DEVELOPMENT OF CORROSION RESISTANT STEEL FOR CARGO OIL TANKS

Transcription

1 DEVELOPMENT OF CORROSION RESISTANT STEEL FOR CARGO OIL TANKS K Kashima, Sumitomo Metal Industries, Ltd., Japan Y Tanino, Sumitomo Metal Industries, Ltd., Japan S Kubo, Sumitomo Metal Industries, Ltd., Japan A Inami, Sumitomo Metal Industries, Ltd., Japan H Miyuki, Sumitomo Metal Industries, Ltd., Japan SUMMARY Recently, ship owners require saving cost of inspection and repair due to corrosion in crude oil tanks of oil tankers. There are two types of corrosion in crude oil tank, the general corrosion at upper deck plate and the pitting corrosion at bottom plate. On the basis of the results of field examinations on several VLCCs by The Shipbuilding Research Association of Japan, simulated corrosion test methods were established and a corrosion resistant steel was developed. Laboratory test results revealed that the developed steel has twice or more corrosion resistance compared with conventional steel in corrosion environments of both upper deck and bottom. Mechanical properties and characteristics of welded joints were equivalent to conventional steel and corrosion resistance of welded joint is also equivalent to base metal. It was found that corrosion resistance is improved by the action of alloying elements in the low ph condensed water on the upper deck plate, and by the formation of protective sulphide film including alloying elements on the bottom plate. NOMENCLATURE [Symbol] [Definition] [(unit)] EPMA Electron Probe Micro Analysis YP Yielding Point (N/mm 2 ) TS Tensile Strength (N/mm 2 ) EL Elongation (%) E -20 Absorbed Energy at -20 C (J) SAW Submerged Arc Welding FCB Flux Copper Backing E 0 Absorbed Energy at 0 C (J) WM Weld Metal FL Fusion Line HAZ Heat Affected Zone 1. INTRODUCTION The solution of the corrosion problem of the cargo oil tanks of oil tankers is indispensable in order to improvement of safety of the ship and the prevention of the environmental pollution. And recently, ship owners require saving cost of inspection and repair due to corrosion in cargo oil tanks. Corrosion environment in cargo oil tank consists of inert gas to prevent from exploding of oil tank, H 2 S originated form crude oil, liquid phase of crude oil and drain water [1]. There are two types of corrosion in crude oil tank, the general corrosion with flaky corrosion products at upper deck plate and the pitting (localized) corrosion at inner bottom plate. Typical examples of corrosion appearances at upper deck plate and inner bottom plate are shown in figure 1. From the viewpoint of the extension of lifetime of oil tanker and improvement of high reliability, countermeasures against corrosion are required. In this paper, simulated corrosion test methods were established on the basis of the corrosion mechanism, and the corrosion resistant steel was developed by the study on the effects of alloying elements on corrosion behavior of steels. Figure 1: Corrosion appearance in cargo oil tank 2. CORROSION MECHANISMS IN CARGO OIL TANKS In vapor space of crude oil tank, inert gas containing O 2 (<5 volume percent), CO 2, SO 2 and H 2 S originated from crude oil exist. According to the field examination of several VLCCs by The Shipbuilding Research Association of Japan Panel #242 (SR242 committee), H 2 S gas was detected in high concentration in vapor space. Maximum concentration of H 2 S was over 0.2 volume percent at full load condition. This co-exist of O 2 and H 2 S is very rare case on the stand point of corrosion science because of reducing property of H 2 S. So 2007: JASNAOE-RINA 5

