JPS6056091A - Manufacture of black cr plated plate for collecting solar heat - Google Patents

Abstract

PURPOSE:To obtain black Cr plating of high quality by using pulse current, periodically interrupting the supply of pulse current, properly regulating the period and by properly regulating the ratio of the integrated supply time at suplying time to the integrated one-supply time at interrupting time. CONSTITUTION:A metallic substrate is immersed in a plating bath contg. Cr ions, and electric current is supplied to manufacture a black Cr plated plate for collecting solar heat. At this time, pulse current is used as the electric current for plating, and the supply of the pulse current is periodically interrupted to form time zones in which the current is not continously supplied. The interval be tween a restarting point of the pulse current and the following restarting point is set as one period, and the period is repeated at 15-100Hz frequency. The ratio of the integrated supply time to the integrated non-supply time in one period is regulated to 0.5-20.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a solar heat collector plate coated with black Cr, and more particularly, to a method for efficiently forming a black Cr plate with excellent selective absorption characteristics on the surface of a metal substrate. be.

Selective absorption characteristics are an important characteristic that governs the performance of solar heat collector plates. In other words, the selective absorption characteristic refers to the characteristic of selectively absorbing light rays in a specific wavelength range.In order to efficiently absorb sunlight, it is necessary to use It is desired that the material absorbs well in the wavelength range and has low emissivity in the infrared region of 2.5 to 20 μm. Various materials are known that satisfy such selective absorption characteristics, but on the other hand, they must have durability against harsh usage conditions (including weather resistance, heat resistance, moisture resistance, salt water resistance, etc.). is also required.

Black Cr plating is attracting attention as a material that satisfies these conditions, but it has been pointed out that in this case, it is extremely difficult to form a plating layer that meets the purpose. In other words, it is known that the film thickness affects the selective absorption characteristics of the black Cr plating layer, but in order to exhibit good selective absorption characteristics, the light absorption rate (α) must be about 0.92 or more. The area that satisfies these conditions is the black Cr plating formed on the surface of the heat collector plate as shown in Figure 1. In controlling the thickness of the layer, it is necessary to make it as uniform as possible in order to satisfy the above-mentioned narrow range, but as shown below, this is extremely difficult.

In other words, the current solar heat collector plate, for example, is shown in Figure 2 (floor plan).
As shown in the figure, the width is about 180+m and the length is about 2000mm.
A unit is made by welding copper tubes 2 to a unit heat collecting plate 1, and each unit is assembled by connecting a plurality of copper tubes 2 at both ends with a header tube, and the black Cr plating treatment is applied to the assembly. This is done by immersing it in a plating bath. In this case, if the distance l between the unit heat collecting plate 1 and the anode 8 can be made constant as shown in Section 3, the current density (A/dm2) between them will also be approximately constant, and each heat collecting plate 1
Since the electric charges ff1 (C/m2) can be made almost equal, the plating thickness becomes uniform. However, since the actual heat collecting plate 1 is irregularly distorted as explained below, it is not possible to keep l constant, that is, to make the amount of charge constant over the entire surface of the heat collecting plate 1, and the plating thickness is irregular. It becomes uniform. That is, the unit heat collecting plate 1 and the copper tube 2 are joined by special TIG welding, but due to welding distortion, the unit heat collecting plate 1 becomes, for example, as shown in Fig. 4 (
4) Transform as shown in (g). Therefore, when attempting to plate such a heat collecting plate 1 with black Cr, for example, as schematically shown in FIG. 5, the distance between the surface to be plated and the anode 8 is 111i!
cannot be kept constant, and in the parts where l is short, the amount of charge becomes too large and the plating thickness exceeds the preferred range, and in the parts where l is long, the amount of charge is insufficient, resulting in insufficient plating, and the overall heat collector plate The selective absorption properties of In addition, after welding the unit heat collecting plate 1, there is a method of straightening it into a waveform as shown in Figure 6 in order to eliminate welding distortion, but in this case, the amount of charge changes between the four parts and the convex part, so The four parts tend to be insufficiently plated, and the convex parts tend to have excessive plating thickness.

Therefore, in order to form a black Cr plating layer with a uniform thickness over the entire thickness in a specific thickness range, it is necessary to eliminate welding distortion of the unit heat collector plate and make it smooth. Since the thickness of the hot plate is extremely thin, about 0.8 mm, it is virtually impossible to eliminate welding distortion. Therefore, it is necessary to develop a smooth plating technique that can form a uniform black Cr plating layer without any problems even when there is some welding distortion.

