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Analysis of corrosion prevention scheme of steel structure in coastal power plant


Large thermal power plants have a large number of steel structures (such as boiler steel frame, plant steel structure, etc.) and equipment, pipes located outdoors. Steel structure has the advantages of light structure and good comprehensive mechanical performance, but the steel exposed to the environment will be subjected to various forms of corrosion, if not protected or isolated corrosion conditions, the steel structure will gradually be oxidized, and finally lose the ability to work. For the power plant located in the coastal area of the seaside, because it has the characteristics of high humidity, high temperature, high salt content in the atmosphere, and the power plant itself fly ash, sulfur dioxide, steam condensation and other local corrosion environment, must fully consider all kinds of corrosion factors, the design of a more appropriate paint anti-corrosion scheme, To achieve long-term corrosion, reduce the number of recoating, prolong the service life of the purpose.
In this paper, a power plant under construction in southeast coastal area two millions of ultra-supercritical п type furnace steel frame as the object, introduce illustrates the current relatively mature zinc-rich coatings, hot-dip zinc, cold spraying zinc protection principle of three kinds of anticorrosion scheme, and the suitable environment, plan construction, anti-corrosion performance, sensors and actuators, the follow-up maintenance and life-cycle cost makes a comprehensive comparison between three kinds of anticorrosion scheme, Finally put forward the optimization proposal scheme.
Design principles of anticorrosive paint for power plant
The design idea of paint anticorROsion is generally according to the different corrosion environment or medium, surface treatment conditions, the use of different components of paint coatings, and according to the protective life requirements and technical and economic comparison results, to determine the coating thickness. "Coatings and varnishes - Corrosion Protection of Protective Paint System on Steel structures"), the atmospheric environment classification of this engineering site belongs to C4 class; According to the durability of the coating, the design life of the coating has short term, medium term, long term 3 standards, most of the current thermal power plant paint design life for 10~15 years.
2. Brief analysis of the project anti-corrosion scheme
2.1 Classification of anti-corrosion schemes
Coating or coating is the most commonly used anti-corrosion way, by coating the steel with a certain thickness of dense material, steel and corrosive medium or corrosive environment separated, so as to achieve the purpose of anti-corrosion. The COATING BEFORE USES DRY OIL OR HALF DRY OIL AND NATURAL RESIN AS MAIN RAW MATERIAL, BECAUSE THIS HABITUALLY CALLS "PAINT". At present, the commonly used paint anticorrosion scheme mainly includes zinc rich coating, hot dip galvanized, cold spray zinc three kinds.
2.2 Hot dip galvanizing solution
Hot dip galvanizing scheme can get dense and thick zinc protective layer, better protection performance. However, the construction process of hot dip galvanized is strict. In the actual operation process, the control of technical parameters of hot dip galvanized process is not good, which will seriously affect the anti-corrosion protection life of hot dip galvanized components. Due to the limited volume and the temperature of 400 ~ 500 ℃ zinc dip plating, steel structure will produce thermal stress changes and even thermal deformation, especially for seamless steel pipe, box structure, etc.; At the same time, hot dip galvanizing is limited by the size of the plating tank and transportation, which makes the construction of many large components very inconvenient; In addition, the process is more polluting and the waste gas treatment cost is also higher. When the zinc layer is consumed for about 15 years, it can not be re-galvanized, can only be allowed to oxidize, there is no other means to ensure the service life of the steel structure.
Due to the above limitations, hot dip galvanizing process has been widely used in power plants only in the steel grating of platform escalator.
2.3 Zinc-rich coating scheme
Because ZINC-RICH PRIMERS HAVE a good shielding function, many projects use EPOXY ZINC-rich paint as a primer for outdoor steel structures, auxiliary engines and pipes. Zinc rich coating process generally according to one epoxy zinc rich primer 50 ~ 75μm, two epoxy cloud iron intermediate paint 100 ~ 200μm, two polyurethane top paint 50 ~ 75μm consideration, the total dry film thickness of 200 ~ 350μm. Under high corrosion environment conditions of power plants in coastal areas, the protection period of ordinary coatings is short. For example, the first phase of Guohua Ninghai Power Plant project and the first phase of Guangdong Haimen Power Plant project, two to three years after completion, a large area of rust occurred. Anticorrosive maintenance has to be carried out many times during the life cycle of the power plant.
2.4 Cold spray zinc solution
Cold spraying zinc is by the purity higher than 99.995% by atomization extracting zinc powder, special agent of fusion of single-component products, dry film coating contains more than 96% of the pure zinc, the combination of hot dip galvanized and spraying zinc (aluminum) and zinc rich coatings, the advantages of protection principle similar to hot dip galvanized, double protection with cathodic protection and barrier protection, Compared with the traditional hot dip zinc hot spray zinc has better corrosion resistance.
Due to the low processing temperature, the oxidation rate of cold injection zinc is greatly reduced. Cold injection construction makes the hole rate of thermal expansion and cold contraction is also very low, so the protection performance of cold injection zinc is better. Cold spray zinc surface treatment requirements are relatively low. Cold spray zinc can not only be painted in the workshop, but also in the field of painting construction, without workpiece size and shape restrictions. Cold spray zinc products do not contain any lead, chromium and other heavy metal components, solvents do not contain benzene, toluene, methyl ethyl ketone and other organic solvents, so the use of safe and sanitary. Based on the above advantages, cold injection zinc process is widely used in outdoor steel structure anticorrosion process of power plants in coastal areas.
2.5 Comparison of anti-corrosion schemes
Table 1 shows the comparison of the above three commonly used anticorrosion schemes in thermal power plants. Taking two steel frames of millions пtype furnace under construction in the power plant in this coastal area as an example, after consulting anti-corrosion coating manufacturers, the results are as follows: if zinc-rich coating scheme is adopted (using "Haihong Old Man" brand paint), with 65μm primer, 80μm topcoat and 180μm intermediate paint, the total material cost is about 7 million yuan; If cold spray zinc scheme is adopted, the thickness of cold spray zinc is 180μm (including sealing paint and topcoat), the cost of using domestic paint materials is about 8 million yuan, and the cost of using imported paint is about 40 million yuan. Considering that the cold-sprayed zinc scheme can be maintained free for 15 years, the zinc-rich paint scheme needs to be repainted and repaired every 5 to 7 years and the maintenance is more difficult, the 15-year economic income of the cold-sprayed zinc scheme is still greater than that of the zinc-rich paint scheme.
From the above analysis and comparison, it can be seen that the cold-sprayed zinc scheme has the advantages of long-term corrosion prevention, avoiding multiple maintenance, good corrosion adaptability, convenient construction and maintenance, and low lifetime cost. For large steel structures such as boiler steel frame, this paper recommends the cold-sprayed zinc corrosion prevention scheme.
3 conclusion

In view of the special environmental and climatic conditions of power plants in coastal areas, it is recommended to give priority to cold-sprayed zinc corrosion prevention scheme for outdoor boiler steel frame and plant steel structure, and hot-dipped zinc scheme for grid plate of power plant platform. It is suggested that owners pay close attention to the price trend of cold spray zinc coating, in the case of affordable cost, priority should be given to the use of cold spray zinc scheme, only when the price exceeds the initial investment estimate too much, consider the zinc rich coating scheme.