PROCESS FOR PRODUCING A PROTECTIVE CHROMIUM LAYER

Abstract

Process for producing a gastight and crack-free protective chromium layer for substrates composed of iron- and nickel- and titanium-based alloys by means of plasma spraying, where the chromium content in the finished layer is at least 70% by weight and a spray powder composed of three components, namely a first component composed of finely particulate chromium powder, a second composed of finely particulate powder of a nickel-based alloy and a third composed of coarsely particulate cristobalite or quartz powder as support for the first and second component, is selected.

Description

The invention relates to a process for producing a protective chromium layer according to the preamble of claim 1 as well as to the use of a plasma-sprayed protective layer.

PRIOR ART

Chromium is one of the most important metals for coatings. Its very high corrosion resistance to many aggressive media in a broad temperature range is comparable with that of noble metals. Depending on how they are produced, chromium coatings have very different properties.

Three types of coatings based on chromium are known:

• • 1. galvanic chromium layers
  • 2. PVD and CVD chromium layers
  • 3. chromium layers formed by high-temperature diffusion

Galvanic chromium layers are the oldest and most widely distributed layers based on chromium. The first description of electrolytic deposition of chromium goes back to A. C. Becquerel in 1843. In 1854, R. W. Bunsen described the deposition of chromium from hot chromium(III) chloride solution with carbon anodes and platinum cathodes. Erik Liebreichs invented chromium deposition in the chromium bath (DE 398054 and DE 448526). Therein the galvanic bath consisted of CrO₃ and H₂SO₄. Heretofore almost all chromium layers have been produced according to this method. In the process, layers of pure chromium with a thickness of <1 μm to approximately 300 μm are applied on completely different substrates (metals, glasses, ceramics, plastics and even wood). Depending on layer thickness, the coating is known as decorative chrome plating (layers<5 μm) or as hard chrome plating (layer thicknesses: 10-200 μm). The properties of galvanically deposited chromium layers consist in very high hardness and brittleness, relatively weak adherence to the substrate and a fine network of cracks at layer thicknesses>5 μm. These properties, together with the fact that chromium has a low coefficient of thermal expansion—well under that of the most important metallic substrates—limit the use of galvanic chromium layers considerably. Because of fine cracks, these chromium layers are basically permeable for gaseous and liquid media, their mechanical durability is relatively low due to weak adherence and high brittleness, and the maximum permissible operating temperature is lower than 500° C., even though chromium as a compact metal can withstand temperatures higher than 1100° C. in air.

PVD and CVD chromium layers are obtained by deposition from the gas phase in the vacuum furnace. A distinction is made between purely physical deposition from chromium vapor (physical vapor deposition, abbreviated PVD) and deposition by means of a chemical reaction between chromium-containing gas and substrate (chemical vapor deposition, abbreviated CVD). Because of this chemical reaction, the CVD chromium layers basically have higher adherence than PVD chromium layers. However, the CVD process requires considerably higher temperatures of 800-1000° C., compared with 200-500° C. for the PVD process. Both processes permit the application of dense thin layers from pure chromium or from chromium nitride (CrN). Compared with galvanic chromium layers, the PVD and especially the CVD chromium layers have very good adherence to the substrate, but on the other hand are much more expensive than galvanic layers and therefore are of only limited use for parts with large surface areas. Moreover, the maximum layer thickness is only approximately 10 μm.

The coating of steels by means of thermochemical diffusion of chromium at temperatures of 1000-1200° C., known by the term “thermal chrome plating”, comprises two different process variants, although they lead largely to the same results: the known (DE 1905717) diffusion of chromium from a solid phase, e.g. chromium powder, and the known gas chromizing (EP 0043742 A1) from a gas phase, e.g. CrCl₃. In both processes, chromium diffuses into a steel surface as far as a depth of approximately 50 μm, thus forming a protective layer. This diffusion layer has a maximum chromium concentration of 50%, together with high corrosion resistance and high hardness of the steel surface. Since the diffusion chromium layers are actually not pure chromium layers such as galvanic or PVD chromium layers, they also have entirely different properties, such as good adherence, good mechanical strength and a coefficient of thermal expansion close to that of steel. These properties permit use of parts coated with diffusion chromium at temperatures higher than 800° C. Compared with layers of pure chromium, however, the high-temperature corrosion resistance of the diffusion chromium layers is much poorer than that for compact metallic chromium. The layers of pure chromium are also superior in terms of wet chemical corrosion resistance. Despite comparatively favorable properties of diffusion chromium layers, their use is only very limited, because of their high complexity.

