JP2008261813A - Corrosion resistance evaluation method of coated steel product, and compound corrosion resistance evaluation method of coated steel product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a simple and accurate evaluation method of corrosion resistance of a coated steel product in an environment near a ground. <P>SOLUTION: The corrosion resistance evaluation method of the coated steel product tests the corrosion resistance of the coated steel product by immersing the coated steel product as an evaluation object for a predetermined time in a test solution with pH of 5.0-11.0 containing 0.02-2.0 mass% of ammonia and 0.1-2.0 mass% of metallic chloride in terms of chloride ion concentration. The accurate corrosion resistance of the coated steel product embedded partially in concrete or the ground and used can be evaluated simply and rapidly, and efficiency of development of an adequate corrosion resistant structure of the coated steel product can be improved. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a corrosion resistance evaluation method for coated steel materials and a composite corrosion resistance evaluation method for coated steel materials, and more particularly to a corrosion resistance evaluation method for coated steel materials used by being partially embedded in concrete or the ground.

  Zinc-based plating is widely adopted as a technique for preventing corrosion of steel materials. In order to further increase the anticorrosion ability, an organic resin coating is often applied on the galvanizing. In this case, it is also well known that the adhesion between the plating and the organic resin is not always good even if the coating is directly applied on the zinc plating. For this reason, phosphate treatment, chromate treatment, etc. The so-called chemical conversion treatment is generally performed.

  Steel structures including such coated steel materials are often used outdoors in the form of being embedded in concrete or the ground. However, it is known that steel structures that are used in this way have severe corrosion directly above the buried portion, that is, at the ground. Hereinafter, this phenomenon is referred to as “ground corrosion”.

The following factors are cited as the cause of this “ground corrosion”.
I) Steel material in concrete is passivated by alkali of concrete. This passivated part and the iron on the surface of the earth form a local battery.
II) The ground part has a structure that is easy to get wet because dew condensation on the steel pipe column falls, and the condensed water has agglomerated salt and the like adhering to the steel pipe column.
III) Animal excrement, insect carcasses, etc. are decomposed by microorganisms to produce ammoniacal nitrogen. This ammoniacal nitrogen dissolves galvanizing and phosphate chemical conversion treatment of steel materials.

As a method for evaluating the corrosion resistance of a steel material having such a corrosion-resistant film, various methods have been implemented.
1) Salt spray test (hereinafter SST) (JIS Z 2371)
This is a method of continuously spraying 5% NaCl water having a pH of 7.0 at 35 ° C., which is easy but has a low correlation with actual outdoor corrosion.
2) Car body exterior corrosion test method defined by the Japan Automobile Manufacturers Association (Non-patent Document 1)
A corrosion cycle test involving spraying 5% NaCl water for 2 hours, with a wetting time of 50% of the total test time.
3) Automobile exterior corrosion test method defined by Society of Automotive Engineers (hereinafter SAE) (Non-patent Document 2)
A test method using a mixed solution of NaCl and CaCl ₂ assuming corrosion by a snow melting agent.
4) Test methods for building materials for acid rain regions in Japan (ISO TC1556 data)
Test method using artificial seawater (ASTM D 1141). Contains NaCl, CaCl ₂ , MgCl ₂ , sulfate radical, nitrate radical.

As can be seen from the above examples, as a method for evaluating the corrosion resistance of a steel material having an anticorrosive film, all of them are mainly chloride ions (hereinafter referred to as Cl ⁻ ) at various temperatures or temperature cycles and at various humidity or humidity cycles. Most of them investigate corrosion due to). Although some have investigated the effect of changing the type of metal ions, it can be considered that corrosion proceeds mainly by chloride ions.

  As an exception, there is known an example in which a corrosion test is performed under a condition that reproduces the environment of ground corrosion as it is (see, for example, Patent Document 1). However, this method requires a long test period of 6 months. Of course, it is considered possible to shorten the test period by taking measures such as placing the test piece in a high-temperature bath. However, the activity of microorganisms is not necessarily increased by raising the temperature, and there are also negative factors such as ammonia being easily diffused. For this reason, it is difficult to greatly shorten the test period without impairing the reliability of the actual environment.

