JP2003329632A - Method of determining occurrence of pitting corrosion - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make performable effective anti-corrosion measures about heat transfer tubes, waste water tanks, discharged water storage tanks, and the like. <P>SOLUTION: A Kelvin probe 2 is placed opposite to, but so as not to be in contact with, the surface of a metallic material covered with a thin water film w. A bias voltage Vbias is measured while the probe 2 is excited, thereby finding effective self-potential E'corr. equal to the bias voltage Vbias in absolute value but reverse thereto in polarity. The occurrence and growth of pitting corrosion is determined by comparing the self-potential E'corr. with pitting corrosion potential Vc. Since the self-potential E'corr. appropriately expresses the state of a metallic material corroded in an actual use environment, the occurrence and growth of a pitting corrosion can be suppressed by anti-corrosion measures that are employed when a sudden fall is detected in the self-potential E'corr.. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of measuring the spontaneous potential of a metal material under conditions where a water film is formed and determining the occurrence / growth of pitting corrosion.

[0002]

2. Description of the Related Art Stainless steel exhibits excellent corrosion resistance because a passivation film of about several tens of liters is formed on the surface. In addition to stainless steel, aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, zirconium, zirconium alloys, magnesium, magnesium alloys, etc. are used as materials whose corrosion is suppressed by the passivation film. Used for purposes. Even in a corrosive environment, in a heat transfer tube, a tank, etc., the surface of a metal material is covered with a water film in which water or an electrolyte solution is condensed. Moreover, the surface of the metal material is not always in contact with the water film, and may be exposed to an atmosphere in which the wet state and the dry state are in contact with each other. Under such an environment, pitting easily occurs on the surface of the metal material. Furthermore, if the stress applied to the pitting occurrence point exceeds a threshold value, it may progress to stress corrosion cracking, which may lead to fatal destruction of the structure itself.

The occurrence of pitting corrosion depends on the self-existence of metallic materials immersed in water.
Natural potential E_corr.(Hereinafter, bulk natural potential E_corr.Say)
Holes that have been measured in advance according to JIS G 0577
Eating potential V_cPredict by comparing with. Bulk
Natural potential E_corr.Is the pitting potential V _cPitting corrosion
Pitting potential V_cPitting corrosion does not occur if it does not exceed
Is determined. For example, the pitting potential of various stainless steels
V_cIs a known value (Table 1).
Rank E_corr.Of pitting corrosion by comparing
I know the existence.

[0004]

[0005]

However, the conventional measurement
Even under the condition that pitting corrosion does not occur by the method,
May cause pitting corrosion. This is
It means that the conditions for pitting corrosion are not fully understood.
It The present inventors have investigated the mechanism of occurrence of such pitting corrosion.
I guessed as follows. A metal surface covered with a thin water film
Natural potential E '_corr.(Hereafter, effective natural potential E '_{co rr.}Toi
U) is the bulk spontaneous potential E_corr.Much higher
And bulk natural potential E_corr.Is the pitting potential V_cIs below
However, pitting corrosion under the water film occurs. In this respect, under the water film
Bulk as an index to predict the possibility of pitting corrosion of metallic materials
Natural potential E _corr.Is not appropriate, and bulk spontaneous potential E_corr.
Therefore, it can be said that the phenomenon of pitting corrosion cannot be accurately grasped.

In addition, a metal material having a passivation film formed on its surface often exhibits water repellency, and even if the spontaneous potential E _corr. Is measured under the condition that a water film is artificially formed,
The measured value tends to be different from that under the water film formed naturally, and the measured value of the natural potential E _corr. Is unreliable. However, it was impossible to accurately measure the spontaneous potential E _corr. Under a thin water film.

[0007]

The present invention has been devised to solve such a problem, and the natural potential E _corr. Measured by the conventional method accurately determines the natural potential under the water film _. Based on the premise that it is not shown, the self-potential measured using a Kelvin probe facing the water film is used as a reference to accurately determine the occurrence and growth of pitting corrosion, and to take necessary anticorrosion measures. The purpose is to enable. In order to achieve the object, the determination method of the present invention measures a bias voltage while vibrating a Kelvin probe that is opposed to a water film of water or an electrolyte solution on the surface of a metal material in a non-contact manner. The effective natural potential is obtained from the Kelvin potential having the same absolute value and the opposite polarity, and the effective natural potential is compared with the pitting potential.

