JPS5810470B2 - Method for preventing metal corrosion in water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a corrosion protection method for metals in water, particularly metals that come into contact with seawater or corrosive liquids of relatively high conductivity in container systems such as ship ballast tanks and various tanks. .

Conventionally, in containers that temporarily or long-term store seawater or corrosive liquids with high comparative conductivity, or that alternately fill with water and drain, the container itself or various attached structural materials, components, etc. Electrolytic protection methods, coating methods, and methods of adding corrosion inhibitors are used as measures to prevent corrosion of metals.

Among these, the cathodic protection method is an excellent corrosion protection method and is currently widely used.Although it can provide a good corrosion protection effect for container systems with a simple structure, For objects, sufficient anticorrosion effects may not always be obtained on the back side of the member, in narrow areas, etc.

Furthermore, although painting methods are gradually becoming more important due to the development and improvement of paints, in the case of complex structures such as those mentioned above, not only is it difficult to paint, but it is also difficult to repair the paint film if it deteriorates. However, this is not a technically or economically sufficient method for conservation.

Corrosion inhibitors, on the other hand, can provide continuous corrosion protection if the content is maintained at a necessary and sufficient appropriate concentration. It is not only efficient but also simple and economical.

In particular, it is difficult to apply it to ephemeral water systems such as seawater cooling systems because the amount of chemicals used is enormous and it is relatively uneconomical. Fully applicable.

However, the addition of corrosion inhibitors is characterized by a mechanism that prevents corrosion by forming a protective film such as an adsorption film or a precipitate film on the metal surface, depending on the properties of the agent. This may lead to insufficient formation of a protective film or destructive deterioration, leading to local corrosion or pitting of the metal body.

Recently, in highly corrosive environments such as seawater and comparatively high conductivity, it is necessary to reduce the amount of corrosion inhibitors added from the standpoint of pollution prevention and environmental conservation. Add a small amount of mold corrosion inhibitor,
Corrosion prevention methods that combine cathodic protection are being researched and are about to be put into practical use.

However, inorganic phosphates inevitably hydrolyze in aqueous solutions with the passage of time and increases in temperature and pH, and it is necessary to replenish and add them in a timely manner to maintain the corrosion inhibiting effect. When used in conjunction with anticorrosion, the pH of the cathode surface rises, resulting in an accelerated decomposition rate on the cathode surface, which has been desired to be improved.

The present inventors have discovered that as a method for preventing corrosion of metals in seawater and other relatively highly conductive aqueous solutions, the corrosion protection effect is greatly improved by using a corrosion inhibitor that forms a precipitated film, such as an inorganic phosphate, in combination with cathodic protection. As a result of continuing research to improve this corrosion prevention method, we found that conventional methods are limited to use in freshwater systems with low conductivity, have no corrosion prevention effect in seawater environments, and are used in large quantities because they actually accelerate corrosion. By using corrosion inhibitors such as organic phosphoric acids and their bases in combination with cathodic protection, especially the galvanic anodic method using a zinc anode, it is possible to eliminate corrosion not only with the corrosion inhibitor or cathodic protection alone, but also with conventional corrosion. We have developed an effective and economical corrosion protection method that can be used in combination with inhibitors and cathodic protection.The main points of this method are as follows.

That is, the corrosion prevention method of the present invention is a method for preventing metal corrosion in seawater or an aqueous solution with comparatively high conductivity, using a combination of an organic phosphoric acid corrosion inhibitor and electrolytic protection using a galvanic zinc anode.

In the present invention, the organic phosphoric acid corrosion inhibitor is based on phosphonic acids, such as aminotrimethylenephosphonic acid (ATMP), 1-hydroxyethylidene-1,1
-diphosphonic acid (HEDP) and its salts.

Next, the configuration and effects of the present invention will be described.

Figure 1 shows that the water was immersed in artificial seawater at 25C for 7 days in an open state.
Mild steel pieces were naturally immersed in organic phosphoric acid (HED) at various concentrations.
This figure shows the results of a comparative test on the anticorrosive effect of adding P) and an inorganic phosphate, that is, small sodium xametaphosphate (SHP).

As is clear from this figure, the corrosion rate gradually decreases up to the addition of 200 ppm, indicating a corrosion-preventing effect.

When the concentration exceeds 200 ppm, inorganic phosphoric acid still has a corrosion inhibiting effect, and at concentrations up to 11,000 ppm, it shows a corrosion prevention rate of 80 to 100% compared to the corrosion rate when no additive is used. In the case of phosphoric acid, the corrosion protection rate peaks at about 50%, and as the concentration increases, corrosion is rather accelerated, and it can be seen that addition alone cannot be used.

However, when cathodic protection is used in combination with cathodic protection, unexpected and remarkable effects are observed, and as a result of many experiments, it has been found that organic phosphoric acid systems are more effective than inorganic phosphoric acid systems when used in combination. did it.

