The compressed air dryer operates in the environment of high temperature and high humidity air compressor station. If it lacks excellent anti-corrosion performance, the service life of the equipment will be significantly shortened. In addition to stainless steel products and chrome plating, zinc plating process, paint protection is the most commonly used anti-corrosion means.
1.Anti-corrosion environment classification
According to the provisions of the national standard GB/T 30790.2-2014, the atmosphere is divided into six corrosive levels, namely C1 very low corrosive, C2 low corrosive, C3 medium corrosive, C4 high corrosive, C5I very corrosive (industrial) and C5M very corrosive (Marine).
Exposure tests on standard specimens are strongly recommended for accurate determination of corrosion grade. Corrosion grade is defined by the loss of mass or thickness of a standard sample made of mild steel and/or zinc after the first year of exposure. For standard samples and how they should be treated before and after exposure, refer to GB/T 19292.4 standard for details.
The use of mass or thickness loss data from less than or more than one year of exposure to extrapolate to one year of exposure is not reliable and is not permitted. When grading corrosivity based on the mass or thickness loss of steel and zinc samples, different results may occasionally be obtained. In this case, it should be classified as the more corrosive class.
If it is not possible to place the standard sample in the actual environment to be considered for exposure tests, then the corrosion level can be assessed by referring to the relevant typical environmental examples, but these examples are for reference only and can sometimes be misleading.
Only by actually measuring mass or thickness loss can an accurate corrosion grade be determined.
Note: The corrosion level can also be estimated by combining environmental factors such as the annual moisture time, the annual average concentration of sulfur dioxide and the annual average precipitation of chloride
2. Surface treatment before spraying
Before painting the workpiece, it is necessary to implement a comprehensive and thorough rust removal and decontamination treatment to ensure that there is no sand dust, iron filings, welding slag, rust, oil and other impurities on the surface of the workpiece, so that it can achieve a smooth and clean state, so as to improve the adhesion of the coating. Sandblasting is a very direct and effective method of rust removal and decontamination, which uses compressed air as a power source to spray materials (such as copper ore, quartz sand, emery, iron sand, Hainan sand) at high speed to the surface of the workpiece to be treated, so as to change the appearance or shape of the workpiece surface. With the impact and cutting effect of abrasive on the surface of the workpiece, the workpiece surface obtains specific cleanliness and different degrees of roughness, and then enhances the adhesion between it and the coating, extends the durability of the coating, and is conducive to the leveling and decorative effect of the coating.
The sand material used for sand blasting should have the characteristics of hard, angular, dry (moisture content < 2%) and free of soil and other impurities. Among them, the choice of abrasive copper ore can achieve better rust removal effect. The particle size of the sand should be 0.5-1.5mm, which should be dried before screening and stored in the shed or indoor. The optimal nozzle pressure is in the range of 6.2MPa-6.9MPa, and the nozzle should be replaced in time when the aperture diameter increases by 25%. The distance between the nozzle and the matrix pipe surface should be controlled at 100-300mm, and the non-sandblasting parts should be shielded before sandblasting. The appropriate Angle between the injection direction and the matrix pipe surface normal is 15° -30 °. The sandblasting treatment grade is SSPC SP10 (Sa2.5), that is, near white sandblasting: the surface should be free of any visible grease, dirt, dust, oxide, rust, paint, oxide, corrosion products and other foreign substances, only a small amount of stains are allowed, and the stain area shall not exceed 5% of the total surface area.
3. Spray painting environment control
Before painting, pay close attention to construction environmental factors, such as dryness, air circulation, light conditions, etc. The ambient temperature should be controlled within the range of not less than 5℃ and not more than 40℃. In terms of relative humidity, the general requirement is not more than 85%, for acrylic intermediate paint, polyurethane topcoat (I-990), the relative humidity should not exceed 80%, inorganic zinc primer (I-22) when the relative humidity is lower than 55%, it is necessary to consider using steam or spray water and other measures to improve the relative humidity. The metal surface temperature should be above 3℃ above the dew point temperature, otherwise the painting construction must be stopped. Outdoor construction is prohibited in rain, snow, fog weather and when the wind exceeds level 4. If there is frost and dew on the surface to be painted, it is strictly prohibited to carry out construction. Air circulation must be ensured in the construction area, and dust and other foreign bodies should be prevented from flying during the painting and curing process.
4. Spray paint thickness control
There is often a misconception that the thicker the paint coating, the better, but this is not the case. Too thick or too thin coating has drawbacks, too thin coating will lead to poor anti-corrosion performance, the workpiece surface is easy to rust, and may cause the paint to fall off due to too brittle; If the coating is too thick, it will cause high internal stress, resulting in cracking and deformation, and it is easy to produce appearance problems such as flow accumulation, and the cost will increase correspondingly. Therefore, the thickness of the paint should be reasonably adjusted to the appropriate range according to the different types of paint used. The paint thickness is usually measured by the coating thickness measuring instrument to ensure that the thickness of each paint coating meets the process requirements.
5.Paint adhesion test
Paint adhesion has a crucial effect on the performance and life of the coating. If the adhesion strength is insufficient, the coating will fall off. Common test methods include grid method and pull test. In order to quickly and easily test paint adhesion, a portable pull-out adhesion tester can be used to detect paint adhesion simply, efficiently and accurately. In general, the opening strength ≥5Mpa can meet the anti-corrosion requirements of the vast majority of the use environment.
6. Spray paint quality requirements
The anti-corrosion primer inside the container and pipeline should be sprayed evenly, and there should be no defects such as flow hanging, particles and bubbles.
The finished paint film should have the characteristics of firm, beautiful and generous, the appearance of the flat surface should be smooth and smooth, the color is uniform, and the phenomenon of missing and miscoating is strictly prohibited.
If the paint film on the surface of the container and pipeline appears foaming, flow hanging, pinholes, color infiltration, orange peel, fading, falling off and peeling, etc., it must be trimmed.
The paint film should reach a dry state, non-stick and have a certain hardness (nail scraping without scratches).
The paint film color should be consistent with the company's product color (by visual contrast with the color card).