DE1161464B - Process for increasing the corrosion resistance of metal objects by applying a metallic coating - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Class: C 23 c

German class: 48 b -17/00

Number:
File number:
Registration date:
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P 21699 VIb / 48 b
November 12, 1958
January 16, 1964

The invention relates to a method for increasing the corrosion resistance of metal objects in corrosive atmosphere, especially in an atmosphere containing vanadium, by applying a coating made of a nickel alloy. S.

It is already known the thermal resistance of alloys with regard to high-temperature corrosion due to the addition of silicon to enhance. In particular, nickel-silicon alloys are used here or nickel-chromium-silicon alloys with a low silicon content of at most a few percent, otherwise the addition of silicon affects the strength properties of the alloys in an undesirable manner. Furthermore are Nickel-silicon spray coatings known. However, these spray coatings do not have sufficiently strong adhesion on the workpieces in question. Finally, it is a nickel-silicon cast alloy with a silicon component between 2 and 16% known, which of course do not lead to the formation of a coating suitable and at which the comparatively high silicon content already a deterioration of the Brings strength properties with it.

The present invention shows how to improve the corrosion resistance of metal objects a way which avoids the disadvantages of the known solutions. The method according to the invention is characterized in that a eutectic nickel-silicon alloy is applied to the surface to be protected is sintered with 12 ° / »silicon.

Sintering takes place at temperatures around 1150 ° C, i.e. just below the melting point of the Eutectic. This avoids porosity of the sintered coatings, as the material flows during sintering to give a high density coating. The sintered coatings after of the invention are almost completely impermeable. On the other hand, these coatings adhere extremely well firmly on the layer to be coated so that the coating cannot tear off.

Metal objects, which are coated according to the method according to the invention, can at high Temperatures are used. Such objects are used, for example, within Gas turbines for the gas turbine blades and other parts called the highly corrosive Are exposed to combustion gases.

The corrosive effects are mainly determined by the composition of the fuels. Since in gas turbines the cheapest possible residual fuels or crude oil with highly corrosive Want to burn components, the lifespan becomes

Process for increasing the corrosion resistance of metal objects
Application of a metallic coating

Applicant:

Power Jets (Research & Development) Limited,

London

Representative:

Dipl.-Ing. R. Holzer, patent attorney,

Augsburg, Philippine-Welser-Str. 14th

Named as inventor:

Norman Stephenson, Farnborough, Hampshire

(Great Britain)

Claimed priority:
Great Britain 12 November 1957
(No. 35 214)

of the machine parts are greatly reduced, thereby reducing the use of residual fuels conditional cheaper operation is largely repealed. As a result, needed one machine parts, which the highly corrosive components of the fuel at high temperatures to be able to resist.

Workpieces treated by the method according to the invention are particularly suitable for Use in an atmosphere containing vanadium when operating temperatures rise up to 700 ° C. The method according to the invention increases the resistance of the machine parts, so that the Intermediate periods between machine overhauls can be increased, resulting in a Can achieve cheaper operation.

It has been shown that the silicon-containing surface layer provides protection against corrosion, which does not impair the mechanical strength of the protected component. The outer surface layer however, must not contain any other element, either by itself or in connection with silicon In the operating temperature range, an oxide layer forms which cancels the protective effect.

This is especially important where fuels containing vanadium are used because the im High temperature range used for components

309 TlVhZl

3 4

Alloys often contain oxidizable elements, subjected to corrosion testing by small amounts

which do not belong to the silicon group, such as powdered vanadium pentoxide intermittently in

wise chrome. It is known that some of these EIe an oxidizing atmosphere at a temperature

Elements together with vanadium pentoxide react at 700 ° C on the surface of the protective coating

and that the presence of such elements in FIG. 5 has been applied.

For this purpose, the sample, on a Destroy or reduce the protective effect of the oxide coating on its flat face by reducing it with platinum would. wire supported in such a way that its concave face

As a result, the after the invention lay up, hung in an electric oven, According to the process, the protective coating of 10 in which a temperature of 700 - C is maintained eutectic nickel-silicon alloy under was obtained. That in powdered form for exclusion other, strongly oxide-forming elements. The associated vanadium pentoxide was then added

Under the strongly oxide-forming element, this is intermittently in portions of 20 mg per day Relation to understand each element, which is applied to the concave face of the sample. This itself or in connection with silicon tends to 15 vanadium pentoxide melted at 700 ° C and unite Oxide layer to form, which with respect to that divided on the whole surface of the sample. the Metal sample located under such an oxide layer was weighed periodically to determine the has no protective effect. Determine weight gain due to oxidation

The object to be provided with the coating men. The results of this weight determination are

can be subjected to a degassing heat treatment beforehand - 20 in the attached drawing in the form of a

are thrown and entered after the application of the coating gram, in which the ordinate the

be heat-treated again to determine the physika- weight gain of the sample in milligrams and the

restore the metallic properties of the metal. The abscissa represents the duration of the experiment in days. the

The method according to the invention was on a curve A symbolizes the degree of oxidation of the

Tried a sample consisting of a nickel alloy. 25 apt sample.

The nickel content was about 60%, while the for comparison purposes was the same

The sample also has a chromium content of 18 to 21 ° / o. Sample from the same alloy without protective coating

an iron content of 5%, a cobalt content of intermittently at a temperature of 700 ° C

2%, a titanium content of 1.8 to 2% and one with vanadium pentoxide treated in the same way.

