GB2206898A

GB2206898A - Chromized coatings containing vanadium

Info

Publication number: GB2206898A
Application number: GB08814998A
Authority: GB
Inventors: Wate T Bakker; Roger A Perkins; Clyde E Lewis
Original assignee: Electric Power Research Institute Inc
Current assignee: Electric Power Research Institute Inc
Priority date: 1987-07-01
Filing date: 1988-06-23
Publication date: 1989-01-18
Anticipated expiration: 2008-06-23
Also published as: JPS6428355A; GB2206898B; GB8814998D0; DE3821767A1; JPH0357184B2

Description

1 1 CHROMIZED COATINGS CONTAINING VANADI The present invention is directed

to an improved corrosion-resistant protective coating for steel substrates containing chromium and vanadium, and a process for simultaneously depositing chromium and vanadium onto the steel substrate to form a diffused surface layer containing chromium and vanadium.

BACKGROUND

Corrosion both at high and low temperatures is a major cause of premature failure of equipment in the generation of electric power. This has led to the development of a large number of corrosion-resistant alloys. Such alloys typically contain chromium as the major alloying element providing corrosion resistance. Typical corrosion-resistant steel alloys contain 12 to 50% chromium. Smaller quantities, 1 to 10% of Mo, Al, Ni, and less frequently, V and Si are added to provide enhanced resistance to specific corrosion mechanisms: Al and Si for high temperature oxidation and sulfidation resistance, Mo, V, and Ni for resistance to pitting and crevice corrosion. Finally, small quantities of alloying elements such as Ti, Nb, Ta, and Cb are added to refine the microstructure of the alloy and to prevent grain boundary segregation.

All of these alloying elements greatly increase the cost of corrosionresistant alloys and may also adversely affect their mechanical properties and weldability. Therefore, the use of protective coatings on alloys with satisfactory mechanical properties is attractive both from a cost and performance point of view. Existing coatings, however, have been successful only in selected applications because of inherent deficiencies. 10 Metallic coatings applied by a spraying, such as flame or plasma spraying, can be formulated to provide the right composition but suf fer from poor adhesion to the substrate and internal porosity. This can lead to corrosion at the substrate -coating 15 interface.

Coatings applied by the pack diffusion process are generally pore-free and have an excellent metallurgical bond between the substrate and the coating. However, the present state of the art is limited to the deposition of one element at a time. The most popular commercially available coatings contain aluminum or chromium. Multielement coatings are, at present, not commercially available. The literature indicates that multi-element coatings are usually prepared by repeated diffusion treatments using single element packs, a process which is usually too expensive to be economical.

chromium containing coatings applied by the pack diffusion process, usually referred to as chromized coatings, are produced by placing the component to be chromized in a sealed retort containing a powder (the pack) consisting of a chromium alloy, usually ferrochrome, an inert material usually alumina, and an "activator" usually a halide salt. When the i I i I i t 1 j i 1 i - 1 1 temperature of the retort is maintained at 170021009F for several hours, chromium metal is transported f rom the pack into the surf ace of the component. When the component consists of a ferritic 5 steel, as commonly used in utility applications, a ferritic stainless steel surface layer or coating is obtained.

Chromium containing coatings applied by the pack diffusion process are generally about 10-20 mils (250-500 m) thick, and contain about 20-40% chromium at the surface and about 12-15% chromium at the substrate/coating interface. The resistance of such a coating to high-temperature oxidation and sulfidation and to aqueous corrosion in mildly acidic liquids is generally good to excellent, except at the grain boundaries. During the chromizing process, carbon from the substrate diffuses into the coating. This can lead to precipitation of chrome carbides at the grain boundaries and local depletion of chromium in the bulk alloy in these areas. Subsequent welding can cause further precipitation of chrome carbides in heat-affected zones. These areas are subject to accelerated oxidation and sulfidation attack at elevated temperatures and pitting and crevice corrosion at room temperature, when exposed to corrosive liquids.

Pitting of chromized coatings can occur, even in the absence of chrome depleted grain boundaries, if the corrosive liquids contain substantial quantities of chlorides. Thus, improvement in chromizing technology and coating composition is needed to provide adequate corrosion protection in most utility applications.

