Molten Al—Si alloy corrosion resistant composite coating and preparation method and application thereof

Abstract

The invention provides a molten Al—Si alloy corrosion resistant composite coating and a preparation method and application thereof. The composite coating layer comprises an aluminized layer and a TiO.sub.2 film layer from a surface of a substrate to the outside in sequence. The preparation method of the coating layer comprises the following steps: (step S1) making a surface treatment to an Fe-based alloy, and then aluminizing with a solid powder penetrant; (step S2) sand-blasting the aluminized Fe-based alloy; (step S3) washing and drying the Fe-based alloy which has been sand-blasted; and (step S4) depositing the TiO.sub.2 film layer on a surface of the dried aluminized Fe-based alloy by using an atom layer vapor deposition. The application of the molten Al—Si alloy corrosion resistant composite coating is used for a solar thermal power generation heat exchange tube.

Claims

1. A preparation method for the molten Al—Si alloy corrosion resistant composite coating, wherein the composite coating comprises an aluminized layer and a TiO.sub.2 film layer from a surface of a substrate to the outside in sequence, the composite coating further comprises an Al.sub.2O.sub.3 film layer prepared by an atom layer vapor deposition, and the A12O3 film layer is located between the TiO.sub.2 film layer and the aluminized layer, wherein comprising the following steps: step S1: making a surface treatment to an Fe-based alloy, and then aluminizing with a solid powder penetrant; step S2: sand-blasting the aluminized Fe-based alloy; step S3: washing and drying the Fe-based alloy which has been sand-blasted; and step S4: depositing the TiO.sub.2 film layer on a surface of the dried aluminized Fe-based alloy by using an atom layer vapor deposition.

2. The preparation method for the molten Al—Si alloy corrosion resistant composite coating according to claim 1, wherein further comprises depositing the Al.sub.2O.sub.3 film layer onto a surface of the aluminized Fe-based alloy in step S3 using an atom layer vapor deposition between the step S3 and the step S4.

3. The preparation method for the molten Al—Si alloy corrosion resistant composite coating according to claim 2, wherein the step of depositing the Al.sub.2O.sub.3 film layer comprises: taking trimethyl aluminum as a precursor and pressure as 0.05-0.2 ton, inflating for 0.01-0.03 s, and then exhausting air for 40-60 s, charging vapor for 0.01-0.03 s, and finally exhausting air for 20-60 s, and repeating trimethyl aluminum inflating-exhausting air-vapor charging-exhausting air to deposit the Al.sub.2O.sub.3 film layer; wherein the repeating times are 50-500 times.

4. The preparation method for the molten Al—Si alloy corrosion resistant composite coating according to claim 1, wherein in the step S1, the solid powder penetrant is a homogeneous mixture comprising the following ingredients: an aluminum powder with a granularity of 200 meshes, a filler consisting of a Al.sub.2O.sub.3 powder and a Cr powder and a powdery NH.sub.4Cl penetration aid, wherein the solid powder penetrant, by mass, comprises 42-74% of the aluminum powder, 20-40% of the Al.sub.2O.sub.3 powder, 5-15% of the Cr powder and 1-3% of the NH.sub.4Cl, the aluminizing conditions include: maintaining a temperature at 400-600° C. for 20-40 min, and then maintaining a temperature at 900° C-1050° C. for 10-15 h, and finally cooling to a room temperature along with a furnace.

5. The preparation method for the molten Al—Si alloy corrosion resistant composite coating according to claim 1, wherein in the step S3, the step of depositing the TiO.sub.2 film layer comprises: taking titanium isopropylate as a precursor and pressure as 0.1-0.3 torr, inflating for 0.1-0.5 s, exhausting air for 30-50s, charging plasma vapor for 0.01-0.03 s, and finally exhausting air for 30-50s and repeating titanium isopropylate inflating-exhausting air-vapor charging-exhausting air for recycling to deposit the TiO.sub.2 film layer, wherein the repeating times are 50-500 times.

6. The preparation method for the molten Al—Si alloy corrosion resistant composite coating according to claim 1, wherein in the step S2, the sand-blasting is conducted at a high pressure nitrogen of 0.6-0.9 MPa, the sand-blasting time is 5-20 min, the sand-blasting abrasive is Al.sub.2O.sub.3 particles with 300-500 meshes, and the sand-blasting distance is 2-6 cm, in the step S1, the surface treatment includes mechanical polishing for the Fe-based alloy before electrolytic polishing, the mechanical polishing comprises: polishing without visually obvious scratches by using an abrasive paper with a granularity of 80-1,200 meshes, ultrasonically washing for 5-20 min with acetone, and then ultrasonically washing with anhydrous ethanol for 5-20 min, and finally drying, the electrolytic polishing means electrolytically polishing an Fe base by taking the Fe-based alloy as an anode and an insoluble conductive material as a cathode, the electrolytic polishing electrolyte comprises concentrated sulfuric acid with a volume fraction of 60-80%, concentrated phosphoric acid with a volume fraction of 15-37% and distilled water with a volume fraction of 3-5%, the electrolytic DC voltage is 5-6 V, the temperature of the electrolyte is 60-80° C., and the electrolytic polishing time is 2-5 min.

