JPS5922777B2 - Surface treatment material for heat exchangers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface treatment material that imparts corrosion resistance to a heat exchanger that uses combustion gas such as city gas, liquefied gas, and kerosene as a heat source.

The combustion gas contains large amounts of nitrogen, oxygen, carbon dioxide, and water vapor, as well as trace amounts of carbon monoxide, sulfur dioxide, nitrogen monoxide, and nitrogen dioxide.

The corrosion phenomenon of the heat transfer surface of a heat exchanger occurs as follows. That is, due to rapid heat exchange, water vapor in the combustion gas condenses on the heat transfer surface, and components in the combustion gas dissolve into the condensed water. Since the condensed water evaporates after the combustion burner is extinguished, metal carbonates, sulfates, nitrates, etc., which are corrosion products caused by components in the gas dissolved in the condensed water, precipitate. As this cycle is repeated, corrosion on the heat exchanger surface progresses. Conventionally, a tin-lead alloy containing about 1% by weight of tin has been generally used as a surface treatment material for the purpose of rust prevention for heat exchangers used in a combustion gas atmosphere.
This was coated on the surface of the heat exchanger by hot-dip plating.

However, as products using this surface treatment material are used in combustion gases, white corrosion products are produced and corrosion gradually progresses, causing damage to the heat exchanger material and pipes wrapped around the heat exchanger. There were problems such as holes forming in the heat exchanger fins, making them unusable, and clogging of the heat exchange fins with corrosion products, resulting in incomplete combustion. In order to eliminate the above-mentioned drawbacks, the present invention was discovered as a result of numerous experiments on the influence of additive elements on the corrosion resistance of lead.The present invention was made by conducting numerous experiments on the influence of additive elements on the corrosion resistance of lead. It is characterized by being made of a ternary alloy of tin-bismuth-lead containing 5 to 14% by weight.

Hereinafter, the present invention will be explained with reference to examples thereof.

First, tin and bismuth were added to molten lead in various proportions to create a master alloy, and this master alloy was hot-dip plated onto a copper plate to create a test piece. For these specimens, C
The following corrosion test was conducted using a gas containing 5% O, 10 pμm of N0, and 5020.1 pμm. Note that this gas concentration setting is based on the analysis results of city gas combustion gas. In order to approximately reproduce the cycle of condensation and drying of water vapor in the combustion gas that induces corrosion of the heat exchanger, the corrosion test was conducted by setting the temperature at 50°C in the above gas concentration atmosphere and holding the sample for 1 hour. After dew condensation occurred, the temperature was set at 30° C., and the sample was held for 3 hours and dried. These steps were repeated as one cycle. FIG. 1 shows the relationship between bismuth content and corrosion in a lead-bismuth-tin ternary alloy containing 1.0% by weight of tin.

It can be seen from the figure that corrosion is significantly reduced when the bismuth content is 0.5% by weight or more. The higher the amount of bismuth, the less corrosion there will be. Further, as shown in FIG. 2, as the amount of bismuth increases, the melting point of the alloy decreases, resulting in improved workability. From the viewpoint of corrosion resistance and workability, it is desirable to contain as much bismuth as possible, but from the viewpoint of use as a surface treatment material for heat exchangers that are exposed to relatively high temperatures, it is important that the melting point is extremely low. It is not desirable, but from the viewpoint of safety and the fact that bismuth is relatively expensive, the content of bismuth is 0.
．． A range of 5 to 14% by weight is particularly desirable. FIG. 3 shows the relationship between the tin content and corrosion in a lead-bismuth-tin ternary alloy containing 0.5% by weight of bismuth.

From the figure, it can be seen that the corrosion resistance is significantly improved by containing 1% by weight or more of tin. In the case of tin, as the content increases, the corrosion resistance improves, and as shown in Figure 4, the melting point tends to decrease, and from the viewpoint of corrosion resistance and workability, the higher the tin content, the better. Although desirable,
For the same reason as in the case of bismuth, the tin content is 1 to 1.
A range of 0% by weight is particularly desirable. Figure 5 shows
It is a comparison diagram of the corrosion resistance effect of the surface treated material according to the present invention and the conventional surface treated material, where curve 1 shows 5% by weight of tin and 0.5% by weight of tin.
When a heat exchanger made of copper fabric is melt-plated with the surface treatment material of the present invention, which is made of bismuth and the balance is lead, and is installed in a gas instantaneous water heater and operated under normal operating conditions, curve 2 is obtained.
A conventional surface treatment material consisting of 1% by weight of tin and the balance of lead was melt-plated onto a copper fabric heat exchanger and was incorporated into a gas instantaneous water heater and operated under the same conditions as in curve 1. This shows the case where

From the figure, it can be seen that the surface treated material according to the present invention has a significantly more corrosion-resistant effect than conventional surface treated materials. As described above, the surface-treated material according to the present invention has excellent corrosion resistance, has a low melting point, and improves workability, and also has superior appearance in terms of surface gloss and other features. It is almost the same as the conventional method, has no problems such as toxicity, and has the advantage that conventional equipment for dipping a surface treatment material onto the surface of a heat exchanger can be used as is, and its value is great.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between bismuth content and corrosion amount in a lead-tin (1.0% by weight)-bismuth ternary alloy, and Figure 2 is a graph showing the relationship between bismuth content and melting point in the same alloy. Figure 3 is a graph showing tin lead-bismuth (0.5
(% by weight) A graph showing the relationship between the tin content and the amount of corrosion in a ternary alloy. Figure 4 is a graph showing the relationship between the tin content and melting point in the same alloy. Figure 5 is a graph showing the relationship between the tin content and the melting point in the same alloy. Figure 5 is a graph showing the relationship between the tin content and the amount of corrosion in the ternary alloy. FIG. 3 is a comparison diagram of corrosion resistance in actual use between a conventional surface-treated material and a conventional surface-treated material.

Claims (1)

[Claims] 1. A surface treatment material for a heat exchanger comprising 0.5 to 14% by weight of bismuth, 1.0 to 10% by weight of tin, and the balance lead.