SUY AND OBJECTS OF THE INVENTIO it is an object of the present invention to provide an improved corrosion- resistant coating by simultaneously depositing chromium and vanadium onto steel substrates to form a diffused surface layer on the steel substrata.

In general, the present invention provides a process for diffusion coating steel substrates with a protective layer of chromium and vanadium comprising the step of subjecting the steel substrata to a diffusion pack composition comprising chromium or a chromiumbearing alloy, vanadium or a vanadium-bearing alloy, an inert material, such as A1 2 0 3 and a halide as the carrier gas. The weight ratio of chromium to vanadium in the pack ranges from about 3 to 13. The diffusion pack is maintained at an elevated temperature and for a time sufficient to simultaneously deposit and diffuse the chromium and vanadium onto and into the steel substrata surface to form a protective layer comprising about 20-50% by weight chromium and about 1.0-8.0% by weight vanadium, and having a thickness of about 0.2 to 0.7 mn.

The diffused surface layer on the steel substrata 25 will contain chromium and vanadium, as well as iron and other minor alloying elements which may be present in the steel substrata. The chromized coating of the present invention is an improvement over the prior art coatings due to the highly beneficial effects of simultaneously diffusing vanadium and chromium into the surface of the steel substrata.

i Because vanadium has a greater affinity to carbon than chromium, vanadium will react with available carbon in the substrate during the diffusion process to form vanadium carbides. Thus, precipitation of chrome carbides does not occur either during the coating process or during subsequent welding. Chrome depletion around precipitated carbides is therefore prevented. This greatly improves the resistance of the coating to intergranular corrosion and pitting, both at elevated temperatures under sulfidizing or oxidizing conditions and at room temperatures, when exposed to condensed liquids containing chlorides and sulfates. Further, vanadium in solid solution in ferritic or austenitic stainless steels containing chromium, iron, and sometimes nickel, improves resistance to pitting and crevice corrosion, similar to the well-known addition of molybdenum to similar alloys.

DETAILED DESCRIPTION OF THE INVENTION

In general, the simultaneous diffusion of chromium and vanadium into the steel substrate is not independent, in that, coatings formed by simultaneous diffusion have a lower concentration of chromium in the surface but greater thickness than those formed by diffusion of chromium alone. The actual amounts of chromium and vanadium present in the diffusion coating depend mainly on the Cr/V ratio present in the pack. The other major factor is the composition of the substrate alloy. By judiciously selecting the Cr/V ratio in the pack and a suitable substrate, coatings containing 20-50% chromium and 1-8% vanadium, the preferred concentrations for the coatings of the present invention, can be produced.

The coatings of the present invention are further characterized by substantially complete absence of precipitated chrome carbides at the grain boundaries. This is due.to the simultaneous diffusion of vanadium into the steel substrate. Dispersed vanadium carbides containing some chromium are usually present throughout the coating and are concentrated near the outer surface of the coating, especially when the vanadium content of the coating is high.

To produce Cr-V coatings with a Cr content between 20-50% and a vanadium content between 1-8%, the Cr/V ratio in the pack should be between about 3 and 13. The preferred chromium content in the coating is about 30-40% and the preferred vanadium content in the coating is about 2-4%. A Cr/V ratio in the pack between about 4 and 11 should produce these preferred concentrations of chromium and vanadium in the coating.

Preferred chromium and vanadium alloys for the 20 diffusion pack composition are commercially available ferro-chrome alloys containing about 50-70% chromium and commercially available ferro-vanadium alloys containing about 40-60% vanadium.

Preferred halides for the diffusion pack composition are NH 4 Cl and NH 4 Br. Inert materials other than Al 2 0 3 powder can be used, such as calcized clay and mullite.

Preferred coating thicknesses for the present invention are in the range of about 0.2-0.7nm, 30 preferrably 0.3-0.6mm.

1 1 i i i The improved coatings of the present invention are particularly useful on low-alloy steel substrates, and ferritic and austenitic stainless steel substrates.

The coatings of the present invention are primarily developed for use as protective coatings for heat exchanger components (water walls, convection bundles, etc.) in syngas coolers of Integrated Coal/Gas Combined Cycle power plants. The coatings are also useful in corrosive areas of conventional power plants, such as super heaters, water walls, and so 2 scrubber components.

