JP4519680B2 - Accelerated testing method for stress corrosion cracking - Google Patents

Description

  The present invention relates to a stress corrosion cracking test method, and more particularly, to a stress corrosion cracking acceleration test method for accelerating a stress corrosion cracking test in order to perform the stress corrosion cracking test in a relatively short period of time.

  Stress corrosion cracking is a phenomenon in which the corrosion of the environment and the mechanical action of the stress applied are superimposed, leading to the destruction of the metal material. It is known that stress corrosion cracking occurs when the metal material, environment, and stress satisfy certain conditions, and are generated in various combinations.

  Stress corrosion cracking occurs even when the working stress is less than the strength of the metal material (= the limit of fracture normally evaluated in a laboratory, not in the actual environment). Therefore, when determining the design strength of a structure, it is necessary to accurately grasp the critical stress of stress corrosion cracking, and its characteristic evaluation technique is extremely important in practice. In addition, laboratory characterization techniques for stress corrosion cracking are indispensable when taking measures to prevent stress corrosion cracking.

  As a test method for stress corrosion cracking, a constant load method, a constant strain method, and the like are known, and are standardized by JIS, ASTM (American Society for Testing Material), and the like. For example, a stress corrosion cracking test method for an aluminum alloy is specified in JIS H8711.

On the other hand, Japanese Patent Application Laid-Open No. 2002-333399 (Patent Document 1) discloses that a plate-like member cut from a member to be tested or a simulated material thereof and subjected to mirror polishing processing has one upper surface end closely adhered to the same surface. Elastic deformation obtained by inserting a plastic deformation portion (high stress portion) that is folded back approximately 180 degrees until it is inserted and a wedge-shaped member made of the same material as the plate-like member between the close contact surfaces of the upper surface end portion and the upper surface. A stress corrosion cracking test method using a stress corrosion cracking test piece provided with a portion (low stress portion) is disclosed.
JP 2002-333399 A

  By the way, in the stress corrosion cracking test by the stress corrosion cracking test method standardized by JIS, ASTM, etc., the test period is long, and especially when it is long, a very long period of tens of thousands of hours is required. For this reason, there is a problem that it takes too much time to grasp the critical load stress necessary for the structure design, which has hindered rapid material development.

  Further, in the stress corrosion cracking test method proposed in Patent Document 1 above, the plate member constituting the test piece is folded back to form a small gap portion where the upper surfaces are close to each other, and the corrosive environment is strictly configured. Therefore, it can be expected that the test time can be shortened, but in the test piece, since the plastic deformation region is formed in the material and the metal structure changes, what is the stress corrosion cracking property when used with the load stress in the elastic region? There is a problem that it does not necessarily correlate.

  The present invention has been made in view of the above circumstances, and its purpose is to evaluate the characteristics in a short period of time against stress corrosion cracking caused in the atmosphere or in an aqueous solution due to chloride. The present invention provides an accelerated test method for obtaining stress corrosion cracking.

To achieve the above object, the accelerated test method for stress corrosion cracking according to the present invention (Claim 1) includes an agent having the action of suppressing pitting chloride ion Cl ^- and a, and the pH is 4 A test piece loaded with stress is placed in an aqueous solution of ~ 9, and the stress corrosion cracking test is accelerated by measuring the time until the test piece breaks due to electrolysis when the test piece is energized It is.

  The accelerated stress corrosion cracking test method according to the present invention (Claim 2) is the stress corrosion cracking accelerated test method according to Claim 1, wherein the test piece is an aluminum alloy. The aluminum alloy mentioned here means 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, 7000 series, 8000 series, and other series of aluminum alloys.

The stress corrosion cracking acceleration test method according to the present invention (Claim 3) is the stress corrosion cracking acceleration test method according to Claim 1 or 2, wherein the agent having an action of suppressing pitting corrosion is:
NO ₃ ⁻ : 0.1 to 10%,
SO ₄ ²⁻ : 0.1 to 10%,
CrO ₄ ²⁻ : 0.1 to 10%,
CH ₃ COO ⁻ : 0.1 to 10%,
H ₂ O _2: 0.1~10%
One type or two or more types selected from the above.

