JP5659982B2 - Corrosion test method of corrosion resistant steel for coal ship and coal / ore combined ship hold and method of predicting the service life of ship using it - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a corrosion test method for corrosion resistance steel for a coal ship and a coal / ore combined ship hold capable of reproducing and evaluating a steel material used for a coal ship and a combined coal / ore ship held in a laboratory.

  In bulk cargo ships, marine accidents became an international issue one after another in the early 1990s. In particular, many accidents have been reported on coal ships and coal / ore combined ships, most of which were caused by damage in the hold (hereinafter simply referred to as “hold”). Bulk cargo ships are loaded directly on the hold, so they are easily affected by corrosive loads, and are locally affected by corrosion inside the hold, especially pitting corrosion on the side walls of coal ships and coal / ore combined ships. In particular, it is considered that the strength decreases. 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 ore and scratches and abrasion caused by heavy machinery during loading and unloading, a sufficient anticorrosion 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 the evaluation of the corrosion resistant steel for holding coal and coal / ore combined ship hold, as shown in Examples of Patent Documents 1 and 2, 60 ° C., 100% RH, 6 hours ⇒ 0.5 at 60 ° C. Soaked in 0.5% NaCl + 0.1% CaCl ₂ + 0.5% Na ₂ SO ₄ solution for 0.5 hour ⇒ Evaluation of coating material with bare material and scratch in 17.5 hour cycle at 60 ° C, 50% RH It is.

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 reported by the Japan Maritime Association as a factor that accelerates corrosion. Since it is not a sulfuric acid environment, it cannot be said that the corrosive environment of the hold of an actual coal ship and a coal / ore combined ship can be 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 it is necessary to evaluate the pitting depth of the bare material because the coating is peeled off due to mechanical damage due to coal or ore and abrasion due to heavy machinery during loading and unloading.

  In order to solve the above-described problems, an object of the present invention is to provide a corrosion test method in which the corrosion environment in the hold of a coal ship / coal and ore combined ship is reproduced in a laboratory.

The present invention is based on the above studies, and the gist thereof is shown below.
1. Coal is placed on the surface of the steel material, maintained at a first temperature in a constant high humidity environment, and then continuously lowered to a second temperature lower than the first temperature. The second temperature is maintained, and then a cycle of continuously increasing the temperature from the second temperature to the first temperature is repeated to cause condensation on the steel surface and evaluate the corrosion resistance. A corrosion test method for corrosion resistant steel for holding a coal ship and a coal / ore combined ship.
2. The first temperature and the second temperature are arbitrarily selected from a temperature range of 20 ° C. to 80 ° C., and the constant high humidity is arbitrarily selected from a range of relative humidity of 80 to 100%. 2. The corrosion test method for the corrosion resistant steel for holding a coal ship and a coal / ore combined ship according to 1.
3. The amount of coal placed on the surface of the steel material is 0.1 to 5 g / cm ² per unit area of the steel material, and the particle size of the coal is in the range of 1 to 20 mm 1 or 2 Corrosion test method for corrosion-resistant steel for holding coal ships and coal / ore combined ships as described.
4). 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 3, wherein the single cycle is 6 to 72 hours.
5. Prediction of ship life characterized by predicting the period of time when the pitting depth reaches the allowable corrosion thickness from the pitting depth of the steel material obtained by the corrosion test method according to any one of 5.1 to 4 Method.

  The present invention reproduces the corrosive environment in the hold of a coal ship / coal and ore combined ship in a laboratory. Can be used.

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.

Below, the form for implementing this invention is demonstrated.
As a result of examining the mechanism of the occurrence of pitting corrosion that most affects the destruction of the ship due to the corrosion in the hold of the coal ship and the coal / ore combined ship, the present inventors have as follows.

The side wall of the bulk carrier is a single hull, and the cargo and seawater are separated from each other only by one piece of steel. 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. Pitting corrosion progresses under the coal that forms the meniscus, and H ⁺ is consumed for corrosion of the steel material in the meniscus portion, 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 at the meniscus portion and pitting corrosion. Will be formed. Based on this mechanism, the following conditions were used to simulate pitting corrosion in the hold of coal ships and coal / ore combined ships.

