Shellac coating for pipe welding and its preparation process

Abstract

The application discloses a shellac coating for pipeline welding and a preparation process thereof, and particularly relates to the field of materials for brazing or welding, wherein, according to proportions, the weight of the matting powder, the glutinous rice powder, the biochar powder and the ethanol is determined to prepare component A; and according to proportions, the weight of the shellac sheet and the ethanol is determined to prepare component B. By constructing a staged response structure with shellac as a matrix, combining a cross-linking gel network and a controllable chelation 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, can directly withstand the high temperature required by welding, and rapidly gasifies above 200 degrees Celsius without leaving residues, thereby eliminating the polishing step; on the other hand, if EDTA solution is sprayed before welding, the calcium ions in the film layer can be selectively chelated, the cross-linking network is destroyed, the whole film layer is slipped or peeled off in an integral block, and the operation mode is adapted to different construction requirements.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of materials for brazing or welding, more particularly, the present application relates to a shellac coating for pipe welding and a preparation process thereof. BACKGROUND

[0002] Pipe is a device for conveying gas, liquid or fluid with solid particles by connecting pipes, pipe connectors and valves; usually, the fluid is pressurized by a blower, compressor, pump and boiler, etc. and flows from high pressure to low pressure of the pipe, or is conveyed by the pressure or gravity of the fluid itself; the pipe has a wide range of applications, mainly used in water supply, drainage, heating, coal gas supply, long-distance transportation of oil and natural gas, irrigation, water conservancy and various industrial devices;

[0003] The connection of the pipe can adopt four ways of threaded connection, flange connection, socket connection and welding, and the connection of the metal pipe is usually in the form of welding in actual application;

[0004] Before welding of the metal pipe, beveling, aligning and centering, cleaning and rust removal are required, in order to prevent the bevel that has been rust removed from rusting, anti-rust paint is applied to the inner and outer surfaces of the bevel; before welding, the anti-rust paint needs to be polished off before welding;

[0005] A skilled welder will also spend a lot of time polishing the anti-rust paint in a day; ordinary anti-rust paint contains formaldehyde and other metal elements, and 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 the welding work;

[0006] The prior art lacks an environmentally friendly and non-toxic anti-rust coating for pipe welding that can be directly eliminated and has no residue during the welding process, therefore, it is urgent to design a shellac coating for pipe welding and a preparation process thereof to solve the above technical problems. SUMMARY

[0007] In order to overcome the above-mentioned defects of the prior art, the embodiments of the present application provide a shellac coating for pipe welding and a preparation process thereof, by constructing a phased response structure with shellac as the matrix, combining a cross-linking gel network and a controllable chelating 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, can directly withstand the high temperature required for welding, and rapidly gasifies above 200 degrees Celsius without residue, eliminating the polishing step; on the other hand, if EDTA solution is sprayed before welding, the calcium ions in the film layer can be selectively chelated, the cross-linking network is destroyed, the whole film layer is peeled off or peeled off in a whole piece, and the working mode is adapted to different construction requirements, so as to solve the technical problems proposed in the above background.

[0008] 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;

[0009] 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);

[0010] 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);

[0011] The mass ratio of component A to component B of the shellac coating is (5-10):100;

[0012] 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.

[0013] 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;

[0014] 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;

[0015] The D component is sprayed in sequence, first spraying the calcium chloride solution, and then spraying the EDTA solution; wherein the calcium chloride solution is used for ion cross-linking reaction with sodium alginate to form calcium ion-alginate cross-linking network, and the EDTA solution is used for pre-spraying before brazing or welding to destroy the calcium ion-alginate cross-linking network and trigger the whole piece peeling;

[0016] The mass fraction ratio of the A component, the B component, the C component and the D component ranges from (5-10):100:(1.5-3.5):(3-6).

[0017] In a preferred embodiment, the shellac coating is hot gasified at a brazing or welding temperature of 200 degrees Celsius or above, and does not need to be polished.

[0018] In a preferred embodiment, the shellac coating is of a coating type structure, and forms a coating layer with a film thickness of 500-1000 microns after one coating.

[0019] In a preferred embodiment, the shellac coating does not contain formaldehyde substances and heavy metal elements, and does not release volatile substances or solid particles that pose a toxic risk to welders during the hot gasification, peeling triggering or polishing process of the shellac coating.

