JPS6191541A - Test method and apparatus for promoting corrosion undercover - Google Patents

Abstract

PURPOSE:To enable the forecasting with the reliability almost equivalent to the actual situation, by exposing a covered metal material to a heating corrosion medium balanced with an atmospheric gas having a higher oxygen partial pressure than the atmospheric air. CONSTITUTION:A test piece 1 is supported by a test piece support means 4 in a medium tank 3 holding a corrosion medium 2. The medium tank 3 is set in a thermostatic cell 5 with which the corrosion medium 2 is kept at a constant temperature, where the corrosion medium 2 comes in contact with an atmospheric gas 6 with a counterflow device 7 so that the atmospheric gas 6 is allowed to be balance dissolved in the corrosion medium 2. The counterflow device 7 feeds the medium 2 to the medium tank 3 through a medium discharge tube 9 provided in the medium tank 3 by way of a reflax means such as pump. The use of such an apparatus enables the forecasting of the long-term durability of the anti-corrosive cover.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention provides an accelerated under-coating corrosion test method and method for accelerating evaluation of the durability of coated metal materials for long-term corrosion protection. The present invention relates to a test device suitable for carrying out this method.

(Prior Art) In general, various types of corrosion protection are applied to the inner surfaces of offshore structures, underground structures, and transport pipes to prevent corrosion damage to these structures over a long period of over 20 years. A coating is applied. Currently, the most commonly used anticorrosion coatings are epoxy coatings, polyethylene linings, and other impervious resin coatings.

Some of these resin-based coatings have already been used for over 20 years, and have received considerable trust. There is also a growing tendency for usage history to be used as a measure of reliability. Emphasizing performance can be said to be an extremely safe strategy, but there are many situations in which this strategy alone cannot be used. For example, measures are needed to compensate for the lack of actual results, such as estimating durability under conditions slightly different from actual results, or predicting lifespan when a promising new coating is used.

One of the strategies is to obtain universal knowledge about the deterioration of resin-based coatings, to accelerate the deterioration in a reasonable manner, and to predict its life.

Incidentally, the situations in which resin-based coating materials are used are roughly divided into those that receive sunlight and those that do not. Sunlight can be said to be the biggest deterioration factor for resin-based materials, and if resin is left exposed to sunlight without any countermeasures, it will crack and break down in a short period of time.

However, because deterioration caused by sunlight is so serious, technologies such as acrylic resin topcoat paints and carbon black to improve the weather resistance of polyethylene have been developed, and now weather resistance is no longer a factor that determines the lifespan. It is coming. Aside from the issue of weather resistance, it is generally possible to select a durable resin based on the research results of various resins as single materials to prevent environmental deterioration of the pond.

The environment that requires corrosion protection generally contains oxygen and water, and the resin coating for corrosion protection has the property of allowing this oxygen and water to permeate, albeit gradually. , the adhesive interface between the coating material and the metal surface deteriorates. Regarding the fact that this deterioration is actually a factor controlling the lifespan, see, for example, Progress in Orga published in 1981.
nic Coatings, vol 9. NILl,
It is described on pages 29 to 46, and the inventors of the present invention generally agree with this.

Now, assuming that the deterioration of the bonding surface between the coating material and the metal is within the durability control criteria, a means for accelerating and evaluating this deterioration is required. The phenomenon of deterioration of the interface described above can first be attributed to a decrease in adhesive strength mainly due to permeated moisture. As a means of accelerating and evaluating this deterioration, a temperature gradient deterioration acceleration test is useful, as described in the Proceedings of the 107th Conference of the Iron and Steel Institute of Japan, published in 1984, page 8448.

Also, an equally important interface deterioration phenomenon is corrosion under the coating. This occurs due to oxygen and moisture permeating the coating material onto the steel surface beneath the coating.

Testing methods for such corrosion include gas-phase corrosion resistance tests targeting particularly corrosive gases such as hydrogen sulfide and sulfur dioxide gas, or introducing an aqueous solution and high-pressure corrosive gas into an autoclave. These tests are commonly conducted and are described in "Publastic Corrosion Prevention Technology" published by Nikkan Kogyo in 1981, pages 50-70, but these tests are beyond the scope of comparative evaluation tests. However, the test results cannot provide quantitative knowledge regarding the life span of coating corrosion.In other words, in the prior art, no truly appropriate means for accelerating coating corrosion has been found.

(Problems to be Solved by the Invention) The present invention solves the
The present invention provides a coating underlayer corrosion acceleration test method and apparatus based on the knowledge that coating corrosion can be accelerated at a desired rate.

