JPH10170470A - Pitting corrosion monitoring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the monitoring of pitting corrosion with a resistance coefficient as an index by detecting the electric current value of each monitor device every predetermined time and calculating the resistance coefficient of a corrosion product by the detected current value. SOLUTION: By the contact between a metal member 30 and water-based medium at a pitting corrosion part, a potential difference occurs between the both of them. By dividing this potential difference by the sum of the resistance of corrosion reaction on the surface of the pitting corrosion part and the resistance of a corrosion product layer 41, the electric current passing through the surface of the pitting corrosion part is calculated. The product of this electric current value and time is multiplied by the atomic weight of the metal and divided by the number of valence electron of the metal involved in the reaction and the Faraday constant to calculate the amount of corrosion. The resistance coefficient of the rust 41 which occurs in a monitor device 29 instead of the rust 41 which occurs in the pitting corrosion part of the metal member 30 is calculated from the electric current value observed by the ammeter of the monitor device 29, and this calculated resistance coefficient is taken as a monitor index.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of monitoring pitting corrosion of a metal member, and more particularly to a method of monitoring the depth of pitting (erosion) due to local corrosion of a heat exchanger or a pipe, and the operation and water flow of the facility. The present invention relates to a pitting monitoring method that can be accurately calculated non-destructively without pausing.

[0002]

2. Description of the Related Art Local corrosion progresses in pipes and heat exchangers to increase the pitting depth, and when it reaches the penetration depth, an unexpected situation such as a stoppage of plant operation may occur. There is a need for a technique for estimating the depth of pitting.

Conventionally, the life of a heat exchanger or a pipe is determined by sampling a part of the operation of the equipment and stopping the flow of water,
It was estimated by measuring the pit depth of the sample.

[0004] However, the above-mentioned conventional method has a drawback that the operation of the plant is affected because the operation of the equipment must be stopped and a part of the equipment must be destroyed for sampling. In addition, there is a disadvantage that it takes a lot of time, labor, and cost until a measurement result is obtained.

[0005] As a method of solving the above drawback and estimating the pit depth of the metal by monitoring the progress rate of the local corrosion of the metal, the local corrosion of the metal member in contact with the aqueous medium is monitored. A liquid reservoir communicating with the aqueous medium via a small hole, and a metal piece of the same material as the metal member provided so as to be in contact with the liquid in the liquid reservoir, wherein the metal Using a monitor device in which the area of the surface of the piece in contact with the liquid in the liquid reservoir is larger than the opening area of the small hole, the metal piece and the metal member are brought into electrical contact with each other, and the current flowing between the two. There is a method of monitoring the local corrosion of a metal member by measuring the
-310452).

Hereinafter, a monitoring method disclosed in Japanese Patent Laid-Open No. 2-310452 will be described with reference to FIG.

Normally, the local corrosion of a metal member progresses by the formation of an oxygen concentration cell, with the potential difference between the metal-dissolved portion (anode) and the surrounding portion where the oxygen reduction reaction occurs (cathode) acting as a driving force.

In the method disclosed in Japanese Patent Application Laid-Open No. 2-310452, as shown in FIG. 2, for example, a columnar metal piece 32 of the same material as the metal member 30 is formed into a liquid reservoir 34 having a circular concave hole, for example.
To form a state of local corrosion in the liquid reservoir 34 in a simulated manner. Reference numeral 33 denotes a non-corrosive member such as vinyl chloride, and an aqueous medium is circulated on the upper surface side of the member 33 in the drawing.

This aqueous medium gradually renews the aqueous medium in the liquid reservoir 34 via the corrosion product (rust) 41 and the small liquid junction 40.

A metal member 30 serving as a cathode and a metal piece 32 serving as an anode are electrically connected by a lead wire 36, and a current flowing through the lead wire is measured by an ammeter 38. Estimate velocity and erosion depth.

According to the method disclosed in Japanese Patent Application Laid-Open No. 2-310452, it is possible to estimate pitting corrosion in a non-destructive manner in real time without stopping the operation of the equipment.

However, in the method disclosed in JP-A-2-310452, the pit depth is directly obtained from the anode current obtained by the monitor device using a short test tube. Actual pit depths may vary significantly with actual sizes of heat exchangers and different lengths of piping.

