JP2012117812A - Corrosion testing method of anti-corrosion steel for hold of coal ship and ship for coal and iron ore and method for predicting usage life of ship using the corrosion testing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion testing method capable of reproducing a corrosion environment in a hold of a coal ship and a ship for coal/ore in a laboratory.SOLUTION: The corrosion testing method of the anti-corrosion steel for the hold of the coal ship and the ship for coal/ore to test the corrosion of a steel material by using an atmosphere adjustment chamber includes: a step for preparing a sulfuric acid-added granular porous substance by adding sulfuric acid to a granular porous substance; a step for mounting the sulfuric acid-added granular porous substance on the steel material and putting the steel material in the atmosphere adjustment chamber; a step for arbitrarily selecting relative temperature in the atmosphere adjustment chamber in a range of 80 to 100% and changing heating and cooling of the temperature in a fixed cycle in a range of 20 to 80°C; and a step for measuring the corrosion quantity or hole corrosion depth of the steel material. In the step for preparing the sulfuric acid-added granular porous substance, the amount of sulfuric acid is 0.01 to 100 mg per 1 g of granular porous substance.

Description

  The present invention relates to a corrosion test method for a corrosion resistance steel for a coal ship and a coal / ore combined-use hold capable of reproducing and evaluating a steel material used in a coal ship and a coal / ore combined-use hold in a laboratory. The present invention relates to a method for predicting the service life of ships.

  In bulk cargo ships, marine accidents became an international issue one after another in the early 1990s. In particular, many accidents were reported on coal ships and coal / iron ore combined ships, most of which were damage in the hold. Bulk cargo ships are loaded directly into the hold (sometimes referred to as “hold”) and are therefore susceptible to corrosive loads. Corrosion in the hold, especially coal ships, coal / iron ore combined It is considered a problem that the strength locally decreases due to pitting corrosion at the side wall in the ship's hold. Cases in which this pitting corrosion has progressed remarkably and cases in which the thickness of the rib portion that ensures the strength of the ship has been extremely reduced have been reported.

  The side wall of the bulk carrier where pitting occurs is a single hull, and the load and seawater are separated from each other only by one steel material. Therefore, due to the temperature difference between the seawater and the cargo hold, dew condensation is likely to occur on the side wall of the hold, and the sulfur component of the coal dissolves in the place, reacts with the dew condensation water to produce sulfuric acid, and sulfuric acid corrosion occurs.

  As a countermeasure against such corrosion in the hold, a modified epoxy coating is applied to the hold with a coating thickness of about 150 to 200 μm. However, since the coating is often peeled off due to mechanical damage caused by coal or iron ore, and scratches and wear caused by heavy machinery during loading and unloading, a sufficient anticorrosive effect cannot be expected.

  Therefore, as a countermeasure against corrosion, methods of repainting and partial repairs are taken regularly, but such methods are very expensive and reduce life cycle costs including ship maintenance costs. It is a problem to make it.

Until now, as an example of evaluation of a corrosion resistant steel for holding a coal ship and a coal / ore combined ship, as shown in Examples of Patent Documents 1 and 2, the temperature is 60 ° C., 100% RH, 6 hours → Immersion in 5% NaCl + 0.1% CaCl ₂ + 0.5% Na ₂ SO ₄ solution for 0.5 hour ⇒ Paint material with bare material and scratch in 60 ° C, 50% RH, 17.5 hour cycle Is being evaluated.

JP 2007-262555 A JP 2008-174768 A

Tatsuro Nakai, Hisao Matsushita, Norio Yamamoto, 2006 Japan Maritime Association (ClassNK) presentation, p.25-37

However, in Patent Documents 1 and 2, the solution used is 0.5% NaCl + 0.1% CaCl ₂ + 0.5% Na ₂ SO _4, which is disclosed in Non-Patent Document 1 as a factor that accelerates corrosion. Unlike the dilute sulfuric acid environmental conditions, it cannot be said that the corrosive environment of the hold of actual coal ships and coal / ore combined ships is simulated. In addition, it is considered that coal ships and coal / ore combined ships have a problem in that local strength decreases due to pitting corrosion of the inner wall of the hold or ribs, leading to destruction. Furthermore, the inside of the hold is painted, but the coating is peeled off due to mechanical damage caused by coal or iron ore and wear by heavy machinery during loading and unloading, so it is necessary to evaluate the pitting corrosion depth of the bare material.

