A hot-rolled L360 grade pipeline steel coil for ground gathering and transportation and a production method thereof

Abstract

The invention relates to an L360 grade pipeline steel hot-rolled plate coil for ground gathering and transportation and a production method thereof. The chemical composition of the steel is C: 0.050%-0.060%, Si: 0.15%-0.25%, Mn: 0.20%-0.28%, P≤0.012%, S≤0.003%, Als: 0.030%-0.045%, Nb: 0.018%-0.023%, Cu: 0.08%-0.12%, Cr: 0.05%-0.10%, N≤0.004%, and the remainder is iron and impurities. The yield strength of the hot-rolled coil product is 378-392MPa, the tensile strength is 514-545MPa, and the elongation after fracture is ≥37.5%; the Charpy impact energy Akv at -40℃ is ≥324J, the corrosion rate is 0.0245-0.0281mm / a, the corrosion potential is ≥-0.462V, and the corrosion current density is ≤1.10×10 ‑5 A.cm ‑2 .

Description

Technical Field

[0001] The present invention relates to the technical field of low alloy hot rolled coil production, and in particular to an L360 grade pipeline steel hot rolled coil for ground gathering and transportation and a production method thereof. Background Art

[0002] Surface gathering and transportation pipelines are primarily used to transport crude oil, sewage, and natural gas. Because the transport medium contains a certain amount of chloride ions, pipeline corrosion failures such as perforations, leaks, and bursts are frequent. Currently, surface gathering and transportation pipelines used by major oil and gas fields mostly use 20# steel, 16Mn, stainless steel, or corrosion-resistant alloys. Stainless steel and corrosion-resistant alloys are prohibitively expensive to use on a large scale. Surface gathering and transportation pipelines made of 20# and 16Mn steel require the addition of large amounts of Mo, Cr, Ni, and Cu to ensure corrosion resistance due to their high carbon design, resulting in relatively high alloy costs. Therefore, there is an urgent need to develop a steel for surface gathering and transportation pipelines that is low-cost, highly corrosion-resistant, and possesses excellent comprehensive mechanical properties.

[0003] Chinese patent application number CN201310368061.9 discloses "Chloride-ion corrosion-resistant steel for pipelines and its preparation method." The steel's composition (by weight percentage) is: C 0.008% to 0.02%, Si 0.05% to 0.015%, Mn 0.50% to 0.85%, S ≤ 0.010%, P ≤ 0.01%, Mo 0.8% to 2.0%, Al 0.001% to 0.020%, Ni 0.50 to 1.2%, W 0.02 to 0.08% by weight, with Cr satisfying the following inequality: 2.4 - 0.49 [Mo] + 11.8 [C] + 12.2 [W] ≤ Cr ≤ 3.2 - 0.57 [Mo] + 9.8 [C] + 7.2 [W]. However, the addition of Mo, Ni, and W to the pipeline steel results in a high alloy cost.

[0004] The Chinese patent application with application number CN201910420753.0 discloses "a steel resistant to corrosion by multiple media with a yield strength of 500 MPa and a preparation method thereof". The composition of the steel is (by weight percentage): C 0.06%~0.10%, Si 0.15%~0.40%, Mn 0.70%~1.20%, P≤0.025%, S≤0.01%, Cu 0.20%~0.40%, Ni 0.20~0.50%, Cr 0.50~1.0%, Sb 0.02~0.12%, Sn 0.005~0.030%, Nb 0.02~0.05%, Ti 0.015%~0.025%, Ca 0.001%0~0.003%, N≤0.006%. The addition of Sb, Sn and Ni to the steel results in a higher alloy cost. The yield strength of the steel is ≥500MPa, but the impact energy at -40°C is only 80J. The strength and toughness of the product are poorly matched, making it not a preferred material for ground gathering and transportation pipelines.

[0005] Chinese patent application number CN201610803301.7 discloses a "thick-gauge X52 pipeline steel and its production method." The steel's composition is as follows: C 0.03%-0.07%, Si 0.10%-0.30%, Mn 1.10%-1.30%, P ≤ 0.020%, S ≤ 0.010%, Nb 0.015%-0.035%, Ti 0.008%-0.019%, and Cr 0.15%-0.30%. The patent application uses a conventional pipeline steel alloy design and does not describe the steel's corrosion resistance.