[0008] Natural potential, while a Kelvin probe held in a position away from the metallic material is vibrated at a constant amplitude measured bias voltage V _bias, the bias voltage V _bias
Kelvin potential V _kp (= -V)
_bias ). The Kelvin potential V _kp between the metal material and the reference electrode is _expressed by a relational expression V _kp = E ' _corr. + K (K: a constant peculiar to the measuring device, which is measured by using a known natural potential E _corr. Substituting into the above), the effective natural potential _{E'corr. Of} the metal material is calculated. Examples of the metal material to be measured include stainless steel, aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, zirconium, zirconium alloys, magnesium, magnesium alloys, etc., on which a passive film is formed.

[0009]

[Function] When a metallic material exhibiting excellent corrosion resistance due to the formation of a passive film is covered with a water film containing corrosive ions such as Cl ⁻ , the passive film is destroyed by the corrosive ions, which becomes a starting point of occurrence of pitting corrosion. . The higher the concentration of corrosive ions contained in the water film, the more likely pitting corrosion will occur. The pitting corrosion generation mechanism in this case is explained as follows. The metal material on which the passivation film is formed generally shows the potential-current curve (polarization curve) of FIG. 1 when the potential and current density of the material surface are measured while applying a potential difference from the outside. At the time of measurement, a cathode or an anode is connected to a metal material by soldering or the like, the potential is changed in the anode direction or the cathode direction, and the potential-current density is measured. In the figure, the solid line shows the anode current density polarization curve, and the broken line shows the cathode current density polarization curve.

Corrosion is a phenomenon in which a metal is ionized from the surface of a metal material and reacts with anions of an external solution to form an oxide or a hydroxide. The anodic current density polarization curve can represent the corrosion state of the metal material surface. In the anodic current density polarization curve, an active state region is formed when the potential is low, and normal rust is generated. As the potential increases, a thin oxide film is formed on the metal material surface,
The formed oxide film becomes a passive region where further oxidation is suppressed, and the current density changes while maintaining a low constant value. When the potential is further increased, the oxide film that has been passivated on the metal material is exfoliated or destroyed all at once, which promotes corrosion and leads to an excessive passivation region where the current density increases.

However, even in the case of a passivated metal material, when a thin water film is formed on the surface, a minute local oxidation region is formed in a part of the passivation film, resulting in corrosion (pitting corrosion). May progress. Once pitting occurs, the current density rises sharply even in the passive region,
Pitting corrosion becomes rapid. The potential at which pitting corrosion starts to occur (pitting corrosion potential) can be measured by JIS or the like. According to the JIS method,
V is the pitting potential when the anode current density is 10 μA / cm ^2.
_It is expressed as _c.10 , and the pitting potential at 100 μA / cm ² is V
It is represented by _c.100 . V _c.10 and V _C.100 without much difference, any pitting potential practically can also be used as pitting reference. Since electric power is supplied from the outside in the measurement of the polarization curve, the cathode current density and the anode current density are not equal. On the other hand, in a natural state in which no power is supplied from the outside, the absolute values of the anode current density i _a and the cathode current density i _c are equal. Therefore, the potential when i _a = | i _c | in the polarization curve of FIG. 1 is referred to as the natural potential E _corr .

If the spontaneous potential E _corr. Is lower than the pitting corrosion potential V _c , pitting corrosion does not occur, but if it exceeds the pitting corrosion potential V _c , pitting corrosion may occur. However, since there is no method to measure the effective spontaneous potential _{E'corr. Of the} metal surface covered with a thin water film, the occurrence of pitting corrosion is predicted by the bulk spontaneous potential E _{corr. Of} the metallic material immersed in water _. There is. However, the water film covering the metal surface has a high concentration of O ₂ , Cl ₂ or the like having a high gas diffusion rate. Therefore, the effective natural potential _E'corr.
Is several hundred mV higher than the bulk natural potential E _corr. Even if corrosion resistance is exhibited in bulk water, pitting corrosion is more likely to occur under the water film. Therefore, the measurement of the effective spontaneous potential _E'corr. Under the water film has an important influence on the material selection when the operating atmosphere of the heat transfer tube, the drain tank, etc. is taken into consideration.

