JP2711455B2 - Backfill for cathodic protection - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a backfill used for cathodic protection, and more particularly, to performing a cathodic protection of a reinforced structure in concrete in a high-resistance environment, the resistivity around the anode. The present invention relates to a backfill for reducing and securing water retention over a long period of time.

[Conventional technology and its problems]

Conventionally, when a metal structure in a relatively high-resistivity electrolyte environment such as soil or fresh water is subjected to electrolytic protection, the resistance around the anode is high. In the electroanode method, the current generated by the anode required for anticorrosion is too small, and it is difficult to maintain an effective potential difference between the anode and the metal structure for a long time, and it is often difficult to maintain the required anticorrosion current. .

For this reason, at the time of cathodic protection for soil environments such as tank bottom plates and buried pipes, the anode periphery is filled with a backfill consisting of bentonite, gypsum, sodium sulfate, etc. to reduce the resistivity of the anode and inactivate the anode. Measures have been taken to prevent this. When installing this backfill, moisture is included from the beginning, or moisture is taken in from the environment,
After that, it is important to maintain the water retention for a long time. Therefore, if the surrounding environment in contact with the anode is in a remarkably dry state, the moisture taken into the backfill escapes to the surrounding environment, causing an increase in the resistivity of the backfill, and as a result, the polarization (nobleness) of the anode increases. This may promote reduction of the generated current, increase of the bath voltage, etc., destabilize the anode performance, and may not be able to expect satisfactory results.

In addition, concrete is mentioned as a high resistance environment other than soil and the like. That is, this is a case where a steel structure covered with concrete is subjected to electrolytic protection. Since concrete has been poorly permeable and alkaline environment,
Corrosion of concrete-coated steel structures such as steel bars and steel pipe piles was almost neglected. However, chloride contained in conventional sand and gravel, which is a concrete component, and chlorides such as foreign seawater splashes from the environment become dissolved water and gradually penetrate into concrete to promote the neutralization of concrete. Eventually, the steel structure is exposed to a corrosive environment. As a result, chloride ions destroy the protective coating on the steel surface, and the amount of corrosive components increases inside the concrete with the progress of corrosion of the reinforcing steel, causing cracking and destruction of the concrete structure. In reality, port concrete facilities such as bridges, bridge girders, floor slabs, steel piles, etc.
At present, the need for replacement is increasing every year. The only means of preventing corrosion of these existing concrete steel structures is the cathodic protection method. For the electric erosion, whether an external power supply system or a galvanic anode system is used, the presence of an electrolyte that conducts an anticorrosion current from the anode is indispensable. In other words, the presence of moisture is an absolute requirement. In general, the conductivity of concrete is extremely small, and the presence of moisture has a great effect. Its resistivity ranges from several thousand Ω · cm to several hundred thousand Ω · cm, and it is not easy to increase the conductivity of such concrete. Therefore, in order to make the effect of cathodic protection effective, it is necessary to minimize the performance of the anode in such an environment, that is, the anode polarization and the anode inertness.

As a means for this, it is conceivable to lower the ground resistance of the anode by surrounding the periphery of the anode with a backfill and generate a sufficient current from the anode. However, the backfill conventionally used in the soil environment as described above has a problem in water retention, and has a problem that sufficient water retention cannot be expected as a backfill for electrolytic protection of concrete reinforced structures. Met.

Therefore, in the present invention, when a steel structure in a high resistance environment such as concrete or soil is subjected to electrolytic protection, a backfill capable of maintaining a stable and long-term anticorrosion current from an anode is provided. An object of the present invention is to provide a backfill in which the ground resistance of the anode is kept low and especially the water retention is enhanced, and to find a structural material of the backfill in which the water retention material is easily available and inexpensive.

[Means for solving the problem]

The present invention has achieved the above-mentioned object by a backfill for electrolytic protection composed of aluminum silicate hydrate, metal sulfate and magnesium chloride.

The metal sulfate in the present invention comprises calcium sulfate alone or calcium sulfate and any one selected from magnesium sulfate, sodium sulfate, and aluminum sulfate.

Such preferred backfill compositions of the present invention include those comprising aluminum silicate hydrate, calcium sulfate, magnesium sulfate or sodium sulfate, and magnesium chloride.

The composition ratio of the backfill having the above composition is aluminum silicate hydrate: calcium sulfate:
The weight ratio of magnesium sulfate or sodium sulfate: magnesium chloride is set to 6-8: 1-3: 1-3: 2-6, and more preferably the ratio is set to 7: 2: 1: 3-5.