2 corrosion environment in cargo oil tank is quite complicated and unique. Figure 2 shows the corrosion mechanism at upper deck plate. The backside of upper deck is exposed in the cyclic wet and dry conditions by the temperature change through day and night, then the ph of the condensate water become lower (approx. 2-4) in the existence of CO 2 and SO 2 in the inert gas. And moreover, elemental sulphur is generated by oxidation of H 2 S with oxygen. Corrosion product on upper deck plate mainly consists of α-feooh and elemental sulphur, and has layered structure of rust and elemental sulphur. According to the field examination by SR242 committee, maximum 60 weight percent of elemental sulphur was detected in corrosion product at upper deck. This indicates that the amount of corrosion product does not correspond to the corrosion loss at upper deck. It seems that existence of H 2 S in vapor space does not affect so much on corrosion but mainly generate elemental sulphur and increase the volume of corrosion product. These mean that the general corrosion of the upper deck plate progressed by the condensate water with a low ph [2][3]. And elemental sulphur that falls off from upper deck plate accelerates pitting corrosion by the action as an oxidizer as follows, S + 2H 2 O H 2 S + 2OH - It is reported that the growth of pits stops at a dry dock because prior to dock inspection inside of cargo oil tank is cleaned and pits are re-coated by new crude oil after inspection. These suggest that the pitting corrosion of the bottom plate occurred and progressed at the defect of the oil coating. Figure 3: Corrosion mechanism at the bottom of cargo oil tank 3. ESTABLISHMENT OF SIMULATED CORROSION TEST METHODS 3.1 SIMULATED CORROSION TEST FOR UPPER DECK OF CARGO OIL TANK Figure 2: Corrosion mechanism at the upper deck of cargo oil tank On the other hand, at the inner bottom of cargo oil tank drain water including high concentrated chloride ion and H 2 S originated from crude oil exist. As shown in figure 3 the bottom plate is covered with oil coating layer containing sludge. In general, oil coating decreases corrosion, but many defects of the oil coating caused by crude oil washing and water drops from above structure exist. Pits originate from these defects of oil coating and grow by creating the corrosion electric cell between defect (anode) and steel surface under the oil coating around defect (cathode) in severe corrosion environment with concentrated chloride ion and H 2 S. In this case anodic and cathodic reactions are described as follows, Anodic reaction : Fe Fe e Cathodic reaction : O 2 + 2H 2 O + 4e 4OH - From the corrosion mechanisms mentioned above, simulated corrosion test methods were studied to reproduce the corrosion which was observed in actual cargo oil tank of oil tanker. Upper deck plate of cargo oil tank is exposed to the corrosion environment that contains O 2, CO 2, SO 2 in inert gas, H 2 S from crude oil and condensation by the temperature change. Simulated test method is shown in Figure 4. The test apparatus consists of two chambers, outer chamber corresponds to the atmosphere and inner chamber corresponds to cargo oil tank. Specimens of 25 mm X 50 mm X 4 mm were set on the upper surface of inner chamber to simulate upper deck plate. Temperature of outer chamber varied between 50 C (20 hours) and 25 C (4 hours) that is a typical temperature change at upper deck. Surface of the specimens condensed by this temperature change. Gas A (13%CO 2-5%O %SO 2 - bal.n 2 ) and gas B (gas A + 0.2%H 2 S) were blown alternatively every 14 days to the inner chamber to simulate condition at ballast and full load. The ph of condensate water of this simulated test was about 2.7, that is similar to the ph measured in actual cargo oil tank [3]. Figure 5 shows cross section of corrosion product after simulated test for 28 days. Corrosion product consists of rust layer, mixture of rust and sulphur, and layered elemental sulphur. The structure of corrosion product 2007: JASNAOE-RINA 6

