Shellac coating for pipeline welding and preparation process of shellac coating

Abstract

The invention discloses a shellac coating for pipeline welding and a preparation process of the shellac coating, and particularly relates to the field of materials for brazing or welding. Extinction powder, glutinous rice flour, biochar powder and ethyl alcohol are weighed according to the proportion to prepare a component A; weighing shellac and ethanol according to a ratio to prepare a component B; by constructing a staged response structure which takes shellac as a matrix and combines a cross-linkable gel network and a controllable chelating and stripping system, the anti-rust coating has two optional response paths before welding: on one hand, under the condition that sodium alginate is not used, the coating is stable in structure, can directly bear high temperature required by welding, and is quickly gasified at 200 DEG C or above; residues are not left, and the polishing step is omitted; and on the other hand, if an EDTA solution is sprayed before welding, calcium ions in the film layer can be selectively chelated, a cross-linked network can be destroyed, overall slippage or overall stripping of the film layer is achieved, and operation modes under different construction requirements are adapted.

Description

Technical Field

[0001] The present invention relates to the technical field of materials used for brazing or welding, and more particularly to a shellac coating for pipeline welding and a preparation process thereof. Background Art

[0002] A pipeline is a device connected by pipes, pipe connectors, and valves for transporting gases, liquids, or fluids containing solid particles. Typically, the fluid is pressurized by blowers, compressors, pumps, and boilers, flowing from high-pressure areas to low-pressure areas in the pipeline. Alternatively, the fluid's own pressure or gravity can be used for transport. Pipelines are widely used, primarily in water supply, drainage, heating, gas supply, long-distance transportation of oil and natural gas, agricultural irrigation, water conservancy projects, and various industrial installations. Pipeline connections can be made in four ways: threaded connection, flange connection, socket connection, and welding. In practical applications, metal pipes are usually connected by welding. Before welding metal pipes, groove processing, alignment, cleaning and rust removal are required. In order to prevent the grooves that have been derusted from rusting, anti-rust paint will be applied to the inner and outer surfaces of the grooves. Before welding, the anti-rust paint needs to be polished off before welding. A skilled welder will spend a lot of time polishing the anti-rust paint during a working day. Ordinary anti-rust paint contains formaldehyde and other metal elements. Inhaling too much formaldehyde will cause harm to the worker's body, and the residue of other metal elements will also affect the quality of welding work. The existing technology lacks an environmentally friendly, non-toxic, special anti-rust coating for pipeline welding that can be directly eliminated during the welding process and leaves no residue. Therefore, it is urgent to design a shellac coating for pipeline welding and its preparation process to solve the above technical problems. Summary of the Invention

[0003] In order to overcome the above-mentioned defects of the prior art, an embodiment of the present invention provides a shellac coating for pipeline welding and a preparation process thereof. By constructing a staged response structure with shellac as the matrix, combining a cross-linkable gel network and a controllable chelating and stripping system, the anti-rust coating has two optional response paths before welding: on the one hand, without using sodium alginate, the coating structure is stable and can directly withstand the high temperature required for welding, and quickly vaporizes above 200 degrees Celsius without leaving any residue, thus eliminating the need for a polishing step; on the other hand, if an EDTA solution is sprayed before welding, the calcium ions inside the film layer can be selectively chelated, the cross-linking network can be destroyed, and the entire film layer can be slipped off or peeled off in one piece, adapting to the operation mode under different construction requirements to solve the technical problems raised in the above-mentioned background technology.

