JPS6012427B2 - Cathodic protection method for aluminum materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cathodic protection of aluminum and aluminum alloy (hereinafter referred to as aluminum) materials.

Aluminum is a metal material that is lightweight, has good thermal conductivity, and is relatively resistant to corrosion in neutral environments, so it has recently been widely used as a structural material for chemical equipment. For example, if it is used in areas that come into direct contact with seawater or industrial water (hereinafter referred to as water), such as heat exchangers or liquid storage tanks, corrosion due to so-called gas chemical causes such as pitting corrosion and intergranular corrosion will occur. It is known to cause uneven corrosion.

In order to prevent electrochemical corrosion of aluminum materials used in places that come in contact with water, methods are used to apply anodized film treatment to the surface of the aluminum material or to paint it with paint. If only film treatment or painting is used, sufficient corrosion prevention effect cannot be expected over a long period of time due to latent defects in the film or painted surface, deterioration of the film during use, and peeling of the painted surface. I couldn't.

Conventionally, as a method to prevent electrochemical corrosion of metal materials immersed in water, a counter electrode is provided near the metal material submerged in water, and an external power source is used between the counter electrode and the metal material. The so-called cathodic protection method is known, in which the electric potential of the metal material to be protected in water is always kept below the pitting corrosion potential by passing a weak anti-corrosion current, and is widely used for corrosion protection of steel and other materials. It is proving effective.

The present inventors applied the cathodic protection method using the above external power source to anodized or surface-painted aluminum materials submerged in water, or to bare aluminum materials to prevent pitting corrosion and intergranular corrosion. Various studies have been conducted on ways to prevent this, but in all cases, it has been found that simply applying the above-mentioned cathodic protection method to aluminum materials will not produce the same satisfactory corrosion protection effect as with steel materials. I couldn't.

The reason for this is that, unlike steel, aluminum is a so-called amphoteric metal that dissolves in acids and alkalis. In other words, in order for an aluminum material to exist stably for a long period of time without being substantially corroded in water, it depends somewhat on the type of alloy of the aluminum material and the erosive environment. The potential is 0.3V to 0.4V higher than the pitting corrosion potential.
It is known that the potential must be kept within a narrow potential range as low as V.

Therefore, in order to protect aluminum materials by cathodic protection, it is necessary to control the cathode current from the counter electrode so that the cathode potential is maintained within the above stable potential range over the entire length of the aluminum material. However, as a practical matter, it is extremely difficult to control the cathode current in this way. In other words, if the voltage of the external power supply is adjusted so that a relatively small potential difference is applied between the aluminum material and the counter electrode, the cathode flowing between the aluminum material and the counter electrode will The potential is properly suppressed by the electric current, but in areas far away from the counter electrode of the aluminum material, the flow of cathode current decreases due to the electrical resistance of water, so the potential cannot be sufficiently suppressed. Therefore, it was difficult to avoid pitting corrosion and intergranular corrosion in this area. In addition, if you try to suppress the potential to a point that is far away from the counter electrode of the aluminum material and adjust the voltage of the external power supply to create a sufficient potential difference between the aluminum material and the counter electrode, the counter electrode of the aluminum material Excessive cathode current flows in nearby areas, which tends to cause alkali corrosion due to so-called over-corrosion protection. In this way, with the conventional cathodic protection method, it is difficult to suppress the cathode potential over a wide range over the entire length of the aluminum material to keep it within a stable range. This made it extremely difficult to apply. The present invention has been made to improve the above-mentioned drawbacks of conventional cathodic protection methods for aluminum materials.

That is, the present invention provides a method for cathodic protection of aluminum materials using an external power source, in which the aluminum material is immersed in water as the material to be protected and the potential difference between the counter electrode and the aluminum material is measured based on the potential of the counter electrode. An aluminum material that intermittently repeatedly suppresses the cathode potential of the aluminum material by passing cathode current from an external power supply for a short time between the counter electrode and the aluminum material each time the potential of the material exceeds a preset upper limit. This is a cathodic protection method. Hereinafter, the method of the present invention will be explained in more detail based on the accompanying drawings.

