JP2017090352A - Life determination method of surface-treated steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new index of life prediction of a surface-treated steel material, and provide a life determination method using the index.SOLUTION: The life determination method of a surface-treated steel material is a method of determining the life of the surface-treated steel material having a coating containing Zn and Al. In this method, the amount of layered double hydroxide in a corrosion product formed on a surface of the steel material is used as an index, and whether the steel material comes to the end of its life is determined.SELECTED DRAWING: None

Description

  The present invention relates to a method for determining the life of a surface-treated steel material, and particularly relates to a method for determining the life of a surface-treated steel material that requires high designability.

  Some surface-treated steel sheets have a surface treatment film containing Al, such as Zn-Al, Zn-Al-Mg, and Zn-Mg-Al-Si. When these surface-treated steel sheets are corroded in the usage environment, the surface-treated film is consumed and eventually disappears.

  Conventionally, the lifetime of these surface-treated steel sheets has been predicted by comparing the amount of corrosion measured after being exposed to a predetermined environment in advance with the measurement result of the corrosion strength of the environment where the structure is installed. . And generally, the lifetime is set based on the prediction result (for example, refer to patent documents 1 to 3).

JP-A 61-22234 JP 2002-318227 A JP 2013-29335 A

  However, in the conventional life prediction method, only the corrosion amount is used as an index indicating the degree of corrosion of the steel material. In order to improve the prediction accuracy, it is necessary to develop a method using the constituents in the material as an index.

  It is an object of the present invention to propose a new index for predicting the lifetime of a surface-treated steel material and to provide a lifetime determination method using the index.

  In order to solve the above-mentioned problems, the present inventors first collected corrosion products from various surface-treated steel materials after performing a corrosion test, and performed precise analysis thereof. As a result, the following knowledge was obtained.

  The composition of the corrosion product depends on the component composition in the surface treatment film. However, it has been found that the movement of components from the surface treatment film to the corrosion product is not constant, and the composition of the generated corrosion product changes according to the degree of progress of corrosion.

  In other words, by investigating the composition of the corrosion products, it is possible to know the residual status of the active ingredients in the surface treatment film without causing damage to the surface treatment steel itself, and whether the surface treatment steel material has reached the end of its life. It becomes possible to determine whether or not.

  The present invention has been made in order to solve the above-described problems, and the gist thereof is a method for determining the life of the following surface-treated steel material.

(1) A method for determining the life of a surface-treated steel material having a coating containing Zn and Al,
A method for determining the life of a surface-treated steel material, wherein it is determined whether or not the steel material has reached the end of life by using the amount of layered double hydroxide in a corrosion product formed on the surface of the steel material as an index.

  (2) When the amount of the layered double hydroxide measured by the X-ray diffraction method is less than 1.0% by mass%, it is determined that the steel material has reached the end of its life, (1) The method for determining the life of a surface-treated steel material according to (1).

  According to the present invention, it is possible to determine whether or not the time of red rust generation is approaching by measuring the component composition of the corrosion product formed on the steel material surface. Therefore, by using the lifetime determination method according to the present invention, it is possible to determine the lifetime of steel sheets used for home appliances, the inner surfaces of buildings, high-end automobiles, and the like with high accuracy.

It is the figure which showed the time-dependent change of the LDH amount according to the progress degree of corrosion.

  The lifetime determination method according to the present invention is intended for a surface-treated steel material provided with a coating containing Zn and Al. Specifically, the coating containing Zn and Al includes plating of Zn—Al, Zn—Al—Mg, and Zn—Mg—Al—Si. These films usually contain 5.0% or more of Al by mass%.

  The surface treatment film is consumed, and a corrosion product is formed on the steel surface. In the present invention, it is determined whether or not the steel material has reached the end of life by using the amount of the layered double hydroxide contained in the corrosion product as an index.

  In addition, in the surface-treated steel material which requires high designability, the designability is remarkably inferior when the occurrence of red rust is recognized on the surface. Therefore, in the present invention, "the surface-treated steel material has reached the end of its life" means that the time when red rust occurs on the surface is sufficiently approached in consideration of a predetermined period for guaranteeing the quality of the coating. Means.

