JPS61130866A - Decision for deterioration in low-alloy steel - Google Patents

Abstract

PURPOSE:To realize a method which is excellent both in the reproducibility of measurement and accuracy of deciding on the deterioration and handy, by immersing a low-alloy steel to be measured into a aqueous solution of an aromatic compound having a hydroxide group (or carboxylic group) and a nitro group in a molecule to perform an electrochemical measurement. CONSTITUTION:When a low-alloy steel such as Cr-Mo steel and Cr-Mo-V steel is brought into contact with a specified acid solution, the corrosion rate under the natural potential changes corresponding to the degree of embrittlement due to heat histeresis of steel. The acid solution herein used shall be a solution in which one or more than one kind of aromatic compounds (e.g. 2, 4, 6 trinitrophenol, 2, 4-dinitro benzoate) having at least one of a hydroxide group or carboxylic group in a molecule and a nitro group. Then, the low-alloy steel is immersed into the solution as test electrode and the polarization resistance Rp and the time constant tau in the potential response when an electric pulse is applied are measured thereby enabling the decision on the degree of embrittlement of the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for evaluating structural changes in metal materials as changes in electrochemical properties, particularly in a high temperature atmosphere.

It relates to a method for non-destructively detecting the degree of deterioration of metal materials that have become brittle due to long-term use in an enclosed environment.

[Technical background of the invention and its problems]

Metal materials such as steel materials are subjected to various heat treatments in order to obtain the necessary mechanical properties depending on the purpose of use, and then used. However, with materials used in relatively high-temperature environments of around several hundred degrees Celsius, there is a phenomenon in which changes occur over time due to the temperature history, and the original mechanical properties deteriorate. This deterioration of mechanical properties is often caused by thermal fatigue due to repeated excessive thermal stress or creep deformation due to steady stress. For example, steam turbine rotors are operated in high temperature ranges, so phosphorus, an impurity element in steel, segregates at grain boundaries and weakens the grain boundary strength. The toughness value decreases significantly (tempering embrittlement),
The most direct method for investigating such changes in metallographic structure is evaluation through microscopic observation, Charpy impact test, etc.

Although these are reliable evaluation methods, they have a major drawback in that they require the collection of test pieces for measurement.

For this reason, changes in the metallographic structure in recent years f,′i! Progress has been made in the development of a vapor-chemically detected evaluation method. All of these conventional electrochemical methods use the object to be measured as a test electrode, and Jfi
The polarization characteristics are measured using a reference electrode and a counter electrode that are also immersed in the electrolyte, and the appropriate parameters during polarization (current density at the maximum or minimum value of the polarization curve, current density at the maximum or minimum value of the polarization curve, Presence or absence of current peak, critical pitting potential,
Detect changes in metallographic structure by comparing repassivation potential, passivation current density, amount of electricity flowing in the target potential range, ratio of parameters during normal sweep and reverse sweep, etc.), or by comparing these parameters with new materials. That is. However, measurements of polarization characteristics generally have poor reproducibility, and although various improvements have been made to compensate for this shortcoming, conventional methods have large variations in measured values and do not adequately correspond to the degree of aging of metals. There was a problem. In addition, when performing electrochemical measurements, the selection of an electrolytic solution is a fundamental issue during evaluation, but solutions have not been sufficiently developed in the past, and this point is also a cause of deteriorating the accuracy of determining the degree of aging. It had become.

[Purpose of the invention]

In view of the above-mentioned drawbacks of the conventional method, the inventor conducted research on a method for non-destructively detecting the deterioration of mechanical properties due to changes in the metal structure of metal materials used at high temperatures by electrochemical measurement, and as a result, he succeeded in reproducing the measurement. sex.

It is an object of the present invention to find a simple method that is both excellent in deterioration determination accuracy and fc, and to provide this method.

[Summary of the invention]

The present invention shows that when low alloy steel such as Cr-Mo steel and Cr-Mo-V steel is brought into contact with a certain type of acidic solution, the corrosion rate at its natural potential corresponds to the degree of embrittlement due to the steel's thermal history. It is based on the discovery that the Here, the acidic solution that causes such a change is a solution in which one or more aromatic compounds having at least one of a hydroxyl group or a carboxyl group and a nitro group are dissolved in the molecule. Such compounds include 0-nitrophenol, m(D nitrophenol, P-nitrophenol, 2,.4-dinitrophenol% 2,4.6
-Dolinitrophenol (pyrinic acid), to! Nitrophenols such as J nitroresorcin, 2.4-dinitrobenzoic acid, 3.5-dinitrobenzoic acid%2.5-dinitrobenzoic acid, 3.4-dinitrobenzoic acid, 〇-nitrobenzoic acid, m -nitrobenzoic acid, P-nitrobenzoic acid, and the like. In addition, the concentration of these solutions is such that the pH is 3.5.
It is preferable to adjust as follows.

It is generally a well-known fact that in various low alloy steels, the mechanical properties of the metal material deteriorate due to changes in the metal structure, and the corrosivity of the button material changes as the mechanical properties of the material deteriorate. This is thought to be due to changes in the seven-node polarization behavior caused by increased local corrosion due to the action of segregated substances and the weakening of the passive film.

