JPH04193963A - Method for composite surface treatment of cast iron material - Google Patents

Abstract

PURPOSE:To form a film excellent in rust proofing effect and having the metallic luster of silver white on the surface of a cast iron material by removing molding sand on the surface and graphite in cast iron parts, thereafter subjecting it to nitriding treatment and oxidizing treatment to form an Fe3O4 film, and successively applying Ni plating thereto. CONSTITUTION:Casting parts of various cast iron contg. graphite in the structure and stuck with molding sand on the surface are immersed in a molten salt bath constituted of NaOH and neutral salt. At first, energizing is executed with it as the negative pole to remove the molding sand and an oxidized film stuck on the surface, and successively, energizing is executed with it as the positive pole to oxidize away the graphite on the surface. Next, the surface is subjected to nitriding treatment by salt bath soft nitriding treatment to form an Fe-N compound layer excellent in corrosion resistance and wear resistance, which is furthermore subjected to heating treatment in a steam atmosphere to form an Fe3O4 film on the surface. This surface is degreased and cleaned, is furthermore subjected to activating treatment, and is thereafter subjected to electroplating so that the thickness of dull Ni plating and bright Ni plating layers shall respectively be regulated to >=1mum and that of both Ni plating layers to >=5mum.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for preventing the formation of red rust on the surface of cast iron castings containing graphite in the matrix.

[Prior art] Cast iron containing graphite as a matrix, such as cored malleable cast iron, spheroidal graphite cast iron, vermicular cast iron, and gray cast iron, is heated in cold water.
When applied to hot water supply and drainage parts, it is necessary to prevent the formation of red rust on the cast iron surface in order to prevent the formation of red water. Currently, stainless steel pipe fittings are used as rust-free pipe fittings, but they are expensive and difficult to use as general-purpose pipe fittings. It is desired to develop a surface treatment method that is inexpensive and exhibits rust prevention performance and metallic luster comparable to that of stainless steel.

Nowadays, nitriding treatment, electric Ni plating, electroless Ni plating, etc. are known as surface treatment methods that exhibit corrosion resistance and wear resistance on a film surface. Additionally, the gas joints are hot-dip galvanized.

Regarding nitriding treatment on cast iron surfaces, see Japanese Patent Application Laid-open No. 6
4-5087 discloses a method of forming triiron tetroxide on a nitrided layer.

Further, US Patent No. 7l-74666S/47 discloses a descaling and degraphitizing method called molten salt electrolytic treatment.

[Problems to be Solved by the Invention] The antirust effect on the surface of cast iron having graphite on the surface is to prevent the occurrence of rust by forming a film on the metal surface that is free of cracks and pinholes. If foundry sand or graphite exists on the cast iron surface, the surface treatment film will not be completely formed on the cast iron surface due to interference from the foundry sand or graphite, and a local battery will be formed between the locally exposed base and red rust will occur. The present inventor has clarified that this occurs. Therefore, for castings with graphite in the base and molding sand adhering to the surface of the cast iron, individual treatments such as nitriding, electric N1 plating, and electroless Ni plating are
Surface treatment methods aimed at preventing rust are not perfect.

Furthermore, gas joints are coated with hot-dip galvanizing, but the JIS standard requires a thick coating of at least 70 μm in order to guarantee anti-corrosion performance. Therefore, since the gas fitting is plated and then threaded, it is ineffective in preventing rust on the threaded portion.

The technology disclosed in Japanese Patent Publication No. 64-5087 is a technique in which a sorbite structure is formed on the surface by heat treatment, and then a nitride layer and a triiron tetroxide layer are formed thereon, and it is applied to base structures such as ferrite, pearlite, and austenite. It is difficult to completely prevent red rust that forms between incomplete surface treatment films due to the interference of graphite because surface treatment technology does not take into account the removal of graphite from the treated surface.

In addition, U.S. Patent No. 71-746f36S/47 is known as a method for degraphitizing the surface of cast iron.
This is used as a pretreatment method to improve the wettability during brazing of cast iron, and has not been studied as a pretreatment for nitriding to improve the rust prevention effect.

