JP2001192870A - Corrosion protecting method for tank bottom plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an efficient and inexpensive cathode protecting method in a high resistance soil environment low in a water content and suitable for a nonelevated tank made of steel. SOLUTION: By adding organic phosphonic acid and zinc chloride to the inside of fundamental soil, at the time of flowing the corrosion protecting current, an insoluble film composed of phosphorous and zinc is deposited on the whole face of the contact face between a tank borrom board 71 and the fundamental soil 83 in a short time, the speed of the cathode polarization of the tank bottom board 71 is promoted, the required corrosion protection maintaining current is reduced, as well, and, moreover, the resistivity of the fundamental soil 83 is reduced, by which the efficiency of the cathode protecting method is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing corrosion of a bottom plate of a steel ground tank for storing oil, chemicals, etc., and more particularly, to a method in which a bottom plate in direct contact with a base soil in which an insoluble anode or a sacrificial anode is embedded is used as a cathode. The present invention relates to an anticorrosion method for a tank bottom plate in which the base soil contains a phosphonate and zinc chloride to improve the anticorrosion effect by cathodic protection, in a method for preventing corrosion using an anticorrosion method.

[0002]

2. Description of the Related Art As shown in FIG. 6, a large steel ground tank 7 for storing oil and chemicals is generally filled with a base soil 83 up to a predetermined position on a base ring wall 84 and compacted. It is installed on a foundation 8.

[0003] Here, since the tank foundation 8 is required to have sufficient strength, sand or fine crushed stone is used for the foundation soil 83, which is sufficiently compacted and the steel tank bottom plate 71 is welded and joined. Be laid.

[0004] By the way, the tank bottom plate 7 with the base soil 83
Conventionally, a waterproof layer 85 such as an asphalt sand layer or an oil sand layer is provided on the upper surface of the base soil 83 so as to prevent the base soil 83 and the tank bottom plate 71 from directly contacting each other. Methods have been taken to prevent corrosion.

However, according to the conventional method, it is difficult to completely prevent corrosion of the tank bottom plate 71 due to aging of the waterproof layer 85 and rainwater intrusion from the outer periphery of the tank bottom plate 71.

Therefore, the base soil 83 filled between the waterproof layer 85 and the impervious layer 82 made of a resin sheet or a concrete slab provided below the base soil 83.
A method in which an insoluble anode 61 or a sacrificial anode is provided therein and a cathodic protection method is used in combination with the tank bottom plate 71 has been attempted.

[0007] However, API Recommended Practice
951 (Cathodic Protection of Aboveground Petrol
eum Storage Tanks, 1997), as described in Section 5.1.4, the waterproof layer 85 is electrically insulating, so that the corrosion protection current from the insoluble anode 61 or the sacrificial anode is shielded and flows into the tank bottom plate 71. Therefore, there was a disadvantage that a sufficient anticorrosion effect could not be obtained.

[0008] Several solutions have been considered and implemented to date to solve this problem.

[0009] In Japanese Patent Application Laid-Open No. 9-272991, a linear insoluble anode is laid on the surface of an asphalt sand layer, and a conductive material such as carbonaceous material is laid on the entire upper surface of the asphalt sand layer. A method of laying a tank bottom plate by laying a sand layer on a substance, that is, a method of forming an anticorrosion current conducting layer between the tank bottom plate and a waterproof layer such as an asphalt sand layer is disclosed.

Japanese Patent Application Laid-Open No. Hei 6-33267 discloses that a zinc film is coated on a surface which is to be in contact with the base soil at a water content of 15% or less and a chlorine ion concentration of 500 ppm or less on a surface to be in contact with the base soil. A method of laying a sprayed zinc-coated steel sheet as a tank bottom plate and a method of using a cathodic protection method in combination with this method are disclosed.

In JP-A-56-44777, a waterproof layer is not provided on a tank base, water containing an inhibitor (corrosion inhibitor) is injected into the base soil and saturated, and the tank bottom plate is used as the base soil. There is disclosed a method of directly laying and preventing corrosion by keeping the tank bottom plate outer surface constantly flooded.

In Japanese Patent Publication No. 3-1385, a method is provided in which an inhibitor layer made of a nonwoven fabric, a woven fabric or other porous mat impregnated with or covered with an inhibitor is provided between a tank bottom plate and a foundation soil to prevent corrosion. Further, a method in which a cathodic protection method is used in combination with this method is disclosed.

