Untitled record

(54) Name: Procedure for pack diffusion coating αν aluminum on iron substrate

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The invention relates to a method of packing diffusion coating of aluminum on iron substrates at temperatures lower than 54 ° C using an aluminum halide as activating agent.

Among the objects of the present invention is to provide a new coating process which is easy to use and highly effective.

This is achieved according to the invention in that the activating agent is applied as an upper layer over the diffusion coating pack, and that the pack is kept in a maximum of 38 cm deep retort.

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The activating agents, aluminum halides suitable for use in the above process are chloride, bromide and iodide, and they can be either in anhydrous or aqueous form. The aqueous forms may be wholly or partially aqueous.

One very effective technique for performing the above diffusion coating is to pack a about 38 cm deep cup-shaped retort up to about 5.8-5.0 cm from its top with activating agent-free packaging agent, previously used for such coating, and with articles, then covering the packaging agent with a layer of about 2.5 cm thick of activating agent-free packaging agent powder containing no articles; then spraying over this layer a thin layer of activating agent in an amount such that anhydrous activating agent corresponding to about 0.2-2% by weight of the total weight of the packing agent used is obtained (weight of the objects not included), and then covering the layer of activating agent with a new layer of wrapping powder and / or with a loosely fitting metal retort lid. This technique makes it unnecessary to follow the usual practice of mixing the activating agent with the packaging powder, thus saving a process step and simplifying the process itself. This saving and simplification is achieved with both newly repaired and used packing agents. Such packing agents for use at low process temperatures according to the present invention may simply be undiluted aluminum powder, in which case no packing powder mixture is necessary at all. The aluminum powder can, if desired, be diluted up to as much as 5θ times its weight with an inert filler, for example alumina powder, in which case additional aluminum can be mixed into the packing agent each time it is reused and to compensate for the aluminum consumed in coating. the process, and thus the composition of the packing agent can be maintained more or less constant during successive coating processes. The packing agent preferably contains 10-40% by weight of aluminum.

During the coating process itself, the packing agent tends to clump in the parts containing activating agents, and instead of decomposing these clumps for use again, they can simply be separated. Thus, the upper 1.25 µm of the packing agent can be removed and discarded after each run.

0m if desired, replacement aluminum or replacement packing can. additive mixture is added as a packing agent coating layer or as a subset to one or both of the packing agent coating layers. This avoids direct contact with the objects without mixing with the packing powder instead automatically

7501684-0 a retort is emptied after a process. In such a modification, preferably the clumped portions of the packing agent are crushed and reused.

If desired, a fine mesh (0, 074 mm) mesh of aluminum, steel, stainless steel, or alloy steel, or any other ferrous metal, may be applied below the activating agent layer to help prevent the activating agent particles from working down into the packing agent. It is not necessary to prevent the activating agent from contacting the retort walls, as proposed in U.S. Patent 3,286,684.

As pointed out above, when aqueous activating agents are used, it is ensured that the water content of the activating agent can be almost half of the total weight of the activating agent and that this water does not contribute to the activating agent function. Usually, the amount of aluminum halide used according to the present invention is approximately the same as is commonly used if its water content is offset.

The activating means is preferably arranged at least 0.6 cm from the articles, in the packaging means, and a distance between them of as much as 2.5 cm is even better. However, an even greater distance between them does not contribute to any additional benefits.

The invention is further described below in the following embodiments.

Example 1. In each of four cup-shaped retorts of unalloyed carbon steel having a diameter of about 60 cm and a height of about 35 cm, a packing agent consisting of a powder containing 20% ​​by weight aluminum and 80% by weight alumina is poured, both with a grain size less than 0.044 mm and uniformly mixed. After the retort bottoms are covered with powder to a height of about 1.25 cm, blades for jet engine compressors and consisting of martensitic stainless steel are arranged on the powder layer at a distance of about 0.3 cm from each other. This bucket layer is then covered with more powder until the surface of the powder is about 1.25 cm above the upper surface of the vanes, after which a new layer of vanes is arranged and this layering is repeated until the total layer height in each retort is approximately 30, 5 cm. Then, additional wrapping powder is arranged on each of the retorts to ensure that each of the top blades is covered by a powder layer of a thickness of about 2.5 cm, after which a very thin layer of crystalline AlCl ^ · 6HgO is sprayed. over each in an amount corresponding to 0.6% by weight of the entire powder weight. The retorts are then filled up with additional powder, and stacked one on the other on the floor of a gas-fired clock oven. This stack does not seal any of

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the retorts completely. The furnace cover, which is provided with gas inlet and gas outlet lines, is submerged over the stack and sealed to the furnace floor and an argon gas flow is slowly passed through the furnace interior to initiate the flushing of the air therein. After the argon gas has vented the furnace, the argon gas is replaced with hydrogen, which is introduced at a rate that allows it to be burned with a small flame as it exits from the end of the outlet pipe. Only a very slow flow rate is required, about 0.28-0.42 Nm 2 / h.