Therefore, the present inventor has proposed that at any time
The pit depth at present and in the future) is described in JP-A-2-3104.
A method of calculating the pit depth that can be calculated more accurately than the method of No. 52 has been proposed in Japanese Patent Application Laid-Open No. 5-215707. The pitting depth calculation method of the same publication is a method of calculating the pitting depth of a metal member such as a heat exchanger or a pipe in contact with an aqueous medium, and a liquid communicating with the aqueous medium through a small hole. A reservoir, and a metal piece of the same material as the metal member provided so as to be in contact with the liquid in the liquid reservoir. The area of a surface of the metal piece in contact with the liquid in the liquid reservoir has an area of the small hole. In a method of calculating the pitting depth of the metal member by measuring the current flowing between the metal piece and the metal member by electrically contacting the metal piece and the metal member using a monitor device larger than the opening area, A resistance coefficient of a corrosion reaction of the metal member is obtained, and a plurality of the monitoring devices are provided on the metal member. The resistance value of the metal member is determined based on a current value of each monitoring device and a resistance coefficient of a corrosion reaction of the metal member. Determine the resistance coefficient of the corrosion product and determine the resistance coefficient of the corrosion reaction. And it is characterized in that calculating the resistance coefficient of the corrosion products, the based on the potential difference generated by the contact pit depth of the metal member and the aqueous medium.

It is known that pitting progresses in various forms such as hemispherical and conical. In the following, the most general form is described in Japanese Patent Application Laid-Open No. The method will be described.

This method calculates the pit depth based on the pitting model shown in FIG. In FIG.
A hemispherical pit 50 is formed, and the pit 50 is covered with a corrosion product layer 51 having a uniform thickness so as to cover the pit.
The metal member is covered with the protective film in portions other than the pit portion, and the metal member and the aqueous medium are not in direct contact.

In the pit portion, a potential difference is generated between the metal member and the aqueous medium due to the contact between the metal member and the aqueous medium. By dividing this potential difference by the total resistance of the corrosion reaction resistance on the surface (hemispherical surface) of the pit portion 50 and the resistance of the corrosion product layer (rust) 51, The current flowing through the surface is calculated. The product of the current value and the time is further multiplied by the atomic weight of the metal and divided by the number of valence electrons involved in the reaction of the metal and the Faraday constant to calculate the amount of corrosion. In the case of this model, by treating the pit portion as a hemisphere, the radius of the pit portion is calculated from the total corrosion amount (ie, the volume of the hemisphere of the pit portion).

The pit portion grows momentarily, but according to the description of Japanese Patent Application Laid-Open No. 5-215707, a certain current flows for a certain day (24 hours), and corrosion proceeds.
On the next day (24 hours), it is assumed that a constant current corresponding to the surface area (surface area of the hemisphere of the pit portion) flows through the pit portion whose diameter has been increased by the energization.

Then, the resistance coefficient of the corrosion reaction and the resistance coefficient of the corrosion product are calculated from the current value actually measured by the monitor device as described later.

1. Pitting corrosion progress model When a metal member having an anticorrosion coating is corroded, pinhole-shaped destruction occurs in the anticorrosion coating for some reason, and a pit (pitting) is formed in a hemispherical shape around the pinhole as shown in FIG. Progress slowly. And as shown in FIG. 6, rust (corrosion product)
51 covers the pit portion.

In the model for predicting the pit depth of the metal member 52, as described above, the pit portion 50 is assumed to be hemispherical, and the rust 51 is accurately formed in a disk shape. The radius of this hemisphere is represented by r, and the height of the rust is represented by h.

2. Pitting corrosion resistance value Rt The destruction of the anticorrosion coating causes a potential difference ΔE at the liquid contact interface of the metal member with the aqueous medium (liquid), and a current It flows between the liquid and the metal member. The current flowing between the liquid and the metal member receives rust resistance and reaction resistance at the liquid contact interface between the liquid and the metal member.

Assuming that the sum of these resistances is the pitting resistance value (total pitting resistance) Rt, Rt = (reaction resistance) + (rust resistance) (1) This reaction resistance depends on the area of the liquid-contact interface (pit corrosion portion 5).
0 hemisphere surface area) is inversely proportional to 2πr ² . Therefore, assuming that the proportionality constant is K ₁ , the reaction resistance is expressed as K ₁ / 2πr ² .

