JP2008039599A - Local corrosiveness evaluation method of low alloy steel material for container of petroleums - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a local corrosiveness evaluation method of a low alloy steel material for a container of petroleums, for rapid and simple evaluation of the local corrosiveness (corrosion resistance) of a mounting machine in the low alloy steel material for a container of petroleum with high accuracy. <P>SOLUTION: In the local corrosiveness evaluation method of the low alloy steel material used in the container for housing petroleum, an aqueous solution containing FeCl<SB>3</SB>and NaCl is dripped on a metal piece produced, using the low alloy steel material and the metal piece on which the aqueous solution is dripped, is held to a constant temperature and the humidity state and is made to be corroded. The average corrosion depth of and the surface roughness measuring value of the corroded metal piece are added to measure the maximum corrosion depth of the corroded metal piece so as to evaluate the local corrosiveness of the low alloy steel material. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for evaluating local corrosion of low-alloy steel materials for petroleum containers used in petroleum containers for storage, transportation, equipment mounting, and the like of crude oil and petroleum-derived oils.

  Used for storage and transportation of crude oil such as crude oil, heavy oil, light oil, kerosene, gasoline, petroleum asphalt, lubricating oil, cutting oil, machine oil, grease, petroleum wax, rust prevention oil, petroleum ether, etc. A container (hereinafter referred to as “petroleum container” as appropriate) is generally made of a metal material such as steel. However, in recent years, a problem has arisen that low alloy steel, which is a metal material used for containers, has been subjected to severe local corrosion due to moisture containing chlorides remaining at the bottom of the tank, leading to early perforation. Yes. Such corrosion of petroleum container materials causes serious accidents such as sinking accidents in, for example, crude oil tankers. Therefore, it is necessary to perform local corrosion evaluation for container design and life prediction such as material selection and wall thickness setting.

For such local corrosion evaluation, the low alloy steel material to be evaluated is immersed in the petroleum used, or exposed to the existing petroleum containers, and the corrosion damage status of the low alloy steel material is evaluated. It is generally well done to investigate. With regard to the corrosion damage situation of such a low alloy steel material, an accurate evaluation can be performed by conducting an exposure test in an actual environment.
In addition, local corrosion that occurs in low-alloy steel materials in petroleum containers often results in pitting corrosion. As a method for simply evaluating such pitting corrosion, ferric chloride solution is used to make stainless steel (steel). A method for examining pitting corrosion resistance is defined in JIS G0578. In this method, a test piece is immersed in a 6% by mass ferric chloride solution at 35 ° C. or 50 ° C. for 24 hours, and the degree of corrosion is evaluated from the mass change.

In addition, assuming the full condition of sour crude oil that maintains a constant temperature as a pitting corrosion occurrence condition, we investigated the occurrence of pitting corrosion on polished steel sheets and black skinned steel sheets to which sulfur was attached. A pitting corrosion reproduction test method in a simulated crude oil tank that has been examined for shape, depth, and growth rate is disclosed (see Non-Patent Document 1).
Maritime Safety Research Area Material Reliability Research Group “Reproduction Test of Pitting Corrosion in Simulated Crude Oil Tanks” Independent Administrative Institution Maritime Technical Safety Research Institute Lecture Meeting June 2003 p. 1-4

However, the conventional methods for evaluating corrosivity have the following problems.
In the exposure test in the actual environment, in addition to the long period of about several years in the test period, there is a problem that a large area test piece is required because local corrosion occurs stochastically. .
The method defined in JIS G0578 is effective as a method for evaluating corrosion resistance in a short time for stainless steel materials, but is not applicable to carbon steel materials and low alloy steel materials due to too severe environmental conditions. There was a problem.
Furthermore, the method described in Non-Patent Document 1 can accurately evaluate local corrosion in a crude oil tank of a tanker. However, in addition to the need for a dedicated evaluation test facility, the evaluation period is also 6 months. There was a problem that it took a relatively long time, although not as much as the exposure test.

  The present invention has been made in view of the above problems, and its purpose is to quickly and easily evaluate local corrosion (corrosion resistance) in an actual machine in a low alloy steel material for petroleum containers used in petroleum containers. Another object of the present invention is to provide a method for evaluating the local corrosivity of low alloy steel materials for petroleum containers, which is performed with high accuracy.

