A gravimetric method for measuring the mass per unit area of ​​phosphorus-free conversion coating on steel surface

Abstract

The present invention discloses a weight measurement method for the mass of a unit area of ​​a phosphorus-free conversion film on a steel surface. The method is as follows: first, 40-50 parts of a stripping agent, 5-10 parts of a accelerator, 1-5 parts of a corrosion inhibitor, 0.1-0.5 parts of a rust inhibitor, 0.1-0.2 parts of a wetting agent, and 34-54 parts of deionized water are mixed and stirred to form a chemical stripping solution. Secondly, a steel specimen of a fixed size is prepared, and the steel specimen is subjected to a phosphorus-free conversion treatment and weighed after drying. After obtaining the initial mass, the steel specimen is immersed in the stripping solution, taken out after stripping, rinsed with deionized water, dried and weighed again, and the mass after stripping is obtained. The mass of the phosphorus-free conversion film per unit area can be calculated based on the mass difference before and after and the total surface area of ​​the steel specimen. The chemical stripping solution of the present invention has a fast stripping speed and almost no corrosion to the steel substrate during the stripping process. The weight method of the present invention measures the mass of the unit area of ​​the phosphorus-free conversion film on the steel surface with high speed and high efficiency.

Description

Technical Field

[0001] The invention belongs to the field of chemical materials and relates to a weight measurement method for a film mass per unit area, in particular to a weight measurement method for a film mass per unit area of ​​a phosphorus-free conversion film on a steel surface. Background Art

[0002] New phosphorus-free conversion coating pretreatment processes primarily utilize zirconium salts and zirconium salt-silane composites. The resulting conversion coatings are collectively referred to as phosphorus-free conversion coatings. These processes offer advantages such as energy conservation, environmental friendliness, low emissions, low costs, minimal sedimentation, low overall costs, and suitability for multi-metal co-processing. Furthermore, they offer compact processes and minimal equipment investment, further aligning with national policy guidelines. Driven by the dual pressures of environmental protection and cost, an increasing number of automobile, auto parts, and household appliance manufacturers are opting for phosphorus-free conversion pretreatment processes in conjunction with cathodic electrophoretic coating, powder coating, and other processes.

[0003] The per-unit-area quality of phosphorus-free conversion membranes significantly impacts the performance of subsequent supporting products, necessitating frequent monitoring. Therefore, establishing an accurate, rapid, and simple measurement method is crucial. First, because the phosphorus-free conversion reaction, unlike the phosphating reaction, does not terminate automatically, the per-unit-area quality of the membrane increases with increasing conversion time. Different treatment times can result in varying per-unit-area quality. Second, the phosphorus-free conversion bath solution does not require heating; room temperature is sufficient. Temperature fluctuations, which are low in winter and high in summer, affect the rate of phosphorus-free conversion film formation and significantly impact the per-unit-area quality of the membrane. Third, per-unit-area quality fluctuates due to variations in the bath solution state, film-forming ion concentration, and pH. A phosphorus-free conversion membrane that is too thin will not provide adequate protection, while a membrane that is too thick will result in a loose film layer, compromising subsequent support. Therefore, monitoring the per-unit-area quality of phosphorus-free conversion membranes is crucial during the production process.

[0004] Currently, the primary method for measuring the per-unit-area mass of phosphorus-free conversion coatings on steel surfaces relies on handheld or desktop XRF (X-ray fluorescence) instrumentation. While this method offers high accuracy, the instrumentation is expensive. The cheapest portable XRF on the market costs 300,000 yuan, while a more comprehensive model can cost nearly 500,000 yuan, and a desktop model can reach over a million yuan. Furthermore, ongoing maintenance and operating fees are significant, placing these costs far beyond the reach of typical manufacturers. Furthermore, XRF requires the creation of a working curve, and there is currently no unified industry standard for calibration, making the implementation process complex. Submitting samples for testing to a specialized laboratory each time can result in significant cumulative costs, and results are often inefficient and unavailable. XRF instrumentation has significant limitations for measuring the per-unit-area mass of phosphorus-free conversion coatings on steel surfaces.

