WO1982001898A1 - Method for coating a metal with a protection layer resistant to hot gas corrosion - Google Patents

Abstract

On an adhesion or intermediate layer, several layers are applied by thermal spraying in which the content of ceramic material increases and, at the same time, the content of metallic material decreases until a total thickness of between 0.5 - 0.8 mm is reached; simultaneously the region of the deposit is cooled so as to obtain a cooling rate of between 2.5 and 30 C/sec. The ceramic material is only partially stabilized so that the layer contains 10 to 20% by volume of unstabilized phases, or else it comprises at least two ranges of grains of different sizes. Microcracks are caused in the diaper in a controlled manner, which reduce the tensions in the diaper. Thus, one can obtain a relatively thick layer of good density and durability, as well as excellent corrosion resistance up to 1200 C.

Description

 METHOD FOR PRODUCING A HOT GAS CORROSION-RESISTANT PROTECTIVE LAYER ON METAL PARTS

The present invention relates to a method for producing a hot gas corrosion-resistant protective layer on metal parts by thermal spraying using a powdered ceramic material, and a hot gas corrosion-resistant protective layer produced by thermal spraying on metal parts, which consists of metallic and ceramic material.

Diesel engines and gas turbines that work with heavy oil are exposed to high levels of hot gas corrosion. At the high combustion temperatures that occur or are striven for, for example, in marine diesel engines, there is a particularly high level of corrosion as a result of the contamination of the heavy oil, which leads, for example, to the formation of sulfur and alkali compounds and vanadium pentoxide. The various parts subject to corrosion, such as exhaust valves, pistons, combustion chambers, injection nozzles, turbine blades, cause high replacement or repair costs, which could not be significantly reduced by the previously known methods of protective coating. In particular, it was not possible to achieve a sufficient layer thickness in the case of thermally sprayed layers of ceramic materials without having to accept the macro cracks which reduce the service life.

The invention has for its object to provide a method for producing a protective layer of ceramic materials with which layer thicknesses of more than 0.5 mm and a very good corrosion resistance at high temperatures up to 1200 ° C can be achieved.

This is achieved according to the invention in that after the application of a metallic adhesive or intermediate several successive layers, each with an increasing proportion of ceramic material and an equally decreasing proportion of metallic material, are sprayed on until finally a purely ceramic cover layer is sprayed on, the total layer thickness being between 0.5 and 8.0 mm, and during spraying with ceramic material, cooling the application area to achieve a cooling rate between 2.5 and 30 ° C / sec. he follows. The ceramic material used is only partially stabilized in such a way that 10-20% by volume of unstabilized phase occurs in the layer formed, or it comprises at least two different grain areas, the maximum grain size of the finer grain area being substantially smaller than the average grain size of the coarser grain area is.

In the method according to the invention, the aforementioned procedural measures and the type of materials used, in a controlled manner, cause microcracks in the layer produced, by means of which the stress states in the layer are reduced and therefore no larger ones which impair the density and durability of the layer Cracks occur.

The protective layer according to the invention consists of a plurality of layers sprayed on top of one another, each of which has a decreasing metallic portion and an increasing ceramic portion from the inside to the outside, the outermost layer being purely ceramic. The layer has 10-20% by volume of unstabilized phases in which microcracks are present, the length of which is at most equal to three times the largest dimension of the storage of unstabilized phase resulting from a spray particle, or it has microcracks, the length of which is at most is two thirds of the largest dimension of the deposited spray particle in which the crack occurs. The microcracks are caused, in particular, by a strong, shock-like cooling during the spraying process, either localization and control of either the inventive presence of unstabilized phases in the application or the even coexistence of larger and smaller lamellar deposits due to the selected grain distribution in the ceramic spray material Size of the cracking occurs. In addition, the tensions between the base material or a metallic intermediate layer and the ceramic cover layer are reduced by a graduated structure of the protective layer. Ni-Cr-Al-Y, Co-Cr-Al-Y, Ni-Al-Ni-Cr-Al or Ni-Cr alloys are used as the adhesive or intermediate layer, and a Cr intermediate layer is also preferably used as a diffusion barrier in the event of corrosion attack by vanadium pentoxide .