[0020] In a preferred embodiment, a preparation process of a shellac coating for pipeline welding comprises:

[0021] S1, preparing the A component by weighing according to the proportion, putting the matting powder, glutinous rice powder, biochar powder and ethanol into a beaker in sequence, standing for 3 hours, and then stirring for 5-10 minutes to obtain the A component;

[0022] S2, preparing the B component by weighing according to the proportion, putting the shellac sheet and ethanol into a beaker in sequence, standing for 3 hours, and then stirring for 5-10 minutes to obtain the B component;

[0023] S3, mixing the A component and the B component according to the mass fraction ratio (5-10):100, low-speed stirring for 5 minutes, and then defoaming for 30 minutes to obtain a shellac coating mother liquor for brazing or welding;

[0024] S4, using a doctor blade coating or spraying method to coat the shellac coating mother liquor on the inner and outer surfaces of the pipeline bevel, manually controlling the thickness of the coating film layer, so that the film layer thickness reaches 500-1000 microns, and the film layer is surface dried after standing at room temperature for 10-20 minutes;

[0025] S5, storing for ≥48 hours without rusting in a humid or outdoor environment; and not needing to be mechanically polished before welding.

[0026] In a preferred embodiment, a preparation process of a shellac coating for pipeline welding comprises:

[0027] S1, prepare the A component by weighing according to the proportion, put the matt powder, glutinous rice powder and biochar powder into a beaker in sequence, stand for 3 hours, then stir for 5-10 minutes, and obtain the A component;

[0028] S2, prepare the B component by weighing according to the proportion, put the shellac sheet and ethanol into a beaker in sequence, stand for 3 hours, then stir for 5-10 minutes, and obtain the B component;

[0029] S3, prepare the C component by weighing according to the proportion, take 1.5-3.5 parts of sodium alginate, and put it into a mixed solution composed of 70% ethanol and 30% deionized water, and magnetically stir for 15 minutes until swelling and dissolution, and obtain a transparent C component solution;

[0030] S4, mix the A component, the B component and the C component in sequence according to the mass fraction ratio (5-10):100:(1.5-3.5), stir at low speed for 5 minutes, and then defoam for 30 minutes, and obtain a shellac coating mother liquor for brazing or welding;

[0031] S5, adopt a scraping or spraying mode to coat the shellac coating mother liquor on the inner and outer surfaces of a pipe bevel, manually control the thickness of the coating film, so that the film thickness reaches 500-1000 microns, and the film is surface-dried after standing at room temperature for 10-20 minutes;

[0032] S6, prepare a calcium chloride solution, dissolve calcium chloride in a mixed solvent composed of 30% deionized water and 70% ethanol, and prepare a solution with a concentration of 0.3 moles per liter, and obtain the calcium chloride solution; spray 1-2 parts of the calcium chloride solution on the surface of the shellac coating in a misting mode, and stand for 1 minute, so that calcium ions and sodium alginate form a calcium ion-alginate crosslinking network in the film, and the adhesion and weather resistance of the film are increased;

[0033] S7, the coating after the crosslinking of calcium ions and sodium alginate does not rust when stored in a humid or outdoor environment for ≥48 hours; and mechanical polishing is not required before welding;

[0034] S8, dissolve EDTA in a mixed solvent containing ethanol and deionized water (the volume ratio of ethanol to deionized water is 6:4 or 7:3 or 8:2) at a concentration of 0.2-0.5 moles per liter, and obtain an EDTA solution after stirring;

[0035] S9, spray 2-4 parts of the EDTA solution in a misting mode 30 seconds before welding; EDTA chelates calcium ions, thereby destroying the calcium ion-alginate crosslinking network, causing the film layer to loosen as a whole, and thus achieving peelability or natural sliding; the residual liquid and the coating gasify at a welding temperature of ≥200 degrees Celsius, and mechanical polishing is not required.

[0036] Technical effects and advantages of the present application:

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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

[0042] Figure 1 The present invention is a flow chart for preparing the shellac coating composed of component A and component B.

[0043] 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

[0044] 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.