  In order to solve the above problems, the present invention provides a corrosion test method that reproduces the corrosive environment in the hold of a coal ship / coal and ore combined ship in a laboratory, and further estimates the life of the ship. With the goal.

The above problems are solved by the following configurations of the present invention.
1. In the steel corrosion test method using the atmosphere adjustment chamber,
Adding sulfuric acid to a granular porous material to produce a sulfuric acid-added granular porous material;
Placing the sulfuric acid-added granular porous material on the steel material, and installing in the atmosphere adjustment chamber;
A step of arbitrarily selecting the relative humidity of the atmosphere adjustment chamber in the range of 80 to 100%, and changing the heating and cooling in a constant cycle in the range of 20 ° C. to 80 ° C .;
A step of measuring the corrosion amount or pitting depth of the steel material, and in the step of forming the sulfuric acid-added granular porous material, the sulfuric acid is in an amount of 0.01 to 100 mg per 1 g of the granular porous material. A corrosion test method for a corrosion resistant steel for holding a coal ship and a combined coal and ore ship.
2. 2. The corrosion test method for a corrosion resistant steel for holding a coal ship and a coal / ore combined ship, according to 1, wherein the granular porous material is activated carbon.
3. 3. The corrosion test method for a corrosion resistant steel for holding a coal ship and a combined coal / ore ship according to 1 or 2, wherein the granular porous material has a particle size in the range of 1 to 20 mm.
4). The amount of the sulfuric acid-added granular porous material is 0.1 to 5 g / cm ² per unit area of the steel material, and the coal ship according to any one of 1 to 3, Corrosion test method for anti-corrosion steel for ore holding ship.
5. 5. The corrosion test method for a corrosion resistant steel for holding a coal ship and a coal / ore combined ship according to any one of 1 to 4, wherein the predetermined period is one day.
6). A method for predicting a service life of a ship, wherein the corrosion test method according to any one of 1 to 5 is used.

  The present invention reproduces the corrosive environment in the hold of a coal ship / coal and ore combined ship in a laboratory, and according to the present invention, it is possible to select and evaluate the steel material for holding the coal ship / coal and ore combined ship. Can be used. Further, it is possible to predict the life of the ship by following the change over time in the corrosion amount and pitting depth of the steel material and extrapolating the results.

The figure which shows the relationship between the diameter and depth of a pitting corrosion formed by the corrosion test method of the Example. The figure which shows the prediction of the pitting depth of a coal ship and a coal and ore combined use ship. The figure which shows the prediction of the average corrosion thickness of a coal ship and a coal and ore combined ship.

Below, the form for implementing this invention is demonstrated.
First, the inventors examined the mechanism of pitting corrosion that has the most influence on the destruction of ships due to corrosion in the hold of coal ships and coal / ore combined ships.

By the way, the side wall portion of the bulk carrier is a single hull, and the cargo and the seawater are separated from each other only by one steel material. Therefore, due to the temperature difference between the seawater and the hold, dew condensation water is generated on the side wall of the hold, the steel material and the surface of the coal are wet, and the H ₂ SO ₄ -derived substance adsorbed on the surface of the coal oozes into the water film. The steel material undergoes pitting corrosion under the coal forming the meniscus, and in the meniscus portion, H ⁺ is consumed for corrosion of the steel material, so the H ⁺ concentration decreases.

On the other hand, since a large amount of H ⁺ exists on the coal surface, a difference in H ⁺ concentration is produced between the coal surface and the meniscus portion. The difference in chemical potential is used as the driving force, and it is considered that H ⁺ is supplied from the coal surface to the meniscus portion. Unreacted H ⁺ adheres to the coal surface again during the drying process, and is used for the corrosion reaction in the next dew condensation process. This process takes place in a long-term cycle, causing more corrosion in the meniscus area and pitting corrosion. Will be formed. Based on this mechanism of pitting corrosion, the present inventors have found the following conditions for simulating pitting corrosion in the hold of coal ships and coal / ore combined ships.