Initially, the furnace is heated at a rate of about 5/6 ° C / min, which speed is measured in each retort by thermocouples connected to external measuring means, and as the thermocouples reach about 149 ° C, the hydrogen gas flow can be reduced. that the discharge flame becomes very small. At this point, the hydrogen gas inflow is preferably less than 0.28 Nm 2 / h.

As the heating proceeds, the temperatures shown by the thermocouples increase uniformly and gradually and chemical vapor begins to appear in the burning exhaust gas. By the time the thermocouple temperatures reach about 232 ° C, the discharge of chemical vapors has slowed. However, this gas flow is continued until the temperatures reach about 454%, at which temperature the oven is then kept.

After 16 hours at this temperature, the furnace is terminated and the furnace is allowed to cool until the thermocouple temperatures reach about 149 ° C, the atmosphere of the furnace is then vented by changing the inflow gas to argon or nitrogen and the furnace cap is then removed from the retorts, allowing to further cool the air. The contents of the cylinders are then poured out of the same, and the blades thus treated exhibit a very uniform coating with a weight of about 2.8 mg per cm of the blade surface, as well as a layer depth of about 0.01 mm. After washing with water and drying, a light blasting with small glass particles propelled by a ferric air stream supplied at a pressure of about 0, 35 ~ 0, 7O kp / cm can be used to clean the coated vanes and prepare them for additional protection coating.

The same coating results are obtained when the packed retorts are enclosed in an outer retort inside the oven. Likewise, during the entire process, the inflowing gas may be argon and this will not substantially affect the coating, although the packing agent may thereafter become less suitable for reuse in new coating using argon. Reuse on coating 7501684-0 using hydrogen gas restores the packing agent to full efficiency. For coating using only argon, the inflow of argon can be kept at an extremely low rate because it. does not burn and no minimum flame retaining speed is needed. Argon flow factor is easily controlled by conducting the gas outlet in the form of a pressure gauge containing water, through which water the effluent argon is bubbled as slowly as desired. As long as the pressure gauge shows a slight overpressure inside the oven, no outflow bubbling is actually needed. During heating, the heat expansion and vapor formation cause gas outflow even when there is no gas inflow, so little or no gas inflow is needed until the furnace approaches its maximum temperature.

Further operable modifications include the replacement of helium or other inert gas for any or all argon, the performance of the diffusion coating in a glass-sealed retort as disclosed in U.S. Patent 3,096,160, as well as the use of a somewhat loose seal sealed retort closure as described in U.S. Pat. U.S. Patent No. 3,764,373. When using these types of retort arrangements, the activating agent must be anhydrous.

The aluminum particles used in the above process can be of a very variable size, from as large as 1 mm to as small as 2 µm. The smaller particles should be handled with care, as they tend to ignite until they are mixed with fillers. The preferred size range is 5-100 µm.

As pointed out above, fillers other than alumina may also be used in the aforementioned packing agent, and kaolin as well as magnesia are such an alternative. On the other hand, while the fillers are not consumed during the coating treatment, usually some of the packing agent is lost during handling, especially when separating the coated articles, and when the packing agent contains fillers, it is desirable to fill the Packaging agent with some filler as well as with new aluminum. This filling can be limited to the cover layers of powder on the packed retort, in case mixing in advance of the filling agent with all the packaging means is not desirable. It is also very helpful, unless the packaging agent contains less than 10 percent filler, to expose completely new packaging material to a

so-called opening heating without any objects.

The alumination process of the present invention is useful for coating any iron base metal, e.g. cast iron, unalloyed carbon7501684-0 steel, low-alloy steel, martensitic stainless steel and other stainless steels and steels comprising precipitation-cured stainless steels, and the process is particularly suitable for making such iron alloys containing at least 1 percent chromium, preferably at least 5 percent chromium, corrosion. Martensitic stainless steels such as A1S1 410 or A1S1 403 stainless steels, Greek Ascoloy and precipitation hardened stainless steels such as steels 17-4PH and I7-7PH are typical examples of alloys in which the alumina of the present invention provides a marked increase in corrosion resistance at temperatures so high as about 595 ° C, which temperatures can rise in the compressor portion of a jet engine flying through salty environments near the sea. Aluminum coatings having a coating weight of about 0.5-7.5 mg 2 per cm are particularly effective for this purpose, and for unalloyed steels or steels with a maximum of 1 percent chromium, as much as 8 mg per cm is desirable. If the iron substrate contains less than 1 percent chromium, it can be chromium plated before the aluminum diffusion and thus enhance the increase in corrosion resistance. The term iron metal refers to metals containing at least 50% by weight of iron.

