JP4690962B2 - Method for evaluating local corrosion of metal materials for petroleum containers - Google Patents

Description

  The present invention relates to a method for evaluating local corrosivity of metal 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 preventive 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, there has been a problem that the metal material used for the container has been subjected to severe local corrosion due to sulfur contained in crude oil or the like and water containing chlorides remaining at the bottom of the tank, leading to early perforation. It has become apparent. Such corrosion of the material of petroleum containers causes a serious accident such as a sinking accident in a crude oil tanker, for example. 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 metal material to be evaluated is immersed in the petroleum to be used or exposed to the existing petroleum container to examine the corrosion damage status of the metal material. Is generally done well. With respect to the corrosion damage situation of such a metal material, an accurate evaluation can be performed by conducting an exposure test in an actual environment.
In addition, local corrosion that occurs in metal materials of petroleum containers often results in pitting corrosion. However, as a method for simply evaluating such pitting corrosion, ferric chloride solution is used to resist the resistance of stainless steel (steel). A method for examining pitting 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 metal material for petroleum containers used in petroleum containers, The object is to provide a method for evaluating the local corrosion of a metal material for petroleum containers with high accuracy.

The method for evaluating the local corrosion property of a metal material for petroleum containers according to the present invention is based on the fact that the local corrosion of a petroleum container is an erosion phenomenon caused by S, and the local corrosion occurrence part of the metal material to be evaluated. By conducting a test that simulates the above, local corrosivity in an actual machine is evaluated quickly and easily with high accuracy.
That is, the local corrosivity evaluation method for a metal material for petroleum containers according to claim 1 is a metal produced by using the metal material in the local corrosivity evaluation method for a metal material used for a container for containing petroleum. pieces, carried out corrosion tests Ru corrode in contact with the solution of a mixture of N NaCl solution and S powder, an average corrosion depth from the mass change of the corrosion test before and after the metal piece, further surface after the corrosion test the roughness was measured sought measurements of, by that summing the measurements of the roughness of the average corrosion depth and the surface, by determining the maximum corrosion depth of the metal piece made by the corrosion, the metal It is characterized by evaluating the local corrosivity of the material.

According to this structure, the metal pieces made of a metal material, is brought into contact with the solution of a mixture of N NaCl solution and S powder, metal pieces may corrode. Then, by adding the measured values of the average corrosion depth and the surface roughness to obtain the maximum corrosion depth of the corroded metal piece, the local corrosivity of the metal material is evaluated. Evaluation of local corrosiveness of the material in the actual machine can be performed quickly and easily with high accuracy.

The local corrosiveness evaluation method for a metal material for petroleum containers according to claim 2 is characterized in that the metal material is a steel material.
According to such a configuration, by using a steel material as the metal material, it is possible to perform a local corrosion evaluation in an actual machine in a petroleum container using the steel material quickly, easily, and with high accuracy.

According to the method for evaluating local corrosion of a metal material for petroleum containers according to the present invention, a test simulating a local corrosion occurrence portion is performed on a metal piece (metal material) to be evaluated, and the average corrosion depth and surface roughness are measured. by summing the measurements to assess the local corrosion of metallic material by determining the maximum corrosion depth of the resultant metal pieces, quickly localized corrosion of the actual machine in petroleum container metallic material In addition, the evaluation can be performed easily and with high accuracy.

Hereinafter, the best mode for carrying out the present invention will be described in detail.
In the evaluation method of the present invention, first, a metal piece produced using a metal material is corroded by being brought into contact with a mixed solution of water or an aqueous NaCl solution (hereinafter referred to as “solvent”) and S powder (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 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 metal material by converter melting, producing a metal material having a predetermined chemical component, and cutting the metal material into a predetermined size. 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, as the metal material for producing the metal piece, it is preferable to use a steel material. Examples of the steel material include stainless steel material, carbon steel material, low alloy steel material, steel material, etc., but aluminum alloy material, titanium alloy material, etc. You may use metal materials other than these steel materials.
As a metal material used for oil containers, steel is often used from the viewpoint of mechanical properties and economy, but by using steel as the metal material of the test piece, it is an actual machine in petroleum containers using steel. The local corrosive evaluation can be performed quickly and easily with high accuracy.

  The mixture of water or NaCl aqueous solution (solvent) and S powder used as the test solution simulates the advanced environment of petroleum containers that cause local corrosion. As the S powder, a commercially available powdered S reagent can be used, and a special grade reagent is particularly preferably used. For example, sulfur powder (Code No. 195) manufactured by Wako Pure Chemical Industries, Ltd. -04651) and the like.

As water, it is preferable to use ion-exchanged water or distilled water from the viewpoint of ensuring the reproducibility of the test.
Further, it is more preferable to use an aqueous NaCl solution instead of water in order to ensure conductivity for the progress of the corrosion reaction.
As the NaCl aqueous solution, it is preferable to add NaCl to the above water so that the NaCl concentration is 0.5 to 30% by mass. When the NaCl concentration is less than 0.5% by mass, it is difficult to reproduce corrosion because of low conductivity. On the other hand, if the NaCl concentration exceeds 30% by mass, salt may precipitate and corrosion of the oil container may not be accurately reproduced. Therefore, the NaCl concentration is preferably 0.5 to 30% by mass, and more preferably 1 to 10% by mass.

