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(11) | EP 2 871 258 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | COMPOSITION OF POWDER MIXTURE FOR THERMAL DIFFUSION GALVANIZING OF PRODUCTS MADE OF ALUMINUM ALLOYS, PREPARATION METHOD THEREOF AND METHOD FOR THERMAL DIFFUSION GALVANIZING OF PRODUCTS MADE OF ALUMINUM ALLOYS |
| (57) The present invention relates to the physicochemical treatment of the surfaces of
articles made of aluminum alloys by thermal-diffusion galvanizing with the use of
powdered mixtures to improve rustproof properties of the articles. The powdered mixture
composition for thermal-diffusion galvanizing articles of aluminum alloys comprises
powdered zinc, an inert filler and an activating agent, the last composed of a mixture
of the components as follows, in % by mass: sodium fluoride 12-15, lithium chloride
20-25, ammonium chloride 10-15, zinc chloride 12-14, potassium chloride to balance,
with the following composition component ratio, in % by mass: Inert filler 17-22 Activating agent 6-8 Powdered zinc to balance. The process for the manufacture of a composition for thermal-diffusion galvanizing articles of aluminum alloys comprises drying the mixture of an inert filler and of an activating agent at a temperature of 60-70°C for 1.5-2.0 hours, mixing all the components in an airtight rotating container at a temperature of 60-70°C to obtain the moisture content of no more than 1%. |
Field of the invention
[0001] The present invention relates to the physicochemical treatment of the surfaces of articles made of aluminum alloys by thermal-diffusion galvanizing with the use of powdered mixtures to improve rustproof properties of the articles. The invention can find application in all the industries: machine building, automobile manufacture, ship building and aircraft construction, chemical and civil engineering, etc., where pieces, blocks, assemblies of mechanisms and articles made of aluminum alloys operate in corrosive media and undergo corrosive damages of various nature.
Background of the invention
[0002] Amongst numerous processes for applying protective coatings onto steel and cast iron parts in order to improve their rustproof properties, the thermal-diffusion galvanizing with powdered mixtures holds a particular place.
[0003] The main material of the powdered mixtures for thermal-diffusion galvanizing is powdered zinc and an activating agent (such as ammonium chloride).
[0004] A number of Russian and foreign papers are known, aimed to improve powdered mixtures and enabling to improve the quality (rustproof properties, thickness and uniformity) of zing plating on iron-carbon steels and alloys.
[0005] It is necessary to point out that heretofore, no success was achieved by any company in the attempts to obtain high-performance rustproof surface layers by thermal-diffusion galvanizing on aluminum alloys. At best, zinc-containing coatings with uneven thickness (5 to 15 µm) were formed on the article surface, characterized by their high accessible porosity, exceeding 80%. In this case, with its high porosity, the coating practically did not have any adhesion to the underlying material, yet in some places the zinc was only deposited onto the surface without any diffusive saturation of the aluminum alloy.
[0006] This fact can be explained by the fact that the surface of articles of aluminum alloys presents some original high-strength film of oxide prohibiting the contact interaction of powdered zinc particles and of its gaseous phase formed at the zinc plating directly with the aluminum alloy surface, which is the main reason of the lack of zinc penetration into the aluminum alloy crystal lattice. A particular difficulty resides in the fact that after a mechanical or chemical elimination of the film of oxide, the same is recreated some seconds later. This procedure is greatly intensified at high temperatures, including during thermal-diffusion galvanizing.
[0007] The above mentioned explains the absence of any scientific publications or substantiated practical recommendations on the subject.
[0008] Another important factor restraining works of thermal-diffusion galvanizing of aluminum alloys is based on the theoretical principles of the electrochemical corrosion of metals and alloys having different electrolytic potentials. For example, while analyzing the scale of standard electrolytic potentials of metals [N.D. Tomashov, G.P. Tchernov. Corrosion and rustproof alloys. - M.: Metallurgya, 1973, p.10-11], one can come to the conclusion of unsuitability to provide rust protection of aluminum alloys with zinc coatings. Nevertheless, we have carried out multiple combined studies to demonstrate that the protective coatings formed at the thermal-diffusion galvanizing are characterized by practically equal electrolytic potentials to that of the aluminum alloy due to the formation of complex chemical compounds of aluminum with zinc in the same, these compounds showing improved rustproof properties compared to aluminum alloys, as a result of a stronger effect of the cathodic alloying. Simultaneously, a strong passivating film is formed on the surface that additionally improves the rustproof properties of aluminum alloys.
[0009] For a better understanding, Fig. 1 shows a view of a specimen of aluminum alloy with a zinc coating after accelerated tests in a chamber containing a neutral salt fog.
[0010] One can observe that the surface of the aluminum alloy (surface B), after the accelerated tests in the neutral salt fog chamber is completely corroded. At the same time, no corrosion damages are observed on the zinc coating (surface A).