Some embodiments of the invention are described in more detail below. example 1

An exhaust valve of a marine diesel engine showed strong corrosion after a long period of operation due to hot gas corrosion due to the sulfur and vanadium content (up to 0.5% S and up to 50 ppm V) of the heavy oil used. A new exhaust valve intended for replacement was coated in a plasma spraying system prior to installation. A Ni - Cr - Al powder was used for the adhesive layer and a partially stabilized zirconium oxide powder made of 80% ZrO ₂ + 20% Y ₂ O _{3 was} used for the cover layer. The application was graded from 100% metal to 100% ceramic by spraying intermediate layers with the proportions 80/20, 60/40, 40/60, 20/80, the total layer thickness being 2 mm. An argon-hydrogen mixture was used as the plasma gas and argon as the powder carrier gas. The electrical power was 52 kW. For cooling liquid CO _{2 was} used. After coating, a sample coated simultaneously for control purposes was examined microscopically in order to determine the length of the microcracks formed in the layer. The measurements showed a microcrack length of at most 2.5 times the length of the embedded particles of the unstabilized phase. In the present case, the mentioned particle length was 5 µm and the maximum micro-crack length was 12 µm.

The percentage of the non-stabilized phase was checked by the known cathode luminescence method and gave a percentage of 11.5% by volume.

After processing by grinding, the coated exhaust valve was installed in the engine and put into operation. No corrosion was found on this valve after 2000 hours of operation. A further check after 5000 operating hours showed only a slight corrosion attack. The running time of the coated part could therefore be increased by at least two and a half times.

Example 2

The piston surface of a diesel engine, which was operated with heavy oil with a contamination of more than 50 ppm vanadium, showed a strong attack by hot gas corrosion.

As was found by experiments, a coating of MgO-stabilized ZrO ₂ with 75% ZrO ₂ + 25% MgO in a layer thickness of 2.0 mm gives the best results with regard to the given corrosion stress.

The piston was coated with a plasma spraying system according to the following scheme:

Preparation by blasting with silicon carbide; Spraying an adhesive layer made of Ni - Cr - Al - Y alloy with a layer thickness of 0.2 mm, whereby as Carrier gas argon is used, the electrical output of the burner is 38 kW and air is used as a cooling medium with a flow rate of 2 1 / min; Spraying a chrome intermediate layer as a diffusion barrier with an electrical output of 45 kW and the same gases and cooling data; Spraying the top layer with an electrical output of 42 kW, the same gases and cooling with CO ₂ from three cooling nozzles, which cause a cooling rate of 18 ° C / sec in the layer surface. The grading of the layer structure of Cr ceramic was carried out as in Example 1. After the coating, the sample which had been injected as a control was checked for the formation of microcracks. It was found that, as desired, the crack length was at most 1.5 times the size of the particles from the unstabilized phase.

To check the distribution of the non-stabilized portion, the sample was examined by the cathode luminescence method and a portion of 13 percent by volume was measured. The running time of the piston could be extended considerably by the applied coating.

Example 3

The injection nozzles of a slow-running diesel engine showed a corrosion attack by hot gas corrosion after a relatively short running time. Based on tests, a 2.0 mm thick layer of 97.5% Ca ₂ SiO ₄ + 2.5 B ₂ O ₃ , which is applied as follows, was considered the best protective layer at this point.

The surface to be coated was prepared by mechanical processing and subsequent blasting with corundum. The autogenous flame spraying process was used for coating. The injection nozzles and corresponding control samples were provided with a metallic adhesive layer made of Ni-Al powder, the thickness of which was 0.15 mm. Then an additional powder was added Conveyor a mixture of 20% Ca ₂ SiO ₄ in the grain size range of 5-45 microns, 77.5% Ca ₂ SiO ₄ in the grain size range of 63-150 microns and 2.5% B ₂ O ₃ in a grain size range of 5-45 microns fed.

With the aid of the two different powder conveying devices, the powder feed was regulated in such a way that a transition from layer to layer between the metallic component corresponding to the adhesive layer and the ceramic component was achieved from the mixture mentioned.

In order to form the desired microcracks and at the same time reduce the internal stresses in the outer layers of the cover layer, with increasing Ca ₂ SiO ₄ content, the cooling was increased with the aid of ring-shaped cooling nozzles so that a cooling rate of when the outer, purely ceramic cover layer was reached 10 ° C / sec was reached on the surface of the spray layer.

After the coating, a microsection was produced from the sample which was also sprayed on, and this was examined for microcracking. It was found that there were microcracks in the applied Ca ₂ SiO ₄ particles of the lower grain area, which were between 1/3 to 1/4 of the size of these applied particles.

In use, the significantly longer service life of the injection nozzles coated in this way confirmed the effectiveness of the coating carried out.

Example 4

In combustion chambers of marine diesel engines, the use of heavy oils contaminated with 0.2% sulfur and 30 ppm vanadium causes strong corrosion attacks by hot gas corrosion, which necessitate early repairs.

In order to reduce this hot gas corrosion attack, the combustion chamber was coated with a protective coating of calcium disilicate, which was stabilized with 3.0% phosphorus pentoxide was provided. A Ni - Cr alloy of 80% Ni and 20% Cr was applied as the adhesive layer.