[0045] Refer to the instruction manual Figure 1The shellac coating for pipe welding and the preparation process thereof comprise the following embodiments and comparative examples.

[0046] Example 1 (minimum value of A component and maximum value of B component):

[0047] Take 5 parts of cancellous bone powder, 15 parts of waxy rice powder, 5 parts of biochar powder and 45 parts of ethanol, and add them into a beaker in sequence. After standing for 3 hours, stir for 5 minutes to obtain the A component. Take 45 parts of shellac tablets and 80 parts of ethanol, and add them into a beaker in sequence. After standing for 3 hours, stir for 10 minutes to obtain the B component. Mix the A component and the B component according to the mass fraction ratio of 5:100, stir for 5 minutes and then deaerate for 30 minutes to obtain the shellac coating mother liquor for brazing or welding.

[0048] Example 2 (maximum value of A component and minimum value of B component):

[0049] Take 15 parts of cancellous bone powder, 25 parts of waxy rice powder, 15 parts of biochar powder and 75 parts of ethanol, and add them into a beaker in sequence. After standing for 3 hours, stir for 10 minutes to obtain the A component. Take 20 parts of shellac tablets and 55 parts of ethanol, and add them into a beaker in sequence. After standing for 3 hours, stir for 5 minutes to obtain the B component. Mix the A component and the B component according to the mass fraction ratio of 10:100, stir for 5 minutes and then deaerate for 30 minutes to obtain the shellac coating mother liquor for brazing or welding.

[0050] Comparative Example 1 (replace waxy rice powder in A component with talc powder):

[0051] Comparative Example 1 and Example 1 have the same amount and ratio, only the 15 parts of waxy rice powder in the A component is replaced with 15 parts of talc powder. The remaining components and processing methods are consistent. The shellac coating prepared shows that the film adhesion is insufficient, and the residual debris phenomenon is obvious after peeling. The effect of no polishing in the example is not achieved.

[0052] Comparative Example 2 (replace shellac tablets in B component with shellac liquid):

[0053] This comparative example and Example 2 have the same amount and ratio, only the 20 parts of shellac tablets in the B component are replaced with commercially available shellac liquid (containing more than 10% shellac solid content). The ethanol remains 55 parts. The final coating film forming process has shrinkage phenomenon, forming bubble fault, and cannot achieve the performance of continuous and uniform film layer in Example 1 and Example 2.

[0054] Comparative Example 3 (replace biochar powder in A component with black iron oxide):

[0055] The comparative example is consistent with example 1, only replace the biochar powder 5 parts in A component with iron oxide black 5 parts, keep the rest components consistent; the obtained shellac coating has odor and obvious carbonization marks when welding hot gasification, and the effect of clean surface after hot gasification in the example is not achieved.

[0056] Referring to the description attached Figure 2 In the shellac coating for pipeline welding and the preparation process thereof, the shellac coating composed of A component, B component, C component and D component includes the following examples and comparative examples:

[0057] Example 3 (EDTA solvent volume ratio 6:4, component minimum combination):

[0058] Take 5 parts of cancellous powder, 15 parts of waxy rice powder, 5 parts of biochar powder and 45 parts of ethanol, add them into the beaker in turn, stand for 3 hours and then stir for 5 minutes to obtain A component; take 20 parts of shellac tablets and 55 parts of ethanol, add them into the beaker in turn, stand for 3 hours and then stir for 5 minutes to obtain B component; take 1.5 parts of sodium alginate, put it into the mixed solvent with a volume ratio of 70%:30% of ethanol and deionized water, and magnetically stir for 15 minutes until it is swelled and dissolved to obtain a transparent C component solution; mix A, B and C components in the mass fraction ratio of 5:100:1.5 in turn, stir for 5 minutes and then deaerate for 30 minutes to obtain a shellac coating mother liquor; prepare a calcium chloride solution (0.3 mol / L) and spray it on the surface of the film layer in an amount of 1 part, crosslink after standing for 1 minute; then dissolve EDTA in the mixed solvent with a volume ratio of 6:4 of ethanol and deionized water at a concentration of 0.2 mol / L, stir uniformly, and spray it in an amount of 2 parts 30 seconds before welding operation to trigger the whole film layer to peel off;