[0011] There are known compositions of powdered mixtures for thermal-diffusion galvanizing of steel articles, obtained by mixing powdered zinc with inert materials (alumina, fireclay, silica sand, etc.) added to the same [E.V. Proskurin, et al. Diffusion zinc coatings. M: Metallurgya, 1972, p. 39].
[0012] There are known compositions of powdered mixtures obtained by mixing powdered zinc or zinc dust in amounts of up to 75 % by mass, an inert filler such as alumina in the amount of up to 23 % by mass, and an activating agent such as ammonium chloride in the amount of up to 2 % by mass of the total composition mass. [Thermo-chemical processing of metals and alloys./ Edited by A.S. Liakhovich. - M.: Metallurgya, 1981]. The above given galvanizing composition does not provide the preparation of an impermeable uniform coating with high indices of corrosive resistance.
[0013] The document SU 1571103, 1990, discloses a composition for thermal-diffusion galvanizing of steel articles, comprising zinc, aluminum and alumina to which nitrilotrimethylphosphonic acid is added to intensify the process, to improve effectiveness by the increase of the number of cycles of use.
[0014] There is a known composition for providing a diffusion coating, comprising a zinc-containing substance, aluminum, ammonium chloride and an inert filler, to which magnesium and carnallite are added to improve the coating corrosive resistance and to reduce generation of gas, and hard zinc is added as a zinc-containing substance [document SU 1521790, 1989].
[0015] There is a known process for the manufacture of a powdered mixture for thermal-diffusion galvanizing by mixing powdered zinc, an inert filler and an activator added in an amount of not more than 2 % by mass of the powdered zinc, the inert filler being previously divided in two portions one of which is modified by mixing with the activator, and the second part of inert filler, the powdered zinc and the activated filler obtained are used as components to prepare a powdered mixture by putting them into a container with parts to be processed. The components mixing is carried out directly during the galvanizing process, with their ratio, in % by mass, as follows:
| Inert filler | 50.0-99.0 |
| Powdered zinc | 0.6-40.0 |
| Activated filler | 0.4-10. |
[0016] As the inert filler, alumina under the form of synthetic corundum or silicon oxide are used. As the powdered zinc, use is made of a powder containing at least 90 % by mass of particles with the size of up to 0.16 mm. As the activator, ammonium chloride is used. The inert filler is used with granularity of no more than 0.2 mm. [patent RU 2180018, 2002, I.V. Frishberg et al.].
[0017] A drawback of the mentioned galvanizing compositions and of the processes of their manufacture resides in the possibility to use them only for rust-preventing processing of articles of carbon and low-alloyed steel, including high-strength steel, of cast iron, of copper. These mixtures cannot be used for rust-preventing processing of articles of aluminum alloys due to the lack of chemical activity of the activating agent and the inert filler used, or to their insufficient chemical activity, the presence of which does not enable to carry out the process of destroying the film of oxide, nor the subsequent protection of the surface of aluminum alloy articles against its formation, in particular under high-temperature processing, which represents in the present case a necessary condition for the galvanizing process course.
[0018] It is necessary to note that the compositions of powdered mixtures disclosed by the Author's certificates SU 1571103, SU 1521790, are rather complicated and labor-consuming in manufacture.
[0019] There is as well a known standardized technology for applying zinc coating onto articles of steel, cast iron and copper according to GOST 9.316-2006 "Thermal-diffusion zinc coatings. General requirements and methods of control". The technological process according to this GOST norm comprises the steps as follows: surface preparing; thermal-diffusion galvanizing (applying a zinc coating) in an airtight rotating container positioned in a furnace at a temperature of 290-390°C for 2-3 hours at the constant container rotation at the rate of 0.03-0.1 s-1; phosphating and additional processing of the coating.
[0020] The closest to the composition and to the process for thermal-diffusion galvanizing of articles of aluminum alloys of the present invention is the composition for galvanizing aluminum surfaces disclosed in the China patent No 102002665, 03.10.2012. The disclosed composition comprises in % by weight: powdered zinc 14.9 - 84.5, inert powdered metal oxide (mixture of SiO2 and Al2O3, in an amount of 14.5 - 84.9, an activating component (ammonium chloride or ammonium nitrate), in an amount of 0.1-0.5 and an auxiliary component NH4NH2SO3 in an amount of 0.1-0.5. The process of thermal-diffusion galvanizing articles according to the China patent No 102002665 comprises the thermal treatment of articles with a powdered mixture in an airtight rotating container at a temperature of 400±10°C for 60-180 minutes, heating and subsequent treatment.
[0021] A drawback of this patent lies in the fact that it uses, at a different percentage, a powdered mixture composition with the traditionally used, for thermal-diffusion galvanizing iron-carbon steels and alloys, cast iron and copper, such components as the activating agent (ammonium chloride) and the inert filler (silicon oxide or alumina), see, for example, the patent RU 2180018, 2012.