7. A preparation method for the molten Al—Si alloy corrosion resistant composite coating, wherein the composite coating comprises an aluminized layer and a TiO.sub.2 film layer from a surface of a substrate to the outside in sequence, the composite coating further comprises an Al.sub.2O.sub.3 film layer prepared by an atom layer vapor deposition, and the Al.sub.2O.sub.3 film layer is located between the TiO.sub.2 film layer and the aluminized layer, wherein the aluminized layer comprises an Fe(Al) phase diffusion layer, an Fe-Al compound layer and an Al.sub.2O.sub.3 layer from the substrate to the outside in sequence, wherein comprising the following steps: step S1: making a surface treatment to an Fe-based alloy, and then aluminizing with a solid powder penetrant; step S2: sand-blasting the aluminized Fe-based alloy; step S3: washing and drying the Fe-based alloy which has been sand-blasted; and step S4: depositing the TiO.sub.2 film layer on a surface of the dried aluminized Fe-based alloy by using an atom layer vapor deposition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1a to 1c illustrate a section morphology of an aluminized steel after subjecting to atom layer vapor deposition compared with the aluminized steel before subjecting to atom layer vapor deposition in Embodiment 3 of the present invention and an EDS energy spectrum analysis diagram of a corresponding point.

(2) FIG. 2 illustrates corrosion rate comparison between a stainless steel containing a composite coating prepared in Embodiments 3 and 4 of the present invention, a stainless steel excluding a coating and a stainless steel containing an aluminized layer prepared in comparative example 1, all of which corrode for 72 h in the molten Al—Si.

DESCRIPTION OF THE EMBODIMENTS

(3) The present invention will be further clarified based on the following figures and embodiments.

(4) According to a molten Al—Si alloy corrosion resistant composite coating provided by the present invention, the composite coating comprises an aluminized layer and a TiO.sub.2 thin film layer from a substrate surface to the outside in sequence. Particularly, the substrate of the present invention is made of a Fe-based material, preferably austenitic stainless steel. The TiO.sub.2 thin film layer is preferably introduced by an atom layer vapor deposition. The TiO.sub.2 thin film layer introduced using this method has a compact surface, and even tissues without microcracks and so on. The TiO.sub.2 thin film layer can be controlled to be in nanometer scale, so as to improve the performance of the composite coating, preferably 5-50 nm. On the one hand, due to high production cost of the precursor for the atom layer deposition, the thickness of the TiO.sub.2 thin film can be controlled within a small scope to lower the production cost of the composite coating while ensuring the coating to exert an effective protection role; on the other hand, the anatase crystal structure is more regular, and gaps between unit cells can be ignored, both of which are very beneficial to preventing the diffusion of the Si element.

(5) The coating further includes Al.sub.2O.sub.3 thin film layer prepared by the atom layer vapor deposition, and the Al.sub.2O.sub.3 thin film layer is located between the TiO.sub.2 thin film layer and the aluminized layer; the Al.sub.2O.sub.3 thin film layer, which is a continuous compact coating prepared by the atom layer vapor deposition, is arranged between the aluminized layer and the TiO.sub.2 thin film layer.

(6) The Al.sub.2O.sub.3 thin film layer has a nanoscale thickness; preferably, the Al.sub.2O.sub.3 thin film layer provided by the present invention has a thickness of 5-50 nm.

(7) The aluminized layer includes an Fe(Al) phase diffusion layer, an Fe—Al compound layer and an Al.sub.2O.sub.3 layer from the substrate to the outside sequentially; the thicknesses of the Fe(Al) phase diffusion layer, the Fe—Al compound layer and the Al.sub.2O.sub.3 layer are in micron scale; the Fe(Al) phase diffusion layer, also called Al-containing Fe diffusion layer, is essentially a diffusion layer formed by diffusing Al to the substrate to replace Fe atoms partially on the surface of the substrate, and is a depleted Al area with a low Al content; an atomic percent of the Al element in the diffusion layer is raised to 8 at. % outermost the Fe(Al) phase diffusion layer from 0 at. % on the substrate surface. The Al.sub.2O.sub.3 layer is a non-continuous coating capable of exerting antioxidant isolation, and but also inducing heat fatigue crack initiation, but also lowering the molten Al corrosion resistance due to oxidation etching grooves on the surface.