The method for evaluating local corrosion properties of low alloy steel materials for petroleum containers according to the present invention is based on the fact that local corrosion of petroleum containers is an erosion phenomenon caused by high-concentration chloride, and is a low alloy steel material to be evaluated. By conducting a test simulating the local corrosion occurrence part, local corrosivity in an actual machine is evaluated quickly and easily with high accuracy.
That is, the local corrosivity evaluation method for a low alloy steel material for petroleum containers according to claim 1 is produced using the low alloy steel material in the local corrosivity evaluation method for a low alloy steel material used for a container containing petroleum. An aqueous solution containing FeCl ₃ and NaCl is dropped on the obtained metal piece, and then the metal piece on which the aqueous solution is dropped is kept in a constant temperature and humidity state to corrode, and the maximum corrosion depth of the corroded metal piece By measuring the local corrosion of the low alloy steel.

According to such a configuration, after an aqueous solution containing FeCl ₃ and NaCl is dropped on a metal piece produced using a low alloy steel material, the metal piece on which the aqueous solution is dropped is kept in a constant temperature and humidity state. As a result, the metal piece of the low alloy steel corrodes. Then, by evaluating the local corrosion of the material to be evaluated by measuring the maximum corrosion depth of this corroded metal piece, it is possible to quickly and easily evaluate the local corrosion in the low-alloy steel material for petroleum containers using a real machine. In addition, it can be performed with high accuracy.

The method for evaluating local corrosion of a low alloy steel material for petroleum containers according to claim 2 is such that the Fe ³⁺ concentration in the aqueous solution containing FeCl ₃ and NaCl is 1 to 5% by mass, and the Cl ⁻ concentration is 5 to 10% by mass. Yes, the temperature in the constant temperature and humidity state is 40 to 80 ° C., and the humidity is 80 to 100%.

According to such a configuration, the corrosion of the metal pieces of the low alloy steel material is promoted by regulating the Fe ³⁺ concentration and the Cl ⁻ concentration in the aqueous solution to a predetermined range, and the difference in corrosion resistance of each metal piece in the test. Becomes easier to appear. Furthermore, by regulating the temperature and humidity in a constant temperature and humidity state within a predetermined range, the corrosion rate increases, corrosion is accelerated, and the test solution is less likely to evaporate.

According to a third aspect of the present invention, there is provided a method for evaluating the local corrosion of a low alloy steel material for petroleum containers, wherein the low alloy steel material is used for a bottom plate of a crude oil tank of a tanker.
According to such a configuration, the local corrosive evaluation in the crude oil tank of the tanker using the low alloy steel material can be performed quickly and easily with high accuracy.

  According to the method for evaluating local corrosion of a low-alloy steel material for petroleum containers according to the present invention, a test simulating a local corrosion occurrence portion in a metal piece (low-alloy steel material) to be evaluated is performed. By evaluating the local corrosion properties of low alloy steels by measuring the maximum corrosion depth, local corrosion properties in low-alloy steel materials for petroleum containers can be evaluated quickly, easily, and with high accuracy. Can do.

Hereinafter, the best mode for carrying out the present invention will be described in detail.
In the evaluation method of the present invention, first, an aqueous solution containing FeCl ₃ and NaCl (hereinafter referred to as “test solution” as appropriate) is dropped on a metal piece of a low alloy steel material which is an evaluation target material, and then this aqueous solution is dropped. The corroded metal piece is kept at a constant temperature and humidity to be corroded (corrosion test). Next, the local corrosion property is evaluated by measuring the maximum corrosion depth of the metal piece corroded in this corrosion test.

[Corrosion test]
The metal piece of the low alloy steel material used as the test piece simulates a local corrosion portion in an actual machine. This metal piece is produced by melting a raw material of a low alloy steel material by melting in a converter to produce a low alloy steel material having a predetermined chemical component, and cutting out to a predetermined size from this steel material. Then, the entire surface of the metal piece is polished and washed with water and acetone to obtain a test piece for a corrosion test.

  If the size of the metal piece is too small, it is not preferable because the measurement accuracy deteriorates in the measurement of mass change before and after the corrosion test. Also, if the size of the metal piece is too large, it takes a long time to measure the roughness after the corrosion test, which is not preferable. Furthermore, when the thickness is small, it is not preferable because an accurate corrosion depth cannot be measured by penetration. From such a viewpoint, the size of the metal piece used as the test piece is preferably in a range of approximately 10 × 10 × 3 mm to 50 × 50 × 50 mm, and preferably in a range of 20 × 20 × 5 mm to 30 × 30 × 10 mm. More preferred.

Here, it is preferable that the low alloy steel material which is an evaluation object material is used for a crude tank bottom plate of a tanker.
Crude oil tankers cause serious accidents such as sinking accidents, so it is important to conduct local corrosion evaluation for container design and life prediction such as material selection and wall thickness setting. Therefore, by using the low-alloy steel material used for the bottom plate of the tanker crude oil tank, it is possible to quickly, easily and highly accurately evaluate the local corrosiveness in the crude oil tank of the tanker using the low-alloy steel material. .