  In addition, as one of the methods for promoting corrosion of coated steel materials, a cycle test with changing the temperature and, in some cases, with freezing conditions is often used. Not suitable for. Moreover, when freezing conditions exist, problems, such as cracking of concrete, also arise.

JP 2006-132128 A JASO M610 Automotive Engineering Society "Automobile parts appearance test method" JASO M610-92 (1992) H. E. Townsend, Corrosion prevention, SP-1265, SAE, 53 (1997)

  For this reason, there is a need for an evaluation method that is simple and has a high correlation with the actual environment in a short period of time.

  However, as described above, in addition to chloride ions, there are two factors in ground corrosion: macro battery formation and dissolution of zinc / chemical conversion film by ammonia. For this reason, the conventional corrosion acceleration test as described above cannot reproduce the phenomenon of ground corrosion. In particular, the corrosion resistance in an environment where a special chemical substance called ammonia is present has not been sufficiently studied, and as described above, the actual situation is that the test is in a form that reproduces the actual environment. Needless to say, it is difficult to develop an effective anticorrosion method in the present situation where such a long-term test is necessary.

  As a case where corrosion in a special environment becomes a problem, there is a case where corrosion caused by an acid gas such as hydrogen sulfide is a problem in a steel pipe used for drilling or transporting crude oil. In this case, since it is only necessary to consider corrosion in such a special environment / substance, it is not difficult to cope with it and the test method. However, in the case of ground corrosion, special factors at the ground must be considered in addition to general corrosion in normal environments.

  An object of the present invention is to provide a simple and accurate method for evaluating the corrosion resistance of a coated steel material in a suburban environment.

In the present invention, attention was focused on ammoniacal nitrogen (hereinafter referred to as ammonia), which is one of the causes of ground corrosion. And as a result of investigating the possibility of dissolution of the plating / chemical conversion coating on the steel with ammonia, the possibility of ammonia formation in the soil, etc., the following was found.
i) Suspended humic soil in simulated urine water (pH: 6.4) made with salt and nitrogen-containing compounds such as uric acid, urea, and creatine, and left at room temperature (about 25 ° C) for 24 hours. analyzed. As a result, about 2000 ppm of ammonia nitrogen, which is hardly present in the simulated urine, was detected, and the pH rose to 9.3. From this, it was confirmed that ammonia is easily generated in a combination that seems to be common in the natural environment of humus + animal excrement.

Next, as an extreme example of a highly corrosive soil environment that produces ammonia, we investigated the compost production process and its analysis results.
ii) During the fermentation process of compost, ammonia nitrogen is first generated and the pH rises to a maximum of about 9.5. Further, it was found that when fermentation progresses, ammonia is oxidized to generate nitric acid, and the pH decreases on the contrary. Further, from the reported analysis results of compost, the pH takes a value of 5.0 to 10.0, and the dry mass ratio of compost is 0 to 1.6% ammonia nitrogen, nitrate nitrogen. Produces from 0 to 1.3%.

(Mass percentage from dry mass of compost)
iii) When the amount of zinc-ammonia complex (hereinafter, zinc ammine complex) is calculated, the sequential complex formation constant of the zinc ammine complex (log K ₁ = 2.18, log K ₂ = 2.25, log K ₃ = 2) .31, log K ₄ = 1.96), the amount of zinc ammine complex is rapidly increased from the vicinity of 0.1 mol / L of ammonia concentration, that is, zinc is likely to be dissolved by ammonia. I understood.

