JP3314644B2 - Pitting depth calculation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calculating a pit depth of a metal member, and more particularly, to a method of calculating the pit depth (erosion) due to local corrosion of a heat exchanger or a pipe, by operating the facility, passing water. The present invention relates to a pit depth calculation method capable of non-destructively calculating accurately 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. 3, 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. The pitting resistance value Rt is caused by the destruction of the anticorrosion coating, and the potential difference ΔE at the liquid contact interface of the metal member with the aqueous medium (liquid).
Is generated, 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 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 Rt is: 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 corrosion d: density of metal such as iron (mg / mm ^{3) In} addition, the meaning of the expression (3) is as follows: Rt = K ₁ / (anode area) + K ₂ ′ · (corrosive metal Amount) / (Pit pit 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) and 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 · (amount of corroded metal) / (pit corrosion frontage area)

[0035]

(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).

[0037]

(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 (24 hours). For these 24 hours,

[0040]

(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 in 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 G ₁ = W ₁ and the pit portion is hemispherical, (pitting depth) is equal to (radius of hemisphere).
That is, the pit depth r ₁ = (3G ₁ / 2πd) ^1/3 = (3W _1/2 / 2d) ^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 first day (first 24 hours) has elapsed is obtained. .

Two days after the occurrence of pitting (after 48 hours) In this new 24 hours, the current I ₁ flowing is represented by the following 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.

[0047]

(Equation 5)

From the above 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.

[0049] (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.

On the other hand, 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.

[0052]

(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).

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 the current day is obtained from the pit depth until the previous day. Then, the amount of pitting occurring on the day is calculated.

Thus, by sequentially calculating r ₁ , r ₂ , r ₃ ... And I ₀ , I ₁ , I ₂ .
The pit depth after n days 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.

[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 pit depth calculating method 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. 5-215707 detects the current value of each monitor device at predetermined time intervals, and calculates the K value based on this current value. ₂ ′ is calculated, and the pitting depth is calculated using this K ₂ ′.

According to the pitting depth calculating method of the present invention, the value of K ₂ ′ is very close to the actual value, so that the error in the calculated value of the pitting depth is extremely small.

[0072]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, pitting is known to progress in various forms such as hemispherical and conical, but in this embodiment, the most common form is hemispherical. A case where the process has proceeded will be described as an example.

The present invention is a method for calculating the pit depth based on the pitting model shown in FIG. In FIG. 4, a hemispherical pit portion 50 is formed, and the pit portion 50 is covered with a corrosion product layer 51 having a uniform thickness so as to cover the pit portion. 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 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 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).

Although the pits grow every moment, in the following description, a certain current flows during a certain day (24 hours), corrosion proceeds, and the next day (24 hours). time)
Treats 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.

In the present invention, 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. Progress gradually like. Then, as shown in FIG. 4, rust (corrosion product) 51 covers the pitting portion. The rust 51 is composed of the first layer 511 formed on the first day and the second layer 511 formed on the second day.
A layer 512 and an n-th layer 51 formed on the n-th day
n.

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 Rt, Rt = (reaction resistance) + (rust resistance) (1) as described above. 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 height h of the rust.
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 '.

[0082] 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. This K ₂ value varies depending on the water quality environment, temperature, and flow conditions.

In this embodiment, K ₂ is assumed to be replaced every day. That is, K ₂ on the first day (first layer 511)
The K ₂₎ of K _21, and the second day of K ₂ (first layer 511 second
K ₂₎ the K ₂₂ of the laminate of the layer 512, ......... day n of K ₂ (first layer, K ₂ of the laminate of the second layer ...... and the n layer)
_Is K _2n . Note that the rust density d 'actually changes. Therefore, the value K _2j / d obtained by dividing the linear resistance proportional coefficient K _2j of rust on the j-th day by the overall density d _j ′ of rust 51 _j on that day.
_{Let j} ′ be K _2j ′.