[0004] To achieve the above object, the present invention provides the following technical solution: a shellac coating for pipeline welding, comprising a shellac coating for brazing or welding, wherein the shellac coating is composed of component A and component B; Component A is composed of the following raw materials in the following mass ratios: 5-15 parts of matting powder, 15-25 parts of glutinous rice flour, 5-15 parts of biochar powder, and 45-75 parts of ethanol (laboratory reagent grade, LR); Component B is composed of the following raw materials in the following mass ratios: 20-45 parts of shellac flakes and 55-80 parts of ethanol (laboratory reagent grade, LR); The mass ratio of component A to component B of the shellac coating is (5-10):100; Shellac flakes are natural polymer materials formed by processing and drying resin metabolites secreted by insects. They have the following characteristics: 1) The main components of shellac are ester polymers of hydroxy fatty acids and resin acid compounds mainly based on shellac acid, which contains about 70%-80% aliphatic and aromatic hydroxy acid units. These units are connected by ester bonds to form a randomly branched natural polyester network structure. In addition, shellac also contains a small amount of wax, pigments (such as shellac red), protein impurities and trace sugars, which affect its color, melting point and mechanical properties; 2) Shellac flakes are sheet materials made from the original shellac crude material after dewaxing, filtration and hot melt pressing. They have excellent thermal softening, gasification, biodegradability and ethanol solubility, and are widely used in food, medicine, electronic insulation and pre-weld protective coating and other fields.

[0005] In a preferred embodiment, the shellac coating is composed of component A, component B and component C, wherein the component C of the shellac coating is used to construct a cross-linked network in the film layer, wherein the mass fraction of component C is: 1.5-3.5 parts of sodium alginate; The invention also includes a component D for triggering the stripping of shellac coatings, wherein the component D comprises the following raw materials in the following weight ratios: 1-2 parts calcium chloride solution and 2-4 parts EDTA solution; wherein the EDTA solution is a chelating agent aqueous solution prepared in water with ethylenediaminetetraacetic acid or its sodium salt as the main component; the EDTA solution is a highly effective metal ion chelating agent solution that can selectively complex polyvalent metal ions and is used in applications such as metal ion removal, buffer system adjustment, and controlled deconstruction of cross-linking systems; The D component is sprayed separately in order, with the calcium chloride solution sprayed first and the EDTA solution sprayed later; the calcium chloride solution is used to undergo an ion cross-linking reaction with the sodium alginate to form a calcium ion-alginate cross-linked network, and the EDTA solution is sprayed before brazing or welding to destroy the calcium ion-alginate cross-linked network and trigger the whole piece to peel off; The mass ratio of the component A, component B, component C and component D is in the range of (5-10):100:(1.5-3.5):(3-6).

[0006] In a preferred embodiment, the shellac coating is thermally vaporized at a brazing or welding temperature of 200 degrees Celsius or above, and does not require grinding.

[0007] In a preferred embodiment, the shellac coating is a coating type structure, and forms a coating with a film thickness of 500-1000 microns after one coating.

[0008] In a preferred embodiment, the shellac coating does not contain formaldehyde and heavy metal elements, and does not release volatile substances or solid particles that pose a toxic risk to welders during the thermal vaporization, triggered stripping or grinding process of the shellac coating.

[0009] In a preferred embodiment, a process for preparing a shellac coating for pipeline welding comprises: S1. Prepare component A by weighing according to the proportion: put matting powder, glutinous rice flour, biochar powder, and ethanol into a beaker in sequence, let it stand for 3 hours, and then stir for 5-10 minutes to obtain component A; S2. Prepare component B by weighing according to the proportion: put the shellac flakes and ethanol into a beaker in sequence, let it stand for 3 hours, and then stir for 5-10 minutes to obtain component B; S3, mixing component A and component B in a mass ratio of (5-10):100, stirring at a low speed for 5 minutes, and degassing for 30 minutes to obtain a shellac coating mother solution for brazing or welding; S4. Apply the shellac coating solution to the inner and outer surfaces of the pipe groove by scraping or spraying. Manually control the thickness of the coating layer to 500-1000 microns. Let it stand at room temperature for 10-20 minutes and then dry. S5. No rusting when stored in a humid or outdoor environment for ≥48 hours; no mechanical grinding is required before welding.

