The metal surface is preserved and rusted, and the old parts are refurbished.
In order to have good cleanliness and roughness of the metal surface, it is necessary to pretreat the surface of the metal structure by rust removal. In the anti-corrosion industry, there is such a sentence "seven-point derusting, three-point painting", which shows the importance of derusting. In view of the particularity of the hydraulic metal structure, such as large volume, the construction environment is basically open air, etc., the sandblasting and rust removal methods are still adopted at this stage. The sandblasting and rust control is mainly reflected in the following aspects.
1. Before the control of sand blasting, the corrosion grade of the surface of the metal structure is evaluated according to the rust grade and rust removal grade of the steel surface before painting. Carefully inspect, remove the deposits such as welding slag, splashing, etc., and clean the surface grease and soluble dirt. The useless welds or joints should also be properly disposed of.
2. Abrasive control sand for sandblasting and rust removal requires hard, angular and dry particles. (After the water content descaling scheme is determined, the abrasive material can be mixed with river sand, quartz sand or both. In the reinforcement of Gangnan Reservoir, According to the pre-spray comparison, the rust removal effect is best when the quartz sand and the river sand are mixed by 1:1.
3. When the tool controls the blasting operation, the air pressure of the air compressor is 6.0×105～6.5×105Pa, and the air pressure is 0.5×105～1.0×105Pa. Generally, the pressure of the sand drum is 4.5×105～5.5×105Pa, which is not less than 4.0×105Pa. Before blasting, check whether the manufacturer of the pressure vessel holds the production license issued by the relevant department, the protective tools worn by the sandblaster, the safety belt (cord) and the oxygen supply device are safe and reliable, and the nozzle wear condition, when the orifice The nozzle should be replaced when the diameter is increased by 25%.
4.Environmental control In view of the blasting and derusting of hydraulic metal structures for open-air operation, in addition to dust and environmental protection during construction, the ambient temperature, humidity and metal surface temperature of the construction site must be checked daily to calculate the dew point on the day. Good construction record. Construction shall not be carried out when the relative humidity of the air is greater than 85% and the surface temperature of the metal is less than 3 °C above the dew point.
5. Process control Compressed air for sandblasting must be treated by cooling device and oil-water separator to ensure dry and oil-free; oil-water separator must be cleaned regularly.
The distance from the nozzle to the surface of the base steel is preferably 100-300 mm, and the non-blasting part should be shielded before sandblasting.
The angle between the jet direction and the normal of the surface of the base steel is preferably 15° to 30°.
After sand blasting and rust removal, before the next process, if it is raining or other conditions that cause the surface of the base steel to be wet, after the environment reaches the construction conditions, dry the compressed air to dry the surface moisture and then rebuild. Sandblasting, do not reduce the abrasive requirements, so as not to reduce the roughness.
Do not stop the nozzle for a long time during sand blasting. The blasting operation should avoid sporadic work, but the spray area should not be too large. Consider the time interval between the coating or thermal spraying process and the surface pretreatment process.
Use manual or power tools to remove rust from areas where the gun cannot be sprayed.
6. Quality control After sandblasting is completed, the sandblasting and rust removal parts should be thoroughly inspected first, followed by the cleanliness and roughness inspection of the surface of the base steel. The focus should be on the parts that are not easily sprayed, and the manual or power tool rust removal parts can be appropriately lowered. When the surface of the base steel is inspected for cleanliness and roughness, it is strictly forbidden to touch by hand; the second is to be carried out under good scattered sunlight or artificial lighting conditions with comparable illumination to avoid missed inspection.
After sand blasting and rust removal, the surface cleanliness of the metal structure should reach Sa21/2. It should be compared with the photos provided in the rust grade and rust removal grade of steel before painting. When comparing, there should be at least one comparison point every 2m2.
After sandblasting and rust removal, the surface roughness of the metal structure should reach Ry60～100μm. According to the technical requirements, the corresponding roughness sample should be taken and compared with at least 7 times magnifying glass. When comparing, there should be at least one comparison point every 2m2. When directly measured by surface roughness meter, there must be at least one evaluation point per 2 m2 surface, the evaluation length is 40 mm, and 5 points are measured within this length range, and the arithmetic mean value is taken as the surface roughness value of the evaluation point.
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