The retorts in which aluminum diffusion coating is carried out may be made of any material, e.g. of steel that can withstand the coating conditions. The surfaces of the retorts will receive an aluminum diffusion coating during use, but this does not weaken excessive steel and does not interfere with the coating. In fact, an aluminum surface of the retort is desirable in that it does not absorb much more aluminum during the coating process and thus does not remove much aluminum from the packing agent. Therefore, a wetting of the packing agent in an unused retort is of much help. The retort metal should preferably not include any low melting metal, e.g. zinc, lead, antimony, bismuth and tin.

The corrosion resistance of the above-mentioned cream-containing alloys can also be increased by an aluminum coating applied by other methods, e.g. such as those disclosed in U.S. Patent 3,787,305. The increased protection that can be achieved by layers with less

O than 1 mg per cm is greatly improved if the aluminum coating is completely continuous over the surface being protected, a result obtained when sheet aluminum is applied in amounts which allow the individual aluminum scales to partially overlap each other over the surface being protected. It is also very helpful, as proposed in US Patent 3 · 7θ7.305, to expose the aluminum-coated iron article to a temperature which causes at least a small portion of the aluminum to diffuse into the iron surface.

Sheet aluminum can be prepared as described in U.S. Patent No. 2,312,088, and is usually characterized by the presence of stearic acid or aluminum stearate or the like as a very thin coating on the surface of the sheet aluminum, a condition which makes it extremely difficult to disperse this aluminum into water. However, a considerable amount of wetting agent makes it possible to obtain a suitable dispersion, although it is easier to achieve such dispersions by also adding diethylene glycol or triethylene glycol or more high polymer ethylene glycols up to about 9000, as described in the US patent. 3,318,716. As shown in this patent, highly effective dispersions of sheet aluminum can be prepared from a concentrate consisting essentially of sheet aluminum, the polymeric ethylene glycol and a wetting agent, the aluminum being present in an amount of about 0.25-1.50 parts by weight. per part by weight of polymer ethylene glycol, and the wetting agent at a concentration of about 5-25% by weight of the concentrate.

This concentrate readily mixes with water in all proportions to provide a water dispersion of almost any desired aluminum content. Thus, a diluted dispersion containing 5 percent aluminum, 6 percent hexaethylene glycol and 7 percent para-n-octylphenyl ether of decethylene glycol is lightly sprayed onto a stator ring of a jet engine compressor to leave a coating weighing 0.5 mg per cm after drying in air for evaporation. water. The thus coated stator is then heated in an air oven until its temperature reaches about 427 ° C. The heating first causes the glycol and wetting agent to evaporate leaving a highly adherent continuous and shiny coating that is similar to polished aluminum and significantly contributes to the corrosion resistance of stator rings even if the heating temperature does not exceed c; a 315 ° C. the increase in corrosion resistance becomes more significant as the heating brings the coating to temperatures of about 482 ° C where a certain diffusion of the aluminum into the iron surface of the stator begins. The diffusion rate and degree of the resulting corrosion resistance is further increased by trapping the coated stator in an atmosphere of aluminum chloride gas while at temperatures above about 371 ° C. The aluminum chloride atmosphere is conveniently provided by a packing treatment described in Example 1, but without any aluminum in the packing agent. However, the stator ring with the sheet aluminum coating need only be hung on a wire in a retort containing no activating agent but no packing agent, and in this way is burned in an otherwise inert atmosphere.

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The sheet aluminum used in the above context preferably has a maximum size of about 50-250 µm, although other sizes can be used.

Non-ionic wetting agents are preferred for dispersing the aluminum as these wetting agents are more easily stripped at high temperatures. However, other types of wetting agents, including those which are not stripped or not completely stripped at 315-4Ö2 ° C, may also be used. Making the alumina coatings heavier than about 4 mg per cm does not contribute significantly to the corrosion resistance, and so

2 as little as 0.10 tng / enr is helpful even if at least about 0.3 mg / cm is preferred,

The use of a sheet aluminum coating also improves the corrosion resistance of iron surfaces, which contain less than 1 percent chromium, especially when these surfaces have an aluminum diffusion coating. The sheet aluminum coating also improves the dry erosion resistance of a coating obtained from mixtures of aluminum particles with phosphoric acid, chromic acid and magnesium, calcium, aluminum, or zinc salts thereof, as described in U.S. Patent 3,248,251. Thus, an exchange of the sheet aluminum, together with sufficient wetting agent and with or without the polymeric ethylene glycol, to bead-shaped aluminum in the blend compositions described in this patent thus contributes to a significant increase in corrosion resistance, especially for cured layers not exceeding 1 mg. per cm. In these mixtures, heating an alumina-containing coating does not provide any significant diffusion of aluminum into an Iron substrate as long as the heating temperature is not higher than about 537 ° C. Above this temperature, the heating tends to adversely affect ferrous metals, especially those used in jet engine compressor parts.