  Corrosion in the hold of coal ships and coal / ore combined ships depends greatly on temperature and humidity. Although the temperature in the hold varies depending on the channel and the type of coal in the cargo, it is about 50 to 80 ° C. during the day and about 20 to 30 ° C. at night. Therefore, in the present invention, the temperature range is preferably 20 ° C. to 80 ° C., and condensation is generated by arbitrarily changing the temperature therebetween.

  Within this range, the high temperature and low temperature of the test temperature were selected. It is desirable to adjust the temperature from high temperature to low temperature or from low temperature to high temperature within a time of 0.3 to 1.0 h (hour). Although a long temperature adjustment time can be adopted, it is preferable that the temperature adjustment is within a time of 0.3 to 1.0 h (hour) as an accelerated test.

  Moreover, since humidity will always be in a highly humid state by the water | moisture content which coal has, or the dew condensation water which arises by the temperature difference in seawater and a hold | maintenance, 80-100% RH (Here, RH is an abbreviation for relative humidity.). ). Further, it is preferable that the fixed cycle is one day and night, that is, one day in order to reproduce the hold environment.

Next, the porous material to be used will be described. The present inventors have revealed that, in the mechanism of the occurrence of pitting corrosion, long-term acid supply to the meniscus portion is necessary for pitting formation. Accordingly, the porous material fixes the added sulfuric acid when drying, and supplies the sulfuric acid to the meniscus portion when wet. The porous material is preferably granular, and by using the granular porous material, the sulfuric acid supply situation of this mechanism can be simulated. However, it is preferable that the porous material has sulfuric acid resistance because sulfuric acid is added. In the present invention, the porous material is a material having fine pores, and particularly refers to a material having a pore volume in the range of 0.1 to 10 cm ³ / g. Here, the pore volume (also referred to as pore volume) can be measured by a known method such as a gas adsorption method or a mercury intrusion method.

  The porous material is used to absorb and impregnate sulfuric acid inside, and the granular material is used to simulate the state of coal and ore in the actual hold environment. Examples of the porous material include porous ceramics, reagent primary silica sand, zirconia, and alumina, and may be coal or ore. Activated carbon is preferable as the coal used in the present invention. Since the activated carbon has a constant quality and is porous, the test results can be obtained stably. However, the present invention is not limited to this, and when using coal, the type of coal expected to be transported can be selected as appropriate.

  Next, the amount of sulfuric acid to be added will be described. The sulfuric acid-added granular porous material is made by adding sulfuric acid to a granular porous material. The properties of coal vary greatly depending on the brand, and the type and content of sulfate radicals contained vary greatly. For example, when the present inventors examined various kinds of coal which is a porous material, 0.04 to 70 mg of sulfate ion was confirmed per 1 g of coal. When this is expressed in terms of sulfuric acid, it corresponds to 0.1 to 100 mg of sulfuric acid added per 1 g of the porous material, and in the present invention, this range is preferable.

  Furthermore, from the viewpoint of the corrosion amount, the ease of measuring the pitting depth, and the variation in the measured values obtained, it is preferable to add 10 mg or more of sulfuric acid per 1 g of the porous material.

  Next, the sulfuric acid-added granular porous material thus produced is placed on the steel material in the atmosphere adjustment chamber. Although it is efficient to place it in the atmosphere adjustment chamber after being placed, the sulfuric acid-added granular porous material can also be placed on the steel material in the atmosphere adjustment chamber.

Further, in the next step, the temperature is changed while the relative humidity inside the atmosphere adjusting chamber is kept constant at high humidity. In this case, condensation occurs when the temperature rises. This is because the temperature difference between the steel material and the atmosphere adjustment chamber is generated because the steel material temperature follows with a delay in the temperature increase in the atmosphere adjustment chamber. This temperature difference changes depending on the type or state of the sulfuric acid-added granular porous material placed on the steel material, and the amount of condensation changes accordingly. Moreover, when the amount of the sulfuric acid-added granular porous material changes, the dilute sulfuric acid supplied to the steel also changes. Accordingly, the amount of corrosion and the degree of progress of pitting corrosion vary depending on the amount of the sulfuric acid-added granular porous material. Therefore, from the viewpoint of more accurate evaluation of the corrosion amount and the pitting depth, the amount of the sulfuric acid-added granular porous material is preferably set to 0.1 to 5 g / cm ² per unit area of the steel material. This is because in this range, the amount of corrosion and the degree of pitting corrosion can be confirmed with good reproducibility.