[0006] Chinese patent application number CN201310585644.7 discloses a "Low-Cost X52 Pipeline Steel Production Method and Pipeline Steel." The steel's composition, by weight, includes C 0.08% to 0.12%, Si ≤ 0.35%, Mn 1.10% to 1.40%, S ≤ 0.025%, P ≤ 0.025%, and Ti 0.008% to 0.022%. The steel contains no Nb and achieves the required strength and toughness solely through Ti microalloying. However, after pipemaking, the product rarely achieves a strength of 360 MPa. Furthermore, as a conventional pipeline steel, it struggles to meet the corrosion resistance requirements of surface gathering and transportation pipelines.

[0007] Chinese patent application number CN201010243258.6 discloses "a production method for X52 pipeline steel", Chinese patent application number CN201910949817.6 discloses "an ultra-thick specification X52 pipeline steel hot-rolled coil and its production method", and the X52 pipeline steel products described in the paper "Research and Development of Hot-Rolled Medium-Wide Strip Steel X52 for Oil and Natural Gas Transmission Pipes" (written by Feng Shaoqiang et al., "Sichuan Metallurgy" Issue 04, 2010) all use the alloy design of traditional pipeline steel, and are also difficult to meet the corrosion resistance requirements of ground gathering and transportation pipelines.

[0008] Chinese patent application number CN201410247308.6 discloses "5Cr corrosion-resistant steel and its production method." The steel's composition (by weight) is: C 0.05% to 0.07%, Si 0.10% to 0.30%, Mn 0.50% to 0.80%, P ≤ 0.020%, S ≤ 0.01%, Cr 4.5% to 5.5%, and N 0.02% to 0.05%. This application relates to hot-rolled rebar steel, and its mechanical performance requirements differ significantly from those of hot-rolled coil products.

[0009] The paper "Corrosion Resistance Screening and Evaluation Test of Oil Casing and Gathering Pipeline Materials" (authored by Duan Fangwei et al., "Oil and Gas Field Environmental Protection" Issue 6, 2018, pp. 24-28) mentions steel for gathering pipelines. The 20 steel, 16Mn steel, and X65 steel described in the paper are all high-carbon, Nb-free, and compositely added with Cr, Ni, Mo, and Cu alloy designs. The uniform corrosion rate data provided show that the corrosion level of the steel reaches moderate or even severe corrosion. In addition, the paper does not record the production process and mechanical properties of the steel.

[0010] The steels for ground gathering and transportation pipelines involved in the above-mentioned public materials are mostly medium and thick plates and seamless products, and the precious metals Mo and Ni are basically added in the alloy design. Except for the patent applications "A steel with a yield strength of 500MPa and resistance to corrosion of multiple media and its preparation method" and "Preparation method of CO2 corrosion-resistant pipeline steel for ground gathering and transportation", no Nb is added. The L360 grade pipeline steel hot-rolled coil for ground gathering and transportation described in the present invention breaks the inherent idea of ​​high-carbon design for ground gathering and transportation steel, and opens up a new way to use low-carbon steel to obtain sufficient strength and good corrosion resistance. Summary of the Invention

[0011] The present invention provides a hot-rolled coil of L360-grade pipeline steel for ground gathering and transportation, and a production method thereof. The coil adopts relatively low contents of C, Mn, and Si, and adds Nb to enhance the effect of grain refinement and strengthening. Economical composite additions of Cr and Cu are used to obtain excellent corrosion resistance, reduce the corrosion rate, and increase the corrosion potential. Low-temperature steelmaking and two-stage controlled rolling and cooling are employed to ensure that the final product obtains a fine and uniform microstructure. The small-thickness intermediate billet rolling technology is employed to overcome the limitation that the thickness of the intermediate billet for pipeline steel must be at least 3.6 times the thickness of the finished product, thereby reducing the reduction in the finishing rolling stage, thereby reducing the dislocation density and further improving the corrosion resistance.