The coating was carried out by autogenous flame spraying in a system with two external powder conveyors. The course of the coating process was as follows:

The surface to be coated was degreased by washing with carbon tetrachloride and then dried. The surface was then cleaned by blasting with silicon carbide with a grain size of 0.5-1.0 mm and roughened.

After preparation, the combustion chamber part was preheated to 150 ° C and the metallic adhesive layer sprayed on from a first powder conveyor. The layer thickness was 0.2 mm.

A mixture of 30% Ca ₂ SiO ₄ with a grain size of 5-37 μm, 67% Ca ₂ SiO ₄ with a grain size of 53-95 μm and 3.0% P ₂ O _{5 was introduced} into the second powder conveying device. After the 0.2 mm adhesive layer had been applied, the setting of the two powder conveying devices was changed so that the proportion of the metallic to ceramic powder per layer was graded in the ratios of 80/20, 60/40, 40/60, 20/80 % occurred. Then a layer of 100% ceramic was sprayed on. The total layer thickness was 2.5 mm.

During the production of the above-mentioned graded transition, the surface of the layer was cooled with CO ₂ by means of several nozzles directed towards the surface in such a way that a cooling rate of 5 ° C./sec in the surface of the layer was achieved when the ceramic cover layer (100% ceramic) was reached has been.

A microscopic examination was then carried out on a sample which had also been injected, and it was found that the stress-relieving microcracking 1/3 of the size of the deposited particles of the lower grain size range (5-37 μm).

When the combustion chamber was checked after a running time of 1000 hours, only a very weak, beginning corrosion attack could be determined.

Example 5

When using heavy oil, which contains an impurity of 0.4% sulfur and 45 ppm vanadium, severe hot gas corrosion damage to the blades occurs on the turbine blades of stationary oil-fired hot gas turbines after a period of 3000 hours, which reduces the performance of the turbine and the turbine blades need to be replaced.

As tests have shown, a protective coating with 88% ZrO ₂ + 12% CaO gives the best results when exposed to the combustion gases of the heavy oil described. A Ni - Al powder with an Al content of 10% was sprayed on as the adhesive layer and intermediate layer. The coating of these turbine blades had to be carried out in a new condition before installation.

A plasma spraying system was used. The preparation was carried out by blasting with corundum with a grain size of 0.25-0.75 mm. After blasting, the surface had a roughness of 35-40 µm.

The Ni - Al powder for the intermediate layer was then sprayed onto the blade surface prepared in this way. During the spraying, the surface was cooled with two air jets. The layer thickness was 0.15 mm. Subsequently, the protective layer made of 88% ZrO ₂ + 12% CaO was applied in stages from the metal layer. For cooling, three CO ₂ nozzles were used, which were attached at a distance of 5 cm from the center of the flame. The cooling rate at the surface was 8 ° C / sec. The total thickness of the coating was 0.9 mm. Together with the turbine blades, two test samples were coated in order to determine the necessary investigations for determining the formation of microcracks, for checking the absence of voltage and the proportion of non-stabilized phases. As expected, it was found on these samples that the microcracks were triggered by the unstabilized phases and their length corresponded to 1.5 times the length of the intercalation resulting from a corresponding particle.

Cathode luminescence tests were carried out to check the proportion and distribution of the unstabilized phases. The volume fraction measured was 15%, the distribution was homogeneous throughout.

The coated turbine blades were installed in the turbine during the next overhaul. After 5000 hours of operation, the turbine blades were inspected and it was found that there was no significant hot gas corrosion attack on the edges and surfaces of the blades.

Example 6

Turbine blades for a hot gas turbine that is operated with heavy oil with a contamination of 0.3% sulfur should be provided with a protective coating against the corrosion of the hot combustion gases.

In preliminary tests, it was found that a powder mixture of 80% Al ₂ O ₃ and 20% non-stabilized ZrO _{2 is} particularly suitable for coating these blades. An important factor with this powder is the homogeneous distribution of the ZrO ₂ in the Al ₂ O ₃ matrix component. The grain size of the aluminum oxide content was 20-75 μm and that of the zirconium oxide was 5-37 μm.

The coating was carried out by the plasma spraying process using two powder feed units, one powder feed device being the Material of the adhesive layer (Ni-Cr-Al-Y) and the other that promoted the top layer (Al ₂ O ₃ + ZrO ₂ ). The coating process was as follows:

The turbine blades were prepared by blasting with corundum with a grain size of 0.25-0.50 mm. After blasting, a Ni - Cr - Al - Y powder was sprayed with an argon / hydrogen plasma, in which the electrical power was 48 kW, without cooling. After this metallic adhesive primer had been sprayed on in a layer thickness of 0.1 mm, the transition to the ceramic cover layer was produced as in Example 4. The plasma gases and the electrical power corresponded to the spraying of the metallic adhesive layer. A cooling rate of 6 ° C./sec was maintained on the surface by cooling nozzles arranged around the plasma torch in order to relieve the internal stresses generated during the application. The layer thickness of the entire layer was 0.8 mm.