Furthermore, recently, a method has been adopted in which a waterproof layer is not provided on the upper surface of the base soil, and the tank bottom plate which is in direct contact with the base soil is protected by the cathodic protection method alone.

According to this method, a substantially good current distribution can be obtained over the entire surface of the tank bottom plate, and it is expected that a reliable anticorrosion effect can be obtained by either the external power supply method or the sacrificial anode method.

[0015]

However, in the above-described anticorrosion method disclosed in Japanese Patent Application Laid-Open No. 9-272991, the construction is complicated and the cost increases, and the insoluble anode is buried in the asphalt layer by compacting the sand layer. As a result, there is a danger that the current will not flow easily, and if the current is applied for a long period of time, the carbonaceous material will be consumed and the sand layer may unequally subside.

The anticorrosion method disclosed in Japanese Patent Application Laid-Open No. 6-33267 has a drawback in terms of reliability because there is no method for monitoring the consumption of the zinc film and the corrosion of the welded portion where the film is not formed. The cost of zinc spraying, including blasting as a pretreatment, is not much different from the cost of cathodic protection, and is less attractive in terms of cost.

The anticorrosion method disclosed in JP-A-56-44777 is not realistic because the strength of the base soil is completely lost in a saturated water-containing state.

Further, in the anticorrosion method disclosed in Japanese Patent Publication No. 3-1835, nonwoven fabric, woven fabric or other porous mats are burned by welding heat input when laying the tank bottom plate. The inhibitor effect cannot be obtained, and even when the cathodic protection method is used, the burned portion may serve as a current shield and the anticorrosion effect may not be obtained.

In a method which has recently been widely used in the background described above, a method in which a tank bottom plate is directly installed on a foundation soil without providing a waterproof layer and corrosion is prevented using only a cathodic protection method. However, the sand and fine-grained crushed stone used as the foundation soil have a low moisture content and easy oxygen diffusion, so that the steel tank bottom plate is less likely to be polarized in such foundation soil. In addition, there is a problem of inefficiency in that it takes a long time of several months from when the anticorrosion current is applied to when the anticorrosion potential is achieved.

Further, since sand and fine crushed stone used as the foundation soil hardly contain soluble salts, the resistivity is extremely high at 104 to 105 Ωcm, and as a result, a large number of materials are required to secure a predetermined anode ground resistance. This required an anode, which increased the cost.

Therefore, the present invention relates to a method of cathodic protection using a cathodic protection method in which a tank bottom plate is installed directly on a base soil without providing a waterproof layer, which is considered to have a high anticorrosion effect among the various anticorrosion methods described above. To promote the polarization of the tank bottom plate and at the same time reduce the resistivity of the base soil to reduce the grounding resistance of the anode, thereby providing an efficient and inexpensive method for preventing corrosion of the tank bottom plate even on a high-resistance base soil having a low water content. With the goal.

[0022]

In order to achieve the above object, the present invention relates to a method of connecting a cathode to an outer surface of a tank bottom plate installed on a base soil having an impermeable layer at a lower portion,
In a method for preventing corrosion of a tank bottom plate in which an insoluble anode or a sacrificial anode is provided in the base soil between the impermeable layer and the tank bottom plate and the outer surface of the tank bottom plate is cathodically protected, 1-hydroxyelene- 1,1-diphosphonic acid or aminotrimethylphosphonic acid or aminotr
Any one of imethylphosphonic acid 5Na-salt
It is characterized by adding each kind of organic phosphonic acid and zinc chloride.

Here, the organic phosphonic acid and zinc chloride are prepared by adding the organic phosphonic acid to the base soil at a weight ratio of 0.6 pp.
m to 20 ppm and the zinc chloride in a weight ratio of 6 ppm to 2
It is preferable to add 00 ppm each.

