as: EP3942087A1; JP2022525915A; CN113557319A; US20220162735A1; BR112021018539A2; MX2021010466A; WO2020188426A1; KR20210127245A

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.

Description

STABLE MANGANOCHROMITE SPINEL ON STAINLESS STEEL SURFACE
FIELD OF THE INVENTION
The present disclosure relates to 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 (Cr2Mn04) in the outer most surface of a steel or a stainless steel.
BACKGROUND OF THE INVENTION
There are a number of patents in the in the name of Benum et al., assigned to NOVA Chemicals including United States patents: 6,436,202 issued August 20, 2002;
6,824,883 issued November 30, 2004; 6,899,966 issued May 31 2005; 7,156,979 issued January 2, 2007; and 7,488,392 issued February 10, 2009. These patents relate to the production of chromium based spinels on high Ni Cr steels. The spinels typically have the formula MnCr204 alone or in combination with oxides of Mn or Si.
The spinels of Benum are generated without the use of a static positive charge on the steel or stainless steel substrate. Further, the coatings of Benum do not contain species such as Cr, 7Feo303.
U.S. patents 8,197,613 issued June 12, 2012 and U.S. 8,568,538 issued October 29, 2013 to Kerber, assigned to Material Interface, Inc., teach applying a solution or dispersion of nano-particles of one or more oxides of cerium, titanium, lanthanum, and aluminum, silicon, scandium, yttrium, zirconium, niobium, hafnium, tantalum, and thorium plus other rare earth elements (e.g., oxides of these metals).
These references do not contemplate the manganochromite spinel (Cr2Mn04) of the present disclosure.
U.K. patents GB 2 159 542 published March 16, 1988 to Zeilinger et al., and GB 2 169 621, published August 3, 1988 to Muhlratzer et al., both assigned to MAN
Maschinenfabrik Nurnberg AG, teach applying an oxidizing atmosphere to a substrate in the absence of a static electric charge to generate the surface coating.
The surface generated on the substrate largely comprises MnCr204, Cr203, FeCr204 and Fe304.
The references fail to teach or suggest applying a static electric charge to the substrate.
U.S. patent 7,396,597 issued July 8, 2008 to Nishiyama and Yamadera, assigned to Sumitomo Metal Industries, Ltd., discloses the depletion of chromium in a substrate is heated in a controlled atmosphere to produce a dense oxide surface. The reference fails to teach applying a static electric charge to the substrate during treatment.
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.
SUMMARY OF THE INVENTION
An embodiment of the disclosure provides a method to enhance the magnochromite (Cr2Mn04) content of the surface to form a treated surface of a mixed metal oxide on the surface of a 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.
In a further embodiment, 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=rn-2.s-1.
In a further embodiment, 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%.
In a further embodiment, after treatment the surface of the treated substrate has a thickness not less than 2pm.
In a further embodiment, the surface of the treated substrate comprises from 9.8 to 20.0 wt% of a compound of the formula Cr203, from 10.4 to 43.3 wt% of a compound of the formula Cr2Mn04, and from 0 to 22.3 wt% of a compound of the formula Cri 7Feo303.
In a further embodiment, the positive static charge on the substrate is from +7.0 to +14.0 kV.
In a further embodiment, the treated surface on the treated substrate covers not less than 70% of the treated substrate.