FIG. 1 is an explanatory diagram illustrating an embodiment of the method of the present invention.

In the figure, 1 is an aluminum material soaked in water as the material to be protected, 2 is a counter electrode provided in the water close to the aluminum material 1, and the counter electrode 2 is made of silicon cast iron, platinum, etc. Conductive materials such as metals or carbon materials can be used, but electrode potentials that have relatively little fluctuation in response to changes in the external environment, such as aluminum, zinc, magnesium, or their alloys, can be used. It is preferable to use the metal electrode or metal alloy electrode shown in FIG. 3 is a potential difference measuring device, in which an aluminum material 1
and the counter electrode 2 are connected to a potential difference measuring device 3 by conducting wires 6, 6, and the potential difference between the two electrodes is constantly measured.

The potential difference measuring device 3 is equipped with a relay device 5 when the potential of the aluminum material 1 measured with respect to the potential of the counter electrode 2 exceeds a preset constant value.
The upper limit potential VU is set so that a signal is emitted. The counter electrode 2 and the aluminum material 1 are connected to an external power source 4 via a relay device 5 to form a cathode current circuit 7, 7. The cathode current circuits 7, 7 are always open, but the potential of the aluminum material 1 based on the potential of the counter electrode 2 measured by the potentiometer 3 exceeds a preset upper limit. Then, the relay device 5 is activated by the signal from the potentiometric measuring device 3, the cathode current circuits 7, 7 are closed for a short time, and a voltage is applied from the external power source 4 between the counter electrode 2 and the aluminum material 1, and during this time ,
Each time a cathode current is passed from the counter electrode 2 to the aluminum material 1, the cathode potential of the aluminum material 1 is suppressed for a short time. 2A and 2B show the counter electrode and a location relatively close to the counter electrode of the aluminum material when the method of the present invention is carried out with the apparatus of FIG. 1 using aluminum of the same quality as the material to be protected as the counter electrode. These are examples of temporal changes in potential.

In the figure, the vertical axis represents the potential of the counter electrode or aluminum material in water (potential with respect to a saturated calomel electrode);
The axis represents the passage of time. Further, FIG. 2C shows the temporal change in the potential difference between the aluminum material and the counter electrode, which was measured using the potential difference fixing device in this case with the counter electrode as a reference. Initially, the aluminum material 1 and the counter electrode 2 immersed in water each have a natural potential V in the water.

o (point co in Figure 2B) and V^. (Figure 2A
(ao point) is shown. In this state, when the relay device 3 is activated to close the cathode current circuits 7, 7 and apply a voltage between the counter electrode 2 and the aluminum material 1 from the external power source 4, the potential of the counter electrode 2 changes from the ao point. The potential of the aluminum material 1 is rapidly lowered from point co to point c. In this case, the potential of the aluminum material 1 based on the potential of the counter electrode 2 measured by the potential difference measuring device 3 shows four points that are the differences in natural potential between the aluminum material 1 and the counter electrode 2 when no voltage is applied. However, when a voltage is applied, it is rapidly lowered from point do to point d. Next, when the cathode current circuits 7, 7 are opened and the voltage application from the external power source 4 is removed, the anode potential V^ of the counter electrode 2 rapidly reaches the natural potential V^.
Returning to (point a2), the cathode potential Vc of the aluminum material 1
As shown by the curves c and →c2, the voltage rises rapidly in the initial stage, but then gradually rises and finally returns to the natural potential Vco. In this case, the potential difference Vc-V^ of the aluminum material 1 with respect to the counter electrode 2 measured by the potential difference measuring device 3 increases along the curves d and -. In this way, the potential of the aluminum material 1 based on the potential of the counter electrode 2 measured by the potential difference measuring device 3 reaches the upper limit potential VU (flat point in FIG. 2 C) set in advance in the potential difference measuring device 3. When you go back to
A signal is sent from the potential difference measuring device 3 to the relay device 5 to close the cathode current circuits 7, 7, and the voltage from the external power source 4 is again applied to the counter electrode 2 and the aluminum material 1, so that the cathode potential of the aluminum material 1 is Vc. is rapidly raised from point c2 to point c3, and then rises along the curve c3→c4 repeatedly. As described above, in the present invention, the potential Vc-V^ of the aluminum material measured by the potential difference measuring device with reference to the potential of the counter electrode is lower than the preset upper limit value V.
This is a cathodic protection method for aluminum materials in which a voltage is applied from an external power supply for a short time between the counter electrode and the aluminum material each time the voltage rises to U, thereby repeatedly suppressing the cathode potential Vc of the aluminum material intermittently. .