The coatings of the present invention can be tailored for various corrosive conditions by adjusting the concentration of vanadium and chromium in the coating. For example, for applications where hightemperature sulfidation is of primary concern, coatings with a low vanadium content of about 1-3% are preferable. This is because compositions with higher vanadium content show more sulfidation attack at elevated temperatures than vanadium-free coatings or coatings with a high Cr/V ratio. Where corrosion by acid condensates is the primary concern, coatings with a high vanadium content of about 3-5% are preferable. Thus, according to the present invention, it is useful to adjust the Cr/V ratio in the pack to obtain a coating with a desired concentration of chromium and vanadium depending on the particular corrosive condition to which the steel substrate will be subjected.

The following examples are provided to illustrate the process of the present invention and the properties of various preferred protective coatings of the present invention.

EXAMPLE.1

Cyclic corrosion tests in which chromized steels were alternately exposed to simulated coal gasification environments at 350-500C and acid condensates containing chlorides and sulfates at room temperature indicated that commercial vanadium-free chromized coatings (0.4 mn, thick, containing 30-40% chromium) resisted high-temperature corrosion in coal gas environments very well, but were readily intergranularly corroded throughout the coating thickness when in contact with acid condensates, and also developed pits up to 0.2 mm deep. Coatings containing 20% or more chromium and 2-8% vanadium and having a thickness in the range of about 0. 4 to 0. 6 mn, were completely free of intergranular corrosion and contained only a few shallow pits. Microscopic examination indicated that these pits were associated with near surface porosity. Apparently a thin layer of coating between the pore and the outer surface was corroded, but there was little or no evidence of corrosion at the bottom of the pit.

SA213-T-11 (composition:

EXAMPLE 2 (wt.%) C 0.15 max, Rn 0.30.6, P 0.03 max, S 0.03 max, Si Oi.5-1.0, Cr 1. 001.50, Mo 0.44-0.65, Fe balance) steel tubing was coated by the pack diffusion process at a temperature within the range of about 2000-2200F using ferro- chrome and ferro-vanadium as the metallic species in the pack. The Cr/V ratios in the pack were 2.7f 5.5f and 11.1, respectively. The coatings produced by the diffusion process had total thicknesses of 0.6 mm, 0.4 mm, and 0.4 mm respectively, chromium contents of P 1 1 i 1 j 1 1 1 1 i 1 i 1 24%, 26%, and 28% and a vanadium content of 8, 3.5, and 2%. Microscopic examination indicated that all coatings were free of precipitated chrome carbide along grain boundaries; dispersed and surface carbides present consisted mainly of vanadium carbide with minor amounts of chromium.

The above coatings were covered with a finely ground coal gasifier slag and alternately exposed for 100 hours to a coal gas containing 0.6% H 2 S and 0.5% HCl at 500C and moist air to simulate service in a syngas cooler of a coal gas if ication-combined-cycle power plant. After 4 cycles, none of the Cr-V coatings showed general or intergranular corrosion.

About 2 shallow pits/mm 2, less than 0.12 mm deep were found on each sample. Microscopic evaluation indicated that these shallow pits were associated with pre-existing surface porosity. A commercial chromized coating on the same substrate (SA213-T11), also about 0.4 mm thick, showed numerous deep pits (about 90/mm 2 with depths up to 0.25 ram) in the same test. Microscopic examination also revealed extensive intergranular corrosion penetrating into the substrate with a commercial vanadium-free chromized coating. Uncoated SS310 (composition: C 0.08 max, Rn 2.0 max, Si 1.5 max, S 0.045 max, P 0.045 max, Cr 24-26, Ni 19-22, Fe balance), a high chromium austenitic stainless steel was also severely pitted in the same test. This example clearly demonstrates the superiority of Cr-V coatings.

EXAMPLE 3

SA213-T-22Nb (composition: C 0.15 max, hn 0.3-0.6, P 0.03 max, S 0.03 max, Si 0.5 max, Cr 1.9-2.6, Mo 0.87-1.13, Nb 0.9-1.1, Fe balance) steel was chromized in the manner described in Example 1. The 1 properties of the resulting coatings are described in the table below.

Coating PrORerties Cr/V Ratio in Pack Thickness. = QLA v % 4.2 0.5 40 5.0 7.5 0.5 40 3.0 7.0 0.5 35 2.0 11.2 0.45 38 1.5 This example shows that the chromium content is largely independent of the Cr/V ratio in the pack, while the V content of the coatings decreases with decreasing V in the pack. Thus, the use of steel substrates containing a Nb stabilizer allows the production of chromized coatings which simultaneously have high Cr and V contents.