[0022] However, the only use of the mentioned components in the activating agent and the inert filler composition, without their determined percentage and ratio, does not provide for the necessary chemical activity of the powdered mixture to carry out the process of saturation with zinc of the aluminum alloy surface in the thermal-diffusion galvanizing.
[0023] It is necessary to note, that introducing the additional component NH4NH2SO3 into the powdered mixture increases very little the powdered mixture activity but mainly serves to increase the number of its reuses thanks to a lower nodulizing process, to its reduced purification required and to the provided prevention of its eventual inflammation.
Summary of the invention
[0024] It is an object of the present invention to provide a powdered mixture composition for thermo-diffusion galvanizing, in particular, of articles of aluminum alloys, a process for manufacturing the same and a process for thermal-diffusion galvanizing with the use of said composition, in order to obtain a high-performance zinc coating with high rustproof properties and to provide an expanded assortment of articles to be processed.
[0025] Taking into consideration the current interest to the problem of improving the corrosion resistance namely for aluminum alloys operating in corrosive media (for example, in sea water), a multi-component powdered mixture with the optimal selected composition and the component percentage has been developed.
[0026] The technical result of the present invention application resides in the fact that using this powdered mixture for thermal-diffusion galvanizing mainly articles of aluminum alloys enables to completely destroy the oxide film on the article surface and to prevent its further formation at high temperatures, which allows to provide high-performance rustproof (the corrosion resistance in a chamber with a neutral salt fog is not less than 720 hours) zinc coatings with the thickness of up to 72 µm (the thickness of the diffusive zinc layer is of up to 65 µm ).
[0027] Said technical result is obtained by the use of a powdered mixture composition for thermal-diffusion galvanizing articles of aluminum alloys, comprising powdered zinc, an inert filler and an activating agent composed of a mixture of the following components, in % by mass: sodium fluoride 12-15, lithium chloride 20-25, ammonium chloride 10-15, zinc chloride 12-14, potassium chloride, to balance, with the following component ratio, in % by mass:
| Inert filler | 17-22 |
| Activating agent | 6-8 |
| Powdered zinc | to balance. |
[0028] As the powdered zinc, the PTsR-1 powder is used (powdered zinc obtained by pulverizing molten zinc with an inert gas), produced on an industrial scale and having the composition as follows, in % by mass: fractions smaller than 63 µm, at least 50%; fractions of 63-160 µm, not more than 40.0%; fractions bigger than 160 µm, not more than 10.0. The metal zinc content is at least 98 % by mass according to the norm GOST 12601-76.
[0029] The PTsR-1 powdered zinc according to GOST 12601-76 is widely used for thermal-diffusion galvanizing iron-carbon steels and alloys, cast iron and copper in the composition of powdered mixtures comprising, when necessary, various activating agents and inert fillers.
[0030] As the inert filler, use is made of a high-strength, porous, high-melting material of organogenic origin composed of silicon, aluminum, iron, calcium oxides as well as of clay and sand impurities, such as diatomite, opoka, tripolite and others, with the density of 0.7 to 1.0 g/cm3, that are an adsorbent, a catalyst and ballast. In this case, at least 80 % by mass of inert filler are used with the fraction size of 0.8 to 1.2 mm, and up to 20.0 % by mass of inert filler can be used with the fraction size lower than 0.8 mm.
[0031] Multiple studies showed that the inert filler according to the present invention, compared to known inert fillers, such as alumina (synthetic corundum) or silicon oxide used in the closest prior art are characterized by increased adsorbing and catalytic properties, which enables to significantly intensify the process of zinc saturation for the crystal lattice of aluminum alloys. Adsorbing capacity of the inert filler is provided due to its low density and high porosity. Just at such a density, it is possible to provide the needed saturation of the inert filler with zinc from the gaseous and solid phases. The high strength and selected particle size enables the reliable disintegration of the oxide film on the surface of aluminum alloys as well as a double effect during the thermal-diffusion galvanizing, such as: the thermo-chemical one (saturation of the aluminum alloy surface from the gaseous phase) and the mechanical one (at the direct contact of the powdered zinc and of the inert filler with the aluminum alloy surface).
[0032] As the activating agent, use is made of a mixture composed of the following components, in % by mass: sodium fluoride NaF 12-15, lithium chloride LiCl 20-25, ammonium chloride NH4Cl 12-15, zinc chloride ZnCl2 12-14, potassium chloride KCl to balance.
