JPS587712B2 - Metal corrosion prevention method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing corrosion of metal surfaces constituting cooling water systems, ship ballast tanks, and other metal containers or equipment that come into contact with corrosive liquids.

Conventionally, various methods such as cathodic protection, metal coating, painting, and anticorrosive agents have been put into practical use to prevent corrosion on the surfaces of metal devices that store or use liquids.

In order to reduce the rate of progress of metal corrosion, it is necessary to suppress both the anodic and cathodic processes in the corrosion reaction.
Although ideal, corrosion protection can be achieved even if either process is suppressed.

Cathodic protection methods include an external power supply method in which a corrosion protection current is applied to the metal surface to be protected using an external DC power supply; There is a galvanic anode method which generates an electric current by short-circuiting and prevents corrosion, and materials such as zinc, aluminum, and magnesium are used as the latter galvanic anode.

Also, among the anticorrosion agents, various corrosion inhibitors are used, which suppress the corrosion of metals in the environment by adding an appropriate amount to a corrosive liquid.

Many types of corrosion inhibitors have already been identified, but those that are put into practical use are quite limited, and their mechanisms of action are often not clear. It is classified into , precipitation film type, adsorption film type, etc.

Oxide film-type corrosion inhibitors are often oxidizing agents that directly oxidize the metal surface to form a metal oxide film to inhibit corrosion reactions, such as nitrites and chromates.

In this case, the corrosion reaction rate decreases due to a so-called passivation phenomenon that moves the corrosion potential (natural potential) of the metal in a noble direction.

Precipitated film-type corrosion inhibitors are soluble in water, but interact with other coexisting ions in the water to form salts that are sparingly soluble or insoluble in water on the metal surface, suppressing corrosion reactions through precipitation or a precipitated film. Examples include polymerized phosphates, phosphates, carbonates, silicates, and benzoates.

Adsorption film-type corrosion inhibitors are organic compounds with polar groups that can be adsorbed onto metal surfaces, and include amines, sulfonic acids, and various surfactant-based agents.

Among the above-mentioned various corrosion prevention methods, in the cathodic protection method, when the object to be protected against corrosion has a relatively simple structure and shape, a uniform current distribution can be obtained and a high corrosion prevention effect can be achieved.

However, in cases where there are many various parts inside the container, a cooling water system with insufficient conductivity, or a ballast tank that is repeatedly filled and drained, the current may not reach a sufficient level, or there may be periods when no current is supplied. In some cases, it may not be possible to obtain a satisfactory corrosion protection effect.

On the other hand, even in the case of corrosion protection using corrosion inhibitors, in recent years most of the chemicals have had to be discontinued or their usage has been forced to be reduced in order to prevent environmental pollution due to their toxicity during drainage.

The present invention was discovered as a result of a series of studies aimed at solving the recent difficulties in corrosion protection technology against the above background.
By using a cathodic protection method using a zinc anode together with an appropriately selected precipitated film type corrosion inhibitor, the amount of corrosion inhibitor used can be reduced, and the cathodic protection current can also be saved. This makes it possible to obtain higher corrosion protection effects than when using either of the cathodic protection methods alone.

When a corrosion inhibitor is maintained at a sufficient concentration during its application, it is possible to obtain a satisfactory high corrosion protection effect.

However, it is known that when used below the required concentration limit, not only is the anticorrosion effect insufficient, but the corrosion is localized, causing so-called localized corrosion.

This is the most difficult point in applying corrosion inhibitors, and this is the point that requires the most care when using them.

Depending on the situation, this tendency to localized corrosion can lead to the growth of pitting corrosion, which poses the danger of perforation in containers and piping systems that are to be protected against corrosion.

Cathodic protection is a process in which when the metal to be protected has a difference in surface potential due to material, environment, etc., a current selectively flows into the more noble potential areas and eventually equalizes the potential of the entire surface. It is well known that corrosion is suppressed by eliminating the potential difference that causes corrosion.

When this corrosion inhibitor and cathodic protection are used together, the metal surface shifts to a negative potential due to the cathodic protection action, and the positively charged corrosion inhibitor is discharged or deposited on the cathodic metal surface, forming an anticorrosion coating. form.

At this time, it is generally known that an increase in cathode polarization on the surface of the cathode increases the pH and is therefore more effective in forming a protective film.

However, in environments with high concentration chloride solutions such as seawater, corrosion inhibitors of the oxide film type and adsorption film type hardly form a good corrosion protection film, and even when cathodic protection is used together, a large amount of corrosion inhibitor Despite the addition of , no significant anticorrosive effect was observed.

Here, a corrosion protection method has been discovered that uses a precipitated film-type corrosion inhibitor and electrolytic protection, which can form an effective corrosion protection film in seawater.

In particular, it has been found that when a galvanic anode method using a zinc anode is applied to cathodic protection, the zinc in the solution participates in the reaction, covering the active surface of the cathode and enhancing the corrosion inhibition effect.

For the above reasons, in environments with high concentration chloride solutions, even if the concentration of the precipitated film-type corrosion inhibitor used is insufficient to achieve corrosion protection alone, cathodic protection using a zinc anode should be used in combination. In some cases, the two have a synergistic effect, which not only eliminates the local corrosion mentioned above, but also provides a high corrosion protection effect at a significantly lower current density than when cathodic protection is applied alone.

In particular, looking at recent developments, the use of low concentrations of corrosion inhibitors is not only becoming an unavoidable trend from the standpoint of preventing environmental contamination and pollution, but is also advantageous in terms of corrosion prevention costs, and at the same time reduces the required current for cathodic protection. The corrosion prevention method of the present invention has the effect of reducing the scale of the power supply device, the anode, etc. and increasing the life span thereof, and the corrosion prevention method of the present invention has great industrial and economic benefits.

