JPH0328680B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to corrosion-resistant surface treatment of a heat exchanger for a water heater that heats water using a heat medium heated by a heat source such as gas, oil, electricity, or solar heat. Conventional technology Traditionally, materials for heat exchangers for water heaters include:
Copper or a copper alloy containing copper as a main component is considered to be optimal because of its excellent durability, workability, and heat transfer properties. However, these metals can be found in aqueous environments (e.g.
The problem was that corrosion such as pitting and dezincing occurred due to water containing a lot of Cl ^- , SO ₄ ^- , NO ₃ ^- , free carbonate, etc., and holes formed in a short period of time. In addition, the amount of copper eluted increases in water with a low pH,
There were also problems in which water quality standards for tap water (1.0 ppm or less) were sometimes exceeded, and laundry and towels were colored blue by eluted copper ions. In order to improve some of these drawbacks, the formation of plating films as surface treatments for metal materials, such as electroplating of nickel and chrome, hot-dip plating of aluminum and zinc, and coating film formation of silicon-based paints, etc., are being considered. It has been. Problems to be Solved by the Invention However, with electroplating of nickel, chrome, etc., corrosion from the pinholes cannot be avoided, and the material is immediately exposed, so the plating effect cannot be obtained. In particular, when a heat exchanger made of copper plated with nickel or the like is used with water containing a lot of free carbon dioxide, the copper in the pinholes first reacts with the free carbonate in the water and becomes basic copper carbonate as shown in the following equation, and then the plated Corrosion progresses to the lower copper (Cu + CO ₂ + H ₂ O → CuCO ₃ · Cu (OH) ₂₎ , and the plating eventually peels off. Under such conditions, green basic copper carbonate would form on the entire surface of the heat exchanger, turning the hot water blue and eventually causing holes. In addition, if a heat exchanger made of a copper alloy, such as brass plated with nickel, is used in water with high conductivity, the zinc in the brass in the pinhole area has a base potential compared to copper, so the zinc There was a drawback that only the zinc was eluted, causing a dezincing phenomenon. Furthermore, in heat exchangers made of copper that has been hot-dip plated with zinc, the zinc on the surface acts as a sacrificial anode and dissolves, immediately exposing the copper material, making the plating ineffective. When applying silicone-based paints, corrosion occurs due to unavoidable pinholes in the paint film, which causes the paint film to blister and peel, leading to even more severe corrosion. Therefore, if we consider only from the perspective of corrosion resistance, it would be possible to improve corrosion resistance by using an expensive material with excellent corrosion resistance, such as stainless steel, but stainless steel lacks machinability and
There are concerns about crevice corrosion, pitting corrosion, stress corrosion cracking, etc., and it has not been easy to solve these problems and use them. Means for Solving the Problems The present invention eliminates these drawbacks by plating the surface of the heat exchanger material with fins with a metal that has a higher ionization tendency than the material, and further ionizes the surface of the heat exchanger material with fins. A metal with a small tendency is plated to form a double plating layer, and the film thickness of the double plating layer is made thicker at the fins (peaks) than at the valleys, thereby eliminating the drawbacks of the conventional method. do. Effects The present invention takes measures to suppress corrosion of copper, which is the material of the heat exchanger, as much as possible through the above-described configuration. In other words, plated nickel and tin have a lower potential than copper and can provide sufficient polarization to protect copper from corrosion. Therefore, even if the environment is such that copper corrodes, the plated nickel and tin will self-corrode first, thereby preventing copper from corroding. Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram of a finned heat exchanger for a water heater according to the present invention, and FIG. 2 is an enlarged sectional view of the main part thereof. In Fig. 2, a nickel layer 2 is formed by electroplating nickel, which has a higher ionization tendency than copper, on the surface of copper 1 as a material for a heat exchanger with fins, and furthermore, a nickel layer 2 is formed on the surface of copper 1, which has a smaller ionization tendency than the above-mentioned nickel. A double plating layer is formed by electroplating 3 tin, which has a higher ionization tendency, and the double plating layer is thicker at the fins (peaks) than at the valleys. in this way,
By plating nickel, which has a higher ionization tendency than the copper material, and then plating tin, which has a lower ionization tendency than nickel, the thickness of the double plating layer is made thicker at the fins than at the valleys, resulting in the following effects: It was confirmed. (1) Measurement of natural potential To determine the natural potential of the heat exchanger with fins,
The natural potential of each sample was measured using water from Yamatokoriyama City, Nara Prefecture as an electrolyte. The results are shown in Table 1.

[Table] From the results in Table 1, it was confirmed that the present invention can best provide the polarization necessary for corrosion protection of copper. (2) Measurement of corrosion amount A finned heat exchanger was subjected to an immersion corrosion test in a 5% NaCl solution. The results are shown in Table 2.

【table】

[Table] From the results shown in Table 2, it was confirmed that the device of the present invention has the greatest anti-corrosion effect for copper. This has a greater tendency to ionize than copper, which is the material for the fins.
It was found that if a large amount of Sn-Ni is applied, the anticorrosive current reaches the valleys and protects the entire heat exchanger with fins. However, with the conventional film with the same thickness (5μ) for both the fins and the valleys, if the film thickness at the fins is insufficient and there are pinholes that cannot be avoided during plating, Ni and Cu on the lower side will elute. I'm sure it will grow bigger. As mentioned above, the heat exchanger with fins of this configuration has extremely excellent corrosion resistance, and is very practical when used in water heaters and the like. Effects of the Invention As described above, a double plating layer is formed by plating nickel, which has a higher ionization tendency than copper, which is the material for the heat exchanger with fins, and then plating tin, which has a lower ionization tendency than nickel, on top of that. By making the thickness of the double plating layer thicker in the fin portions than in the valley portions, the following effects can be obtained. (1) By thickly plating the fins with a metal that has a higher ionization tendency than the material of the heat exchanger with fins, the anti-corrosion current can reach the valleys and the entire heat exchanger with fins can be protected from corrosion. . (2) Since tin has a smaller tendency to ionize than nickel, plating the surface (parts that come in contact with water) makes it difficult to dissolve. (3) Since tin is a metal that is easy to plate, pinholes are less likely to form, and the effect of covering pinholes in the underlying nickel plating can be achieved. (4) Nickel has a strong ionization tendency and is a metal suitable for polarizing the copper material to a stable region, but it is easily corroded by reacting with SO ^2- ₄ , NO ^- ₃ , Cl ^-, etc. contained in water. . On the other hand, tin is relatively resistant to water environments, so if there is a plating layer on the surface (water contact area) of tin, the durability in water will be stronger.

[Brief explanation of the drawing]

FIG. 1 is a front view of a finned heat exchanger for a water heater according to an embodiment of the present invention, and FIG. 2 is an enlarged cross-sectional view of the same essential part. 1...Copper of the heat exchanger material with fins, 2...Nickel plating layer, 3...Suzmeteki layer.

Claims (1)

[Claims] 1. Plating a metal with a higher ionization tendency than the heat exchanger material, and then plating a metal with a smaller ionization tendency than the plated metal to form a double plating layer. A heat exchanger for water heaters in which the thickness of the double plating layer is thicker at the fins than at the valleys. 2. The heat exchanger for a water heater according to claim 1, wherein a double plating layer is formed by plating nickel on the copper material and then plating tin with excellent corrosion resistance thereon.