WO2013075769A1 - Process for producing a protective chromium layer - Google Patents

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

 description

Method for producing a chromium protective layer

The invention relates to a method for producing a chromium protective layer according to the preamble of patent claim 1 and to the use of a plasma-sprayed protective layer.

State of the art

 Chromium is one of the most important metals for coatings. Its very high corrosion resistance against many aggressive media in a wide range

Temperature range is comparable to that of precious metals. Depending on how they are made, the properties of chrome coatings vary widely.

 Three types of chromium-based coatings are known:

1. galvanic chrome layers

 2. PVD and CVD chrome layers

 3. Chromium layers, which are formed by high-temperature diffusion

Galvanic chrome layers are the oldest and most widely used chromium-based layers. The first description of the electrolytic deposition of chromium from 1843 is from AC Becquerel. In 1854, RW Bunsen described chromium deposition from hot chromium (III) chloride solution with anodes of coal and methods of platinum. Chromium deposition in the chromium bath was invented by Erik Liebreich (DE398054 and DE448526). After that the galvanic bath is based on Cr0 ₃ and H ₂ S0 ₄ . To date, almost all chromium layers are produced by this process. Here, layers of pure chromium are applied with a thickness of <Ιμπι to about 300 m on very different substrates (metals, glasses, ceramics, plastics and even wood). Depending on the layer thickness, this is referred to as decorative chrome plating (layers <5 π or hard chrome (layer thicknesses: 10-200 μm).) The special features of galvanically deposited chromium layers are very high hardness and brittleness, relatively weak adhesion to the substrate and a fine Cracking network with layer thicknesses> 5μπι These features and the fact that chromium has a low coefficient of thermal expansion - well below that of the most important metallic substrates - limits the use of

galvanic chrome layers significantly. Because of their fine cracks, these chromium layers are basically permeable to gaseous and liquid media, their mechanical strength is relatively low due to poor adhesion and high brittleness, and the maximum permissible operating temperature is less than 500 ° C, although chromium as a compact metal temperatures above 1100 ° C in air can withstand.

PVD and CVD chromium layers are obtained by deposition from the gas phase in a vacuum oven. One distinguishes between a purely physical separation of Chromium vapor (physical vapor deposition, PVD short) and a deposition by means of a chemical reaction between chromium-containing gas and substrate (chemical vapor deposition, short CVD). Because of this chemical reaction, the CVD chromium layers generally have higher adhesion than PVD chromium layers. However, the CVD process requires significantly higher temperatures of 800-1000 ° C compared to 200-500 ° C for the PVD process. Both methods allow the application of dense thin layers of pure chromium or chromium nitride (CrN). In comparison with galvanic chromium layers, the PVD and especially the CVD-chromium layers have a very good adhesion to the substrate, but are much more expensive compared to galvanic layers and therefore only limited usable for large-area parts. In addition, the maximum layer thickness is only about ΙΟμπ

The coating of steels known by the term "thermal chromium plating ^" by means of a thermochromic chromium diffusion at temperatures of 1000-1200 ° C. comprises two different process variants which, however, lead in principle to the same results: the known (DE1905717) chromium diffusion solid phase eg chromium powder, and the well-known

Gas-chromating (EP0043742 AI) from a gas phase eg CrCl ₃ . In both processes, chromium diffuses into a steel surface to a depth of about 50 microns and thus forms a protective layer. In this diffusion layer, a chromium concentration of max. 50% ranges, with high corrosion resistance and high hardness of the steel surface. In fact, since the diffusion chromium layers are not pure chrome layers such as electroplated or PVD chromium layers, they also have very different properties such as good adhesion, good mechanical strength, and a coefficient of thermal expansion close to that of the steel. These properties allow the use of diffusion chrome coated parts at temperatures higher than 800 ° C. Compared with pure chromium layers, however, the high-temperature corrosion resistance of the diffusion chromium layers is far below that for compact metallic chromium. Even with wet-chemical corrosion resistance, the layers of pure chromium are superior. Despite comparably favorable properties of diffusion chromium layers, these are only used to a very limited extent because of their high complexity.