3 after this test was quite similar to that of cargo oil tank. And composition of corrosion products was similar to that of cargo oil tank as shown in table 1. These results suggest that the structure and the composition of corrosion product were reproduced by the laboratory corrosion test. Figure 4: Simulated corrosion test apparatus for upper deck of cargo oil tank 3.2 SIMULATED CORROSION TEST FOR BOTTOM OF CARGO OIL TANK As mentioned above, inner bottom plate of cargo oil tank is covered with oil coating that suppresses corrosion but many defects exist caused by crude oil washing. Pits initiate at the defects and grow rapidly in severe corrosion environment at the bottom. Figure 6 shows test apparatus for pitting corrosion at bottom. The test apparatus consists of two chambers and temperature of outer chamber was kept at 40 C. Inner chamber corresponds to cargo oil tank and specimens of 25 mm X 50 mm X 4 mm were set in synthetic sea water (ASTM-D ) simulated drain water containing chloride ion at the bottom. Simulated inert gas (13%CO 2-5%O %SO 2 -bal.n 2 ) and 0.2% H 2 S were blown to inner chamber continuously. According to the research by SR % of sludge in cargo oil tank is oil and others mainly consist of rust shown in table 2, α-feooh and Fe 3 O 4. So simulated oil coating that was composed of a mixture of crude oil, α- FeOOH and Fe 3 O 4 was coated on the specimens. To reproduce the pitting corrosion at the fixed portion of bottom plate, artificial circle defect of 5mm in diameter is induced [4]. Figure 5: Cross sectional morphology and distribution of elements by EPMA in corrosion product of specimen after simulated corrosion test for upper deck Table 1: Composition of corrosion products on the simulated test for upper deck analyzed by X-ray diffraction method (mass %) α-feooh β-feooh Cargo oil tank 37 0 Simulated test 30 0 γ-feooh Fe 3 O 4 Elemental S Others Figure 6: Simulated corrosion test apparatus for bottom of cargo oil tank and surface appearance of a test specimen after corrosion test Appearance of specimen after pitting corrosion test for 14 days is shown in figure 6. Pit was observed at the defect of simulated oil coating and corrosion rate under oil coating is very low. This corrosion morphology is quite similar to pitting corrosion in actual cargo oil tank. Corrosion products of specimen were similar to that of the bottom in cargo oil tank as shown in table 3. Pit depth after 28 days test was about 0.9 mm, that was over 10 times larger than thickness loss of specimen without oil coating, 0.08 mm. It means that corrosion electric cell was created in this laboratory test and pitting corrosion grew by the same mechanism as cargo oil tank. These results suggest that pitting corrosion observed on actual ship was reproduced by laboratory test. 2007: JASNAOE-RINA 7

4 Table 2: Examples of compositions of sludge in crude oil tanks (mass %) Sludge Oil Solid substances A B Composition of solid substances Sludge α-feooh Fe 3 O 4 others A B Table 3: Composition of corrosion products on the simulated test for the bottom analyzed by X-ray diffraction method (mass %) α-feooh β-feooh Cargo oil tank Simulated test conventional steel in the environment of bottom plate for 56 days. It was found that corrosion resistance is improved by the action of alloying elements which are effective to corrosion resistance to low ph water on the upper deck plate [5][6]. It seems that corrosion product layer containing alloying elements also suppress corrosion because corrosion loss of developed steel tended to decreases with test period. At the bottom pit depths of conventional and developed steel were comparable after 7 days test, but pit depth of developed steel decreased with test period. This indicates that the growth of pit was suppressed on developed steel. It was found that corrosion product just on steel surface of developed steel contains high concentration of S and alloying elements by cross sectional observation shown in figure 8. This sulphide film including alloying elements was observed only on the developed steel. It seems that the sulphide film formed on developed steel suppresses H 2 S concentration at inner corrosion product layer and permeation of chloride ions to the steel surface. γ-feooh Fe 3 O 4 Elemental S Others FeCO 3, FeS FeCO 3, FeS In actual tank pit depth varies widely because of thickness of oil coating, size of defects and incubation time to pit initiation. And bottom plate can be investigated only in a dry dock and pit stops at a dock as mentioned in section 2, so time dependence of pit depth can not be measured in actual cargo oil tank. On the other hand, in this laboratory test method pit depth does not vary so much because all specimens have almost same thickness and defect of oil coating. And time dependence of pit depth can be measured because pitting corrosion starts at a same time on the all specimens. From that point of view this laboratory test method is suitable for comparison of pit depth among specimens. On the basis of the study for the effects of alloying elements on corrosion behavior of steels in simulated corrosion test environments, the corrosion resistant steel which contain Cu, Ni and W which improve corrosion resistance in the environments of upper deck and bottom was developed. 4. CHARACTERISTICS OF DEVELOPED STEEL 4.1 CORROSION RESISTANCE Simulated corrosion test results of the upper deck plate and the bottom plate are shown in figure 7, which show that the developed steel has about twice corrosion resistance compared with conventional steel in corrosion environments of upper deck plate for 84 days. Pitting rate of developed steel was also about a half compared with Figure 7: Simulated corrosion test results 2007: JASNAOE-RINA 8