(Means and operations for solving the problems in question 2) The gist of the present invention is to expose a coated metal material to a corrosive medium at elevated temperature equilibrated with an atmosphere having a higher partial pressure of oxygen than the atmosphere. A test method for accelerated corrosion under coating, characterized by
There is a device for embodying the method.

The present invention will be explained in detail below.

First, in the present invention, coated metal materials are various steel materials, plated steel materials, metal sprayed steel materials, non-ferrous metal materials, or metal materials that have undergone chemical conversion treatment, and are used to prevent corrosion of these metal materials over a long period of time. For purposes such as coating with epoxy, polyolefin, and polyolefin resins,
Refers to materials coated with resin mortar, polymer cement mortar, etc.

Next, in the present invention, the test is carried out by exposing the coated metal material in a corrosive medium, and the corrosive medium referred to here may be, for example, pure water, fresh water, tap water, or seawater. ,
A general term for aqueous solutions containing salts, acids, alcohols, etc., or those in the form of a shower, mist, or vapor phase. The means for placing the metal material in the medium or pouring the medium over it can be selected as appropriate depending on the environment in which the coated metal material is to be used.

Now, in the present invention, the main point is to raise the temperature of the corrosive medium that strips the coated metal material and to balance it with a partial pressure of 2m higher than that of the atmosphere. , based on the finding that coating corrosion can be accelerated by a desired factor by increasing the oxygen partial pressure of the equilibrium atmosphere.

Such knowledge was obtained through the following experiment.

In other words, Figure 1 shows the oxygen permeability characteristics of a 0.4 M thick cured film made from powdered epoxy paint, one of the typical coating materials, measured by the isobaric method and plotted against temperature. It is something. As can be seen from the figure, the oxygen permeation rate of the coating material quadrupled by raising the temperature of the system from room temperature to 50°C.

Since it is already known that the oxygen permeation rate through a resin membrane is roughly proportional to the oxygen partial pressure on the inlet and outlet sides, the equilibrium atmosphere surrounding the coated metal material is normally atmospheric (oxygen content). pressure 0.2 at, m ) to pure oxygen (oxygen partial pressure 1 at
m), the oxygen permeation rate of the coating increases approximately five times. If this is multiplied by 4 times due to temperature rise, it is expected that coating corrosion will be accelerated by a total of 20 times.

In this section 5, an example regarding a specific temperature increase and a specific oxygen partial pressure was described, but by changing the degree of temperature increase and the oxygen partial pressure, the promotion rate can be set in any manner. That is, by raising the temperature appropriately and increasing the oxygen partial pressure in the equilibrium atmosphere, coating corrosion can be accelerated to a desired rate.

In addition, in order to cause corrosion under the coating, it is necessary to supply moisture to the oxygen pond. As previously discussed, this provision is accomplished by the passage of environmental moisture through the coating. For this purpose, the moisture conditions in the environment must be appropriately set. For example, for applications where the test piece is immersed in the sea, the test piece is immersed in saline water or artificial seawater as a corrosive medium, and for the inner surface of a water pipe, the test piece is immersed in tap water as a corrosive medium. After raising the temperature of the liquid, the atmospheric gas having the partial pressure of oxygen is equilibrated.

In addition, if artificial seawater or fresh water equilibrated with atmospheric gas with high oxygen partial pressure is sprayed in a pattern as a corrosive medium, and the test piece is exposed in this medium, it can be used as a spray zone in the ocean or on land. This is an accelerated test that corresponds to a rainy environment.

However, when the corrosive environment contains, for example, glycol,
The liquid may be used as a corrosive medium.

This will allow accelerated testing in an environment where the saturated water vapor pressure has dropped to suit the actual situation. Furthermore, in order to promote and evaluate durability in the atmosphere of a region with little rainfall, for example, an atmosphere with relative humidity representative of the local environment is created9, the oxygen partial pressure of this atmosphere is increased, and this is used as a corrosive medium. Expose the test piece in this.

In order to equilibrate the corrosive medium with the atmospheric gas having a high oxygen partial pressure, for example, if the corrosive medium is an aqueous solution, it is desirable to bubble the atmospheric gas into the liquid and, preferably, to countercurrent the water flow and the gas flow. It is a good idea to add

In addition, in order to create a constant humidity atmosphere with a high oxygen partial pressure, for example, a solute may be added to an aqueous solution to lower the saturated water vapor pressure, or the temperature may be lowered so that the saturated water vapor pressure is lower than that of the test chamber temperature. A high oxygen partial pressure atmospheric gas may be brought into contact with fresh water by a method such as bubbling, and this atmospheric gas may be introduced into the test chamber. Furthermore, the temperature within the test chamber can be increased or controlled in the same manner as in a normal thermostat.