[0010] In a preferred embodiment, a process for preparing a shellac coating for pipeline welding comprises: S1. Prepare component A by weighing according to the proportion: put matting powder, glutinous rice flour, biochar powder, and ethanol into a beaker in sequence, let it stand for 3 hours, and then stir for 5-10 minutes to obtain component A; S2. Prepare component B by weighing according to the proportion: put the shellac flakes and ethanol into a beaker in sequence, let it stand for 3 hours, and then stir for 5-10 minutes to obtain component B; S3. Prepare component C by weighing according to the proportion: weigh 1.5-3.5 parts of sodium alginate, add it to a mixture of 70% ethanol and 30% deionized water, and stir magnetically for 15 minutes until it swells and dissolves, to obtain a transparent component C solution; S4. Component A, component B, and component C are sequentially mixed in a mass ratio of (5–10):100:(1.5–3.5), stirred at a low speed for 5 minutes, and degassed for 30 minutes to obtain a shellac coating mother solution for brazing or welding; S5. Apply the shellac coating solution to the inner and outer surfaces of the pipe groove by scraping or spraying. Manually control the thickness of the coating layer to 500-1000 microns. Let it stand at room temperature for 10-20 minutes and then dry. S6. Prepare a calcium chloride solution by dissolving calcium chloride in a mixed solvent consisting of 30% deionized water and 70% ethanol to a concentration of 0.3 mol / L to obtain a calcium chloride solution; spray 1-2 parts of the solution onto the surface of the shellac coating by atomization and let stand for 1 minute to allow calcium ions and sodium alginate to form a calcium ion-alginate crosslinked network within the film to increase adhesion and weather resistance to welding; S7. The coating cross-linked by calcium ions and sodium alginate will not rust when stored in a humid or outdoor environment for ≥48 hours; and no mechanical grinding is required before welding; S8, dissolving EDTA at a concentration of 0.2-0.5 mol / L in a mixed solvent containing ethanol and deionized water (the volume ratio of ethanol to deionized water is 6:4, 7:3, or 8:2), and stirring to obtain an EDTA solution; S9. 30 seconds before welding, spray 2-4 parts of EDTA solution by atomization; EDTA chelates calcium ions, thereby destroying the calcium ion-alginate cross-linking network, causing the entire film layer to loosen, allowing it to be peeled off or slide off naturally; the residual liquid and coating will vaporize at a welding temperature of ≥200 degrees Celsius, and no mechanical grinding is required.

[0011] Technical effects and advantages of the present invention: By constructing a controllable cross-linked gel network with shellac as the matrix and combining it with sodium alginate and calcium ions, and further supplemented by the chelating effect of EDTA solution, the entire coating can be quickly peeled off before welding operations, fundamentally solving the problem of additional mechanical grinding of anti-rust coatings in existing technologies, reducing the operating burden of welders and avoiding the interference of metal residues on weld quality.

[0012] By mixing shellac and ethanol to form component B, and combining it with glutinous rice flour, matting powder, and biochar powder in component A, the initial coating film quality and adhesion stability are enhanced, so that the anti-rust effect remains reliable under high humidity and air contact conditions, meeting the long-term storage requirements before welding.

[0013] By introducing sodium alginate as the C component and subsequently spraying and cross-linking with calcium chloride to form a calcium ion-alginate cross-linked network, the coating has good integrity after spraying and has controllable film adhesion, which provides a structural basis for subsequent responsive peeling and improves the reliability of the film protection before welding.

[0014] By adjusting the concentration of EDTA and configuring the ethanol / water ratio to form component D, and atomizing and spraying it within 30 seconds before activating the welding heat source, it can chelate calcium ions and destroy cross-linking points in a short period of time, achieving overall loosening of the film layer without heating, thereby enhancing the controllability and operational flexibility of the stripping process.

[0015] By introducing combustible and highly adsorbent biochar powder into component A, it can completely disappear with thermal vaporization during the welding process without leaving any particles or scorch marks. This eliminates adverse factors on the quality of the heat-affected zone of the weld from the material system itself, effectively improving the compatibility of the welding process. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The present invention is a flow chart for preparing the shellac coating composed of component A and component B.