Next, Table 1 shows examples of the combined anticorrosion method of the present invention together with comparative examples.

Table 1 shows the corrosion inhibitor concentration and corrosion protection current density for 7 days of cathodic protection alone, corrosion inhibitor alone, and both in combination for steel plates using artificial seawater and applying a weak flow to the liquid by blowing air at 25°C. and a comparison of their anti-corrosion effects.

For cathodic protection, in addition to a zinc anode, for comparison, an aluminum anode and a magnesium anode were used in the galvanic anode method, and a platinum electrode was used in the external power supply method.

In addition to polymerized sodium phosphate, corrosion inhibitors include
For comparison, sodium nitrite and sodium chromate were used for the oxide film type, and triethanolamine and sodium sulfonate were used for the adsorption film type.

As is clear from Table 1, in order to obtain a corrosion protection rate of 90%, a corrosion protection current of 180 mA/m2 is required when using the external power source method, and 180 mA/m2 when using the galvanic anode method using a zinc anode.
It requires 10mA/m2.

Further, when adding a corrosion inhibitor using polymerized sodium phosphate, an addition concentration of 200 ppm is required.

With cathodic protection alone, if the corrosion protection current is below these values, not only will sufficient corrosion protection not be obtained, but corrosion will remain in some areas, and even with corrosion inhibitors alone, if the concentration is insufficient, corrosion may become localized. It's happening.

In addition, when an external power supply method using platinum electrodes and a corrosion inhibitor using polymerized sodium phosphate are used together as a cathodic protection method,
Although a considerable reduction in current is observed, a corrosion protection current that is 3 to 4 times higher is required compared to when a zinc anode is also used.

This current reduction when a zinc anode is used together indicates the contribution of zinc ions eluted from the zinc anode to the corrosion inhibiting effect.

Similarly, when cathodic protection using an aluminum or magnesium anode is combined with a corrosion inhibitor using polymerized sodium phosphate, the current reduction is not as remarkable as when using a zinc anode, and the anticorrosion film is porous, discontinuous, and adhesive. It has disadvantages such as poor performance.

On the other hand, when cathodic protection using zinc anode is used together with sodium nitrite, sodium chromate, triethanolamine, sodium sulfonate, etc., the corrosion inhibitor
Even if added in a large amount of 000 ppm, the corrosion protection rate will be 32 to 65% at a current density of 20 mA/m2, and no corrosion protection effect can be expected at all.

On the other hand, in the corrosion prevention method using a corrosion inhibitor using polymerized sodium phosphate and a zinc anode according to the present invention, even when the corrosion inhibitor is added at a low concentration of 10 to 150 ppm,
95-100 if used with ~15mA/m2 anti-corrosion current
%, and the anticorrosive film has features such as being dense, continuous, and having good adhesion.

In this combined corrosion protection, the higher the concentration of the corrosion inhibitor added, the lower the required current for corrosion protection, but it goes without saying that the most economical combined conditions exist in between.

Next, Table 2 shows the effect on the performance of zinc anodes when used in combination with corrosion inhibitors.

As a result of flowing a low current with an anode current density of 0.1 mA/m2 for 30 days in a static solution at 25°C using artificial seawater,
The zinc anode improves the anode current efficiency by 4 to 6% compared to the case of cathodic protection alone, and the anode potential changes by 15 to 30 mV.

The precipitated film type corrosion inhibitor used in this case was polymerized sodium phosphate.

The reason for this improvement in anode performance is that the eluted zinc is used to form a complex with the corrosion inhibitor, which reduces the hydrolysis of zinc ions and suppresses the drop in pH near the anode. This is recognized to have contributed to reducing self-corrosion and improving current efficiency.

At this time, the anode potential becomes the most basic when the concentration of the corrosion inhibitor added is low, and the degree of basification is small when the concentration is high.

Conversely, when added in large amounts, it exhibits a noble potential.

It is thought that lowering the potential reduces self-corrosion and reduces the amount of dissolved products that cause film resistance, thereby preventing the potential from becoming nobler.

In addition, pitting corrosion and dissolution of the zinc anode is observed when used in combination with high concentration addition, but when used in combination with low concentration, there is uniform dissolution and no pitting corrosion, and there is no concern that the anode life will be shortened.

It should be noted that the corrosion prevention method of the present invention can be similarly used for precipitated film-type corrosion inhibitors other than polymerized sodium phosphate, such as sodium phosphate, sodium silicate, sodium benzoate, and soda carbonate. It was similarly effective in an electrolyte solution with high conductivity, for example, fresh water of 3000 Ω·cm.

As detailed above, the corrosion protection method of the present invention, which uses a precipitated film type corrosion inhibitor in combination with a cathodic protection method using a zinc anode, enables the use of a significantly lower concentration than when a corrosion inhibitor is used alone. , and can be applied at a significantly lower current density than when cathodic protection is used alone, and it can achieve a higher corrosion protection effect than either of these methods alone, and can prevent localized corrosion. Since it can also reduce the impact on the environment when using an inhibitor, it is an extremely useful, economical and effective corrosion prevention method, and can be said to have great industrial benefits.

Claims (1)

[Claims] 1. A metal corrosion prevention method that uses a combination of a corrosion inhibitor that forms a precipitated film in seawater or an electrolyte with relatively high conductivity and cathodic protection using a galvanic zinc anode.