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Development of oil-slot backplane with corrosion resistant steel

Time:2016-10-19 17:08:45
Crude oil carrier tank (Cargo Oil Tank referred to as COT) floor is prone to pitting (ie, local corrosion), which may cause the leakage of crude oil. In the use of 2.5 years after the ship maintenance, found on the floor pitting the maximum depth of 10mm, must repair the number of pitting up to thousands, check and repair the workload is also great. With the newly developed corrosion resistant steel from Kobe Steel, the maximum pitting depth is reduced to less than 1/4 of that of conventional steels. Development of steel welding can be used conventional welding methods to meet the requirements of shipbuilding specifications. The development of steel for the COT floor will increase the safety of oil tankers, reducing the use of oil tank costs.
 
The SR242 of the Japan Shipbuilding Research Association has found that the mechanism of the pitting corrosion on the COT floor on an oil tanker is as follows: Although there is water (high concentration brine) necessary for electrochemical reaction of corrosion on the bottom plate, Of the surface layer can be called the oil film to form a protective role of the coating layer, so that the steel plate in the corrosive environment has been protected. However, when the oil film local damage, the sulfur content of crude oil or low pH (acid strong) chloride solution will act on the oil film damage, accelerate the local corrosion of steel plate and pitting.
 
Sometimes also painted on the floor as a response, but found that the coating defects are often caused by pitting growth, so the user should have a real anti-corrosion effect of steel.
 
As a fundamental anti-corrosion measures, Kobe Steel developed a corrosion-resistant steel plate for the bottom of the crude oil tank greatly reduces the growth rate of pitting corrosion, to achieve the level of dry dock inspection without repair, and the product has been practical.
 
The following describes the development of steel corrosion resistant design and steel corrosion resistance and mechanical properties.
1 corrosion resistant design
 
In the development of the corrosion resistant steel for the bottom plate of crude oil tank, two effective corrosion factors (elemental sulfur and low pH chloride solution) identified by SR242 were taken into account and effective components were designed. The results show that the elemental sulfur, which is one of the corrosion factors, is precipitated from the gas phase in the COT, and is present in the steel layer near the steel, which contributes to the corrosion of the bottom plate in the oil film defect. The electrochemical test confirmed that the corrosion reaction was promoted due to the direct contact between elemental sulfur and steel. In the development of steel composition design is based on the steel surface to produce a stable protective film corrosion products, to prevent elemental sulfur and steel contact corrosion inhibition program based on the program.
 
In addition, a chloride solution having a pH value (2 to 4) lower (i.e., stronger acidity) than the external pH (4 to 8) was observed in the bottom plate pitting. The presence of a low pH solution promotes hydrogen evolution. In the development of steel on the use of effective alloy components inhibit hydrogen generation reaction; and dissolved in the alloying elements can increase the pH value of the solution within the pitting, which can significantly inhibit the corrosion of steel reaction.
 
Even in the case where the above-mentioned corrosion factors are simultaneously applied, the developed steel also generates a stable protective film and obtains a pH alleviation effect, so that the corrosion reaction of the steel is effectively suppressed.
 
2 Experimental methods
 
Based on the corrosion mechanism of the above COT substrate, the following test method was established to evaluate the corrosion resistance of the steel.
 
In Test A, NaCl was mixed with a sulfur powder of a special grade (purity ≥99.5%) as a simulated solution in the pitting, and the pitting growth rate was evaluated according to the corrosion rate of the impregnated sample. In addition, in the actual ship survey, is considered to be sludge (dust) elemental sulfur concentration is a few percent. In this test, the corrosion resistance of the sample was evaluated under the harsh environmental conditions where the elemental sulfur concentration was controlled to a higher level than that of the actual ship.
 
Test B is the use of low pH chloride solution to evaluate the corrosion resistance for corrosion. As the pitting internal simulation solution on the sample, a FeCl3 + NaCl solution adjusted to pH 1.0 (i.e., strongly acidic) was dropped to maintain constant temperature and humidity (i.e., 333 K and 95% RH-relative humidity) The corrosion rate of pitting growth rate was evaluated. Test B increased the temperature, lowered the pH value, and promoted corrosion compared to the actual ship condition, so that the corrosion resistance of the steel could be evaluated under more severe conditions than the actual ship environment.
 