  Moreover, in this corrosion mechanism, pitting corrosion of an actual ship occurs when coal and coal / ore come into contact with steel materials, and sulfuric acid derived from coal is concentrated in a meniscus portion formed by condensed water accumulated there. Therefore, the shape of pitting corrosion formed varies depending on the particle size of coal. On the other hand, according to Non-Patent Document 1, it is reported that the pitting corrosion of the hold side wall portion is 8: 1 to 10: 1 in the aspect ratio of the pitting diameter and the depth, regardless of the size. Yes. Incidentally, the pitting corrosion of the tanker has a ratio of the pitting diameter to the depth of 4: 1. From this, it was decided that the aspect ratio between the pitting corrosion diameter and the depth would be 8: 1 to 10: 1 as a criterion for judging whether or not this corrosion test method reproduces pitting corrosion of an actual ship. Here, the measurement targets all pitting corrosion appearing on the test piece, and the diameter of the pitting corrosion is measured by a depth meter (manufactured by Mitutoyo: Model No. CD-15C), and the average of the major axis and the minor axis is measured. went. Moreover, the depth of pitting corrosion was measured with a depth meter (manufactured by TECLOCK: DMD-215).

  Here, in order to form pitting corrosion of this shape, it is preferable that the particle size of the granular porous material is 1 to 20 mm. This is because, within this range, the shape of pitting corrosion of an actual ship with clearness of the above-described reproducible evaluation criteria can be obtained.

  Here, the particle size in this range of the granular porous material can be selected and adjusted by sieving in the range of 1 to 20 mm, for example, according to JIS8801.

  Furthermore, the present invention predicts the pitting corrosion depth after 10 years and 20 years by observing the pitting corrosion depth or corrosion amount of the steel material obtained by the test over time, and extrapolating it. Life expectancy can be predicted.

  That is, first of all, tracking the pitting depth of a steel material over time is to measure the pitting depth of each period by performing this test while changing the period. Predicting the period to reach the thickness is to extrapolate the pitting depth obtained by the change over time and calculating the period to reach the corrosion-acceptable plate thickness. Ship life means that the pitting depth reaches the switching thickness of steel as specified in the Common Structural Rules for Bulk Cargo Ships (CSR-B Rules).

  Examples are set forth below, but embodiments of the present invention are not limited thereto.

  The molten steel which becomes a component shown in Table 1 was made into a slab by continuous casting after melting or converter melting in a vacuum melting furnace. Next, the slab was charged into a heating furnace and heated to 1200 ° C., and a 25 mm thick general shipbuilding steel plate was obtained by hot rolling at a finish rolling finishing temperature of 800 ° C.

A test piece of 5 mm ^t × 50 mm ^W × 75 mm ^L was taken from this steel plate, and the surface of the test piece was shot blasted to remove surface scale and oil. After coating the back and end faces with a silicon-based seal, it is fitted into an acrylic jig, and a granular porous material to which a predetermined amount of sulfuric acid has been added is placed, and relative temperature is controlled by a constant temperature and humidity chamber AGX-325. A temperature and humidity cycle with constant humidity and varying temperature was given for 28 days. After the test, the rust of each test piece was peeled off using a rust remover and the amount of corrosion was measured.

  Moreover, it measured using the generated maximum pitting depth depth meter. The test conditions performed are shown in Table 2, and the results are shown in Table 3. In Table 2, for example, no. In the case of 1, the humidity was kept constant at 95% RH, the temperature was kept at 80 ° C. for 20 h (hours), and then the temperature was lowered to 20 ° C. over 0.5 h (hours). Furthermore, it hold | maintained at 20 degreeC for 3 h (hour), and heated up to 80 degreeC over 0.5 h (time) after that. This means that the above process was performed for 24 hours (one day), and this cycle was performed for 28 days (day).