[0017] Figure 2 The present invention is a flow chart for preparing the shellac coating composed of component A, component B, component C and component D. DETAILED DESCRIPTION

[0018] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0019] Refer to the instruction manual Figure 1 In the shellac coating for pipeline welding and its preparation process of the present invention, the shellac coating composed of component A and component B includes the following embodiments and comparative examples:

[0020] Example 1 (minimum value of component A, maximum value of component B): 5 parts of unglazing powder, 15 parts of glutinous rice flour, 5 parts of biochar powder, and 45 parts of ethanol were weighed, added to a beaker in sequence, and allowed to stand for 3 hours, followed by stirring for 5 minutes to obtain component A; 45 parts of shellac flakes and 80 parts of ethanol were weighed, added to a beaker in sequence, allowed to stand for 3 hours, and stirred for 10 minutes to obtain component B; component A and component B were mixed in a mass ratio of 5:100, stirred for 5 minutes, and then degassed for 30 minutes to obtain a shellac coating mother liquor for brazing or welding.

[0021] Example 2 (maximum value of component A, minimum value of component B): 15 parts of unglazing powder, 25 parts of glutinous rice flour, 15 parts of biochar powder, and 75 parts of ethanol were weighed, added sequentially into a beaker, allowed to stand for 3 hours, and then stirred for 10 minutes to obtain component A; 20 parts of shellac flakes and 55 parts of ethanol were weighed, added sequentially into a beaker, allowed to stand for 3 hours, and stirred for 5 minutes to obtain component B; component A and component B were mixed in a mass ratio of 10:100, stirred for 5 minutes, and then degassed for 30 minutes to obtain a shellac coating mother solution for brazing or welding.

[0022] Comparative Example 1 (replacing glutinous rice flour in component A with talcum powder): Comparative Example 1 maintained the same dosage and ratio as Example 1, except that 15 parts of glutinous rice flour in component A was replaced with 15 parts of talc; the remaining ingredients were treated in the same manner; the prepared shellac coating showed insufficient film adhesion after spraying, and left obvious debris after peeling, failing to achieve the effect of not requiring polishing in the example.

[0023] Comparative Example 2 (replacing the shellac flakes in component B with shellac liquid): This comparative example maintained the same dosage and ratio as Example 2, except that 20 parts of shellac flakes in component B were replaced with commercially available shellac liquid (containing a shellac solid content of 10% or more); 55 parts of ethanol were maintained; the resulting coating exhibited shrinkage during film formation, forming bubble fractures, and failed to achieve the continuous and uniform film performance of Examples 1 and 2.

[0024] Comparative Example 3 (replacing the biochar powder in component A with black iron oxide): This comparative example is consistent with Example 1, except that 5 parts of biochar powder in component A is replaced with 5 parts of black iron oxide, while the other components remain the same; the resulting shellac coating has an odor during welding heat vaporization and retains obvious carbonization traces, failing to achieve the surface cleanliness effect after heat vaporization in the example.

[0025] Refer to the instruction manual Figure 2 In the shellac coating for pipeline welding and its preparation process of the present invention, the shellac coating composed of component A, component B, component C and component D includes the following embodiments and comparative examples:

[0026] Example 3 (EDTA solvent volume ratio 6:4, minimum component combination): Weigh 5 parts of glaze powder, 15 parts of glutinous rice flour, 5 parts of biochar powder, and 45 parts of ethanol, add them to a beaker in sequence, let them stand for 3 hours, and then stir for 5 minutes to obtain component A; weigh 20 parts of shellac flakes and 55 parts of ethanol, add them to a beaker in sequence, let them stand for 3 hours, and stir for 5 minutes to obtain component B; weigh 1.5 parts of sodium alginate, place it in a mixed solvent of ethanol and deionized water with a volume ratio of 70%:30%, and stir magnetically for 15 minutes until it swells and dissolves to obtain a transparent component C solution; Mix components A, B, and C in a ratio of 5:100:1.5, stir for 5 minutes, and then degas for 30 minutes to obtain a shellac coating mother liquor. Prepare a calcium chloride solution (0.3 mol / L) and spray one part of it onto the film surface. Let it sit for 1 minute to allow crosslinking. Then, dissolve EDTA at a concentration of 0.2 mol / L in a mixed solvent of ethanol and deionized water in a volume ratio of 6:4. Stir thoroughly and spray two parts of it onto the film surface 30 seconds before welding to trigger the entire film to peel.