[0084] By using this K _2j ', pitting resistance Rt _j in the j-th day is the equation (3) as, Rt _j = (reaction resistance) + (rust resistance) = K ₁ / (hemisphere surface _{area) + K 2j · h = K} 1 / 2πr 2 + K 2j · ((2/3) πr 3 d / d j ') / πr 2 = K 1 / 2πr 2 + K 2j' · (2/3) πr 3 d / Πr ² = K ₁ / 2πr ² + K _2j ′ · (2/3) r · d (3j) Where K ₁ is a proportional coefficient, that is, a resistance coefficient of a corrosion reaction (Ω · mm ^{2), and} K _2j ′ is a proportional coefficient of rust 51 on the jth day, that is, a rust resistance coefficient (Ω / mm · (mg / mm ³⁾ ). r: radius of pitting corrosion d: density of metal such as iron (mg / mm ^{3) In} addition, the meaning of the expression (3j) is as follows: Rt _j = K ₁ / (anode area) + K _2j ′ · (amount of corroded metal) / (Pit pit frontage area) (4j)

In the corrosion monitor (FIG. 2), the metal piece 32 is corroded in the same manner as the metal member 52 (FIG. 4) to generate the same rust 41. Therefore, also in the corrosion monitor of FIG. 2, Rt _j is calculated as in the above equation (4j).
The anode area (that is, the liquid contact area S _a of the metal piece 32), the pitting opening area (the opening area S _b of the liquid junction 40), and the metal piece 3
It is expressed as follows by the amount of corrosion D j for 2 days _j .

Rt _j = K ₁ / S _a + K ₂ ′ · D _j / S _b (4.5j) 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 ₁
Before the main test, the same water as the main test water was used, the corrosion monitor was measured as the anode, and the metal member was measured as the cathode, and this value was treated as constant during the entire main test.

K _2j ′ can be determined by the local corrosion monitor shown in FIG. That is, according to Ohm's law, ΔE = I _j · R
t, and ΔE and I _j can be measured, so that Rt _j is obtained. In the equation (4.5j), the areas S _a and S _{b on the} right side are known, and K ₁ is measured by a measurement method such as the linear polarization resistance method. The corrosion amount D _j is measured by the ammeter 38 of the corrosion monitor.
It is calculated from the integrated value of the current value flowing through the element, the metal atomic weight, the valence, and the Faraday constant. (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 (4.5j), the (4.5j)
K _2j ′ is obtained from the equation.

Thus, the resistance coefficient K _{1 of the} corrosion reaction is obtained.
And the resistance coefficient K _2j of the corrosion product (rust) on day j
'Is found.

4. Calculation of Pitting Volume from Pitting Voltage ΔE and Pitting Current It The pitting volume W _j when a current I _j flows for one day can be obtained 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, as the corrosion amount G _n after n days elapsed since the pitting initiation the

[0090]

(Equation 9)

Is obtained.

This I _j, ie, I ₁ , I ₂ , I ₃ ... Can be calculated based on the above K ₁ and K _2j ′.

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.

I _j = ΔE / Rt _j , and the above equation (4)
j), Rt _j = K ₁ / (anode area) + K _2j ′ · (amount of corroded metal) / (pitting area)

[0096]

(Equation 10)

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 at} t = ₀ is given by the following equation (8j).

[0098]

[Equation 11]

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

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

[0101]

(Equation 12)

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 G ₁ = W ₁ and the pit portion is hemispherical, (pitting depth) is equal to (radius of hemisphere).
That is, the pit depth r ₁ = (3G ₁ / 2πd) ^1/3 = (3W _1/2 / 2d) ^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 is represented by the following equation (7) because the radius of the pit is r _1. by placing the r _1, it will be as follows.

[0108]

(Equation 13)

From the above 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.

[0110] (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.

[0111] On the other hand, 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 the occurrence of pitting, the current I _{2 of} the following equation (13j) similarly flows constantly for one day (24 hours) Continue, part 2
The amount of corrosion W _{3 for} 4 hours is given by the following equation (13.5).

[0113]

[Equation 14]

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 Equation (13j), I ₂ is obtained. Then, W ₃ is also obtained from equation (13.5).

Total corrosion amount G ₃ = W ₃ + W 3 days after the occurrence of pitting corrosion
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 the current day can be obtained from the pit depth until the previous day. Then, the amount of pitting occurring on the day is calculated.

In this way, by sequentially calculating r ₁ , r ₂ , r ₃ ... And I ₀ , I ₁ , I ₂ .
The pit depth after n days can be calculated.

[0118] Note that the current I _m of the day are those determined by the wetted area of the pitting unit that occurred the day before (2πr ² _m-1) and the resistance coefficient of rust 51m K _2m of the day ', bother without calculating the _{I m, r 1, r 2} , r 3
... And K ₂₁ ′, K ₂₂ ′, K ₂₃ ′.
The pit depth after the passage of days can be calculated.

This will be described below.

[0120] 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.

