JPS6217636A - Method for high sensitivity detection of grain boundary sensitivity degree of stainless steel - Google Patents

Abstract

PURPOSE:To detect a grain boundary sensitivity degree with high sensitivity, by applying anode electrolysis to stainless steel in an electrolyte, which is prepared by adding org. acids, acid anhydrides, amines or a specific complex forming compound having solubility to a non-aqueous solvent, two or more times in a constant potential state. CONSTITUTION:One or more of org. acids, acid anhydrides, amines or beta-diketone having solubility to a non-aqueous solvent and one or more of lithium chloride or quaternary alkylammonium halide are simultaneously added to constitute a non-aqueous solvent type electrolyte. Anode electrolysis is applied to stainless steel in thus formed non-aqueous solvent type electrolyte two or more times in a constant potential state. By this method, the grain boundary sensitivity degree of stainless steel can be detected with high sensitivity.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a highly sensitive method for detecting the degree of grain boundary sensitization in stainless steel, and particularly to the formation of extremely small grain boundary chromium-deficient layers in various stainless steels. The present invention relates to a method for detecting grain boundary sensitization with high sensitivity.

(Conventional technology) Conventionally, the method for detecting the degree of grain boundary sensitization in stainless steel is as follows:
The 10% oxalic acid etch test method (JISGO571) is often used, but based on the surface optical microscope observation results after the oxalic acid etch treatment, the sulfuric acid/ferric sulfate corrosion test method (
JISGO572), 65 nitric acid corrosion test method (JIS
GO573), Nitric acid/Hydrofluoric acid corrosion test method (JI
SGO574) or sulfuric acid/copper sulfate corrosion test method (J
The necessity of long-term thermal acid tests such as ISGO575) is being determined. In recent years, as an electrochemical detection method for grain boundary sensitization, F:JPR (E1electrochem4
cal [electrochemical] Potentiokl
netic (potentiokinetic) React
Ivatjon (react page, n) test method is also J
IS work is underway. These are all industrially established test methods that detect the degree of grain boundary sensitization by performing corrosion in an aqueous corrosive solution under corrosion potential (natural potential) or electrochemically controlled potential. be.

However, even in materials that are determined to have no grain boundary sensitization by various JIS test methods using this solution system, grain boundary corrosion, which is thought to be caused by grain boundary sensitization,
In some cases, a decrease in low-temperature toughness may occur due to intergranular stress corrosion cracking accidents or intergranular carbide precipitation, and there is a need to develop a means that can detect extremely slight grain boundary sensitization.

On the other hand, some of the present inventors previously proposed a means of performing constant potential anodic electrolysis using a non-aqueous electrolyte in order to observe precipitates in metal materials in JP-A-55-107934. In addition, Japanese Patent Application No. 72,234/1980 has proposed various types of non-aqueous electrolytic solutions for use in such electrolysis.

However, all of these methods aim to make water-soluble or water-insoluble precipitates in the metal material appear on the metal surface by dissolving the surrounding metal, and are difficult to detect grain boundary sensitization. The feasibility of this has not yet been considered.

(Problems to be Solved by the Invention) Therefore, the present inventors have studied various means to enable the detection of extremely slight sensitization that cannot be detected by conventional aqueous solution-based intergranular corrosion testing methods. We have discovered a method to detect sensitization in stainless steel with high sensitivity.

In other words, the present inventors performed potentiostatic anodic electrolysis in a non-aqueous electrolyte solution as previously proposed.
The present invention was made based on the completely new knowledge that by conducting the test more than once, it is possible to detect for the first time very slight sensitization that cannot be detected by conventional testing methods.

(Means for Solving the Problems) The present invention has been made based on the above findings, and its gist consists of organic acids, acid anhydrides, and amines that are soluble in non-aqueous solvents. Anodic electrolysis of stainless steel two or more times at constant potential in a non-aqueous electrolytic solution to which one or more of β-types or β-dyquents and one or more of lithium chloride or tetraalkylammonium halides are simultaneously added. A highly sensitive method for detecting the degree of grain boundary sensitization in stainless steel is provided.

The present invention will be explained in detail below.

First, in the present invention, stainless steel refers to ferritic steel,
It is a general term for commonly used stainless steels such as austenitic or duplex stainless steels.Next, grain boundary sensitization as referred to in the present invention refers to carbon removal in the steel at the grain boundaries of the various stainless steels mentioned above. . Chromium carbide (
This refers to a phenomenon in which the amount of chromium near the grain boundaries decreases due to precipitation as Cr7C3 or Cr2sCb), and the intergranular corrosion resistance or intergranular stress corrosion cracking resistance deteriorates.