EXAMPLE 4

SS304 (composition: C 0.04-0.10, Mn 2.0 max, P 0.04 max, S 0.03 max, Si 0.75 max, Ni 8-11, Cr 18-20, Fe balance) and SA213-T-91 (composition: C 0.08-0.12, Mn 0.3-0.6, P.002 max, S 0.01 max, Si 0.2-0.5, Cr 8- 9.5, Mo 0.85-1.05, V 0.18-0.25, Nb 0.06-0.1, Fe balance), representing an austenitic stainless steel and a ferritic stainless steel, respectively, were chromized as described in Example 1. The properties of the resulting coatings are described in the tables below.

Chromized SS304 Coating-Properties Cr/V Ratio in Pack Thickness. mm Cr v 1 i i 1 1 1 i i 1 1 j i i i i 4.2 0.3 30 5.0 6.2 0.3 30 4.0 7.0 0.3 35 3.0 11.0 0.32 42 1.5 Chromized T-91 Coating Properties Cr/V Ratio in Pac Thickness. rm P-LA v % 4.2 0.6 30 6.0 6.2 0.6 45 2.5 7.0 0.6 40 2.0 8.8 0.5 35 1.0 13.7 0.6 42 1.0 This example illustrates that Cr-V diffused coatings are not limited to low-alloy steels but can also be applied to ferritic and austenitic stainless steels.

Claims

CLAIMS:

1. A process for diffusion coating a steel substrate with a corrosionresistant protective layer of chromium and vanadium comprising the step of simultaneously depositing and diffusing chromium and vanadium onto and into the steel substrate surface by subjecting the steel substrate to a diffusion pack composition comprising chromium or a chromium-bearing 10 alloy, vanadium or a vanadium-bearing alloy, wherein the weight ratio of chromium to vanadium is in the range of from 3 to 13, and further comprising an inert material and a halide, at an elevated temperature and for a time sufficient to deposit and is diffuse the chromium and vanadium onto and into the steel substrate surface to form a protective layer comprising from 20-50% by weight chromium and from 1-8% by weight vanadium, and having a thickness in the range of from 0.2 to 0.7mm.

2. A process as claimed in claim 1, wherein the elevated temperature is in the range of from 2000 to 22000F.

3. A process as claimed in claim 1 or claim 2J, wherein the chromium-bearing alloy is a ferro-chrome alloy containing from 50 to 70 weight % chromium and the vanadium-bearing alloy is a ferro-vanadium alloy containing from 40 to 60 weight % vanadium.

4. A process as claimed in any one of the preceding claims wherein the chromium/vanadium weight ratio in the diffusion pack composition is in the range of from 4 to 11.

5. A process as claimed in any one of the i i 1 i I 1 i 1 preceding claims wherein the chromium content of the protective layer is in the range of from 30 to 40% by weight and the vanadium content is in the range of from 2 to 4% by weight.

6. A process as claimed in any one of the preceding claims, wherein the steel substrate is a low alloy steel, a ferritic stainless steel, or an austenitic stainless steel.

7. A diffusion pack composition comprising chromium or a chromium-bearing alloy, vanadium or a vanadium-bearing alloy, wherein the weight ratio of chromium to vanadium is in the range of from 3 to 13, and further comprising an inert material and a halide.

8. A corrosion-resistant protective coating diffused into the surface of a steel substrate which is a low alloy steel or a ferritic or austenitic stainless steels, which comprises a chromium content in the range of from 20 to 50% by weight, a vanadium content in the range of from 1.0 to 8.0% by weight, and a thickness in the range of from 0.2 to 0.7mm.

9. A process as claimed in claim 1 substantially as hereinbefore described with reference to any one of the Examples.

10. A steel product having a corrosion-resistant protective coating of diffused chromium and vanadium, whenever produced by a process as claimed in any one or claims 1 to 6, or claim 9.

Published 1988 at The Patent Office, State House. 66 71 High Holborn. London WC1R 4TP. Further copies may be obtained from The Patent Office. Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD Printed by MWtiplex techniques ltd- St Mary Cray, Kent. Con. 1187.