[0121]

(Equation 15)

G _n is obtained by substituting the calculated values of r ₁ , r ₂ , r ₃ ... And K ₂₁ ′, K ₂₂ ′, K ₂₃ ′. However, because it is _{G n = (2/3) πr n} 3 · d ... (23), the pitting depth _{r n = (3G n / 2πd} ) becomes ^1/3 ... (24). Since G _n is calculated from equation (22n),
By substituting the _Gn value of the equation (22n) into the equation (24), r
_n is calculated. Note that when representing a r _n in the general formula, (2
The following equation (25n) is obtained by substituting equation (22) into equation (4).

[0123]

(Equation 16)

In this manner, r ₁ , r ₂ , r ₃ ... Are sequentially calculated, and are calculated by substituting them into the equation (25) to calculate the pitting depth r _n after elapse of an arbitrary n days. can do.

In the above description, the calculation method has been described on the assumption that the pit portion progresses in a hemispherical shape. However, even when the pit portion progresses in a conical shape or a contact lens shape, the calculation is performed in the same way. be able to.

FIG. 5 is a cross section of the pit portion showing a case where the pit portion 50 is formed in a partially spherical shape composed of a part of a sphere having a radius R.

When the depth of the pit portion 50 is r, the radius of the frontage of the pit portion 50 is ar. This a is assumed to be constant irrespective of the change of r in this embodiment.

In this case, (anode area) = (surface area of partial sphere) = (a ² +1) πr ² (area of pit opening) = (area of circle with radius ar) = a ² πr ² (area of pit section) volume) = amount (volume of the ^{part-spherical) = (3a 2 +1) ·} πr 3/6 ( metal (element, which serves as rust due to corrosion)) = (volume of pitting unit) × (density) = (3a ² +1) · πr ³ · d / 6 By the way, the content of the denominator of the target expression of Σ in the equations (19n) to (25n) is (reaction resistance) + (rust resistance). This (reaction resistance) is K ₁ / (anode area), that is, K ₁ / (a ² +1) πr ² .

(Rust resistance) is (rust height h) · K _2. This is obtained by replacing K ₂ with the rust resistance coefficient K _2j ′ on the j-th day. / (Pit frontage area)] · K _2j ′ or [((3a ² +1) πr ³ · d / 6) a ² πr ² ] · K
_2j ′ = [(3a ² +1) r · d / 6a ² ] · K _2j ′.

[0130] Thus, pit depth r _n n days after a case pitting of the part-spherical type as shown in FIG. 8 is composed as follows in the (26n) equation.

[0131]

[Equation 17]

For a, a corrosion test was carried out in advance, and for a number of holes formed, the (width of frontage) and the (depth) were determined.
Are calculated with high accuracy by calculating and averaging the ratios.

[0133]

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

Example 1 A test was performed using the test apparatus shown in FIG.

The heat exchanger 10 is provided with 32 heat transfer tubes 13 between the inflow side header 11 and the outflow side header 12. A liquid to be cooled flows around the heat transfer tube 13. This heat transfer tube 13 is made of STB-35
It has a length of 240 mm, an outer diameter of 19 mm, and a wall thickness of 1.6 mm.

Water cooled by the cooling tower 14 is supplied to the inflow side header 11 by the pump 15. Water from the outlet header 12 is sent to the cooling tower 14 via the local corrosion monitor 16 provided on the pipe 17.

A plurality of local corrosion monitors 16 (five in this embodiment, only three are shown in FIG. 1A) are provided.

The local corrosion monitor 16 is shown in FIG.
As shown in the figure, a substantially cylindrical body portion 20 screwed into a female screw hole 19 formed in the water pipe 17, a metal rod 21 inserted into the body portion 20, and the metal rod 21 are pressed. And a metal plug 22.

The tip of the body portion 20 is a liquid junction 23 composed of a small hole having a diameter of 2 mm, and a portion following the liquid junction 23 is a liquid reservoir 24 having a diameter of 2 mm. A cylindrical metal rod 21 having a diameter of 3 mm and a length of 15 mm is inserted into the liquid reservoir 24. The plug 22 is screwed into the body 20 to prevent the metal rod 21 from coming out and leaking.

The material of the pipe 17 is arbitrary, but is made of synthetic resin here. The body 20 is made of vinyl chloride. The metal rod 21 is S
It is made of TB-35. The plug 22 is made of SUS304. The plug 22 is in direct contact with the metal rod 21 and both are electrically connected.

The plug 22 of each local corrosion monitor 16 is electrically connected to the heat transfer tube 13 via a lead wire 25, and the lead wire 25 is provided with an ammeter 26. The detected value of the ammeter 26 is input to a calculator (not shown).

