How to choose paint for wet surface under atmospheric exposure environment

Since the beginning of this century, with the rapid development of economy, more and more dynamic equipment and long-distance pipelines appear in petrochemical, power and infrastructure construction, which form the main part of production and operation and play a central role in industrial development. At the same time, due to frequent exposure to the atmosphere, ultraviolet rays from sunlight can damage the coating on the surface of the equipment and pipes, causing aging, powdering and other phenomena, and water vapor in the atmosphere will condense on the surface of the equipment and pipes, eroding the coating and gradually penetrating until the coating foams, cracks and falls off. Nitrogen oxides and sulfur oxides in industrial exhaust gases in the petrochemical and power industries also create harsher acidic gas corrosion environments
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in various types of atmospheric corrosion environment, the most common and unavoidable, is the cohesion of water vapor. Because the atmosphere of water vapor everywhere, the four seasons cycle, temperature and relative humidity often change, equipment and pipe surface from time to time there will be water vapor condensation, wall. Some equipment and the outer surface of the pipe, both often exposed to the sun's strong ultraviolet radiation, but also from time to time water vapor condensation, such as overhead municipal gas pipeline, in the pressure section, the outer surface of the pipe will form a layer of water film. In this kind of corrosion environment, to achieve coating protection, is extremely difficult, and the owner's request is: not yellowing, not powdering, not foaming. Therefore, the selection of suitable products for wet surfaces and the use of appropriate coating techniques are two of the challenges in protecting wet surfaces.
1 Analysis of the wet surface corrosion environment of overhead gas pipelines
Zhu Wei of Tongji University carried out a detailed analysis and calculation of the temperature field of steel gas pipelines, and finally concluded that the distribution of the temperature field of gas pipelines is mainly related to pipeline burial methods, pipeline materials and conveying media, and the overall temperature rise (or temperature drop) of the pipeline is larger than when the transmission of artificial gas. Taizhou PetroChina Kunlun Gas Co., Ltd.'s Lu Lei on the natural gas station high-pressure pipeline wall condensate causes a detailed analysis, summarized as: natural gas before entering the station site, the upstream pipeline needs to be pressure-down, this operation will be in the pipeline high-speed gas The flow produces a significant throes-saving effect, which makes the gas temperature in the pipe drop sharply, and the temperature of the outer surface of the pipe decreases by heat transfer, reaching the condensation point of the condensate (which is the dew point), which will cause condensate to appear outside the pipe wall and freeze at too low a temperature.
According to ISO 12944-2:2017 and the corrosion environment analysis standard ISO 9223 cited, the corrosion environment of the wet surface exposed by the atmosphere is not the same as the pure atmospheric exposure corrosion environment, but not the pure immersion corrosion environment, is a kind of corrosion environment between the condensation high humidity (C5) atmospheric corrosion environment and the water immersion (fresh water, Im1) environment. Its specific performance is: when the surface temperature of the steel gas pipeline is lower than the dew point temperature (the air relative humidity is large), the surface of the pipe is the condensate film cover state, that is, drying with a rag, a short period of time (such as 1min inside) the surface will gradually appear condensate film cover phenomenon, just like people "sweating", see Figure 1. Conversely, the pipe surface will not be able to form condensate, the pipe surface appears a low-temperature, dry state. The gas velocity in the pipe affects the gas temperature, which affects the surface temperature of the steel gas pipe. If the outside temperature is high, it will affect the evaporation rate of the condensate film on the surface of the pipe. Because the gas velocity and external temperature in the pipe change rapidly, the corrosion environment on the pipe surface is essentially alternating and changing between C5 and Im1.
Figure 1 Microscopic simulation of wet surface water beads
2 Hazards of corrosion of wet surfaces under atmospheric exposure
Defects in steel equipment and pipes are often divided into two categories, one is appearance defects. Mainly refers to the paint in the long-term sunlight (mainly the ultraviolet light) under the sun, such as color change, powdering, loss of luster and other phenomena, so as to show the overall color dim, appearance is not coordinated, such defects are due to the sun's ultraviolet light on the coating polymer chemical bonds damage. Wet and dry alternate steel structure surface, if in the atmospheric exposure environment, its special place, lies in different parts of the surface powder degree is not the same, no water film cover of the appearance of flour serious, and the surface covered with water film will retain some degradation powdered paint film particles, reduce the visual presentation of powdering, powdering degree in general, visual difficult to detect.
second is the endurability defect. Mainly refers to the whole anti-corrosion coating system (including paint) cracking, peeling, foaming, and even rust and other phenomena. The emergence of these problems will inevitably affect the service life of steel structure. The main reasons for such defects are the result of the combination of steel structure and external environmental factors. Wet and dry alternating steel structure surface, due to long-term immersion, its most intuitive performance is the outerst layer of paint quickly form a bubble, shovel off the foam, flow out of tiny droplets and intermediate coating or bottom coating, serious places will appear peeling or rusting, see Figure 2.
