一、Corrosion condition of steel structure of ship lift

　　Tilowa shipyard is located in the tropical area of Myanmar, with a 5000t ship lift of 148m long and 22m wide. It is the main ship repair equipment of the shipyard. It has been moored by the river all the year round and relies on 52 winches to lift and sink the steel structure of the platform. According to the environmental conditions and the working principle of the ship lift in Myanmar, the corrosion conditions of the steel structure paint film of the ship lift are as follows: 1. In rainy season in Myanmar, the relative humidity of the air is as high as 95% - 100%; the annual average temperature is above 30 ℃, the maximum temperature in summer is 40 ℃, and the surface temperature of the steel can be as high as 50 ℃ - 60 ℃ when the sun is direct.

　　2. The content of chloride ion in the air is relatively high in Myanmar offshore area; due to welding, gas cutting, acid pickling and other processes, there are corrosive gases and dust such as SO2 and H2S in the air near the shipyard.

　　3. The steel structure paint film of ship lift often sinks in the water and alternates between dry and wet frequently; the river water has a fast flow velocity and a high content of sediment in the river water, so the paint film should withstand the impact and friction of sediment.

　　4. The steel structure of the ship lift is processed into parts in Shanghai, with a long process, which needs to be shipped to Myanmar. Chloride ions in sea wind and sea water will also corrode the paint film.

二、Recommended paint matching instructions

　　According to the requirements of the owner's paint film quality assurance for more than 5 years, three paint supporting systems are designed, as shown in table 1-3.

　　Description of supporting system:

　　1. Under the correct and reasonable construction technology conditions, the service life of the above three matching systems is guaranteed to be more than 5 years.

　　2. Inorganic zinc silicate primer is selected for the above three supporting systems for the following reasons: the paint has strong anti-corrosion performance; it is suitable for gas cutting, electric welding and other operations with less environmental pollution; it increases the friction of bolts and nuts to facilitate fastening; it is suitable for steel plate assembly line operation.

　　3, in the supporting system 2 and 3, the use of anti-corrosion and other aspects of better performance of epoxy zinc base paint and reusable polyurethane topcoat, so the design of the film thickness is slightly lower.

　　4. In matching system 1, chlorinated rubber primer and acrylic top coat are all single component paint, which is convenient for construction, no limit of recoating interval, and is conducive to the repair of paint film.

三、Surface treatment grade requirements

　　Steel surface treatment includes two aspects: first, steel cleanliness; As shown in Table 1, the requirement of zinc silicate shop primer for steel surface cleanliness is SIS 055900 Sa2.5 (equivalent to SSPC SP 10 of the United States, which is close to the white standard), and its text definition is "after sand blasting, the steel surface is free of oil, rust, oxide skin and other dirt, or only leaves light trace, 95% of the steel surface is impacted by steel shot, exposing the metal color". The second is to control the roughness. According to the thickness of steel plate of ship lift and the specified thickness of paint, the average roughness after shot blasting (spraying) is 40-70m. If the zinc salt produced on the primer surface is removed by sand or steel brush in the process interval, the salt and loose zinc powder shall be removed and cleaned with water; for the two times of rust spots produced locally, the specified requirements of SIS ST3 level shall be met when they are polished and cleaned by hand.

　　The above content is reproduced in "anticorrosive coating and coating" collected by Mr. Li Minfeng