Cold-coated zinc coating technology.

0 Introduction
Cold-coated zinc content of zinc powder reached more than 95%, single-component system, with a variety of coatings have a good matching, is a long anti-corrosion life anti-rust primer. In particular, CNOOC Changzhou Coatings and Chemical Research Institute Co., Ltd. promulgated HG/T 4845-2015 standards, more domestic paint factories have developed and developed cold-coated zinc coatings, a large number of applications in bridges, power plants, airports, subways and other key projects. However, many projects more or less appear coating ills

. Analysis of the reasons, in addition to the cold zinc resin type and the impact of the coating package, mainly cold zinc coating technology application is not appropriate. In order to better promote and apply cold-coated zinc coating, this paper on the beijing Yongdingmen Bridge steel beam outside the correct supporting program, cold-coated zinc surface treatment, airless spraying and other coating technical features, make a description for peer reference.
1 Anti-corrosion coating design principle
anti-corrosion coating supporting system of the core content of the designer coating design, anti-corrosion coating and coating industry at home and abroad has accumulated rich experience. See ISO 12944-5-2017 "Color paint and varnish - protection of steel structures in the paint system".
includes various coating supporting systems and what kind of corrosive environmental conditions these supporting systems can be applied to, the expected coating protection life, the basic chemical composition and film-forming process of the main film-forming substances of the relevant coatings, and the requirements of each coating for substrate surface treatment.
the Beijing Yongding Gate Bridge application of zinc and AkzoNobel coating matching programs see Table 1, process flow see Figure 1.
table 1 Beijing Yongding Gate Bridge supporting program
f diagram 1 coating process
In this coating process, the most critical is the weld and steel structure surface defect treatment and cold coating zinc pre-painting two key processes. In particular, it should be emphasized that steel components on-site installation, can no longer move open fire, welding, fear of causing damage to the cold-coated zinc layer, repair more trouble.
2 Surface pre-treatment
Before coating, remove the attachment to the surface of the work piece, the resulting oxides to improve surface roughness, improve the adhesion of the work surface and coating, or give the surface a certain corrosion resistance process. In the above process, the following 3 points are the key points.
2.1 workshop primer
in the bridge production process, the steel plate is bound to enter the pre-treatment process, blast rust removal, grade to ISO8501-Sa21/2 level;
2.2 Treatment of surface defects in welds and steel structures
Chang'an Bridge is in the atmospheric environmental corrosion C4 level specified by ISO 12944-2, according to GB/T 8923.3-2009, the defect treatment level must reach P2 level. The correspondence between steel surface treatment requirements and the service corrosion environment can be found in Table 2.
Table 2 Steel structure surface treatment level and corrosion environment corresponding table
for cold-coated zinc has the characteristics of sharp edge high shrinkage, the following parts must be focused treatment. According to GB/T 8923.3 standard, to achieve P2 level.
(1) welding slag: the surface should be free of welding slag;
(2) pores: holes in the surface should be fully opened for the paint to seep in or the holes to be worn off;
(3) biting edges: surfaces should be free of sharp or deep bite edges;
(4) welding splashes: surfaces should be free of loose and slightly attached welding splashes;
(5) arc pits: arc pits should have no sharp edges.
2.3 Blast Process
(1) Abrasives
are natural or synthetic solid materials used as injection treatment media. The type and size of the abrasive is the key to determining roughness size and shape. Unlike conventional heavy anti-corrosion paint primers, cold-coated zinc blasting must achieve an average roughness of Rz-40 to 65 sm, which should not be too large. The matching scheme for new equipment blasting abrasives can be found in Table 3.
Table 3 New Equipment Abrasive Patching Scheme
Abrasives should meet the requirements of the ISO 11124 and ISO 11126 series of standards and be free of corrosive ingredients and dirt that affect coating adhesion. The use of abrasives that have been permanently contaminated is not permitted.
(2) preparation of the blasting process
visual inspection of the presence of oil, lipids, salts and similar contaminants on the work piece. Mask areas that are intended not to be cleaned by injection. Suitable methods for removing contaminants can be found in ISO 12944-4.
(3) blasting technical conditions can be found in Table 4.
4 Blast Technical Conditions
Through the blasting process, the average roughness of the surface of the steel plate is reached Rz-40 to 65 sm, and the rust removal grade reaches ISO8501-Sa21/2.
3 Pre-coated
pre-coated to achieve better paint film coverage, in the bridge steel structure of the port is not easy to spray everywhere, flowing water holes, exhaust holes, narrow corners, back-side difficult to spray, hand welds and other difficult parts must be pre-coated (see Figure 2).
particular need to emphasize that pre-painting needs to use brush coating method, can not use roller coating method to prevent leakage or coating thickness is insufficient.
4 Improving airless spraying technology for cold-coated zinc
4.1 Airless spraying is the most important construction method for cold-coated zinc

