A metal coating resistant to sulfuric acid and acid dew point corrosion and a preparation process thereof

Abstract

The application discloses a metal coating resistant to sulfuric acid and acid dew point corrosion and a preparation process thereof. The metal powder used in the metal coating comprises 56-65% of Ni, 30.0-34.5% of Cr, 6.0-7.5% of Si and 5.5-6.6% of B. Compared with the prior art, the coating material can meet the self-melting requirement of induction heating, the metal coating is prepared by using a new coating process method of coating preparation + electromagnetic induction cladding, the coating can meet the corrosion requirement of sulfuric acid and acid dew point environment, the bonding strength between the coating and the substrate is greater than 300 MPa, the porosity of the coating is eliminated, and the new corrosion-resistant metal coating can be used in strong acid corrosion working conditions and high-temperature equipment.

Description

Technical Field

[0001] This invention relates to a metal anti-corrosion coating for high-temperature equipment and pipelines under sulfuric acid or acid dew point corrosion conditions and its preparation process. Background Technology

[0002] In boilers and industrial heating furnaces fueled by sulfur-containing heavy oil or sulfur-containing gas, the sulfur powder is oxidized during combustion. This oxidation occurs when the sulfur condenses with water in low-temperature areas such as flue gas ducts and air preheaters, resulting in corrosion known as flue gas acid dew point corrosion or sulfuric acid dew point corrosion. Typically, heavy oil used as fuel contains 2%–3% sulfides. Combustion produces SO2, of which approximately 1%–2% is catalyzed by metal oxides in the soot to form SO3. This SO3 then combines with moisture (approximately 5%–10%) in the combustion gases to form sulfurous acid or sulfuric acid, which then combine to form wet sulfates. These wet sulfates can condense at temperatures much higher than the normal dew point, leading to severe corrosion. Studies have shown that even combustion products containing as little as 0.025% (volume percentage) of SO3 have a dew point of 171°C. Corrosion rates are generally highest within the temperature range of 20–45°C below the acid dew point. Simultaneously, sulfuric acid condensing on equipment surfaces reacts with dust in the flue gas to form stubborn scale, affecting heat transfer and lowering the surface temperature of the pipe walls, further accelerating condensation and promoting corrosion. This "dew point" corrosion of sulfurous or sulfuric acid often occurs during shutdowns because the flue gas contains a certain amount of water vapor. When the temperature drops to the dew point during shutdown, water accumulates in areas prone to accumulation, causing severe corrosion to equipment and connected pipelines. Current solutions include: firstly, increasing the exhaust gas temperature during design and operation, but this results in significant energy losses and is uneconomical; secondly, using non-metallic linings or coatings for equipment surface treatment and material selection, which are not only expensive but also have uncertain effectiveness. Therefore, there is an urgent need to find a new type of corrosion-resistant coating material that can withstand highly corrosive conditions and be widely used. Summary of the Invention

[0003] To overcome the aforementioned shortcomings of existing technologies, this invention proposes a metal anti-corrosion coating resistant to sulfuric acid or acid dew point corrosion and its preparation process. This coating is suitable for metal coating materials used in H2SO4+H2O or SO2+H2O media conditions at temperatures not exceeding 200℃. Furthermore, a novel coating preparation process combining pre-treatment and post-processing is employed. This invention effectively overcomes the drawbacks of poor corrosion resistance in existing non-metallic linings or coatings used in equipment.

[0004] The technical solution adopted by the present invention to solve its technical problem is: a metal coating resistant to sulfuric acid and acid dew point corrosion, wherein the metal powder used includes: 56% to 65% Ni, 30.0% to 34.5% Cr, 6.0% to 7.5% Si, and 5.5% to 6.6% B.

[0005] This invention also provides a process for preparing a metal coating resistant to sulfuric acid and acid dew point corrosion, comprising the following steps:

[0006] Step 1: Prepare alloy powder PB-Ni60CrSi:

[0007] Metal mixtures are prepared by screening alloys or pure materials according to the proportions, and then fully mixed by vacuum melting. The mixture is then produced by gas atomization to obtain alloy powder PB-Ni60CrSi with a spherical or near-spherical shape and a particle size of 100-300 mesh or finer.

[0008] Step 2: Pre-treat the parts of the equipment's inner wall that require protection;

[0009] Step 3: Pre-coat the internal surfaces of the equipment that come into contact with the corrosive medium:

[0010] (1) Thermal spraying prefabrication: After drying the alloy powder PB-Ni60CrSi in an oven at 105-115℃ for 6-10 hours, it is loaded into the powder feeding system. Then, the air compressor, water cooling system and flame spraying equipment are started. On the pre-treated and qualified substrate, the metal underlayer is first sprayed with oxyacetylene flame; then the metal intermediate layer or top layer is sprayed with supersonic flame until the prefabrication thickness reaches the specified thickness.

