JPS58167783A - Cathodic corrosion-proofness of aluminum material - Google Patents

Abstract

PURPOSE:To prevent the generation of non-uniform corrosion such as pitting or interstitial corrosion, in a cathodic corrosion-proof method utilizing an Al material used in water as a cathode, by applying cathode pulse voltage to the Al material intermittently. CONSTITUTION:An anodic oxidation film is formed on the surface of a member made of Al or an Al alloy according to a usual method and fine pores in the film are subjected to sealing in boiled water. In using this Al member used in sea water, voltage is applied between the Al member as a cathode and an opposed electrode to apply cathodic corrosion-proofness. In this case, when the potential of the Al material in sea water is negative potential of 0.4-0.8V and the pH of sea water is within a range of 4-9, the Al member is stable and is no corroded. When the potential of the Al member is approaches a point A in the vicinity of 0.4V during cathodic corrosion-proofness, pulse voltage is applied to once reduce the potential of the Al member to a point B and, if the application of pulse voltage is stopped, potential is rapidly returned to a point C to enter an immobilized region generating no corrosion while is gradually moved to the point A. By repeating this process, the Al member is prevented from pitting generation.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cathodic protection of aluminum materials, and more particularly to a method for preventing corrosion by using an aluminum material as a cathode and intermittently applying cathodic pulse voltage to the cathode using a sacrificial anode or a counter electrode.

Aluminum material has excellent corrosion resistance, and with the emphasis on @ wire equipment for workability and energy saving, it is being used in new fields that were not previously available, fields that are exposed to more severe corrosive environments. It has come to be.

Corrosion prevention methods for aluminum materials include anodic oxide coating, painting, and a combination of both.Also, aluminum-magnesium alloys have good corrosion resistance even in seawater, and are suitable for fields where corrosion resistance is required as a corrosion-resistant aluminum alloy. used in Even with such surface-treated aluminum materials and corrosion-resistant aluminum alloys, as the environments in which they are used become more severe, problems such as deterioration of the surface layer and occurrence of pitting corrosion have become a problem. It is also known that so-called crevice corrosion occurs when there are minute gaps in a structure. This is an unavoidable issue for expanding the uses of aluminum.

Sacrificial anodes are generally used as a corrosion protection method when aluminum or aluminum alloys, the surfaces of which have been anodized or electrodeposited (hereinafter referred to as aluminum materials), come into contact with water. There is. However, it is known that it is not stable due to fluctuations in temperature, impurity storage, the presence or absence of water flow, and corrosion often occurs even when a sacrificial anode is provided. This is because there is an upper limit to the current that can be supplied by the sacrificial anode, so if the environmental conditions on the surface of the aluminum material change, the sacrificial anode cannot cope with it.

There is a cathodic protection method that uses electric power or an external power source (the amount of power that can be supplied is limited), but simply keeping the potential distribution of the aluminum material in the passive region will also cause pH fluctuations, which can lead to alkaline corrosion. Even if the cathode potential could be fixed under conditions where pitting corrosion could be prevented, there was a risk of accelerating alkaline corrosion, which was problematic for industrial use. Even in the sacrificial anode method, it has been considered impossible to use a magnesium alloy, which has a lower potential than a zinc alloy, as an anode because it causes alkali corrosion in the case of aluminum.

The present application improves the above-mentioned drawbacks of the cathodic protection method, and the potential of the aluminum material in water is lower than the pitting corrosion potential and is higher than the potential at which alkaline corrosion occurs, and the pH of the solution is 4 to 4.
This method focuses on the fact that it is stable within the range of 9, and applies a cathode pulse voltage to the aluminum material so as to maintain it approximately within that range.

If the potential of the aluminum material is higher than the pitting corrosion potential, pitting corrosion will occur, so the potential of the aluminum material must always be kept below the pitting corrosion potential. Furthermore, if the pl+ of the aqueous solution is 9 or more, there is a risk of alkali corrosion occurring. This alkaline corrosion dissolves the surface uniformly, and for a short period of time,
In particular, structures (this is not a problem).

As has been known in the past, the safe range for corrosion is determined by the pH of the solution and the potential of the cathode aluminum; as shown in Figure 1, the vertical axis represents the potential.

In an aluminum corrosion diagram with pH on the horizontal axis, the stable range depends on the type of aluminum alloy, but generally, when the hydrogen electrode is referenced, the potential is α4 to 08V and the pH is 4 to 9.
Within the rectangular range surrounded by ', the part where the pH is greater than 9 forms an alkaline corrosion area, and it is quite difficult to maintain the potential of the entire aluminum material within the rectangular range.