Of the known chromium layers described in the foregoing, only diffusion chromium layers are suitable for use at high temperatures above 800° C. Because of their relatively low chromium content of at most 50%, however, they do not attain the desired resistance of pure chromium layers. Relative to the layer thickness, all known chromium layers are inadequate, since the maximum layer thickness of dense crack-free chromium layers is limited to approximately 10 μm.

From EP 2006410 A2 there is further known a thermally sprayed protective layer for metallic substrates, wherein the spray powder comprises at least two components, of which the first is a silicatic mineral or rock and the second is a metal powder and/or a further silicatic mineral or rock.

Furthermore, DE 69313456 T2 describes a coating material of ceramic composition, wherein the applied metal layer may contain quartz glass among other components.

Finally, WO 2003/031672 A1 discloses a spray powder composed of ceramic particles, including quartz, and a metal powder consisting of Ni, Cr, Fe and Si.

The objective of the present invention is to exploit the advantages of metallic chromium as a material for protective layers against high-temperature corrosion without having to accept its disadvantages. The desired improvement is intended to relate to the following properties of chromium layers:

• • continuous use in air should be possible up to 1000° C.
  • the layer should be resistant to thermal shock
  • the chromium content of the layer should be at least 70%
  • good adherence to the substrate should be assured
  • high gas-tightness of the layer should be achieved by appropriate freedom from cracks
  • layer thicknesses up to approximately 1 mm should be possible.

In particular, the object according to the present invention is to find a solution to the effect that fine-grained chromium powder can be used without disadvantages, i.e. adequate adherence can be achieved despite extensive oxidation of the fine chromium particles; high kinetic energy and thus pore-free and sound microstructure can be obtained and good free-flowing ability of the powder can be achieved despite fine chromium particles; a further object is to find additives for the chromium powder that reduce the brittleness of the layer and increase its coefficient of thermal expansion; finally, another object is to develop a method for heat treatment of the coated substrates that leads to a strong metallurgical bond between layer and substrate.

This object is achieved according to the body of claim 1 by the combined use of the plasma-spray process with the inventive powder mixture and if necessary a subsequent diffusion heat treatment.

The application of the chromium layers by thermal spraying according to claim 1 permits low costs even for large parts. Moreover, it permits the application of quite thick layers, which would not be conceivable with known techniques such as galvanization, PVD, CVD, gas chromizing and inchromizing.

Because of the high melting point of chromium, approximately 1900° C., plasma spraying is optimally selected. Under the process conditions according to claim 1, it permits melting of the chromium powder but prevents scorching (oxidation) thereof in the flame.

Accordingly, the inventive process provides for melting the chromium particles and accelerating them against a substrate. Furthermore, it relates to creating oxidation protection both for the free-flowing powder and for the surface of the substrate under the flame. Since the temperature of the particles in the plasma is determined mainly by their size during plasma spraying, the chromium particles should be sufficiently fine-grained to ensure that they will melt. On the other hand, the use of fine-grained chromium powder means that it will be very susceptible to oxidation, because of its large surface area. The particles of chromium oxide Cr₂O₃ formed in the process cannot be reduced in the plasma but instead are melted and accelerated together with chromium particles against the substrate. A consequent disadvantage is that the chromium oxide hinders the adherence between metallic substrate and chromium, since a metallurgical bond cannot be formed.

A further disadvantage of fine-grained chromium powder consists in a low kinetic energy of the small particles in the flame, with the consequence that the layer microstructure formed has inadequate adherence and is neither pore-free nor sufficiently sound.

This and further disadvantages due to plasma spraying of pure fine-grained chromium powder are overcome by the inventive process by virtue of the composition of the spray powder according to claim 1.