(8) Preferably, the Fe—Al compound layer has a thickness of 60-200 μm; the Fe(Al) phase diffusion layer has a thickness of 50-160 μm; the Al.sub.2O.sub.3 layer has a thickness of 10-30 μm; the Fe—Al compound comprises FeAl, FeAl.sub.2 and Fe.sub.3Al.

(9) A method for preparing the molten Al—Si corrosion resistant coating provided by the present invention, comprising the following steps:

(10) step S1: making a surface treatment to an Fe-based alloy, and then aluminizing with a solid powder penetrant;

(11) step S2: sand-blasting the aluminized Fe-based alloy;

(12) step S3: washing and drying the Fe-based alloy which has been sand-blasted;

(13) step S4: depositing the TiO.sub.2 thin film layer on a surface of the dried aluminized Fe-based alloy by using an atom layer vapor deposition.

(14) In this solution, the Fe-based alloy is made of an alloy plate, preferably austenitic stainless steel.

(15) Between the step S3 and the step S4, there is also a step of depositing the Al.sub.2O.sub.3 thin film layer onto the aluminized Fe-based alloy surface obtained in the step S3 by the atom layer vapor deposition, wherein the Al.sub.2O.sub.3 thin film layer introduced by the method is compact and uniform, and is capable of strengthening the isolation of the substrate, remedying the defect that the Al.sub.2O.sub.3 layer has oxidation etching grooves on the surface of the aluminized coating obtained by an aluminizing method, etc. This defect not only easily induces the heat fatigue crack initiation, but also lowers the molten Al corrosion resistance. According to the present invention, the Al.sub.2O.sub.3 thin film layer subject to atom deposition is deposited by the atom deposition to supplement the non-continuous Al.sub.2O.sub.3 thin film layer on the penetrated layer, so that the continuous and compact Al.sub.2O.sub.3 film covers a specimen surface, which realizes the effect of blocking the Al atom diffusion, and provides a good surface environment to subsequently deposit the TiO.sub.2 thin film, so as to prevent other interfere elements from affecting the deposition effect; and meanwhile, this is good for lowering the interfacial stress and improving the bonding force and stability between the coatings.

(16) In the step S1, the solid powder penetrant is a homogeneous mixture comprising the following ingredients: an aluminum powder with a granularity of 200 meshes, a filler consisting of a Al.sub.2O.sub.3 powder and a Cr powder and a powdery NH.sub.4Cl penetration aid, wherein the solid powder penetrant, by mass, comprises 42-74% of the aluminum powder, 20-40% of the Al.sub.2O.sub.3 powder, 5-15% of the Cr powder and 1-3% of the NH.sub.4Cl. The tissue controlling precision can be effectively improved by the solid powder penetrant using this ingredient, so as to further improve the tissue compactness and integrity.

(17) The aluminizing conditions include: drying for 2 h at 150° C.; maintaining a temperature at 400-600° C. for 20-40 min, wherein a heating rate is 10° C./min; maintaining a temperature at 900° C.-1050° C. for 10-15 h; and finally cooling to the room temperature along with the furnace. The control for the thicknesses and microstructures of the Fe(Al) phase diffusion layer, the Fe—Al compound layer and the Al.sub.2O.sub.3 layer can be further improved to obtain the aluminized coating with more even tissues, small internal stress and more tight to bond with the composite coatings.

(18) In the step S4, the step of depositing the TiO.sub.2 thin film layer comprises: heating a cavity to 300-450° C.; taking titanium isopropylate (purity of 99.99%) as the precursor and pressure as 0.1-0.3 torr, inflating for 0.1-0.5 s, exhausting air for 30-50 s, charging plasma vapor for 0.01-0.03 s, and finally exhausting air for 30-50 s, depositing the TiO.sub.2 thin film layer, and repeating titanium isopropylate inflating-exhausting air-vapor charging-exhausting air for recycling to deposit the TiO.sub.2 thin film layer; the times of controlling the cycle are 50-500 times, and thus the TiO.sub.2 thin film layer of different thicknesses can be generated.

(19) The step of depositing the Al.sub.2O.sub.3 thin film layer comprises: placing an aluminized steel as a substrate into an equipment cavity, and heating the cavity to 150-300° C.; taking trimethyl aluminum (TMA) (purity of 99.99%) as a precursor and pressure as 0.05-0.2 torr, inflating for 0.01-0.03 s, and then exhausting air for 40-60 s, charging vapor for 0.01-0.03 s, and finally exhausting air for 20-60 s, and depositing the Al.sub.2O.sub.3 thin film layer; repeating trimethyl aluminum inflating-exhausting air-vapor charging-exhausting air to deposit the Al.sub.2O.sub.3 thin film layer; the times of controlling the cycle are 50-500 times, and thus the Al.sub.2O.sub.3 thin film layer of different thicknesses can be generated.