Therefore, the upper limit potential VU to be set in the potentiometric measuring device in the above-mentioned present invention varies somewhat depending on the type of alloy of the aluminum material to be protected, the environment of use, etc. It is appropriate to set the value within the range between the difference in natural potential at and a potential that is about 60 V lower than this.

For example, when an aluminum alloy of the same quality as the material to be protected is used as the counter electrode, the natural potential of the aluminum material as the material to be protected and the counter electrode are the same, so the upper limit potential is 0 to about 60 V. It is sufficient to set the potential to be low, that is, within the range of 0 to 16 V. Also, the natural potential in water is 35m higher than that of aluminum material.
When a zinc material with a low V is used as a counter electrode, the upper limit potential may be selected between 350 V and 290 V, which is about 60 mV lower than this, that is, within the range of 350 to 290 V. In the method of the present invention, when the potential of the aluminum material based on the potential of the counter electrode measured by the potentiometric measuring device exceeds the upper limit potential VU, the voltage of the external power supply applied between the counter electrode and the aluminum material is: Generally, it is appropriate to adjust the cathode potential Vc of the aluminum material so that it is approximately 0.3 to 0.8 V lower than its natural potential Vco, that is, the potential of the aluminum material when no negative voltage is applied. However, especially in an environment where alkali corrosion is difficult to occur, a decrease to about IV can be tolerated.

In addition, the voltage application time in this case varies somewhat depending on the magnitude of the applied voltage, the material of the aluminum material, the corrosive environment, etc., but it will take a long time for the aluminum material to undergo alkali corrosion due to the box and deterioration. In order to avoid exposure, it is desirable to keep the time to a few seconds or less, and more preferably 0.
The time should be adjusted to about 0.01 to 2 seconds. The above method is performed by adjusting the voltage of the external power supply and its application time in order to apply an appropriate negative voltage to the aluminum material. However, instead of adjusting the voltage of the external power supply, the potential difference measurement device In addition to the potential value VU, a lower limit potential value VL is set, and when the potential of the aluminum material measured with the potential of the counter electrode as a reference decreases to the lower limit potential value VL by applying a negative voltage, potential difference measurement is performed. A signal from the device can also actuate a relay device to automatically open the cathode current circuit.

When such a method is used, cathodic protection can be continued more stably. As described above, the method of the present invention measures the potential difference between the aluminum material immersed in water and the counter electrode, and each time this potential difference exceeds a preset upper limit value based on the potential of the counter electrode, This method applies a short-time voltage between the counter electrode and the aluminum material from an external power source to suppress the cathode potential of the aluminum material intermittently and repeatedly, and the potential of the aluminum material in water is always kept lower than its natural potential. Therefore, there is no risk of pitting corrosion or intergranular corrosion even if there are slight changes in the external environment, and as shown in Figure 2 C, the application time of negative voltage applied to the aluminum material is is short, and when the negative voltage is removed, the potential of the aluminum material recovers quickly at the initial stage and then rises slowly. Therefore, the time during which the potential of the aluminum material is essentially in the alkali corrosion castle is short. This is an extremely short period of time, and alkaline corrosion has a concession period, so it is almost impossible to experience alkaline corrosion during this period.

Therefore, compared to the conventional cathodic protection method using an external power source that constantly supplies a cathode current, even if the negative voltage applied to the aluminum material is increased, there is less risk of alkaline corrosion caused by over-protection, and severe This is an excellent method that can provide corrosion protection over a wide range of areas over the entire length of the aluminum material even in harsh usage environments. Next, examples of the present invention will be shown. Example 1 An AI100 aluminum plate (length: 80 m x width: 10 m x thickness: 1 willow) was used as a specimen to be subjected to corrosion protection, and was used in an experiment.