Moreover, it has become clear that both the concentration of the corrosion inhibitor added and the required corrosion protection current density can be significantly reduced by the combination of the two methods, rather than by the corrosion protection methods using each alone.

Hereinafter, the effects and characteristics will be described in more detail with reference to Examples.

Example 1 Figure 2 shows that when used in conjunction with cathodic protection using a zinc galvanic anode,
25 shows the relationship between the concentration of corrosion inhibitor and the required corrosion protection current density for cathodic protection when the corrosion protection rate is 95% or more.
This is the result of immersing a mild steel piece in C1 artificial seawater for 7 days while sucking air at 133 rl/min.

The results were compared using an organic phosphoric acid type (HEDP soda) and an inorganic phosphoric acid type (SHP) as corrosion inhibitors.

As shown in the figure, the relationship between the corrosion inhibitor concentration required to obtain a corrosion protection rate of 95% and the corrosion protection current density is better for organic phosphates than for inorganic phosphates, and the corrosion inhibitory concentration remains constant. If so, the required corrosion protection current density is sufficient to be about 1/2, and the current density is less than 1/10 compared to the case where no inhibitor is added. It is very clear that this is more pronounced in organic phosphates.

In general, high molecular weight phosphoric acid corrosion inhibitors combine with calcium in seawater to form colloidal cations, and when electrolytic protection is used together, it is thought that they tend to form an anticorrosive film at the cathode.

In particular, organic phosphonic acid-based corrosion inhibitors combine with zinc to inhibit scale crystal growth, prevent coarsening, and promote densification of the film, increasing coating effectiveness and improving corrosion protection. This not only makes the corrosion prevention current density even more pronounced.

In addition, organophosphate corrosion inhibitors are hardly decomposed even when the pH of the aqueous solution increases;
This is thought to be the reason for obtaining a higher corrosion protection rate than inorganic phosphoric acid systems when used in combination with cathodic protection, which is accompanied by an increase in corrosion protection.

Example 2 Table 1 shows a comparison of the effects with the corrosion prevention method of the present invention under seawater ballast tank environmental conditions.

As the organic phosphoric acid corrosion inhibitor, 1-
Hydroxyethylidene-1,1-diphosphonate (HE
For comparison, inorganic polymerized phosphate,
Specifically, sodium hexametaphosphate (SHP) and sodium nitrite, as well as organic sodium sulfonate and triethanolamine were used, and artificial seawater was used for a period of 10 days.
These are the test results for a total of 100 days with a ballast ratio of 50 days (temperature 40° C.), a drainage period of 10 days (temperature 35° C., humidity 93%), and 5 ballast cycles.

In the external power source method for cathodic protection, a predetermined DC current controlled from an external DC power source is applied using a platinum electrode, and in the galvanic anode method, a zinc anode, a magnesium anode, and an aluminum anode are used in the same way. A constant current was applied for a predetermined period of time.

From this result, in the case of a corrosion prevention method using a corrosion inhibitor alone, 200
At a concentration of ppm, the inorganic type has a higher anticorrosion effect than the organic type, but the corrosion protection rates are 57% and 44%, respectively, which cannot be said to be sufficient.

In particular, in the case of inorganic systems, according to past experiments by the inventors of the present application, in static seawater conditions at room temperature, addition of 200 ppm will reduce the
It is clear that a corrosion protection rate of 5% or more can be obtained, and even in the case of 40C, a corrosion protection effect of 85% or more can be achieved.

On the other hand, the corrosion conditions in this test where ballast seawater is repeatedly filled and drained are severe, and the corrosion protection rate is extremely low at 44%.

In addition, in the case of cathodic protection alone, at a corrosion protection current density of 80 mA/m2, which is the design value for a current actual ship, the corrosion protection rate with zinc galvanic anodes exceeds 60%, but with the external power supply method, the corrosion protection rate is as low as 45%.

However, when a corrosion inhibitor and cathodic protection method are used in combination, the corrosion protection rate is higher than that of each corrosion protection method alone, and a synergistic effect is recognized by the combination. A combination corrosion prevention method that applies the electrode anodic method is recognized to be more effective.

That is, in the combination corrosion prevention method of the present invention, 20 ppm of organic phosphate (HEDP soda) and 40 mA of zinc galvanic method are used.
/m2, a corrosion protection rate of 83% is obtained.

On the other hand, when other galvanic anodes, such as magnesium anodes or aluminum anodes, are used together, the corrosion protection effect is significantly inferior to that of the present invention, with the magnesium anode showing only 51% and the aluminum anode 48%. The film has drawbacks such as being porous, discontinuous, and having poor adhesion.

Furthermore, when the external power supply method was also used, film formation was insufficient and the corrosion protection rate was 67%.

On the other hand, in a comparison using the zinc galvanic method at 50 mA/n2 in combination with various corrosion inhibitors, it was found that when used in combination with inorganic sodium nitrite, 1
Even if 1000 pp is added, only 47% corrosion protection rate is obtained, 30% when used in combination with organic sodium sulfonate, and 33% when triethanolamine is used, which is almost ineffective.