The stress corrosion cracking acceleration test method according to the present invention (Claim 4) is the stress corrosion cracking acceleration test method according to any one of Claims 1 to 3, wherein the current density is 0.1 to 10 mA / cm. ^{2 is applied} for 10 minutes or more, and then the current density is changed to 0.001 to 0.1 mA / cm ² and the current is supplied.

  According to the accelerated test method for stress corrosion cracking according to the present invention, stress corrosion cracking can be evaluated in a short period of time, so that it is possible to speed up the structure design and material development in an environment where stress corrosion cracking is a concern.

  Hereinafter, an accelerated test method for stress corrosion cracking according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a conceptual diagram of an apparatus applied to an accelerated test method for stress corrosion cracking according to the present invention.

The container 1 contains an aqueous solution 2 containing a drug having an action of suppressing pitting corrosion and a chloride ion Cl ⁻ , which will be described in detail later, and having a pH of 4 to 9. A test piece 3 loaded with stress to be a stress corrosion cracking test piece and an electrode 4 as a counter electrode of the test piece 3 are inserted into the aqueous solution 2, and the test piece 3 and the electrode 4 are connected to a power source 5. . By this connection, the test piece 3 is subjected to electrolytic action, and stress corrosion cracking is accelerated.

  Hereinafter, an accelerated test method for stress corrosion cracking according to the present invention will be described in detail.

(1) About a drug having an action of suppressing pitting corrosion in an aqueous solution A drug having an action of suppressing pitting corrosion is pitting corrosion when a stress corrosion cracking specimen is immersed in an aqueous chloride solution to which the chemical is added. Either the number or the maximum pitting depth is a drug that can be reduced by 20% or more compared to a simple aqueous chloride solution. For example, when an aluminum alloy is used as a stress corrosion cracking test piece, either the number of pitting corrosion or the maximum pitting corrosion depth when the test piece is immersed in a 3% NaCl aqueous solution at 30 ° C. to which the agent is added for 100 hours. However, it is a drug that is reduced by 20% or more compared to a simple 3% NaCl aqueous solution. Examples of such drugs include NO ₃ ⁻ : 0.1 to 10%, SO ₄ ²⁻ : 0.1 to 10%, CrO ₄ ²⁻ : 0.1 to 10%, CH ₃ COO ⁻ : 0. 1 to 10%, H ₂ O ₂ : 0.1 to 10%, and the like. Among these, NO ₃ ^- and H ₂ O ₂ are most preferable because they have a large pitting corrosion inhibitory effect. It seems that the site of pitting corrosion is passivated to effectively suppress the progress of pitting corrosion. Moreover, these chemical | medical agents have the effect | action which promotes generation | occurrence | production and progress of the stress corrosion cracking by a chloride ion, and can use 1 type (s) or 2 or more types in combination.

  The reason why the above chemicals are included in the aqueous solution is that, in order to accelerate the stress corrosion of the stress corrosion cracked specimen and accelerate the time to break, it is necessary to generate many grain boundaries early. Grain boundaries are not generated. Therefore, in order to generate grain boundaries, a drug having an action of suppressing pitting corrosion is included so that many grain boundaries are generated in the test piece quickly.

(2) About chloride ion Cl ^{− in} aqueous solution Although stress corrosion cracking is known to occur in various combinations of materials and environments, it often occurs especially in an environment containing chloride ions. Chloride ions, as well as marine and beach environments, fly as sea salt particles in rural areas far from the coast and are also contained in snow melting agents, etc., so structural materials that form structures in a wide range Is inevitable to be exposed to the risk of stress corrosion cracking in contact with chloride. When chloride ions come into contact with many metal materials through water, they form metal chlorides, and hydrochloric acid (HCl) is generated by the hydrolysis reaction of the metal chlorides. It is thought that it will be achieved.

In view of the above, chloride ions Cl ⁻ is contained in the aqueous solution. The concentration of chloride ions in the aqueous solution is recommended to be 0.5% to 10% as Cl ⁻ . The reason is that if Cl ⁻ is less than 0.5%, the amount of HCl produced by the hydrolysis reaction is small, so that it is easily neutralized with moisture in the solution, and stress corrosion cracking cannot be reproduced. If Cl ⁻ exceeds 10%, the metal chloride concentration on the surface of the test piece locally exceeds the saturation concentration and precipitates on the surface, and the deposited metal chloride acts as a barrier to the electrochemical reaction of corrosion. This is because there is a concern that stress corrosion cracking will hardly occur.