In this connection, the powder for plasma spraying is preferably generated by simple dry mixing of three components:

chromium powder<20 μm (d50<10 μm): 30-50 wt %

powder of a nickel-base alloy (e.g. 80Ni20Cr)<20 μm (d50<10 μm): 5-10 wt %

cristobalite or quartz powder 50-100 μm (d50=70-90 μm): the rest

In this mixture, lightweight (2.3 g/cm³) coarse-grained cristobalite powder functions as carrier for the heavy fine-grained chromium and nickel-chromium powders: large cristobalite particles, with a volume amounting to more than 70% of the mixture (<30 vol % Cr+NiCr), are covered on the surface with fine chromium-base and nickel-base particles. These fairly large, round agglomerates make the powder efficiently free-flowing. In the plasma, the agglomerates are heated sufficiently at the surface that all metallic particles melt. Under these conditions the large refractory (1720° C.) cristobalite core basically remains solid. By virtue of its size and its weight, an agglomerate particle consisting of a cristobalite core ensheathed with a molten metallic “crust” acquires high kinetic energy in the plasma flame. When it collides with the substrate, the following events take place:

The solid cristobalite core bursts into small fragments, which rebound from the substrate and are carried away by the gas stream. Only a fraction of the original amount of cristobalite, namely approximately 1-5% of the layer mass, is “also pulled in”. These cristobalite residues then form small uniformly distributed inclusions (<20 μm) in the finished metallic layer. In contrast, almost the entire metallic fraction of the spray powder remains “sticking” on the substrate, thus forming a fine-structured dense layer. The large cristobalite particles fulfill yet another advantageous function: upon colliding with the substrate or with inner strata, the hard and brittle grains act as a kind of sandblasting material, which “strips” oxide layers (Cr₂O₃) immediately during coating. Thereby the adherence to the substrate and between individual strata and the layer microstructure becomes stronger, and only minimum contents of Cr₂O₃ remain. Since the nickel-base alloy has a much lower melting temperature than chromium, it solidifies later than the chromium. Thus fine nickel-base lamellas are formed on the surface of the chromium particles, meaning that hard chromium particles in the finished layer are ensheathed by a fine “network” of soft nickel-base alloy. The “network” of soft nickel-base alloy significantly increases the ductility of the layer. Stresses developing during solidification of molten chromium no longer lead to crack formation, but are dissipated by the plastic deformation of the nickel-base lamellas.

The resulting layer has the following composition:

• • chromium: 70-90 wt %
  • nickel-base alloy: 7-25 wt %
  • cristobalite: 1-5 wt % (3-15 vol %)

Even with an admixture of quartz powder, the finished layer contains only cristobalite, because quartz is transformed to cristobalite in the plasma.

Since cristobalite has a very high coefficient of thermal expansion, approximately 50×10⁻⁶ K⁻¹, the coefficient of thermal expansion of the layer reaches approximately 9-10×10⁻⁶ K⁻¹ for the layer as a whole (compared with approximately 6.2×10⁻⁶ K⁻¹ for pure chromium). This value is already close to values for some steels, nickel-base alloys and titanium alloys, with the result that no harmful stresses can develop in the layer during cooling.

The adherence of the layer on iron-base and nickel-base alloys can be improved even more by heat treatment of the coated parts. This is carried out in the furnace at temperatures of 900° C. and higher in air. This heat treatment for up to approximately 5 hours leads to diffusion of the chromium from the layer into the substrate to approximately 5 μm. By virtue of this diffusion, the layer and the substrate are “welded together”, as it were. At the same time, the heat treatment anneals out the residual stresses present in the layer after plasma spraying.

EXAMPLES

Example 1

Use in highly-stressed valves of large diesels running on heavy fuel oil: corrosion protection for nickel-base alloys against aggressive fused ashes (sodium vanadate) in combination with SO₂-containing exhaust gases and temperatures up to approximately 900° C.

