The adaptability of nano-modified epoxy powder coating in The Tahe oil field was studied

Xiao Wei 1, Jiang Yufa 1, Yang Jianb 2, Shi Teng 1, Gao Dolong 1, Cai Rui 3
1. Northwest Oilfield Branch of China Petrochemical Co., Ltd., Key Laboratory for Improving Harvest rate of Sinopec Seam Cave-type Oil and Gas Reserve;
2. China Petroleum Guangdong Petrochemical Corporation;
3. China National Petroleum Corporation Pipe Research Institute, National Key Laboratory for Service Behavior and Structural Safety of Petroleum Pipes and Equipment Materials.abstract
: Using high temperature autoclave to simulate the service conditions of the nano-modified epoxy powder coating in an oil field, taking the thermal scanning characteristics of the coating as the entry point, the corrosion resistance in neutral brine, high acid and strong alkali is studied, as well as the corrosion resistance in the three phases of gas, oil and water under simulated operating conditions, and the electrochemical impedance spectrum characteristics of the coating are analyzed under 6 simulated operating conditions. The results show that the glass transition temperature of the coating after soaking 30d in neutral brine, high acid and strong alkali exceeds 90 degrees C, which can be used at higher temperatures, and has better characteristics of chemical corrosion resistance and condition corrosion. Even after testing 30d in simulated operating conditions, the electrochemical impedance value is still higher than the 107 s cm2

. The anti-corrosion mechanism of coating is discussed.
0 Foreword
As an important replacement area for Sinopec's western resource strategy and one of the main production positions of crude oil, the Tahe oil field, because the extraction liquid has the characteristics of "high H2O, high CO2, high Cl, low pH", the medium corrosion is strong, the corrosion of the ground collection and transmission system presents "in-house corrosion-based, weak external corrosion, point corrosion-based, uniform corrosion is weak" characteristics. Ground-based pipeline corrosion perforation accidents occur frequently, with corrosion rates of up to 5.8mm/a. This not only directly affects and threatens the safe production of oil fields, but also causes a substantial increase in rescue maintenance and management costs. Therefore, the reasonable choice of metal pipeline protection technology is to extend its service life and avoid major accidents (safe production) an important measure.
coating has the characteristics of simple process, easy coating and low cost, and is the most effective and practical means of protection. Organic coating can effectively extend the service life of the pipe because it can prevent the corrosion medium from seeping into the steel substrate. Among them, epoxy resin molecules have a large number of active and polar groups, can be cross-cured with different types of curing agents, and become the most important anti-corrosion coating of oil and gas pipelines. However, in the more demanding conditions and complex service environment, the problem of coating failure is constant. Such as the Tahe oil field part of the coating also due to the increase in service life and aging, cracking, peeling and other conditions, resulting in partial nudity of metal pipes, causing serious corrosion and perforation accidents. Therefore, how to improve the corrosion resistance of epoxy resin has become the focus of scholars and technical personnel at home and abroad. It is reported that the main ways to improve the corrosion resistance of epoxy coating include the following two aspects: (1) chemically modifiing epoxy resin molecules, and (2) optimizing the performance of epoxy coatings with nano-inorgic fillers. Among them, small size nanoparticles are not only conducive to stress transfer, its extremely strong activity can also promote cross-linking reaction with epoxy resin, so that the bonding force between molecules is improved, enhance the tightness of the coating, and thus improve the corrosion resistance of epoxy resin. However, the corrosion resistance and service life of the coating depend not only on the composition and structure of the coating itself, but also on its service environment. Therefore, in this paper, the nano-modified epoxy powder coating was prepared in the melting of 20 steel surfaces, combined with the working conditions of the Tahe oil field, the corrosion-resistant characteristics and applicability of the coating were studied in depth and systematically.
1 Experiment
1.1 Materials
Test coating material for the market-purchased nano-modified epoxy powder, produced in Beijing, mainly composed of epoxy resin, phenolic resin, nanoSiO2, nano ZnO, leveling agent, defoulant, anti-sink agent, composite surfactant, couple agent, sulfuric acid, calcium carbonate and talcum powder. The substing material is 20 steel sheets with a size of 150mm × 30mm × 4mm.
1.2 Instrument equipment
complete set of powder electrostitive spraying device: Tianjin Honghuada Coating Equipment Co., Ltd.; YT-6050 blaster: Shenzhen Billion Automation Equipment Co., Ltd.; ZHD100 electronic moisture-proof cabinet: Hangzhou Zhishu Electronic Technology Co., Ltd.; 101 type electric heat blower drying box: Beijing Section Wei Yongxing Instruments Co., Ltd.; Elcometer456 Magnetic Dry Film Thickness Meter: Elcometer, United Kingdom; C276 High Temperature Autoclave: CORTEST, United States; P4000A Electrochemical Workstation: Princeton Appleid Research, United States; DSC3 Differential Scan Heater: Mettler-Toledo.