The rust resistance is proportional to the rust height h.
The rust height h is (rust volume) / (rust bottom area),
Rust volume, calculated by multiplying the ratio of the pitting has been total volume 2.pi.r ^3/3 density of the metal of the density d and rust (volume of a hemisphere) d of metal '.

[0024] That is, the height h of the rust is ^{h = ((2/3) πr 3} d / d ') / πr 2 ... (2), when the line resistance proportional coefficient of rust and K _2, rust The resistance is K ₂ · h. (Note that this K ₂ value varies depending on the water quality environment, temperature, and flow conditions.
In the invention of No. 7, these environmental conditions are always constant during the monitoring test period. Therefore, the pitting resistance value (total resistance) Rt is as follows: Rt = (reaction resistance) + (rust resistance) = K ₁ / (hemispheric surface area) + K ₂ · h = K ₁ / 2πr ² + K ₂ · ((2 / 3) πr ³ d / d ')
/ Πr ² . Assuming that K ₂ / d ′ in the second term on the right side is K ₂ ′, Rt = K ₁ / 2πr ² + K ₂ ′ · (2/3) πr ³ d / πr ² = K ₁ / 2πr ² + K ₂ ′ · (2/3) r · d (3) Here, K ₁ is a proportional coefficient, that is, a resistance coefficient of corrosion reaction (Ω · mm ^2). K ₂ ′: A proportional coefficient, that is, a resistance coefficient of rust (Ω / mm · (mg /
mm ³⁾ ) r: radius of pitting d: density of metal such as iron (mg / mm ³⁾ The meaning of equation (3) is pitting resistance value (total resistance) = K ₁ / (anode area) + K ₂・ · (amount of corroded metal) / (pit pit opening area) (4)

In the corrosion monitor (FIG. 2), the metal piece 32 is corroded in the same manner as the metal member 52, and the similar rust 41 is formed.
Is caused. Therefore, in this corrosion monitor, Rt is the anode area (that is, the liquid contact area S _a of the metal piece 32), the pit frontage area (the frontage area S _b of the liquid junction 40), and Rt, as in the above equation (4). the amount of corrosion D _j of the j-day metal strip 32 is expressed as follows.

Rt = K ₁ / S _a + K ₂ ′ · D _j / S _b (4.5) Resistance coefficient of resistance and rust corrosion reaction the K ₁ is linear polarization resistance method, impedance measurement, potentiostatic polarization measurements,
It can be determined by a constant current polarization measurement method or the like.

K ₂ ′ can be determined by a local corrosion monitor. That is, assuming that the pitting current is It, ΔE = It · Rt according to Ohm's law, and ΔE and It can be measured, so that Rt is obtained. In equation (4.5), the area S on the right side
_a, S _b are known, K ₁ is measured by a measuring method such as the linear polarization resistance method. Corrosion amount D _j is calculated from the integrated value and a metal atomic weight and the Faraday constant value of current flowing in the ammeter 38 of the corrosion monitoring. (The calculation of the amount of corrosion D _j is the same as the calculation of W _{j in} the following equation (5)). Therefore, by substituting these S _a , S _b , K ₁ and D _j into the equation (4.5), K ₂ ′ is obtained from the equation (4.5).

Thus, the resistance coefficient K _{1 of the} corrosion reaction is obtained.
And the resistance coefficient K ′ _{2 of the} corrosion product (rust) is determined.

4. Pitting voltage ΔE, Pitting current I _j on day _j
Pitting amount W _j when calculating I _j becomes a current pitting amount flows 1 days is determined as follows. W _j = I _j · (3600 · 24) · (M / Z · F) (5) M: atomic weight of metal such as iron constituting metal member 32 F: Faraday constant Z: charge number (in case of iron) 2) 1 day from the time the pitting has occurred (between 0-24 hours) the current flows comprising I ₁ on average, 2 days (24 hours to 48
On the other hand, during the time period, a current of I ₂ flows on average, and on the third day (4
For 8 hours to 72 hours) flows a current becomes I _3, ... when the j-th day and that flow I _j becomes current, corrosion amount G _n after n days elapsed since the pitting initiation

[0030]

(Equation 1)

## EQU1 ##

This I _j, ie, I ₁ , I ₂ , I ₃ ... Can be calculated based on the above K ₁ , K ₂ ′.