Of the known chromium layers described above, only diffusion chromium layers are suitable for use at high temperatures above 800 ° C. Because of their relatively low chromium content of max. However, 50% do not achieve the desired resistance of pure chromium layers. Regarding the

Layer thickness are all known chromium layers insufficient, because the maximum layer thickness of dense crack-free chromium layers is limited to about μθμπι.

From EP 2 006 410 A2 a thermally sprayed protective layer for metallic substrates is also known, 4a wherein the spray powder comprises at least two components, of which the first is a silicate mineral or rock and the second is a metal powder and / or another silicate mineral or rock.

Then DE 693 13 456 T2 describes a ceramic composite coating material, wherein the applied metal layer u.a. May have quartz glass.

Finally, WO 2003/031 672 A1 discloses a spray powder consisting of ceramic particles, i.a. Quartz, and a metal powder consisting of Ni, Cr, Fe and Si is composed.

The aim of the present invention is to provide the advantages of the metal chromium as a material for protective coatings against

5

To use high-temperature corrosion without its disadvantages. The intended improvement should refer 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 adhesion to the substrate should be ensured

 high gas-tightness of the layer should be given by appropriate cracking

 Layer thicknesses up to approx. 1mm should be possible

In particular, the object of the present invention comprises finding a solution to be able to use fine-grained chromium powders without disadvantages, i. to be able to produce sufficient adhesion despite increased oxidation of the fine chromium particles; in spite of fine chromium particles, to obtain a high kinetic energy and thus nonporous and firm structure as well as to achieve a good flowability of the powder;

it is also important to find additives for the chromium powder which reduce the brittleness of the layer and increase its thermal expansion coefficient;

finally, it is a question of developing a method for heat treatment of the coated substrates, 6 which leads to a solid metallurgical bond between layer and substrate.

This object is achieved according to the invention by the combined use of the plasma spraying method according to the characterizing part of patent claim 1

Powder mixture and optionally a subsequent diffusion heat treatment dissolved.

The application of the chromium layers by thermal spraying according to claim 1 allows low cost even for large parts. In addition, it allows for the application of quite thick layers, which, when using known techniques such as e.g. by plating, PVD, CVD, gas chromating and inchromating would be unthinkable.

Due to the high melting point of chromium of approx. 1900 ° C, plasma spraying is optimally selected. It allows under the process conditions of claim 1, the melting of the chromium powder, but prevents its burning (oxidation) in the flame.

Accordingly, the inventive method provides to bring the chromium particles to melt and to accelerate against a substrate. It also involves the production of an oxidation protection for both the flying powder and the surface of the substrate under the flame. As in plasma spraying, the temperature of the particles in the plasma is mainly 7 is determined by their size, the chromium particles should be sufficiently fine-grained to ensure their melting. On the other hand, the application of fine-grained chromium powder means that it is because of its large

Surface is very susceptible to oxidation. The resulting particles of chromium oxide Cr ₂ 0 ₃ can not be reduced in the plasma but are melted and accelerated together with chromium particles against the substrate. The disadvantage here is that the chromium oxide hinders the adhesion between the metallic substrate and chromium, since no metallurgical compound can form.

 Another disadvantage of fine-grained chromium powder consists in a low kinetic energy of the small particles in the flame, with the result that in insufficient adhesion, a layer structure is formed, which is neither pore-free nor sufficiently solid.

These and other disadvantages of the plasma spraying of pure fine-grained chromium powder are overcome by the inventive method by the composition of the spray powder according to claim 1.

In this case, the powder for plasma spraying is preferably produced by simply dry mixing of three components:

 Chromium powder <20μτη (d50 <ΙΟμπι): 30-50 wt. %

Powders of nickel base alloys, (e.g.