1.3 coating preparation
the surface of the hanging sheet is polished with Sandpaper No. 180 before coating, rinsed with acetone and alcohol and blow-dried. The surface of the hanging film is then blasted by a blaster, and the anchor depth is controlled at 50 to 100 m, and the surface dust is removed. After the sample is warmed up, the nano-modified epoxy powder is evenly sprayed on the sample surface with an electrostatial spray gun, and after completion, it is placed in an electrothermal drying box at 230 degrees C to cure 25min and remove it to room temperature. The thickness of the coating is measured using a thickness gauge, which is controlled at (200±50) sm.
1.4 performance test and charactertation
heating the sample from (20±5) to (180±10) at a rate of 20 degrees C/min, using a differential scan thermostat to test the thermal characteristics of the coating. The coating after simulating the environmental immersion test of high acid and strong alkali (see Table 1) was also tested for thermal characteristics.
Using high temperature autoclave, reference standard NACETM0185-2006 "Tube organic corrosion-resistant coating autoclave test method" for the coating simulation corrosion test, corrosion media are composed of crude oil phase, gas phase, water phase (field water). After placing 15d at the starting temperature of 75 oC, the sample is removed for observation, and then the temperature is changed to 90 oC and 15d is continued, other conditions are found in Table 2.
uses Princeton's classic three-electrode system (platinum electrode, Ag/AgCl electrode, coating test for working electrode) for electrochemical impedance spectrometry testing at frequencies from 100kHz to 5mHz with an amplitude of 10mV.
2 results and discussion of the thermal characteristics of the
2.1 coating
obtained its glass transition temperature (Tg) based on the tested coating thermal curve (shown in Figure 1): 94.4 degrees C before the test, high acidity After environmental immersion, it is 91.3 degrees C, 15% of NaOH strong alkali environment is 92.8 degrees C, and 30% NaOH strong alkali environment is 92.4 degrees C after immersion.
results show that the nano-modified epoxy powder coating is greater than 90 degrees C under different media conditions, and higher Tg means relatively good corrosion resistance and suitability at higher temperatures. However, there are still some differences in Tg under different conditions. The research shows that the glass transition temperature of epoxy coating is closely related to the molecular structure, curing degree and water content of epoxy. Therefore, after the coating is soaked by the chemical medium, its molecular chain structure is affected to some extent, and different Tgs also reflect the different degrees of effect of different chemical media.the corrosion resistance of the
2.2 coating
2.2.1 corrosion resistance to chemical media
as can be seen in Figure 2, after soaking 30d in 3% NaCl solution, the coating surface defects, and in 10% NaOH solution and 10% H2SO4 solution after soaking foaming or falling off.
2.2.2 Corrosion resistance
Figures 3 to 8 show the macro-shape of the nano-modified epoxy powder coating before and after immersion under Table 2 operating conditions.
As can be seen from Figure 3, in condition 1 (flow rate 0.5m/s), H2S-led corrosion environment, the coating in the oil phase, gas phase and water phase corrosion after the color has changed, but the surface is intact, there are no bubbles and other defects.
as shown in Figure 4, in the corrosive environment in which H2S-CO2 coexists under condition 2 (flow rate 0.5m/s), the color of the coating changes after corrosion in the oil, gas and water phases, but the surface remains intact.
as can be known from Figure 5, in condition 3 (flow rate 1.0m/s), CO2-led corrosion environment, the coating in the oil phase, gas phase and water phase corrosion after the color change is not obvious, and the surface is intact.
As can be seen from Figure 6, in condition 4 (flow rate 2.0m/s), H2S-led corrosion environment, the coating in the oil phase, gas phase and water phase corrosion after the color has changed significantly, but the surface still maintains good integrity, there is no large area of foaming phenomenon.
as shown in Figure 7, in the corrosive environment where condition 5 (flow rate 2.0m/s) and H2S-CO2 coexist, the color of the coating changes after corrosion in the oil, gas and water phases, but the surface integrity remains good.
as shown in Figure 8, in condition 6 (flow rate 3.0m/s), CO2-led corrosion environment, the coating in the oil phase, gas phase and water phase corrosion after the color change is not obvious.
From the macro-form after the coating corrosion test, it can be seen that the nano-modified epoxy powder coating sample surface did not appear bubbles and other defects, maintaining good integrity, indicating that the coating in the simulated environment has good corrosion resistance.