Next, a method of calculating I ₁ , I ₂ ,... Will be sequentially described.

It is assumed that the current I ₀ when pitting occurs (time t = 0), the anode area when pitting occurs = A. A is an extremely small value (for example, 0.0001 mm ² ). Pitting depth r ₀ = 0. In this state of t = 0, (anode area) = (pitting area) = A.

Since I = ΔE / Rt, and from the above equation (4), Rt = K ₁ / (anode area) + K ′ ₂ · (corrosion metal amount) / (pit corrosion frontage area)

[0036]

(Equation 2)

Is as follows. When t = 0, (anode area) = (pitting area) = A as described above,
Since (the amount of corroded metal) = 0, the current value I ₀ when t = ₀ is as shown in the following equation (8).

[0038]

(Equation 3)

In the equation (8), K ₁ and ΔE are known, and A is set to a predetermined value, so that I ₀ is obtained from the equation (8).

One day after the occurrence of pitting corrosion (after 24 hours). For these 24 hours,

[0041]

(Equation 4)

It is assumed that the pit portion has a constant radius r ₁ as a result.

The corrosion amount W ₁ during this day calculated from the current value I ₀ is as shown in the following equation (9).

W ₁ = I ₀ · (3600 · 24) · (M / Z · F) (9) (M: atomic weight of metal, F: Faraday constant) The amount of corroded metal during this one day is (2 / 3) is πr ₁ ³ d. Here, d is the density of the metal.

Since the total amount of corrosion is G ₁ = W ₁ and the pit portion is a hemisphere, (pit depth) is equal to (radius of hemisphere).
That is, the pit depth r ₁ = (3G _{1 /} 2πd) ^1/3 = (3W _{1/2 /} 2πd) ^1/3 (10).

By substituting the value of W ₁ calculated from Equation (9) for W ₁ in Equation (10), the pit depth r ₁ after the lapse of the first day (first 24 hours) is obtained. .

Two days after the occurrence of pitting (after 48 hours) In this new 24 hours, the current I ₁ flowing becomes r ₁ in the equation (7) because the radius of the pit portion at that time is r _1. by placing the r _1, it will be as follows.

[0048]

(Equation 5)

From equation (10), r ₁ is obtained, and G
_{Since 1} (= W ₁ ) is obtained from equation (9), this (11)
Current I ₁ is calculated from the equation.

[0050] (between 24 hours to 48 hours) pitting the second day this I ₁ continues to flow in one day (24 hours) constant. The amount of corrosion W ₂ for one day is W ₂ = I ₁ · (3600
24) Calculated from (M / Z · F). And G
₂ (= W ₂ + W ₁ ) is also obtained from this.

Meanwhile, since the corrosion amount G ₂ after a lapse of 2 days is _{G 2 = W 2 + W 1} = (2/3) πr 2 3 d ... (11.5), the pitting depth r ₂ is expressed by the following equation Required by

R ₂ = (3G ₂ / 2πd) ^1/3 (12) On the third day of pitting, the next current I ₂ is kept flowing for one day (24 hours) in the same manner, and for 24 hours amount of corrosion W ₃ of is as follows.

[0053]

(Equation 6)

W ₃ = I ₂ · (3600 · 24) · (M / Z · F) (13.5) r ₂ is obtained from the above equation (12), and G ₂ is also obtained from the equation (11.5). From the equation (13), I ₂ is obtained. Then, W ₃ is also obtained from equation (13.5).

The total amount of corrosion three days after the occurrence of pitting corrosion G ₃ = W ₃ + W
Since a ₂ + W _1, Motomari is G _3, the pitting depth r ₃ is determined by the following equation.

R ₃ = (3G ₃ / 2πd) ^1/3 (14) Even after the fourth day, the current value of that day can be obtained from the pitting depth up to the previous day. Then, the amount of pitting occurring on the day is calculated. Thus, r ₁ , r
By sequentially calculating ₂ , r ₃ ... And I ₀ , I ₁ , I ₂ ..., The pit depth after n days has elapsed can be calculated.