From these survey results, it was confirmed that the so-called “international corrosion” was caused by the fact that ammonia was generated by the action of microorganisms in the soil at the subsurface site, which dissolved zinc. As a result of further testing,
iv) A low concentration of ammonia alone has little corrosiveness to the surface-treated steel material, but in the presence of chloride ions, exhibits an effect of promoting corrosion,
v) When the pH is increased to 11 or more in order to accelerate the corrosion test, it is not realistic that the zinc phosphate chemical conversion coating film is rapidly dissolved by ammonia or the steel material is passivated and does not rust. End up in a corrosive environment,
vi) Locally severe corrosion occurs in the vicinity of the surface-treated steel material where humus is attached,
vii) In an ammoniacal environment, the occurrence of white rust normally observed when galvanized steel is corroded is not observed,
I understood.

Based on the above investigation results, a simple and quick method for accurately evaluating the corrosion resistance of a coated steel material used in a suburban environment in a special environment containing ammonia or producing ammonia has been established. That is, the present invention is as follows.
(1) In a test aqueous solution having a pH of 5.0 to 11.0 containing 0.02 to 2.0% by mass of ammonia and 0.1 to 2.0% by mass of metal chloride at a chloride ion concentration. Further, the corrosion resistance evaluation method for the coated steel material is characterized by testing the corrosion resistance of the coated steel material by immersing the coated steel material to be evaluated for a predetermined time.
(2) The method for evaluating corrosion resistance of a coated steel material according to (1), wherein the ammonia concentration is 0.1 to 1.0% by mass.
(3) The method for evaluating corrosion resistance of a coated steel material according to (1), wherein the metal chloride is an alkali metal chloride or an alkaline earth metal chloride.
(4) The corrosion resistance evaluation method for a coated steel material according to (1), wherein the pH of the test aqueous solution is 7.0 to 10.0.
(5) The method for evaluating corrosion resistance of a coated steel material according to (1), wherein the test aqueous solution further contains 0.1 to 2.0% by mass of a nitrate radical.
(6) The method for evaluating corrosion resistance of a coated steel material according to (5), wherein the concentration of the nitrate radical is 0.1 to 1.0% by mass.
(7) Subsequent to the corrosion resistance test according to any one of (1) to (6), a water vapor wetting test, a drying test, and a freezing test are sequentially performed on the coated steel material to be evaluated, and the corrosion resistance test, A method for evaluating composite corrosion resistance of a coated steel material, characterized by repeating a water vapor wetting test, the drying test and the freezing test a predetermined number of times.

  According to the present invention, it is possible to easily and quickly evaluate the accurate corrosion resistance of a coated steel material that is partially embedded in concrete or the ground, and further improve the efficiency of developing an appropriate anticorrosion structure for the coated steel material. it can.

  Hereinafter, the present invention will be described in detail.

  The evaluation method of the present invention is of two types: a simple immersion method and a method called a cycle test in which the temperature and humidity are changed.

  First, the evaluation method by the dipping method will be described. The immersion method is a test method in which a coated steel material is continuously immersed in an ammonia-containing aqueous solution in a stationary state. The conditions of the aqueous solution in this case are as follows.

  First, as described above, the ammonia concentration is estimated to have reached 0.2% by mass under the active condition of microorganisms even in an actual environment. At this concentration, the amount of zinc complex formed by ammonia is small, and the effect of ammonia directly dissolving zinc is small. However, when a corrosion test is actually conducted by immersing a galvanized steel coated with this ammonia concentration, the coating film peels off while almost no white zinc rust normally seen in the corrosion test of galvanized steel occurs. To do. For this reason, it turns out that corrosion of the galvanized steel material by ammonia is accelerated | stimulated also in this density | concentration. For this reason, 0.02 mass% which is a concentration lower than this ammonia concentration is made the lower limit of the test conditions. Actually, it is realistic as an accelerated test to be performed at a concentration of 0.2% by mass or more as described above. In the case where priority is given to the actual environment rather than the promotion, the content is set to 0.02 to 0.2% by mass. Next, the upper limit concentration is 2.0% by mass. This is about twice the maximum value of the amount of ammonia produced in compost, and was set higher in this way due to the nature of the accelerated test. Thus, the ammonia concentration is in the range of 0.02 mass% to 2.0 mass%. In an actual test, it is desirable that it is 0.1 mass% or more and 1.0 mass% or less.