However, in general, anode polarization behavior is susceptible to subtle changes in the electrode surface and has poor reproducibility. Therefore, no attempt has been made to determine the degree of embrittlement of low-alloy steel from the difference in corrosion rate at natural immersion potential.

On the other hand, the change in corrosion rate depending on the degree of embrittlement at the natural immersion potential, which is the basis of the present invention, is based on a mechanism different from the above-mentioned change in corrosivity. This point will be explained in detail below.

On low alloy steels such as Cr-Mo steel and Cr-Mo-V steel,
When brought into contact with a solution that has strong etching properties and can dissolve and remove the oxide film on the metal surface, more specifically, an aqueous solution of an aromatic compound that has at least one of a hydroxyl group, a carboxyl group, and a nitro group in its molecule, The segregated substances of various impurity elements contained therein come into direct contact with the solution and sensitively exhibit their activity during electrode reactions. Cr-Mo steel,
In the case of low-alloy steels such as Cr-Mo-'I steel, it is well known that P contained as an impurity element segregates at grain boundaries and causes embrittlement. Iron has a small hydrogen overvoltage, and therefore, it is thought that the reduction reaction of hydrogen, which is a cathode reaction, will be more active in the brittle material. This relationship can be seen in the cathode polarization curve C in Figure 1.
(non-embrittleable material) and C' (embrittleable material). Furthermore, in this case, it is considered that the anode dissolution reaction is not affected by the segregated IP to a great extent.

For convenience, both the non-embrittling material and the embrittling material are represented by the same 7-node polarization curve A. Therefore, the corrosion current density increases from 10°rr to 10°rr as the material becomes embrittled. The correlation between the corrosion rate and the degree of embrittlement found by the inventors is thought to be based on the change in the cathode polarization curve related to the hydrogen generation reaction. Local corrosion increases due to
This is fundamentally different from a corrosive change based on a change in the anode polarization curve that makes passivation difficult. Furthermore, as can be inferred from the mechanism described above, if adsorbent substances, especially surfactants, coexist in the solution used in the present invention, a hybrid reaction dominated by the anode reaction may occur, and hydrogen generation from the segregated substances may occur. It was also found that due to the decrease in reaction activity, the above-mentioned change in cathode polarization behavior could not be sufficiently detected as a change in corrosion rate, making it difficult to determine the degree of embrittlement.

In the present invention, the parameters to be measured to determine the corrosion rate at natural immersion potential are polarization resistance (p), time constant (tau) of potential response when an electric pulse is applied, and iron ion concentration (C) dissolved in the electrolyte. .

[Embodiments of the invention]

Next, examples of the present invention will be described.

[Example 1] An aqueous solution of 2,4-dinitrobenzoic acid was prepared as an aromatic compound having a nitro group and a carboxyl group in the molecule. The pH of the solution was set to 1i2.2 fc, and three types of Cr-Mo-V steels having different degrees of embrittlement were immersed in this test solution, and the polarization resistance 'fLp was determined by the Courostat method.

The companion device used for the measurement was HK-201 manufactured by Hokuto Denko.

FIG. 2 shows the correspondence between the obtained Rp value and the degree of embrittlement. In Figure 2, the horizontal axis is the ductile-brittle transition temperature difference ΔFATT, which is a parameter of the degree of embrittlement of the steel, and the vertical axis is the polarization resistance Rp value.As is clear from Figure 6, the Rp value changes as the embrittlement progresses. You can see that it becomes smaller.

[Example 2] As an aromatic compound having a hydroxyl group and a nitro group in the molecule, 2, 4, and 6-1 linitrophenol prepared at pH 2, 2 were used, and three types with different degrees of embrittlement were used as in Example 1. C of
The r-Mo-V steel was immersed, a constant electrical pulse was applied, and the time constant τ of the potential response was measured. The time constant was measured using the same coulostat device HK-201, and the relationship between the time constant τ and ΔFATT is shown in FIG. In this case as well, it can be seen that the more the embrittlement progresses, the smaller the time constant becomes.

As shown in the above examples, the degree of embrittlement of the steel can be determined by measuring the polarization resistance Rp and the time constant τ of the response upon application of an electric pulse using low alloy steel as a test electrode.

〔Effect of the invention〕

As mentioned above, the present invention can detect the difference in corrosion rate at natural immersion potential by detecting the polarization resistance or the time constant of the potential response when an electric pulse is applied. However, by using a specific acid solution, unlike previous electrochemical methods,
Since changes in cathode polarization behavior are detected, the degree of embrittlement of low alloy steel can be determined with precision.

[Brief explanation of drawings]

Figure 1 shows the mechanism of change in the corrosion rate of low alloy steel according to the present invention due to embrittlement *t - a schematic number (Evans diagram) to explain it, where A is the anode polarization curve and C is the non-embrittlement curve. 2 and 3 are diagrams showing examples of measurement results according to the present invention. Figure 1 Figure 2 4F, 47r, 'C Procedure Amendment (Voluntary) 1986/i! -2, days within

Claims (1)

[Claims] The time constant of the potential response when an aqueous solution of an aromatic compound having at least one of a hydroxyl group or a carboxyl group and a nitro group in the molecule is brought into contact with a low alloy steel to be measured, and a polarization resistance RP or an electric pulse is applied. A method for determining deterioration of low alloy steel, which determines the degree of embrittlement due to thermal history of the low alloy steel by measuring.