80Ni20Cr) <20μπι (d50 <ΙΟμ ^η ι): 5-10 wt. % 8th

• Cristobalite or quartz powder 50-100μπι (d50 = 70-90μηι): balance

In this mixture serves light (2.3 g / cm ³ ) coarse-grained cristobalite powder as a carrier for the heavy fine-grained chromium and Nickelchrompulver: Large

Cristobalite particles whose volume is more than 70% of the mixture (<30 vol.% Cr + NiCr) are covered on the surface with fine chromium and nickel-based particles. These, quite large, round agglomerates make the powder very free-flowing. In plasma, the agglomerates on the surface are heated so that all metallic particles melt. The large refractory (1720 ° C) cristobalite core remains basically solid. An agglomerate particle consisting of a cristobalite core encased in a molten metal ^" crust" gets high kinetic energy in the plasma flame because of its size and weight. During its impact on the substrate, the following happens:

 The solid cristobalite core breaks into small pieces that bounce off the substrate and are carried away by the gas flow. Only a fraction of the original amount of cristobalite is "incorporated" into the layer, namely approximately 1-5% of the layer mass

Cristobalitreste then form small evenly distributed inclusions (<20 μπι) in the finished metallic layer. By contrast, almost the entire remains

metallic portion of the spray powder on the substrate "stick" and forms a finely textured density 9

Layer. The large cristobalite particles fulfill another advantageous function: upon impact with the substrate or on inner layers, the hard and brittle grains act in the manner of a sandblasting material, which ^" radiates away ^" oxide layers (Cr ₂ O ₃ ) directly during coating. As a result, the adhesion to the substrate and between individual layers increases and the layer structure becomes stronger, wherein it contains only minimal amounts of Cr ₂ 0 ₃ . Since the nickel-based alloy has a much lower melting temperature than chromium, it later solidifies as chromium. This creates fine nickel-based lamellae on the surface of the chrome particles, ie in the finished layer are hard

Chrome particles encased in a fine ^" mesh ^" of soft nickel-based alloy. The ^" net ^" of soft nickel-based alloy significantly increases the ductility of the layer. Tensions that arise when solidifying molten chrome, no longer lead to cracking, but are removed by the plastic deformation of the nickel-base fins.

The resulting layer has the following composition:

 • Chrome: 70-90 wt. %

 • Nickel-based alloy: 7-25 wt. %

 • Cristobalite: 1-5 wt. % (3-15 vol.%)

Even with admixture of quartz powder, the finished layer contains only cristobalite, because quartz is transformed into cristobalite in the plasma. 10

Since cristobalite a very high coefficient of thermal expansion of about 50xlO ^{"has 6} K ^_1, for the entire layer has a thermal expansion coefficient of the layer of about ^{9-10xlO" 6} K ^_1 (compared to approximately β ^ ^ χΙΟ Κ ^"1 for pure chromium This value is already close to values for some steels, nickel-base alloys and titanium alloys, so that during cooling no dangerous stresses can occur in the layer.

An even better adhesion of the layer to iron and nickel base alloys is obtained by heat treatment of the coated parts. This is done in the oven at temperatures from 900 ° C in air. This heat treatment to about 5 hours leads to a diffusion of chromium from the layer into the substrate up to about 5μπι. As a result of this diffusion, the layer and the substrate are to a certain extent ^" welded together ^" . At the same time, the heat treatment causes a reduction of residual stresses that are present in the layer after the plasma spraying.

11

At games

 Example 1.

Use in heavily loaded valves of heavy diesel engines operated by heavy fuel oil: Corrosion protection of nickel-base alloys against aggressive molten ash (sodium vanadate) in combination with S0 ₂ -containing exhaust gases and temperatures up to approx. 900 ° C.

