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

[0001]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[0002]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.[0003]Three types of coatings based on chromium are known:[0004]1. galvanic chromium layers[0005]2. PVD and CVD chromium layers[0006]3. chromium layers formed by high-temperature diffusion[0007]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. ...

AI Technical Summary

Benefits of technology

The purpose of this invention is to use metallic chromium as a protective layer against high-temperature corrosion without succumbing to its disadvantages. The goal is to improve the properties of chromium layers by addressing their shortcomings.

Problems solved by technology

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.

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.

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.

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.

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.

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.

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.