[0033] The selection of activating agent components and their percentage are provided with the multiple experiments carried out for thermal-diffusion galvanizing aluminum alloys in order to obtain high-performance, maximum-thickness rustproof coatings: potassium chloride KCl and sodium fluoride NaF increase the thermal-diffusion activity of zinc, enable dissolving aluminum and zinc oxides; lithium chloride LiCl promotes the diffusion process acceleration thanks to the increased thermodynamic activity of zinc, increases the density and the corrosion resistance of zinc coatings, dissolves aluminum and zinc oxides; ammonium chloride NH4Cl promotes formation of a protective atmosphere in the furnace, promotes the galvanizing reaction acceleration due to the active mixing of reacting materials, to the density increase of the zinc coating; zinc chloride ZnCl2 promotes the zinc diffusion process acceleration due to the increase of its thermodynamic activity, to dissolution of aluminum and zinc oxides, to the reduction of zinc.
[0034] The content of chemically active components in the activating agent provides for reliable protection of the aluminum alloy surface against the formation of an oxide film at galvanizing high temperatures, enabling by the fact, together with the inert filler of the present invention, the formation of high-performance, rustproof zinc coatings.
[0035] Said technical result is achieved by the use of a process for the manufacture of a powdered mixture composition for the thermal-diffusion galvanizing articles of aluminum alloys, comprising the steps of: preliminary drying the powdered mixture of an inert filler and of an activating agent in a dewatering box at a temperature of 60-70°C for 1.5-2.0 hours followed by mixing in an airtight rotating container at constant warming up to 60-70°C to obtain a uniform mass with the moisture content of no more than 1%, with the component ratio as follows, in % by mass:
| Inert filler | 17-22 |
| Activating agent | 6-8 |
| Powdered zinc | to balance. |
[0036] The powdered mixture components can be stirred both just before its use and before its storage in an airtight package in the presence of silica gel (at the rate of 100 g of silica gel per 1 kg of mixture).
[0037] A lower content if only of one of the components does not provide for the needed technical result. An exceeding content results in adhering of some powdered mixture to the article surface, which leads to lower quality and lower characteristics of the coating.
[0038] The activating agent and the inert filler of the present invention are characterized by a high water-absorbing capacity. The inert filler and the activating agent components are dried in a dewatering box at a temperature of 60-70°C for 1.5-2.0 hours. Said conditions namely provide for the admissible moisture content (no more than 1%) in the activating agent and in the inert filler. Higher moisture content causes the formation of lumps in the powdered mass, whereas at the thermal-diffusion galvanizing, no full contact of the powdered mixture with the aluminum alloy surface is observed, the process being accompanied by adhering of the same to the article surface. At higher temperatures of the drying operation, the activating agent components can become depleted of their chemically active substances. Lower temperatures require a significant increase of the drying time, which is economically disadvantageous.
[0039] It is necessary to note that the powdered mixture composition of the present invention and the method for the manufacture of the same can be used as well to increase the rustproof resistance in the thermal-diffusion galvanizing of parts made of a wide range of iron-carbon steels and alloys, cast iron and copper.
[0040] The use of the powdered mixture composition of the present invention, manufactured by said process enables, during the thermal-diffusion galvanizing of aluminum alloy articles, to completely destroy the oxide film on the aluminum parts surface and to prevent further formation of the same at high temperatures, and, by these means, to obtain high-performance, corrosion-resistant zinc coatings with the thickness of up to 72 µm (the thickness of the diffusive zinc layer is of up to 65 µm ), as well as to provide stability of the same in a chamber containing a neutral salt fog, at this thickness, for at least 720 hours.
[0041] All the above disclosed allows to state that the powdered mixture composition for the thermal-diffusion galvanizing of aluminum alloy articles, the process for the manufacture of the same and the process of the thermal-diffusion galvanizing with the use of the said composition, is characterized not only by new essential features such as the activating agent composition, the numerical ratio of the mixture components and by the process for the manufacture of the same, but provides as well the achievement of the needed technical result that is to obtain a high-performance, rustproof zinc coating on articles of aluminum alloys, and to expand the assortment of articles to be processed.
[0042] The application of protective zinc coatings is carried out as follows: previously degreased specimens of the size 100x100x50 mm, made of the aluminum alloy AMg6 and the necessary amount of powdered mixture for the thermal-diffusion galvanizing with the component ratio of the present invention, in % by mass: inert filler 17-22, activating agent 6-8, powdered zinc of brand PTsR-1 to balance, are loaded into a container (a retort) with the possibility of rotation. The container is sealed, loaded into a furnace and the thermal-diffusion galvanizing is started at a temperature of 300-450°C for 60 min. After the end of the thermal-diffusion galvanizing process and of the container cooling, the specimens are removed and their surface is cleaned to eliminate the residues of powdered mixture.
[0043] The study of the zinc coating quality was performed on prismatic test specimens sized 100x100x50 mm, made of aluminum alloy AMg6 (GOST4784-74).
[0044] The zinc coating thickness was determined by a metallographic method on cross-sectional micro section specimens with the use of the MMR-4 microscope, the micro section specimen etching being carried out in a 1% hydrofluoric acid solution.