An aqueous solution (test solution) containing FeCl ₃ (ferric chloride, iron (III) chloride) and NaCl dropped on a metal piece (test piece) simulates the advanced environment of a petroleum container that causes local corrosion. is there. From the viewpoint of ensuring the reproducibility of the test, it is preferable to use a test solution in which a special grade reagent of FeCl ₃ and NaCl and ion-exchanged water or distilled water are mixed. As for the blending of the test solution, it is preferable to mix FeCl ₃ and NaCl so that the Fe ³⁺ concentration in the test solution (aqueous solution) is 1 to 5 mass% and the Cl ⁻ concentration is 5 to 10 mass%.
As the special grade reagent for FeCl ₃ and NaCl, a commercially available special grade reagent can be used. For example, FeCl ₃ · 6H ₂ O (code No. 095-00875) manufactured by Wako Pure Chemical Industries, Ltd. And NaCl (code No. 191-01665) manufactured by Wako Pure Chemical Industries, Ltd.

Fe ³⁺ in the test solution is necessary for promoting corrosion by adding the reaction formula of the following formula (1) as a cathodic reaction of corrosion. When the Fe ³⁺ concentration is less than 1% by mass, The accelerating action is small, and it takes a long time for the evaluation test. On the other hand, if the Fe ³⁺ concentration exceeds 5% by mass, the corrosion promotion is too intense, and the difference in corrosion resistance between the low alloy steel materials in the test hardly appears. From such a viewpoint, the Fe ³⁺ concentration in the solution is preferably 1 to 5% by mass, and more preferably 2 to 4% by mass.
Fe ³⁺ + e ⁻ → Fe ²⁺ (1)

Cl ⁻ in the test solution is necessary for destroying the passive film formed on the surface of the low alloy steel and promoting local corrosion. If the Cl ⁻ concentration is less than 5% by mass, The accelerating action is small and it takes a long time for the evaluation test. On the other hand, if the Cl ⁻ concentration exceeds 10% by mass, the corrosion promotion is too intense, and the difference in corrosion resistance of the low alloy steel material hardly appears. From such a viewpoint, the Cl ⁻ concentration in the solution is preferably 5 to 10% by mass, and more preferably 6 to 9% by mass.

In the local corrosion evaluation method, the aqueous solution is dropped onto the test piece to corrode, but the local corrosion evaluation method of the present invention devise solution conditions (concentration, amount) to be dropped on the test piece. Thus, the environmental conditions which are problems of the corrosion test with ferric chloride of JIS G0578 are improved.
The amount of the aqueous solution to be dropped is preferably 0.1 to 1 L per 1 m ² of the test area. When the dropping amount is less than 0.1 L / m ² , Fe ³⁺ is consumed in a short time by the reaction formula (1), and sufficient corrosion promotion cannot be obtained. On the other hand, when the dripping amount exceeds 1 L / m ² , the difference in corrosion resistance between the low alloy steel materials hardly appears because the corrosive environment is too severe. From such a viewpoint, the amount of the aqueous solution to be dropped is preferably 0.1 to 1 L, more preferably 0.2 to 0.9 L per 1 m ² of the test area.

  In the local corrosion evaluation method, the test piece to which the aqueous solution is dropped is kept in a constant temperature and humidity state to be corroded. The temperature at this time is preferably 40 to 80 ° C., and the humidity is preferably 80% or more (100% or less). When the temperature is lower than 40 ° C., the corrosion rate is low, the corrosion promotion is not sufficient, and the evaluation test tends to take a long time. In addition, the higher the temperature, the greater and better the corrosion promotion. However, when the temperature exceeds 80 ° C., even if water is replenished by adjusting the humidity, the evaporation of droplets occurs and it is difficult to reproduce the actual corrosion state. From such a viewpoint, the temperature is preferably 40 to 80 ° C, and more preferably 50 to 70 ° C. In addition, when heating is performed to promote corrosion in this way, the test solution (droplets) on the test piece evaporates and the moisture disappears, and conversely, corrosion does not progress. In order to prevent such droplet evaporation, the humidity needs to be adjusted to 80% or more, preferably 90% or more. The upper limit of humidity is 100%.