[0005] Therefore, it is urgent to develop a low-cost, fast, efficient, simple and feasible method for measuring the mass of the phosphorus-free conversion film per unit area on the steel surface. Summary of the Invention

[0006] The present invention aims to provide a method for measuring the mass per unit area of ​​a phosphorus-free conversion coating on steel surfaces. This method uses a chemical stripping solution to rapidly remove the phosphorus-free conversion coating from the steel surface without corrosively affecting the base metal during the stripping process. This method enables rapid and accurate gravimetric measurement of the mass per unit area of ​​the phosphorus-free conversion coating on the steel surface. This method addresses the difficulty faced by manufacturers using novel phosphorus-free conversion coating pretreatment processes in measuring the mass per unit area of ​​the phosphorus-free conversion coating on steel surfaces. It provides rapid and effective data support for monitoring the quality of the phosphorus-free conversion coating on workpiece surfaces and for controlling the quality of the entire coating after subsequent coating.

[0007] A method for measuring the mass per unit area of ​​a phosphorus-free conversion film on a steel surface by weight, comprising the following steps:

[0008] (1) Prepare steel test pieces of fixed size and complete the phosphorus-free conversion treatment; or cut the steel workpiece with the phosphorus-free conversion coating into test pieces of fixed size;

[0009] (2) Dry the specimen after the phosphorus conversion treatment and weigh it with an analytical balance to obtain the mass m1 of the specimen with and without phosphorus conversion coating;

[0010] (3) Immerse the specimen in a chemical stripping solution, strip, rinse, dry, and weigh using an analytical balance to obtain the mass m2 of the specimen after stripping. The chemical stripping solution consists of the following mass components:

[0011] 40-50 parts of film stripping agent

[0012] 5-10 parts of accelerator

[0013] 1-5 parts of corrosion inhibitor

[0014] 0.1-0.5 parts of rust inhibitor

[0015] Wetting agent 0.1-0.2 parts

[0016] 34-54 parts deionized water

[0017] (4) The mass per unit area of ​​phosphorus-free conversion film is calculated as follows:

[0018] m A = (m1 - m2) / A

[0019] The above formula is converted into the following order of magnitude according to the selected units:

[0020] m A =1000000×(m1-m2) / A

[0021] Where: m A ——The mass per unit area of ​​phosphorus-free conversion film, in mg / m 2

[0022] m1——mass of the specimen with or without phosphorus conversion coating, in mg

[0023] m2——mass of the specimen after stripping, in mg

[0024] A——total surface area of ​​the specimen, in mm 2

[0025] Preferably, the steel test piece prepared in step (1) has a size of 100 mm × 150 mm × (0.3-1.0) mm.

[0026] Preferably, an analytical balance is used for weighing in step (2) and step (3), and the weighing accuracy is not greater than 0.1 mg.

[0027] Preferably, in step (2) and step (3), weighing is performed alternately using an analytical balance and drying in a dryer until a constant mass is achieved.

[0028] Preferably, in step (2) and step (3), when weighing, the standard for achieving constant mass is that the difference between two adjacent weighings is less than 0.1 mg.

[0029] Preferably, in step (3), the test piece is immersed in a chemical stripping solution and maintained at 45° C. for 2 minutes.

[0030] Preferably, in step (3), when cleaning, first use clean running water for cleaning, and then use deionized water for rinsing.

[0031] Preferably, the main stripping agent of the chemical stripping solution has a free acidity of 80 points to 120 points, a pH value <1, and a fluoride ion content of 1000 ppm to 1400 ppm.

[0032] Preferably, the main film stripping agent is prepared by mixing nitric acid, chromic anhydride, fluoride and organic acid.