In the case of a sacrificial anode (although the same applies when using an external power source)
Near the sacrificial anode, the aluminum material has a sufficiently negative potential, but the farthest part of the aluminum material does not reach a negative potential below the pitting potential.In order to bring the potential below the pitting potential even in this area, the supply current of the sacrificial anode must be adjusted. In order to increase the current, the only options are to use a magnesium alloy (there is a limit) or to increase the current using an external power supply. However, when lowering the potential of the aluminum material, the pH of the solution also changes along a curve sloping downward to the right, as shown by curve 1 in the aluminum corrosion diagram in FIG. A sacrificial anode or an external power source (V = constant) is often set at the very edge of the stable range, as shown at point C in Figure 1, but if the environmental conditions in the aluminum material's water, such as temperature or flow rate, fluctuate, If the pitting corrosion potential is taken out of the stable range and well below the pitting corrosion potential, it means that it is set in the alkaline corrosion range, and if exposed to the alkaline corrosion range for a long time, corrosion will be accelerated.

The present application improves the above-mentioned drawbacks, and will be explained based on Figure 1. When set under the condition of 0 point of the song, Iil, the potential of the aluminum material becomes close to point A due to fluctuations in environmental conditions. At this time, a pulse potential is applied to lower the voltage to point B. Even when point B is reached, when the voltage application is removed, the potential of the aluminum material returns to point 0 fairly quickly, and thereafter gradually increases from point C to point A. Therefore, this hysteresis is mostly occupied by the stable region.

The method of the present invention can respond well to changes in environmental conditions. For example, in FIG. 1, point A becomes A', point B becomes B', and so on. Since the time spent in the alkali corrosion is short and the induction period is dependent on the alkaline corrosion, there is almost no effect on the corrosion rate of the aluminum material.

The pitting potential depends on the type of alloy of the aluminum material and the immersion conditions, but when a saturated calomel electrode is used as the reference electrode, it is approximately -α60V for A1100.

In the present invention, the pulse voltage application time is t.

If the interval until the application of LI ( is T and the applied voltage is V, then
T > t, preferably T > 10 t and t=1
/100 seconds to 1 second.

T-α 1 minute to 10 minutes.

V: 2 volts ~ 4 volts be selected as appropriate within the range.

The voltage may be applied by providing an external power source between the counter electrode and the aluminum material, or by energizing the sacrificial anode for one hour. If a counter electrode is used, the counter electrode is placed at one end or both ends of the aluminum material with a slight distance between them.

In addition, the actual application of pulse voltage is 5Q from the pitting potential.
It would be safer to do this on the mV negative potential side.

According to the method of the present invention, since the aluminum material is always kept at a negative potential below the pitting corrosion potential, pitting corrosion can be reliably prevented, and the time of exposure to the alkaline corrosion region is extremely short, so the aluminum material can be sacrificed. Not only in the case of using an anode, but also in the case of cathodic protection using an external power source, alkaline corrosion caused by excessive corrosion can be avoided, greatly expanding the field of application of aluminum materials.

Hereinafter, the present invention will be further explained in detail with reference to Examples.

Example 1 A rolled plate of aluminum alloy A 1100 with a thickness of 1 mm was pretreated by a conventional method, and then anodized in a 15% sulfuric acid solution to form a 20μ anodic oxide film on the surface. Test No. 11, which was subjected to a sealing treatment in boiling water, was subjected to an experiment.

The specimen is 1 mm x 100 mm x 800
mm, the width of the water flow path on both sides of the specimen was set to approximately 5 mm, and a counter electrode with a diameter of lQmmφ and a length of lQmm for corrosion protection was installed at a position approximately lQcm away from one end of the plate. A corrosion protection device was installed between the electrode and the test piece to allow electricity to be applied intermittently so that the test piece served as a cathode. The water used was natural seawater, which was flowed along the channel at a flow rate of 20 cm/sec.

The anti-corrosion device checks the potential of the specimen at 2-minute intervals with reference to a reference electrode with a thin film on the opposite side of the anti-corrosion electrode.
A method was adopted in which a voltage was applied for about 1 second using a constant voltage source of Z5V only when the potential of the sample exceeded a preset critical pitting potential and reached the anode side.

