The failure of the anti-corrosion layer on the edge plate of the storage tank is mainly caused by the deformation of the storage tank due to changes in load and temperature. The edge plate at the bottom of the storage tank mainly refers to the part where the base of the tank body is combined with the cement base. The bottom of the tank body is made of carbon steel, and the base material is concrete, with different materials and coefficients of expansion. With changes in external environmental temperature, significant expansion and contraction often occur, resulting in horizontal displacement. When the storage tank is fully loaded and unloaded, the tank body will undergo certain deformation under the pressure of the liquid column, and the edge plate parts will also deform together. At the same time, the foundation of coastal tank areas belongs to soft soil layers that are prone to settlement. Severe environmental conditions such as falling ice impact in high cold winter environments and infiltration of high salinity chemical media can cause phenomena such as warping, peeling, shelling, and material failure of the anti-corrosion layer on the edge plates of storage tanks
2.1 Anti corrosion scheme for edge plates of storage tanks
The reality of long-term exposure to sunlight and rain on the edge plate of large storage tanks requires that the anti-corrosion materials used must have waterproof and UV resistant properties. At the same time, considering the deformation of storage tanks with changes in load and temperature, the materials used must have elasticity, toughness, and good plasticity. In order to prevent damage to the anti-corrosion layer caused by daily maintenance and winter ice falling, anti-corrosion materials must also have mechanical impact resistance characteristics. A new anti-corrosion layer structure has been adopted to address the environmental impact and damage to the edge plates of storage tanks. The specific anti-corrosion scheme is shown in Figure 2.The composite structure of the edge plate of the storage tank is a combination of an inner anti-corrosion layer and an outer protective layer. The inner anti-corrosion layer is sequentially equipped with an anti-corrosion primer layer, a middle filling layer, and an anti-corrosion tape layer from the inside out. The outer protective layer includes a polyethylene jacket outer protective body, flanges, and matching installation bolts.
The petroleum grease primer layer is mainly uniformly applied to the tank wall panels, tank edge panels, the upper surface of the cement base, and the side of the cement base to improve the surface quality of the concrete foundation and tank steel plates. The middle filling layer is a transition layer filled with mineral grease cement from the inside out at the concave corner of the tank wall, forming a slope on the concrete foundation to guide rainwater to flow out. The oil belt layer is a dense oil-based anti-corrosion layer formed by tightly covering two layers of mineral grease oil belts on the construction area of mineral grease oil and mineral grease adhesive. The outer cover of the jacket is tightly covered with mineral grease tape, providing reliable mechanical protection for the anti-corrosion layer.
2.2 Composite structural materials for edge plates of storage tanks
(1) petrolatum paste
petrolatum paste is an artificial light brown oily paste like corrosion inhibitor that contains rust conversion agents. It can convert the rust layer on the metal surface into a hard chelating compound, forming a protective sealing layer that can play a dual role in rust removal and rust prevention. During construction, the material is directly applied to the surface of the steel structure, which has good anti-corrosion performance in humid environments and can effectively and stably protect the steel structure from corrosion in harsh corrosive environments for a long time.
(2) petrolatum mastic
A cementitious mixture composed of inert mineral grease, corrosion inhibitor, filler, and reinforced short fibers, which is stable, non hardening, non curing, and has excellent flexibility, plasticity, insulation, and corrosion resistance. It is used as a filling and sealing material and forms a certain slope.
(3) petrolatum tapeMineral oil belt is a strip of anti-corrosion material made by impregnating non-woven fabric as a carrier in special mineral grease containing composite rust inhibitors, thickeners, lubricants, fillers, etc. For the uneven surface of corroded steel, the mineral oil strip can fully cover the steel surface and completely match, while also having good density. Mineral oil tape and mineral oil primer are the same series of anti-corrosion materials, which are compatible and bonded together to completely isolate the substrate from contact with air and water, thus protecting the substrate. Petrolatum oil tape is a flexible material that does not require solidification, making construction convenient and allowing for continuous construction.
(4) Polyethylene jacket outer protection
The outer sheath of high-density polyethylene jacket is a high crystallinity, non-polar thermoplastic resin with good heat and cold resistance, good chemical stability, high rigidity, toughness, and mechanical strength, and resistance to environmental stress cracking
2.3 Characteristics of anti-corrosion schemes
(1) The inner layer material is made of non curing material, which has excellent flexibility and can overcome the deformation effects caused by temperature and load changes in the storage tank;
(2) The outer layer is made of high-density polyethylene material, which has good weather resistance, strong ultraviolet radiation resistance, high cold resistance, and rapid temperature difference changes. It has good impact resistance and can effectively resist the impact of falling ice from high places;
(3) The composite structure has good sealing and waterproof performance. The petrolatum paste and mineral grease tape are both hydrophobic materials, which can block the entry of water vapor into the channel;
(4) The composite structure provides multiple protective effects on the edge plate, and the three-layer structure complements each other, resulting in a better protective effect than a single anti-corrosion layer;
(5) The connection method adopts flange and stainless steel bolt fastening, with adjustable tightness for easy on-site replacement and maintenance.