[0033] Preferably, the organic acid has a large dissociation constant in water and can dissolve metal oxides.

[0034] Preferably, the fluoride is any one or more of hydrogen fluoride, ammonium bifluoride, and ammonium fluoride.

[0035] Preferably, the corrosion inhibitor is a compound of polynaphthalene sulfonate and water-soluble phosphate ester.

[0036] Preferably, the corrosion inhibitor is polynaphthalene sulfonate and water-soluble phosphate in a mass ratio of 1: (0.5-1.5).

[0037] Preferably, the accelerator is a compound of organic polyphosphonic acid and polycarboxylate.

[0038] Preferably, the mass ratio of the organic polyphosphonic acid to the polycarboxylate is (1.5-2.5):1.

[0039] Preferably, the rust inhibitor is a reactant of a medium- to long-chain dibasic acid anhydride and an amine, and has a pH value of 8-9.

[0040] Preferably, the wetting agent is a non-ionic surfactant with strong permeability.

[0041] Preferably, the wetting agent is fatty alcohol polyoxyethylene ether with an HLB value of 11.5-12.5.

[0042] The present invention also provides a method for preparing any of the above-mentioned chemical stripping liquids, which is obtained by uniformly mixing a stripping main agent, an accelerator, a corrosion inhibitor, a rust inhibitor, a wetting agent and deionized water in proportion.

[0043] The present invention also protects the use of any of the above-mentioned chemical stripping liquids for stripping the phosphorus-free conversion film on the surface of steel to measure the film mass per unit area.

[0044] The present invention has the following advantages and beneficial effects:

[0045] The main component of the phosphorus-free conversion film on the steel surface is zirconium oxide, which is difficult to dissolve in both acid and alkali. Traditional chemical stripping solutions, whether alkaline or acidic, are difficult to remove the phosphorus-free conversion film on the steel surface. It takes more than 10 hours to completely remove the phosphorus-free conversion film on the steel surface when the stripping solution temperature is 90℃-100℃. The phosphorus-free conversion film on the steel surface has a light weight, usually 100 mg / m 2 Slight corrosion of the base metal during the film stripping process can cause large measurement errors. For example, for a specimen with a size of 100 mm (width) × 150 mm (length) × (0.3-1) mm (thickness) and a phosphorus-free conversion coating weight of 100 mg / m 2 For a specimen with a thickness of 0.5 mg, assuming the corrosion of the base metal during the stripping process is 0.5 mg, the resulting measurement error exceeds 15%. The technical difficulty of this invention is that the chemical stripping solution can remove the phosphorus-free conversion film on the steel surface without corroding the base metal.

