Recommended application plan for anti-corrosion coatings in the petrochemical and metallurgical industries

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Recommended Solutions for the Application of Corrosion-Resistant Coatings in the Petrochemical and Metallurgical Industries 2008-08-05 Public information from China Anti-Corrosion Equipment Network. Font options: http://www.chinaae.net/article/images/font_big_h.gif http://www.chinaae.net/article/images/font_mid.gif http://www.chinaae.net/article/images/font_sma_h.gif Abstract: The quality of corrosion-resistant coatings on steel structures and non-metallic surfaces is closely related to the treatment of those surfaces, as well as the quality of the coatings used and the standard of application; therefore, the proper application of corrosion-resistant coatings is particularly important. This article covers the selection, matching, application, and inspection of coatings, and recommends practical reference solutions. Keywords: anti-corrosion coatings; steel structures; management; anti-corrosion solutions; surface treatment. 0 Introduction: In accordance with the anti-corrosion requirements of sectors such as petroleum, petrochemicals, metallurgy, and power generation, high-performance heavy anti-corrosion coatings are recommended as part of the anti-corrosion solution for steel structures, and the coating application must be carried out strictly in accordance with all the requirements of this solution. Surface treatment and coating operations shall be carried out through consultation between Party A and Party B to ensure corrosion resistance. 1. Recommended supporting solutions: Tables 1–8 present 8 supporting solutions for the reader’s reference (attached at the end). 2 Painting Requirements 2.1 Surface Treatment Quality Requirements For ordinary steel, sandblasting is used to remove rust, scale, and other impurities. The quality control of surface treatment should meet the Sa2.5 standard specified in GB8923 \"Corrosion Grades and Rust Removal Grades of Steel Surfaces before Painting\", with a surface roughness of 35–75 μm; all dust must be removed after sandblasting. The fixed parts can be manually derusted, strictly in accordance with the above standards. The anti-corrosion solutions for buried pipes (ordinary steel, cast iron, galvanized steel) are shown in Table 1. The actual amount of paint used is estimated by the contractor based on factors such as experience, skill level, and the working conditions; it is approximately 1.5 times the theoretical amount required. If zinc salts appear on the surface of the paint film, these must be removed before applying the next coat of paint – this can be done by brushing or sanding the surface with a dry cloth or sandpaper. If the interval between coating applications is exceeded, the surface of the paint film should be sanded to create a rough texture before applying the subsequent coat, in order to improve the adhesion between the layers of paint. For galvanized surfaces, it is necessary to use light sandblasting to remove impurities such as dust and oil, followed by the application of epoxy iron oxide anti-corrosion paint. 2.2 Painting environmental conditions The painting environment has a significant impact on the quality of the paint film. To ensure good painting quality, the following requirements are imposed on the painting environment: (1) Outdoor painting operations should not be carried out under direct sunlight, or in weather conditions with rain, fog, or snow. Construction is not suitable when the relative humidity is above 85%; the substrate temperature must be at least 3°C above the dew point before painting can proceed. However, it has little impact on long-lasting zinc-rich coatings using inorganic phosphates. (2) Construction cannot be carried out during summer when the sun shines directly and the substrate temperature exceeds 60°C. When the winter temperature is not lower than -5°C, epoxy iron red anti-corrosion paint, epoxy limonite rust-proof paint, and ultra-thick paste-type epoxy asphalt rust-proof paint should not be used for outdoor applications during winter. (3) Work must not be carried out when dust is generated during the painting process and the drying of the paint film. Zinc-rich coatings with inorganic phosphates that dry in just 3 minutes are not included in this category. Preparation of the coatings: (1) First, verify that the type and name of the coating meet the requirements for use; (2) After opening the container, the coating must be thoroughly mixed to ensure that any precipitates are evenly distributed; (3) For two-component coatings, it is necessary to prepare them on-site according to the proportions specified in the instructions, the amount required for the project, and the allowed time for application – prepare only as much as is needed. (4) Different methods are used for formulating coatings depending on the type. ①For the epoxy zinc-rich anti-rust primer, the mixing ratio of Component A to Component B is 4:1 (by mass). Component B should be added to the large container containing Component A, mixed thoroughly, and left to mature for 20 minutes before it can be used. ②For the epoxy limonite anti-rust paint, the mixing ratio of Component A to Component B is 4:1 (by mass). The Component B (curing agent), which is in smaller containers, should be added to the larger container containing Component A, and then mixed thoroughly. It must be allowed to mature for 20 minutes before it can be used. ③For the epoxy iron oxide anti-corrosion coating, the mixing ratio of Component A to Component B is: Component A : Component B = 4 : 1 (by mass). Add the small-bucket of Component B (curing agent) to the large bucket containing Component A, stir well, and let it sit for 20 minutes to mature before use. ④Ultra-thick epoxy asphalt anti-rust paint: The mixing ratio of Component A to Component B is 4:1 (by mass). Add Component B (curing agent) to the large container containing Component A, stir well, and let it sit for 20 minutes to mature before use. ⑤Chlorinated rubber topcoat: The chlorinated rubber topcoat is a one-can paint; it can be used after being mixed well once the can is opened. 