Furthermore, with the various conventional surface treatment methods mentioned above, even if an anti-rust effect is achieved, the color of the treated surface is black or gray. is unreasonable to apply.

An object of the present invention is to provide a surface treatment method that has an anticorrosive effect and imparts a silvery white metallic luster.

[Means for Solving the Problems] In order to solve the above problems, the present invention first uses a molten salt electrolytic treatment method to expose the surface of cast iron to a depth of about 50 μm, which interferes with the formation of a surface treatment film. After removing the molding sand and graphite, a nitriding treatment, for example, a salt bath soft nitriding treatment is performed to improve corrosion resistance and wear resistance, and a Fe--N compound layer is formed on the surface of the cast iron with a thickness of about 1 to 30 μm. Furthermore, by performing heat treatment in a steam atmosphere etc., a film of 1 to 3 Fe3O4 is formed on the surface of the Fe-N compound layer.
A thickness of approximately 0 μm is formed. That is, by removing the graphite on the surface of the cast iron that obstructs the formation of a rust preventive film through molten salt electrolytic treatment, it is possible to form a uniform and complete rust preventive film over the entire surface of the cast iron. Thereafter, at least one N1 plating film with a total thickness of about 5 μm to 30 μm is coated on the treated surface for the purpose of imparting a silvery white metallic luster. Of course, the Ni plating treatment may be performed on the Fe--N compound layer without performing the Fe3O4 film formation treatment on the nitrided film.

[A single surface treatment on the cast iron surface that fosters graphite in the active core did not show a complete rust prevention effect, but after removing foundry sand and graphite on the cast iron surface that interfere with the formation of the surface treatment film, Fe50. The composite of the present invention is characterized in that the treated film is coated with at least one or more thin Ni plating film for the purpose of making the surface beautiful. By applying surface treatment, it exhibits an excellent rust prevention effect and has an excellent appearance that retains a silvery white metallic luster comparable to that of stainless steel. In addition, when the thickness of the Ni plating film is 5 μm or less, no metallic luster is exhibited and the base color appears through the plating film.

[Example] An example of the surface treatment method of the present invention will be explained according to the following example.

The method of this example consists of the steps of (a) degraphitization, (b) nitridation, (c) surface oxidation, (d) electrolytic Ni plating, and (e) film stabilization heat treatment. Each step will be explained in detail below.

(a) Degraphitization process by molten salt electrolysis treatment.As a pretreatment, a bath consisting of sodium hydroxide and neutral salts is used at a temperature of 460°C for the purpose of removing graphite from the surface of the cored malleable cast iron sample.
The sample is immersed in an alkaline redox bath at °C, and the sample surface is cleaned by electrolytic treatment. The electrolysis procedure is (1)
The sample is held at a cathode, and sand and oxide film on the sample surface are removed by reducing action. (2) The sample is held at the anode and the graphite on the surface of the sample is removed by reaction with a salt bath agent by oxidation. (3) The oxide film formed on the surface of the sample in step (2) is removed by reduction by holding the sample again at the cathode.

After that, after cooling to room temperature by air cooling and water cooling,
Wash with 0°C hot water.

(b) Nitriding process In order to impart corrosion resistance and wear resistance to the sample surface, the sample was preheated to about 350"C, and then subjected to salt bath nitrocarburizing treatment at a temperature of about 580°C for about 1 hour to coat the surface with Fe. -N compound layer 5
The thickness is approximately 30 μm to 30 μm.

(c) Surface oxidation treatment step After the treatment step (b) above is completed, the sample is immersed in a salt bath with a bath temperature of about 380°C for about 15 minutes, and the sample is immersed on the surface of the Fe-N compound layer or inside the compound layer. Add a 1μm Fe3O4 layer to
After forming about 30 μm, the film is cooled to room temperature with water, washed with hot water at 60° C., and then dried.

In addition to this oxidation treatment step, a Fe3O4 layer may be formed on the surface or in the Fe-N compound layer by oxidation treatment in a steam atmosphere at about 400°C.