[Action]

Bentonite, which is an aluminum silicate hydrate (Al ₂ O ₃ · mSiO ₂ · nH ₂ O), is the main component of the backfill and has the effect of retaining moisture for a long time and the effect of reducing the grounding resistance . The gypsum (CaSO ₄ · 2H ₂ O) and sodium sulfate (Na ₂ SO ₄₎
Sulfates such as are effective in reducing the resistivity of the backfill and maintaining water retention. However, these components can dissipate moisture into the environment over a prolonged period of dryness, which increases the resistivity of the backfill and promotes the polarization of the anode, reducing the current generated and increasing the bath voltage. As a result, the performance of the anode becomes unstable. On the other hand, when magnesium chloride is added to these components as in the present invention, the magnesium chloride suppresses the escape of water even when the dry state is long,
In addition, since the above-mentioned factors that make the anode performance unstable are suppressed in order to easily absorb moisture, an effect of maintaining extremely stable anode performance for a long period of time is achieved. This magnesium chloride is usually easily available and inexpensive.

Although calcium chloride may be added in place of magnesium chloride, calcium chloride is effective as a dehydrating agent, but is not sufficient in terms of water retention. That is, it is not effective in keeping moisture for a long period of time even in a dry environment. As described above, magnesium chloride has a great difference from calcium chloride in water retention, and its function as a backfill additive cannot be replaced by calcium chloride.

 Examples will be described below.

Example A specimen was prepared in which a zinc plate anode was fixed to the surface of a mortar (cover thickness 7 cm) in which an iron plate (100 × 70 × 0.5 mm) was embedded via a backfill, and the iron plate in the mortar was used as a counter electrode. Anode electrolysis at a constant current of 10 μA / cm ² was performed.

Backfill is made of bentonite, gypsum, magnesium sulfate (7: 2: 1 by weight), bentonite,
Gypsum, magnesium sulfate, calcium chloride (7: 2: 1: 4)
Mixed system, bentonite, gypsum, magnesium sulfate,
Four kinds of mixed systems of magnesium chloride (7: 2: 1: 4), bentonite, gypsum, and magnesium sulfate (7: 2: 4) were used.

FIG. 1 shows the results of chronologically measuring the anode potential when each of these mixed backfills was used. As can be seen from FIG. 1, the specimen via the magnesium chloride-containing backfill has a stable anodic potential at a base potential without anodic polarization for a long period of time. The performance of the backfill containing magnesium chloride was remarkably superior to that of the comparative example.

Next, the anode and cathode of each specimen are made conductive,
FIG. 2 shows the results of measuring the current (current density) when the anticorrosion current was generated. As can be seen from FIG. 2, the specimen through which the magnesium chloride-containing backfill according to the present invention and the test specimen via the magnesium chloride-containing backfill had a larger current as compared with the conventional example and the calcium chloride-containing backfill.
This shows that this backfill system has high performance.

Also, put the mixture of the four types in a wide-mouthed 100ml beaker, impregnate it with a certain amount of moisture, put it in a thermo-hygrostat (temperature constant at 25 ° C), and adjust the humidity to 30%, 50%, 70% and 90%. After setting, each was left for 7 days. When the weight before and after the constant humidity setting was measured, the result shown in FIG. 3 was obtained. This third
As can be seen from the figure, it can be said that the mixture containing magnesium chloride has a small amount of water lost and is excellent as a backfill.

〔The invention's effect〕

As described above, according to the present invention, an inexpensive and readily available magnesium chloride is added to and mixed with a backfill composed of bentonite, gypsum, and magnesium sulfate (sodium sulfate), which has been widely used in the past, so that a dry state is obtained. It suppresses the deviation of water even over a long period of time and makes it easy to absorb water, so it can maintain extremely stable anode performance for a long period of time, and is suitable for use in electrolytic protection of steel structures in concrete environments. Backfill is obtained.

[Brief description of the drawings]

FIG. 1 is a relationship diagram showing a change over time of the anode potential of each specimen in the example. FIG. 2 is a relationship diagram showing a change over time in the current density of each specimen in the example. FIG. 3 is a relationship diagram showing a weight change when the humidity is changed at a constant temperature of each specimen in the example.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-59-69462 (JP, A) JP-B-63-16471 (JP, B2)

Claims (7)

(57) [Claims] 1. An anticorrosion backfill for a reinforced concrete structure comprising aluminum silicate hydrate, metal sulfate and magnesium chloride. 2. The backfill for cathodic protection against reinforced concrete structures according to claim 1, wherein the metal sulfate comprises calcium sulfate and any one selected from magnesium sulfate, sodium sulfate and aluminum sulfate. 3. The backfill according to claim 1, wherein the metal sulfate comprises calcium sulfate. 4. The backfill according to claim 1, comprising aluminum silicate hydrate, calcium sulfate, magnesium sulfate and magnesium chloride. 5. The backfill according to claim 1, comprising aluminum silicate hydrate, calcium sulfate, sodium sulfate and magnesium chloride. 6. The weight ratio of aluminum silicate hydrate: calcium sulfate: magnesium sulfate or sodium sulfate: magnesium chloride is 6-8: 1-3: 1-3: 2-6. A backfill for cathodic protection against a reinforced concrete structure as described. 7. The backfill according to claim 5, wherein the weight ratio is 7: 2: 1: 3 to 5.