Figure 2 Endurability Defects
3 Choice of wet surface protection coatings of the wet and wet alternate type
Taking into account the wet and dry alternate service environment on wet surfaces, the technical requirements of the coating should meet two objectives at the same time: to withstand ultraviolet light exposure and to resist the erosion of water molecules in wet environments.
3.1 Typical coating systems and products for wet surfaces of alternate types of wet and dry
according to ISO 12944-5:2018, im1 coatings are available in the options shown in Table 1.
Table 1 is sprayed to clean the coating system under the impregnation corrosive grade Im1 by injection
according to ISO 12944-5:2018, the optional options for C5 coatings, as shown in Table 2.Table
2 Rated dry film thickness data for coating systems under atmospheric corrosive level C5 on carbon steel substrates
from Table 1, Table 2 available, two types of products common to the corrosive environment: zinc-rich primer, epoxy or polyurethane finish, zinc-rich primer including ethyl silicate zinc-rich primer, zinc-rich primer, polyurethane-rich zinc-rich primer. Since ethyl silicate zinc-rich primer and polyurethane zinc-rich primer do not have the convenience or cost-effective advantages of construction, the varieties of coatings commonly used include: zinc-rich primer, epoxy medium paint or finish, polyurethane medium paint or finish.
options for marine structural splashes and Tidal Zone (CX and IM4) coatings are available in accordance with ISO 12944-9:2018, as shown in Table 3.Table
3 Minimum requirements for the protection coating system and initial performance of the splash and tidal zone on carbon steel substrates by injection
in general, the chloride ion content in the land atmosphere corrosion environment is much lower than in offshore buildings (offshore facilities), so the marine structure splash zone and tidal zone are an extreme of the onland wet or wet alternating wet surface steel equipment and pipe corrosion environment.
Because polyurethane finish curing agent isocyanate is a substance with strong water solubility (6.7 percent at 20 degrees C) and easy to react with water, polyurethane is not the best choice for the outermost coating (except for specialty polyurethanes with minimal yield), which can easily cause defects such as foaming. As a result, epoxy coatings are the best choice for finishes on wet surfaces. It is important to note that the epoxy coating performs slightly less well than the polyurethane coating in terms of color preservation performance.The comprehensive analysis of
shows that on wet surfaces alternating wet and dry, the protective coating system should reach the design age of 15-25a, and the two typical supports are: (1) epoxy zinc-rich primer and epoxy resin finish (system total dry film thickness 360-450 m) ;( II) epoxy resin primer and epoxy finish (system total dry film thickness 380-450 m).
3.2 Technical requirements for wet surface coatings of alternating types of wet and dry
due to the condensation of water vapor from time to time on alternating surfaces of wet and dry, according to ISO 12944, two typical products (zinc-rich primers, epoxy finishes) and their coating systems require a range of good performance to meet high durability requirements in harsh corrosion environments.
specific to the product category, more factors need to be considered, in addition to being able to withstand prolonged immersion, but also to minimize maintenance purposes. In wet steel equipment and pipe operation, which alternates in wet and dry, it is easy to see some locations not coated according to design requirements, resulting in early surface defects such as foaming.
analysis, the epoxy coating selected for dry and wet alternating surfaces needs to have the following characteristics in order to cope with the rapidly changing wet steel surface, as shown in Table 4.
Table 4 Minimum technical requirements for wet surface coating products of alternating types of wet and wet
through comprehensive study of coating defects and protection, combined with international standards, analysis and definition of wet surface corrosion environment, based on the problems in the wet surface service environment, put forward the basis for the selection of coating products, construction precautions, focusing on the typical coating system (epoxy zinc-rich primer, epoxy finish) technical requirements and other protective systems.
References (slightly)
For more information, see Coatings and Protection No. 3 of 2019
Original title: Technical Requirements for The Selection of Coatings for Wet Surfaces in Atmospheric Exposure environment
Ma Jianhua (Guangzhou Shibao Technology Co., Ltd., Guangzhou 510510)