3.1 Problems Existing on Site
Three issues were found on site that will affect the implementation of the construction plan. (1) There are some irregular structural components around the edge plate of the storage tank that affect construction, mainly grounding points and cathodic protection cables; (2) The width of the edge of the concrete foundation varies, and the width of the slope varies greatly. The width is generally around 300mm, with the widest position being 370mm and the narrowest position being 280mm. However, the prefabricated jacket protection in the factory is a fixed length finished product, which will affect the adhesion and continuity of the outer protective layer; (3) The diameter of the tank body is large, the circumference is long, and the protection area is large. Simply using bolts for fastening lacks a strong force point, and the outer protective layer of the jacket is prone to loosening and falling off. Corresponding measures must be taken to address these three issues before installation. The on-site photos of the grounding point are shown in Figure 3.
3.2 Handling of Existing Problems
In response to the existing problems, corresponding measures were taken on site: (1) re welding the grounding flat steel on the tank wall to ensure that there is a certain gap (more than 2cm) between the angle steel on the horizontal side of the concrete foundation and the platform foundation, and the space is sufficient for the use of the outer protective layer construction, as shown in Figure 4. The grounding wire should be readjusted; (2) Measure the overall edge plate of the storage tank, mark the areas with significant deviations between the slope width and the standard width, and customize the polyethylene jacket according to the actual width on site, with proper labeling to prevent the use of wrong materials; (3) Weld six channel steels evenly distributed around the entire bottom tank wall as support points for jacket installation and fastening points for bolt connections,
4.2 Surface treatment
The temperature during construction should be at least 3 ℃ higher than the dew point temperature. In windy and snowy weather, without reliable protective measures, on-site construction should not be carried out. Before construction, remove the original anti-corrosion layer of the storage tank and then treat the surface. The surface of cement should be cleaned thoroughly, free of ash, soil, and impurities. Due to severe edge warping or uneven cement surface of the storage tank, cement mortar was used for filling pre-treatment. Before rust removal, use solvents and brushes to remove oil, grease, and other impurities from the metal surface. The floating rust and loose paint on the metal surface have been cleaned, and the previously tightly adhered paint has been retained. After rust removal, sharp parts that may puncture the anti-corrosion layer should be polished and abrasive and dust adhering to the metal surface should be removed.
The time interval between the pre-treatment of the metal surface and the application of mineral grease primer should be controlled within 4 hours to prevent moisture and contamination of the metal surface. Before applying mineral grease base oil, if rust or surface contamination occurs, surface pretreatment must be carried out again.
4.3 Applying petrolatum paste (Primer)
Apply petrolatum paste evenly on the tank wall and metal edge plate, and stir thoroughly before using the primer. The primer should be applied evenly without any defects such as omissions, clumps, or sagging. Each kilogram of primer can be applied to 3-5 m2 of surface, and the time interval between applying primer and applying oil tape should not exceed 1 hour. The height of applying mineral grease bottom oil on the tank wall is 15cm.
4.4 Filling withpetrolatum mastic putty
Fill the gap between the edge plate of the storage tank and the cement base with petrolatum adhesive, and fill the transition from the concave corner of the tank wall from the inside out with petrolatum tape. The surface should be flat and smooth, and a certain slope (greater than 8 °) should be formed from the inside out to avoid water storage areas. The filling process and effect are shown in Table 6.
4.5 Covering petrolatum tape (grease tape)
Covering petrolatum tape is the core link of the entire construction process and a key step in ensuring anti-corrosion quality. When pasting, the anti-corrosion tape should be applied from a height of 100mm extending downwards from the cement tabletop. The overlap between the anti-corrosion tapes should be maintained at 55% overlap, and the height of the mineral grease tape on the tank wall should be 15cm. Starting from the cement facade section, this can ensure that the oil belt and oil belt are in an upward and downward shape, forming a structure similar to a "eaves board". The oil belt anti-corrosion layer should not produce defects such as wrinkles and bubbles. As the coating is applied, flatten the oil strip by hand. The vertical part of the oil belt should be compacted and leveled, so that the oil belt is evenly adhered to the cement surface, metal surface, and tank wall. The overlap between the two oil belts should not be less than 100mm, and the overlap between the oil belts should be maintained at 55% to ensure the effect of double-layer winding
4.6 Installation of polyethylene outer protective jacket
The polyethylene jacket serves as the first outer protective layer to block the erosion of external rain, snow, and ultraviolet rays. The outer protective scheme used in this project adopts a three-stage fixing method, which involves flange fixation at the tank wall, edge plate, and side of the edge plate foundation. The overlap is carried out in a "two-stage pressure in the middle" manner, where the tank wall is overlapped on the edge plate, and the side of the edge plate foundation is overlapped on the edge plate. The outer protective body of the polyethylene jacket is connected to the flange by plastic welding. The outer protective height of the jacket attached to the tank wall panel shall not be less than 150mm and shall be at least 5cm higher than the height of the anti-corrosion layer. The outer protective height of the jacket attached to the side of the cement base shall not be less than 50mm and shall be at least 5cm higher than the height of the anti-corrosion layer. The outer protective width of the polyethylene jacket attached to the surface of the tank edge plate, side, and cement base shall be consistent with the on-site dimensions.
The polyethylene jacket and flange are prefabricated in the factory and transported to the site according to the size of the storage tank, and connected and assembled with 316L stainless steel bolts. Before installation, check the specifications and dimensions of each part. The length of each prefabricated section in the factory is about 3 meters, but on-site measurements have found that the distance between each grounding point is not the same, which may lead to the possibility of flange connections overlapping with the grounding point. A short distance compensation patch transition connection has been made separately, and overlapping points should be avoided as much as possible during installation. During installation, pay attention to the changes in the curvature of the storage tank and make adjustments at any time, while leaving room for later adjustments. The installation photo of the outer protective jacket is shown in Figure 8.