A new process for surface treatment and painting of steel structure

-A new process for surface treatment and coating of steel structure Zhu Yaqin (Sifang Locomotive and Rolling Stock Factory Measuring and Physical Testing Center, Qingdao 266031, Shandong) test process, the application shows that the process can fully meet the design and process requirements 1 Introduction The second big speed increase, the requirements for the design and production level of railway locomotives are also getting higher and higher. The products have been replaced by the top priorities of the railway locomotive and vehicle manufacturing plants. The design of the steel structure for locomotives is increasingly using high quality and high performance The surface treatment and coating process of steel plates and traditional steel structures can no longer meet the requirements of production development, and there is an urgent need to find a new surface treatment process that meets the design and process requirements.

2 The main points of surface treatment and coating process Before the steel structure is painted, in order to make the coating and the substrate have good adhesion performance, the surface of the steel to be painted and the surrounding area must be pretreated. The surface must be clean and not allowed There are dust, oil stains, water stains or other dirt and loose oxides. Second, when spraying, the combination of the coating and the substrate is generally mechanically combined. Therefore, the coating and the substrate should be as much as possible without affecting the quality of the spray. The large contact area of ​​the ground and the appropriate roughness help to improve the adhesion of the coating film and extend the life of the coating. Relevant information introduction: Alkyd paint is used for manual surface derusting, the service life of its coating is 2 ~ Ba, and the service life of sand blasting reaches Sa2.5 level can be 10a. The quality of steel surface pretreatment directly affects the coating engineering table The quality and service life of the coating, and the cleanliness and roughness of the surface treatment of the steel structure directly affect the service life of the coating.

In order to maximize the contact area between the coating and the substrate, mechanical roughening and sandblasting roughening methods are generally used. Mechanical roughening includes shot peening, manual grinding and polishing. Sandblasting roughening is to choose dry and angular yellow sand or quartz sand with a diameter of 1 ~ 2mm, use compressed air to spray sand particles from the nozzle of the sandblasting gun to the steel surface at high speed, and use the impact and friction effect of the sand particles to spray the steel surface The process of removing scale, rust, paint film grease and other contaminants, and obtaining a surface with a certain roughness. In general, the higher the steel grade, the higher the surface treatment requirements. The surface of the steel structure after sandblasting treatment, surface area It is about 90 times the surface area of ​​steel without sandblasting, which greatly increases the contact area between the coating and the substrate. This has a great influence on the adhesion of some coatings, especially inorganic zinc-rich coatings. 3 original process Prior to the existing problem, the surface treatment method for steel materials (including plates and profiles) for the steel structure of locomotives and vehicles was one of which was to simply remove rust, after cold and hot processing, group weld into a steel structure, and then to the steel Spray primer on the structure surface. This process method has been proved to be inferior in anti-corrosion effect by field application of various vehicle models. Another process method is that the steel surface is first chemically treated (generally phosphating), then directly sprayed with primer, and then assembled into a steel structure. The disadvantage is that there is often a problem of poor compatibility between the primer and the phosphating film, which leads to the phenomenon of reduced adhesion of the primer. In recent years, a process has also been adopted. The steel is first sandblasted, and then the primer is sprayed directly. There is also a problem of poor corrosion protection, which even affects the next process. 4 The determination of a new process plan is aimed at the problems of the original process. In order to meet the requirements of process design, taking into account the inheritance and continuity of the process, based on the above theory, Through the test, it was decided to use a combination of sandblasting and precoat primer surface treatment and painting process.

4.1 Test materials (1) Substrate model: choose the high-quality chromium-rich steel plate used in recent years, the Bachelor of Engineering, majoring in applied chemistry, currently engaged in applied chemistry, a new process for surface treatment and coating of steel structures 4003), specifications There are 24 claws for 150 claws, 80 claws and 1.5 claws.