[0046] The organic acid selected in the present invention is a polyhydroxy acid, which contains both carboxyl groups and a relatively high number of hydroxyl groups. It has a large dissociation constant in water, provides both H+ and hydroxyl groups, and synergistically acts with the inorganic acid to dissolve the zirconium oxide in the phosphorus-free conversion film and rapidly complexes with the zirconium ions dissolved into the solution, rapidly reducing the zirconium ion concentration at the interface and facilitating further film stripping. The added fluoride dissociates to produce fluoride ions, which have a small radius and strong permeability, accelerating the peeling of the phosphorus-free conversion film on the steel surface. The fluoride ions can also complex with zirconium ions. Organic polyphosphonic acids have both phosphonic acid and carboxylic acid groups. After the addition of polycarboxylates, they combine the strong chelation of organic phosphonic acids with the high dispersibility of polymers, further strengthening the chelation of zirconium ions entering the solution and greatly promoting the dissolution and peeling of the phosphorus-free conversion film on the steel surface. Polynaphthalene sulfonates and water-soluble phosphates, when compounded, have extremely strong corrosion inhibition capabilities for steel. Addition prevents corrosion and dissolution of the steel substrate during film stripping, reducing measurement errors. Adding reactants of medium- and long-chain dibasic acid anhydrides and amines prevents rusting during the water washing process after film stripping and reduces measurement errors. The added fatty alcohol polyoxyethylene ether has low surface tension and good interfacial wettability, which is beneficial to the uniform dissolution of the phosphorus-free conversion film. Its permeability is also very strong, which helps to peel off the phosphorus-free conversion film on the steel surface. The chemical stripping liquid of the present invention was used to conduct parallel stripping tests on bare steel plates that were degreased but not treated for phosphorus-free conversion. The quality of the bare steel plates did not change before and after stripping, confirming that the added corrosion inhibitor effectively prevented corrosion of the steel substrate during the stripping process. At the same time, the unit area film mass of the phosphorus-free conversion film was measured for steel test plates of different models using the XRF method and the present method respectively. The unit area film mass of the phosphorus-free conversion film measured by the two methods for the same model of steel test plates differed by <5%, which met industry requirements. The present invention achieves the removal of the phosphorus-free conversion film on the steel surface in a relatively short time (2 minutes to 5 minutes) at a relatively low temperature (25°C to 45°C) by selecting and compounding an organic acid and an inorganic acid, and supplemented with a metal ion chelating agent with strong chelating properties. At the same time, the selection and combination of a high-efficiency corrosion inhibitor solves the problem of base metal corrosion during the film removal process. The present invention achieves the gravimetric determination of the mass per unit area of ​​the phosphorus-free conversion film on the steel surface. The measurement process is convenient and efficient, the measurement results are accurate, and the measurement cost is low. This solves the problem of measuring the mass per unit area of ​​the phosphorus-free conversion film on the steel surface faced by manufacturers who use a new phosphorus-free conversion coating pretreatment process. DETAILED DESCRIPTION

[0047] In order to better understand the present invention, the following examples are provided to further illustrate the present invention, but the present invention is not limited to the following examples.

[0048] Example 1:

[0049] Chemical stripping solution composition:

[0050] Film stripping agent (free acidity 105 points, pH 0.55, fluoride ion 1180 ppm) 50 parts

[0051] Accelerator (HEDP: A25-CL = 2:1) 7.5 parts

[0052] Corrosion inhibitor (DA-P:H66 =1:1) 5 parts

[0053] Rust inhibitor (reaction product of dodecenylsuccinic anhydride and ethanolamine, pH 8.5) 0.4 parts

[0054] Wetting agent (JFC-6, polyoxyethylene ether) 0.1 part

[0055] 37 parts deionized water

[0056] The preparation method of the chemical stripping solution includes the following steps:

[0057] Step 1: Add an appropriate amount of deionized water to a beaker, add nitric acid, chromic anhydride, fluoride, and organic acid to the beaker in proportion, stir to dissolve, measure and adjust the free acidity, pH value, and fluoride ion, add the remaining deionized water and stir evenly to obtain the main stripping agent;

[0058] Step 2: Add 90 parts of deionized water to a beaker, add 10 parts of HEDP (hydroxyethylidene diphosphonic acid) and A25-CL (sodium polyacrylate) in a mass ratio of 2:1 to the beaker, and stir at room temperature for 15 minutes to obtain the accelerator.

[0059] Step 3: Add 90 parts of deionized water to a beaker, add 10 parts of DA-P (naphthalenesulfonate oligomer) and H66 (phosphate potassium salt) to the beaker in a mass ratio of 1:1, and stir at room temperature for 15 minutes to obtain a corrosion inhibitor.

[0060] Step 4. Add 37 parts of deionized water into a beaker in proportion, and add 50 parts of the main stripping agent obtained in step 1, 7.5 parts of the accelerator obtained in step 2, 5 parts of the corrosion inhibitor obtained in step 3, 0.4 parts of the rust inhibitor, and 0.1 parts of the wetting agent into the beaker in proportion, and stir evenly to obtain a chemical stripping liquid for the phosphorus-free conversion film on the steel surface, which can be used to determine the film mass per unit area of ​​the phosphorus-free conversion film on the steel surface.