[0011] (2) Pre-coating: On the pre-treated substrate, first apply the metal paste evenly from top to bottom and from left to right using a spray gun or brush to ensure uniform coating thickness. The thickness of a single coating should not exceed 0.15 mm. After the coating surface is completely dry, apply the intermediate layer and top layer in the same way until the pre-coated thickness reaches the specified thickness.

[0012] Step 4: Apply electromagnetic induction heating to the pre-formed coating using a continuous scanning heating method;

[0013] Step 5: Surface quality inspection.

[0014] Compared with the prior art, the positive effects of the present invention are:

[0015] The coating material of this invention can meet the requirements of induction heating for material self-fluxing. The metal coating is prepared by a novel spraying process of coating prefabrication + electromagnetic induction cladding. It not only meets the corrosion requirements of sulfuric acid and acid dew point environments, but also greatly improves the bonding strength between the coating and the substrate (not less than 300MPa) and eliminates the porosity of the coating (generally 0). It is an economical new corrosion-resistant material that can withstand strong corrosion conditions and can be widely used in high-temperature equipment.

[0016] The high-temperature equipment anti-corrosion coating provided by this invention can replace the expensive high-alloy steel pure materials or composite materials such as 316L, 904, 2205, 254SMo, and C-276 commonly used in H2SO4 and SO2 dew point corrosion environments. The main alloying elements used in the metal coating of this invention include: 56% to 65% Ni, 30.0% to 34.5% Cr, 6.0% to 7.5% Si, and 5.5% to 6.6% B. The Ni-Cr-Si material can self-fuse to form a dense coating. The brazing process is used for local reinforcement or overall protection to achieve long-term protection. Detailed Implementation

[0017] A high-temperature equipment anti-corrosion coating resistant to sulfuric acid and acid dew point corrosion is disclosed. The metal powder used comprises: 56%–65% Ni, 30.0%–34.5% Cr, 6.0%–7.5% Si, and 5.5%–6.6% B. The metal powder is produced by gas atomization and has a spherical or near-spherical shape with a particle size of 100–300 mesh or finer, making it suitable for metal coating.

[0018] The preparation process of the metal coating described in this invention includes the following steps:

[0019] Step 1: Prepare alloy powder PB-Ni60CrSi and metal paste

[0020] (1) Select alloys or pure materials and prepare metal mixtures according to the following proportions: 56%–65% Ni, 30.0%–34.5% Cr, 6.0%–7.5% Si, and 5.5%–6.6% B. All metal powders are sintered together and then produced by gas atomization to obtain PB-Ni60CrSi alloy powder with spherical or near-spherical shape and a particle size of 100–300 mesh, suitable for flame spraying.

[0021] (2) Metal coating preparation: The adhesive is a composite adhesive such as PVA+HPMC, with a ratio of 5%-12%, and the alloy powder PB-Ni60CrSi accounts for 88%-95%. It is mixed at high speed for 20-40 minutes.

[0022] Step 2: Pre-treat and remove rust from the inner walls of the equipment that require protection.

[0023] (1) Pre-treatment of the inner surface of the equipment: smooth the unevenness and non-circular transition of the substrate surface; clean the burrs, welding slag, dust and loose material on the substrate surface; clean any original surface treatment layer (such as nitriding layer, electroplating layer, etc.) on the substrate surface.

[0024] (2) Load the abrasive into the sandblasting equipment, place the sandblasting gun into the pressure vessel through the operating port, start the air compressor, and manually or automatically operate the sandblasting gun to remove rust from the inner wall of the pressure vessel where protection is required, ensuring that the surface rust removal grade is not lower than Sa3.0. Abrasive selection: Based on the relationship between coating thickness and abrasive grain diameter, select 14-mesh or 16-mesh brown corundum according to the designed coating thickness to ensure good adhesion between the coating and the pressure-bearing equipment.

[0025] Step 3: Pre-form a corrosion-resistant metal coating on the surfaces inside the pressure equipment that come into contact with the corrosive medium.