In the present invention, the effective spontaneous potential E _corr. Under the water film is measured in a non-contact manner as follows. The spontaneous potential measuring device has a circuit composed of a test piece s covered with a thin water film w, a thin reference electrode immersed in the water film 1, a Kelvin probe 2, and a bias battery 3. The cathode 4 side of the Kelvin probe 2 and the bias battery 3 is the lead wire 4, and the bias battery 3
The anode side and the test piece s are connected by the lead wire 5. The Kelvin potential V _kp means a voltaic potential difference or a contact potential difference with the test piece s covered with a thin water film with the Kelvin probe 2 as a reference. It was found from the results of the study by the present inventors that the natural potential _E'corr. Is not equivalent to the Kelvin potential V _kp and has a relationship of V _kp = E ' _corr. + K.
In fact, the surface of stainless steel is passivated, so E'when the surface is covered with a water film of 1000 μm
_corr.> E _{c orr.} and was not considered.

The Kelvin potential V _kp is the effective natural potential E '.
_corr. (corrosion potential) V _kp = E _{'c orr between.} + K a relationship of (Constant) (DENNKI Gakkai Volume 60, No. 5 (1998) The
493). _{E'corr. And} K are constants K, which are values unique to the circuit, from the linear function of E _corr. / V _kp obtained by measuring the Kelvin potential V _kp , which is known as the corrosion potential E _corr. Desired. For example, 0.1M with a thickness of about 0.5mm
The constant K can be determined by measuring the potential difference (E _corr. ) Between the test piece s covered with the liquid film of the NaCl solution and the reference electrode 1 with a potentiostat and simultaneously measuring the Kelvin potential V _kp . Since the Kelvin potential V _kp depends on the material and environment of the Kelvin probe 2, the reference electrode 1
It is preferable to measure the potential of the pitting corrosion-occurring portion on the passive film from the value based on the above. The reference electrode 1 is used for measuring the potential necessary for calculating the constant K, but not for measuring the effective natural potential _E'corr .

Bias voltage V by Kelvin probe method
_In the measurement of _bias , since there is a gap between the Kelvin probe 2 and the test piece s, the measurement by the direct current used in the conventional method such as the potentiostat method is not suitable, and even if there is a gap, the electric circuit An alternating current is supplied to configure the. The AC current may be supplied by an AC current generated by a power supply unit, but a method of inducing an AC current component by exciting a DC current supplied from the power supply unit by a Kelvin probe is preferable. For the vibration of the Kelvin probe 2, a method of varying the distance from the surface of the metal material to the tip of the Kelvin probe 2 or the area of the Kelvin probe 2 facing the surface of the metal material is adopted. By vibrating the Kelvin probe 2, a capacitor is formed between the Kelvin probe 2 and the test piece s, and a weak alternating current is generated.

Although the bias voltage V _bias is generated due to the alternating current, the bias voltage V _bias is measured in the present invention. The bias voltage V _bias has the same absolute value as the Kelvin potential V _kp and the opposite polarity (−V _bias = V _kp ). The Kelvin potential V _kp is calculated from the bias voltage V _bias and the effective natural potential _{E'corr. Is} calculated to derive the natural potential derived from the increase in the diffusion rate of O ₂ , Cl _{2 and the} like diffused in the thinned water film. The shift of E _corr. Can be measured accurately, and a more realistic measured value than the spontaneous potential EE _corr. Measured value obtained by the conventional dipping method can be obtained. As a result, a highly reliable effective natural potential E′E _corr. Is obtained, and it is possible to accurately determine whether pitting corrosion has occurred in the metal material covered with the liquid film.

[0017]

[Example 1] SUS304 (18% Cr-8% Ni), SUS316
(18% Cr-8% Ni-2% Mo), 329J4L (25%
Cr-7% Ni-3% Mo-0.5% Cu-0.3% W-
A 0.15% N) stainless steel was used for the test piece s, and a water film w was formed on the surface of the test piece s. A stable water film w was not formed in the initial stage where the test piece s had a water-repellent fresh surface, but the water film w was stabilized to a constant film thickness due to changes with time. When the water film w is stabilized, Kelvin probe 2
Was made to face the water film w, and the spontaneous potential E _{corr. Of the} test object was determined by the spontaneous potential measuring device of FIG. During the measurement, the distance from the surface of the test piece s to the tip of the Kelvin probe 2 was set within a fluctuation range of 3 ± 0.5 mm, and the Kelvin probe 2 was vibrated at a cycle of 1000 Hz. When the measurement results were sorted by the film thickness of the water film w, as shown in FIG. 3, when the film thickness of the water film w was 1000 μm or more, the effective spontaneous potential E ′ _corr. Decreased regardless of the steel type. Reduced effective spontaneous potential E '
_{The corr.} value corresponds to the conventional value obtained by measuring the spontaneous potential E _corr. of the metallic material immersed in the bulk solution.