Airless spray spraying efficiency is high, up to 1,000 m2 per hour, because the paint does not contain air, the coating quality is good, especially at the corners of the film is good, a spray paint can obtain 65 to 80 m film thickness, reduce the number of sprays, is an effective measure for energy conservation and environmental protection.
4.2 must use a special airless paint machine rich in zinc primer
cold coated zinc contains a large amount of zinc powder, the deposition speed is fast, need to be fully stirred, so that zinc powder evenly dispersed, spraying process to avoid precipitation. Use a special airless paint sprayer.
spray operation is as follows: the distance between the
(1) spray gun and the work piece is controlled between
2
50 and 400
mm;
4.3 Nozzle selection and timely exchange
according to the construction viscosity of the coating and the size of the coated work piece to choose high-pressure airless spray equipment. This is the key to ensure the quality of the coating and paint consumption. Due to material differences, it is recommended to use foreign manufacturers, such as GRACO, WIWA, WAGNER and other companies' equipment. Replace one nozzle for every spray of approximately 5,000 m2. When one of the following occurs, it is recommended to replace the nozzle in time: poor spray effect of paint atomization, narrowing of paint consumption, construction efficiency significantly reduced the moment of buckle gun, pressure loss of more than 20%.
the corresponding relationship between the wear degree of the nozzle and the spray flow can be seen in Table 5, and the change in the atomization amplitude of the coating is shown in Figure 3.
Table 5 gun nozzle wear and spray flow
Figure 3 paint atomization amplitude change
4.4 stirring and dilution
General cold-coated zinc in airless spraying, need to use a special blender for full stirring, pay attention to the speed of stirring, not too fast, so as not to cause the coating foaming.
5 Control of environmental conditions
spraying cold-coated zinc: steel surface clean, dry, temperature greater than 10 degrees C, relative humidity 75% to 85%, steel surface temperature above dew point temperature of more than 3 degrees C;
after painting, to maintain a certain amount of ventilation, otherwise it is easy to cause the middle paint to fall off,
cold zinc in the northern winter must have a insulation device, storage temperature of 5 to 30 degrees C conditions, the general storage period of 1a.
6 Quality Assurance Clause
In accordance with iso12944-5 (2017 edition), the content of C411, epoxy zinc-rich primer, inorganic zinc-rich primer in 65 to 80 m, the total film thickness of the paint reached 260 m, there is a coating life greater than 25a. The content of cold-coated zinc is as high as 95%, and the anti-corrosion performance is relatively high. C309 and C411 cold-coated zinc matching scheme and durability estimates are in Table 6.
table 6 C309 and C411 cold-coated zinc matching scheme and durability assessment
Beijing Yongding River Bridge using C411 supporting scheme, coating has a service life of >25a.
three-point coating, seven-point coating. Do a good job of cold-coated zinc coating, play the biggest role of cold-coated zinc, reduce expensive medium-coated maintenance, to LCC full-life analysis method, evaluation of the rationality of the project.
service life of the coating, also known as coating durability. It is not a commercial shelf life, but a technical indicator, is the expected design life, is the development of supporting coatings to reach the first overhaul before the time. The first overhaul is usually performed when 10% of the surface of the coating state reaches iso4268-3 standard Ri3. At this point the overhaul is the most cost-effective.
7 Conclusion
To Beijing Chang'an Bridge cold-coated zinc supporting design as a model, pointed out the key points of the coating process, design the correct coating process, serious surface treatment, pre-painting, adhere to airless spray method, so that the coating to achieve the longest service life of >25a.
References (slightly)
For more information, see Coatings and Protection No. 10 of 2019
Author / Wang Qingjun 1, Li Minfeng 2
(1. Shandong Qilu Paint Co., Ltd.; 2. China Paint Industry Association)
.