2 In a further embodiment, the treatment is at a temperature from 700 C to 750 C.
In a further embodiment, the treated surface of the treated substrate comprises from 9.0 to 11.0 wt% of a compound of the formula Cr203, from 40.0 to 44.0 wt%
of a compound of the formula Cr2Mn04, and from 20.0 to 22.5 wt% of a compound of the formula Cri 7Fe0.303, the sum of the components adding up to 100 wt%.
In a further embodiment, the positive static charge on the substrates is from +9.0 to +10.0 kV.
In a further embodiment, the thickness of the treated surface of the treated substrate is from 2 pm to 5 pm.
In a further embodiment, the substrate comprises from 13 to 50 wt% of Cr, from to 50 wt% of Ni, and the balance is substantially Fe.
In a further embodiment, 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 15 and all other trace metals; and C in an amount of less than 2.0 wt%.
In a further embodiment, 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 one or more of the trace elements to bring the composition up to 100 wt%.
20 In a further embodiment, 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%.
In a further embodiment, 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%.
In a further embodiment, 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%.
An embodiment of the disclosure provides a stainless steel substrate having on at least one surface a treated surface having a thickness of not less than 2pm comprising from 26.1 to 69.6 wt% of a compound of the formula Cro ioFeo.65Nio.25, from 9.8 to 20.0 wt% of a compound of the formula Cr203, from 10.4 to 43.3 wt% of a compound of the formula Cr2Mn04, and from 0 to 22.3 wt% of a compound of the formula Cri 7Fe0303, the sum of the components adding up to 100 wt%.
In a further embodiment, the thickness of the treated surface of the substrate is from 2pm to 5 pm.
In a further embodiment, 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.
In a further embodiment, 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%.
In a further embodiment, 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 one or more of the trace elements to bring the composition up to 100 wt%.
In a further embodiment, 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%.
In a further embodiment, 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%.
In a further embodiment, 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%.
BRIEF DESCRIPTION OF THE DRAWINGS
Error! Reference source not found. is a Scanning Electron Microscopy image of AlS1310 surface after treatment at 200 C.
Error! Reference source not found. is a Scanning Electron Microscopy image of AlS1310 surface after treatment at 740 C.
Error! Reference source not found, is a Scanning Electron Microscopy image of AlS1310 cross section after treatment at 200 C.
Error! Reference source not found, is a Scanning Electron Microscopy image of AlS1310 cross section after treatment at 740 C.

4 DETAILED DESCRIPTION
In a number of industries and particularly the chemical industry stainless steel substrates are used to form equipment (e.g. furnace tubes, steam reforming reactors, heat exchangers and reactors) used in harsh environments which may result in coking of the stainless steel surface. In an ethylene furnace, the furnace tubes may be a single tube or tubes and fittings welded together to form a coil which may be subject to coke build-up, or coking. In hydrocarbon reformers the reactors and piping are subject to similar coking issues. In fluidized catalyst crackers, particularly in the downcomers, there are similar issues. In the piping for gases generated in iron ore reduction process and particularly fluidized bed iron ore reduction, there are similar issues. In gas powered turbines (e.g., jet engines) there are also coke build up issues on components in the turbine.
The substrate may be any material to which the composite coating will bond.
The substrate may be a carbon steel or a stainless steel typically comprising not less than 15 wt% Cr which may be selected from the group consisting of wrought stainless, austentic stainless steel and HP, HT, HU, HW and HX stainless steel, heat resistant steel, and nickel-based alloys. The substrate may be a high strength low alloy steel (HSLA); high strength structural steel or ultra high strength steel. The classification and composition of such steels are known to those skilled in the art.
In one embodiment the stainless steel, preferably heat resistant stainless steel, typically comprises from 18 to 50, preferably 20 to 50, most preferably from 22 to 38 weight % of chromium. The stainless steel may further comprise from 15 to 50, preferably from 25 to 50 most preferably from 25 to 48, desirably from about 30 to 45 weight % of Ni. The balance of the stainless steel is substantially iron and small amounts of minor components disclosed below. Expressed in mole % the above composition ranges would be: Cr 25 to 35 mole %; Fe 15 to 50 mole %; and Ni 18 to 42 mole %.
The present invention may also be used with nickel and/or cobalt based extreme austentic high temperature alloys (HTAs). Typically, the alloys comprise a major amount of nickel or cobalt. Typically, the high temperature nickel based alloys comprise from about 50 to 70, preferably from about 55 to 65 weight % of Ni;
from about 15 to 20 weight % of Cr; from about 10 to 20 weight % of Co; and from about 5 to 9 weight % of Fe and the balance one or more of the trace elements noted below to bring the composition up to 100 weight %. Typically, the high temperature cobalt