(3) pH of the aqueous solution When the pH of the aqueous solution becomes low, that is, when the H ⁺ ion concentration becomes high, hydrogen atoms enter the test piece and cause hydrogen embrittlement cracking. It may not be possible to evaluate accurately. In the evaluation of stress corrosion cracking, the pH is preferably 4 or more in order to eliminate the influence of such hydrogen embrittlement cracking. If the pH is too high, alkali brittle cracking may occur, and the stress corrosion cracking characteristics of the material may not be accurately evaluated. In order to eliminate the influence of such alkali brittle cracking, the pH is preferably 9 or less. For these reasons, the pH of the aqueous solution is recommended to be 4-9.

(4) Material of test piece and stress load The material of the test piece is not particularly limited as long as it is a metal material that causes stress corrosion cracking, but it is mainly used in the development process leading to the present invention. It is developed for aluminum alloy and is considered to be applicable to other metal materials that cause stress corrosion cracking such as steel. In addition, about the quantitative condition in this embodiment, it can apply enough for aluminum alloy.

  There is no particular limitation on the stress loading method to the test piece. About this stress loading method, stress is applied to a U-bend test piece, C-ring test piece, 3-point bend test piece, 4-point bend test piece, etc., which are generally used in the past, by applying a predetermined stress by bolt tightening. It is possible to use a method. Moreover, it is also possible to apply stress to a round bar or plate-shaped test piece in which a notch is formed as required by a load loading device such as a tensile tester.

(5) Method for energizing the test piece The method for energizing the test piece is not particularly limited as long as the electrolysis can be obtained, but the conditions for comparing the results by conducting a plurality of tests are as follows. As a standard, a method of constant current electrolysis using a suitable insoluble counter electrode (for example, platinum) or a method of constant potential electrolysis using a reference electrode (for example, a silver-silver chloride electrode) is recommended.

In the energization method for the test piece, the recommended value for the current during constant current electrolysis varies depending on the material of the test piece, but a value of about 0.001 to 10 mA / cm ² is suitable. Further, the recommended value of the potential during constant potential electrolysis varies depending on the material of the test piece, but a value of about steady corrosion potential +50 to 500 mV is preferable. In the case of the constant current electrolysis, the current density is changed to 0.001 to 0.1 mA / cm ² after performing constant current electrolysis at a relatively large current density of 0.1 to 10 mA / cm ^{2 in the} initial stage. Desirably, stress corrosion cracking is further promoted. This is because the surface is locally activated by electrolysis with an initial high current. Such local activation requires more than 10 minutes. If electrolysis is performed for a long time at a high current, the entire surface of the test piece is activated and the overall corrosion becomes conspicuous, and stress corrosion cracking hardly occurs. Therefore, an electrolysis time of 2 hours or less is recommended.

(6) About temperature of aqueous solution Although the temperature of aqueous solution is not specifically limited, The temperature equivalent to a real machine or room temperature-50 degreeC is recommended. When the temperature of the aqueous solution is too low, the stress corrosion cracking progresses slowly, which is disadvantageous for promotion. However, when the temperature is too high, cooling is required due to problems such as evaporation of the solution.

  Examples of the stress corrosion cracking acceleration test method according to the present invention will be described below together with comparative examples.

Example 1
Using a 7000 series aluminum alloy (Al-4.5% Zn-1.5% Mg) as a test material, a J2B C-ring specimen (plate thickness: 3.04 mm, outer diameter: 38 mm) described in JIS H8711 was prepared. Then, stress corrosion cracking specimens loaded with various stresses by bolt tightening were prepared. This test piece was covered with a silicon sealant except for a width of 10 mm across the apex (maximum stress point) so that bolts and nuts were not electrolyzed. This test piece was subjected to an accelerated test for stress corrosion cracking using a test apparatus having the configuration shown in FIG. In the test, constant current electrolysis was performed at a current density of 5 mA / cm ² in an aqueous solution containing 8% NaCl + 3% Na ₂ CrO ₄ (temperature: 30 ° C., pH: 5.0), and cracks were observed on the test surface. The time until was measured. HCl was used to adjust the pH of the aqueous solution. The test results are as shown in FIG. 2. When the load stress was as high as 350 MPa, the fracture occurred in a very short time within 1000 s, and the crack limit stress was about 200 MPa. However, the test was terminated when 10 ⁵ s was passed.