First, the reason why the corrosion environment condition is limited to the above range in the present invention will be described.
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, it is preferable to select the highest expected temperature during the day and the lowest expected temperature during the night, and in the present invention, the temperature range is 20 ° C to 80 ° C. 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 for the accelerated test that the temperature adjustment is within a time of 0.3 to 1.0 h (hour).

  Moreover, humidity will always be in a highly moist state by the water | moisture content which coal has, or the dew condensation water produced by the temperature difference in seawater and a hold | maintenance. Therefore, the humidity condition is preferably 80 to 100% RH (where RH is an abbreviation for relative humidity). In the present invention, the accelerated test is carried out by setting it to be a constant high humidity, but there is no problem with slight fluctuations in humidity due to deviations or variations in the timing of humidity control and response. However, in order to make the test reproducible, it is preferable to eliminate variations for each cycle as much as possible.

  In addition, the sulfur component dissolved in the condensed water is more corroded when it is concentrated in the drying process. In order to reproduce this concentration process in this test, one cycle of 6 hours or more is required. Preferably, in order to further enhance the effect of the acceleration test, it is preferable that one cycle is 72 hours or less.

Further, when the temperature is changed in a constant high humidity environment, condensation occurs when the temperature rises. This is because the temperature of the steel material follows the temperature rise of the outside air, and a temperature difference occurs between the steel material and the outside air. This temperature difference and the amount of dew condensation vary depending on the amount of coal placed on the steel surface. Further, when the amount of coal placed on the steel material surface is large, the amount of dilute sulfuric acid supplied to the steel material increases, so that the corrosion amount of the steel material and the degree of pitting corrosion change. Therefore, from the viewpoint of more precise corrosion amount and clarity of evaluation criteria with reproducibility of pitting depth, the coal amount is preferably 0.1 to 5 g / cm ² per unit area of the steel material. However, the amount of coal is not limited to this.

  Next, the coal to be used will be described. In general, coal is greatly different in nature and state depending on its brand and place of origin, but it is said that coal is a causative substance that accelerates corrosion because it contains a sulfur component. Since this invention pays attention to the corrosiveness which coal has, when implementing this invention, the brand etc. are not limited. However, from the viewpoint of easy measurement and variation in corrosion amount and pitting depth, 5 g of coal is immersed in 100 ml of distilled water at room temperature for 2 hours so that the corrosion progresses, followed by filtration and 200 ml with distilled water. It is preferable to use coal that has a pH of 4 or less when the volume is increased. However, the test target coal is not limited to this.

  In addition, as a result of the experiment, the present inventors have found that pitting corrosion observed on an actual ship is caused by the concentration of dilute sulfuric acid derived from coal in the meniscus portion where coal contacts steel and is formed by condensed water. We have knowledge. In addition, according to Non-Patent Document 1, it is reported that the pitting corrosion of the hold side wall portion has an aspect ratio of 8: 1 to 10: 1 between the pitting corrosion diameter and the pitting corrosion depth regardless of the size. . Incidentally, the ratio of the pitting corrosion diameter to the pitting corrosion depth of the pitting corrosion of the tanker is 4: 1. From this, it is considered that the ratio of pitting diameter to pitting depth is almost constant depending on the corrosive environment, regardless of the size of pitting corrosion, and this corrosion test method reproduces the pitting corrosion of the actual ship. The criterion for determining whether the aspect ratio between the pitting diameter and the pitting depth is 8: 1 to 10: 1. Here, the measurement is performed on all pitting corrosion appearing on the test piece, and the diameter of the pitting corrosion is measured with a caliper (manufactured by Mitutoyo: Mitutoyo Digital Caliper CD-15C), and the average of the major axis and the minor axis is measured. It was. The depth of pitting corrosion was measured with a depth meter (manufactured by TECLOCK: Digital Depth Gauge DMD-215).

Here, the particle size of coal is not particularly limited, but in order to easily obtain the above pitting corrosion shape, the particle size is preferably 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 of the coal in this range can be selected and adjusted with a sieve having a mesh opening of 1 to 20 mm, for example, according to JIS8801.