Since the environmental conditions are assumed to be constant and therefore K ₂ ′ is constant, the current _Im
Is the liquid contact area (2π
r ² _m-1 ), and the pit depth after n days can be calculated only from the values of r ₁ , r ₂ , r ₃ ... without calculating I _m. .

This will be described below.

[0059] In the pitting n days after corrosion by (n-1) day -th to I _n-1 becomes the current one day flow constant over (24 hours), the amount made new W _n in the 1 day Due to the progress, the total amount of corrosion becomes _Gn . W _n = I
_{Since n−1} · (3600 · 24) · (M / Z · F), G _n is as follows.

[0060]

(Equation 7)

By substituting the calculated values of r ₁ , r ₂ , r ₃ ... Into this equation (22), G _n is obtained.

By the way, since G _n = (2/3) πr _n ³ · d) (23), the pit depth r _n = (3G _n / 2πd) ^1/3 (24)

Since G _n is calculated from equation (22), r _n is calculated by substituting the G _n value of equation (22) into equation (24).

When r _n is represented by a general formula, (24)
The following equation (25) is obtained by substituting equation (22) into the equation.

[0065]

(Equation 8)

In this way, r ₁ , r ₂ , r ₃ ... Are sequentially calculated, and they are substituted into equation (25) to calculate the pitting depth r _n after the elapse of an arbitrary n days. can do. Then, it is possible to monitor the pitting this pitting depth r _n as an index.

[0067]

The problem to be solved by the present invention is disclosed in Japanese Patent Application Laid-Open No. 5-215.
In the method of No. 707, the resistance of the corrosion product rust 51 is treated as a constant value.

However, in an actual water system, the water quality fluctuates every day, and the resistance of the rust 51 often fluctuates every day. In such a case, the method of Japanese Patent Application Laid-Open No. 5-215707, in which the resistance of the rust 51 is always a constant value (specifically, it is assumed that K ₂ ′ is constant as described above), is the pitting depth. The error of the calculated value from the actual pit depth becomes large.

An object of the present invention is to solve such a problem and to provide a more accurate method for monitoring pitting corrosion than the method disclosed in Japanese Patent Application Laid-Open No. 5-215707.

[0070]

According to the method of calculating the pit depth of the present invention, the method of Japanese Patent Application Laid-Open No. HEI 5-215707 detects the current value of each monitor device at predetermined time intervals, and uses the current value to determine the corrosion rate. The resistance coefficient of the product is calculated, and pitting corrosion is monitored using the resistance coefficient as an index.

According to the pit depth calculation method of the present invention, the value of the resistance coefficient of the corrosion product becomes very close to the actual value, so that the pitting monitoring accuracy is high.

[0072]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, in a pit portion,
The contact between the metal member and the aqueous medium causes a potential difference between the two. By dividing this potential difference by the total resistance of the resistance of the corrosion reaction on the surface of the pit portion and the resistance of the corrosion product layer (rust), the current flowing on the surface of the pit portion is calculated. . The product of the current value and the time is further multiplied by the atomic weight of the metal and divided by the number of valence electrons involved in the reaction of the metal and the Faraday constant to calculate the amount of corrosion.

The rust grows momentarily, but in the following description, it is assumed that a certain current flows during a certain day (24 hours) and corrosion proceeds.

In the present invention, for the rust generated by the monitor device instead of the rust generated at the pit portion of the metal member (piping) 30, the resistance coefficient is calculated from the current value actually measured by the ammeter of the monitor device. The resistance coefficient is used as a monitoring index.

1. FIG. 1 is an enlarged view of a main part of the monitor device 29 shown in FIG. Also in this embodiment, it is assumed that the rust 41 as a corrosion product is formed in a columnar shape having the same diameter as the liquid junction 40. The rust 41 includes a first layer 411 formed on the first day, a second layer 412 formed on the second day, an n-th layer 41n formed on the n-th day, Consists of Let h be the height of the rust 41.

2. Corrosion resistance in the monitor device Potential difference ΔE at the liquid contact interface between the metal piece 32 and the aqueous medium (liquid)
Occurs, and a current flows between the liquid and the metal piece 32. The current flowing between the liquid and the metal piece 32 receives the resistance of the rust 41 and the reaction resistance of the liquid contact interface between the liquid and the metal piece 32.