[0012] In order to achieve the above object, the present invention adopts the following technical solutions:

[0013] The invention discloses an L360 grade pipeline steel hot-rolled coil for ground gathering and transportation. The chemical composition of the steel is, by weight percentage, C: 0.050%-0.060%, Si: 0.15%-0.25%, Mn: 0.20%-0.28%, P≤0.012%, S≤0.003%, Als: 0.030%-0.045%, Nb: 0.018%-0.023%, Cu: 0.08%-0.12%, Cr: 0.05%-0.10%, N≤0.004%, and the remainder is iron and unavoidable impurities.

[0014] A method for producing L360 grade pipeline steel hot-rolled coils for ground gathering and transportation includes the following process steps: molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, slab heating, rolling, laminar cooling, and coiling; wherein the following processes are controlled:

[0015] 1) Converter smelting; using top blowing or top and bottom combined blowing;

[0016] 2) Refining outside the furnace: light desulfurization and calcium treatment in LF furnace;

[0017] 3) Continuous casting; using dynamic soft reduction method;

[0018] 4) Slab heating: Continuous casting slab is heated to 1143-1155°C;

[0019] 5) Rolling; adopt two-stage controlled rolling and controlled cooling; the rough rolling and finishing temperature is 998-1050℃, the rough rolling is performed for at least 5 passes, the first pass reduction rate is 19%-20.3%, the subsequent pass reduction rate gradually increases, and the final pass reduction rate is 21%-22.9%; after rolling, the temperature is allowed to drop below 980℃ before entering the finishing rolling, the finishing rolling temperature is 795-808℃, the finishing rolling is performed for at least 7 passes, the first pass reduction rate is 18%-19%, the subsequent pass reduction rate gradually decreases, and the final pass reduction rate is 8.1%-9.8%;

[0020] 6) Laminar cooling and coiling: Cool to 565-580℃ at a rate of 22.9-25.9℃ / s for coiling, and finally air cool to room temperature.

[0021] Furthermore, the yield strength of the hot-rolled coil product is 378-392 MPa, the tensile strength is 514-545 MPa, and the elongation after fracture is ≥37.5%; the Charpy impact energy Akv at -40°C is ≥324 J, the corrosion rate is 0.0245-0.0281 mm / a, the corrosion potential is ≥-0.462 V, and the corrosion current density is ≤1.10×10 -5 A.cm -2 .

[0022] Furthermore, in step 5), the ratio of the thickness of the rolling intermediate billet to the thickness of the finished steel plate is ≤3.0.

[0023] Furthermore, the thickness of the continuous casting slab is 172-175 mm, the thickness of the rolling intermediate slab is 54-56 mm, and the thickness of the finished steel plate is 18-20 mm.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1) Breaking the inherent idea of ​​high-carbon design for existing surface gathering and transportation steel, we adopt a low-carbon, low-manganese, low-sulfur and low-phosphorus design to ensure the product has excellent corrosion resistance and weldability;

[0026] 2) Add a small amount of Nb to play its role in grain refinement;

[0027] 3) No precious metals Ni and Mo are added, but Cu and Cr are added in combination to ensure the corrosion resistance and strength of the product, while giving the product a greater advantage in terms of economy;

[0028] 4) Low-temperature steelmaking and two-stage controlled rolling and cooling are used to ensure that the final product has a fine and uniform microstructure;

[0029] 5) Accurately allocating the reduction rate for each pass, combined with the small-thickness intermediate billet rolling technology, breaks the limitation that the thickness of the intermediate billet for pipeline steel should be more than 3.6 times the thickness of the finished product, reduces the reduction in the finishing rolling stage, thereby reducing the dislocation density and further improving the corrosion resistance of the steel. DETAILED DESCRIPTION

[0030] The present invention discloses an L360 grade pipeline steel hot-rolled coil for ground gathering and transportation, wherein the chemical composition of the steel is, by weight percentage, C: 0.050% to 0.060%, Si: 0.15% to 0.25%, Mn: 0.20% to 0.28%, P≤0.012%, S≤0.003%, Als: 0.030% to 0.045%, Nb: 0.018% to 0.023%, Cu: 0.08% to 0.12%, Cr: 0.05% to 0.10%, N≤0.004%, and the remainder is iron and unavoidable impurities.