[0061] The steps for determining the mass per unit area of ​​the phosphorus-free conversion coating on the steel surface by gravimetric method using the above-mentioned chemical stripping solution are as follows:

[0062] (1) Prepare steel specimens, size: 100 mm × 150.4 mm × 0.7 mm;

[0063] (2) Dry the specimen after phosphorus-free conversion and weigh it using an analytical balance. The mass is m1 = 81232.3 mg.

[0064] (3) XRF was used to measure the mass per unit area of ​​the phosphorus-free conversion film (m) A1 =113 mg / m 2 ;

[0065] (4) Immerse the specimen in a chemical stripping solution and keep it at 45 °C for 2 minutes; take out the specimen and immediately rinse it in clean running water, then rinse it with deionized water; dry it quickly and weigh it again. Repeat this operation until a constant mass is reached (the difference between the two times is <0.1 mg). The mass is m 2= 81228.8mg;

[0066] (5) Calculation

[0067] The mass per unit area of ​​phosphorus-free conversion film measured by gravimetric method:

[0068] m A =1000000×(81232.3-81228.8) / (100×150.4×2) =116 mg / m 2 ;

[0069] Comparison of the unit area film quality of phosphorus-free conversion film measured by two methods:

[0070] (116-113) / 113=2.7%<5%.

[0071] Example 2:

[0072] Chemical stripping solution composition:

[0073] 40 parts of film stripping agent (free acidity 116 points, pH 0.45, fluoride ion 1350 ppm)

[0074] Accelerator (HEDP:A25-CL=1.8:1) 6.3 parts

[0075] Corrosion inhibitor (DA-P:H66 =1.2:1) 3 parts

[0076] 0.3 parts of rust inhibitor (reaction product of dodecenylsuccinic anhydride and ethanolamine, pH 8.5)

[0077] Wetting agent (JFC-6, polyoxyethylene ether) 0.1 part

[0078] 50.3 parts deionized water

[0079] The preparation method of the chemical stripping solution includes the following steps:

[0080] Step 1: Add an appropriate amount of deionized water to a beaker, add nitric acid, chromic anhydride, fluoride, and organic acid to the beaker in proportion, stir to dissolve, measure and adjust the free acidity, pH value, and fluoride ion, add the remaining deionized water and stir evenly to obtain the main stripping agent;

[0081] Step 2: Add 90 parts of deionized water to a beaker, add 10 parts of HEDP (hydroxyethylidene diphosphonic acid) and A25-CL (sodium polyacrylate) to the beaker in a mass ratio of 1.8:1, and stir at room temperature for 15 minutes to obtain the accelerator.

[0082] Step 3: Add 90 parts of deionized water to a beaker, add 10 parts of DA-P (naphthalenesulfonate oligomer) and H66 (phosphate potassium salt) to the beaker at a mass ratio of 1.2:1, and stir at room temperature for 15 minutes to obtain a corrosion inhibitor.

[0083] Step 4. Add 50.3 parts of deionized water into a beaker in proportion, and add 40 parts of the main stripping agent obtained in the step, 6.3 parts of the accelerator obtained in step 2, 3 parts of the corrosion inhibitor obtained in step 3, 0.3 parts of the rust inhibitor, and 0.1 parts of the wetting agent into the beaker in proportion, stir evenly, and obtain a chemical stripping liquid for the phosphorus-free conversion film on the steel surface, which can be used to determine the film mass per unit area of ​​the phosphorus-free conversion film on the steel surface.