5 Table 5: Impact test result of welded joint (plate thickness: 16.5mm) Welding procedure Notch position WM SAW (3-electrode FCB method) FL HAZ 1mm HAZ 3mm HAZ 5mm E 0 (J) Figure 9: Corrosion resistance of weld metal Figure 8: Cross sectional distribution of elements by EPMA in corrosion product on developed steel after simulated corrosion test for bottom These results indicate that newly developed steel has good corrosion resistance in both upper deck and bottom that have different corrosion environments and mechanisms 4.2 MECHANICAL PROPERTIES OF DEVELOPED STEEL As shown in table 4, it was confirmed that tensile and impact properties of the developed steel were equivalent to conventional steel, and they satisfied the specification of DH36 grade of classes. And it was found that large heat input welded joint had good impact properties as shown in table 5. Furthermore, regard to corrosion resistance of the welded joint, it was equivalent to the base metal shown in figure 9. In the result of appreciation these good properties, the developed steel was certificated of classification LR, NK, ABS, and DNV. Especially, LR approved corrosion resistance of developed steel in his technical paper. 5. EXPOSURE TEST RESULTS OF DEVELOPED STEEL IN ACTUAL CRUDE OIL TANKS Newly developed steel has been applied to upper deck and bottom of cargo oil tanks of actual ships. Test coupons have been also exposed. 3 test coupons exposed to vapour space for a year in 3 cargo oil tanks of 2 aframax tankers as covers of tank cleaning holes were taken out and investigated. Figure 10 shows the results of exposure test in cargo oil tanks for 1 year. It found that corrosion rate of developed steel was about % of conventional steel. It was confirmed that developed steel has good corrosion resistance compared with conventional steel in actual tanks. Table 4: Mechanical properties of developed steel (Plate thickness: 16.5mm) Developed steel DH36 spec. YP (N/mm 2 ) TS (N/mm 2 ) EL (%) E -20 (J) Figure 10: Results of exposure test for actual upper deck corrosion environment 2007: JASNAOE-RINA 9

6 6 CONCLUSIONS Countermeasures against corrosion problems in cargo oil tank from the viewpoint of material were investigated. Results obtained are as follows. Simulated corrosion test methods for upper deck and bottom of cargo oil tank were established on the basis of corrosion mechanism, and corrosion on actual ship was reproduced by laboratory corrosion test. New steel was developed which has good corrosion resistance in the corrosion environment of both upper deck plate and bottom plate. Developed steel has about twice corrosion resistance compared with conventional steel. Mechanical properties of the base metal and the characteristics of the welded joint are equivalent to conventional steel. Corrosion resistance of weld metal is equivalent to base metal. Developed steel has good corrosion resistance after exposure test for 1 year in vapor space of actual cargo oil tanks. 7. REFERENCES 1. H.MIYUKI et al., European Federation of Corrosion Publications No.26, Advances in Corrosion Control and Materials in Oil and Gas Production 188, H.YOSHIKAWA, Zairyo-to-Kankyo, 53, 388, K.KASHIMA et al., Proceeding of 40th Jpn. Conf. Materials and Environments, 73, K.KASHIMA et al., Proceedings of JSCE Materials and Environments 2007, 89, K.KASHIMA et al., Conference proceedings The society of naval architects of Japan, 131, Y.TANINO et al., Asia Steel international conference- 2006, 758, AUTHORS BIOGRAPHIES Kazuyuki Kashima holds the current position of researcher at plate and structural steel research and development department, corporate research and development laboratory. He is responsible for research of corrosion mechanism and development of corrosion resistant steel for ships and bridges and other steel structures. 2007: JASNAOE-RINA 10