[0026] (1) Thermal spraying pre-treatment: After drying the metal powder in an oven at 105-115℃ for 6-10 hours, it is loaded into the powder feeding system. The air compressor, water cooling system, and flame spraying equipment are started. First, the metal base layer is sprayed with an oxy-acetylene flame. The flow rate of the negative pressure gravity powder feeder is adjusted to 78-80 g / min, ensuring a spraying distance of 80-100 mm. The oxygen pressure is controlled at 0.8-0.9 MPa and the oxygen flow rate at 16-20 L / min. The acetylene pressure is controlled at 0.1-0.12 MPa and the acetylene flow rate at 11-13 L / min to form an oxidizing flame. The 100-300 mesh PB-Ni60CrSi metal powder is melted and sprayed into a metal base layer with the required performance parameters by manual or automatic operation of the spray gun. Then, the metal intermediate layer or top layer is sprayed with supersonic spraying. The nitrogen gas is adjusted. The powder feeder pressure is 0.7–0.80 MPa, and the nitrogen flow rate is 0.3–0.4 L / min, ensuring a spraying distance of 100–120 mm. The oxygen pressure is controlled at 0.8–0.9 MPa, and the oxygen flow rate at 30–32 L / min. The propane pressure is controlled at 0.6–0.7 MPa, and the propane flow rate at 16–20 L / min, forming an oxidizing flame. The flame blowing air pressure is controlled at 0.6–0.7 MPa, and the air flow rate at 70–80 L / min. The 100–300 mesh alloy powder is melted and sprayed into a metal intermediate or top layer with the required performance parameters by manual or robotic operation of the spray gun.

[0027] (2) Pre-coating: On the pre-treated substrate, use metal powder with a mesh size of not less than 300 mesh or finer; first, apply the metal paste mixed in proportion using a spray gun or brush, in the order from top to bottom and from left to right, to ensure uniform coating thickness, with a single coating thickness not exceeding 0.15 mm; after the coating surface is completely dry (moisture content less than 10%), apply the intermediate layer and top layer in the same way until the pre-coated thickness reaches the specified thickness.

[0028] During the coating prefabrication process, a thickness gauge is used to check the thickness of the metal coating to ensure that the thickness of the corrosion-resistant coating meets the design requirements (generally 0.3mm to 0.8mm); if the thickness is greater than or equal to the specified thickness of 0.1mm or more, the excess coating can be removed by grinding machinery.

[0029] Step 4: Post-treatment to improve coating adhesion strength and eliminate porosity.

[0030] Place the induction heater trajectory control system inside the container, ensuring that the induction heater can move in a circular motion along the inner wall of the pressure vessel (50–120 mm / min), move axially (0–4.0 m), and adjust radially (1–3 mm from the coating surface). Activate the tooling trajectory control system and the high-frequency induction heating control system (set frequency 80–300 kHz, power 50–200 kW) to ensure the coating consistently reaches the induction remelting temperature (910–1020 °C) and the heat-affected zone thickness is no greater than 2 mm, guaranteeing no incomplete melting or over-melting defects in the metal coating. After each cladding pass, adjust the tooling so that the induction gun moves approximately 3 / 4 of the induction coil width, ensuring seamless overlap between coating passes. Repeat the above steps manually or robotically until all areas requiring coating are completed.

[0031] Step 5: Surface quality inspection

[0032] The coating surface should be subjected to ultrasonic and dye penetrant testing to ensure that it is free of defects such as cracks, pores, and inclusions. Otherwise, the defective area should be manually ground down to the substrate for local repair until the coating passes the inspection.

[0033] Working principle of the invention

[0034] With over 56% nickel and over 30% chromium, the alloy exhibits excellent acid corrosion resistance. The addition of boron forms a nickel-boron low-temperature eutectic self-fluxing alloy. The addition of silicon prevents alloy oxidation and forms and strengthens the Cr2O3-SiO2 protective film. Simultaneously, it ensures good wettability between the metal coating and the base metal material, as well as self-fluxing properties between the alloys. This results in a metal coating with matching hardness and a smooth surface, while eliminating porosity within the coating. The absence of carbides in the alloy ensures the coating's plasticity. Combined with flame spraying followed by induction heating post-treatment or metal paste coating followed by induction heating post-treatment, this process guarantees both the corrosion resistance of the PVB-Ni60CrSi high-temperature equipment metal coating and good bonding strength between the coating and the substrate, as well as between the coating components themselves, during use.

Claims

1. A metal coating resistant to sulfuric acid and acid dew point corrosion, characterized in that: The metal powder used includes: 56%–65% Ni, 30.0%–34.5% Cr, 6.0%–7.5% Si, and 5.5%–6.6% B; The process for preparing a metal coating resistant to sulfuric acid and acid dew point corrosion using the aforementioned metal powder includes: Step 1: Prepare alloy powder PB-Ni60CrSi: Weigh the metal powder according to the ratio, achieve full mixing through vacuum melting, and produce PB-Ni60CrSi alloy powder with spherical or near-spherical shape and particle size of 100-300 mesh or finer by gas atomization method; Step 2: Pre-treat the parts of the equipment's inner wall that require protection; Step 3: Pre-coat the internal surfaces of the equipment that come into contact with the corrosive medium: Step 4: Electromagnetic induction heating is used to continuously scan and heat the pre-coated layer for cladding. First, it is ensured that the induction heater can move in a circular motion of 50-120 mm / min along the inner wall of the pressure vessel, move 0-4.0 m axially, and be adjusted radially within 1-3 mm from the coating surface; so that the coating always reaches the induction remelting temperature of 910-1020°C. Step 5: Surface quality inspection.

2. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 1, characterized in that: Step three includes the following steps: (1) Thermal spraying prefabrication: After drying the alloy powder PB-Ni60CrSi in an oven at 105-115°C for 6-10 h, it is loaded into the powder feeding system. Then, the air compressor, water cooling system and flame spraying equipment are started. On the pre-treated and qualified substrate, the metal underlayer is first sprayed with oxyacetylene flame; then the metal intermediate layer or top layer is sprayed with supersonic flame until the prefabrication thickness reaches the specified thickness. (2) Pre-coating: On the pre-treated base surface, first apply the metal paste evenly from top to bottom and from left to right using a spray gun or brush to ensure uniform coating thickness. The thickness of a single coating should not exceed 0.15 mm. After the coating surface is completely dry, apply the intermediate layer and top layer in the same way until the pre-coated thickness reaches the specified thickness.

3. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 1, characterized in that: The method for pre-treating the parts of the equipment's inner wall that require protection, as described in step two, is as follows: (1) Grind the uneven and non-circular transition of the substrate surface to make it smooth; clean the burrs, welding slag, dust and loose material on the substrate surface; clean the original surface treatment layer on the substrate surface. (2) Load the abrasive into the sandblasting equipment, place the sandblasting gun into the pressure vessel through the operating hole, start the air compressor, and operate the sandblasting gun to remove rust from the parts that need to be protected, ensuring that the surface rust removal grade is not lower than Sa3.

0.

4. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 3, characterized in that: The abrasive is 14-mesh or 16-mesh brown corundum.

5. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 1, characterized in that: In step three, during the prefabrication of the metal coating, a thickness gauge is used to check the thickness of the metal coating to ensure that the thickness of the metal coating meets the design requirements. If the thickness is greater than or equal to the specified thickness by more than 0.1 mm, the excess coating can be removed by grinding machinery.

6. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 2, characterized in that: In step three, the method of using oxyacetylene flame spraying to coat the metal underlayer is as follows: adjust the flow rate of the negative pressure gravity powder feeder to 78-80 g / min, ensure the spraying distance to 80-100 mm, control the oxygen pressure to 0.8-0.9 MPa and the oxygen flow rate to 16-20 L / min, control the acetylene pressure to 0.1-0.12 MPa and the acetylene flow rate to 11-13 L / min, and after forming an oxidizing flame, operate the spray gun to melt and spray 100-300 mesh alloy powder PB-Ni60CrSi into a metal underlayer with performance parameters that meet the requirements.

7. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 2, characterized in that: In step three, the method of supersonic spraying of the metal intermediate layer or top layer is as follows: adjust the nitrogen powder feeder pressure to 0.7-0.80 MPa and the nitrogen flow rate to 0.3-0.4 L / min, ensuring a spraying distance of 100-120 mm; control the oxygen pressure to 0.8-0.9 MPa and the oxygen flow rate to 30-32 L / min; control the propane pressure to 0.6-0.7 MPa and the propane flow rate to 16-20 L / min to form an oxidizing flame; control the flame blowing air pressure to 0.6-0.7 MPa and the air flow rate to 70-80 L / min; and use a spray gun operated manually or by a robot to melt and spray 100-300 mesh alloy powder PB-Ni60CrSi into a metal intermediate layer or top layer with performance parameters meeting the requirements.

8. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 2, characterized in that: In step three, the method for preparing the metal paste is as follows: use composite adhesive PVA+HPMC as a binder at a ratio of 5%-12%, and mix it with alloy powder PB-Ni60CrSi at a ratio of 88%-95% for 20-40 minutes by high-speed stirring.

9. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 1, characterized in that: Step four of the method for cladding the pre-fabricated coating using electromagnetic induction heating with continuous scanning heating further includes: Start the tooling trajectory control system and the high-frequency induction heating control system, with a frequency of 80-300 KHz, a power of 50-200 Kw, and a heat-affected layer thickness of less than or equal to 2 mm, to ensure that there are no defects in the metal coating that are not fully melted or over-melted. After each cladding pass is completed, adjust the tooling so that the induction gun moves 3 / 4 of the width of the induction coil to ensure that there is no gap between each coating pass. Then repeat the above steps until all areas that need to be sprayed are completed.

10. The metal coating resistant to sulfuric acid and acid dew point corrosion according to claim 1, characterized in that: The surface quality inspection method described in step five is as follows: ultrasonic and colorimetric tests are performed on the coating surface. If the coating has cracks, pores, or inclusions, the defective area is manually ground down to the substrate and local repair is carried out until the inspection is qualified.