(d) Electro Ni plating treatment process In order to impart a silvery white metallic luster to the sample surface, the above (
After completing the step c), the sample is subjected to degreasing treatment, pickling treatment, etc. at least once to clean and activate the surface, and then electrolytic Ni plating treatment is applied to the sample surface to a thickness of approximately 5 μm to 3 μm.
Apply with a film thickness of approximately 0 μm. Plating treatment is performed at a liquid temperature of 60°C.
In a bright Ni plating bath with current density IA~3A/dm2
It was conducted under the following conditions. The processing time was selected depending on the required film thickness.

Further, in the electro-Ni plating process, the first layer may be made of corrosion-resistant semi-bright Ni plating, and the second layer may be made of bright Ni plating.

If the film thickness is less than 5 μm, the color of the base will show through, making it impossible to obtain metallic luster, and if it is more than 30 μm, the total film thickness will become too thick to be used.

By performing the above composite surface treatment, it is possible to form a surface treatment film on the cast iron surface with a total thickness of approximately 50 μm or less, which has rust prevention properties and a silvery white metallic luster. .

In this way, by keeping the total thickness of the surface treatment film that has an anti-corrosion effect to 50 μm or less, preferably 30 μm or less, it can be applied to areas where the accuracy of combination with threaded parts is a problem. becomes possible.

Figure 1 shows the state of the surface treatment film of the sample treated according to the above example, and the surface treatment film invades the holes where graphite that was exposed on the cast iron surface was removed. It shows the situation. Furthermore, the film itself maintains continuity.

Figure 2 shows the state of the surface treatment film in a sample in which the degraphitization treatment step (a) in the above example was omitted. It is difficult to form a film, and the film shows discontinuities 6 due to graphite.

Figure 3 shows the average occurrence of red rust on the surfaces of 10 samples after being immersed in tap water at a water temperature of 25°C for one week. In sample A, which was subjected to electroplating and Ni electroplating, rust formed on the sample surface although the metallic luster was maintained, whereas in sample B, which was subjected to degraphitization and nitriding, the occurrence of rust was significantly reduced. However, the metallic luster disappears. On the other hand, in Sample C of the present invention, almost no rust was observed even after being immersed for one week, and the metallic luster on the surface remained as it was at the beginning of the test.

[Effects of the Invention] As is clear from the above description, the composite surface treatment method of the present invention can be applied to cast iron castings, which are cheaper than stainless steel, to prevent red rust from forming on the surface. It also maintains a silvery white metallic luster similar to that of stainless steel, and can be used as a replacement for the expensive stainless steel pipe fittings that are currently used as rust-free pipe fittings, and for cold water and hot water applications where the generation of red water is a problem. It becomes possible to apply it to water supply and drainage parts, etc.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the state of the surface treatment film of a sample that was subjected to degraphitization treatment on cored malleable cast iron, followed by nitriding treatment and electroplating treatment. FIG. 2 is a diagram showing the relationship between the surface treatment film and graphite of a sample in which cored malleable cast iron was subjected to nitriding treatment and electrolytic N1 plating without being subjected to degraphitization treatment. FIG. 3 is a diagram showing the average occurrence of red rust and metallic luster on the surface of a sample when immersed in tap water at a water temperature of 25° C. for one week. 1: Electrolytic Ni plating layer 2: Fe30A layer 3: Fe-N layer 4; Graphite 5: Fe base 6: Graphite drop-off trace 7: Plating discontinuity Fig. 1 Fig. 2/ Fig. 3 Sample

Claims (3)

[Claims] 1. (b) A process of removing casting sand and/or graphitization from the surface of a cast iron material having graphite in its matrix by molten salt electrolysis treatment, (b) A process of performing a nitriding process, (c) Treatment of the cast iron material 1. A composite surface treatment method for cast iron materials, comprising the step of coating the surface with at least one Ni plating film each having a thickness of 1 μm or more, and each layer having a total thickness of 5 μm or more. 2. After forming a Fe_3O_4 film on the surface treatment film formed in steps (a) and (b) above (c)
A composite surface treatment method for cast iron materials, characterized by applying the following steps. 3. A composite surface treatment method for cast iron materials, characterized in that a decarburization treatment step by heat treatment is performed before the step (a), and then the steps (a) to (c) are performed.