  As the concentration of the other components, the chloride concentration is preferably the same as that of animal excrement, particularly the urine of mammals. Therefore, the chloride concentration is preferably 0.1 to 2.0% by mass. As the counter ion, it is desirable that sodium, potassium, calcium, magnesium and the like are mixed, but it may be said that only the salt concentration actually acts on corrosion. For this reason, as a simple method, the immersion bath may be adjusted only with sodium chloride.

  In addition, the nitrate radical had almost no effect on the dissolution of the zinc plating within the examined range, but there was a tendency for red rust to easily occur on the exposed iron surface due to progress of corrosion. For this reason, it is desirable to add nitrate radical, but it may be omitted when only the initial corrosion of the coated steel material is investigated. As for the density | concentration in the case of adding, 0.1-2.0 mass% is desirable similarly to the analysis result of compost. Similarly to the analysis result of compost, in an actual test, the concentration in the case of adding nitrate radicals is desirably 0.1% by mass or more and 1.0% by mass or less.

  The pH of the solution is between 5.0 and 11.0. The low pH is based on the analysis result of compost, and it is unlikely that the pH will be lower than that even in an actual suburban environment. The high pH side is 11.0, but if the pH is higher than this, the chemical conversion treatment becomes an unstable region. Since the environment of ground corrosion is open and difficult to reach a high pH, and the actual compost has a pH of 10 or less, the pH of the ground environment is considered to be about 10. Higher pH is more effective as an accelerated test, but in the pH region where the chemical conversion treatment is unstable, the peeling and corrosion of the coating may proceed due to a mechanism different from the actual local surface. 11.0. However, in terms of reality, it is actually desirable to set it to 10.0 or less. On the low pH side, a low value of 5 to 7 is observed in the compost environment, but it is preferably 7 or more in consideration of the effective concentration of ammonia, shortening of the test evaluation period, and the like. Therefore, the pH of the solution is limited to 5.0 to 11.0, and is preferably 7.0 to 10.0.

  The materials to be tested are steel materials that are embedded in the ground or concrete, etc., and usually coated steel materials by zinc-based plating, or zinc-based plating on steel materials, and phosphate treatment, etc. as necessary This is a coated steel sheet that has been subjected to a chemical conversion treatment and then painted. It is desirable that the test material is previously filled with wrinkles that reach the iron surface. In the case of an intact painted surface, the test period is too long until the evaluation result is known, which is not desirable. In this case, as is often done in corrosion tests, a thin streaky wrinkle is made using a cutter knife in an X shape, and the coating film / plating layer is stripped / removed in a line with a wide width of 1 mm or more, or A wrinkle such as a dent may be put by an appropriate method. In any case, it goes without saying that it is important to have reproducibility in the degree of wrinkles.

  When the prepared test piece is immersed in the above-mentioned ammoniacal solution, there are a method of immersing the entire test piece and a method of immersing only a part of the test piece. According to the test results so far, any method has almost no influence on the evaluation result of ammonia resistance.

  Test temperature shall be 10 degreeC or more and 40 degrees C or less. If it is less than 10 ° C., the progress of corrosion is slow and it takes too much test time. As an upper limit, it is first important that the temperature is equal to or lower than the Tg (glass transition temperature) of the coating film. In addition, even when the Tg of the coating film is high, considering that the higher the temperature is, there is a problem that ammonia tends to be volatilized and lost, and that the actual temperature in the soil is unlikely to be high. However, the limit is around 40 ° C. For this reason, when considering acceleration and compatibility with the natural environment, 25 to 35 ° C. is more appropriate.