[Measurement of maximum corrosion depth]
As a method of measuring the maximum corrosion depth, the average corrosion depth is obtained from the mass change of the test piece (metal piece) before and after the corrosion test, the surface roughness after the corrosion test is measured, and the measured value is obtained. It is preferable to measure the maximum corrosion depth by adding up the measured values of the average corrosion depth and the surface roughness. Further, in order to perform measurement with high accuracy, it is preferable to remove the corrosion products before measuring the mass and the roughness of the surface after the corrosion test. As a method for removing the corrosion product, it is possible to use a method of immersing in an appropriate removal solution such as an acid to which an inhibitor is added, a cathode electrolysis method using an aqueous solution of diammonium hydrogen citrate, a water jet method, or the like. is there.

  In the local corrosion evaluation method, it is possible to estimate the maximum corrosion depth of an actual machine by corroding in a corrosion test and performing data analysis using the extreme value analysis method for the maximum corrosion depth of a metal piece. Assess by measuring the corrosion depth. That is, a plurality of test pieces are used for the corrosion test, the maximum corrosion depth of each test piece is obtained, and extreme value plotting is performed to obtain the maximum corrosion depth of the actual machine. This is based on the fact that the maximum corrosion depth in local corrosion follows a Gumbel distribution.

In the evaluation method of the present invention, when the corrosion occurrence area ratio of a certain material is known, it is possible to obtain the maximum corrosion depth corresponding to the area used in the actual machine using the recursion period (T). is there. Here, assuming that the use area in the actual machine is A, the corrosion occurrence area rate is B, and the test piece area used in the evaluation test is C, the maximum corrosion depth corresponding to “T = A × B / C” is the maximum in the actual machine. This is an estimate of the corrosion depth. This estimation is based on the precondition that the corrosion occurrence area ratio does not change even when the material type is different.
In addition, the usage area in the actual machine means the total area of the material in contact with petroleum in the environment that is the object of evaluation in the present invention, that is, in the actual machine. Further, the corrosion occurrence area means the total area of the local corrosion that has occurred.

Next, examples of the method for evaluating local corrosion of a low alloy steel for petroleum containers according to the present invention will be described with reference to the drawings.
In the drawings to be referred to, FIG. 1 is an image diagram of a test piece dropped with a test solution, FIG. 2 is an explanatory diagram showing the maximum corrosion depth in the cross section of the test piece after the corrosion test, and FIG. It is an extreme value plot figure which shows a value plot.
In addition, in order to show the consistency between the evaluation result in the evaluation method of the present invention and the evaluation result in the conventional evaluation method, the evaluation result in the conventional evaluation method (exposure test) is also shown.

<Evaluation method according to the present invention>
[Test method]
The raw material of the low alloy steel is melted by converter melting to produce a low alloy steel having the chemical components A to C shown in Table 1, and the size of this steel is 30 × 30 × 5 (mm). A metal piece was cut out. The entire surface of the cut metal piece was polished with a wet rotary polishing machine (abrasive paper; # 600), washed with water and washed with acetone to obtain a test piece for a corrosion test. Surfaces other than the test surface were coated with a silicon sealant to prevent corrosion. Using this test piece, the following corrosion test was conducted.

The test solution for the corrosion test was FeCl ₃ · 6H ₂ O special grade reagent (Wako Pure Chemical Industries, Ltd .: Code No. 095-00875) and NaCl special grade reagent (Wako Pure Chemical Industries, Ltd.). : Code No. 191-01665) 40 g was added to ion-exchanged water and mixed so that the total was 1000 g, and Fe ³⁺ concentration and Cl ⁻ concentration were 3.1 mass% and 8.3 mass, respectively. %. 0.7 mL (0.78 L / m ² ) of the test solution was dropped on one test piece, and the test solution was spread over the entire surface of one side of the test piece so as not to spill using the interfacial tension (FIG. 1). reference). Next, the test piece to which the test solution was dropped was placed in a constant temperature and humidity tester maintained at a temperature of 60 ° C. and a humidity of 95% RH to corrode the test piece. The test time is 168 hours.

  And as shown in FIG. 2, average corrosion depth (A) is calculated | required from the mass change before and behind a corrosion test, Furthermore, the three-dimensional roughness measurement of the test piece surface after a test is performed, and roughness measured value (B) These were added together to determine the maximum corrosion depth (A + B) of the test piece. In addition, the corrosion product produced | generated on the test piece surface after completion | finish of a test was removed by the cathodic electrolysis method in 10 mass% diammonium hydrogen citrate aqueous solution.

