JPH02128151A - Evaluation of corrosion for metallic material - Google Patents

Abstract

PURPOSE:To enable evaluation of corrosiveness with a high reliability by generating a dissolution current noise to measure a recovery time at a latter half indicating a process in which a corroded hole of the material is repaired. CONSTITUTION:A metallic material desired to be evaluated is dipped into a desired aqueous solution and the metal material undergoes a constant potential electrolysis at a desired potential in a potential area from a natural dip potential to active dissolution to generate a dissolution current noise. Then, time (recovery time) is measured actually from the generation of a peak current of the dissolution current noise to the recovery of a corroded hole on the surface of the metallic material to evaluate corrosiveness depending on the length of the recovery time. The corrosiveness may be evaluated by a recovery speed in stead of the length of recovery time.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for evaluating corrosion resistance of any metal material.

[Prior Art] Conventionally, as a method for evaluating the corrosion resistance of metal materials, the salt water jet test method (, zs
Z 2371) or a pitting potential measurement method for evaluating corrosion resistance by measuring the pitting potential observed in the anode polarization curve of a passivated alloy such as stainless steel.

These evaluation methods accelerate corrosion phenomena by exposing metal materials to a harsher corrosive environment than the actual corrosive environment.
This is a so-called accelerated testing method that predicts the corrosion resistance in the actual environment from changes in the sample at that time.

These evaluation methods are considered to be powerful evaluation indicators that are close to the characteristic values of the metal material to some extent, but the evaluations obtained often differ depending on the measurement conditions, and they are only accelerated tests and do not measure corrosion resistance in the actual environment. There is often a significant difference. For example, crystalline 5US304 stainless steel (crystal grain size approximately 1 μm) produced by the roll quenching method is
It is known that the corrosion resistance in an actual corrosive environment is far superior to that of crystalline 5US304 stainless steel (crystal grain size approximately 30 to 50 μm) produced by conventional methods, but when evaluated by the pitting potential method. , the corrosion potential of the former is 400mV, while that of the latter is 200mV.
V, and the difference is not so clear compared to the actual difference in corrosion resistance, and it cannot be said to be a highly reliable evaluation method.

Furthermore, although it is an accelerated test, a salt spray test usually requires a test period of several months, and even the pitting potential method requires a considerable amount of time if it is to be carried out strictly.

Furthermore, these evaluation methods are destructive tests, which is also one of the causes of limitations in their applicability.

On the other hand, as an electrochemical measurement method other than pitting corrosion potential,
A method of forcibly polarizing the metal to the anode side and evaluating the corrosion resistance of the metal under severe corrosive conditions (polarization resistance method), a method of measuring the insulation of the film and evaluating the corrosion resistance by insulation rate (film resistance method), A method of evaluating corrosion resistance by immersing a metal material in water and measuring potential changes relative to a reference electrode (corrosion potential measurement method) is attracting attention as a new corrosion testing method.

These methods have the following advantages: (1) real-time measurement, (2) non-destructive testing, and (2) continuous measurement.

However, these evaluation methods using electrochemical measurement methods have the following problems.

That is, the polarization resistance method is applicable only to electrochemical corrosion in active state corrosion, and is easily interfered with by the high resistance of the film and solution.

Furthermore, in the film resistance method, there are large measurement differences depending on the target metal material.

It is not possible to quantitatively understand the corrosion rate using the corrosion potential measurement method.

After all, these electrochemical measurement methods cannot be said to have high reliability either.

[Problem to be solved by the invention] The purpose of the present invention is to solve the problem in a short time and without destroying the sample.
The purpose of the present invention is to provide a method for evaluating corrosion resistance of metal materials that can be applied to any alloy and can evaluate corrosion resistance with high reliability.