â‘ Pre-coating primer: use magnet epoxy ester anti-rust paint. Because the pre-treated steel needs to be welded, cut and polished when assembled into a steel structure, the surface of its paint layer inevitably has a lot of scratches and Defects such as shedding, so choose the current high temperature resistant, low heat influence, strong anticorrosive magnet epoxy ester anticorrosive paint commonly used in the steel structure of locomotive vehicles as the precoat primer on the steel surface.

â‘¡Primer: After welding the steel structure with phosphoric acid polyurethane primer, in order to repair its surface defects, and also to prepare for the next step of putty or topcoat, it is generally necessary to spray the steel structure one more time. The paint has better compatibility with the putty and top coat used in the next process. After production verification, the zinc phosphate polyurethane primer can fully meet this requirement 4.2 Test process 4.2.1 Surface treatment of the sample plate The sample plate is divided into two groups of AB, 12 pieces each, of which group A is sandblasted and group B is not sprayed For sand treatment, only use 240 grit cloth to clean the surface of the two groups of samples with organic solvent, blow dry with cold air, and put it into the dryer.

4.2.2 Preparation of model paint film Group 6 samples of Group AB were taken and sprayed with 1 zinc phosphate polyurethane primer, and dried for use; then another 6 samples of Group AB were taken and sprayed with 1 layer of magnet epoxy ester antirust paint, After drying for 48h, spray another layer of zinc phosphate polyurethane primer, dry and toss the thickness of 4.2.3 performance test, and then do the following performance test: â‘  Adhesion test: use adhesion tester to test, according to GB / T 2 primers The test board is used for the process matching test.

Sample preparation process paint film thickness / 4m adhesion level salt water resistance process supporting sanding, sanding, spraying 1 zinc phosphate polyurethane primer 5d blistering, non-rust sandblasting, spraying 1 zinc phosphate polyurethane primer 5d blistering , No rust sanding cloth polishing, spray polyurethane primer 1 magnet epoxy ester anti-rust paint and 1 zinc phosphate 8d blistering, non-rust matching good sandblasting treatment, spray polyurethane primer 1 magnet epoxy ester anti-rust Rust paint and 1 zinc phosphate foaming for 10d, good rust-free matching (1) Adhesion The corrosion resistance of the plate after coating depends on the size of the coating film adhesion, the greater the adhesion, the stronger the corrosion resistance of the plate. 1 It can be seen that the thickness of the paint film and the blasted model are thicker than those without blasting; the adhesion and the blasted model are not smaller than those without blasting.

(2) Salt water resistance Salt water resistance refers to immersing the sample in salt water, soaking at a specified temperature, and observing the degree of erosion. Salt water resistance is an important indicator to measure the anticorrosion ability of paint films and materials. It can be seen from Table 1: Similarly, after sandblasting and spraying primer, the model with sprayed precoat primer is more corrosion resistant than the model without sprayed precoat primer Strong; also spray 2 primers, the anti-corrosion ability of the template after sandblasting is stronger than that without sandblasting (3) Process matching paint The process matching means that when the two paints are applied together, the layer to layer Whether the phenomenon of "bite bottom", blister, wrinkle, etc. does not appear between these times does not appear, which means that the matching is good. It can be seen from Table 1: No matter whether it is a sandblasted model or a model without sandblasting, the compatibility of the magnet epoxy epoxy ester anti-rust paint of different paint types with the zinc phosphate polyurethane primer is good, without blistering, iron Epoxy ester anti-rust paint is used as a pre-treated steel. After assembly and welding into a steel structure, a layer of zinc phosphate polyurethane primer can be sprayed. 5 Conclusions After testing, a new process for surface treatment and coating of the steel structure was finally determined, namely The surface treatment process using a combination of sandblasting and pre-coating primer: the steel is firstly sand-blasted with large roughness, and then sprayed with a magnet epoxy ester anti-rust paint as a pre-coat primer to prevent the steel from being sand-blasted After the steel structure is assembled and welded, a zinc phosphate polyurethane primer is sprayed.

This process has been successfully applied to several models such as Sri Lanka EMU since 1999, without any quality problems, indicating that the process fully complies with the design and process requirements and achieved the expected results

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