5 based alloys comprise from 40 to 65 weight % of Co; from 15 to 20 weight % of Cr;
from 13 to 20 weight % of Ni; less than 4 weight % of Fe and the balance one or more trace elements as set out below and up to 20 weight % of W. The steels also comprise small amounts of minor components as disclosed below. The sum of the components is 100 weight %. The compositions expressed as molar ratios would be as follows:
a) For high temperature nickel based alloys; 56 to 60 mol% Ni, 20 to 22 mol%
Cr, to 18 mol% Co, 5 to 8 mol% Fe;
b) For high temperature cobalt based alloys; 48 to 60 mol% Co, 22 to 24 mol%
Cr, 14 to 20 mol% Ni, less than 4 mol% Fe.
10 In some embodiments of the invention the substrate may further comprise at least 0.2 weight %, up to 3 weight % typically 1.0 weight %, up to 2.5 weight %
preferably not more than 2 weight % of manganese from 0.3 to 2, preferably 0.8 to 1.6 typically less than 1.9 weight A) of Si; less than 3, typically less than 2 weight % of titanium, niobium (typically less than 2.0, preferably less than 1.5 weight %
of niobium) and all other trace metals; and carbon in an amount of less than 2.0 weight %.
The protective coating should cover not less than 75%, preferably more than 85%, desirably more than 95% of the surface area of the treated surface(s) of the substrate.
In some embodiments the surface layer or coating has a thickness up to 10 microns, in some instances 7 microns typically 5 or less microns, in some embodiments at least 1.5 microns, preferably 2 microns thick. The surface has a crystallinity of not less than 40%, preferably greater than 60% and an average crystal size up to 7 microns, preferably less than 5 microns, typically less than 2 microns. The surface covers at least about 70%, preferably 85%, most preferably not less than 95%, desirably not less than 98.5% of the surface of the substrate.
The substrate may be shaped in to an industrially useful part or component such as a tube or pipe, an agitator, a static mixer, a heat exchanger, or even turbine blades for a compressor and similar such parts or components.
As part of the treatment the substrate is subjected to positive static electric charge from +7.0 to +14.0 kV, in some embodiments from +8.0 to +11.5 kV, in further embodiments from +9.0 to +10.0 kV. Electrostatic generators and their methods of use are well known in the art.
The surface of the part or component exposed to the hydrocarbon environment is treated by passing a mixture of steam and an oxidizing gas such as air or a mixture

6 of oxygen and an inert gas (oxygen 15-25 vol% or mole%, inert gas (nitrogen or argon) from 75 to 85 vol% or mole%). The ratio of steam to oxidizing gas typically comprises 50 to 80 wt% steam and 20 to 50 wt% oxidizing gas, in some embodiments from 60 to 90 wt% steam and from 10 to 40 wt% oxidizing gas, in further embodiments from 75 to 85 wt% steam and from 15 to 25 wt% of oxidizing gas.
The mixture of steam and oxidizing gas is passed over the surface of the substrate to generate a treated surface. The treated surface is the surface in contact with the hydrocarbon or other material. For example, in the case of a pipe or vessel the treated surface would be the internal surface. For heat exchangers the treated surface would be on the external surface of the heat exchanger.
The mixture of steam and oxidizing gas is passed over the substrate at a temperature from 200 C to 750 C, in some embodiments at a temperature from 700 C
to 750 C, in further embodiments at a temperature from 700 C to 740 C.
The dosing (flow rate) of the oxidant may range from 0.05 to 0.100 g=
(grams per square meter per second), in some embodiments from 0.075 to 0.095, in further embodiments from 0.085 to 0.090 g=m-2=s-1, desirably 0.088 g=
The dosing (flow rate) for the steam may range from 0.500 g=rn-2.s-1 to 1.000 g=m-2=s-1, in some embodiments from 0.850 to 0.95 g=rn-2.s-1, in further embodiments from 0.870 to 0.900 g=m-2=s-1, desirably 0.881 g=m-2.s-1.
The overall dosing (flow rate) for the gaseous stream may range from 0.550 to 1.100 g=m-2=s-1, in some embodiments from 0.925 to 1.045 g=m-2=s-1, in further embodiments from 0.955 to 0.990 g=m-2=s-1, desirably 0.969 g=
The time of treatment depends on a number of factor including temperature, gas composition and intricacy of the surface being treated (flat to finned), and the thickness of the treated surface being produced. The treatment may be conducted for a period of time from about 2 to 40 hours per square meter of surface, typically from 5 to 30 hours per square meter of surface.
The resulting treated surface will form a surface coating on the substrate not less than 2.0 pm thick, in some embodiments up to 10 pm thick, in further embodiments less than 7 pm thick, typically less than 5 pm thick, and in some embodiments less than 4 pm thick.
The resulting treated surface on the substrate should cover not less than 70%
of the substrate surface which was treated, in some embodiments not less than 85%