A powder mixed together from 40 wt. % Chromium <20μπι, 10 wt.% 80Ni20Cr <20μπι and 50 wt. % Cristobalit 50 μm, was sprayed onto a valve disk made of Nimonic 80A by means of Axial-3 plasma spraying equipment from Thermico GmbH using the following parameters:

Nozzle: 3/8 ^"

 Current: 200A (burner power: 95 kW)

 Plasma gas: argon - 200 L / min, nitrogen - 55 L / min,

Hydrogen - 12 L / min

 Powder gas: Nitrogen - 10 L / min

 Amount of powder: 20 g / min

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

After coating and heat treatment, a pore-free and crack-free layer 800 μm thick was formed on the valve disk surface with the following composition:

 Chrome: approx. 82 vol. %

80Ni20Cr: approx. 12 vol. % 12

Cr ₂ 0 ₃ : about 3 vol. %

 Cristobalite: approx. 3 vol. %

Example 2.

Use in heavily loaded pipes of waste incineration plants: Corrosion protection for steels against chloride and sulphate ashes in combination with S0 ₂ and HC1 containing exhaust gases and temperatures up to approx. 600 ° C.

A powder mixed together from 40 wt. % Chromium <20μτη, 10 wt.% 80Ni20Cr <20μτη and 50 wt. % Cristobalit 50- ΙΟΟμτη, was sprayed by means of Axial-3 plasma spraying system from Thermico GmbH with the following parameters on a boiler tube made of steel 37:

Nozzle: 3/8 ^"

 Current: 200A (burner power: 95 kW)

 Plasma gas: argon - 200 L / min, nitrogen - 55 L / min,

Hydrogen - 12 L / min

 Powder gas: Nitrogen - 10 L / min

 Amount of powder: 20 g / min

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

After coating and heat treatment, a pore-free and crack-free layer having the following composition was formed on the pipe surface:

Chrome: approx. 82 vol.%

 80Ni20Cr: approx. 12 vol.%

Cr ₂ 0 ₃ : about 3 vol. % 13

Cristobalite: approx. 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 approx. 800 ° C.

A powder, mixed together from 40 wt.% Chromium <20μπ, 10 wt.% 80Ni20Cr <20μηι and 50 wt.% Cristobalit 50- ΙΟΟμΐϊΐ, by means of Axial-3 plasma spray system by Thermico GmbH with the following parameters on a valve plate and stem of Ti6Al2Sn4Zr2Mo sprayed on:

Nozzle: 3/8 ^"

 Current: 200A (burner power: 95 kW)

 Plasma gas: argon - 200 L / min, nitrogen - 55 L / min, hydrogen - 12 L / min

 Powder gas: Nitrogen - 10 L / min

 Amount of powder: 20 g / min

After coating, a pμιτι thick pore-free and crack-free layer with the following composition was formed on the entire valve surface:

Chrome: approx. 84 vol. %

 80Ni20Cr: approx. 12 vol. %

Cr ₂ 0 ₃ : ca 1 vol. %

 Cristobalite: approx. 3 vol. %

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

Claims

14 claims 1. A method for producing a gas-tight and  Crack-free chromium protective layer for substrates of iron and nickel and titanium-based alloys by plasma spraying, wherein the chromium content in the  Layer is at least 70% by weight and a  Spray powder is selected from three components, a first component of fine-grained  Chromium powder, a second one of fine-grained powder of a nickel-based alloy, and a third of coarse-grained cristobalite or quartz powder as a carrier for the first and second components. 2. The method of claim 1, wherein the second  Component of the spray powder is mixed to improve the mechanical properties of the protective layer. 3. The method of claim 1, wherein the third  Component of the spray powder is mixed to increase the thermal expansion coefficient of the protective layer. 4. The method according to claim 1, characterized by a subsequent heat treatment of  Protective layer in air at temperatures higher 900 ° C. 15 Method according to one of the preceding Claims, wherein the thickness of the protective layer is selected to 1mm. Use of a plasma-sprayed protective layer according to one of the preceding claims, on corrosion-endangered substrates of components of internal combustion engines, gas turbines, steam turbines, engine compressors or heat exchangers.