[0027] Example 4 (EDTA solvent volume ratio 7:3, component median combination): Weigh 10 parts of glaze powder, 20 parts of glutinous rice flour, 10 parts of biochar powder, and 60 parts of ethanol, add them to a beaker in sequence, let them stand for 3 hours, and then stir for 7 minutes to obtain component A; weigh 32.5 parts of shellac flakes and 67.5 parts of ethanol, add them to a beaker in sequence, let them stand for 3 hours, and stir for 7 minutes to obtain component B; weigh 2.5 parts of sodium alginate, place it in a mixture of ethanol and deionized water with a volume ratio of 70%:30%, and stir magnetically for 15 minutes until it swells and dissolves, to obtain component C solution. ; Mix components A, B, and C in sequence at a mass ratio of 7:100:2.5, stir for 5 minutes, and degas for 30 minutes to obtain a mother liquor; prepare a calcium chloride solution with a concentration of 0.3 mol / L, and spray it on the surface of the film layer in 1.5 parts by atomization to form a cross-linked network; dissolve EDTA at 0.35 mol / L in a mixed solvent of ethanol and deionized water in a volume ratio of 7:3, and spray 3 parts 30 seconds before welding to destroy the cross-linked structure and make the film layer peelable.

[0028] Example 5 (EDTA solvent volume ratio 8:2, component maximum combination): Weigh 15 parts of unglazing powder, 25 parts of glutinous rice flour, 15 parts of biochar powder, and 75 parts of ethanol, add them to a beaker in sequence, let them stand for 3 hours, and then stir for 10 minutes to obtain component A; weigh 45 parts of shellac sheets and 80 parts of ethanol, add them to a beaker in sequence, let them stand for 3 hours, and stir for 10 minutes to obtain component B; weigh 3.5 parts of sodium alginate, place it in a mixture of ethanol and deionized water with a volume ratio of 80%:20%, and stir it magnetically for 15 minutes until it is completely dissolved to obtain component C solution; mix components A, B, and C in a mass ratio of 10:100:3.5, stir for 5 minutes, and then degas for 30 minutes to obtain a mother liquor; prepare calcium chloride solution (0.3 mol / L), and spray it by atomization in an amount of 2 parts for crosslinking; dissolve EDTA at a concentration of 0.5 mol / L in a mixed solvent with a volume ratio of 8:2, stir to form a solution, and spray 4 parts 30 seconds before welding to trigger complete peeling of the coating and complete vaporization of the residue.

[0029] Comparative Example 4 (reference to Example 3, replacing sodium alginate with sodium carboxymethyl cellulose): In this comparative example, based on Example 3, 1.5 parts of sodium alginate in component C was replaced with the same mass of sodium carboxymethyl cellulose, while the other components and usage procedures remained unchanged. The prepared coating could form a film on the surface, but no effective cross-linking structure was formed after treatment with calcium chloride, resulting in the film layer before welding having warped edges and uneven peeling in some cases.

[0030] Comparative Example 5 (Reference to Example 4, replacing EDTA with citric acid) In this comparative example, based on Example 4, the EDTA solution is replaced with a citric acid solution of equimolar concentration, and the other components are consistent with the processing steps; although the film layer becomes loose after spraying before welding, the whole piece cannot be peeled off, and some areas still need to be mechanically polished or scraped off.

[0031] Comparative Example 6 (Refer to Example 5, replacing calcium chloride with sodium sulfate) In this comparative example, based on Example 5, 1.5 parts of the calcium chloride solution was replaced with an equimolar sodium sulfate solution, while the other components and operating procedures remained unchanged. No effective crosslinking was found after the film layer was treated, indicating poor adhesion and a fragile structure of the film layer, which failed to meet the requirements for wet storage and complete welding peeling.