Although effectiveness is recognized when used in combination with inorganic SHP, the combined use effect of the present invention cannot be achieved even when the additive concentration and anticorrosive current density are higher than when used in combination with HBDP.

In the end, electrolytic protection using zinc galvanic anodes is expected to be more effective in conventional seawater than any other combination of various anodes used for cathodic protection and various corrosion inhibitors considered to be effective in normal seawater. The combination corrosion prevention method of the present invention, which combines an organic phosphate corrosion inhibitor, which was thought to be impossible, showed the highest corrosion protection rate and was the most advantageous.

As described above, the corrosion protection method of the present invention exhibits a higher corrosion protection effect than the conventional method under corrosive environmental conditions in which the tank has a drainage period, and the residual effect of the corrosion protection film extends even during the drainage period when no anticorrosive current flows. It shows.

In addition, in the corrosive environment of a ballast tank where the period of one water filling is as short as about 10 days and the ballast rate is about 50%, as in this test condition, the current cathodic protection method has a corrosion protection rate of about 50%. A value of about 60 to 70 is not sufficient, and anti-corrosion coatings have drawbacks in terms of workability and cost, and improvements have been desired.

Furthermore, as mentioned above, the organophosphate corrosion inhibitor of the present invention is not susceptible to decomposition, but the fact that the amount added is small and decomposition is slow is said to reduce the amount of phosphorus discharged during drainage. This can be said to have a great advantage in reducing the tendency towards eutrophication in rivers, ports and oceans.

In this respect, the present invention is a corrosion prevention method that is significantly more advanced from the standpoint of environmental conservation than the application of conventional inorganic phosphate corrosion inhibitors.

Example 3 In a new corrosion prevention method that uses the organophosphate corrosion inhibitor of the present invention in combination with a zinc galvanic anode, change in performance of the zinc galvanic anode when the concentration of the organophosphate corrosion inhibitor is varied An example is shown in Table 2.

Artificial seawater 25C1 Anode current density 0.1 in static state
These are the results of a constant current test in which a low current of mA/cmd was applied for 7 days.

As a corrosion inhibitor, the same 1-hydroxyethylidene-1,1-diphosphonate (HEDP soda) as in Example 2 was used.
was used.

20 to 1100pp compared to the case without corrosion inhibitor added
By adding an organic phosphate corrosion inhibitor, the anode potential was increased to 2.
It showed a base value of 5 to 45 mV, and the anode efficiency was improved by 6%.

It was also revealed that the adhesion products on the anode dissolution surface were significantly reduced, and as a synergistic effect, the performance of the zinc galvanic anode was improved.

This is because the organic phosphoric acid corrosion inhibitor suppresses hydrolysis of eluted zinc, prevents a drop in pH of the solution, suppresses self-corrosion of the anode, and contributes to improvement of anode efficiency.

At the same time, it prevents the dissolved products from depositing on the surface of the anode and prevents the anode potential from increasing.

As described above, the present invention adds an extremely low concentration of an organic phosphate corrosion inhibitor, which conventionally has no corrosion inhibiting effect when added alone, to seawater or a relatively highly conductive and highly corrosive aqueous solution. This is a new corrosion protection method whose main feature is that it enables corrosion prevention with an unexpectedly high corrosion protection rate by combining low current density cathodic protection using a zinc galvanic anode. It can be applied in aqueous environments with a wide pH range, exhibiting a unique synergistic corrosion protection effect on the cathode surface where the pH has increased due to cathodic protection, and is also suitable for corrosion conditions such as ballast tanks, which are repeatedly filled and drained. By combining the cathodic protection method, which has a large corrosion protection effect under still water conditions, and the corrosion protection method with corrosion inhibitor addition, which is effective under flowing conditions, each of them exhibits its characteristics and compensates for its shortcomings. It has the characteristic of uniformly obtaining high corrosion prevention effects even on the back side of the member, gaps, flow velocity areas, etc.

Moreover, in the combined corrosion prevention method of the present invention, the amount of the organic phosphate corrosion inhibitor added is very small and is difficult to decompose, so that problems of water pollution and eutrophication caused by wastewater can be solved.

As described above, the present invention provides an innovative composite anticorrosion method based on unique functions and effects, and provides a technology widely useful to the industrial world.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between corrosion inhibitor concentration and corrosion rate when a corrosion inhibitor is added alone. FIG. 2 is a diagram showing the relationship between corrosion inhibitor concentration and corrosion protection current density when a corrosion inhibitor and a zinc anode are used together.

Claims (1)

[Claims] 1. A metal corrosion prevention method characterized by adding an organic phosphate corrosion inhibitor in seawater or corrosive water with high comparative conductivity, and using electrolytic protection using a zinc galvanic anode.