After spraying, the test samples sprayed with the turbine blades were examined for the formation of microcracks and it was found that the length of the microcracks in the embedded ZrO ₂ particles corresponded to half the particle diameter and that the ZrO _{2 was} homogeneously distributed. The examination was again carried out using the known cathode luminescence method.

When checking after the use of the turbine blades in the hot gas turbine, a significant improvement was found compared to the non-coated blades.

Claims

PATENT CLAIMS 1. A process for producing a hot gas corrosion-resistant protective layer on metal parts by thermal spraying using a powdered ceramic material, characterized in that after the application of a metallic adhesive or intermediate layer, several successive layers, each with an increasing proportion of ceramic material and an equally decreasing proportion be sprayed onto metallic material until finally a purely ceramic top layer is sprayed on, the total layer thickness being between 0.5 and 8.0 mm and cooling of the application area during spraying with ceramic material in order to achieve a cooling rate between 2.5 and 30 ° C / sec. takes place, and that the ceramic material used is only partially stabilized, such that 10-20% by volume of non-stabilized phases occur in the layer formed. 2. A process for producing a hot gas corrosion-resistant protective layer on metal parts by thermal spraying using a powdered ceramic material, characterized in that after the application of a metallic adhesive or. Intermediate layer, several successive layers, each with an increasing proportion of ceramic material and an equally decreasing proportion of metallic material, are sprayed on until a purely ceramic cover layer is sprayed on, the total layer thickness being between 0.5 and 8.0 mm and during the spraying ceramic material cooling the application area to achieve a cooling rate between 2.5 and 30 ° C / sec. and that the ceramic material used comprises at least two different grain size ranges, the maximum grain size of the finer grain area is significantly smaller than the average grain size of the coarser grain area. 3. The method according to claim 1 or 2, characterized in that the thickness of the layer is between 2 and 7 mm. 4. The method according to claim 1 or 2, characterized in that a chromium layer is first applied as a diffusion barrier to the adhesive layer. 5. The method according to claim 1, characterized in that the ceramic material is partially stabilized ZrO ₂ , 5-30% Y ₂ O ₃ , 5-40% CaO or 5-40% MgO being used as the stabilizing additive . 6. The method according to claim 5, characterized in that the components of the ceramic material are homogeneously mixed. 7. The method according to claim 1, characterized in that the ceramic material from a homogeneous mixture of - in weight percent - 70-95% Al ₂ O ₃ and 5-30% non-stabilized ZrO ₂ exists. 8. The method according to claim 7, characterized in that the proportions of Al ₂ O ₃ 80-90 & and ZrO _{2 are} 10-20%. 9. The method according to claim 2, characterized in that the ceramic material made of calcium silicate Ca ₂ SiO ₄ . and / or CaSiO ₄ exists. 10. The method according to claim 9, characterized in that a mixture of powder with the grain size range of 1-45 µm and of powder with the grain size range of 63-150 µm is used. 11. The method according to claim 9 or 10, characterized in that the powder with the finer grain size 0.5-5.0 weight percent B ₂ O ₃ or P ₂ O ₅ are added. 12. The method according to claim 2, characterized in that the ceramic material made of molten Spinel Al ₂ O ₃ + MgO exists, the MgO content being 15-30 percent by weight. 13. The method according to claim 2, characterized in that the ceramic material consists of aluminum silicate and / or magnesium silicate. 14. Hot gas corrosion-resistant protective layer on metal parts, which is produced by hot spraying, and consists of metallic and ceramic material, characterized in that it consists of several layers sprayed on top of one another, each with a decreasing metallic component from the inside and an increasing ceramic component from the inside have, the outermost layer is purely ceramic, and that it has 10-20% by volume of non-stabilized phases in which microcracks are present, the length of which is at most equal to three times the largest dimension of the storage of non-stabilized phase resulting from a spray particle. 15. Hot gas corrosion-resistant protective layer on metal parts produced by thermal spraying, which consists of metallic and ceramic material, characterized in that it consists of several layers sprayed on top of one another, each with a decreasing metallic component from the inside and an increasing ceramic component from the inside have, the outermost layer is purely ceramic, and that it has microcracks, the length of which is at most two thirds of the largest dimension of the deposited spray particle in which the crack occurs.