Further, the organic phosphonic acid and zinc chloride are added to water for tightening the base soil and impregnated in the base soil, or a resin pipe extending from the base soil to the outside of the base soil is laid, After the tank is constructed, it is poured into and impregnated from the outside of the base soil using the resin tube, or is previously mixed with the base soil.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Summary of the Invention) In the present invention, an organic phosphonic acid and zinc chloride are contained in a base soil in a method of installing a tank bottom plate directly on a base soil and using a cathodic protection method. It is characterized by:

By adding an organic phosphonic acid and zinc chloride to the base soil, when an anticorrosion current is applied to the tank bottom plate, an insoluble film made of phosphorus and zinc is formed on the entire surface of the tank bottom plate in a short time, This coating increases the polarization resistance of the tank bottom plate. Therefore, the cathode polarization of the tank bottom plate is promoted, and the required current is reduced.

Further, since organic phosphonic acid and zinc chloride are electrolytes, their addition lowers the resistivity of the base soil.

Further, the organic phosphonic acid and zinc chloride are inexpensive and chemically stable, and the corrosiveness to steel when added to the sand constituting the base soil is as follows when distilled water is added to the sand constituting the base soil. It does not accelerate natural corrosion of the tank bottom plate until the anticorrosion current is applied.

Although the resistivity of the base soil decreases with the addition of any electrolyte, chlorides other than zinc chloride hinder cathodic polarization, increase the required current density, and furthermore, increase the natural current during the period until the start of the application of the anticorrosion current. It is not preferable because it accelerates corrosion.

(Embodiment) An embodiment of a method for preventing corrosion of a tank bottom plate according to the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a schematic view of a test apparatus according to the present invention. The following test was performed using this test apparatus.

An organic phosphonic acid and zinc chloride (ZnCl 2) are placed on an insoluble anode plate 11 in which titanium is covered with a platinum group oxide.
₂ ) A sand (natural silica sand) 12 which was added with an aqueous solution to have various concentrations and water content ratios and adjusted to a height of 350 mm, filled and tamped, and a tank coated with a portion other than the sand contact surface A 100 × 100 mm steel test piece 13 simulating a bottom plate was placed, and a 7 kg weight 14 was loaded. 1-hydroxye, commercially available as organic phosphonic acid
lene-1,1-diphosphonic acid, aminotrimethylphosphon
Three types of ic acid and aminotrimethylphosphonic acid 5Na-salt were tested.

Also, a saturated copper sulfate reference electrode 15 was inserted and installed in the center hole of the test piece so that the liquid junction was located on the contact surface with the sand of the test piece.
The cathode was polarized from the natural potential to -1.0 V at a potential sweep rate of 1 minute.

For comparison, the same measurement was carried out for sand in which only water was added to adjust the water content.

From the comparison of the cathodic polarization characteristics, one of the above three kinds of organic phosphonic acids was added to sand in a weight ratio of 0.1%.
6ppm ~ 20ppm, zinc chloride is also 6p by weight ratio
pm to 200 ppm, the water content is 3% to
It has been found that the cathode polarization increases in the range of 19% (saturation).

Further, the test piece was set to -0.0.
The current density at 85 V was measured. FIG. 2 shows an example in which 1-hydroxyelene-1,1-dip is used as the organic phosphonic acid.
The measurement results when hosphonic acid and zinc chloride are added are shown.

As shown in FIG. 2, the polarization was slow in the sand a in which the water content was adjusted to 3% by adding only distilled water and the sand b in which the water content was adjusted to 10%, and the polarization was constant even after 72 days. Although the convergence does not occur, the current density of the sand c in which the water content is adjusted to 3% by adding the organic phosphonic acid and zinc chloride and the sand d in which the water content is adjusted to 10% decreases in a short period of time to become a constant value. It was confirmed that the anticorrosion potential was maintained at a current density of about 1/2 that of the case where no addition was made.

FIG. 3 shows the relationship between the water content of the sand and the resistivity of the test apparatus.

100 ppm of organic phosphonic acid and zinc chloride 1
The sand b to which the mixed aqueous solution of 000 ppm was added had a low water content of 3% which is comparable to that of desert soil (in terms of weight ratio to sand, 3 ppm of organic phosphonic acid and 30 pp of zinc chloride).
In m), the resistivity of the organic phosphonic acid and zinc chloride is less than half that of the sand a to which only distilled water is added due to the function as an electrolyte.

As an organic phosphonic acid, aminotrimeth
ylphosphonic acid or aminotrimethylphosphoni
When c acid 5Na-salt was used, the same results as described above were obtained.