[0031] The present invention provides a method for producing a hot-rolled coil of L360 grade pipeline steel for ground gathering and transportation, comprising the following process steps: molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, slab heating, rolling, laminar cooling, and coiling; wherein the following processes are controlled:

[0032] 1) Converter smelting; using top blowing or top and bottom combined blowing;

[0033] 2) Refining outside the furnace: light desulfurization and calcium treatment in LF furnace;

[0034] 3) Continuous casting; using dynamic soft reduction method;

[0035] 4) Slab heating: Continuous casting slab is heated to 1143-1155°C;

[0036] 5) Rolling; adopt two-stage controlled rolling and controlled cooling; the rough rolling and finishing temperature is 998-1050℃, the rough rolling is performed for at least 5 passes, the first pass reduction rate is 19%-20.3%, the subsequent pass reduction rate gradually increases, and the final pass reduction rate is 21%-22.9%; after rolling, the temperature is allowed to drop below 980℃ before entering the finishing rolling, the finishing rolling temperature is 795-808℃, the finishing rolling is performed for at least 7 passes, the first pass reduction rate is 18%-19%, the subsequent pass reduction rate gradually decreases, and the final pass reduction rate is 8.1%-9.8%;

[0037] 6) Laminar cooling and coiling: Cool to 565-580℃ at a rate of 22.9-25.9℃ / s for coiling, and finally air cool to room temperature.

[0038] Furthermore, the yield strength of the hot-rolled coil product is 378-392 MPa, the tensile strength is 514-545 MPa, and the elongation after fracture is ≥37.5%; the Charpy impact energy Akv at -40°C is ≥324 J, the corrosion rate is 0.0245-0.0281 mm / a, the corrosion potential is ≥-0.462 V, and the corrosion current density is ≤1.10×10 -5 A.cm -2 .

[0039] Furthermore, in step 5), the ratio of the thickness of the rolling intermediate billet to the thickness of the finished steel plate is ≤3.0.

[0040] Furthermore, the thickness of the continuous casting slab is 172-175 mm, the thickness of the rolling intermediate slab is 54-56 mm, and the thickness of the finished steel plate is 18-20 mm.

[0041] The L360 grade pipeline steel for surface gathering and transportation described in the present invention adopts a C-Mn-Nb-Cr-Cu alloy design. Low-temperature sintering technology is used to obtain a uniform and fine ferrite-pearlite (FP) structure to ensure that the product has excellent strength and toughness. Cu and Cr are added in combination to ensure that the product has excellent corrosion resistance. The functions and reasons for the selection of the main elements in the steel are as follows:

[0042] Carbon is the most economical element in steel for increasing its strength. However, as the carbon content increases, the steel's toughness and weldability gradually deteriorate. Excessive carbon content also negatively impacts corrosion resistance. Therefore, a low carbon content is essential for ensuring excellent strength, toughness, weldability, and corrosion resistance in surface gathering and transportation pipeline steel. In this invention, the carbon content is controlled within a range of 0.050% to 0.060%.

[0043] Si: It is an important reducing agent and deoxidizer in the steelmaking process. Many carbon steels contain less than 0.5% Si. Si can significantly improve the strength of the ferrite-pearlite structure type, but a high Si content will reduce the plasticity and toughness of the material. Therefore, the present invention controls the Si content to a lower level, that is, the Si content is controlled at 0.15% to 0.25%.

[0044] Mn: Manganese has a solid solution strengthening effect and can also reduce the γ-α phase transformation temperature, thereby refining the ferrite grains. However, manganese is prone to segregation. Excessive manganese content will cause severe segregation, resulting in a loss of toughness and corrosion resistance. Therefore, the present invention controls the manganese content to 0.20% to 0.28%.

[0045] P: It is very easy to highly segregate during solidification of molten steel, forming a banded FP structure. P also significantly reduces the benefits of reducing carbon in the steel, reducing the steel's toughness and corrosion resistance. As a harmful element, P in steel should be minimized. In the present invention, the P content is controlled to below 0.012%.

[0046] S: is an inevitable impurity element in steel that reduces the toughness and corrosion resistance of the steel. Generally, the lower the better, but too low a requirement will increase production costs. Therefore, the present invention controls S to ≤ 0.003%.