A powder, mixed together from 40 wt % chromium<20 μm, 10 wt % 80Ni20Cr<20 μm and 50 wt % cristobalite 50-100 μm, was sprayed by means of the Axial-3 plasma spraying system of Thermico GmbH with the following parameters onto a valve disk of Nimonic 80A:

Nozzle: ⅜″

Current: 200 A (burner power: 95 kW)

Plasma gas: Argon—200 L/min, nitrogen—55 L/min, hydrogen—12 L/min

Powder gas: Nitrogen—10 L/min

Powder flow: 20 g/min

The coated valve was heat-treated at 1020° C. for one hour in air.

After the coating process and the heat treatment, the layer formed on the surface of the valve disk was 800 μm thick, free of pores and cracks, and had the following composition:

chromium: approximately 82 vol %

80Ni20Cr: approximately 12 vol %

Cr₂O₃: approximately 3 vol %

cristobalite: approximately 3 vol %

Example 2

Use in highly-stressed pipes of garbage incineration systems: corrosion protection for steels against chloride and sulfate ashes in combination with exhaust gases containing SO₂ and HCl and temperatures up to approximately 600° C.

A powder, mixed together from 40 wt % chromium<20 μm, 10 wt % 80Ni20Cr<20 μm and 50 wt % cristobalite 50-100 μm, was sprayed by means of the Axial-3 plasma spraying system of Thermico GmbH with the following parameters onto a boiler pipe of steel 37:

Nozzle: ⅜″

Current: 200 A (burner power: 95 kW)

Plasma gas: Argon—200 L/min, nitrogen—55 L/min, hydrogen—12 L/min

Powder gas: Nitrogen—10 L/min

Powder flow: 20 g/min

The coated pipe was heat-treated at 900° C. for five hours in air.

After the coating process and the heat treatment, the layer formed on the pipe surface was 100 μm thick, free of pores and cracks, and had the following composition:

chromium: approximately 82 vol %

80Ni20Cr: approximately 12 vol %

Cr₂O₃: approximately 3 vol %

cristobalite: approximately 3 vol %

Example 3

Use in highly-stressed titanium valves of racing engines: oxidation protection for all titanium alloys and titanium aluminides at temperatures up to approximately 800° C.

A powder, mixed together from 40 wt % chromium<20 μm, 10 wt% 80Ni20Cr<20 μm and 50 wt % cristobalite 50-100 μm, was sprayed by means of the Axial-3 plasma spraying system of Thermico GmbH with the following parameters onto a valve disk and stem of Ti6Al2Sn4Zr2Mo:

Nozzle: ⅜″

Current: 200 A (burner power: 95 kW)

Plasma gas: Argon—200 L/min, nitrogen—55 L/min, hydrogen—12 L/min

Powder gas: Nitrogen—10 L/min

Powder flow: 20 g/min

After the coating process, the layer formed on the complete valve surface was 100 μm thick, free of pores and cracks, and had the following composition:

chromium: approximately 84 vol %

80Ni20Cr: approximately 12 vol %

Cr₂O₃: approximately 1 vol %

cristobalite: approximately 3 vol %

This layer on the valve stem also functions as a wear-resistant running layer.

Claims

1. A process for producing a gas-tight and crack-free protective chromium layer for substrates of alloys based on iron and nickel and titanium by plasma spraying, wherein a spray powder is selected from three components, a first component of fine-grained chromium powder, a second component for improvement of the mechanical characteristics of the protective layer of fine-grained powder of a nickel-base alloy and a third component for improvement of the coefficients of thermal expansion of the protective layer of coarse-grained cristobalite or quartz powder as carrier for the first and second components, and wherein the chromium content in the formed protective layer is at least 70 wt %, the content of the nickel-base alloy is 7-25 wt % and the cristobalite content is 1-5 wt %.

2. (canceled)

3. (canceled)

4. A process according to claim 1, characterized by a subsequent heat treatment of the protective layer in air at temperatures higher than 900° C.

5. A process according to claim 1, wherein the thickness of the protective layer is selected up to 1 mm.

6. The use of a plasma-sprayed protective layer according to claim 1 on substrates threatened by corrosion from components of internal combustion engines, gas turbines, steam turbines, propulsion-unit compressors or heat exchangers.