(20) In the step S2, the sand-blasting is conducted at a high pressure nitrogen of 0.6-0.9 MPa; the sand-blasting time is 5-20 min; the sand-blasting abrasive is Al.sub.2O.sub.3 particles with 300-500 meshes; the sand-blasting distance is 2-6 cm. Fluffy surface layer and impurities on the aluminized layer surface can be effectively removed by controlling sand-blasting pressure and sand-blasting time, so as to obtain even aluminized tissues with strong bonding force, and provide the aluminum oxide film subject to atom layer deposition with the excellent substrate doped with no external elements, thereby improving the bonding efficiency between the reactive precursor and the substrate.

(21) In the step S1, the surface treatment includes mechanical polishing for the Fe-based alloy before electrolytic polishing; the mechanical polishing comprises: polishing without visually obvious scratches using an abrasive paper with a granularity of 80-1,200 meshes, ultrasonically washing for 5-20 min to remove oil with acetone, and then ultrasonically washing with anhydrous ethanol to remove the stains for 5-20 min, and finally drying; the electrolytic polishing means electrolytically polishing the Fe base by taking the Fe-based alloy as an anode and an insoluble conductive material as a cathode; the electrolytically polished electrolyte comprises concentrated sulfuric acid with a volume fraction of 60-80%, concentrated phosphoric acid with a volume fraction of 15-37% and distilled water with a volume fraction of 3-5%; the electrolytic DC voltage is 5-6 V, the electrolyte temperature is 60-80° C., and the electrolytic polishing time is 2-5 min.

(22) The electropolishing treatment is as follows: connecting a 321 austenitic stainless steel plate to the anode at a distance of 50 mm from the cathode made of an insoluble conductive material (graphite plate); heating the electrolyte to 60-80° C. (optionally, by water bath) and placing the anode and the cathode in the electrolyte; switching on a 5-6 V DC voltage and taking out the plate after 2-5 min; and then rinsing and drying the plate. The electrolyte comprises the following compositions (by volume fraction): 60-80% of concentrated sulfuric acid (98% pure), 15-37% of concentrated phosphoric acid (85% pure) and 3-5% of distilled water.

(23) The step S3 is as follows: placing the test piece in a beaker containing 3-8 L of deionized water and heating and oscillating for 2-7 min to remove residual fines from the surface of the test piece; transferring the test piece to another beaker containing 2-5 L of acetone for 5-10 min heating and oscillation; and then drying the test piece for 10-30 min by means of a drying oven.

(24) According to the preparation method for the composite coating with high resistance to a molten Al—Si alloy corrosion provided by the present invention, the aluminum-diffused stainless steel is subject to an atom layer vapor deposition to obtain a coating in multilayer structure, comprising a TiO.sub.2 thin film with a thickness of 5 nm-50 nm, an Al.sub.2O.sub.3 thin film with a thickness of 5 nm-50 nm, a discontinuous Al.sub.2O.sub.3 layer with a thickness of 10-30 μm, an Fe—Al outer aluminized layer with a thickness of 60-200 μm (FeAl, FeAl.sub.2 and Fe.sub.3Al), a diffusion layer comprising Fe(Al) phase with a thickness of 50-160 μm and a substrate from the outside inwards. The layers of the composite coating are tightly integrated with sharply defined and regular boundaries and free of crack. The Al.sub.2O.sub.3/TiO.sub.2 thin film or TiO.sub.2 thin film obtained by atom layer vapor deposition is controllable in thickness and grows in a uniform and smooth way with a good step coverage. With compact surface, the thin film is firmly adhered on the aluminized layer structure without changing the aluminized layer structure. The 72-hour molten Al—Si alloy corrosion test was conducted at 620° C. and revealed a corrosion rate of 0.35×10.sup.−5 g/mm.sup.2.Math.h for the obtained stainless steel with the Al.sub.2O.sub.3/TiO.sub.2 thin film coating structure and 0.23×10.sup.−5 g/mm.sup.2.Math.h for the stainless steel with the TiO.sub.2 thin film coating structure, which decreased by 73.1% and 82.3% with respect to the corrosion rate of austenitic stainless steel (1.3×10.sup.−5 g/mm.sup.2.Math.h). Therefore, the obtained stainless steel has excellent resistance to corrosion of the molten Al—Si alloy, guarantees the compatibility of the molten Al—Si alloy (as a heat reservoir medium) to a heat exchange tube for solar thermal power generation, and promises extraordinary scientific merits and industrial application.