In the test, particularly when the ammonia concentration is high, the ammonia odor is strong, so the solution system must be sealed in some way. In this case, there is no problem with the loss due to odor and volatilization of ammonia, but when oxygen is consumed in the corrosion reaction, the system becomes oxygen-deficient and the reaction may be delayed. For this reason, it is necessary to cancel the sealed state at the rate of about once a day or every two days when observing the test piece. Similarly, the absolute amount of ammonia is also required for the reaction to proceed. For this reason, the air and test liquid confined in the system to be sealed must have a sufficient amount. The chemical formula of the zinc-ammonia complex is Zn (NH ₃ ) ₄ , and zinc consumes an equivalent mass of ammonia when forming the complex. For this reason, for example, when the test solution having an ammonia concentration of 0.02 mass% is 100 mL, the absolute amount of ammonia is 0.02 g, and the dissolved zinc amount is also about 0.02 g. In the case of a galvanized steel material of 200 g / m ² , this is a 1 cm ² plating adhesion amount, and when the length of the test piece wrinkle is 10 cm, there is a possibility that the coating film peeling of about 1 mm width may be limited. For this reason, especially when the ammonia concentration is low, about 1 L of a test solution is required for a tub length of 10 cm. Oxygen consumes two molecules of oxygen for ionization of zinc, so that an equivalent amount of oxygen to zinc is still required for electrochemical dissolution of zinc. Since the mass of oxygen contained in 1 L of air is about 0.28 g, a few hundred mL is sufficient for the amount of air per test piece on the assumption that air is periodically replaced.

There are various conditions as a test (hereinafter abbreviated as cycle corrosion test) to which conditions such as steam wetting, drying and freezing are added, but the ammonia-containing liquid immersion may be within the range described above. For wet conditions, the temperature ranges from 20 ° C. to 60 ° C., and the humidity is 70% or more. For freezing conditions, the temperature ranges from −5 ° C. to −30 ° C. For dry conditions, the temperature ranges from 30 ° C. to 70 ° C. The range and humidity are preferably 60% or less. As the test time, since a change in temperature and humidity is large and a physical operation of immersion and pull-up is included, a sufficient transition time is required although it depends on the capability of the tester or the test technique. For this reason, since it is considered that at least 2 to 3 hours or more are required for each step, it is desirable to set a cycle of about 1 cycle / day. As an example,
Ammonia-containing liquid immersion (30 ° C, 4 hours)
→ Wet (40 ° C, RH 95%, 4 hours)
→ Freezing (-20 ° C, 4 hours)
→ Dry (50 ° C., RH 40%, 12 hours)
The conditions such as repeating the process can be considered.

  Simple immersion in an ammonia solution is a test in which electrochemical corrosion with chlorine, chemical conversion treatment with ammonia, and dissolution and removal of zinc and zinc corrosion products proceed simultaneously. However, in actual subsurface corrosion, these two reactions do not necessarily occur simultaneously. Electrochemical corrosion due to chlorine or the like is considered to occur continuously under humid conditions, but it is difficult to think that ammonia is continuously generated. For this reason, this cyclic corrosion test is considered to be a more realistic corrosion condition than the continuous immersion test.

  In the freezing process, moisture contained in the film freezes and expands to promote peeling of the coating film. However, in reality, it is unlikely that ammonia will be generated by microorganisms in an environment (time) in which the soil freezes. For this reason, the freezing process is not necessarily required from the viewpoint of approaching the actual environment.

  Since the corrosion rate varies depending on the ammonia concentration, the test period cannot be generally stated, but it requires one week to ten days for a high ammonia concentration and two months to six months for a low ammonia concentration. In high-concentration tests, the chemical treatment film and galvanized direct dissolution by ammonia are large, so this result can only be considered as a reference test. As a reproducibility test and acceleration test for ground corrosion due to the combined action of chlorine and ammonia in an actual environment, an evaluation test with a low ammonia concentration and at least one month is desirable.

As an evaluation method after the corrosion test,
(A) Presence / absence of white rust / red rust (non-destructive inspection)
(B) The swollen area of the paint film or the swollen width from the collar (nondestructive inspection)
(C) Peeling area of coating film or peeling width from heel (destructive inspection)
(D) Measurement of chemical conversion film weight loss / plating weight loss after removing paint film (destructive inspection)
Etc.