Another result of the use of sheet aluminum is the improved appearance of the objects. By substituting this type of aluminum for that shown in the composition of Example 1 of U.S. Patent 3,248,251 using the aforementioned polyglycol wetting agent composition, not only is a product having better conosist resistance but also with a luminous aluminum sheen. During the heating of the new mixture compositions and for curing thereof, gases are emitted, which shows that the polyglycol and wetting agent evaporates, and no significant reduction of 6-value chromium to 5-value state appears to take place.

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The aluminum diffusion coatings produced by the aluminum halide activating agent in the arrangements illustrated in Example 1 are extremely uniform and particularly free of defects. A similar freedom from defects, although with slightly less uniformity, is achieved when the retorts are not more than 38 cm deep and the aluminum halide activating agent is trapped in porous containers embedded in the packaging agent and not in contact with the article or any object above or below the containers. . These porous containers may be made of stainless steel or aluminum mesh, whereby the mesh is rolled into a pipe, and the pipe ends are then collapsed for locking of the rotor onto plutters. The F 'shelves should preferably be elongated and quite narrow so that they can be in phase, with their longitudinal shafts vertically in the packing means and in this way do not experience too much horizontal space. They can then be inserted into the packing medium after the retorts have been partially or fully packed, but to achieve the best results, no object is placed below or above the containers, nor is any object located at a distance less than about 0.63 cm. preferably not less than about 1.2 cm, from the side of the containers.

Maximum efficiency is achieved when the containers containing the activating agent are limited to a relatively small portion of the horizontal cross-section of the packing agent. Thus, an annular retort having a depth of about 33 cm, an inner diameter of about 20 cm, and an outer diameter of about 76 cm can have a set of 6 activating containers, each about 1.25 cm in diameter, embedded in the packing medium against the inner wall of the retort. Alternatively, a simple cup-shaped retort having the same depth and outer diameter may have similar activating agent containers packed in retorts at its center or at any other convenient location, such as a group located immediately adjacent to each other, all objects being spaced therefrom. The activating agent can be either anhydrous or aqueous aluminum halide, and the following examples illustrate this.

Example 2. An annular steel retort of SAE 1010 having a rectangular cross-section and lying on its planar side is designed to have a depth of about 35 cm, internally measured, with the packing receiving ring having an inner diameter of c: about 15 cm and an outer diameter of about 8 cm. An amount of the packing agent is prepared from a pre-burned mixture of 18 wt.% Aluminum and 82 wt.% Alumina, the pre-firing being carried out at about 482 ° C from uniformly mixed particles having a size of about 40 µm.

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0.3 percent anhydrous aluminum chloride added as an activating agent and distributed throughout the mixture before burning. The firing lasted for 15 hours in a hydrogen atmosphere, driving about 80 percent of the activating agent.

A layer having a thickness of about 1.25 cm of the pre-burnt and then cooled packing agent was poured over the bottom of the retort and a layer of articles was then placed over that packing layer but no closer than about 2.5 cm from the inner wall of the retort. Further pre-burned packing agent was then poured over the layer of articles until it covers all of them and reaches a height of about 1.25 era above the same. A new layer of objects is then applied in the same way as the first layer, and this layering is repeated until the retort is fully filled. Five activating containers are then prepared by rolling two turns of aluminum mesh with about 80 threads per cm to produce a tube having a diameter of about 0.96 about and a length of about 20 cm. One end of each tube is compressed, about 1/5 of all the activating agent (anhydrous aluminum chloride) required for the entire packing agent, about 0.3% by weight of the packing agent, is then incubated in each tube through the uncompacted end, end is clamped to hold the activator in place. The tubes were then inserted into the packing medium adjacent to the inner wall of the retort and the retort is now ready for filling. A number of such retorts can be stacked one on top of the other and then heated as described in US Pat. No. 3 · 7θ5 · 85 °, the peak heating temperature being about 468 ° C for 14 hours. uniform aluminum diffusion coating of a thickness of about 0.015 mm, and the number of objects that must be discarded or re-treated does not exceed percent.

As with the activation arrangement of Example 1, substantially the same results are obtained if anhydrous aluminum bromide or iodide, or aqueous aluminum chloride, bromide or iodide is used instead of anhydrous chloride activating agent, with the aluminum content of the packing agent varying from 100 percent down to 2 percent. For aluminum diffusion carried out at a temperature lower than about 482 ° C, at least 4 percent aluminum is preferred in the packing agent.

Instead of using aluminum of relatively pure composition, this aluminum may be an alloy containing significant quantities of favorable ingredients, e.g. silicon. A content of 12 percent silicon, for example, will improve the resistance to high temperature wax dating of ferrous metals exposed to dioxide coating by such an alloy.

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