The present inventors focused on the above-mentioned circumstances, and by multiplying the plating method several times, it is possible to form a uniform black Cr plating layer even on a unit heat collector plate that has some welding distortion. Furthermore, various studies have been conducted in an effort to establish a plating technology that can improve the selective absorption characteristics of heat collecting plates. As a result, if the base-treated metal substrate is immersed in a plating bath containing Cr ions and electroplated using pulsed current, uniform and high-performance coating can be achieved even on unit heat collector plates with slight welding distortion. It was discovered that a black Cr plating layer was formed,
Based on this fact, I first filed a patent application (Japanese Patent Application
7-71085).

Currently, the most widely studied method for obtaining a black chromium plated layer with excellent selective absorption properties is to modify the plating bath composition. Publication No. 56-27599 and JP-A-56
There have been many proposals, including JP-A-112494, and some of these have clarified that plating temperature, current density, etc. affect selective absorption characteristics.

However, even with these known plating techniques, when applied to a unit heat collector plate that has been deformed due to welding distortion as described above, the amount of charge on the plated surface becomes uneven, so it is difficult to form a uniform plating layer over the entire surface. You can't get it.

On the other hand, in normal Cr plating processing, the plating coverage is poor, so it is considered preferable to use full direct current with less ripple as the plating current. For example, use a rectifier such as an 8-phase rectifier to minimize the ripple and smooth it. It has become common sense to use a rectified power supply.

However, as the inventors confirmed in a previous experiment,
If the plating current is an intermittent pulsed current, the range of allowable current density (hereinafter simply referred to as allowable current density) that can provide a plating layer with excellent selective absorption characteristics will be greatly expanded, resulting in a Even if the electrode is distorted as shown in Figures 4 and 6 and the distance j from the anode (see Figure 5) is uneven, it can be selected by forming a nearly uniform glazed layer over the entire surface. It has become clear that black Cr plating with excellent absorption properties can be obtained. The invention of the prior application was completed based on these results, and it can be said that the technical significance of making it possible to realize an excellent black Cr plated layer by expanding the allowable current density range is extremely great. Furthermore, in the prior invention, the ratio of (T1/T2) between the total conduction time (T1) of the pulse current and the edge-off time (T2) of the current, and the frequency (1/[T1 + T2)) are appropriately controlled. It was also made clear that the above objective could be achieved more reliably by doing so. Furthermore, it is clear that this purpose can be achieved not only when the pulse current is applied continuously (hereinafter referred to as uninterrupted pulse), but also when the pulse current supply itself is periodically interrupted (hereinafter referred to as interrupted pulse). I made it.

By the way, among the techniques disclosed in the above-mentioned prior invention, the preferred ranges of the (T1/T2) ratio and frequency in interrupted pulse plating are the same as in the case where uninterrupted pulse plating is adopted.
It was determined based on the current waveform of the pulse current itself. However, in subsequent experiments, when setting suitable conditions for interrupted pulse plating, we used the reversing pattern of the pulse energization time and pulse current interruption time as the reference for frequency setting, and also determined the integrated energization by adding the above pulse current interruption time. time and 81
It was confirmed that the black Cr plating conditions could be managed more appropriately by determining the time ratio of [energized/de-energized] based on the ratio of the calculated non-energized time, and as a result of further pursuit, the present invention was achieved. Reached.

That is, the present invention is a method for manufacturing a Taishangshu heat collection plate plated with black Cr by immersing a metal substrate in a plating bath containing Cr ions and applying current, in which a pulsed current is used as the plating current, and The supply of the pulse current itself is periodically interrupted to form a time period during which the current is not continuously supplied, and one period is 15 days from the restart point of the pulse current to the next restart point.
~100) The gist is that black Cr plating is performed repeatedly at a frequency of Iz, and the ratio of the cumulative energizing time (ΣTa) to the cumulative non-energizing time (ΣTz) within one cycle is 0.5 to 20. be.

That is, in the present invention, for example, a pulse current as shown in FIG.
), the pulse current itself is periodically interrupted to form a non-supply time period, and one period is defined as the period from the restart point of the phraseless current to the next restart point. At the same time, the cumulative energizing time (ΣTa) and the cumulative de-energizing time (ΣTa) are adjusted when the current is energized and interrupted.
The objective is to obtain good black Cr plating by appropriately adjusting the ratio (ΣTa/ΣTz) of Tz). The ratio (ΣTa/ΣTz) and frequency can be determined as follows. For example, Kano 8 diagram (6), (6)
As shown in < , is, when the energization time of the J less current is set as T1, the non-energization time is set as T2, and a energization pattern with a repeating cycle in which 8 pulses are omitted (or negative voltage is supplied) after 14 continuous pulses is adopted, 1 The cumulative energization time (ΣTa) in the repetition unit is [1-14XT1'), the cumulative de-energization time (ΣTz) is [14T2+3(T1+T2)], and the (ΣTa)/(ΣTx) ratio is (t4-71/(8T).
1+1772)-) can be entwined. Also, the frequency (3) where one cycle is the repetition of the above 1a can be determined as the value obtained by dividing the frequency (X) of the pulse current by [14 pulses + (8 pulses for omission)], that is, (x/17). or from the above (ΣTa) and (ΣTz), [1/(Σ'ra
+ΣTz)': can be expressed as l.