Approximately 5 willow channels were created in the flow path along the longitudinal direction on both sides of the specimen, and an AIlOO aluminum electrode (dimensions:
A cathode current circuit was installed between the counter electrode and the test piece to allow electricity to be applied intermittently so that the test piece became a cathode.

The water used is natural seawater (temperature approximately 20q○), and approximately 20ml
It was flowed along the channel at a flow rate of skin/sec. Using a potential difference measurement device, measure the potential difference between the specimen and the counter electrode, and measure the potential difference every time the potential of the specimen with respect to the counter electrode rises to a preset upper limit potential (-50 mV). A signal from the device automatically sets the cathode current circuit to 0.
Close it for 1 second and connect an external power supply (25V) between the mystery piece and the opposite electrode.
A voltage from a constant voltage power supply (regulated to

As a result, the potential of the specimen is approximately 50% lower than its natural potential.
With a potential lower than mV as the upper limit, further approximately 550 V
The potential was suppressed by repeating raising and lowering at intervals of about 1.0 to 1.2 steps at a low voltage. In this way 3
Continuous testing was conducted for several months, but no pitting corrosion occurred during this period, and no alkaline corrosion was observed.

Example 2 An AI100 aluminum plate (dimensions are the same as in Example 1) was used as a specimen to be protected against corrosion.

Channels similar to those in Example 1 were made on both sides of the specimen, and a zinc electrode with the same dimensions as in Example 1 was used as a counter electrode for corrosion protection at a position approximately 10 degrees away from one end of the channel.

The water used was natural seawater as in Example 1, and was flowed at a flow rate of approximately 20 seconds. Every time the potential of the specimen with respect to the counter electrode measured by the potentiometric measuring device exceeds a preset upper limit potential (300 mV), the potentiometric measuring device emits a signal, closes the cathode current circuit, and connects the specimen to the counter electrode. A voltage is applied from an external power supply between
When the voltage dropped to 50 mV), a continuous test was performed while intermittent and repeated potential control was performed by opening the cathode current circuit and releasing the voltage application using a signal from the potentiometer. In this example, the upper limit potential value of 30 mV and the lower limit potential value -750 mV set in the potential Misato setting device were determined as follows.

That is, the actual measured values of the natural potential in water of the aluminum material as the material to be protected and the zinc material as the counter electrode used in this example are approximately -700 V and -10 V, respectively.
50 V (both potentials are based on the saturated calomel electrode), the upper limit potential value VU is the natural potential difference Vc-V^ of the specimen with the counter electrode as the reference, that is, (-700 V).
V) - (-1050 V) = 50 V from 350 mV
I set it to a low 300V.

Further, the lower limit potential value VL was determined based on the actual value of the potential Vc-V^ of the mysterious piece with respect to the opposite electrode, assuming that the cathode potential Vc of the mysterious piece was lowered by about 600 mV from its natural potential Vco due to the application of voltage. A continuous test was conducted for three months as described above, but no pitting corrosion occurred during this period, and no alkaline corrosion was observed at all.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram illustrating an embodiment of the method of the present invention, and FIG. 2 A, B, and C are a counter electrode, an aluminum material, and an aluminum material measured based on the counter electrode, respectively, according to the method of the present invention. This is an example of the change in potential over time. In the figure, 1...aluminum material, 2...counter electrode, 3...
Potential difference measuring device, 4...external power supply, 5...relay device,
6, 6... Conductor wire, 7, 7... Cathode current circuit.

Claims (1)

[Claims] 1 In the cathodic protection method for aluminum materials using an external power source, the potential of the aluminum material is set in advance based on the potential of the counter electrode measured from the potential difference between the aluminum material as the material to be protected immersed in water and the counter electrode. The aluminum material is characterized in that the cathode potential of the aluminum material is intermittently and repeatedly suppressed by passing a cathode current from an external power supply for a short time between the counter electrode and the aluminum material each time the specified upper limit value is exceeded. Method of cathodic protection of wood.