[0045] The coating corrosion-resistance test was carried out in a chamber containing a neutral salt fog according to GOST 9.308.
Brief description of the drawings
[0046]
Fig. 1 shows the view of an aluminum alloy specimen with zinc coating (according to the present invention) after an accelerated corrosion test in a chamber containing a neutral salt fog. One can see that the aluminum alloy surface (surface B) after said accelerated corrosion test is completely corroded, whereas the zinc coating (surface A) is free of corrosion damages.
Fig. 2 shows the view of the zinc-plated surface (according to the present invention) of an aluminum alloy specimen. The coating is continuous, smooth, without pores or defects.
Fig. 3 shows the view of a zinc coating (according to the present invention) on a
cross-section micro section (a 500X magnification) for a specimen of aluminum alloy.
The zinc coating is shown as dark and light areas (denoted by 1); the diffusion layer
as a light area (denoted by 2); the aluminum alloy is denoted by 3. One can see that
the coating is continuous, with uniform thickness (68 - 72µm), without pores or defects,
the diffusion layer is (60 - 65µm).
Fig. 4 shows the view of a zinc coating in an aluminum alloy specimen obtained according to the patent RU 2180018. The coating is not continuous, presents pores and damages. The dark areas represent the zinc-containing coating (denoted by 1), the light areas represent the aluminum alloy.
Fig. 5 shows the view of a zinc coating (according to the patent RU 2180018) on a cross-section micro section (a 500X magnification) for a specimen of aluminum alloy. One can see that the zinc coating (denoted by 1) is not uniform in thickness (5-15µm), it presents damages, no diffusion saturation with zinc is observed, the aluminum alloy being denoted by 2.
Examples of embodiments of the invention
Example 1 (comparative)
[0048] As an inert filler, use is made of diatomite, tripoli earth or opoka, with the fraction size of 0.8-1.2 mm in the percentage of at least 80.0% and with the density of 0.7-1.0 g/cm3, in an amount providing for the mass ratio of 10.0-15.0 % by mass. As an activating agent, a mixture is used composed of the following components: sodium fluoride NaF (12-15), lithium chloride LiCl (20-25), ammonium chloride NH4Cl (10-15), zinc chloride ZnCl2 (12-14), potassium chloride KCl to balance, in an amount providing the mass ratio of 3.0-5.0 % by mass, the powdered zinc PTsR-1 being added to balance. To obtain a uniform mass with the moisture content of no more than 1%, the powdered zinc, the inert filler and the activating agent are mixed in a closed mixer permanently heated to 60-70°C. The powdered mixture is loaded into a container containing the specimens to be processed. The container is sealed, put into a furnace and the thermal-diffusion galvanizing is carried out according to the technological chart. After the end of the galvanizing process and after cooling the container, the specimens are extracted and their surfaces cleaned to remove the powdered mixture residues.
Example 2 (comparative)
[0051] As an inert filler, use is made of diatomite, tripoli earth or opoka, with the fraction size of 0.8-1.2 mm in the percentage of at least 80.0% and with the density of 0.7-1.0 g/cm3, in an amount providing for the mass ratio of 25.0-30.0 % by mass. As an activating agent, a mixture is used composed of the following components: sodium fluoride NaF (12-15%), lithium chloride LiCl (20-25%), ammonium chloride NH4Cl (10-15%), zinc chloride ZnCl2 (12-14%), potassium chloride KCl to balance, in an amount providing the mass ratio of 10.0-15.0 % by mass, the powdered zinc PTsR-1 being added to balance. To obtain a uniform mass with the moisture content of no more than 1%, the powdered zinc, the inert filler and the activating agent are mixed in a closed mixer permanently heated to 60-70°C. The powdered mixture is loaded into a container containing the specimens to be processed. The container is sealed, put into a furnace and the thermal-diffusion galvanizing is carried out according to the technological chart. After the end of the galvanizing process and after cooling the container, the specimens are extracted and their surfaces cleaned to remove the powdered mixture residues.
Example 3 (The best embodiment of the present invention)
[0054] As an inert filler, diatomite, tripoli earth or opoka are taken, with the fraction size of 0.8-1.2 mm in the percentage of at least 80.0% and with the density of 0.7-1.0 g/cm3, in an amount providing for the mass ratio of 17-22 % by mass. As an activating agent, a mixture is used composed of the following components: sodium fluoride NaF (12-15), lithium chloride LiCl (20-25), ammonium chloride NH4Cl (12-15), zinc chloride ZnCl2 (12-14), potassium chloride KCl to balance, in an amount providing the mass ratio of 6.0-8.0 % by mass, the powdered zinc PTsR-1 being added to balance. To obtain a uniform mass with the moisture content of no more than 1%, the powdered zinc, the inert filler and the activating agent are mixed in a closed mixer permanently heated to 60-70°C. The powdered mixture is loaded into a container containing the specimens to be processed. The container is sealed, put into a furnace and the thermal-diffusion galvanizing is carried out according to the technological chart. After the end of the galvanizing process and after cooling the container, the specimens are extracted and their surfaces cleaned to remove the powdered mixture residues.
