JPS59113186A - Cathodic corrosion protection method of aluminum material - Google Patents

Abstract

PURPOSE:To prevent effectively corrosion over a wide range even if the environment for using an Al material changes drastically in a titled method using an external power source by suppressing intermittently the cathode potential of the Al material in accordance with the cathode potential of said material with respect to a reference electrode provided in water. CONSTITUTION:A reference electrode 2 is provided in water in proximity to an Al material 1, which is a material to be protected and is immersed in the water. The cathode potential V of the material 1 with the electrode 2 as a reference is measured by a potential measurement device 3, via circuits 7, 7. A signal is emitted from the device 3 to a relay device 5 at every rise of the potential V to a preset upper potential Vu. A counter electrode 4 and the material 1 are connected, via the device 5, to an external power source 6, whereby cathode current circuits 8, 8 are formed. The circuits are normally held open but when the potential V rises up to the Vu, the device 5 is operated by the above-mentioned signal, by which the circuits 8, 8 are closed for a short time. The cathode current is run between the material 1 and the electrode 4 during this time and the potential V is intermittently suppressed.

Description

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

Aluminum material has good thermal conductivity.

Moreover, since it is a metal material that is relatively resistant to corrosion in a neutral environment, it has recently been used in a wide range of industrial fields as a structural material for chemical equipment. It is known that when used in locations that come into direct contact with seawater or industrial water (hereinafter referred to as water), such as in liquid storage tanks, so-called non-uniform corrosion such as pitting corrosion and intergranular corrosion often occurs.

The pitting corrosion and intergranular corrosion that occur in aluminum materials used in areas exposed to water are due to electrochemical phenomena caused by the potential difference that occurs at the interface between the aluminum material and water. Corrosion can be prevented to a certain extent by applying a chemical film or anodic oxide film to the surface, or by painting it with paint, etc. However, such a protective film or painting is not necessarily perfect. However, due to latent defects in the film and painted surface, as well as deterioration of the paint film, it was not possible to expect sufficient corrosion protection over the long term.

Conventionally, as a method for preventing corrosion of metal materials immersed in water, a sacrificial anode is attached to the metal material, and a weak anti-corrosion current is applied between the counter electrode placed in the water and the metal material using an external power source. A so-called cathodic protection method is known in which the electric potential of metal materials in water is suppressed to below the pitting corrosion potential by flowing water, thereby completely preventing electrochemical corrosion of gold striped materials.

It is widely used and effective as a corrosion prevention method for steel materials.

The present inventors have repeatedly conducted various experiments to apply the above cathodic protection method to aluminum materials immersed in water.
In the case of aluminum materials, simply applying the conventional cathodic protection method described above does not necessarily produce the same satisfactory effects as with steel materials. This Kari 1 Yu is
This is because, unlike steel, aluminum is a so-called amphoteric metal that dissolves in acids and alkalis.

In other words, the conditions for an aluminum material to exist stably for a long period of time without being substantially corroded in water depend to some extent on the type of alloy of the aluminum material and the immersion environment. It is known that the potential in the water must be maintained within a narrow potential range from the pitting potential as an upper limit to a potential lower by about 0.3 to 114 V. Therefore, in order to protect alkynium materials by cathodic protection, the cathode current must be controlled so that the cathode potential of the aluminum material is always kept within the above stable potential range. The potential of a material is subject to considerable fluctuations due to changes in the temperature and flow rate of the water that comes into contact with it, and in harsh environments with such potential fluctuations, it is difficult to use conventional cathodic protection methods. However, it is extremely difficult to control the cathode potential of the aluminum material over a wide range within the narrow range described above. That is,
If you try to keep the potential low in order to prevent pitting corrosion and intergranular corrosion caused by the increase in potential of aluminum over a wide area,
Excessive suppression is often performed in the vicinity of the anode, which tends to cause melting of the aluminum material due to potential drop, that is, alkali corrosion.

The present invention prevents the drawbacks of the conventional cathodic protection methods as described above, and can respond well to severe changes in the usage environment and prevent both pitting corrosion and alkali corrosion over a wide range. The present invention provides a cathodic protection method for aluminum materials that can effectively suppress corrosion.

In other words, 1. In the cathodic protection method for aluminum materials using an external power source, each time the cathode potential of the aluminum material measured with reference to a reference electrode set in water rises to a preset upper limit potential, , by applying a negative voltage from an external power source to the aluminum material for a short period of time.