[Means for Solving the Problems] The gist of the present invention is to immerse a metal material to be evaluated in an arbitrary aqueous solution, and immerse the metal material at an arbitrary potential in the potential range from natural immersion potential to active dissolution. A method for evaluating corrosion resistance of metal materials is characterized in that a dissolution current noise is generated by electrolyzing at a constant potential, a repair time tr is actually measured, and the corrosion resistance is evaluated based on the magnitude of the repair time tr.

Note that instead of the magnitude of the repair time tr, the corrosion resistance may be evaluated using the repair speed V expressed by the following equation (1). In this case, the larger V means better corrosion resistance, which makes evaluation easier.

v-f/1r(1) [Function] The function of the present invention will be explained below along with the findings obtained in making the present invention.

Immerse the metal material to be evaluated in any aqueous solution,
When the metal material is subjected to constant potential electrolysis at an arbitrary potential in the potential range from natural immersion potential to active dissolution, dissolution current noise is generated as shown in FIG. 1.

The present inventor got the idea that corrosion resistance could be evaluated by using this melting current noise, and analyzed the melting current noise in detail. As a result, it was found that the dissolution current noise can be divided into the first half, which indicates the process in which corrosion holes are formed, and the second half, which indicates the process in which the corrosion holes are repaired, starting from the time when the peak current value is indicated. Further analysis revealed that the first half of the melting current noise does not necessarily indicate the characteristics of the metal material, as the points where corrosion holes occur are affected by the surface treatment status of the metal material used as the test piece. , On the other hand, the second half shows the process in which eluted ions precipitate and accumulate and repair the active dissolution part within the corrosion hole, and is the part that shows the original characteristics of the metal material without being influenced by other factors. I discovered that.

The present invention has been made based on this discovery. That is, the repassivation time of this latter half, that is, the repair time t
By measuring r and evaluating the corrosion resistance based on its magnitude, it is possible to correct the inherent characteristics of metal materials, and to evaluate the corrosion resistance of metal materials with high reliability in a short period of time, non-destructively, and without accelerating corrosion. This makes it possible to accurately evaluate corrosion resistance.

Note that since the current noise corresponding to the formation of a plurality of corrosion holes takes a complicated form, a simple form corresponding to the formation of one corrosion hole as shown in FIG. 1 is preferable as the noise to be measured. Further, it is preferable that the peak current value is approximately 1 μA or less.

Note that the present invention can be applied to all metal materials,
In addition, the metal materials mentioned here are not limited to pure metal materials.
This concept also includes alloy materials.

[Example] (Example 1) Amorphous alloys are reported to have high corrosion resistance due to the formation of a uniform passive film, and in fact, they self-passivate even in extremely harsh corrosive environments such as concentrated hydrochloric acid. There are also alloy electrodes that do this. Here, we will show the results of measurements made using one of these, an Fe-Cr-5t-B-based amorphous stainless steel alloy, as a sample.

Using the roll quenching method, F with a width of 5 mm and a thickness of 30 μm
e Sac r 20S l +oB +sAmorphous stainless steel alloy (hereinafter simply referred to as amorphous alloy)
After washing this amorphous alloy with ethanol, it was immersed in a 3% NaCJ2 aqueous solution 9 as shown in FIG. 2, and the repair time V was measured using a saturated Amagi standard 4 as a reference electrode. Note that a PT plate 5 was used as the counter electrode.

To describe the measurement procedure in more detail with reference to the measurement system shown in Figure 2, constant potential electrolysis is performed at 0 to 200 mV with a saturated Amagi reference
The dissolution current noise generated during the process was measured for 1 ms.
AD converts the time using AD converter and controls the measurement.
Analysis was performed by recording on a floppy disk via a personal computer 8 having a recording function.

The results shown in FIG. 1 were obtained as an example of dissolution current noise corresponding to one corrosion hole. As shown in Fig. 1, the peak current value ip is approximately 0.6μA, and the repair time tr is 0.02μA.
It was sec. As mentioned above, when the repair speed V was calculated from the measurement of the repair time tr and equation (1), the repair time V
was 50/sec.