[0047] AlS is an inevitable deoxidizing element in steel. Adding an appropriate amount of Al can form fine and dispersed AlN particles, which is beneficial for grain refinement and improving the strength and toughness of steel. Therefore, the AlS content of the present invention is controlled to be 0.030% to 0.045%.

[0048] Nb: A key element for controlled rolling in modern microalloyed pipeline steels, it can lower the γ-α transformation temperature, increase the austenite recrystallization temperature, and promote austenite and ferrite refinement, resulting in grain refinement and strengthening, while also improving the steel's strength and toughness. Furthermore, the NbC formed after hot rolling can also contribute to precipitation strengthening. However, Nb is a precious element, and its strengthening effect is diminished beyond a certain level. Therefore, the present invention limits the Nb content to 0.018% to 0.023%.

[0049] Cu: When an appropriate amount of copper is added to steel, it will precipitate secondary precipitation on its surface to form a cathode, promoting anodic passivation of the steel in contact with it and forming a rust layer on the surface, thereby increasing its corrosion resistance. Copper can also increase the overall potential of the steel, thereby improving its corrosion resistance. However, when Cu is greater than 0.2%, the tendency of copper to become brittle increases. To ensure that the steel has good corrosion resistance and mechanical properties, the present invention controls the Cu content to 0.08% to 0.12%.

[0050] Cr: It effectively improves hardenability and increases product strength, especially tensile strength. It also effectively enhances steel's oxidation resistance, forming a dense passivation film and thus increasing the steel's corrosion resistance. However, excessive addition can lead to a loss of plasticity and toughness. Therefore, the present invention limits the Cr content to 0.05% to 0.10%.

[0051] N: Increasing the nitrogen content significantly increases the strength of steel, but also significantly reduces plasticity, especially toughness, weldability, and cold brittleness. Therefore, the lower the nitrogen content, the better. However, too low a nitrogen content increases steelmaking costs. Therefore, in the present invention, N is controlled to ≤ 0.004%.

[0052] The present invention describes a method for producing L360-grade hot-rolled coils of pipeline steel for ground gathering and transportation. The production process includes: molten iron pretreatment - converter smelting - refining (LF+Ca treatment) - continuous casting - slab heating - rolling - laminar cooling - coiling. Unlike existing technologies, the following processes are controlled:

[0053] 1) Smelting to continuous casting process: After molten iron pretreatment, converter smelting adopts top blowing or top and bottom combined blowing; LF furnace refining is adopted for refining outside the furnace, and light desulfurization treatment and calcium treatment are used to control the number and morphology of inclusions to improve the plasticity and toughness of the steel; dynamic light reduction method is adopted for continuous casting to reduce central porosity and improve the quality of continuous casting billets.

[0054] 2) Heating, rolling and laminar cooling process: The continuous casting slab is heated to 1143-1155°C in a walking beam furnace. During the slab heating process, the original austenite structure grows as the heating temperature increases. Since the steel of the present invention contains a small amount of Nb, and the Nb-containing steel can increase the coarsening temperature of the original austenite grains; in addition, the thermodynamic conditions for grain growth are insufficient when the slab heating temperature is low, so low-temperature steel sintering will refine the original austenite grain structure; then a two-stage controlled rolling and controlled cooling process is adopted, with the rough rolling and finishing rolling temperature of 998-1050°C, and at least 5 rough rolling passes. The first pass has a reduction rate of 19%-20.3%, and the reduction rate of each subsequent pass gradually increases. The reduction rate of the last pass is 21% to 22.9%; after rolling, the temperature is lowered to below 980°C before finishing rolling, and the final rolling temperature of finishing rolling is 795 to 808°C. Finishing rolling is performed for at least 7 passes, and the reduction rate of the first pass is 18% to 19%. The reduction rate of each subsequent pass gradually decreases, and the reduction rate of the last pass is 8.1% to 9.8%; in the finishing rolling stage, there are fewer dislocations caused by deformation (defects such as dislocations are microscopic substructures that are weak to corrosion) and the dislocation density is reduced, which can improve the corrosion resistance of the finished product; then, laminar cooling is used to cool the steel to 565 to 580°C at a speed of 22.9 to 25.9°C / s for coiling, and finally air-cooling to room temperature.