  Hereinafter, the present invention will be described in detail using examples.

Example 1
The steel material used for the test is a hot-rolled and pickled steel plate having a length of 150 mm, a width of 75 mm, and a plate thickness of 2.3 mm. This specimen was hot dip galvanized at about 120 g / m ^{2 on} one side by the flux method. A part of this plated test piece was subjected to a normal immersion-type phosphate chemical conversion treatment for a coating base, and further spray-coated.

  The paint used was a tar epoxy paint, and the film thickness was 150 to 180 μm.

  The test steel material was dipped in various test solutions shown in Table 1 after putting a heel reaching a steel layer having a length of 100 mm with a cutter knife. The bath temperature was 25 ° C. and the immersion time was 6 weeks. After the test was completed, the peeled coating film around the buttock was removed, and the peel width from the buttock was measured. As comparative tests, the tests listed in Non-Patent Documents 1 and 2 were performed on the same processed specimens. The results are summarized in Table 1.

  As shown in Table 1, it can be seen that the corrosion resistance of the surface-treated steel material in the presence of ammonia varies depending on the pH, the ammonia concentration, the presence of other ions, and the presence or absence of chemical conversion treatment.

  With simple high-concentration ammonia water, the coating film peels off entirely from the steel material that has been subjected to chemical conversion treatment, and corrosion does not proceed with steel material that has not undergone chemical conversion treatment. ), Which is obviously inappropriate as a method for evaluating corrosion resistance. Moreover, in the sodium hydroxide aqueous solution (NO. 7, 8) having the same pH as the ammonia water, the corrosion hardly progresses, so it is clear that the ammonia has a great influence on the corrosion. However, corrosion does not proceed with a low concentration, low pH ammonia solution (NO. 7 to 10). NO. Except for 10, in the immersion test, corrosion progressed only in the low concentration ammonia + sodium chloride solution (NO. 15 to 18) in the example of the present invention. It is clear that the corrosion of the obtained steel material is promoted. In addition, the results of the salt spray test (hereinafter abbreviated as SST) and the car body outer surface corrosion test show that the peel width varies greatly depending on the presence or absence of chemical conversion treatment, and the effect of chemical conversion treatment with salt water, which is a general corrosion condition, is obvious. Appears. However, NO. As can be seen from the results of 9 and 10, the chemical conversion film has low ammonia resistance. Under the test conditions of the example of the present invention, the factor of low chemical resistance of the chemical conversion treatment is large, and the corrosion resistance is worse when the chemical conversion treatment film is provided.

  Therefore, it can be seen that in order to investigate the corrosion resistance of the surface-treated steel material in an ammoniacal environment, the present invention requires the coexistence of ammonia and chloride ions.

(Example 2)
The steel material used for the test is a steel pipe made of a hot-rolled steel sheet having a length of 500 mm, a diameter of 80 mm, and a wall thickness of 2.3 mm. This test material was hot dip galvanized at about 80 g / m ^{2 on} one side by a flux method. A part of the plated test piece was subjected to a normal immersion-type phosphate chemical conversion treatment for a coating base, and further sprayed with a tar epoxy paint. The film thickness was 50 to 70 μm.

  The above test steel materials were subjected to a test method according to the example of the present invention and an actual environmental test embedded in actual concrete. The test piece was filled with scissors reaching the steel surface with a length of 100 mm with a cutter knife. In the test method of the present invention, an immersion test and a cycle test were performed under the conditions shown in Table 1. The conditions of the cycle test were a corrosion resistance test, that is, ammonia solution immersion was performed at 25 ° C. in a test solution having a pH of 9.5 containing ammonium nitrate having a concentration of 0.8% by mass and sodium chloride having a mass of 0.5% by mass. The test was performed by immersing the test piece for a time. The water vapor wetting test was conducted at a temperature of 40 ° C. and a humidity of 95% for 8 hours. The freezing test was performed at a temperature of −20 ° C. for 4 hours. The drying test was conducted at a temperature of 50 ° C. and a humidity of 30% for 8 hours. In addition, as an actual burial test, a test in which the concrete was buried in the soil using a concrete in a mountain area and a test in which the concrete was buried in a pot of 500 mmφ × 300 mm depth and installed on the roof of the building were performed. . The firewood was set so that about half was buried in concrete. In addition, the test material embedded in the soil was covered with an amount of humus so that no soot was visible on the concrete. The evaluation results are shown in Tables 2 and 3.