Moreover, 20-80 degreeC is preferable and the processing temperature at the time of making a metal piece contact a mixture of a solvent and S powder and 24 to 240 hours are preferable. Furthermore, it is preferable to mix S with the mass ratio of 0.1-1 with respect to the solvent 1. FIG.
If the treatment temperature is too low, the progress of the local corrosion reaction is slow, and it takes a long time to obtain a local corrosion depth that allows accurate measurement. On the other hand, if the treatment temperature is too high, the local corrosion is too intense, and the difference in the local corrosion characteristics of the metal material hardly appears. Therefore, the treatment temperature is preferably 20 to 80 ° C, more preferably 25 to 60 ° C.

  The treatment time varies depending on the treatment conditions such as the S mixing ratio of the solution used and the treatment temperature, but if the treatment time is too short, the accuracy of measurement of the local corrosion depth tends to be low. On the other hand, if the treatment time is too long, the local corrosion penetrates the test piece, so that the accurate local corrosion depth cannot be measured. Therefore, the processing time is preferably about 24 to 240 hours, but this processing time is determined in consideration of other processing conditions.

  If the mass ratio of S to solvent 1 is less than 0.1, S is too small to cause local corrosion, whereas if it exceeds 1, local corrosion is too severe and a difference in the local corrosion characteristics of the materials appears. Disappear. Therefore, it is preferable that the mixing ratio of the solvent and S is a mass ratio and that S is in the range of 0.1 to 1 with respect to the solvent 1.

[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 the actual machine by analyzing the data of the maximum corrosion depth of the metal piece corroded in the corrosion test using the extreme value analysis method. Assess by measuring the maximum corrosion depth. That is, using a plurality of test pieces in 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 metal material 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 explanatory diagram showing a maximum corrosion depth in a cross section of a specimen after a corrosion test, and FIG. 2 is an extreme value plot diagram showing an extreme value plot of the maximum corrosion depth.
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]
Steel materials are melted by converter melting to produce steel materials having chemical components A to D shown in Table 1, and metal pieces with a size of 30 × 30 × 5 (mm) are cut out from the steel materials. It was. 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. Using this test piece, the following corrosion test was conducted.

  The corrosion test solution was prepared by using an NaCl special grade reagent (manufactured by Wako Pure Chemical Industries, Ltd .: Code No. 191-01665) and ion-exchanged water to prepare an 8% by mass NaCl aqueous solution. On the other hand, a powdery S grade reagent (Wako Pure Chemical Industries, Ltd .: Code No. 195-04651) is mixed at a mass ratio of 0.5. In the corrosion test, the test piece was immersed in a solution mixed with the powder S to be corroded. The solution temperature at this time is 30 ° C., and the test time is 168 hours. And as shown in FIG. 1, 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 depth of the steel materials A, B, C, and D obtained by performing the corrosion test. This extreme value plot (Gumbel distribution) is shown in FIG. FIG. 2 shows the results of obtaining the maximum corrosion depth for 30 test pieces, B for 6 pieces, C for 10 pieces, and D for 8 pieces, and arranging them in ascending order by the average rank method. 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 × 5 mm, the test piece area C used is 2.4 × 10 ³ mm ² . For example, if the area of the actual container (area used in the actual apparatus) is 2.4 × 10 ⁸ mm ² and the corrosion occurrence area ratio is 0.1%, T = 2.4 × 10 ⁸ × 0.001. ÷ The maximum corrosion depth corresponding to 2.4 × 10 ³ = 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. 2 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, C, and D is evaluated to be 35.7%, 30.0%, and 28.1% of the steel material A, respectively.

<Conventional evaluation method (exposure test)>
[Test method]
Steel materials A, B, C and D shown in Table 3 were exposed in a crude oil storage container, and the correlation with the evaluation method according to the present invention was examined. The test piece used had a size of 1000 × 1000 × 19 (mm) and was placed in parallel with the bottom of the storage container and exposed to crude oil (produced by Kuwait). In addition, the used steel materials were produced by the same method as the evaluation method according to the present invention. After the exposure for 5 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, C, and D 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 a metal material for petroleum containers according to the present invention, the maximum corrosion depth in the use area of various steel materials in an actual machine can be easily obtained. Based on this maximum corrosion depth, the corrosion resistance evaluation with an actual machine for a metal material for petroleum containers can be performed quickly, easily, and with high accuracy from the difference 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 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 metal material)
A Mass change (average corrosion depth)
B Roughness measurement

Claims (2)

In the method for evaluating local corrosivity of metal materials used in containers containing petroleum,
The metal piece manufactured using the metal material performs N NaCl solution and S powder and mixed solution to the contacted with Ru corrosion test corrode, average corrosion depth from the mass change of the corrosion test before and after the metal piece Further, the surface roughness after the corrosion test is measured to obtain a measurement value, and the average corrosion depth and the surface roughness measurement value are summed to obtain the maximum corrosion depth of the corroded metal piece. The local corrosivity evaluation method of the metal material for petroleum containers is characterized by evaluating the local corrosivity of the metal material by determining the thickness.   The said metal material is steel materials, The local corrosivity evaluation method of the metal material for petroleum containers of Claim 1 characterized by the above-mentioned.

Images