4 Construction methods and parameters (1) The airless high-pressure spraying method or roller coating method can be used, as well as manual brushing or air spraying. (2) To ensure the proper thickness of the paint film at areas such as welds, edges, and corners, a coat of rust preventive paint should be applied first using a paint brush before carrying out large-scale painting. (3) During painting, the operator should constantly measure the thickness of the wet film using a wet film thickness gauge in order to control the thickness of the paint film. (4) When applying the coating, a back-and-forth painting method—first top and bottom then left and right, or first left and right then top and bottom—should be used to ensure a smooth and even film with uniform thickness. (5) It is necessary to read the construction parameters in the instructions before starting work in order to understand the key points of construction; it is also possible to use the base coat, intermediate coat, and top coat provided for each type of product. 5 Repair painting of damaged paint films: If the paint film suffers mechanical damage during the painting process, resulting in damage to the underlying paint layer and local rusting, it must be polished using manual or power tools to reach the St2–St3 grade specified in standard GB8923—88 before repair can be carried out using H06 epoxy zinc-rich anti-rust primer and the various subsequent coating layers. 6 Inspection: (1) The quality of surface treatment is inspected by comparing with the color photos in the GB8923—1988** standard. (2) Inspection of the paint film appearance. Visually inspect the surface condition of the paint film; it should be free from defects such as pinholes, bubbles, cracks, peeling, sagging, and incomplete coating. (3) Control of paint film thickness: The dry film thickness is measured using a paint film thickness gauge; the thickness of the paint film should be measured after each coating application, and the total thickness of the paint film must be determined once coating is complete. One point is measured every 1–3 m2 (or as specified otherwise). The film thickness at over 90% of the measurement points must meet the specified value; for those points where the thickness does not meet the specified value, it must still be at least 90% of that specified value. Otherwise, it should be repainted or an additional coat applied. 7 Corrosion protection for temperature-resistant parts of steel structures and pipelines (1) For the corrosion protection of temperature-resistant steel structural components and pipeline parts, appropriate high-temperature resistant primers, topcoats, as well as silicone-modified and inorganic-modified high-temperature resistant coatings should be used. (2) Before applying high-temperature resistant coatings, the steel surface must first be treated with sandblasting, shot blasting, degreasing, and rust removal before the primer and topcoat can be applied. In the case of unconditional sandblasting or shot blasting, an oil-removing and rust-removing phosphating agent can be applied as a first coat; after 24 hours, it can be wiped with a dry cloth before applying the epoxy-silicone high-temperature resistant primer. The best approach is to treat the steel surface in the same way as normal paint, then apply one coat of ET-98 inorganic phosphate zinc-rich coating, which can withstand temperatures of 200–600°C. After that, apply two more coats of high-temperature resistant topcoats designed for temperatures of 200–300°C, 400°C, 500°C, and 600°C. Generally, one base coat and two top coats are sufficient for high-temperature resistant coatings, with a dry film thickness of around 100μm. (3) For high-temperature resistant coatings, the color of the topcoat can be any color below 400°C; at temperatures around 400–600°C, silver gray, medium gray, light gray, black, medium yellow, medium green, and similar colors are recommended. (4) For galvanized areas and non-metallic areas, silicone-modified high-temperature resistant paint can be used; it does not need to be applied directly to the base surface, and generally three coats are sufficient. For the fire-resistant parts of steel structures, the first coat should be an epoxy zinc-rich coating or an epoxy iron oxide primer; alternatively, ET-98 inorganic phosphate zinc-rich coating can also be used. Apply several layers of thin-type fire-resistant coating to the steel structure, with the number of layers depending on the required fire resistance time. For coating anti-corrosion purposes, the requirements for the primer apply as specified in Tables 1 to 8; the requirements for the intermediate coats remain unchanged. It is also possible to use the primers and intermediate coats specific to various types of paints. The top coats are available in standard, medium-grade, and high-grade anti-corrosion versions, and for areas requiring special anti-corrosion protection, top coats of different types can be used to achieve the best anti-corrosion results. 8 Conclusion Anti-corrosion coatings are not a single type of product; they generally refer to coating systems that are used in harsh corrosive environments, have a total film thickness of over 100μm, and consist of base, intermediate, and top coats. To fully utilize the corrosion-resistant properties of anti-corrosion coatings, a proper coating system must be employed during their application. Table 1: Anti-corrosion solutions for buried pipes (ordinary steel, cast iron, galvanized steel)
Table 2: Anti-corrosion solutions for exposed pipes without insulation (ordinary steel, cast iron)
Table 3: Anti-corrosion solutions for exposed pipes made of galvanized steel without insulation
Table 4: Anti-corrosion solutions for exposed pipes with insulation, as well as for ordinary steel and cast iron
Table 5: Anti-corrosion solutions for exposed pipes made of galvanized steel with insulation
Table 6: Anti-corrosion solutions for chemical plants and factory structures
Table 7: Recommended anti-corrosion solutions for chemical equipment and high-temperature components
Table 8: Anti-corrosion coating solutions for internal and external surfaces of oil storage tanks