However, when the film thickness of the water film w is less than 1000 μm, the effective natural potential _{E'corr. Suddenly} changes to the noble side. For example, the effective natural potential E'of stainless steel
_corr. is a system in which a water film of 3% NaCl solution is present.
0 mV (vs. Ag / AgCl electrode standard), 1000 pp
mCl ^-. In a system where water film exists of a solution 350 mV (vs A
g / AgCl electrode standard). When the effective natural potential E in the presence of water film _'corr is compared with pitting potential (Table 1), 1000ppmCl ^_-. Risk of SUS304 in pitting occurs when the water film of the solution is present, water 3% NaCl solution It can be seen that SUS304 and SUS316 have a high risk of pitting corrosion when a film is present.

[0019]

[Example 2] Before SUS304 stainless steel causes pitting corrosion
After that, the effective natural potential E '_corr.To investigate the changes over time
It was Effective natural potential E '_corr.In the measurement of
SUS304 with a thin water film formed on the surface using a position measuring device
Natural potential E'of stainless steel _corr.Was measured continuously.
As shown in the survey results in Fig. 4, under the water film, SUS304
Effective self-potential E'when pitting corrosion occurs in stainless steel_corr.But
It turns out that it drops sharply. In other words, effective natural electricity
Rank E '_corr.The sharp decrease in the
That the pitting corrosion has occurred or that the pitting corrosion that has occurred is growing.
To taste.

Effective natural potential E'indicating the occurrence and growth of pitting corrosion
The sharp decrease in _corr. is also used as an index for implementing anticorrosion treatment. Specifically, when the effective natural potential _{E'corr. Drops} sharply, the stainless steel surface near the measurement point is cleaned, or a repassivation promoting agent such as sodium nitrate solution, a corrosion inhibitor such as molybdate ion, etc. And / or by injecting a neutralizing agent such as sodium hydroxide solution in the vicinity of the measurement point, the SUS304 stainless steel is protected from pitting corrosion.

[0021]

As described above, the effective natural potential _{E'corr. Of} a metal material covered with a thin water film is obtained while vibrating the Kelvin probe, and the effective natural potential _E'corr. Is _pitted. The occurrence and growth of pitting corrosion can be predicted by comparing with the potential V _c . This method differs from the conventional method that predicts the occurrence of pitting corrosion from the bulk spontaneous potential of a metallic material immersed in bulk water, and accurately determines the surface potential of a metallic material whose surface is covered by a thin water film of condensed water. It can be measured practically and enables effective anti-corrosion measures for heat transfer pipes, wastewater tanks, drainage storage tanks, metallic infrastructure, etc.

[Brief description of drawings]

FIG. 1 is a graph showing a potential-current curve of a metal material.

FIG. 2 is a schematic view showing a non-immersion type self-potential measuring device.

FIG. 3 is a graph showing the effect of the thickness of a water film on the surface of a metal material on the spontaneous potential.

FIG. 4 is a graph showing that the spontaneous potential of SUS304 stainless steel sharply decreases when pitting corrosion occurs.

[Explanation of symbols]

1: Reference electrode 2: Kelvin probe 3: Bias battery 4,5: Lead wire s: Test piece w: Water film

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI Theme Coat (Reference) G01N 27/46 M

Claims (3)

[Claims] 1. A bias voltage is measured while vibrating a Kelvin probe facing a water film of water or an electrolyte solution on the surface of a metal material in a non-contact manner, and the bias voltage has the same absolute value and opposite polarity. A method for determining the occurrence of pitting corrosion, which comprises obtaining an effective spontaneous potential from a Kelvin potential and comparing the effective spontaneous potential with a pitting potential. 2. The determination method according to claim 1, wherein the Kelvin probe is excited by varying the distance from the surface of the metal material to the Kelvin probe or the area of the Kelvin probe facing the surface of the metal material. 3. The determination method according to claim 1, wherein a metal material whose surface is covered with a passivation film is a measurement target.