Embodiment 1

(25) According to a molten Al—Si alloy corrosion resistant composite coating provided by the present invention, the composite coating comprises an aluminized layer, an Al.sub.2O.sub.3 thin film layer and a TiO.sub.2 thin film layer from a substrate surface to outside in sequence. The Al.sub.2O.sub.3 thin film layer with thickness of 5 nm and the TiO.sub.2 thin film layer with thickness of 20 nm are introduced by means of an atom layer vapor deposition; the aluminized layer comprises an Fe(Al) phase diffusion layer, an Fe—Al compound layer and an Al.sub.2O.sub.3 layer from the substrate surface to outside in sequence. The Fe(Al) phase diffusion layer, the Fe—Al compound layer and the Al.sub.2O.sub.3 layer have micro-sized thicknesses.

(26) A preparation method for the molten Al—Si alloy corrosion resistant composite coating of this embodiment, comprising the following steps:

(27) (1) Surface mechanical polishing: polishing a hot-rolled austenitic stainless steel plate test piece to remove visible scratches using an abrasive paper with a granularity varying from 80 to 1200 meshes, ultrasonically washing the test piece in acetone for 5 min to remove oil, then transferring the test piece to an absolute ethyl alcohol to ultrasonically remove the stain for 5 min, and drying the test piece in a drying oven at 80° C. for 20 min; and the 321 austenitic stainless steel is a rolled plate, comprising the following chemical components (by mass fraction): 0.04% of C, 0.38% of Si, 1.08% of Mn, 17.02% of Cr, 9.06% of Ni, 0.05% of N, 0.03% of P, 0.22% of Ti and the rest of Fe.

(28) (2) Electropolishing: connecting the 321 austenitic stainless steel plate to an anode at a distance of 50 mm from a cathode made of an insoluble conductive material (graphite plate), heating an electrolyte to 60° C. (optionally, by water bath), immersing the anode and the cathode in the electrolyte simultaneously, switching on a 5 V DC voltage, and then rinsing and drying the test piece after 2 min; and the electrolyte comprises the following compositions (by volume fraction): 60% of concentrated sulfuric acid (98% pure), 37% of concentrated phosphoric acid (85% pure) and 3% of distilled water.

(29) (3) Aluminizing: a solid powder penetrant comprises an aluminum source, a filler and a penetration aid (activator); the aluminum source comprises an aluminum powder (200 meshes), the filler comprises an Al.sub.2O.sub.3 powder and a Cr powder, and the penetration aid comprises a powdered NH.sub.4Cl; and the solid powder penetrant is prepared by fully mixing these components (percentage by weight): 5 wt % of Cr, 64 wt % of Al, 28 wt % of Al.sub.2O.sub.3 and 3 wt % of NH.sub.4Cl. The penetrant is placed in a heat-resistant stainless steel charging bucket with the test piece in a compacted manner, and then sealed using a refractory mortar for aluminizing: heating up the test piece with the furnace to 150° C. and drying for 2 h, maintaining a temperature at 400° C. for 20 min and 900° C. for 15 h at a rate of 10° C./min, and then cooling the test piece with the furnace to room temperature;

(30) (4) Sandblasting treatment: with Al.sub.2O.sub.3 particles (300 meshes) as an abrasive, sandblasting the aluminized test piece by a 0.6 MPa high-pressure nitrogen for 5 min at a distance of 6 cm to remove loose aluminized layer and impurities;

(31) (5) Washing and drying with organic solvents: placing the test piece in a beaker containing 3 L of deionized water and heating and oscillating for 7 min to remove residual fines from the surface of the test piece; transferring the test piece to another beaker containing 5 L of acetone for 10-min heating and oscillation; and then drying the test piece for 30 min by means of a drying oven.

(32) (6) Al.sub.2O.sub.3/TiO.sub.2 thin film by means of atom layer vapor deposition: placing the aluminized steel into a chamber of equipment as a substrate, heating up to 150° C., taking trimethyl aluminum (TMA, 99.99% pure) as a precursor, inflating for 0.03 s and then vacuumizing for 40 s under a pressure of 0.05 torr, introducing a vapor for 0.01 s, and then vacuumizing for 30 s to deposit the Al.sub.2O.sub.3 thin film; and repeating 50 cycles of inflation-vacuumization-vapor introduction-vacuumization in the presence of the TMA until the Al.sub.2O.sub.3 thin film reaches the desired thickness of 5 nm; with the aluminized steel/Al.sub.2O.sub.3 thin film as a substrate, heating up the chamber to 300° C., taking a titanium isopropylate (99.99% pure) as a precursor, inflating for 0.5 s and then vacuumizing for 30 s under a pressure of 0.1 torr, introducing a water-vapor plasma for 0.01 s, and then vacuumizing for 30 s to deposit the TiO.sub.2 thin film; and repeating 200 cycles of inflation-vacuumization-vapor introduction-vacuumization in the presence of the titanium isopropylate until the TiO.sub.2 thin film reaches the desired thickness of 20 nm.