  For comparison, the same test piece as described above was subjected to an air exposure test (Kobe City, distance of about 5 km from the coast), and the time to break was measured. The test results are as shown in FIG. In this atmospheric exposure test, it takes a long time to crack, and it can be seen that the present invention is advantageous in terms of the evaluation period. Moreover, the crack limit stress obtained by the test of the present invention shows the same value as that obtained by the atmospheric exposure test, and it is clear that the limit stress required for the design can be evaluated by the stress corrosion crack test of the present invention.

(Example 2)
Two types of 2000 series aluminum alloys (Material A: Al-4.3% Cu-0.80% Si-0.65% Mn, Material B: Al-5.5% Cu-0.30% Pb-0. 25% Bi) and two types of 7000 series aluminum alloys (material C: Al-5.2% Zn-0.80% Mg-0.20% Cr, material D: 5.5% Zn-2.6% Mg) -1.8% Cu) was used to measure the crack limit stress by the stress corrosion cracking test and the atmospheric exposure test of the present invention. The test piece was prepared in the same manner as described above (Example 1). The electrolysis conditions were an aqueous solution (temperature: 40 ° C., pH: 7.5) containing 10% NaCl + 2% NaNO ₃ + 1% CH ₃ COONa, and the current density was initially 10 mA / cm ² for 30 minutes. Then, the current density was changed to 0.1 mA / cm ² and constant current electrolysis was continued, and the time until cracks were observed on the test surface was measured. NaOH was used to adjust the pH of the aqueous solution. The air exposure test was performed in the same exposure field (Kobe City, distance from the coast of about 5 km) as described above (Example 1). The test results are as shown in FIG.

As is clear from FIG. 4, the crack limit stresses of the test pieces are very well correlated, and the order of the materials evaluated in a short time (within 10 ⁵ s) in the stress corrosion cracking test of the present invention is atmospheric. It can be seen that the stress corrosion cracking property (superior or inferiority) in the real environment can be correctly judged in accordance with that by exposure.

It is a conceptual diagram of the apparatus applied to the accelerated test method of stress corrosion cracking according to the present invention. It is a test result by the accelerated test method of the stress corrosion cracking which concerns on this invention, Comprising: It is a figure which shows the relationship between load stress and the time which led to the crack. It is a test result by the stress corrosion cracking test method by the atmospheric exposure used as a comparative example, Comprising: It is a figure which shows the relationship between load stress and the time which led to the crack. It is a figure which compares and shows the crack limit stress of the accelerated test method of the stress corrosion cracking which concerns on this invention, and the stress corrosion cracking test method by the atmospheric exposure of a comparative example.

Explanation of symbols

1: Container 2: Aqueous solution 3: Test piece 4: Electrode 5: Power supply

Claims (4)

A test piece loaded with stress is placed in an aqueous solution containing a drug having an action of suppressing pitting corrosion and chloride ion Cl ⁻ and having a pH of 4 to 9, and the test piece is energized by electrolytic action. An accelerated stress corrosion cracking test method characterized by accelerating a stress corrosion cracking test by measuring a time until a specimen breaks.   The accelerated test method for stress corrosion cracking according to claim 1, wherein the test piece is an aluminum alloy. Drugs that suppress pitting corrosion
NO ₃ ⁻ : 0.1 to 10% by mass (hereinafter simply referred to as “%”),
SO ₄ ²⁻ : 0.1 to 10%,
CrO ₄ ²⁻ : 0.1 to 10%,
CH ₃ COO ⁻ : 0.1 to 10%,
H ₂ O _2: 0.1~10%
The accelerated test method for stress corrosion cracking according to claim 1 or 2, wherein the method is one or more selected from two or more. Energized, after the constant-current supply for at least 10 minutes at a current density 0.1~10mA / cm ^2, of claims 1 to 3, energization by changing the current density 0.001~0.1mA / cm ² Any one of the accelerated test methods of stress corrosion cracking.

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