3 experimental results
 
In order to evaluate the corrosion resistance to elemental sulfur, a laboratory-promoted test A was carried out. As a result, it was found that the growth rate of pitting corrosion of the developed steels was reduced to 1/4 of the original steel. After the development of the steel surface after the formation of more dense than the original steel corrosion products. Because the former can prevent the chloride ion from the environmental side of the corrosion products of intrusion, suggesting that the higher environmental blocking, it can inhibit the elemental sulfur on the steel substrate corrosion rate.
 
As described above, the corrosion resistance of the developed steel to the elemental sulfur and the low pH (i.e., strong acid) chloride solution is high, and it can effectively suppress the growth of pitting on the COT substrate.
 
Here, the growth rate of pitting represents the fluctuation of the probability theory, according to the Gumbel distribution which is the distribution of the maximum value. This means that the larger the area, the deeper the pitting will be generated. From a small area of ​​the laboratory test results, a large area of ​​the actual pitting growth test has been in oil and gas pipelines and oil storage tank to get more applications. Even on the tanker COT bottom plate, the pitting growth rate was confirmed to follow the above-described Gumbel distribution by the corrosion investigation of a real ship. This shows that the probability of the means for the evaluation of the maximum pitting corrosion growth rate is very effective. The corrosion resistance (ie, the maximum pitting growth rate) of the equivalent area of ​​a real ship is also evaluated by using the extreme value analysis of the Gumbel distribution.
 
By comparing the maximum pitting growth rates of the developed and the original steels in Runs A and B, it is shown that the maximum pitting growth rates in both experiments follow the Gumbel distribution well and reproduce the same probability distributions of the real ship Corrosion growth, thus showing the appropriateness of the test. In either case, the Gumbel distribution of the developed steels is larger than that of the original steel, indicating that there is no large pitting corrosion on the developed steel, ie, the corrosion resistance of the developed steel is effective .
 
Here, the average pitting diameter of the VLCC (crude oil carrier) was 10mm, and the number of the pits was set at 2000. The pitting growth rates of the samples were 65.4 and 174 for the test cycles A and B, respectively Equivalent to the maximum growth rate of the actual ship. In either test, the maximum pitting growth rate for a given area of ​​steel equivalent to a real ship is 1.2 mm per year, about one-quarter of the original steel, ie, the maximum pitting corrosion during dry-dock repairs over 2.5 years Depth of 1.2mm / years × 2.5 years = 3mm, indicating that the maintenance standards can be controlled under the depth of 4mm. This indicates that the developed steel exhibits excellent corrosion resistance even under the conditions of a substantial area of ​​the vessel.
 
The maximum pitting depth predicted by the oil tanker at the dry dock at intervals of 2.5 years on the original steel is 10 mm, usually> 4 mm, and> 7 mm for surfacing repair. The investigation by SR242 showed that pitting corrosion on the COT soleplate was stopped by the dry-dock repaired. The reason is that in the repair, the removal of corrosive products in the pitting, compared with the general site to produce a thicker film, it is better corrosion resistance. Therefore, if the maximum pitting depth ≯ 4mm is used for the COT base plate, it is not necessary to repair the oil tanker during the use of the COT.
 
The strength and toughness of the developed steels all reached the same level as the original steels, which satisfied the requirements of the class specification (AH32). The toughness of FCB welds made with ordinary welding materials and heat input of 130kJ / cm To achieve the same level of the original steel to meet and exceed the requirements of AH32 specifications, so the development of steel can be completely the same with the original steel welding method.
 
4 Conclusion
 
Corrosion-resistant steel used in Kobe Steel's crude oil tank bottom plate has a corrosion resistance of 4 times that of the original steel, and can greatly suppress the growth of pitting corrosion. Development of the use of steel can reduce the cost of painting and regular maintenance costs, while reducing the risk of oil leaks and improve the safety of oil tankers. In addition, the excellent corrosion resistance of the developed steel is also confirmed in practical use.
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