[0032]

[0033]

[0034]

[0035]

[0036] It should be noted that the thermal vaporization residual rate in Table 1 is calculated by collecting the ratio of the mass of the pyrolysis residue to the mass of the original dry film after welding heat treatment of the coating film layer of standard area, that is, thermal vaporization residual rate = (mass of the residue after heat treatment ÷ mass of the dry film before heat treatment) × 100%; The thermal vaporization residual rate is used as an indicator to measure the completeness of the vaporization of the coating at a welding temperature of ≥200 degrees Celsius. The lower the residual rate, the more complete the thermal decomposition, the cleaner the surface after welding, and no need for grinding; The film adhesion is measured by vertically peeling the film within a standard test area using the pull-off method (tensile bond strength test), and recording the maximum tensile stress at the interface between the film and the substrate. This is: film adhesion = maximum peel force (Newtons) ÷ effective test area (square meters), expressed in MPa. The film adhesion is used to reflect the degree of adhesion between the film layer and the pipe surface. The higher the value, the more stable the coating is and the less likely it will fall off. In Table 2, the coefficient of variation of film uniformity is obtained by measuring the film thickness (unit: μm) at multiple measurement points within a fixed area after film formation. The ratio of the standard deviation of these thickness data to the mean is calculated using the formula: Coefficient of variation of film uniformity (%) = (standard deviation of film thickness ÷ average thickness) × 100%. The coefficient of variation of film uniformity is used to evaluate the distribution consistency of film thickness. The smaller the coefficient of variation, the more uniform the film. The residual film rate is calculated based on the mass ratio of the film remaining after thermal vaporization or stripping. The specific method is to heat the film to the set welding temperature within a specified area (Table 2 was measured at 200°C), measure the mass of the residue, and compare it with the initial dry film mass. The calculation formula is: Residual film rate (%) = (Residual mass after heat treatment ÷ Initial dry film mass) × 100%. This value reflects whether the coating can be fully vaporized or stripped during the welding process. The lower the residual film rate, the better the weld cleanliness. In Table 3, the fixed area peeling area ratio is obtained by spraying an equal amount of EDTA solution on a standard size (5 cm x 5 cm) film area, and performing visual identification and image analysis after a standard time. The area ratio of the peeled area to the original film area is calculated and expressed as a percentage to measure the peelability and response integrity of the film layer, reflecting the destruction efficiency of the cross-linked network after EDTA triggering; The film integrity score is manually graded based on the film's morphology after peeling. It primarily examines whether the film has detached in continuous chunks, or if there are any fragments, cracks, or residual traces. Table 3 shows the average value of the scores independently assessed by seven experienced personnel based on their own criteria. The maximum score is 10, with higher scores indicating a more complete film, neater edges, and less residual film during the peeling process. In Table 4, the peeling trigger response time is measured using a stopwatch method. Timing begins immediately after spraying the EDTA solution. Peeling is initiated and timing stops when visible signs of warping, bulging, or overall loosening appear at the edge of the film. This time is the peeling trigger response time, expressed in seconds. If no noticeable peeling occurs 30 seconds after spraying, a value greater than 30 is recorded, indicating a failed or insignificant peeling response.

[0037] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A shellac coating for pipe welding, including a shellac coating for brazing or welding, wherein the shellac coating comprises component A and component B, and is characterized in that: Component A is composed of the following raw materials in the following mass ratios: 5-15 parts of matting powder, 15-25 parts of glutinous rice flour, 5-15 parts of biochar powder, and 45-75 parts of ethanol; Component B is composed of the following raw materials in the following mass ratios: 20-45 parts of shellac flakes and 55-80 parts of ethanol; The mass ratio of component A to component B of the shellac paint is (5-10):

100.

2. The shellac coating for pipeline welding according to claim 1, characterized in that: The shellac coating is composed of component A, component B and component C. Component C of the shellac coating is used to construct a cross-linked network in the film layer, wherein the mass ratio of component C is: 1.5-3.5 parts of sodium alginate; Also included is a D component for triggering stripping of shellac coating, wherein the D component comprises the following raw materials in the following mass ratios: 1-2 parts of calcium chloride solution and 2-4 parts of EDTA solution; The D component is sprayed separately in order, with the calcium chloride solution sprayed first and the EDTA solution sprayed later; the calcium chloride solution is used to undergo an ion cross-linking reaction with the sodium alginate to form a calcium ion-alginate cross-linked network, and the EDTA solution is sprayed before brazing or welding to destroy the calcium ion-alginate cross-linked network and trigger the whole piece to peel off; The mass ratio of the component A, component B, component C and component D is in the range of (5-10):100:(1.5-3.5):(3-6).