[0084] The steps for determining the mass per unit area of ​​the phosphorus-free conversion coating on the steel surface by gravimetric method using the above-mentioned chemical stripping solution are as follows:

[0085] (1) Prepare steel specimens, size: 100 mm × 150 mm × 0.7 mm;

[0086] (2) Dry the specimen after phosphorus-free conversion and weigh it with an analytical balance. The mass is m 1= 80788.0 mg;

[0087] (3) XRF was used to measure the mass per unit area of ​​the phosphorus-free conversion film (m) A1 =112 mg / m 2 ;

[0088] (4) Immerse the specimen in a chemical stripping solution at 45°C for 2 minutes; remove the specimen and immediately rinse it in clean running water, then rinse it with deionized water; dry it quickly and weigh it again. Repeat this operation until a constant mass is reached (the difference between the two measurements is less than 0.1 mg). The mass is m2 = 80784.5 mg.

[0089] (5) Calculation

[0090] The mass per unit area of ​​phosphorus-free conversion film measured by gravimetric method:

[0091] m A=1000000×(80788.0-80784.5) / (100×150×2) =117 mg / m 2 ;

[0092] Comparison of the unit area film quality of phosphorus-free conversion film measured by two methods:

[0093] (117-112) / 112=4.5%<5%.

[0094] Example 3:

[0095] Chemical stripping solution composition:

[0096] 48 parts of film stripping agent (free acidity 92 points, pH 0.6, fluoride ion 1050 ppm)

[0097] Accelerator (HEDP: A25-CL = 2.2:1) 9 parts

[0098] 2 parts of corrosion inhibitor (DA-P:H66=0.8:1)

[0099] 0.2 parts of rust inhibitor (reaction product of dodecenylsuccinic anhydride and ethanolamine, pH 8.5)

[0100] Wetting agent (JFC-6, polyoxyethylene ether) 0.1 part

[0101] 40.7 parts deionized water

[0102] The preparation method of the chemical stripping solution includes the following steps:

[0103] Step 1: Add an appropriate amount of deionized water to a beaker, add nitric acid, chromic anhydride, fluoride, and organic acid to the beaker in proportion, stir to dissolve, measure and adjust the free acidity, pH value, and fluoride ion, add the remaining deionized water and stir evenly to obtain the main stripping agent;

[0104] Step 2: Add 90 parts of deionized water to a beaker, add 10 parts of HEDP (hydroxyethylidene diphosphonic acid) and A25-CL (sodium polyacrylate) to the beaker at a mass ratio of 2.2:1, and stir at room temperature for 15 minutes to obtain the accelerator.

[0105] Step 3: Add 90 parts of deionized water to a beaker, add 10 parts of DA-P (naphthalenesulfonate oligomer) and H66 (phosphate potassium salt) to the beaker at a mass ratio of 0.8:1, and stir at room temperature for 15 minutes to obtain a corrosion inhibitor.

[0106] Step 4. Add 40.7 parts of deionized water into a beaker in proportion, and add 48 parts of the main stripping agent obtained in step 1, 9 parts of the accelerator obtained in step 2, 2 parts of the corrosion inhibitor obtained in step 3, 0.2 parts of the rust inhibitor, and 0.1 parts of the wetting agent into the beaker in proportion, and stir evenly to obtain a chemical stripping liquid for the phosphorus-free conversion film on the steel surface, which can be used to determine the film mass per unit area of ​​the phosphorus-free conversion film on the steel surface.

[0107] The steps for determining the mass per unit area of ​​the phosphorus-free conversion coating on the steel surface by gravimetric method using the above-mentioned chemical stripping solution are as follows:

[0108] (1) Prepare steel specimens, size: 100 mm × 150.3 mm × 0.7 mm;

[0109] (2) Dry the specimen after phosphorus-free conversion and weigh it using an analytical balance. The mass is m1 = 80980.2 mg.

[0110] (3) XRF was used to measure the mass per unit area of ​​the phosphorus-free conversion film (m) A1 =116 mg / m 2 ;

[0111] (4) Immerse the specimen in a chemical stripping solution at 45°C for 2 minutes; remove the specimen and immediately rinse it in clean running water, then rinse it with deionized water; dry it quickly and weigh it again. Repeat this operation until a constant mass is reached (the difference between the two measurements is less than 0.1 mg). The mass is m2 = 80976.8 mg.