Embodiment 2

(33) According to a molten Al—Si alloy corrosion resistant composite coating provided by the present invention, the composite coating comprises an aluminized layer, an Al.sub.2O.sub.3 thin film layer and a TiO.sub.2 thin film layer from a substrate surface to outside in sequence. The Al.sub.2O.sub.3 and the TiO.sub.2 thin film layers with thicknesses of 30 nm and 50 nm are introduced by means of an atom layer vapor deposition; the aluminized layer comprises an Fe(Al) phase diffusion layer, an Fe—Al compound layer and an Al.sub.2O.sub.3 layer from the substrate surface to outside in sequence. The Fe(Al) phase diffusion layer, the Fe—Al compound layer and the Al.sub.2O.sub.3 layer have micro-sized thicknesses.

(34) A preparation method for the molten Al—Si alloy corrosion resistant composite coating of this embodiment, comprising the following steps:

(35) (1) Surface mechanical polishing: polishing a hot-rolled austenitic stainless steel plate test piece to remove visible scratches using an abrasive paper with a granularity varying from 80 to 1200 meshes, ultrasonically washing the test piece in acetone for 10 min to remove the oil, then transferring the test piece to an absolute ethyl alcohol to ultrasonically remove the stain for 10 min, and drying the test piece in a drying oven at 80° C. for 30 min; and the 321 austenitic stainless steel is a rolled plate, comprising the following chemical components (by mass fraction): 0.04% of C, 0.38% of Si, 1.08% of Mn, 17.02% of Cr, 9.06% of Ni, 0.05% of N, 0.03% of P, 0.22% of Ti and the rest of Fe.

(36) (2) Electropolishing: connecting the 321 austenitic stainless steel plate to an anode at a distance of 50 mm from a cathode made of an insoluble conductive material (graphite plate), heating an electrolyte to 70° C. (optionally, by water bath), immersing the anode and the cathode in the electrolyte simultaneously, switching on a 5 V DC voltage, and then rinsing and drying the test piece after 5 min; and the electrolyte comprises the following compositions (by volume fraction): 70% of concentrated sulfuric acid (98% pure), 26% of concentrated phosphoric acid (85% pure) and 4% of distilled water.

(37) (3) Aluminizing: a solid powder penetrant comprises an aluminum source, a filler and a penetration aid (activator); the aluminum source comprises an aluminum powder (200 meshes), the filler comprises an Al.sub.2O.sub.3 powder and a Cr powder, and the penetration aid comprises a powdered NH.sub.4Cl; and the solid powder penetrant is prepared by fully mixing these components (percentage by weight): 15 wt % of Cr, 44 wt % of Al, 40 wt % of Al.sub.2O.sub.3 and 1 wt % of NH.sub.4Cl. The penetrant is placed in a heat-resistant stainless steel charging bucket with the test piece in a compacted manner, and then sealed using a refractory mortar for aluminizing: heating up the test piece with the furnace to 150° C. and drying for 2 h, maintaining a temperature at 600° C. for 40 min and 1050° C. for 10 h at a rate of 10° C./min, and then cooling the test piece with the furnace to room temperature;

(38) (4) Sandblasting treatment: with Al.sub.2O.sub.3 particles (400 meshes) as an abrasive, sandblasting the aluminized test piece by a 0.8 MPa high-pressure nitrogen for 10 min at a distance of 4 cm to remove loose aluminized layer and impurities.

(39) (5) Washing and drying with organic solvents: placing the test piece in a beaker containing 8 L of deionized water and heating and oscillating for 7 min to remove residual fines from the surface of the test piece; transferring the test piece to another beaker containing 5 L of acetone for 10-min heating and oscillation; and then drying the test piece for 30 min by means of a drying oven.

(40) (6) Al.sub.2O.sub.3/TiO.sub.2 thin film by means of atom layer vapor deposition: placing the aluminized steel into a chamber of equipment as a substrate, heating up to 300° C., taking a TMA (99.99% pure) as a precursor, inflating for 0.01 s and then vacuumizing for 60 s under a pressure of 0.2 torr, introducing a vapor for 0.03 s, and then vacuumizing for 50 s to deposit the Al.sub.2O.sub.3 thin film; and repeating 300 cycles of inflation-vacuumization-vapor introduction-vacuumization in the presence of the TMA until the Al.sub.2O.sub.3 thin film reaches the desired thickness of 30 nm; with the aluminized steel/Al.sub.2O.sub.3 thin film as a substrate, heating up the chamber to 450° C., taking titanium isopropylate (99.99% pure) as a precursor, inflating for 0.5 s and then vacuumizing for 50 s under a pressure of 0.3 torr, introducing a water-vapor plasma for 0.03 s, and then vacuumizing for 50 s to deposit the TiO.sub.2 thin film; and repeating 500 cycles of inflation-vacuumization-vapor introduction-vacuumization in the presence of the titanium isopropylate until the TiO.sub.2 thin film reaches the desired thickness of 50 nm.