  The particle size of the porous material of 3 to 5 mm is obtained by adjusting coal having a particle size of several tens of μm to several tens of mm using a sieve having openings of 3 mm and 5 mm. Further, the amount of corrosion measured in Table 3 was measured by subtracting the weight of the sample after the test from the weight of the sample before the test, and the maximum pitting corrosion depth was measured with a depth meter. The pitting diameter and depth ratio were measured with calipers. “O” in the column of reproduction of pitting corrosion on an actual ship means a test condition in which the ratio between the diameter and depth of the pitting corrosion is 8: 1 to 10: 1, which is the same as that of an actual ship.

  From the results in Table 3, it can be seen that pitting corrosion has occurred in the test conducted within the range specified in the present invention. Further, FIG. 1 plots the maximum pitting corrosion diameter and depth aspect ratio in each example. From this figure, it can be seen that the ratio of the pitting corrosion diameter to the depth is 8: 1 to 10: 1 which is equivalent to the pitting corrosion of the actual ship. In other words, it is considered that this corrosion test method can simulate the actual corrosive environment of the coal / ore combined ship hold.

  Here, Example No. 3, no. Extrapolation of the maximum pitting corrosion depth of 29-33 by power approximation, hole in actual ship after 12 years (4380 days), 13 years (4745 days), 14 years (5110 days), and 20 years (7300 days) A graph compared with the depth of pitting is shown in FIG.

  The extrapolation curve of the maximum pitting depth in this figure obtained in this test is close to the value of the pitting depth appearing on the actual ship, so the pitting corrosion of the actual ship can be accurately predicted. I understand.

  By using this corrosion test method, the maximum pitting depth after several decades can be predicted by following the change in the maximum pitting depth over time, and this value is set to hold coal and ore combined ships. By comparing the corrosion allowance, it is possible to predict the life.

  Similarly, the amount of corrosion (average corrosion thickness: mm) after several decades can be predicted using the obtained corrosion amount, the test area of the steel material, and the density of the steel material. In FIG. 3, the solid line is an extrapolated line. In this test example, the average corrosion thickness is predicted to be 1.92 mm after 20 years and 2.16 mm after 25 years.

  The present invention simulates a corrosive environment in a coal ship and a coal / ore combined ship hold, and the pitting corrosion depth is tracked with time and extrapolated for 10 years or 20 years ahead in the hold. Therefore, it can be used for the evaluation of corrosion resistant steel for holding coal ships and coal / ore combined ships.

Claims (6)

In the steel corrosion test method using the atmosphere adjustment chamber,
Adding sulfuric acid to a granular porous material to produce a sulfuric acid-added granular porous material;
Placing the sulfuric acid-added granular porous material on the steel material, and installing in the atmosphere adjustment chamber;
A step of arbitrarily selecting the relative humidity of the atmosphere adjusting chamber in a range of 80 to 100%, and changing heating and cooling in a constant cycle in a temperature range of 20 ° C. to 80 ° C .;
A step of measuring the corrosion amount or pitting depth of the steel material, and in the step of forming the sulfuric acid-added granular porous material, the sulfuric acid is in an amount of 0.01 to 100 mg per 1 g of the granular porous material. A corrosion test method for a corrosion resistant steel for holding a coal ship and a combined coal and ore ship.   The corrosion test method for a corrosion resistant steel for holding a coal ship and a coal / ore combined ship according to claim 1, wherein the granular porous material is activated carbon.   The corrosion test method for a corrosion resistant steel for holding a coal ship and a coal / ore combined ship according to claim 1 or 2, wherein a particle diameter of the granular porous material is in a range of 1 to 20 mm. 4. The coal ship according to claim 1, wherein an amount of the sulfuric acid-added granular porous material is 0.1 to 5 g / cm ² per unit area of the steel material. Corrosion test method for corrosion resistant steel for holding coal and ore ships.   The corrosion test method for a corrosion resistant steel for holding a coal ship and a coal / ore combined ship according to any one of claims 1 to 4, wherein the constant cycle is one day.   A method for predicting the service life of a ship, wherein the corrosion test method according to claim 1 is used.

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