[0112] (5) Calculation

[0113] The mass per unit area of ​​phosphorus-free conversion film measured by gravimetric method:

[0114] m A =1000000×(80980.2-80976.8) / (100×150.3×2) =113 mg / m 2 ;

[0115] Comparison of the unit area film quality of phosphorus-free conversion film measured by two methods:

[0116] (116-113) / 113=2.7%<5%.

[0117] The above description is only a preferred embodiment of the present invention, which certainly cannot be used to limit the scope of rights of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and changes can be made without departing from the principles of the present invention. These improvements and changes are also considered to be within the scope of protection of the present invention.

Claims

1. A method for measuring the mass per unit area of ​​a phosphorus-free conversion coating on a steel surface, characterized in that: The steps include: (1) Prepare steel test pieces of fixed size and complete the phosphorus-free conversion treatment; or cut the steel workpiece with the phosphorus-free conversion coating into test pieces of fixed size; (2) Dry the specimen after the phosphorus conversion treatment and weigh it with an analytical balance to obtain the mass m1 of the specimen with and without phosphorus conversion coating; (3) Immerse the specimen in a chemical stripping solution, strip, wash, dry, and weigh using an analytical balance to obtain the mass m2 of the specimen after stripping; Chemical stripping solution consists of the following components by mass: 40-50 parts of film stripping agent 5-10 parts of accelerator 1-5 parts of corrosion inhibitor 0.1-0.5 parts of rust inhibitor Wetting agent 0.1-0.2 parts 34-54 parts deionized water (4) The mass per unit area of ​​phosphorus-free conversion film is calculated as follows: m A =(m1- m2) / A Where: m A ——Non-phosphorus conversion film unit area mass m1——mass of the specimen with or without phosphorus conversion coating m2——mass of the specimen after film removal A - total surface area of ​​the specimen; In step (3), the main agent for film stripping is prepared by compounding nitric acid, chromic anhydride, fluoride and organic acid; The fluoride is any one or more of hydrogen fluoride, ammonium bifluoride, and ammonium fluoride; The corrosion inhibitor is a compound of polynaphthalene sulfonate and water-soluble phosphate; The accelerator is a compound of organic polyphosphonic acid and polycarboxylate; the rust inhibitor is a reactant of medium-long carbon chain dibasic acid anhydride and amine; the wetting agent is a nonionic surfactant; the wetting agent is fatty alcohol polyoxyethylene ether with an HLB value of 11.5-12.

5.

2. The method for measuring the mass per unit area of ​​the phosphorus-free conversion coating on the surface of steel according to claim 1, characterized in that: The main agent of the chemical stripping solution has a free acidity of 80 points to 120 points, a pH value of less than 1, and a fluoride ion content of 1000 ppm to 1400 ppm.

3. The method for measuring the mass per unit area of ​​the phosphorus-free conversion coating on the surface of steel according to claim 1 or 2, characterized in that: In step (2) and step (3), an analytical balance is used for weighing, and the weighing accuracy is not greater than 0.1 mg.

4. The method for measuring the mass per unit area of ​​the phosphorus-free conversion coating on the surface of steel according to claim 1 or 2, characterized in that: In step (3), the immersion temperature of the chemical stripping solution is 25°C-45°C, and the immersion time is 2 minutes-5 minutes.

5. The method for measuring the mass per unit area of ​​the phosphorus-free conversion coating on the surface of steel according to claim 1 or 2, characterized in that: In step (3), when cleaning, first use clean running water to clean, and then use deionized water to rinse.

6. The method for measuring the mass per unit area of ​​the phosphorus-free conversion coating on the surface of steel according to claim 1 or 2, characterized in that: In step (2) and step (3), weighing is performed alternately using an analytical balance and drying in a dryer until a constant mass is achieved.