Embodiment 3

(41) According to a molten Al—Si alloy corrosion resistant composite coating provided by the present invention, the composite coating comprises an aluminized layer, an Al.sub.2O.sub.3 thin film layer and a TiO.sub.2 thin film layer from a substrate surface to outside in sequence. The Al.sub.2O.sub.3 thin film layer with thickness of 10 nm and the TiO.sub.2 thin film layer with thickness of 10 nm are introduced by means of an atom layer vapor deposition; The aluminized layer comprises an Fe(Al) phase diffusion layer, an Fe—Al compound layer and an Al.sub.2O.sub.3 layer from the substrate surface to outside in sequence. The Fe(Al) phase diffusion layer, the Fe—Al compound layer and the Al.sub.2O.sub.3 layer have micro-sized thicknesses.

(42) A preparation method for the molten Al—Si alloy corrosion resistant composite coating of this embodiment, comprising the following steps:

(43) (1) Surface mechanical polishing: polishing a hot-rolled austenitic stainless steel plate test piece to remove visible scratches using an abrasive paper with a granularity varying from 80 to 1200 meshes, ultrasonically washing the test piece in acetone for 20 min to remove oil, then transferring the test piece to an absolute ethyl alcohol to ultrasonically remove the stain for 20 min, and drying the test piece in a drying oven at 80° C. for 40 min; and the 321 austenitic stainless steel is a rolled plate, comprising the following chemical components (by mass fraction): 0.04% of C, 0.38% of Si, 1.08% of Mn, 17.02% of Cr, 9.06% of Ni, 0.05% of N, 0.03% of P, 0.22% of Ti and the rest of Fe.

(44) (2) Electropolishing: connecting the 321 austenitic stainless steel plate to an anode at a distance of 50 mm from a cathode made of an insoluble conductive material (graphite plate), heating an electrolyte to 80° C. (optionally, by water bath), immersing the anode and the cathode in the electrolyte simultaneously, switching on a 5 V DC voltage, and then rinsing and drying the test piece after 3 min; the electrolyte comprises the following compositions (by volume fraction): 80% of concentrated sulfuric acid (98% pure), 15% of concentrated phosphoric acid (85% pure) and 5% of distilled water.

(45) (3) Aluminizing: a solid powder penetrant comprises an aluminum source, a filler and a penetration aid (activator); the aluminum source comprises an aluminum powder (200 meshes), the filler comprises an Al.sub.2O.sub.3 powder and a Cr powder, and the penetration aid comprises a powdered NH.sub.4Cl; and the solid powder penetrant is prepared by fully mixing these components (percentage by weight): 10 wt % of Cr, 58 wt % of Al, 30 wt % of Al.sub.2O.sub.3 and 2 wt % of NH.sub.4Cl. The penetrant is placed in a heat-resistant stainless steel charging bucket with the test piece in a compacted manner, and then sealed using a refractory mortar for aluminizing: heating up the test piece with the furnace to 150° C. and drying for 2 h, maintaining a temperature of 500° C. for 30 min and 950° C. for 12 h at a rate of 10° C./min, and then cooling the test piece with the furnace to room temperature;

(46) (4) Sandblasting treatment: with Al.sub.2O.sub.3 particles (500 meshes) as an abrasive, sandblasting the aluminized test piece by a 0.9 MPa high-pressure nitrogen for 5 min at a distance of 2 cm to remove loose aluminized layer and impurities;

(47) (5) Washing with organic solvents: placing the test piece in a beaker containing 5 L of deionized water and heating and oscillating for 5 min to remove residual fines from the surface of the test piece; transferring the test piece to another beaker containing 4 L of acetone for 8-min heating and oscillation; and then drying the test piece for 20 min by means of a drying oven.

(48) (6) Al.sub.2O.sub.3/TiO.sub.2 thin film by means of atom layer vapor deposition: placing the aluminized steel into a chamber of equipment as a substrate, heating up to 200° C., taking a TMA (99.99% pure) as a precursor, inflating for 0.02 s and then vacuumizing for 45 s under a pressure of 0.1 torr, introducing a vapor for 0.015 s, and then vacuumizing for 45 s to deposit the Al.sub.2O.sub.3 thin film; and repeating 90 cycles of inflation-vacuumization-vapor introduction-vacuumization in the presence of the TMA until the Al.sub.2O.sub.3 thin film reaches a thickness of approximately 10 nm; with the aluminized steel/Al.sub.2O.sub.3 thin film as a substrate, heating up the chamber to 370° C., taking titanium isopropylate (99.99% pure) as a precursor, inflating for 0.25 s and then vacuumizing for 40 s under a pressure of 0.2 torr, introducing a water-vapor plasma for 0.02 s, and then vacuumizing for 40 s to deposit the TiO.sub.2 thin film; and repeating 90 cycles of inflation vacuumization-vapor introduction-vacuumization in the presence of the titanium isopropylate until the TiO.sub.2 thin film reaches a thickness of approximately 10 nm.