On the other hand, for comparison, crystalline 5
The repair time was measured for US304 stainless steel (crystal grain size 30 to 50 μm) (hereinafter this alloy will be referred to as normal alloy in this specification) in the same manner as for the amorphous alloy described above. As a result, the repair speed for normal alloys is 7/s
e c was obtained.

Table 1 shows the results obtained for the amorphous alloy and the normal alloy. Comparing both results, it can be seen that the repair speed V of the amorphous alloy is one order of magnitude faster than that of the normal alloy. In other words, the fact that the repair speed of amorphous alloys is much faster than that of normal alloys means that in an actual corrosive environment,
This means that the corrosion resistance of amorphous alloys, which is overwhelmingly superior to that of ordinary alloys, has been evaluated as expected. in the end,
In this example, it was possible to accurately evaluate the high corrosion resistance of an amorphous alloy in a short time and without destroying the sample.

Table 1 (Example 2) Molten metal of 5US304 stainless steel was rapidly solidified by the roll quenching method to produce a ribbon-shaped metal material with a width of 5 mm and a thickness of 70 μm (hereinafter, this metal material is referred to as a quenched alloy). did. The X-ray diffraction results confirmed that this metal material was crystalline, and the crystal grain size was approximately 1 μm.
Met.

Regarding this rapidly solidified alloy, the repair time was actually measured using the same method as in the first example.

In addition to measuring the repair time, the potential for pitting corrosion was also measured. The results are combined with the results of the first example and the second
Shown in the table.

Table 2 Ep: Pitting corrosion generation potential V based on saturated sweetness in 3XNaCl aqueous solution; Repair speed As seen from Table 2, rapidly solidified alloys have a pitting corrosion generation potential of only about 200 mV at a saturated sweetwood electrode than normal alloys. However, the repair speed of the rapidly solidified alloy is close to the repair speed of the amorphous alloy obtained in the first example, which is similar to that in the actual corrosive environment. It meets the corrosion resistance of From this, the high corrosion resistance of the rapidly solidified alloy in the passive region could be evaluated with high reliability.

Although the corrosion potential method does not show any significant difference in corrosion resistance between normal alloys and rapidly solidified alloys, the method of the present invention shows that there is a significant difference in corrosion resistance detected by X-ray diffraction. The microstructural structure of the extreme surface layer of the alloy is considered to be due to the amorphous structure, but the corrosion resistance cannot be evaluated using the conventional pitting potential method, which evaluates by destroying the passive film. can now be evaluated using the method of the present invention.

[Effects of the Invention] As explained above, according to the present invention, it is possible to perform a faster and more accurate evaluation of the object to be measured than with conventional techniques, and therefore it is possible to evaluate the object to be measured non-destructively. It has a wide range of uses, such as predicting the lifespan of metal articles and expensive items, and has the advantage of being an omnipotent corrosion resistance evaluation method.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between electrolysis current and electrolysis time for explaining the present invention in detail. FIG. 2 is a schematic diagram illustrating a measurement system in an embodiment of the present invention. 1...Peak current value (Ip) of dissolution current noise, 2.
・Repair time (tr) of corrosion holes generated on the metal surface, 3
...Metal material (amorphous stainless steel alloy sample), 4...
・Saturated Amagi electrode, 5... Counter electrode (Pt plate), 6... Constant potential electrolyzer, 7... AD converter, 8... Personal computer (for measurement control and recording), 9.3%N
a(, ft aqueous solution.

Claims (1)

[Claims] Immerse the metal material to be evaluated in any aqueous solution,
Dissolution current noise is generated by subjecting the metal material to constant potential electrolysis at an arbitrary potential in the potential range from natural immersion potential to active dissolution, and from the point when the peak current Ip of the dissolution current noise is generated, the metal material 1. A method for evaluating corrosion resistance of a metal material, comprising actually measuring the time until a corrosion hole on the surface of the metal material is repaired (hereinafter referred to as repair time tr), and evaluating corrosion resistance based on the magnitude of the repair time tr.