Furthermore, according to the present invention, by tracking the pitting corrosion depth of the steel material obtained by the test over time, it is possible to predict the period during which the pitting corrosion depth reaches the corrosion-acceptable plate thickness and to predict the life of the ship. .
The pitting depth is measured after removing rust. Therefore, it is difficult to follow changes with time of the same part by pitting corrosion. Therefore, tracking the pitting corrosion depth of a steel material over time is to measure the pitting corrosion depth for each period by performing this test by changing the period with a plurality of test pieces. Predicting the period to reach the corrosion-acceptable plate thickness is to predict by extrapolating the pitting corrosion depth obtained by the change over time and calculating the period to reach the corrosion-acceptable plate thickness. Further, the life of the ship means that the pitting depth reaches the switching plate thickness of the steel material defined in the common structure rule for bulk carriers (steel ship rule CSR-B).

  Here, the target steel material can be tested in various states, such as a bare steel material, or a steel material having a whole surface coating or a partially painted portion, and is not particularly limited. As described above, assuming a use environment with a large mechanical damage, the corrosion of the steel material and the life of the ship can be accurately predicted if the test is performed with a bare steel material or a steel material having a partially painted portion. It can be appropriately selected assuming the use environment.

  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 steel plate having a thickness of 25 mm was formed 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 collected from the general shipbuilding steel shown in Table 1 as a steel plate, and the surface of the test piece was shot blasted to remove scale and oil on the surface. After coating the back and end surfaces with silicone seals, fit them in an acrylic jig, spread the coal on them, make the relative humidity constant with a low temperature and humidity chamber (Advantech: AGX-325), and adjust the temperature. A varied temperature and humidity cycle was applied for 28 days. The pH of the coal was measured by immersing 5 g of coal in 100 mL of distilled water at room temperature for 2 hours, filtering, and making up to 200 mL with distilled water.

  After the test, the rust of each test piece was peeled off using a rust remover and the amount of corrosion was measured. Further, the maximum pitting depth produced was measured using a 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 coal particle size 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. ○ in the column of reproduction of pitting corrosion of actual ship means that the ratio of the diameter and depth of pitting corrosion is 8: 1 to 10: 1, which is the same as the actual ship, and x is within the range. It means the test condition that was not entered.

  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 diameter to the depth is 8: 1 to 10: 1 which is equivalent to the pitting corrosion of the actual ship. Here, Example No. 3, no. The maximum pitting depth of 26-30 is extrapolated by a power approximation, and the actual ship hole 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. In FIG. 3, no. 26 to 30 and represents a power approximation curve. 3, no. It is the plot point of the maximum pitting corrosion depth of 26-30, and the actual ship pitting corrosion represents the plot point of the pitting corrosion depth of the actual ship reported from the Japan Maritime Association.

  In addition, the extrapolation curve of the maximum pitting depth in this figure obtained in this test is close to the value of the pitting depth that appears on the actual ship. It can be predicted. Here, the maximum pitting corrosion depth is defined as the deepest pitting corrosion occurring in one sample of this test. In addition, No. which does not cause dew condensation by repeated dry and wet, which is important in the pitting mechanism considered by the present inventor. In 31 and 32, the amount of corrosion was small compared to other conditions, the pitting corrosion was shallow, and the pitting corrosion shape was different from the actual ship.

  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 (4)

Coal having a particle size of 1 to 20 mm is placed on the steel surface at a rate of 0.1 to 5 g / cm ² per unit area of the steel material , maintained at a first temperature in a constant high humidity environment, and then the first The temperature is continuously lowered to a second temperature lower than the first temperature, then held at the second temperature, and then continuously raised from the second temperature to the first temperature. A corrosion test method for a corrosion resistant steel for holding a coal ship and a coal / ore combined ship holding, characterized in that the cycle is repeated to cause condensation on the surface of the steel material and evaluate the corrosion resistance.   The first temperature and the second temperature are arbitrarily selected from a temperature range of 20 ° C. to 80 ° C., and the constant high humidity is arbitrarily selected from a range of relative humidity of 80 to 100%. The corrosion test method for the corrosion resistant steel for holding a coal ship and a coal / ore combined ship according to claim 1. 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 the single cycle is 6 to 72 hours. A ship life prediction method characterized by predicting a period during which the pitting depth reaches the corrosion allowance from the pitting depth of the steel material obtained by the corrosion test method according to any one of claims 1 to 3 .

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