The sum of these resistances is referred to as the pitting resistance value (total resistance).
Then, as described above, the total pitting resistance = (reaction resistance) + (rust resistance) (1) The right reaction resistance of the first term is inversely proportional to the area S _a of the wetted interface. Therefore, a proportionality constant is K _1,
Reaction resistance is expressed as K ₁ / S _a .

The rust resistance in the second term on the right side is the rust height h.
Is proportional to The height h of the rust is (volume of rust) / (bottom area of rust), rust volume, pitting metal density d and rust corrosion total weight G _j of j-day metal strip 32 Multiplied by the ratio to the density d '.

That is, the rust height h is given by h = G_jD / d '... (26) where the linear resistance proportional coefficient of rust is K_TwoThen the rust resistance is
K_TwoH. This K _TwoValues are water quality environment, temperature, flow
Varies depending on conditions.

In this embodiment, K ₂ is assumed to fluctuate every day. That is, the first day of K ₂ a (K ₂ of the first layer 411) K _21, second day of K ₂ (the first layer 411 and K ₂ of the laminate between the second layer 412) K ₂₂ ……… K on the nth day
₂ (K ₂ of the laminate of the first layer, the second layer,... And the n-th layer) is represented by K _2n . Note that the rust density d 'actually changes. Therefore, a value K _2j / a value obtained by dividing the linear resistance proportional coefficient K _2j of rust on the j-th day by the entire density d _j ′ of rust 41 _j on that day
Let d _j ′ be K _2j ′.

By using this K _2j ′, the total resistance of the monitor device on the j-th day can be _calculated by the following equation (1): (total resistance of monitor device) = (reaction resistance) + (rust resistance) = K a _{1 / S a + K 2j ·} h = K 1 / S a + K 2j · G j · d / d j '= K 1 / S a + K 2j' · G j · d ... (27). Where K ₁ is a proportional coefficient, that is, a resistance coefficient of corrosion reaction (Ω · mm ²⁾ K _2j ′ is a proportional coefficient of rust 41 on the jth day, that is, a resistance coefficient of rust (Ω / mm · (mg / mm ³⁾ ) d: Density of metal such as iron (mg / mm ³⁾ The (total resistance of the monitor device) of the expression (27) is obtained by calculating the current detected by the ammeter 38 on the jth day in the local corrosion monitor device of FIG. _{If j} , then △ E = I according to Ohm's rule
_j (Total resistance), so (Total resistance of monitor device) =
ΔE / I _j

This K ₁ is obtained in advance by a linear polarization resistance method, an impedance measurement method, a constant potential polarization measurement method, a constant current polarization measurement method, and the like, and this value is treated as constant during the entire test period. This K ₁ takes a different value by the quality of the test water to be measured is measured using a water continuous water passage test. If water of the cooling water, K ₁ is usually 10~30KΩ
・ Take the value of mm.

The resistance coefficient K ₁ of the corrosion reaction is determined by, for example, a potentiostatic polarization measurement method using the equipment (1) and the test water (2) according to the procedure (3).

[0084] (1) Corrosion measurement instrument resistance coefficient K ₁ of the reaction (a) 1L beaker (b) pitting sensor (anode part only) (c) STB-35 halved tube (used as a cathode) (d) Silver / silver chloride electrode (Specified as reference electrode) (e) Potentiometer (f) Ammeter (g) Magnetic stirrer (2) Test water: The water used in the water flow test is used as test water. (3) Procedure: Put test water into a 1 L beaker, and add (b), (c),
(D) is immersed. Stir using a magnetic stirrer. Using (e) above, 50, 10 between (b) and (c)
A potential difference of 0, 150, 200 mV is created. The current flowing between (b) and (c) is measured by (f). A graph is created with the potential difference on the horizontal axis and the current value on the vertical axis. Since the slope of this graph is the resistance, the resistance is obtained by calculating the slope of the graph. Incidentally, the anode electrode area = 7.0
Since the resistance is 7 mm ² , the resistance is divided by the anode area to obtain “anode reaction resistance per unit area = K1”. G _j is
The sum of the corrosion amounts W _j of the metal pieces 32 per day when the current I _j flows for one day on the _j day, that is, G _j = W ₁ + W ₂ +
... + W _j . This W _j is obtained as follows using the above equation (5).