As a result of various experiments, it was found that by adjusting the ratio (ΣTa/ΣTz) to be in the range of 0.5 to 20 and setting the frequency (3) in the range of 15 to 1.00 iIz, black Cr metal The allowable current density range has been significantly expanded,
It was confirmed that even if there were some differences in the distance l between the surface to be plated and the anode, a uniform black Cr plating layer could be formed.

Incidentally, Table 1 shows the frequency (1) and (ΣTa/
This figure shows the experimental results of plating properties when the ΣTz) ratio was variously changed. However, all other plating conditions were as follows.

[Plating conditions]

Plating time: 28 minutes Bath temperature: 15-17℃ Base metal: Copper plate (thickness 0.8m+, average roughness Ra =
0.1μm) Anode: Lead (anode area/cathode area = 1) Bath composition: Chromic anhydride 801'//+catalyst (boric acid or nitrate) Treatment procedure: Immersion of grease → water washing → pickling → water washing - base ( Ni
) Plating → Water washing → Black Cr plating → Water washing → Drying [Selective absorption characteristic measurement method] Absorption rate α: Wavelength range measured by integrating sphere reflector: 0.8 to 2.5 μm Reflectance ε: Wavelength range measured by specular reflection device :2.6 to 25 μm For comparison, Table 1 also shows the results when a DC power source was used and the results when a continuous pulse current (without interruption) was used.

As is clear from Table 1, when a DC power source is used, the appropriate current density range is extremely narrow, and it is impossible to form a uniform black Cr plating layer on the surface of the plated material, which has a considerable width in l. It is possible. On the other hand, if a continuous pulse power source without interruption (uninterrupted pulse) is adopted (prior invention), T1/
By appropriately adjusting the T2 ratio and 1/[T1+T2], the appropriate current density range can be greatly expanded, and it is possible to form a uniform black Cr plating layer despite the fluctuation of l. Become. Furthermore, if the method of the present invention (interrupted pulse) in which periodic interruptions are performed using a pulsed power source is adopted, the (ΣTa/ΣTz) ratio and 1/[ΣTa+ΣT
It is understood that by appropriately adjusting z), the appropriate current density range can be further expanded, the permissible range of variation in l can be further expanded, and the uniformity of the black Cr plating layer can be further improved. However, ΣTa/ΣTz)
If the conditions of the ratio and 1/[ΣTa+ΣTZ) are incorrectly set, the effects of the present invention will be halved as described below.

That is, from the results in Table 1, the (ΣTa/ΣTz) ratio and 1/
Looking at the tendency that [ΣTa + ΣTz) has on plating formation, it can be considered as follows.

(ΣTa/ΣTz) ratio: If it is less than 0.5, plating becomes difficult to adhere. However, if it exceeds 20, the current interruption time becomes extremely short and there is no significant difference from when using a DC power supply, making it impossible to sufficiently expand the allowable current density range. Based on these trends, the preferable range of the (ΣTa/ΣTz) ratio is considered to be 0.6 to 20.

Frequency (1/[Σ'ra+ΣTz)): As the frequency decreases, the coloring limit gradually shifts to the higher current density side, making it difficult for plating to adhere to low-charge areas (concave areas). Furthermore, if the frequency becomes too large, the blackness of the plating layer will decrease and the selective absorption characteristics will become poor. In view of the results in Table 1, the preferred frequency is considered to be in the range of 15 to 100 H2.

In this way, in the present invention, by periodically discontinuing the pulse current while passing the current, the allowable current density range can be greatly expanded. Uniform black Cr even when
A plating layer can be formed. Note that if the command is issued when the current is not flowing during uninterrupted pulse or interrupted pulse operation, it may not be possible to completely reduce the current to zero due to equipment and economic constraints. In other words, if the current is completely reduced to zero, the effective current will be smaller than the peak current, so it may be necessary to flow a small amount of current within a range that can substantially ensure the effect of intermittent current, or even to ) It is also possible to flow a small amount of negative current. Note that as the effective current decreases, it becomes necessary to increase the peak current, and the power supply device must be enlarged, which increases the economic and equipment burden.