Example 4 (comparative)
[0058] The powdered mixture content composition is similar to that of example 3. The activating agent content is similar to that of the example 3. The inert filler density is similar to that of example 3. Mixing powdered zinc, inert filler and activating agent is similar to that of example 3. The technology of the thermal-diffusion galvanizing is similar to that of example 3.
Example 5 (comparative)
[0061] The powdered mixture content composition is similar to that of example 3. The activating agent content is similar to that of the example 3. The inert filler density is similar to that of example 3. Mixing powdered zinc, inert filler and activating agent is similar to that of example 3. The technology of the thermal-diffusion galvanizing is similar to that of example 3. The inert filler is used with the fraction size of 1.3 - 1.5 mm.
Example 6 (comparative)
[0063] The powdered mixture content composition is similar to that of example 3. The activating agent content is similar to that of the example 3. The inert filler fraction size is similar to that of example 3. Mixing powdered zinc, inert filler and activating agent is similar to that of example 3. The technology of the thermal-diffusion galvanizing is similar to that of example 3. The inert filler is used with the density of 0.4 - 0.6 g/cm3.
Example 7 (comparative)
[0065] The powdered mixture content composition is similar to that of example 3. The activating agent content is similar to that of the example 3. The inert filler fraction size is similar to that of example 3. Mixing powdered zinc, inert filler and activating agent is similar to that of example 3. The technology of the thermal-diffusion galvanizing is similar to that of example 3. The inert filler is used with the density of 1.2 - 1.5 g/cm3.
Example 8 (comparative)
[0067] The powdered mixture content composition is similar to that of example 3. The inert filler content is similar to that of the example 3. The inert filler fraction size and the density are similar to that of example 3. The activating agent content is similar to that of the example 3.
[0069] Mixing powdered zinc, inert filler and activating agent is carried out in a closed mixer at its continuous heating to 60-70°C to obtain uniform mass with the moisture content of more than 1%.
Example 9 (comparative)
[0071] The powdered mixture content composition is similar to that of example 3. The inert filler content is similar to that of the example 3. The inert filler fraction size and the density are similar to that of example 3. As the activating agent, a mixture is used composed of the following components: sodium fluoride NaF (8-10%), lithium chloride LiCl (15-18%), ammonium chloride NH4Cl (6-8%), zinc chloride ZnCl2 (7-10%), potassium chloride KCl to balance. Mixing powdered zinc, inert filler and activating agent is similar to that of example 3. The technology of the thermal-diffusion galvanizing is similar to that of example 3.
Example 10 (comparative)
[0073] The powdered mixture content composition is similar to that of example 3. The inert filler content is similar to that of the example 3. The inert filler fraction size and the density are similar to that of example 3. As the activating agent, a mixture is used composed of the following components: sodium fluoride NaF (18-20%), lithium chloride LiCl (28-32%), ammonium chloride NH4Cl (18-20%), zinc chloride ZnCl2 (17-20%), potassium chloride KCl to balance. Mixing powdered zinc, inert filler and activating agent is similar to that of example 3. The technology of the thermal-diffusion galvanizing is similar to that of example 3.
Example 11 (comparative, according to the China patent No 102002665, 03.10.2012)
[0075] 10 kg of previously degreased specimens sized 100x100x50 mm, made of aluminum alloy AMg6 (GOST4784-74) are charged into a container.
[0076] The preparation of the powdered mixture, its composition, the component ratio in % by mass, the component fraction size and the regimes of thermal-diffusion galvanizing were provided in accordance with the process of the China patent No 102002665,03.10.2012).
[0080] Thus, the composition and the process for the manufacture of a powdered mixture for thermal-diffusion galvanizing aluminum alloys enable to provide a high-performance, rustproof zinc coating on articles of aluminum alloys (Tables 1-6, example 3). With this process, continuous, pore-free zinc coatings are provided, with their uniform thickness (zinc coating thickness of 68-72 µm), with no defects (bulges, pockets, cracks, build-ups, stripping, embedment), with good corrosion resistance (resistance in a chamber containing a neutral salt fog of at least 720 hours).
[0081] However, as shown by multiple experiments and as one can see from the Table 1 (examples 1, 2), the Table 2 (examples 4, 5), the Table 3 (examples 6, 7), the Table 4 (example 8), the Table 5 (examples 9, 10), even a minor change of the composition and in the process for the manufacture of the powdered mixture, in particular, of the component mass ratio in the powdered mixture (Table 1, examples 1 and 2), of the inert filler fractions (Table 2, examples 4 and 5), of the inert filler density (Table 3, examples 6 and 7), of the moisture content in the powdered mixture (Table 4, example 8), of the activating agent component content (Table 5, examples 9 and 10) does not provide the solution of the task formulated nor the necessary technical result.