This is a cathodic protection method for aluminum materials that suppresses the cathode potential by repeating it intermittently.

Hereinafter, the method of the present invention will be described in more detail based on the accompanying drawings.

FIG. 1 is an explanatory diagram illustrating two embodiments of the method of the present invention. In the figure, (1) is an aluminum material immersed in water and serves as a hole protection material, and (2) is an aluminum material + 1
A reference electrode is placed in water close to 1. As the reference electrode (2), a reference electrode such as a calomel electrode can be used, but is not necessarily limited to this, and an electrode that is relatively stable against fluctuations in the external environment such as zinc or magnesium can be used. Electrodes made of metals or metal alloys exhibiting electrical potential may also be used. (3) is a potential measuring device, in which the aluminum material (1) and the reference electrode (2) are connected to the potential measuring device R (3) by a conductive wire to form a potential measuring circuit (7) (7). and a potential measuring device (
When the cathode potential V of the aluminum material (1) rises to the preset upper limit potential Vu with respect to the reference electrode measured in step 3), a signal is emitted from the potential measuring device (3) to the relay device (5). It will be done. (4) is a counter electrode placed in water, and the counter electrode (4) is lined with an insoluble conductive material such as magnetic iron oxide or platinum coated titanium material. Counter electrode (4) and aluminum material (1
) is connected to an external power source (6) via a relay device (6) to form a cathode current circuit (8).

The cathode current circuit (8) (8) is normally open, but the potential of the aluminum material (1) is the upper limit potential vu,
When the current level rises to , the relay device (5) is activated by the signal from the position measuring device (3) and the cathode current circuit (8) is activated.
(8) is closed for a short time, during which time the alkynium material (1
) and the counter electrode (4), a cathode current from the outside * m (6) is passed, and the cathode potential v' of the alkynium material fil is
It is structured to suppress 6.

FIG. 2 shows an example of the temporal change in cathode potential in an aluminum material [11] as a material to be protected when the method of the present invention is carried out using the apparatus shown in FIG. 1. In the figure, the vertical axis represents the cathode potential of the aluminum material with respect to a saturated calomel electrode, and the horizontal axis represents the passage of time. That is, in FIG.

Aluminum material (1) measured by reference electrode (2)
) rises and reaches point a1 of the preset upper limit potential Vu, the voltage 1 position measuring device (
The relay device (5) is activated by the signal from 3).

The cathode current circuit (8) (8) is closed and the external power supply (6
), a negative voltage is applied for a short time to the aluminum material +11, and the cathode potential V is quickly lowered to the 61 point.

When the application of voltage from the external power source (6) is removed, the cathode potential V immediately starts to rise. As shown by curve 1 b1→a, the cathode potential (2) at this time recovers rapidly at the initial stage, and then continues to rise slowly. When the cathode potential V returns to the upper limit potential vuOa1 point in this way, the relay device (6) is activated by a signal from the potential measuring device (3).

After the cathode potential V is rapidly lowered to the 1 point again,
turn to rise.

As can be seen from the above, in the second invention, when the cathode potential of the aluminum material rises to a preset upper limit potential vu, a negative voltage is applied to the aluminum material from an external power supply for a short time, and the cathode potential is intermittently increased. This is a method for cathodic protection of aluminum materials by repeatedly lowering the upper limit potential Vu, which is set in advance in the present invention.
as.

Although it varies depending on the type of aluminum alloy and the environment in which it is used, the pitting potential of the aluminum alloy material used is around (for example, about -0.70 V with respect to a saturated calomel electrode for human 1100 materials in seawater). Alternatively, it is desirable to set the pitting potential to within a voltage range about 50 mV lower than the pitting potential. In addition, the applied voltage is such that the cathode potential of the aluminum material is further −α6 than the upper limit potential.
It is desirable that the voltage decreases to a certain extent, and in order to apply such a negative voltage to the all-aluminum material, it can also be done by adjusting the voltage of the external power supply to a constant level.
A lower limit potential VL is set in advance for the amount p of adjusting the external power supply, and the cathode potential V of the aluminum material measured by the reference electrode (2) changes to the lower limit potential VLK'! by applying a negative voltage. x position measuring device (3
) by a signal from.

Relay (5) t-work automatically cathode current circuit (8)
(8) It can also be done by opening it.

The application time of one negative voltage applied to the aluminum material is
This varies depending on the magnitude of the negative voltage and the usage environment, but this is done to prevent aluminum materials from being exposed to alkaline corrosion areas for long periods of time due to a drop in potential.

