JPS6316471B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the composition of a backing film for galvanic anodes used for cathodic protection.

When electrolytically protecting a metal structure in an electrolyte environment with relatively high resistivity such as soil or fresh water, if a galvanic anode is installed and used as is, the grounding resistance of the galvanic anode is large and the anode generated current may become too small. at the same time,
An inert film forms on the surface of the anode, making it difficult to maintain an effective potential difference between the anode and the structure for a long period of time, often making it difficult to maintain the required anticorrosion current.

As a known countermeasure against this problem, in soil environments, backfilling (electrode filling) made of a combination of bentonite, gypsum (calcium sulfate CaSO ₄ 2H ₂ O) and Glauber's salt (sodium sulfate Na ₂ SO ₄ ) is used around the galvanic anode. The electrical conductivity of the salts is used to prevent the galvanic anode from being inactivated, and to reduce the ground resistance due to the electrical conductivity of the salts.

In the practical use of this conventional backing film, the weight ratio of bentonite, gypsum and mirabilite is 6:
Fill a cloth bag made of cotton with a 3:1 mixture so that a galvanic anode such as a magnesium anode is located in the center, add sufficient water or moisture, and place it into the soil at a certain distance from the object to be protected. It is usually buried.

However, when the moisture content of the surrounding soil is extremely small or the grain size of the surrounding soil is not fine, the water contained in the backfill dissipates into the surrounding soil, causing the resistance of the backfill to increase. As the rate increases, the anodic potential becomes nobler, the generated current decreases, the dissolution surface becomes uneven,
Problems such as a decrease in anode efficiency may occur, and satisfactory anode performance cannot always be achieved.

In particular, when using a zinc anode, which has a smaller effective potential difference than a magnesium anode, there were problems such as a decrease in generated current due to a decrease in the effective potential difference as the anode potential became nobler. Furthermore, when applying galvanic anodes to prevent corrosion on the outer surface of tank bottom plates, it is not possible to reduce the current density required for bottom plate corrosion protection under conditions where conventional back-filters come into direct contact with the steel plate to be protected. This resulted in a disadvantageous situation either technically or technically.

The present invention was discovered as a result of a series of development studies aimed at improving the above-mentioned practical drawbacks, and the gist is that aluminum silicate hydrate
40 to 95% and metal sulfate 5 to 60%, or 4 to 40% of metal sulfate and 1 to 35% of one or two of metal carbonate and metal phosphate salts added. This is a backing film for galvanic anodes made of.

Aluminum silicate hydrate, which is a component of the backing film of the present invention, has an extremely remarkable effect of retaining moisture for a long time, and has the property of reducing ground resistance and stabilizing the current generated by the anode, and its weight content is If it is less than 40%, water retention will be insufficient, and
If it exceeds 95%, the surface of the backing foil becomes soft and easily washed away.

Aluminum silicate hydrate (Al ₂ O ₃・mSiO ₂・
nH ₂ O) includes the montrillonite group, which is a type of clay mineral, such as montrillonite (Al ₂ O ₃・4SiO ₂・H ₂ O), pyrophyllite (Al ₂ O ₃・4SiO ₂・H ₂ O). ), Bidenite (Al ₂ O ₃・
3SiO ₂ · nH ₂ O) or the kaolinite family, such as kaolinite (Al ₂ O ₃ · 2SiO ₂ · 2H ₂ O), dateskite (Al ₂ O ₃ · 2SiO ₂ · 2H ₂ O), nacrite (Al ₂ O ₃・2SiO ₂・2H ₂ O), halloysite (Al ₂ O ₃・2SiO ₂・2H ₂ O), rectorite (Al ₂ O ₃・2SiO ₂・H ₂ O), newtonite (Al ₂ O ₃・2SiO ₂・5H ₂ O) and mixtures thereof are effective, and bentonite or acid clay whose main component is montmorillonite is particularly suitable.
In addition, other clay minerals such as alohan (Al ₂ O ₃
SiO ₂・5H ₂ O), natrolite (Al ₂ O ₃・3SiO ₂・
NaO・2H ₂ O) etc. may also be used.

Examples of metal sulfates that are other components of the backfill of the present invention include calcium sulfate (CaSO ₄ 2H ₂ O);
Magnesium sulfate (MgSO ₄ ), aluminum sulfate (Al ₂ (SO ₄ ) ₃ ) or a mixture of these.
These are extremely effective in reducing the resistivity of backing foil, and also contribute to maintaining low resistivity and water retention over a long period of time.If their weight content is less than 5%, the resistivity will not decrease sufficiently. However, if it exceeds 60%, its solubility is low, so it becomes excessive and is wasted.

In addition, if the content of the metal sulfate is 4 to 40%, the same effect can be achieved by adding one or both of metal carbonate or metal phosphate. be.

Metal carbonates, such as calcium carbonate (CaCO ₃ ), magnesium carbonate (Mg CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), or mixtures thereof, or metal phosphates, such as phosphoric acid, which are other components of the backup film of the present invention. Calcium dihydrogen (Ca(H ₂ PO ₄ ) ₂ ), magnesium phosphate (Mg H
PO ₄ ), magnesium dihydrogen phosphate (Mg
(H ₂ PO ₄ ) ₂ ) or a mixture thereof each has an equivalent effect.