[0059] Example 4 (EDTA solvent volume ratio 7:3, component medium combination):

[0060] Take 10 parts of cancellous powder, 20 parts of waxy rice powder, 10 parts of biochar powder and 60 parts of ethanol, add them into the beaker in turn, stand for 3 hours and then stir for 7 minutes to obtain A component; take 32.5 parts of shellac tablets and 67.5 parts of ethanol, add them into the beaker in turn, stand for 3 hours and then stir for 7 minutes to obtain B component; take 2.5 parts of sodium alginate, put it into the mixed solution with a volume ratio of 70%:30% of ethanol and deionized water, and magnetically stir for 15 minutes until it is swelled and dissolved to obtain a C component solution; mix A, B and C components in the mass fraction ratio of 7:100:2.5 in turn, stir for 5 minutes and then deaerate for 30 minutes to obtain a mother liquor; prepare a calcium chloride solution with a concentration of 0.3 mol / L, spray it on the surface of the film layer in an amount of 1.5 parts to form a crosslinked network; dissolve EDTA in the mixed solvent with a volume ratio of 7:3 of ethanol and deionized water at a concentration of 0.35 mol / L, and spray 3 parts 30 seconds before welding to destroy the crosslinked structure and realize the peelability of the film layer.

[0061] Example 5 (EDTA solvent volume ratio 8:2, component maximum combination):

[0062] Take 15 parts of light powder, 25 parts of glutinous rice powder, 15 parts of charcoal powder, and 75 parts of ethanol, add them into a beaker in turn, stand for 3 hours, and then stir for 10 minutes to obtain component A; take 45 parts of shellac sheet and 80 parts of ethanol, add them into a beaker in turn, stand for 3 hours, and then stir for 10 minutes to obtain component B; take 3.5 parts of sodium alginate, put it into a mixed solution with a volume ratio of 80%:20% of ethanol and deionized water, and magnetically stir for 15 minutes until it is completely dissolved to obtain component C solution; mix components A, B, and C according to a mass fraction ratio of 10:100:3.5, stir for 5 minutes, and then deaerate for 30 minutes to obtain a mother liquor; prepare a calcium chloride solution (0.3 mol / L), and spray 2 parts of it for crosslinking; dissolve EDTA in a mixed solvent with a volume ratio of 8:2 at a concentration of 0.5 mol / L, stir to form a solution, and spray 4 parts of it 30 seconds before welding to trigger complete peeling of the coating and complete gasification of the residue.

[0063] Comparative Example 4 (reference to Example 3, replace sodium alginate with sodium carboxymethyl cellulose):

[0064] This comparative example is based on Example 3, replacing 1.5 parts of sodium alginate in component C with the same amount of sodium carboxymethyl cellulose, and keeping the remaining components and use process unchanged; the prepared paint can form a film on the surface, but after calcium chloride treatment, no effective crosslinking structure is formed, resulting in the film layer before welding, and in some cases, the edge is raised and the peeling is not neat.

[0065] Comparative Example 5 (reference to Example 4, replace EDTA with citric acid)

[0066] This comparative example is based on Example 4, replacing the EDTA solution with an equimolar concentration of citric acid solution, and the other components and treatment steps are consistent; after spraying before welding, the film layer is loose, but it cannot achieve the effect of peeling off in one piece, and some areas still need to be mechanically polished or removed.

[0067] Comparative Example 6 (reference to Example 5, replace calcium chloride with sodium sulfate)

[0068] This comparative example is based on Example 5, replacing 1.5 parts of calcium chloride solution with an equimolar concentration of sodium sulfate solution, and the remaining components and operation process are unchanged; no effective crosslinking is found after film layer treatment, and the state is that the film layer has poor adhesion and fragile structure, which cannot meet the requirements of wet storage and complete peeling during welding.

[0069]

[0070]

[0071]

[0072]

[0073] It should be noted that the thermal gasification residual rate in Table 1 is calculated by collecting the mass ratio of the pyrolysis residue after the standard area coating film is subjected to welding heat treatment to the original dry film mass, that is, thermal gasification residual rate = (residue mass after heat treatment ÷ dry film mass before heat treatment) x 100%;

[0074] The thermal gasification residual rate is used as an index to measure the gasification completeness of the coating at a welding temperature of ≥200 degrees Celsius. The lower the residual rate, the more complete the thermal decomposition, and the cleaner the surface after welding without the need for grinding.