[Test results]
An extreme value plot was performed for the maximum corrosion depths of the steel materials A, B, and C obtained by performing the corrosion test. This extreme value plot (Gumbel distribution) is shown in FIG. FIG. 3 shows the results of obtaining the maximum corrosion depth for 24 specimens for A, 10 specimens for B, and 10 specimens for C, and plotting them by the average rank method by arranging them in ascending order. The vertical axis represents the recursion period and the cumulative probability, and the horizontal axis represents the maximum corrosion depth. The maximum corrosion depth varies stochastically and follows the Gumbel distribution. That is, it means that the larger the use area in the actual machine, the deeper the maximum corrosion depth.

Next, the maximum corrosion depth corresponding to the use area in the actual machine is obtained. In this example, since the size of the test piece is 30 × 30 mm, the test piece area C used is 9.0 × 10 ² mm ² . For example, if the area of the actual container (area used in the actual apparatus) is 9.0 × 10 ⁸ mm ² and the corrosion occurrence area ratio is 0.01%, T = 9.0 × 10 ⁸ × 0.0001. The maximum corrosion depth corresponding to ÷ (9.0 × 10 ² ) = 100 can be estimated as the maximum corrosion depth in the actual machine.

  Table 2 shows the results of estimating the maximum corrosion depth corresponding to the area used in the actual machine from the above experimental results.

Table 2 is obtained by extrapolating the extrapolated line obtained by the least square method of the plot of FIG. 3 and reading the maximum corrosion depth in the recursion period T = 100.
In addition, according to this result, the maximum corrosion depth of the steel materials B and C is evaluated as 48.6% and 34.4% of the steel materials A, respectively.

<Conventional evaluation method (exposure test)>
[Test method]
The steel materials A, B and C shown in Table 3 were exposed in a heavy oil storage container, and the correlation with the evaluation method according to the present invention was examined. The test piece used was 1000 × 1000 × 19 (mm) in size, and was placed parallel to the bottom of the heavy oil storage container and exposed to heavy oil. In addition, the used steel materials were produced by the same method as the evaluation method according to the present invention. The heavy oil in the contents is a heavy oil classified as Type 3 No. 2 in JIS K2205. After the exposure for 3 years, the test piece was taken out, corrosion products such as rust were removed by the water jet method, and the corrosion depth of the corroded portion was measured using a depth gauge.

[Test results]
Table 3 shows the maximum corrosion depths of the steel materials A, B, and C obtained by performing the test.

As shown in Table 3, the ratio of the maximum corrosion depth is a result that almost coincides with the evaluation result according to the present invention (Table 2), and the present invention is quick and simple, and with high accuracy as in the exposure test. It can be seen that the corrosion situation can be evaluated.
As described above, according to the local corrosion evaluation method for low-alloy steel materials for petroleum containers according to the present invention, the maximum corrosion depth in the use area of various steel materials in the actual machine can be easily obtained. Based on this maximum corrosion depth, it is possible to evaluate corrosion resistance in actual equipment in low alloy steel materials for petroleum containers quickly and easily with high accuracy due to differences in the maximum corrosion depth of each steel material. .

  The preferred embodiments and examples of the present invention have been described above. However, the present invention is not limited to the above-described embodiments and examples, and various changes and modifications can be made within the scope that can meet the spirit of the present invention. These are all included in the technical scope of the present invention.

It is an image figure of the test piece which dripped the test solution. It is explanatory drawing which shows the maximum corrosion depth in the test piece cross section after a corrosion test. It is an extreme value plot figure which shows the extreme value plot of the maximum corrosion depth.

Explanation of symbols

1 Test piece (metal piece of low alloy steel)
2 Test solution (aqueous solution)
A Mass change (average corrosion depth)
B Roughness measurement

Claims (3)

In the local corrosion evaluation method for low alloy steel used for containers containing petroleum,
After an aqueous solution containing FeCl ₃ and NaCl is dropped on a metal piece produced using the low alloy steel material, the metal piece on which the aqueous solution is dropped is kept in a constant temperature and humidity state to be corroded and corroded. A method for evaluating the local corrosivity of a low alloy steel material for petroleum containers, wherein the local corrosion property of the low alloy steel material is evaluated by measuring the maximum corrosion depth of the metal piece. The Fe ³⁺ concentration in the aqueous solution containing FeCl ₃ and NaCl is 1 to 5% by mass, the Cl ⁻ concentration is 5 to 10% by mass, the temperature in the constant temperature and humidity state is 40 to 80 ° C., and the humidity is 80 to 100%. The local corrosion evaluation method for low alloy steel for petroleum containers according to claim 1, wherein   The method for evaluating local corrosion of a low alloy steel for petroleum containers according to claim 1 or 2, wherein the low alloy steel is used for a bottom plate of a crude oil tank of a tanker.

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