2.3 The electrochemical impedance spectrum characteristics of the coating
first of all, the nano-modified epoxy powder coating simulated operating conditions corrosion test 30d, that is, at the starting temperature of 75 degrees C test 15d, after taking out the observation to change the temperature to 90 degrees C to continue the test 15d, and then the sample for electrochemical testing, respectively, in the gas phase, oil phase and water phase after the test electrochemical impedance spectrum map 9 to 11.
before and after corrosion, the Nyquist chart consists mainly of a time constant, with a tail attached to individual conditions. The high-frequency zone's tolerance arc resistance is related to the transfer of charge between the sample and the electrolyte, and the larger the tolerance arc, the better the corrosion resistance of the sample. The presentation of anti-arc in the low frequency zone indicates that the corrosive medium has been transmitted to the coating/base metal interface, and the substrate metal has begun to corrode, and electrochemical reaction is the control step of the corrosion process. The arc radius is gradually reduced, reflecting the gradual decrease of the charge transfer resistance of the base metal, indicating that the corrosion rate of the metal is gradually increasing and the corrosion is becoming more and more serious.
barrier representing diffusion characteristics at low frequency ends means that the transfer process of coating/carbon steel interface corrosion reactions can be a rate control step for the entire corrosion system due to the blocking of nano-powders. The dispersion of low frequency data may be caused by the dispersion effect caused by severe local corrosion.
Coating samples in the gas phase, oil phase, water phase after undergoing 6 kinds of conditional corrosion impedance spectrum compared to the non-corrosive samples are significantly reduced, and in the gas phase, oil phase and water phase change trend is consistent, indicating that the coating corrosion resistance after gas phase, oil phase and water phase corrosion is reduced, and the law is consistent. The impedance spectrum decreases the most after corrosion of the water phase sample, followed by the oil phase sample, the gas phase sample is the smallest, indicating that the coating has the worst corrosion resistance in the water phase, followed by the oil phase, the corrosion resistance in the gas phase is relatively best. This is consistent with the coating failure mechanism. There are water molecules and ions in the water phase migration, they are easy to enter the coating caused by failure, the oil phase and gas phase in the ions are relatively few, the degree of corrosion is much lighter. In general, the impedance of the coating under H2S-led corrosion conditions is relatively minimal, followed by the co-existence corrosion conditions of H2S-CO2 and the largest impedance under CO2 corrosion conditions, which is consistent with the color variation characteristics of the sample surface. On the other hand, under the same H2S/CO2 pressure conditions, the flow rate has little effect on the corrosion resistance of the coating, which is different from the corrosion characteristics of the metal material, the coating belongs to organic polymer material, in a considerable period of time the surface will hardly dissolve to produce corrosion products, and the corrosion medium is entered into the coating through micro-holes, the change of flow rate will not affect its ion migration and exchange speed, so the change of flow rate will not have a great impact on the corrosion of the coating.
There are studies showing that when the coating resistance in the system is maintained at 108 to 109 s.cm2, the metal organic coating system has good corrosion resistance, the coating resistance is less than 107 s.cm2 indicates that the corrosion resistance of the system has decreased, and when the coating resistance is reduced to 106 s.cm2, the resistance of the coating to particles such as water is already very low, and electrochemical corrosion reactions may occur in the coating/metal interface.
nano-modified epoxy powder coating impedance between 107 and 109 s. cm2 under 6 conditions, such as immersion in the oil phase 0, 15, 30, 60, 90d after coating sample electrochemical impedance is 7.3× The nano-modified epoxy powder coating is very corrosion resistant ×109, 1.9×109, 7.2×108, and 5.3×108 s. cm2. Because electrochemical impedance testing is carried out on a complete, defect-free coating after corrosion, the position where the coating is not blistered or broken after corrosion still has good protective properties.
2.4 Anti-corrosion mechanism of the coating
Liu Ye et al. believe that there are three main forms of coating failure: the coating and substate binding force (i.e., the adhesion of the coating) is not easy to cause it to fall off;
Currently, the researchers propose a failure mechanism such as a bubble caused by volume expansion after the coating absorbs water, a coating that contains gas that causes the bubble, an electrical penetration that causes the bubble, and a permeable pressure that causes the bubble, but none of which can successfully explain the various phenomena associated with the failure of the coating. In contrast, the combined effect of osmosis pressure and absorbent volume expansion may result in coating bubbles being more generally accepted.
that produces permeable pressure are contaminants at the coating/metal substation interface, which are ubiquitous and difficult to avoid. These contaminants can be residual hydrophobic solvents, residual salts after phosphation or blasting treatment, SO24-ions generated by SO2 reactions in the atmosphere, grease-based substances, etc. Semi-permeability is a prerequisite for the production of osmosis pressure, and the membrane providing semi-permeability must have sufficient mechanical strength to resist osmosis pressure. The non-defective coating is largely permeable without the addition of voltage, but the water is passable and therefore meets the conditions of semi-penetration. For the bubbles that occur at the defect of the coating, because the hole is the direct channel of material transmission, the presence of the hole is not conducive to the formation of osmosis pressure, where there should not be a bubble. In practice, however, it is sent at the micro-holes in the coating