Next, FIG. 4 shows an example in which the present invention is applied to an actual tank.

In the figure, reference numeral 7 denotes a bottom plate 71 having a diameter of 17.5 m,
This is an oil tank having a surface area of 240 m ² , and a base soil 83 of washing sand is formed on a non-permeable layer 82 of a high-density polyethylene sheet having a thickness of 0.5 mm to a predetermined position of the base concrete 81, and a tank bottom plate 71 is formed. Welded.

The water content of the base soil 83 measured before the construction was 3.2%, the resistivity was 150,000 Ωcm, and the saturated water content was 20%.

In the base soil 83, ribbon-shaped insoluble anodes 61 in which titanium is coated with a platinum group oxide are laid at intervals of 1.5 m, and the anode and the titanium feeder 62 orthogonal to the anode are spot-welded at intersections. Are joined.

Reference numerals 91 and 92 denote saturated copper sulfate permanent reference electrodes for measuring the potential.
The reference electrode 92 is located closest to the anode 61, that is, on the anode 61, at a position farthest from 1, that is, between the anodes.
Each is installed at a position of 0.2 m directly below the tank bottom plate 71, and is connected to a measuring instrument 90 via a lead wire 93.

In the tank A having the above structure to which the present invention is applied, after the base soil 83 is compacted, the organic phosphonic acid (1-
0.8 kg of hydroxyelene-1,1-diphosphonic acid) and 8.0 kg of zinc chloride were dissolved in 8.0 m ³ of tap water and sprayed on the base soil 83.

The amount of base soil 83 used was 84.2 tons, and the amount of organic phosphonic acid and zinc chloride used was 8
3 ppm and 30 ppm in weight ratio to 3 respectively
Is equivalent to

After the application of the organic phosphonic acid and the zinc chloride solution, a period until the tank bottom plate 71 is laid is a base soil 83.
A waterproof sheet was laid on top and cured.

Next, a tank B having the same size and structure as that of the tank A was compared with a tank A adjacent to the tank A, which was sprayed with 8.0 m ³ of tap water without any addition to the base soil 83 and closed with water. Built as an example.

The rating of the DC power supply 60 is 24 V-5 A for both the tank A and the tank B.

Twelve months after the foundation work, the application of the anticorrosion current was started when the tank was completed, and the results of measuring the current and the potential (instant-off potential) of the tank bottom plate 71 are shown in the table of FIG.

Here, the measurement point 1 is 5.25 from the center of the bottom plate.
m, and the measurement point 2 is the reference electrode 9 installed at the center of the bottom plate.
No. 2.

At the same time, the application of a 3.6 A anticorrosion current to both the tank A to which the present invention was applied and the conventional tank B was started.

The potential of the tank bottom plate immediately after the start of energization was −0.51 V at measurement point 1 of tank A, and −0.53 V at measurement point 2.
V, which was -0.52 V at the measurement point 1 of the tank B and -0.53 V at the measurement point 2, so that there was no significant difference between the potentials of the bottom plates of the tanks A and B in a natural state.

However, a clear difference was already observed in the potential measurement results 7 days after the start of energization. At the measurement points 1 and 2 of the tank A, the anticorrosion potential (-0.80 V and -0.83 V, respectively) was smoothly increased. −0.85 V), whereas in tank B, at measurement point 1 and measurement point 2, −0.54 V and −0.60 V, respectively, indicating that there is not much change from the values immediately after the start of energization. .

Further, 14 days after the start of energization, tank A reached a sufficient anticorrosion potential at both measurement points and the current was able to be reduced from 3.6 A to 2.4 A, whereas tank B was measured at measurement point 1 and measurement point In both of the measurement results, the current was increased from 3.6 A to 4.8 A because there was no significant change from the value immediately after the start of energization.

One month after the start of energization, the potential was again lower than the anticorrosion potential at both measurement points of tank A, so the current was increased to 2.4 A.
To 1.8 A, and then maintain the anticorrosion potential with this current until the measurement three months after the start of energization. In tank B, however, the anticorrosion potential reached only at measurement point 2 two months after the start of energization, and the start of further energization It was confirmed that the anticorrosion potential reached the measurement point 1 three months later.

