Protective coating for metal surfaces

Abstract

A method for heating or cooling a carbon containing reducing gas having a carbon monoxide content of at least 0.5 vol %, wherein the gas is heated to a temperature of at least 400 C. or wherein the gas is cooled from a temperature exceeding 400 C., wherein the gas is passed along a surface of a heating or cooling unit having a heat conductive metal or metal alloy body and a protective layer, which protective layer provides said surface, and which protective layer is made from a coating composition including colloidal amorphous silicate and crystalline oxide particles.

Claims

1. A method for heating or cooling a carbon containing reducing gas having a carbon monoxide content of at least 0.5 vol %, wherein the gas is heated to a temperature of at least 400 C., the method comprising: heating the gas by passing the gas along a surface of a heating or cooling unit comprising a heat conductive metal or metal alloy body and a protective layer that reduces corrosion caused by carburization or metal dusting, wherein the protective layer provides said surface, and is made from a coating composition comprising: colloidal amorphous silicate comprising colloidal silica and amorphous silicate; crystalline oxide particles; optionally an emissivity agent; and optionally a stabiliser/binding agent.

2. A method according to claim 1, wherein the reducing gas has a content of carbon monoxide of at least 1 vol %.

3. A method according to claim 1, wherein the reducing gas has a content of carbon monoxide of at least 2 vol %.

4. A method according to claim 1, wherein the reducing gas comprises hydrogen (H.sub.2) and carbon monoxide (CO) as the major components.

5. A method according to claim 4, wherein carbon monoxide and hydrogen in the reducing gas are reacted to form hydrocarbon.

6. A method according to claim 1, wherein the reducing gas comprises 20-80 vol. % H.sub.2 and 5-50 vol. % CO.

7. A method according to claim 6, wherein the reducing gas further comprises at least one of 1-15 vol. % CO.sub.2, 1-40 vol. % CH.sub.4 and 1-75 vol. % H.sub.2O.

8. A method according to claim 1, wherein the reducing gas is synthesis gas.

9. A method according to claim 1, wherein the heating or cooling method is used in a process selected from the group consisting of coal liquefaction, coal gasification, synthesis gas production, and processing a synthesis gas.

10. A method according to claim 1, wherein the heating or cooling unit is a tubular heating or cooling unit, the gas is passed through the tube and the protective layer is provided on the inner surface of the tube.

11. A method for heating a gas according to claim 1, wherein the gas is heated to a temperature of 400-900 C.

12. A method according to claim 1, wherein the gas comprises a carbon compound selected from the group consisting of carbon monoxide, alkanes, and carbon dioxide.

13. A method according to claim 1, wherein the gas is essentially free of molecular oxygen.

14. A method according to claim 1, wherein the metal or metal containing alloy is a nickel alloy.

15. A method according to claim 1, wherein the colloidal amorphous silicate comprises at least one of an amorphous colloidal alkali metal silicate and an alkaline earth metal silicate.

16. A method according to claim 1, wherein the coating composition comprises, based on dry weight of the composition: 5-35 wt. % colloidal amorphous silicate; 23-79 wt. % crystalline oxide particles; 0-20 wt. % emissivity agent; 0-5 wt. % stabiliser/binding agent.

17. Method according to claim 1, wherein the colloidal amorphous silicate is selected from the group comprising sodium silicate, potassium silicate, calcium silicate and magnesium silicate.

18. A method according to claim 1, wherein the crystalline oxide particles are selected from the group consisting of silicon dioxide, aluminium oxide, titanium dioxide, magnesium oxide, calcium oxide, boron oxide, lithium oxide and zirconium oxide.

19. A method according to claim 1, wherein the coating comprises colloidal amorphous sodium silicate particles, colloidal silica and crystalline aluminium oxide.

20. A method according to claim 1, wherein an emissivity agent is present selected from the group consisting of the silicon hexaboride, boron carbide, silicon tetraboride, silicon carbide, molybdenum disilicide, tungsten disilicide, zirconium diboride, cupric chromite and metallic oxides.

21. A method according to claim 1, wherein a stabiliser agent is present and selected from the group consisting of bentonite, kaolin, magnesium alumina silica clay, tabular alumina and stabilized zirconium oxide.

22. A method according to claim 1, wherein the protective layer is non-porous.

23. A method according to claim 1, wherein the protective layer is glassy.

24. A method according to claim 1, wherein the heating or cooling method is used in a steam reforming synthesis gas production process.

25. A method according to claim 1, wherein the heating or cooling method is used in a synthesis gas process selected from the group consisting of an ammonia production process, a direct reduction process, and a Fischer-Tropsch process.

26. The method according to claim 1, wherein the protective layer comprises an amorphous silicate in amount of 5-35 wt. % and crystalline oxide in an amount of 23-79 wt. %, based on total dry weight of the coating.

27. A method according to claim 1, wherein the gas further comprises hydrogen gas.

28. A method according to claim 1, further comprising: cooling the gas from a temperature exceeding 400 C. in a unit comprising the protective layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is further illustrated by corrosion tests of which results are shown in FIGS. 1 to 5.

(2) FIG. 1 shows an uncoated test surface for a metal dusting experiment before exposure (Uncoated Stainless steel 19Cr8Ni2Mn67Fe (304H)).

(3) FIG. 2. shows the state of the uncoated test surface of FIG. 1 after 500 hours exposure to a reducing gas.

(4) FIG. 3. shows a coated test surface for a metal dusting experiment before exposure.

(5) FIG. 4. Shows the state of the coated test surface for a metal dusting experiment after 500 hours exposure.

(6) FIG. 5 shows the EDX analysis of the coating material

DESCRIPTION OF EMBODIMENTS

(7) Stainless steel 19Cr8Ni2Mn67Fe (304H) test plates with dimensions of 25 mm80 mm (FIG. 1) were provided for exposure to a reducing gas. The gas comprised 47 vol % H.sub.2, 37 vol % CO, 7 vol % CO.sub.2, 9 vol % H.sub.2O. The effect of the reducing gas on a uncoated plate was compared with the effect of the reducing gas of a plate that had been coated with a coating composition comprising colloidal amorphous silica and crystalline silica. The thickness of the coating was about 50 m. FIG. 5 shows the EDX analysis of the coating material. The coating contain: O: 61 wt % Na: 2 wt % Al: 0.5 wt % Si: 36 wt %

(8) The plates were exposed to the reducing gas for 500 hours at 600 C. As illustrated by FIG. 2, the surface of the uncoated stainless steel shows pitting as well as uniform surface corrosion. The weight loss of the plate was 1 g.

(9) However, the corrosion of the plate provide with the protective coating was negligible, as illustrated by FIG. 4. The weight of the plate before and after exposure was the same.