[0082] As one can see from the data given in Table 6 (example 11), the closest process to that of the present invention does not enable to resolve the task of the present invention nor to achieve the necessary technical result in the thermal-diffusion galvanizing of aluminum alloy articles.
[0083] It was established that in the closest process to that of the present invention, the zinc-containing coating formed on the surface of specimens of aluminum alloys has no uniform thickness (5-15 µm), is highly porous and has defects (bulges, pockets, cracks, build-ups, stripping, embedment).
[0084] With this, it is to be stated in particular that due to the multiple defects in the zinc coating, the corrosion resistance in the present case will be practically determined by the corrosion characteristics of the aluminum alloy proper.
[0085] Thus, the application of the present invention allows to provide the thermal-diffusion galvanizing of aluminum alloy articles, to obtain a high-performance zinc coating with high rustproof properties, to expand the assortment of articles to be processed.
Table 1
| Effect of the component ratio in the powdered mixture on the zinc coating properties (% by mass) | ||||||||
| No | Component ratio in the powdered mixture, % by mass | Properties of the zinc coating | Characteristics of the zinc coating | |||||
| Thickness, µm | Corrosion resistance, hrs | |||||||
| Examples | Inert filler | Activating agent | Powdered zinc | max | min | |||
| 1 | Example 1 (comparative) | 10.0-15.0 | 3.0-5.0 | to balance | 50 | 45 | at least 480 | Uniform in thickness, continuous, smooth, little porosity, without defects |
| 2 | Example 2 (comparative) | 25.0-30.0 | 10.0-15.0 | to balance | 60 | 10 | no more than 190 | Nonuniform in thickness, continuous, rough, porous, with defects |
| 3 | Example 3 (present invention) | 17.0-22.0 | 6.0-8.0 | to balance | 72 | 68 | at least 720 | Uniform in thickness, continuous, smooth, no porosity, without defects |
Table 2
| Effect of the fraction size in the inert filler on the zinc coating properties | ||||||
| No | Examples | Fraction size, mm | Properties of the zinc coating | Characteristics of the zinc coating | ||
| Thickness, µm | Corrosion resistance, hrs | |||||
| max | min | |||||
| 1 | Example 4 (comparative) | 0.5-0.7 | 12 | 5 | no more than 190 | Nonuniform in thickness, continuous, rough, porous, with defects |
| 2 | Example 5 (comparative) | 1.3-1.5 | 53 | 10 | no more than 240 | Nonuniform in thickness, continuous, rough, porous, with defects |
| 3 | Example 3 (present invention) | 0.8-1.2 | 72 | 68 | at least 720 | Uniform in thickness, continuous, smooth, no porosity, without defects |
Table 3
| Effect of the inert filler density on the zinc coating properties | ||||||
| No | Examples | Density g/cm3 | Properties of the zinc coating | Characteristics of the zinc coating | ||
| Thickness, µm | Corrosion resistance, hrs | |||||
| max | min | |||||
| 1 | Example 6 (comparative) | 0.4-0.6 | 10 | 4 | no more than 190 | Nonuniform in thickness, continuous, rough, porous, with defects |
| 2 | Example 7 (comparative) | 1.2-1.5 | 50 | 12 | no more than 240 | Nonuniform in thickness, continuous, rough, porous, with defects |
| 3 | Example 3 (present invention) | 0.7-1.0 | 72 | 68 | at least 720 | Uniform in thickness, continuous, smooth, no porosity, without defects |
Table 4
| Effect of moisture content in the powdered mixture on the zinc coating properties | ||||||
| No | Examples | Moisture content, % | Properties of the zinc coating | Characteristics of the zinc coating | ||
| Thickness, µm | Corrosion resistance, hrs | |||||
| max | min | |||||
| 1 | Example 8 (comparative) | more than 1% | 20 | 15 | no more than 240 | Nonuniform in thickness, not continuous, rough, porous, with defects |
| 2 | Example 3 (present invention) | no more than 1% | 72 | 68 | At least 720 | Uniform in thickness, continuous, smooth, no porosity, without defects |
Table 5
| Effect of component ratio in the activating agent on the zinc coating properties | ||||||||||
| N o | Examples | Component ratio in the activator, % | Properties of the zinc coating | Characteristics of the zinc coating | ||||||
| Thickness, µm | Corrosion resistance, hrs | |||||||||
| NaF | LiCl | NH4Cl | ZnCl2 | KCl | max | min | ||||
| 1 | Example 9 (comparative) | 8-10 | 15-18 | 6-8 | 7-10 | to balance | 50 | 45 | At least 480 | Uniform in thickness, continuous, smooth, no porosity, without defects |
| 2 | Example 10 (comparative) | 18-20 | 28-32 | 18-20 | 17-20 | to balance | 72 | 68 | At least 720 | Uniform in thickness, continuous, smooth, no porosity, without defects, economically disadvantageous |