It is necessary to keep the time at least several seconds or less, preferably one second or less.

As described above, in the method of the present invention, each time the cathode potential of the aluminum material immersed in water rises to the vicinity of pitting corrosion, a negative voltage is applied for a short period of time to lower the cathode potential. The potential of the material is always kept lower than the pitting corrosion potential, which not only reliably prevents pitting corrosion and intergranular corrosion.

If the negative voltage applied to the aluminum material is not applied for a short time, the negative voltage may be removed.

This is because the potential of the aluminum material recovers quickly in the initial stage and then increases slowly.

The time that the potential of the aluminum material is substantially in the alkaline corrosion range is extremely short, and
Since there is an induction period during an eclipse, there is little chance of alkaline corrosion during this period.

Therefore, compared to the conventional cathodic protection method that uses an external power source that constantly supplies cathode current, there is less risk of alkaline corrosion caused by over-protection even if the negative voltage applied to the aluminum material is increased, and it is less severe. This is an excellent method that can exert a corrosion-preventing effect over a wide range of aluminum materials even in the usage environment.

Next, an example of the present invention will be shown.

Example 1 A 1100 aluminum plate (length 800II11 x width 100 m x thickness j m) was used as a specimen to be protected against corrosion and subjected to an experiment.

9. Create a channel with a channel width of approximately 5 strips along the longitudinal direction on both sides of the specimen.9. A counter electrode (ferrite material) with a diameter of low and a length of 104 for anticorrosion was installed at a position approximately 10 cm away from one end of the plate, and the test piece intermittently served as a cathode between this electrode and the test piece. A cathode current circuit was installed to allow current to flow. The water used was natural seawater (approximately 20°C), which was flowed along the channel at a flow rate of 20 cuts/sec.

A reference electrode (calomel electrode) is installed on the opposite side of the specimen to the anticorrosion electrode (counter electrode), and the potential of the specimen measured with this as a reference is set to a preset upper limit potential (-α
70V) by the signal from the potential measuring device.

The cathode current circuit was automatically closed for a short time (set to 006 seconds), and a voltage from an external power source (2.5 V constant voltage power source) was repeatedly applied between the specimen and the counter electrode.

During this time, the cathode potential of the specimen was set at the upper limit potential and then at a voltage of about Q, 6V lower than this, at intervals of about 2 to 5 seconds.
It went up and down intermittently.

Continuous testing was conducted in this manner for 10 months, during which no pitting corrosion occurred, and no rust or alkali corrosion was observed.

For comparison, a similar test piece was tested without applying an anti-corrosion current, and pitting corrosion was observed one week after the seawater started flowing.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram illustrating nine embodiments of the method of the present invention, and FIG. 2 is an illustration of temporal changes in the cathode potential of an alonium material according to the method of the present invention. (1)...Aluminum material, (2)...Reference electrode,
(3)...Potential measuring device, (4)...Counter electrode, (5)
... Relay device, (6) ... External power supply + (7) (
7)... Potential measurement circuit + (8) (s)... Cathode current circuit. Patent Applicant Nippon Light Metal Co., Ltd. Figure 1 Figure 2 Ginma-ku Ei - Procedural Amendment June 3, 1980 Commissioner of the Japan Patent Office Kazuo Wakasugi 1 Indication of Case Patent Application No. 222980 1988 2 Invention Name: Cathodic protection method for aluminum materials 3, Relationship with the case of the person making the amendment Patent applicant address: 7-3-5-5, Ginza, Chuo-ku, Tokyo Number of inventions to be increased by the amendment None 6, Specifications subject to the amendment Contents of Amendment 2 Contents of Amendment 1 as shown in the attached sheet. 1- o, b v in the first line from the bottom of page 8 of the specification
It says J. [Corrected to 0.3-0.8 V J. 2 On page 9 of the same book, the first line from the top says ``desirably.''"Although it is desirable, a drop of 1v or more can be tolerated, especially in environments where alkali corrosion is difficult to occur." Above 423-

Claims (1)

[Claims] +11 In the cathodic protection method for aluminum materials using an external power source, Reference 1! Each time the cathode potential of the aluminum system, measured with reference to the pole, rises to the preset upper limit '11'. A cathodic protection method for aluminum material, characterized in that the cathode potential is intermittently and repeatedly suppressed by applying a short-time voltage to the aluminum material from an external power source.