Black hexavalent chromium-free plating treatment system
Inactive Publication Date: 2004-10-07
NITTO SEIKO KO
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Benefits of technology
[0007] The object of the present invention is to provide a black trivalent chromate treatment which can yield plated products having a black trivalent chromate-treated coating superior in corrosion resistance and which has less bad influence on human bodies and the environment.
[0008] In order to achieve this object, in one black hexavalent chromium-free plating treatment system of the present invention, the surface of a metal part as a base is plated with zinc in a zinc plating treatment step 10 and the surface of the zinc-plated coating is activated in a treatment solution in a dilute nitric acid activation treatment step 11. Then the activated metal part is rinsed, or is washed with water, so that nitric acid components are removed. Subsequently, a black coating is formed on the rinsed metal part in an inorganic salt solution containing trivalent chromium and iron components as main ingredients in a black chromate treatment step 20. After the subsequent rerinsing, the rerinsed black-coated metal part is subjected to a finish treatment by forming a conversion coating on the rerinsed metal part in a solution of inorganic salt and organic acid containing trivalent chromium and silica as main ingredients in a finish treatment step 30. The resulting metal part is dried in a drying step 34. Thus, a black trivalent chromate-treated coating is obtained which improves the corrosion resistance of the surface of the metal part.
[0009] In addition, in order to achieve the object previously mentioned, in another black hexavalent chromium-free plating treatment system of the present invention, the surface of a metal part as a base is plated with zinc in a zinc plating treatment step 10 and the surface of the zinc-plated coating is activated in a treatment solution in a dilute nitric acid activation treatment step 11. Then the activated metal part is rinsed, so that nitric acid components are removed. Subsequently, a black regulation coating is formed on the rinsed metal part in a solution of inorganic salt and organic acid containing trivalent chromium and silica components as main ingredients in a chemical conversion treatment step 13 which is arranged next to the rinsing step. After a rinse of the coated metal part, a black coating is formed on the rinsed coated metal part in an inorganic salt solution containing trivalent chromium and iron components as main ingredients in a black chromate treatment step 20. Subsequently, the black coated metal part is rerinsed and then the rerinsed metal part is subjected to a finish treatment in a finish treatment step 30 by forming a conversion coating on the rerinsed metal part in a solution of inorganic salt and organic acid which contains trivalent chromium and silica as main ingredients and is less concentrated than the solution used in the foregoing conversion treatment step 13. Then the finished metal part is dried in a drying step 34. This procedure can also yield a black trivalent chromate-treated coating which improves the corrosion resistance of the surface of the metal part.
[0010] It is preferable that the aforementioned finish treatment step comprises an initial finish treatment step 31 in which a conversion coating is formed in a solution of inorganic salt and organic acid which contains trivalent chromium and silica as main ingredients and a final finish treatment step 33 in which after rinsing following the initial finish treatment, the rinsed metal part is immersed in any one of an overcoat treatment solution containing silica and cobalt as main ingredients and a water-soluble anti-corrosive solution. This allows the corrosion resistance of the black trivalent chromate-treated coating to be held for a long term.
Problems solved by technology
However, a sufficient corrosion resistance cannot be achieved by only the plating and a treatment with chromic acid including hexavalent chromium, namely chromate treatment is usually carried out after the plating treatment.
However, it has lately been found that a prolonged contact of the human skin with the metal part treated with chromate including hexavalent chromium may cause the risk that the chromate is absorbed and accumulated in the body to cause chromium tumor, chromium allergy or the like.
Moreover, when the parts or products are buried in the ground as waste or are used for a long time, that component is eluted into the ground or harmful substances penetrate into the ground, causing soil pollution, which leads to environmental pollution such as pollution of water sources and penetration into plants.
However, regarding a metal part applied with this treatment, only a coating in white or in pale blue near white is obtained on the surface of the base of a metal part and any glossy black surface coating, the demand for which is increasing in order to impart heavy feeling to parts, has not been obtained.
Therefore, existing challenges include to urgently obtain a black coating having a corrosion resistance equal or similar to that of a black coating by a hexavalent chromate treatment by a trivalent chromium treatment like the above conventional example.
Method used
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, designed to be a two-step finish treatment including an initial finish treatment step 31 and a final finish treatment step 33 in place of the finish treatment step 30 in the treatment process shown in the Second Example. The details of the finish treatment are the same as those of the Third Example and, therefore, the explanation of the treatment is omitted here. The Fourth Example is the best treatment process by which corrosion resistance can be demonstrated most effectively in the formation of a black trivalent chromate coating 6.
[0025] Regarding the corrosion resistance of the bases obtained in the manners described above, the time required for 5% of white corrosion product of zinc to be formed was visually measured according to JIS H 8502 "Methods of corrosion resistance test for metallic coatings." The results are shown in Table 1. The condition for the measurement is salt spray test using ten screws as samples for each test.
1TABLE 1 RANGE OF WHITE APPEARANCE OF CORROSION PR...
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Abstract
The present invention provides a black hexavalent chromium-free plating treatment system in which the corrosion resistance of a surface of a metal part is improved by activating the surface of a zinc-plated coating of the metal part in a treatment solution in a dilute nitric acid activation treatment step, subsequently forming a black coating in an inorganic salt solution containing trivalent chromium and iron components as main ingredients in a black chromate treatment step, forming a conversion coating in a solution of inorganic salt and organic acid containing trivalent chromium and silica as main ingredients in a finish treatment step, and drying the resulting metal part. Therefore, even in a black trivalent chromate coating, sufficient corrosion resistance and sufficient strength are obtained. Moreover, a black trivalent chromate coating with heavy feeling is obtained for the first time. In addition, because the coating is one which is made mainly of trivalent chromium, less penetration into the ground occurs and the probability of occurrence of soil pollution caused by penetration of harmful substances into the ground is reduced.
Description
[0001] 1. Field of the Invention[0002] The present invention relates to a black hexavalent chromium-free plating treatment system which is so designed that a corrosion resistance equal or similar to that of metal parts applied with hexavalent chromate treatment is maintained and environmental pollution is eliminate as much as possible not by application of conventional hexavalent chromate treatment, but by application of black trivalent chromate treatment to the surface of a metal part.[0003] 2. Description of the Related Art[0004] As a method for preventing the surface of metal from corrosion, generally adopted is to apply zinc or zinc alloy plating to the surface of the base of a metal part to maintain the corrosion resistance. However, a sufficient corrosion resistance cannot be achieved by only the plating and a treatment with chromic acid including hexavalent chromium, namely chromate treatment is usually carried out after the plating treatment. However, it has lately been foun...
Claims
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Login to View More IPC IPC(8): C23C2/26C23C22/30C23C22/53C23C22/73C23C28/00
CPCC23C2/26C23C22/53C23C22/73C23C28/00C23C2222/10
InventorASOU, YOSHITAKASHIKATA, YASUHIROSHIKATA, TOSHIOFUJIWARA, RYUICHI
OwnerNITTO SEIKO KO