The quality of the cooling water circulating through the cooling tower 14, the heat exchanger 10, and the local corrosion monitor 16 fluctuates between the following.

[0143] M alkalinity = 200~230mg · CaCO _3/1 calcium hardness = 220~265mg · CaCO _3/1 chloride ion ^{= 102~108mg · CL - / 1 FFZ} = 1.0~1.7mg · Zn / 1 ZenRinsan ^{_{= 4.9~5.8mg · PO 4 3- / 1}} SiO 2 concentration = 95~97mg / 1 in this way the anode current flowing through each local corrosion monitoring leads 25 in each ammeter 26 Measured over 377 days. Then, K _2j ′ was calculated once a day based on the detection value of the ammeter 26 based on the above-mentioned equation (4.5j). By substituting the K _2j 'in (25n) equation was used to calculate the pitting depth r _n.

Further, from the above measurement results, the results of
According to the method of No. 215707, the pitting corrosion depth was calculated by substituting the measured anode current value into the pitting corrosion model formula.

The STB constituting the heat transfer tube 16
The K ₁ of −35 was 47000 Ω · mm ² . Also,
The charge number was Z = 2, d = 7.86 mg / mm ³ . The A value (the area of the pinhole in the initial pitting) is 0.000.
1 mm ² . The value of A is 0.1 mm ² or 0.00
00001mm ² little effect on the calculated value of even pitting corrosion depth by changing the digit such as.

The maximum pit depth was calculated by performing extreme value statistics on the calculated value of the pit depth and the measured value of the pit depth actually measured by removing the heat transfer tube 13. The extreme value statistics were calculated using commercially available software ("EVAN" supervised by the Corrosion Protection Association). Table 1 shows the results.

[0147]

[Table 1]

The following is clear from the above results.
That is, in the method of JP-A-5-215707, there is a large error with respect to the measured value of the maximum pit depth, whereas the calculated value obtained by the method of the present invention has a very small error with respect to the measured value.

[0149] Incidentally, 'but seeking, K _2j more than once a day' K _2j by a current value measured once a day in this example seek, K _2j daily average these 'Also good.

[0150]

As described above in detail, according to the method for calculating the pit depth 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 pipes and the flow of water. It is possible to accurately estimate the pit depth at any time due to local corrosion of the pipe.

According to the method of the present invention, the progress of pitting can be easily and accurately estimated in real time during operation of the equipment by accurately calculating the pitting depth. By controlling the amount, the progress of local corrosion can be suppressed.

The remaining life can be estimated from the progress of local corrosion.

Inspection at the time of operation stop is unnecessary.

It is possible to prevent a penetration or leakage accident due to local corrosion.

The effects as described above are achieved, and it is possible to safely and stably operate various plants and extend the life of the metal device members.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a test apparatus according to a first embodiment.

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 sectional view showing a pitting corrosion model.

FIG. 5 is a sectional view showing a pitting corrosion model.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Heat exchanger 13 Heat transfer tube 14 Cooling tower 16 Local corrosion monitor 21 Metal rod 23, 40 Liquid junction part 24, 34 Liquid storage part 25, 36 Lead wire 26, 38 Ammeter 32 Metal piece

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Akihide Hirano 3-4-7 Nishi-Shinjuku, Shinjuku-ku, Tokyo Inside Kurita Kogyo Co., Ltd. (56) References JP 5-215707 (JP, A) JP JP-A-2-310452 (JP, A) JP-A-10-170470 (JP, A) JP-A-4-28888 (JP, A) JP-A-4-66859 (JP, A) JP-A-5-142140 (JP , A) JP-A-55-149049 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/26 351 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A method for calculating a pitting depth 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 monitor provided with the metal member provided to be in contact with the liquid in the part and a metal piece of the same material, and a surface area of the metal piece in contact with the liquid in the liquid reservoir is larger than an opening area of the small hole. A method of electrically contacting the metal piece and the metal member, measuring a current flowing between the two, and calculating a pitting depth of the metal member. A resistance coefficient of the reaction is determined, and a plurality of the monitoring devices are provided on the metal member. The resistance of the corrosion product of the metal member is determined based on the current value of each monitoring device and the resistance coefficient of the corrosion reaction of the metal member. The coefficient of resistance of the corrosion reaction and the corrosion product In a pitting depth calculation method for calculating a pitting depth based on a resistance coefficient and a potential difference caused by contact between the metal member and an aqueous medium, a current value of each monitor device is detected at predetermined time intervals. A method of calculating a pit depth, wherein a resistance coefficient of the corrosion product is obtained from a current value.