W _j = I _j · (3600 · 24) · (M / Z · F) (5) M: atomic weight of metal such as iron constituting metal piece 32 F: Faraday constant Z: valence number for iron 2) 1 day from the time of installing the monitoring device (between 0-24 hours) is I ₁ becomes current flows on average, during the second day (24 hours to 48 hours) are averaged The current I ₂ flows,
On the day (between 48 hours and 72 hours), a current I ₃ flows, and on the j day, a current I _j flows.
The total corrosion amount G _j after j days from the start of pitting corrosion is as described above.

[0086]

(Equation 9)

## EQU10 ##

From equation (27), K _2j ′ = [(total resistance of monitor device) −K ₁ · S _a ] / G _j · d (28) K _2j ′ is calculated as follows. Is done.

[0089]

(Equation 10)

The K _2j ′ obtained in this manner is monitored as an index, and when K _2j ′ falls below the reference value,
Water treatment is performed to delay the progress of pitting.

[0091] In the present invention, it may be an indicator of the logarithm of 'K _2j instead of' K _2j.

[0092]

EXAMPLES The present invention will be specifically described below with reference to examples.

[0093]

Example 1 Pitting corrosion of a pipe was monitored using the test apparatus shown in FIG. FIG. 6 shows a cross-sectional view of the vicinity of the monitor device in this test apparatus.

The water in the tank 10 is
And a metal (STB-35) tube 12 circulated by pumps 13 and 14. A monitor device 29 is installed near each metal tube 12.

The configuration of the monitor device 29 is the same as that of FIG. 2, and the same components are denoted by the same reference numerals. It should be noted that a water channel in which three tubes 12 are connected in series is provided in two systems in parallel. The discharge flow rates of the pumps 13 and 14 are made different, and the liquid flows at different flow rates in each system. Reference numeral 15 denotes a flow meter, and 16 denotes a makeup water tank. The main test conditions are as follows. Metal (STB-35) pipe 300 mm length x 19 mm inner diameter System flow rate 1 m / sec System flow rate 0.3 m / sec Water quality pH 8.5 Calcium hardness 250 mg / L M alkalinity 250 mg / L Magnesium hardness 100 mg / L Silica 100 mg / L Anticorrosive Crisette S112 70 mg / L (Crisette S112 is a registered trademark of Kurita Water Industries Co., Ltd.) Water temperature 20-25 ° C Test period 30 days Monitor device 2 mm in diameter of liquid junction 40 mm Contact area S _a of metal piece 32 7.1 mm ⁽²⁾ Potential difference due to liquid contact ΔE 0.15 V Atomic weight of STB-35 M 55860 mg / mol Charge number of STB-35 Z 2 Resistance coefficient of corrosion reaction K ₁ 24 KΩ · mm K _2j ′ (K _2j) obtained under the above conditions The logarithm of / d ') was defined as the pitting inhibition index and is shown in FIG.

As shown in FIG. 5, K _2j is larger in a system having a higher liquid flow rate. In addition, the average value of the pitting depth generated in each metal tube 12 after passing water for 30 days is as follows, and is smaller in the system.

Average Pitting Corrosion Depth of System 0.99 mm Average Pitting Corrosion Depth of System 1.84 mm Example 2 A test was conducted using the actual apparatus shown in FIG.

The cooling water inlet and outlet of the heat exchanger 20 having 1065 heat transfer tubes are connected to the cooling tower 23 by pipes 21 and 22.

From the pipes 21 and 22, branch pipes 24 and 25 were branched, and five STB-35 tubes 26 were connected in series to the branch pipes 24 and 25, respectively. Each tube 26 was installed in the heat exchanger 20.

The same material and the same diameter as the heat transfer tube (outer diameter 19 m)
m), the same thickness (1.6 mm) and the same length (500 mm
L), and water is supplied to each tube 26 at the same flow rate as the heat transfer tube. A local corrosion monitoring device 29 is provided at the outlet end of each tube 26 in the same manner as in FIG.
Is installed.

The outputs of all the local corrosion monitoring devices 29 are input to a monitoring computer (not shown).