  The pot embedding of the exposure test is not a ground corrosion but a simple dry and wet repetition and corrosion due to the formation of batteries inside and outside the concrete, and the test in the static bath of the comparative example is also different from the ground corrosion.

  In both experiments, sacrificial corrosion protection of galvanization functions and white rust occurs before red rust occurs. On the other hand, in the case of embedding in the soil of the exposure test, red rust has occurred without seeing the occurrence of white rust. Even in the present invention examples of the still bath immersion and cycle test, no white rust is generated and red rust is generated, and in this respect, it can be seen that the test method of the present invention reproduces dissolution of zinc by ammonia.

  Next, looking at the effect of the chemical conversion treatment, in the case of pot embedding in the exposure test, the corrosion does not reach red rust, but the chemical resistance is clearly better with the chemical conversion treatment. Even in the comparative example of immersion in the still bath, the swollen width is smaller when there is a chemical conversion treatment. However, in the still bath immersion test of the present invention example, it can be seen that the chemical conversion treatment is inferior in corrosion resistance, and from the viewpoint of ammonia resistance, the chemical conversion treatment is counterproductive. However, in the soil embedding test of the comparative example and the cycle test of the example of the present invention, the corrosion resistance is slightly better with the chemical conversion treatment. By the way, the difference by the presence or absence of the chemical conversion treatment is not as great as the pot embedding test and the still bath immersion test of the comparative example. This is because the corrosion in the actual suburban environment is not only due to ammonia, but also has a large element of electrochemical corrosion due to other chloride ions and the like.

  From the above results, the stationary bath immersion test containing ammonia and chloride ions of the present invention example is a test method for mainly evaluating the ammonia resistance of the anticorrosion structure, and the cycle test including immersion in the same solution is ammonia It can be seen that this is a comprehensive corrosion resistance promotion evaluation test in an environment including The weighting of the ammonia resistance evaluation in this cycle test can be changed by changing the conditions of the ammonia solution and the immersion time, and the wet conditions and the time.

As mentioned above, although preferred embodiment of this invention was described, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

Claims (7)

  Evaluation was performed in a test aqueous solution having a pH of 5.0 to 11.0 containing 0.02 to 2.0% by mass of ammonia and 0.1 to 2.0% by mass of metal chloride at a chloride ion concentration. A method for evaluating the corrosion resistance of a coated steel material, comprising testing the corrosion resistance of the coated steel material by immersing the coated steel material as a target for a predetermined time.   The method for evaluating corrosion resistance of a coated steel material according to claim 1, wherein the ammonia concentration is 0.1 to 1.0 mass%.   The method for evaluating corrosion resistance of a coated steel material according to claim 1, wherein the metal chloride is an alkali metal chloride or an alkaline earth metal chloride.   The method for evaluating corrosion resistance of a coated steel material according to claim 1, wherein the pH of the test aqueous solution is 7.0 to 10.0.   The method for evaluating corrosion resistance of a coated steel material according to claim 1, wherein the test aqueous solution further contains 0.1 to 2.0% by mass of a nitrate radical.   6. The method for evaluating corrosion resistance of a coated steel material according to claim 5, wherein the concentration of the nitrate radical is 0.1 to 1.0% by mass. Subsequent to the corrosion resistance test according to any one of claims 1 to 6, a water vapor wetting test, a drying test and a freezing test are sequentially performed on the coated steel material to be evaluated, and the corrosion resistance test, the water vapor wetting test, A composite corrosion resistance evaluation method for a coated steel material, wherein the drying test and the freezing test are repeated a predetermined number of times.