The time interval for each equivalent amount during which cathodic protection is actually carried out is 6 to 8
Minute intervals were sufficient, and a continuous test was conducted for 5 months, but no decrease in the thickness of the oxide film was observed.

The measurement results of the film thickness are shown in Table 1 below, and the measured values are the average values of 10 measurement points measured using a Lamitz meter.

Further, there was no occurrence of pitting corrosion.

Table 1 Film thickness (μ) Before experiment 21.2 After 1 month 21.6 After 2 months 2B4 After 3 months 2L9 After 4 months 2Z0 After 5 months 22.2 Example 2 Rolling of JIS A 1100 used in Example 1 After pre-treating the board in a conventional manner, it was anodized in a 15% sulfuric acid bath, then an acrylic melamine electrodeposition coating was applied, and baked at 200°G, the same as in Example 1. A test specimen with dimensions of 1 was prepared, and an experiment was conducted using the method described in Example 1.

The time interval at which cathodic protection was actually performed was 2 minutes, and there was no pitting corrosion even after 5 months.

Table 2 shows the results of film thickness measurements conducted in the same manner as in Example 1. As is clear from Table 2, no decrease in film thickness was observed.

Table 2 Film thickness (μ) Before experiment 161 1-After the evening moon 162 2 months later 16.4 3 months later 161 4 months later 1 & 4 5 months later 165

[Brief explanation of drawings]

FIG. 1 is a corrosion diagram of an aluminum material shown by the potential of the aluminum material and the pH of the immersion liquid. In the figure, the parallel hatched area indicates the corrosion stable region of aluminum material. Patent Applicant Nippon Light Metal Co., Ltd. Agent Patent Attorney Keiji Matsumizu 1 Figure 1 J Procedural Amendment July 30, 1980 Case against Mr. Kazuo Wakasugi, Commissioner of the Japan Patent Office 1982 Patent Application No. 50010 2 Invention NameRelationship with the Case of Person Who Amends Cathodic Protection Method 3 for Aluminum MaterialsRepresentative Patent Applicant Yoshimasa MatsunagaRepresentative Address: 156 Telephone: (03) 322-4850 Address: 3-27-27 Matsubara, Setaga-ku, Tokyo 7. The "Claims" column, "Detailed Description of the Invention" column of the specification to be amended, and the attached drawings & contents of the amendment (1) As per the Claims appendix (2) Detailed description of the invention Explanation 1, page 1 of the specification, lines 13 to 14, "by means of a sacrificial anode or a counter electrode" is deleted. 2゜ Same, page 3, lines 12 to 16 ``The sacrificial anode method... has been considered impossible. Delete J. & Same, page 5, line 7 [ Correct the current supplied by an external power source to 1 by using an external power source. 4 Correct the current supplied by an external power source to I. 4. 5th contribution of the same, line 17, ``This means that it has been set,'' is changed to ``-1 because there is a risk of setting it.'' do. 5 Same page 5, line 18, ``When exposed'' is added to ``If exposed, add 1 to J[:,''. a Same 7th line 2nd line to 4th line t = 1/100 seconds to 1 second. T201 minutes to 10 minutes. V=2vnlt-4volt J to 1-t=1/200 seconds to 2 seconds. T21/1000 minutes to 10 minutes. V := 1 volt to 4 volt l 2 correction. 7, page 7, line 12, 150 mV, I is corrected to about 50 m V J. a Same page 8 Add the following literary giant between the first and second lines. [-In addition, water in the present specification refers to at least 51
) Refers to an aqueous solution containing chlorine ions of pm, such as seawater, tap water, treated waste water, etc. 1 9, same page 8, line 11, ``Water on both sides of 1'' is corrected to ``Water 1'' by creating channels on both sides of [. 1G Same 8th line 18th line [Correct the anti-corrosion electrode 1 to "counter electrode for anti-corrosion". (3) As shown in Appendix 2, Claim 1, Cathodic protection method in which an aluminum material immersed in water is used as a cathode and a counter electrode is connected to an external power source (in this case, the electrode potential of the aluminum material is A cathodic protection method for aluminum materials, which is characterized by applying a cathodic pulse voltage to the aluminum material when the corrosion potential is exceeded.

Claims (1)

[Claims] 1. When the electrode potential of the aluminum material exceeds the pitting corrosion potential in the sacrificial anode method or the cathodic protection method using an external electrode with a counter electrode in which an aluminum material immersed in water is used as a cathode, a cathode pulse is applied to the aluminum material. A cathodic protection method for aluminum materials that involves applying voltage.