The quality of the cooling water circulating through the cooling tower 23 and the heat exchanger 20 fluctuates between the following.

[0104] Temperature 3.3 to 17 ° pH 8.3-8.9 M alkalinity = 147~206mg · CaCO _3/1 calcium hardness = 200~270mg · CaCO _3/1 chloride ion = 106~135mg · CL ^{- / 1 FFZ = 1.0~1.7mg · Zn} / 1 ZenRinsan ^{_{= 5.5~8.8mg · PO 4 3- / 1}} SiO 2 concentration = 76~100mg / 1 in this way each station corrosion The anodic current flowing through the monitor lead wire 25 was measured with the ammeter 26 over 361 days. Then, K _2j ′ was calculated once an hour based on the detection value of the ammeter 26. The logarithm l of this K _2j ′
ogK _2j ′ was used as a pitting corrosion monitoring index, and a corrosion inhibitor was injected so that the log K _2j ′ value was always 5.5 or less.

FIG. 8 shows a part of the test results.

Around May 1994, the pitting corrosion inhibition index was 5.5.
It became the following. The residual chlorine concentration was lower than 0.1 mg / L, which was considered to promote pitting corrosion due to the influence of slime. Then, when the injection amount of the disinfectant was increased so that the residual chlorine concentration became 0.2 mg / L or more, the pitting corrosion inhibition index increased to 5.5 or more.

One year after the start of the test, 12 tubes were removed from the heat exchanger and the maximum pit depth was measured every 20 cm. The maximum pit depth was 0.30 mm. In general, it is considered that the pit depth of mild steel progresses to a third power with respect to time, so that the maximum pit depth after 10 years is 0.65 mm.

Since the heat exchanger uses a 1.6 mm-thick tube, if the pitting inhibition index is 5.5 or more, the pitting depth for 10 years can be suppressed to less than half of the wall thickness. Is possible.

[0109]

As described in detail above, according to the pitting monitoring method of the present invention, the operation of the heat exchanger and the pipes can be performed non-destructively without stopping the operation of the heat exchangers and the pipes. It is possible to accurately monitor the pitting corrosion at any time due to local corrosion.

According to the method of the present invention, it is possible to achieve safe and stable operation of various plants and extend the life of metal equipment members.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a main part of a monitor device.

FIG. 2 is a cross-sectional view illustrating a conventional monitoring method.

FIG. 3 is a sectional view showing a pitting corrosion model.

FIG. 4 is a system diagram of a water flow test device.

FIG. 5 is a graph showing test results of the water flow test device of FIG.

FIG. 6 is an enlarged view of a main part of the apparatus of FIG.

FIG. 7 is a system diagram of an actual machine subjected to a water flow test.

FIG. 8 is a graph showing the results of a pitting corrosion test.

[Explanation of symbols]

 12, 26 STB-35 tube 23 Cooling tower 29 Local corrosion monitor 32 Metal piece 34 Liquid reservoir 36 Lead wire 38 Ammeter 40 Liquid junction (small hole)

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Akihide Hirano 3-4-7 Nishi-Shinjuku, Shinjuku-ku, Tokyo Inside Kurita Kogyo Co., Ltd.

Claims (1)

[Claims] 1. A method for monitoring pitting corrosion of a metal member such as a heat exchanger or a pipe in contact with an aqueous medium, comprising: a liquid reservoir communicating with the aqueous medium via a small hole; A metal piece provided with the metal member provided so as to be in contact with the liquid, using a monitor device in which the area of the metal piece in contact with the liquid in the liquid reservoir is larger than the opening area of the small hole. A method for monitoring the pit depth of a metal member by electrically contacting the metal piece and the metal member and measuring a current flowing between the metal member and the metal member, wherein a resistance coefficient of a corrosion reaction of the metal member is determined in advance. And a potential difference caused by the contact between the metal member and the aqueous medium, and a plurality of the monitor devices are provided on the metal member, and the current value of each monitor device is detected at predetermined time intervals. And the resistance coefficient of the corrosion reaction of the metal member Based determined every predetermined time the resistance coefficient of the corrosion products of the metal member, the method of monitoring the pitting, which comprises monitoring the pitting of the resistance coefficient as an index.