Embodiment 4

(49) A molten Al—Si alloy corrosion resistant composite coating provided by the present invention, characterized by excluding the Al.sub.2O.sub.3 thin film layer introduced by means of the atom layer vapor deposition, based on comparison with the composite coating of Embodiment 3.

(50) A preparation method for the molten Al—Si alloy corrosion resistant composite coating of the embodiment, characterized in that the method is basically identical with that of Embodiment 3, except that the Al.sub.2O.sub.3 thin film layer introduced by means of the atom layer vapor deposition is deleted from step (6), and the TiO.sub.2 thin film layer is directly formed on the surface of the aluminized steel by means of the atom layer vapor deposition.

Comparative Example 1

(51) A molten Al—Si alloy corrosion resistant composite coating, characterized by comprising the aluminized layer only rather than the Al.sub.2O.sub.3 and the TiO.sub.2 thin film layers introduced by means of the atom layer vapor deposition, based on comparison with the composite coating of Embodiment 3.

(52) A preparation method for the molten Al—Si alloy corrosion resistant composite coating of this comparative example, characterized by excluding the step (6), based on comparison with the composite coating of Embodiment 3.

(53) FIG. 1b shows the results of SEM analysis on the surface of the composite coating obtained in Embodiment 3. There is no significant difference in surface topography between the composite coating and the aluminized steel prior to the atom layer vapor deposition (FIG. 1a), indicating that the atom layer vapor deposition makes no change to the aluminized surface structure. The energy spectrum analysis was carried out on point A in FIG. 1b to detect Fe, Cr, Al, Ti and O atoms. In addition to the atoms existing in the coating, the Cr atom and the Fe atom in the substrate material were observed as an EDS penetrated into the substrate. Only trace amount of Ti element was measured by means of the EDS since the TiO.sub.2 in nano-scaled thickness was introduced by means of the atom layer vapor deposition.

(54) FIG. 2 is a histogram of corrosion rates of the 321 stainless steel containing the ALD Al.sub.2O.sub.3/TiO.sub.2 composite aluminized coating of Embodiment 3, the 321 stainless steel containing the ALD TiO.sub.2 composite aluminized coating of example 4, as well as the 321 stainless steel containing the aluminized layer and the 321 stainless steel without the coating of comparative example 1, in the presence of the molten Al—Si alloy at 620° C. for 72 hours.

(55) The corrosion with extents varying with corrosion time on the metal test piece is measured by means of a weight loss method. The extent of corrosion on materials is directly characterized by the weight loss of the test piece. The metal test piece corrosivity is evaluated based on the corrosion rate V(g/mm.sup.2.Math.h), as shown in Formula (1):

(56) $\begin{array}{cc}V=\frac{W^0-W}{A\times t}& \left(1\right)\end{array}$

(57) In the above formula, A means the surface area of the test piece (mm.sup.2); Wo means the mass of the test piece before corrosion (g); W means the mass of the test piece after corrosion (g); t means the corrosion time (h). As shown in FIG. 2, compared with the corrosion rate of the stainless steel resistant to the molten Al—Si alloy, it seems that the corrosion rate of the test piece with a single aluminized coating decreases by 63.1%, the test piece with an ALD TiO.sub.2 composite aluminized coating decreases by 73.1%, and the test piece with an ALD Al.sub.2O.sub.3/TiO.sub.2 composite aluminized coating decreases by 82.3%. Therefore, the composite coating provided by the present invention has high resistance to corrosion of the molten Al—Si alloy. In addition, the coating introduced by means of the ALD Al.sub.2O.sub.3/TiO.sub.2 composite aluminizing shows a greater improvement in the resistance to corrosion than that introduced by means of the ALD TiO.sub.2 composite aluminizing, and satisfies the compatibility of the molten Al—Si alloy (as the heat reservoir medium) to the heat exchange tube for solar thermal power generation.

(58) The above examples are only preferred embodiments of the present invention and not used to limit the present invention. Any person skilled in the art, without departing from the scope of the technical solution of the present invention, is capable of taking advantage of the above-described technical content to make a plurality of possible variations and modifications of the technical solution, or equivalent embodiments with equivalent changes. Therefore, all the contents without departing from the technical solution of the present invention, based on any simple modification, equivalent variations and modifications made by the technical spirit of the present invention for the above embodiments, would be incorporated in the protection range of the technical solution of the present invention.