7 of the substrate surface which was treated, in further embodiments not less than 90%
of the substrate surface which was treated.
The composition of the surface layer, excluding the underlying metal matrix may have the following compositions, as shown in TABLE 1 and TABLE 2. In some embodiments the surface layer may substantially comprise from about 60 to 65 wt% of Cr204 and from 30 to 40, in some embodiments from 30 to 35 wt% of Cr2Mn04.
This coating may comprise up to about 5, preferably less than 3 wt% of the substrate metal. Preferably the surface layer further comprises Cr1.7Feo303. The surface coating generally comprises from 8 to 15 wt% of Cr203, from 40 to 60 wt% of Cr2Mn04 and from about 18 to 30 wt% of Cr1.7Feo.303 (the sum of the components adding up to 100 wt%). In some embodiments the surface layer may comprise from 9.5 to 14 wt% of Cr203, from 42 to 59 wt% of Cr2Mn04 and from about 20 to 28 wt% of Cri 7Fe0.303 (the sum of the components adding up to 100 wt%).

Surface Coating Ranges ' Surface Element Min wt% Max wt%
Cr 40 45 Fe 18 21 Ni 6 10 Mn 2.5 10 Normalized Composition of the Surface Eskolaite (Cr203) 0.66 , 0.15 0.13 Cr2Mn04 0.34 ' 0.55 0.59 Cr1.7Fe0.303 0.00 0.30 0.28 An embodiment of the disclosure provides a method to enhance the magnochromite (Cr2Mn04) content of the surface to form a treated surface of a mixed

8 metal oxide on the surface of a 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.
In a further embodiment, 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.
In a further embodiment, 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%.
In a further embodiment, after treatment the surface of the treated substrate has a thickness not less than 2pm.
In a further embodiment, the surface of the treated substrate comprises from

9.8 to 20.0 wt% of a compound of the formula Cr203, from 10.4 to 43.3 wt% of a compound of the formula Cr2Mn04, and from 0 to 22.3 wt% of a compound of the formula Cr1.7Feo303.
In a further embodiment, the positive static charge on the substrate is from +7.0 to +14.0 kV.
In a further embodiment, the treated surface on the treated substrate covers not less than 70% of the treated substrate.
In a further embodiment, the treatment is at a temperature from 700 C to 750 C.
In a further embodiment, the treated surface of the treated substrate comprises from 9.0 to 11.0 wt% of a compound of the formula Cr203, from 40.0 to 44.0 wt%
of a compound of the formula Cr2Mn04, and from 20.0 to 22.5 wt% of a compound of the formula Cr1.7Fe0303, the sum of the components adding up to 100 wt%.
In a further embodiment, the positive static charge on the substrates is from +9.0 to +10.0 kV.
In a further embodiment, the thickness of the treated surface of the treated substrate is from 2 pm to 5 pm.
In a further embodiment, the substrate comprises from 13 to 50 wt% of Cr, from 20 to 50 wt% of Ni, and the balance is substantially Fe.