Here, the layered double hydroxide (LDH) is a compound represented by [M ²⁺ _1-x M ³⁺ _x (OH) ₂ ] [A ⁿ⁻ ] _{x / n} · zH ₂ O. Examples of the ^{M 2+,} Zn ^2+, contains one or more selected from ^{Mg 2+} and ^{Fe 2+,} examples of the ^{M 3+,} contains one or more selected from ^{Fe 3+} and ^{Al 3+,} wherein ^{A n −} Includes at least one selected from OH ⁻ , CO ₃ ²⁻ , Cl ⁻ and SO ₄ ²⁻ .

  As described above, the composition of the corrosion product changes depending on the degree of progress of corrosion. Specifically, the amount of LDH in the corrosion product decreases according to the degree of progress of corrosion. In the present invention, when the LDH amount measured by the X-ray diffraction method is less than 1.0% in mass%, it is preferable to determine that the steel material has reached the end of its life. The basis for setting the LDH amount threshold to 1.0% is shown below.

A test piece having a size of 70 mm × 150 mm was collected from a surface-treated steel material having a coating composition of Zn-11% Al-3% Mg-0.2% Si and a basis weight of 45 g / m ² / single side. The sample was subjected to a cyclic corrosion test specified in 609. In the salt spray test (SST) included in the cycle corrosion test, a test solution prepared by adjusting the sodium chloride concentration to 50 g / L as defined in JIS K 5600-7-1 (1999) was used.

Further, the amount of LDH in the film was measured using XRD. In the measurement, as well as assuming the presence of some oxides, the ^{M 2+} in the above structural formula of LDH ^{Mg 2+,} the ^{M 3+ Al 3+,} and ^{A n-} a _CO ^{3 2-} and the x 0.33 As a result, a Rietveld analysis was performed. The oxides assumed to exist are basic zinc chloride (Simonkoleite: ZnCl ₂ [Zn (OH) ₂ ] ₄ H ₂ O), zinc oxide (ZnO), hydrozincite: Zn ₅ (OH) ₆ (CO ₃ ) ₂ ), and hydrotalcite: [Mg ²⁺ _0.67 Al ³⁺ _0.33 (OH) ₂ ] [CO ₃ ²⁻ ] _0.165 · zH ₂ O).

  FIG. 1 is a diagram showing the change over time in the amount of LDH corresponding to the degree of progress of corrosion. From Table 1, it can be seen that the amount of LDH decreases as the number of cycles in the cycle corrosion test increases. And generation | occurrence | production of red rust was confirmed when the LDH amount fell below 0.9%. From this, taking into consideration ensuring the stability of the quality, when the LDH content in the film is less than 1.0%, it is judged that the quality of the film has deteriorated and the surface-treated steel material has reached the end of its life. can do.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Test No. Corrosion tests were performed using 1 to 9 film-treated steel materials, and thereafter the life was determined. The corrosion test was the same cycle corrosion test as described above, the corrosion product was measured using XRD, and the composition was quantified by La Bal analysis based on the same assumptions as described above. The results are shown in Table 1.

  As can be seen from Table 1, as a result of the determination, it was determined that the test No. About 1-5, generation | occurrence | production of red rust was not recognized. On the other hand, test No. determined to have reached the end of its life. 6-9, test no. For 6, 8, and 9, red rust was observed. In addition, Test No. Although the presence of red rust was not recognized for No. 7, it is presumed that red rust will soon occur if the use is continued as it is.

  According to the present invention, it is possible to determine whether or not the time of occurrence of red rust is approaching by measuring the component composition of the corrosion product formed on the steel material surface. Therefore, by using the lifetime determination method according to the present invention, it is possible to determine the lifetime of steel sheets used for home appliances, the inner surfaces of buildings, high-end automobiles, and the like with high accuracy.

Claims (2)

A method for determining the life of a surface-treated steel material having a coating containing Zn and Al,
A method for determining the life of a surface-treated steel material, wherein it is determined whether or not the steel material has reached the end of life by using the amount of layered double hydroxide in a corrosion product formed on the surface of the steel material as an index.   The method according to claim 1, wherein when the amount of the layered double hydroxide measured by the X-ray diffraction method is less than 1.0% by mass%, it is determined that the steel material has reached the end of its life. The life judging method of the surface treatment steel materials of description.

Images