Treatment of metal surface structure by sand blasting and rust removal process

In order to make the metal surface to achieve the ideal roughness and cleanliness, the rust removal process must be carried out on the surface of the metal structure. There is a saying in the anti-corrosion industry that "seven points of rust removal and three points of coating" show the importance of rust removal. In view of the particularity of hydraulic metal structure, such as large volume, the construction environment is basically open air, etc., the method of sand blasting derusting is still used at this stage, and the control of sand blasting derusting is mainly reflected in the following aspects.

I. Pre-control: before sand blasting, the corrosion grade of the surface of the metal structure substrate shall be evaluated according to the provisions of the Steel Surface Rust Grade and Derusting Grade before Coating. Carefully check, remove welding slag, spatter and other attachments, and clean the surface grease and soluble dirt, the useless welding body or joint should also be properly treated.

II. Tool control: during sandblasting operation, the air pressure of the air compressor is 6.0 × 105~6.5 × 105Pa, and the air pressure amplitude is 0.5 × 105~1.0 × 105Pa. Generally, the air pressure of sand barrel is 4.5 × 105~5.5 × 105Pa, which shall not be less than 4.0 × 105Pa. Before sandblasting, check whether the manufacturer of the pressure vessel holds the production license issued by the relevant department, whether the protective tools, safety belts (ropes) and oxygen supply devices worn by sandblasting workers are safe and reliable, and whether the nozzle is worn. When the orifice diameter increases by 25%, the nozzle should be replaced.

III. Abrasive control: sand used in sandblasting derusting process shall be hard, angular, dry (water content <2%), free of soil and other impurities; Quartz sand is better, coarse river sand is also acceptable; At the same time, factors such as sand transportation conditions, unit price, recovery capacity and the like should be considered. Sand particle size should be 0.5~1.5mm. Sand should be dried in the sun before screening and stored in sheds and rooms. The sieve size is: coarse sieve 40~48 holes/cm2 (particle size 1.2mm), fine sieve 372~476 holes/cm2 (particle size 0.3mm). After the derusting scheme is determined, the abrasive can be river sand, quartz sand or a mixture of the two. In the reinforcement of the Gangnan reservoir, the pre-spray comparison, the quartz sand and river sand by 1: 1 mixed with the use of rust removal effect is the best.

IV. Process control: The compressed air used for sandblasting must be treated by a cooling device and an oil-water separator to ensure dry and oil-free; the oil-water separator must be cleaned regularly. The distance from the nozzle to the surface of the base steel should be 100~300mm, and the non-sandblasting part should be shielded before sandblasting. The angle between the spraying direction and the normal line of the base steel surface should be 15 ° ~ 30 °. After sand blasting derusting and before the next process, in case of rain or other conditions that cause the surface of the base steel to be wet, after the environment reaches the construction conditions, dry the surface moisture with dry compressed air shall be used for construction. If sand blasting is required again, abrasive requirements shall not be reduced so as not to reduce roughness. When sandblasting, the nozzle should not stay in a certain place for a long time. Sandblasting operation should avoid sporadic operation, but the spraying area should not be too large at one time. The time interval between coating or thermal spraying process and surface pretreatment process should be considered. Manual or power tools shall be used to remove rust from the parts where the spray gun cannot spray.

V. Environmental control: in view of the hydraulic metal structure sandblasting derusting is an open-air operation, in addition to dust prevention and environmental protection, the construction site environment temperature, humidity and metal surface temperature must be detected daily, the dew point of the day is calculated, and construction records are made. When the relative humidity of the air is greater than 85%, the metal surface temperature is lower than the dew point above 3 ℃ shall not be construction.

VI. Quality control: After the sandblasting is completed, the sandblasting and derusting parts should be fully inspected, and the cleanliness and roughness of the surface of the base steel should be checked. Focus on the parts that are not easy to spray, and the requirements for manual or power tool rust removal can be appropriately reduced. When inspecting the cleanliness and roughness of the substrate steel surface, one is strictly prohibited to touch by hand; the other is to perform under good scattered sunlight or artificial lighting conditions with equivalent illuminance to avoid missed inspection. After sand blasting and derusting, the cleanliness of the surface of the metal structure shall reach Sa21/2, and the comparison shall be made with the photos provided in "Rust Grade and Derusting Grade of Steel Surface before Coating", and there shall be at least one comparison point per 2m2 for comparison.

After the sand blasting and derusting process, the surface roughness of the metal structure shall reach Ry60 ~ 100 μm. According to the technical requirements, the corresponding roughness sample shall be taken and compared with at least 7 times magnifying glass. During comparison, there shall be at least one comparison point per 2m2. When the surface roughness meter is used for direct measurement, there shall be at least one evaluation point per 2m2 surface, and the evaluation length shall be 40mm. Five points shall be measured within this length range, and the arithmetic mean value shall be taken as the surface roughness value of this evaluation point.