Comparing the potential difference between the measurement points 1 and 2 at the same time in the tanks A and B, the potential difference between the two measurement points in the tank A is almost the same as that in the tank B. At each measurement, the potential at the measurement point 2 is clearly lower than the potential at the measurement point 1, and a large potential difference occurs.

That is, according to the table of FIG.
It took 3 months from the start of energization until the bottom plate reached the anticorrosion potential, and the current required to maintain the anticorrosion potential was 4.8 A, whereas in tank A to which the present invention was applied, only 1 A was used.
The anticorrosion potential was reached in 4 days, and the maintenance current was 1.8 A, 1/3
Further, in the tank A, the potential difference due to the difference in the distance of the bottom plate from the anode can be obtained and a good potential distribution can be obtained, and it can be determined that the efficiency of the cathodic protection method has been sufficiently improved by implementing the present invention. .

Although the above embodiment has described the cathodic protection method using an external power supply method, the present invention can be similarly applied to a sacrificial anode method.

[0061]

As described above, according to the present invention,
By applying an anti-corrosion current to the tank bottom plate, an insoluble film made of phosphorus and zinc is formed on the entire tank bottom plate surface in a short time, which promotes cathodic protection and reduces the required current, and furthermore, the resistivity of the base soil And a good anticorrosion state can be achieved in a short period of time even with a low current value, so that the cost of anticorrosion equipment and the cost of current adjustment on site can be reduced, and an industrially useful effect can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a test apparatus for testing the effect of the present invention.

FIG. 2 is a graph showing a change over time in current density in the test apparatus of FIG.

3 is a graph showing the relationship between the water content of sand and the resistivity in the test apparatus of FIG.

FIG. 4 is a schematic diagram of an actual steel ground-mounted tank to which the present invention is applied.

FIG. 5 is a diagram showing a comparison result of the anticorrosion effect between the tank to which the present invention is applied and the conventional tank when the cathodic protection method is applied to the tank bottom plate.

FIG. 6 is a schematic diagram of an actual steel ground-mounted tank to which a conventional cathodic protection method is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Insoluble anode plate 12 Base soil 13 Steel test piece 14 Weight 15 Saturated copper sulfate reference electrode 16 Automatic polarizer 7 Tank 8 Tank base 60 DC power supply 61 Insoluble anode 62 Titanium feeder 71 Steel tank bottom plate 81 Base ring wall 82 Impervious layer 83 Base soil 84 PVC leak detection tube 85 Waterproof layer 86 Asphalt shoulder 90 Measurement box 91 Reference electrode 92 Reference electrode 93 Lead wire

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mituya Hirado 2-3-1 Minatomirai, Nishi-ku, Yokohama-shi, Kanagawa F-term within JGC Corporation 4K060 AA02 AA03 EA06

Claims (5)

[Claims] 1. A cathode is connected to an outer surface of a tank bottom plate directly laid on a base soil having an impermeable layer at a lower portion,
A method for protecting a tank bottom plate in which an insoluble anode or a sacrificial anode is provided in a base soil between the impervious layer and the tank bottom plate to cathodic protect the outer surface of the tank bottom plate, wherein an organic phosphonic acid (1-hydroxyeleneene) is added to the base soil. -1,1-dipho
sphonic acid or aminotrimethylphosphonic aci
d or aminotrimethylphosphonic acid 5Na-salt
One of the above) and zinc chloride, respectively. 2. The weight ratio of the base soil is 0.6 ppm to 20 ppm.
ppm of said organic phosphonic acid and a weight ratio of 6 ppm to 2
The method for preventing corrosion of a tank bottom plate according to claim 1, wherein 00 ppm of the zinc chloride is added. 3. The method for preventing corrosion of a tank bottom plate according to claim 1, wherein the organic phosphonic acid and zinc chloride are added to water for tightening the base soil to impregnate the base soil. 4. A resin pipe extending from the base soil to the outside of the base soil, laying the organic phosphonic acid and zinc chloride in the base soil from the outside of the base soil using the resin tube after the tank is constructed. The method for preventing corrosion of a tank bottom plate according to claim 1, wherein the tank bottom plate is injected and impregnated. 5. The anticorrosion method for a tank bottom plate according to claim 1, wherein the organic phosphonic acid and the zinc chloride are preliminarily mixed with the base soil to form a tank base.