| 3 | Example 3 (present invention) | 12-15 | 20-25 | 12-15 | 12-14 | to balance | 72 | 68 | At least 720 | Uniform in thickness, continuous, smooth, no porosity, without defects |
Table 6
| Effect of the process for the manufacture of the powdered mixture on the zinc coating properties | |||||
| No | Examples | Properties of the zinc coating | Characteristics of the zinc coating | ||
| Thickness, µm | Corrosion resistance, hrs | ||||
| max | min | ||||
| 1 | Example 3 (present invention) | 72 | 68 | At least 720 | Uniform in thickness, continuous, smooth, no porosity, without defects |
| 2 | Example 11 (comparative, of the China patent No 102002665) | 15 | 5 | no more than 120 | Nonuniform in thickness, rough, porous, with defects |
1. Powdered mixture composition for thermal-diffusion galvanizing articles of aluminum
alloys, comprising powdered zinc, an inert filler and an activating agent wherein,
as the activating agent, use is made of a mixture of the components as follows, in
% by mass: sodium fluoride 12-15, lithium chloride 20-25, ammonium chloride 10-15,
zinc chloride 12-14, potassium chloride to balance, with the following composition
component ratio, in % by mass:
| Inert filler | 17-22 |
| Activating agent | 6-8 |
| Powdered zinc | to balance. |
2. Composition of claim 1 wherein, as the inert filler, use is made of silicon, aluminum,
iron, calcium oxides with clay and sand impurities.
3. Composition of claim 1 wherein, as the inert filler, use is made of diatomite, opoka,
tripolite with the fraction size of no more than 1.2 mm and with the density of 0.7-1.0
g/cm3.
4. Composition of claim 1 wherein at least 80 % by mass of inert filler is used with
the fraction size of 0.8 to 1.2 mm, and up to 20.0 % by mass of inert filler is used
with the fraction size lower than 0.8 mm.
5. Process for the manufacture of a powdered mixture composition for thermal-diffusion
galvanizing articles of aluminum alloys, wherein it comprises preliminary drying the
mixture of an inert filler and of an activating agent at a temperature of 60-70°C
for 1.5-2.0 hours and mixing all the composition components in an airtight rotating
container at a temperature of 60-70°C to obtain a homogeneous mass with the moisture
content of no more than 1%, with the component ratio as follows, in % by mass:
wherein, as the activating agent, use is made of a mixture of the following components,
in % by mass: sodium fluoride (12-15), lithium chloride (20-25), ammonium chloride
(10-15), zinc chloride (12-14), potassium chloride to balance.
| Inert filler | 17-22 |
| Activating agent | 6-8 |
| Powdered zinc | to balance, |
6. Process of claim 5 wherein, as the inert filler, use is made of silicon, aluminum,
iron, calcium oxides with clay and sand impurities, mostly diatomite, opoka, tripolite
with the fraction size of no more than 1.2 mm and with the density of 0.7-1.0 g/cm3.
7. Process of claim 5 wherein at least 80 % by mass of inert filler is used with the
fraction size of 0.8 to 1.2 mm, and up to 20.0 % by mass of inert filler is used with
the fraction size lower than 0.8 mm.
8. Process of thermal-diffusion galvanizing articles of aluminum alloys, comprising heat
treatment of the articles with a powdered mixture in an airtight rotating container,
cooling and subsequent processing, wherein the heat treatment of the articles with
a powdered mixture is carried out in an airtight rotating container placed into a
furnace with the temperature of 300-450°C for 60 minutes, the powdered mixture composition
for the thermal-diffusion galvanizing of claim 1 being used as the powdered mixture.
9. Process of claim 8 wherein the powdered mixture for the thermal-diffusion galvanizing
articles of aluminum alloys is manufactured by the process of claim 5.
10. Process of claim 8 wherein the subsequent processing of the articles of aluminum alloys
comprises phosphating and/or treatment with oils, greases, means for temporary rust
protection, paint and varnish coatings, siliceous compositions.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.
Patent documents cited in the description
- SU1571103 [0013] [0018]
- SU1521790 [0014] [0018]
- RU2180018 [0016] [0021] [0046] [0046]
- CN102002665 [0020] [0020] [0076] [0085]
Non-patent literature cited in the description
- N.D. TOMASHOVG.P. TCHERNOVCorrosion and rustproof alloysM.: Metallurgya, 1973, 10-11 [0008]
- E.V. PROSKURIN et al.Diffusion zinc coatingsM: Metallurgya, 1972, 39- [0011]
- Thermo-chemical processing of metals and alloysM.: Metallurgya19810000 [0012]