Had a carbon content of 0.1%. The sample in _{Figure 30} Curve B represents the degree of oxidation of this sample

Shape of a massive, essentially cylindrical figure, and it can be seen that the sample is oxidized

Body of 22.2 mm diameter had a weight by using the protective device according to the invention

from 36 g. One end face of the sample was flat, while the coating is considerably reduced,

The other end face of the sample is slightly concave. The sintering time and the sintering temperature

was trained. Before the protective coating on the 35 can be changed, with limits on such

Sample was applied, the same was first given a change only by a

Degassing heat treatment in a vacuum at 900 ° C continuous coating is sought and the

Subjected for a period of 24 hours and so diffusion of chromium or other harmful oxides

then cleaned with abrasive compound. On the sample-forming elements from the metal to the outside

Then a deposit of about 0.076 mm thickness 40 would have to be prevented.

applied, which consisted of a mixture of pulverized The isothermal heat treatment can consist of tem nickel and pulverized silicon, whose any temperature within the temperature composition of the nickel-silicon eutectic ture range between 900 and 1000 ^c C carried out with a silicon content of 12% corresponded. It has been shown that, for the purpose, the powder was used in a suspension of xylene to achieve optimum corrosion resistance, after which the xylene was dried off. The ability of the sample to undergo heat treatment when the sample is coated in this way was 1000 ° C should be avoided as much as possible, while then heated in an electric furnace in a vacuum to 1150 ° C, the preferred temperatures in the areas between and 10 minutes exposed to this temperature. Apparently 900, 950 and 1025 ° C are, although self-closing the sample was cooled. Understandably, other temperatures also resulted, a surface coating in the form of a can be used. In general, the hanging sintered layer of the nickel-silicon layer shows that yawing during isothermal heat treatments. above 1025 ° C the corrosion resistance of the

It has been shown that the sintering process of the coating decreases.

an adverse effect on fatigue strength and 55 It has also been shown that the relationship

had the fatigue strength of the sample. LJm here between the small particles of the protective coating

to achieve the original properties again, strongly curved surfaces, for example on the

was the sample of isothermal heat treatment blade roots of turbine blades, during the

treatment, which in a cooling of the sintering is first stronger than the connection

Surface coating provided specimen of 60 between the coating and the through this over-

the sintering temperature was 900 ° C, including the object to be protected. As a consequence,

after this temperature is maintained for 20 hours that between the sintering coating and the to

and the sample was then cooled to room temperature the protective object was not touched,

became. The sample was then used for the duration of this undesirable property.

16 hours at a temperature of 700 ° C should be avoided that on the object to be protected

tet, and then the sample was allowed to cool. stood or on the strongly curved surfaces of this

The object in this way with a surface coating first under essentially the same

The coated and treated sample was then thin coated under the conditions set out above

of the nickel-silicon eutectic is sintered, the thickness of which should not be thicker than 0.0127 mm.

If the alloy to be provided with the protective coating contains chromium, it can be advantageous to to treat the object to be protected beforehand in such a way that before application of the nickel-silicon coating the surface of the object to be protected is freed from chromium. In modification a nickel coating can also be applied to the object to be protected before this the nickel-silicon coating is applied to the object to be protected. Through these two Pre-treatment is followed by the diffusion of chromium from the alloy of the object to be protected suppressed outside during the sintering process. The content of strong oxide-forming elements in the outermost surface layer of the object to be protected can also be reduced by that the object is repeatedly covered with such a sintered coating. This can ao several times happen, with a complete covering of the object to be protected with the Protective coating is achieved.

The elements nickel and silicon can already be alloyed with one another before the coating is applied and then brought into powder form. Such a mass can just as well be used for production of the protective coating can be used like the mixture of nickel powder and silicon powder described above.

According to a modified method according to the invention, the powdery mixture or Alloy of nickel and silicon by any spraying process on the object to be protected be sprayed on in the form of a protective coating. After that, if necessary, a High-temperature heat treatment of the article sprayed in this way in an inert atmosphere connect, whereby a further sintering of the covering is brought about.

In cases where fuels containing vanadium are not used and consequently vanadium corrosion does not cause any difficulties, the composition of the combustion products can be such that an outer layer containing chromium or other strongly oxide-forming elements does not have any harmful effects and that their presence, together with silicon, even increases their resistance to oxidation still improved. In this case one leaves the chromium or the other concerned strongly oxide-forming Element deliberately penetrate into the outer silicon-containing layer, which is thereby further promoted can be that the sintering temperature is increased or the sintering time is extended. In modification this can also form the protective coating for the object to be protected Mixture or alloy strongly oxide-forming elements are added, such as Chromium, beryllium or aluminum mixtures.

Claims (3)

Patent claims: 1. Process for increasing the corrosion resistance of metal objects in corrosive, especially in an atmosphere containing vanadium by applying a coating made of a nickel alloy, characterized in that that a eutectic nickel-silicon alloy with 12% silicon is sintered onto the surface to be protected. 2. The method according to claim 1, characterized in that one in a volatile medium suspended mixture of powdered nickel and silicon is applied and the sintering is carried out at a temperature of about 1150 ° C in a vacuum. 3. The method nadh claim 1 and 2, characterized in that when using one chromium-containing metal object whose surface before the application of the coating of a Is subjected to treatment which causes a reduction in its chromium content. Considered publications:
British Patent Nos. 295,971, 514,432;
U.S. Patent Nos. 1346188, 1514 064, 840; "Materials and Methodes", 1956, No. 6, p. 87;
"Plating", 1953, p. 1127. 1 sheet of drawings 309 779/127 1.64 © Bundesdruckerei Berlin