[0075] The film layer adhesion is measured by vertically peeling the film layer in the standard test area using the pull-off method (tensile bond strength test), and recording the maximum tensile stress value when the film layer and the substrate interface separate, that is, film layer adhesion = maximum peeling force (Newton) ÷ effective test area (square meter), with the unit being MPa.

[0076] The film layer adhesion is used to reflect the degree of adhesion between the film layer and the surface of the pipeline. The higher the value, the more stable the coating and the less likely it is to fall off.

[0077] In Table 2, the film layer uniformity coefficient of variation is calculated by measuring the thickness of the film layer (unit: μm) at multiple measurement points in a fixed area after film formation, and calculating the ratio of the standard deviation to the average of these thickness data, with the formula being: film layer uniformity coefficient of variation (%) = (film layer thickness standard deviation ÷ average thickness) x 100%. The film layer uniformity coefficient of variation is used to evaluate the consistency of the distribution of the film layer thickness. The smaller the coefficient of variation, the more uniform the film layer.

[0078] The residual film rate is calculated based on the mass proportion of the residual amount of the film layer after thermal gasification or peeling. Specifically, the film layer is heated to a set welding temperature (200°C in Table 2) in a specified area, the mass of the residue is measured and compared with the initial dry film mass, and the calculation formula is: residual film rate (%) = (residual mass after heat treatment ÷ initial dry film mass) x 100%. This value reflects whether the coating can fully gasify or peel during the welding process, and the lower the residual film rate, the better for weld cleaning.

[0079] In Table 3, the peeling area proportion of the fixed area is calculated by spraying an equal amount of EDTA solution in the standard size (5 cm x 5 cm) film layer area, visually identifying and image analyzing after a standard time, and calculating the area ratio of the peeled area to the original film layer area, expressed in percentage, to measure the peelable effect and response integrity of the film layer, and reflect the damage efficiency of the cross-linked network after EDTA triggering.

[0080] The film layer integrity score is based on the morphology of the film layer after peeling, mainly investigating whether the film layer is continuously peeled off in blocks, whether it is fragmented, cracked or has residual traces. Table 3 is the average value obtained by 7 people with construction experience after independent evaluation according to individual standards, with a full score of 10 points. The higher the score, the more complete, regular and less residual the film layer is in the peeling process.

[0081] In Table 4, the peeling trigger response time is determined by a stopwatch timing method. Timing starts immediately after spraying the EDTA solution. When the film layer edge shows visible signs of warping, bulging or overall loosening, it is determined that the peeling starts, and the timing stops. This time is the peeling trigger response time, with units of seconds. If there is no obvious peeling phenomenon after 30 seconds, it is recorded as greater than 30, indicating that the peeling response fails or is not significant.

[0082] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

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. A shellac coating for pipe welding, including a shellac coating for brazing or welding, wherein the shellac coating comprises a shellac coating mother solution and a D component, and is characterized in that: The shellac coating mother liquor is composed of component A, component B and component C, wherein 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; wherein 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; component C of the shellac coating mother liquor is used to construct a cross-linked network in the film layer, wherein component C is composed of the following raw materials in the following mass ratios: 1.5-3.5 parts of sodium alginate; component D is used to trigger the peeling of the shellac coating mother liquor, wherein component D includes the following raw materials in the following mass ratios: The invention relates to a method for preparing a shellac coating having a raw material ratio of 1-2 parts of calcium chloride solution and 2-4 parts of EDTA solution. After applying a shellac coating mother solution, the D component is sprayed separately in a sequential order, with the calcium chloride solution sprayed first and the EDTA solution sprayed later. The calcium chloride solution is used to undergo an ion crosslinking reaction with sodium alginate to form a calcium ion-alginate crosslinking network, and the EDTA solution is sprayed before brazing or welding to destroy the calcium ion-alginate crosslinking network and trigger whole-piece peeling. The mass ratio of the A component, the B component, the C component and the D component 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 will vaporize at a welding temperature of ≥200 degrees Celsius, and no mechanical grinding is required.

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