Next, in the present invention, non-aqueous solvents include methyl alcohol,
Ethyl alcohol and n-hexane alkaline refer to non-aqueous solvents such as methyl acetate, and all of these solvents must be able to dissolve organic acids, acid anhydrides, amines, or β-diketones. It is. The reason why the non-aqueous solvent is required here is to prevent the formation of a surface passive film due to moisture during constant potential anodic electrolysis. Furthermore, all organic acids are complex salt-forming compounds that are soluble in the above-mentioned non-aqueous solvents, and examples of organic acids include maleic acid, succinic acid, lactic acid, citric acid, and tartaric acid, and examples of acid anhydrides include: Suitable maleic anhydride and amines include triethanolamine, jetanolamine, monoethanolamine, etc., and β-diketones include methyl acetoacetate, ethyl acetoacetate, acetylacetone, methyl salicylate, Methyl maltol, ethyl maltol, etc. are suitable.

Where it is necessary to add one or more of the above complex salt forming compounds, iron, chromium,
Second, there are ways to dissolve Kel as a metal complex ion.

Further, in the present invention, one or more of lithium chloride, rl+, and 1-alkylammonium halide is added as a tt<dissolved substance for the purpose of imparting electrical conductivity to the solution. The tetraalkylammonium halide mentioned here refers to a substance represented by the general formula R4NX,
In the same formula, R is CH3, C2H5, C3H.

represents any of C4H9, and X is a halogen element C
It represents any one of t, Br, and I. The addition of one or more of the electrolyte substances mentioned above is for imparting electrical conductivity to the non-aqueous solution to enable constant potential anodic electrolysis.

The non-aqueous solvent electrolyte in the method of the present invention is constituted by the non-aqueous solvent to which the above-mentioned complex salt-forming substance and electrolyte substance are added. In this case, 1^ of each additive substance is not particularly specified, but approximately 1 to 15 wt% for one or more complex salt forming compounds, and approximately 0.5 to 10 wt% for electrolyte substances. be.

Next, the most important point in the present invention is that stainless steel is electrostatically anodically electrolyzed two or more times using the non-aqueous electrolyte. In this case, the adoption of this electrolytic method was based on the following reasons.

That is, when stainless steel is first anodically electrolyzed in the non-aqueous electrolytic solution, since it does not contain water, it completely dissolves without forming a passive film. In this case, grain boundary sensitization cannot be detected because chromium dissolves uniformly regardless of the amount of chromium in the steel. However, after complete dissolution by the first anodic electrolysis in a non-aqueous electrolyte, washing and drying in methyl alcohol or ethyl alcohol and forming a passive skin Mk in the atmosphere were performed. When the second anodic electrolysis was carried out using an aqueous electrolyte, significant intergranular corrosion grooves were detected due to grain boundary sensitization. Furthermore, if the generation of intergranular corrosion grooves is not sufficient with the second anode 1i solution, the entire intended purpose can be achieved by repeating the above-mentioned cleaning, drying, and electrolysis. .

In this case, the anodic electrolysis is desirably carried out at a potential of 7070-13O from the natural potential (8i! dipping potential),
If it is less than 0 mV, it is difficult to achieve anode amelioration, and if it exceeds 130 mV, pitting corrosion will occur and it will be difficult to detect intergranular corrosion grooves.

As described above, in the present invention, stainless steel is first electrostatically anodically electrolyzed in a non-aqueous electrolyte to obtain a surface without a passive film, and then a passive film is generated in the atmosphere. The difference in the dependence of the protective property on the amount of chromium is detected as the difference in solubility due to the second and subsequent anodic electrolysis.

The effects of the present invention will be described in more detail below based on Examples.

(Example) Table 1 shows 0.012 wt% and 0.007 w, respectively.
t% weight 0.013 wt% (5 containing D carbon
US304L, SUS430 and Fe-18w
The results of investigating the presence or absence of grain boundary etching using the method of the present invention and the conventional method for lightly sensitized materials of t%Cr-5,6wt%Ni steel are shown. Grain boundary etching is detected in both cases, but no grain boundary etching is detected by the conventional grain boundary sensitization detection method.

(Effects of the Invention) As described above, according to the method of the present invention, extremely slight sensitization that cannot be detected by conventional grain boundary sensitization testing methods can be detected. It is possible to determine the cooling conditions after heat treatment that do not cause sensitization, to detect initial sensitization when using stainless steel products, or to detect slight sensitization of the weld heat-affected zone, and the industrial effect is extremely significant. It is.

Claims (1)

[Claims] In a non-aqueous electrolytic solution to which one or more organic acids, acid anhydrides, amines, or β-diketones and one or more lithium chloride or tetraalkylammonium halides that are soluble in non-aqueous solvents are simultaneously added. A highly sensitive method for detecting the degree of grain boundary sensitization in stainless steel, which is characterized by subjecting stainless steel to anode electrolysis two or more times in a constant potential manner.