[0055] The L360 grade pipeline steel hot-rolled coil for ground gathering and transportation disclosed in the present invention has excellent strength, toughness and corrosion resistance: yield strength of 378-392 MPa, tensile strength of 514-545 MPa, elongation after fracture ≥37.5%; -40°C Charpy impact energy (average of 3 samples) Akv ≥324 J, corrosion rate of 0.0245-0.0281 mm / a, corrosion potential ≥-0.462 V, corrosion current ≤1.10×10 -5 A.cm -2 .

[0056] The present invention combines conventional corrosion tests and electrochemical corrosion tests to evaluate the corrosion performance of the product; the corrosion test conditions are as follows:

[0057] 1) Place the sample in a saturated sodium chloride solution for a full immersion test for 6 months. After the test, calculate the corrosion rate in mm / a.

[0058] 2)2) Electrochemical tests were carried out in a 3.5% NaCl solution as the corrosive medium using a three-electrode system. The working electrode was the test sample, the auxiliary electrode was a Pt wire mesh, and the saturated calomel electrode was the reference electrode. The corrosion potential and corrosion current were calculated using the Tafel curve.

[0059] The following examples are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the protection scope of the present invention is not limited to the following examples.

[0060] [Example]

[0061] The chemical composition of the steel in each example is shown in Table 1, the rolling process parameters of the steel in each example are shown in Table 2, the main pass reduction ratio of each example is shown in Table 3, and the main performance parameters of the products in each example are shown in Table 4.

[0062] Table 1 Chemical composition of steel (wt, %)

[0063]

[0064] Table 2 Main process parameters of steel

[0065]

[0066] Table 3 Main pass reduction rates of various examples

[0067]

[0068] Table 4 Main performance parameters of the product

[0069]

[0070] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for producing L360 grade pipeline steel hot-rolled coil for ground gathering and transportation, characterized in that: The chemical composition of the steel is as follows by weight: C: 0.050% to 0.060%, Si: 0.21% to 0.25%, Mn: 0.20% to 0.28%, P≤0.012%, S≤0.003%, Als: 0.039% to 0.045%, Nb: 0.018% to 0.023%, Cu: 0.08% to 0.12%, Cr: 0.05% to 0.10%, N≤0.004%, and the rest is iron and unavoidable impurities; The production method of L360 grade pipeline steel hot-rolled coil includes the following process: molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, slab heating, rolling, laminar cooling and coiling; wherein the following processes are controlled: 1) Converter smelting; using top blowing or top and bottom combined blowing; 2) Refining outside the furnace: light desulfurization and calcium treatment in LF furnace; 3) Continuous casting; using dynamic soft reduction method; 4) Slab heating: Continuous casting slab is heated to 1143-1155°C; 5) Rolling; adopt two-stage controlled rolling and controlled cooling; the rough rolling and finishing temperature is 998-1050℃, the rough rolling is carried out for at least 5 passes, the first pass reduction rate is 19%-20.3%, the reduction rate of each subsequent pass gradually increases, and the final pass reduction rate is 21%-22.9%; the ratio of the thickness of the rolling intermediate billet to the thickness of the finished steel plate is ≤3.0; after rolling, the temperature is lowered to below 980℃ before finishing rolling, the finishing rolling temperature is 795-808℃, and the finishing rolling is carried out for at least 7 passes, the first pass reduction rate is 18%-19%, and the subsequent passes reduction rate gradually decreases, and the final pass reduction rate is 8.1%-9.8%; 6) Laminar cooling and coiling: Cool to 565-580℃ at a rate of 22.9-25.9℃ / s, then coil, and finally air cool to room temperature; The thickness of the continuous casting slab is 172-175 mm, the thickness of the rolling intermediate slab is 54-56 mm, and the thickness of the finished steel plate is 18-20 mm. The yield strength of the hot-rolled coil product is 378-392MPa, the tensile strength is 514-545MPa, and the elongation after fracture is ≥37.5%; the Charpy impact energy Akv at -40℃ is ≥324J, the corrosion rate is 0.0245-0.0281mm / a, the corrosion potential is ≥-0.462V, and the corrosion current density is ≤1.10×10 -5 A.cm -2 .

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