These components not only contribute to reducing the resistivity of the backing film, but also when the backing film is in direct contact with or relatively close to the steel plate to be protected, as in the case of cathodic protection of the outer surface of the tank bottom plate. is the surface of the object to be protected (steel plate)
It has the function of improving the electrocoating effect on. In other words, the formation of a film of magnesium and calcium components on the surface of the object to be protected is promoted, making it possible to significantly reduce the corrosion protection current, and by adding one or more of these components, the weight content can be reduced to 1. If it is less than 35%, the effect of forming a film on the object to be protected will be insufficient and the reduction in corrosion protection current will not be realized, and if it exceeds 35%, the anode performance will deteriorate due to the film formed on the surface of the galvanic anode.

The most important feature of the backing film of the present invention is that it has extremely good water retention compared to conventional backing film. Since it stably retains the necessary moisture over a long period of time, electrocoating improves the effectiveness of the cathode and reduces the anti-corrosion current.
For galvanic anodes, it prevents the formation of an inert film and improves the uniformity of dissolution, resulting in increased current efficiency and extended anode life.

In actual application of the backing film of the present invention, it is desirable to mix the composition as described above and then install it with sufficient water content in advance. The rate is
50~80Ω. Cm.

The backing film of the present invention is of course effective as a grounding resistance reducing agent for magnesium anodes, but is particularly suitable for zinc anodes for which no suitable backing film has conventionally been available.

Since the bag film of the present invention contained a magnesium component and a calcium component in its composition,
In electrolytic protection of the outer surface of a tank bottom plate where the backing film is located relatively close to the steel to be protected, it not only promotes the formation of a lime/magnesium electrocoating on the cathode surface, but also makes the distribution of the anticorrosive current uniform. It shows remarkable effects such as promoting the reduction of anti-corrosion current.

In addition, by applying the backing film of the present invention, it is possible to prevent uneven dissolution of the zinc plate anode, which is observed in the case of the corrosion protection method that is currently being implemented for corrosion prevention of the outer surface of the tank bottom plate by direct contact between the steel bottom plate and the zinc plate anode. Since there is no separation involved, it not only prevents the disadvantages of uneven current distribution and uneven corrosion protection, but also has the advantage of installing zinc plate anodes in or in close contact with concrete to strengthen reinforcing bars in bridges and road slabs. This method has extremely beneficial effects, such as making it possible to prevent shortening of the life of the anode due to self-corrosion of zinc due to the alkaline components of concrete, which is harmful when performing cathodic protection of iron materials.

Example 1 A steel plate and a zinc anode plate of the same shape and size are placed 50mm above and below in a plastic transparent tank of 400 x 250 x 300 mm.
They were separated and faced to each other, and buried with the bag film of the present invention interposed therebetween.

The composition of the backing film of the present invention is bentonite 60
%, calcium sulfate 25%, magnesium sulfate 10%
and 5% calcium dihydrogen phosphate, and water was added to this mixed powder so that the water content was 40% by weight.

The above steel plate and zinc anode plate are 50×100×2mm
Then, only the opposing surfaces were exposed bare, and the side (thickness) and opposite surfaces were coated with insulation. A conducting wire was connected to one end of the plate shape in advance, and this part was sealed with paint, and the conducting wire was connected to the outside and short-circuited via a zero-resistance ammeter.

The results of the three-month test under these test conditions are shown in FIG. 1 as changes over time in the anticorrosion current density, steel plate potential, and zinc anode potential. For comparison, a similar test was also conducted on a conventional backup file.

As is clear from Fig. 1, the anti-corrosion current density of the back film of the present invention is reduced to 5 mA/ ^m2 , one-eighth of that of the conventional back film, and the potential of the steel plate is reduced to 5 mA/m2.
The zinc anode potential was 230 mV base, and the zinc anode potential was also 20 mV base, and the zinc anode was uniformly dissolved.The dissolved product was a small amount of soft material, and there was no concern about the formation of an inert film that would cause surface resistance. Further, the water content after the test was reduced to 25% in the conventional backing film, whereas the water content in the backing film of the present invention remained almost unchanged at 38%, showing extremely good water retention.

Example 2 Bentonite 70 as a backing film of the present invention
%, calcium sulfate 25% and calcium carbonate 5%
Example 1 except that a composition consisting of % was used.
It is similar to

As is clear from FIG. 1, the backup film of the present invention having the above composition exhibited substantially the same performance and effects as the backup film of the present invention of Example 1.

[Brief explanation of the drawing]

FIG. 1 shows the changes over time in the anticorrosive current density, steel plate potential, and zinc anode potential when the backfill of the present invention and the conventional backfill were applied.

Claims (1)

[Scope of Claims] 1 Consisting of 40-95% aluminum silicate hydrate and 5-60% metal sulfate, or 4-40% of the metal sulfate and any one of metal carbonate and metal phosphate. A backing film for galvanic anodes consisting of one or two species to which 1 to 35% of salts have been added. 2. The backing film for a galvanic anode according to claim 1, wherein the aluminum silicate hydrate is a montmorillonite group, a kaolinite group, or a mixture thereof. 3. The backing film for a galvanic anode according to claim 2, wherein the montmorillonite group is bentonite or acid clay whose main component is montmorillonite. 4. The backing film for a galvanic anode according to claim 1, wherein the metal sulfate is calcium sulfate, magnesium sulfate, aluminum sulfate, or a mixture thereof. 5. The backing film for a galvanic anode according to claim 1, wherein the metal carbonate is calcium carbonate, magnesium carbonate, sodium carbonate, or a mixture thereof. 6. The backing film for a galvanic anode according to claim 1, wherein the metal phosphate is calcium dihydrogen phosphate, magnesium phosphate, magnesium dihydrogen phosphate, or a mixture thereof.