In a further embodiment, 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%.
In a further embodiment, 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 one or more of the trace elements to bring the composition up to 100 wt%.
In a further embodiment, 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%.
In a further embodiment, 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%.
In a further embodiment, 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%.
An embodiment of the disclosure provides a stainless steel substrate having on at least one surface a treated surface having a thickness of not less than 2pm comprising from 26.1 to 69.6 wt% of a compound of the formula Cro ioFeo 65Ni0 25, from 9.8 to 20.0 wt% of a compound of the formula Cr203, from 10.4 to 43.3 wt% of a compound of the formula Cr2Mn04, and from 0 to 22.3 wt% of a compound of the formula Cr1.7Fe0.303, the sum of the components adding up to 100 wt%.
In a further embodiment, the thickness of the treated surface of the substrate is from 2pm to 5 pm.
In a further embodiment, 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.
In a further embodiment, 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%.
In a further embodiment, 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 one or more of the trace elements to bring the composition up to 100 wt%.

In a further embodiment, 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%.
In a further embodiment, 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%.
In a further embodiment, 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%.
EXAMPLE
The present disclosure will now be illustrated by the following non¨limiting example.
A 0.5" OD wrought tube of A1S1310 was charged with a wire lead directly to the OD of the tube through the direct application of a positive electrostatic charge (+9.5 kV). An atmosphere comprising a 10:1 steam:air by volume was passed through the charged pipe at temperatures of 200 C, 710 C and 740 C for a period of time of hours. More nnanganochromite was generated at higher temperatures. The .. composition of the crystalline surface on the inner surface of the pipe was identified by (GI-XRD) spectroscopy supported by an Energy-Dispersive x-ray Spectroscopy (EDS). TABLE 3 shows the composition of the crystalline phases on the surface of the material.

Compound , 200 C 710 C 740 C
Cr203 20.0 11.0 9.8 CuMn04 10.4 40.1 43.3 Cr1.7Feo.303 22.3 20.9 Error! Reference source not found. and Error! Reference source not found. show that the oxide surface is well distributed and provides complete coverage of the bulk alloy at the higher temperature range. Error! Reference source not found. and Error! Reference source not found. show cross sectional images of the surface showing that the oxide layer is up to 2 pm thick when treated at 200 C
(Error!
Reference source not found.) and up to 4 pm thick when treated at 740 C
(Error!
Reference source not found.) respectively. Error! Reference source not found.
also shows annealing twins that are characteristic of high temperature treatments.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method to enhance the magnochromite (Cr2MnO4) content of the surface to form a treated surface of a mixed metal oxide on the surface of a 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.

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.cndot.m-2.s-1 air;
0.5 to 1.0 g.cndot.m-2.s-1 steam; and an overall flow rate from 0.55 to 1.10 g.cndot.

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 2µm.

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 Cr2O3, from 10.4 to 43.3 wt% of a compound of the formula Cr2MnO4, and from 0 to 22.3 wt% of a compound of the formula Cr17Fe0.3O3.

6. The method according to claim 5, wherein the positive static charge on the substrate is from +7.0 to +14.0 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 Cr2O3, from 40.0 to 44.0 wt% of a compound of the formula Cr2MnO4, and from 20.0 to 22.5 wt% of a compound of the formula Cr17Fe0.3O3, 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 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 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 on at least one surface a treated surface having a thickness of not less than 2pm comprising from 26.1 to 69.6 wt% of a compound of the formula CroloFe0.65Ni0.25, from 9.8 to 20.0 wt% of a compound of the formula Cr2O3, from 10.4 to 43.3 wt% of a compound of the formula Cr2MnO4, and from 0 to 22.3 wt% of a compound of the formula Cr1.7Fe0.3O3, 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 2pm to 5 pm.

20. The method 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 method 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 method 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 one or more of the trace elements to bring the composition up to 100 wt%.

23. The method 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 method 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 method 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%.