According to the present invention, the selective absorption characteristics of the entire black Cr plating layer can be improved by expanding the allowable current density range as described above, but in addition, as described below, impurity cations in the plating bath Another advantage is that a suitable state of plating can be maintained for a long period of time without being significantly affected by In other words, when mass producing heat collecting plates, as the processing temperature increases, Nl + Pb + Sn and Fe IC in the plating bath increase.
Impurity cations such as u 2+ , Ba 2+ , A g , etc. increase. Among these, Ni+ adheres to and mixes into the treated material from the Ni plating tank in the pretreatment process, Pb+, Sn+, and Fe3+ are eluted from the anode electrode, Cu2+ is eluted from the treated material, and furthermore, B a2
+ and A g+ are added to the bath as impurity anions (S04, C1-, etc.) removers, and if these impurity cations are mixed into Cr 6 +, the conventional method using direct current as the plating current cannot be selected. The allowable current density range for obtaining absorption characteristics becomes narrower. For example, when plating is performed using a new solution or a solution contaminated with impure cations as described above, the allowable current density range for the new solution is 1
The range is 2 to 27 A/dm2, while that of contaminated liquid containing 1 to 4 f)/l of each impurity cation is 9 to 11 A/dm2.
It is extremely narrow. On the other hand, when the plating method according to the present invention is adopted, the allowable current density range becomes about 10 times wider than that of the conventional method even when a contaminated liquid is used. That is, by turning on and off the plating current, the influence of impure cations is significantly reduced, making it possible to drastically reduce the frequency of replacing the plating solution.

The present invention is roughly constructed as described above, and its effects can be summarized as follows.

+1) Since the allowable current density range is wide, even when applied to heat collector plates with uneven or deformed surfaces, it is possible to form a black Cr plating layer with excellent selective absorption characteristics over the entire surface, reducing yield due to plating defects. The decrease can be reduced to almost zero.

(2) Since there is little influence from fluctuations in the plating solution composition, plating current, etc., the control standards for these fluctuation factors can be lowered, and the plating schedule becomes easier.

(3) Since the allowable current density range for obtaining good selective absorption characteristics is wide and the coloring state of the plating layer is uniform, a high-quality heat collecting plate without color unevenness can be obtained.

(4) Since it is almost unaffected by impure cations in the bath, there is no change in selective absorption characteristics over time, and the frequency of replacing the pinning liquid can be drastically reduced.

(5) Since the same effect is exhibited almost regardless of the component composition of the plating solution, it can be applied to existing black Cr plating equipment without any problems.

(6) Even a black Cr plated layer with an extremely low emissivity of 0.1θ or less can be easily obtained by adjusting the current density. In other words, such low emissivity black Cr plating is extremely suitable for vacuum tube solar collectors, but at present it is extremely difficult to form such black Cr plating stably in mass production. It is said to be a thing. However, the demand for vacuum tube type solar collectors for high-temperature heat collection is expected to continue to increase in the future, and as mentioned above, the present invention can be applied to black chromium heat collectors, which are fully applicable to such applications. It is possible to obtain a J- value.

In addition, the absorption rate of the black Cr plating layer required for ordinary flat solar heat collecting plates is about 0.92 or more, and the emissivity is about 0.15 or less, but in vacuum tube type solar heat collecting plates, the heat collecting surface is Since the convective loss from the black Cr plating layer is almost zero, it is necessary to keep the remaining radiation loss as low as possible (that is, lower the emissivity).The required characteristics of the black Cr plating layer are an absorption rate of 0.85 or more, An emissivity of 0.10 (preferably 0.05) or less is considered to be a standard for practical quality.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the thickness and charge of the black Cr plating layer and the absorptivity and emissivity, Figure 2 is a sketch illustrating a solar heat collector plate, and Figure 3 is an ideal unit heat collector plate. An explanatory diagram showing the licking situation, Fig. 4 (A) to @) is an explanatory diagram showing the welding distortion state of the heat collecting plate, Fig. 5 is an explanatory diagram showing the actual plating situation, and Fig. 6 is an explanatory diagram showing the welding distortion state of the heat collecting plate. A sketch diagram illustrating a straightened unit heat collecting plate, FIGS. 7(A) and (B) are diagrams showing pulse current patterns used in the present invention, and FIGS. 8(A) and (6).
, (Q is a diagram showing the current waveform adopted in the experiment. 1... Unit heat collection plate, 2... Thin tube. Applicant Kobe Steel, Ltd. Sugibuki &#(Su)i 4 volumes () ) Ku taste w 1) 〉

Claims (1)

[Claims] The metal substrate is immersed in a plating bath containing Cr ions and energized,
A method for manufacturing a solar heat collecting plate plated with black Cr, which uses a pulsed current as the plating wt flow, and periodically interrupts the supply of the pulsed current to stop the continuous supply of current. The period from the restart point of the pulse current to the next restart point is defined as one cycle for 15 to 100 H.
A method for manufacturing a black Cr-plated solar heat collecting plate, characterized in that the ratio of the cumulative energized time to the cumulative non-energized time within one cycle is 0.5 to 20 by repeating the process at a frequency of z.