3. The shellac coating for pipeline welding according to claim 1 or 2, characterized in that: The shellac coating is thermally vaporized at a brazing or welding temperature of 200 degrees Celsius or more and does not require grinding.

4. The shellac coating for pipeline welding according to claim 1 or 2, characterized in that: The shellac paint is a coating type structure, and forms a coating with a film thickness of 500-1000 microns after one coating.

5. The shellac coating for pipeline welding according to claim 1 or 2, characterized in that: The shellac coating does not contain formaldehyde and heavy metal elements, and does not release volatile substances or solid particles that pose a toxic risk to welders during the process of thermal vaporization, triggered stripping or grinding of the shellac coating.

6. The process for preparing a shellac coating for pipeline welding according to claim 1, characterized in that: S1. Prepare component A by weighing according to the proportion: put matting powder, glutinous rice flour, biochar powder, and ethanol into a beaker in sequence, let it stand for 3 hours, and then stir for 5-10 minutes to obtain component A; S2. Prepare component B by weighing according to the proportion: put the shellac flakes and ethanol into a beaker in sequence, let it stand for 3 hours, and then stir for 5-10 minutes to obtain component B; S3, mixing component A and component B in a mass ratio of (5-10):100, stirring for 5 minutes, and degassing for 30 minutes to obtain a shellac coating mother solution for brazing or welding; S4. Apply the shellac coating solution to the inner and outer surfaces of the pipe groove by scraping or spraying. Manually control the thickness of the coating layer to 500-1000 microns. Let it stand at room temperature for 10-20 minutes and then dry. S5. No rusting when stored in a humid or outdoor environment for ≥48 hours; no mechanical grinding is required before welding.

7. The process for preparing a shellac coating for pipeline welding according to claim 2, characterized in that: S1. Prepare component A by weighing according to the proportion: put matting powder, glutinous rice flour, biochar powder, and ethanol into a beaker in sequence, let it stand for 3 hours, and then stir for 5-10 minutes to obtain component A; S2. Prepare component B by weighing according to the proportion: put the shellac flakes and ethanol into a beaker in sequence, let it stand for 3 hours, and then stir for 5-10 minutes to obtain component B; S3. Prepare component C by weighing according to the proportion: weigh 1.5-3.5 parts of sodium alginate, add it to a mixture of 70% ethanol and 30% deionized water, and stir magnetically for 15 minutes until it swells and dissolves, to obtain a transparent component C solution; S4. Component A, component B, and component C are sequentially mixed in a mass ratio of (5–10):100:(1.5–3.5), stirred for 5 minutes, and degassed for 30 minutes to obtain a shellac coating mother solution for brazing or welding; S5. Apply the shellac coating solution to the inner and outer surfaces of the pipe groove by scraping or spraying. Manually control the thickness of the coating layer to 500-1000 microns. Let it stand at room temperature for 10-20 minutes and then dry. S6. Prepare a calcium chloride solution by dissolving calcium chloride in a mixed solvent consisting of 30% deionized water and 70% ethanol to a concentration of 0.3 mol / L to obtain a calcium chloride solution; spray 1-2 parts of the solution onto the surface of the shellac coating by atomization and let stand for 1 minute to allow calcium ions to react with sodium alginate to form a calcium ion-alginate crosslinked network within the coating; S7. The coating cross-linked by calcium ions and sodium alginate will not rust when stored in a humid or outdoor environment for ≥48 hours; and no mechanical grinding is required before welding; S8, dissolving EDTA at a concentration of 0.2-0.5 mol / L in a mixed solvent containing ethanol and deionized water (the volume ratio of ethanol to deionized water is 6:4, 7:3, or 8:2), and stirring to obtain an EDTA solution; S9. 30 seconds before welding, spray 2-4 parts of EDTA solution by atomization; EDTA chelates calcium ions, thereby destroying the calcium ion-alginate cross-linking network, causing the entire film layer to loosen; the residual liquid and the coating are vaporized at a welding temperature of ≥200 degrees Celsius, and no mechanical grinding is required.

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