FIG. 2 is a front view showing an embodiment of the device of the present invention.

In the figure, reference numeral 1 denotes a test piece, which is supported by a test piece support means 4 in a medium tank 3 containing a corrosive medium 2. The medium tank 3 is installed in a constant temperature bath 5 that keeps the corrosive medium 2 at a constant temperature, and the corrosive medium 2 and the atmospheric gas 6 come into contact with each other in the countercurrent device 7, so that the atmospheric gas is dissolved in equilibrium in the corrosive medium. I am made to do so.

The countercurrent device 7 feeds the medium 2 into the medium tank 3 from a medium discharge pipe 9 provided in the medium tank 3 via a circulation means 8 such as a pump, for example. In addition, although not shown, various mechanisms such as a temperature measuring device for measuring the temperature of the medium and a temperature control mechanism for controlling the temperature of the constant temperature bath according to the temperature change can be added as appropriate. Needless to say.

The above explanation has been made regarding the case where the medium 2 is liquid, but if the medium 2 is in the form of a picture or vapor, the countercurrent means may be changed appropriately depending on the properties of the medium. .

As the atmospheric gas 6, a gas with a controlled oxygen partial pressure is used, and in addition to pure oxygen, a mixed gas of oxygen and nitrogen in a desired ratio can be used as appropriate. Further, as the test piece supporting means 4, a pedestal or a hanging mechanism as shown in the figure can be appropriately employed.

EXAMPLES Hereinafter, the effects of the present invention will be illustrated in more detail with reference to Examples.

(Example) Using the apparatus shown in FIG. 2, trichloride was circulated as a corrosive medium, the system temperature was set at 50° C., and pure oxygen was flowed at 250 ml/min as an equilibrium atmosphere gas.

Next, a test piece of 2.3 t x 50 was placed in this medium tank.
After grid plasting, a steel plate of X I OO (rxs) was coated with the following four types of coatings A, B, C, and D, and then immersed for 1.5 months.

A Nigaras flake polyester 1.3UB
: Epoxy resin mortar 3.2°C: Polyester resin mortar 32°C: Polyurethane paint 4.6u These test pieces were also immersed in actual seawater for 30 months.

The test piece immersed in both of the above conditions was withdrawn, the amount of oxygen under the coating was analyzed, and the corrosion product was determined to be Fe2O3/H2O, which was converted to the corrosion thickness of the steel plate, and the accelerated test was conducted for 1.5 months. A comparison of the amount of corrosion of the coating after 30 months of underwater testing resulted in the results shown in Figure 3, and it was recognized that corrosion could be accelerated by 20 times compared to actual underwater testing.

(Effects of the Invention) As is clear from the above examples, the method and apparatus of the present invention make it possible to accelerate coating corrosion at a desired rate. As a result, it has become possible to predict the long-term durability of new or improved anticorrosive coatings with a reliability close to the actual situation.

In addition, it will make a great contribution to fields where data accumulation is slow, such as the corrosion fatigue problem of metal materials coated with anti-corrosion coatings, because it takes a long time, and the industrial effect will be greatly improved. Very noticeable.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a custom-made oxygen permeation row of resin membranes, Fig. 2 is a front view showing an embodiment of the device of the present invention, and Fig. 3 shows the correlation between the method of the present invention and the underwater immersion test. This is a diagram. 1: Test piece 2: Corrosive medium 3: Medium bath 4: Test piece support means 5: Constant temperature chamber 6: Atmospheric gas 7: Countercurrent device 8
: Reflux means 9: Medium discharge pipe Fig. 1 C'Cn 103/T (l/k)

Claims (1)

[Scope of Claims] 1. An under-coating corrosion acceleration test method characterized by exposing a coated metal material in a heated corrosive medium equilibrated with an atmospheric gas having an oxygen partial pressure higher than that of the atmosphere. 2. A countercurrent device for equilibrating the corrosive liquid with an atmospheric gas having a higher partial pressure of oxygen than the atmosphere, a corrosive liquid tank connected to the countercurrent device, a constant temperature bath surrounding the corrosive liquid tank, and a liquid in the corrosive liquid tank. An under-coating corrosion acceleration test device comprising a reflux device for sending the flow to the countercurrent device.