Abstract
Aims
This paper aimed to build an accelerated life test scheme and carry out quantitative analysis between accelerated life test in the laboratory and actual service for the coating composed of epoxy primer and polyurethane paint on structure components of some kind of radar served in the coastal environment of South China Sea.
Methods
The accelerated life test scheme was built based on the service environment and failure analysis of the coating. The quantitative analysis between accelerated life test and actual service was conducted by comparing the gloss loss, discoloration, chalking, blistering, cracking and electrochemical impedance spectroscopy of the coating.
Results
The main factors leading to the coating failure were ultraviolet radiation, temperature, moisture, salt fog and loads, the accelerated life test included ultraviolet radiation, damp heat, thermal shock, fatigue and salt spray. The quantitative relationship was that one cycle of the accelerated life test was equal to actual service for one year.
Conclusions
It was established that one cycle of the accelerated life test was equal to actual service for one year. It provided a precise way to predict actual service life of newly developed coatings for the manufacturer.
Keywords
Introduction
The coating system of radar structure components plays a vitally important role in controlling and protecting against the structural corrosion. However, with the time of radar operation and parking increasing, the physical and chemical properties of the coating in structure components will gradually degrade, caused by environmental damages, then the durability of structure components will degrade and finally the functionality of protection will deteriorate (1-2-3).
Most studies have been done about the structure components of aircrafts, especially in a coastal environment. Environmental factors such as temperature, damp heat, salt fog, ultraviolet radiation and pollution will accelerate the aging of coating. Corrosion becomes serious when metallic substrate and environment mediums directly contact, which consequently affects the reliability and safety of structure components (4, 5). Chinese and overseas researchers have studied the accelerated life test of coating to a certain extent based on the failure analysis. Chang et al (6) have studied the life variation rule of coating samples in NaCl solution by means of recording self-corrosion potential along with cyclic number, and put forward the expression of coating corrosion-fatigue life in certain experimental conditions. The research made by Yang and Liu (7) presents the accelerated corrosion environmental spectra of surface coating on the important parts of civil aircraft, and the spectrum is composed of damp heat, ultraviolet radiation, low-temperature fatigue and salt spray. Guseva et al (8) built the coating life assessment model under the stress condition of temperature, ultraviolet and salt spray through studying the loss of coating gloss under accelerated life test and actual service. Brunner et al (9) developed a device of accelerated climatic test, to simulate the actual working environment condition, and gain coating life data by exerting the stress of ultraviolet, temperature, humid and acid spray.
In brief, most researches aim at the failure condition of aircraft structural components. Furthermore, evaluating service life of radar surface coating is an important issue for formulating a coating maintenance plan and prolonging service life of the coating. However, there are few relevant researches and the integrated, reliable and applicative analysis and assessment methods are not put forward. Therefore, building the scheme of accelerated life test and confirming the quantitative relationship between accelerated life test and actual service for the coating on the radar structure components are the key points and difficulties in this field. The present work in this article intends to build an accelerated life-test scheme by analyzing the failure of radar surface coating. At the same time, the quantitative relationship between accelerated life test in the laboratory and actual service is established in the present research by comparing the gloss loss, discoloration, chalking, blistering, cracking and the electrochemical impedance spectroscopy of the coating.
Coating failure analysis for some kinds of radar equipment in service
Serious coating failure has been found in some kinds of radar equipment served in the coastal environment of South China Sea at the first time of overhaul, as shown in Figure 1.
Macro failure morphology of coating on the radar structure components.
The metallic substrate of radar structure components is carbon steel. The coating system of the structure components are epoxy primer and polyurethane paint, and the service environment is a tropical marine climate, which includes severe sunlight radiation, maximum temperature of 36°C, relative humidity between 70% and 80%, chloride ion with content of around 0.2 mg·m−3. It is obvious that the main factors triggering coating ageing are ultraviolet radiation, temperature, humidity and environmental medium in the coastal environment. Under the co-actions of factors mentioned above, the performance of the coating gradually degrades. Meanwhile, the phenomenon of blistering, peeling and cracking obviously appears and results in the final failure. Coating failure causes are analyzed in detail as follows.
The proportion of ultraviolet rays in sunshine is approximately 3%, and the energy of ultraviolet rays accounts for 310∼420 kJ·mol−1 under the condition of coastal environment while the oxidation activation energy of most polymers in the coating is 40∼170 kJ·mol−1, and chemical bond dissociation energy is 60∼410 kJ·mol−1. Therefore, molecular chains of polymers in the coating will break down after long time exposure to ultraviolet irradiation. The polymer molecules will become activated state molecules after absorbing the ultraviolet light quantum, and then photochemical reaction of some activated state molecules occurs, which results in the rupture of carbon-oxygen bond and the formation of small molecular compounds generated by the break of polymers.
In the coastal environment, coating gradually degrades under continuous thermal cycling conditions caused by temperature difference under different times and locations. Therefore, the thermal property, insulating property and mechanical property of coating decreases continually. In addition, oxidation rate of resin in organic coating is accelerated, reaction activation energy decreases, and main chains rupture when exposed to high temperatures over a long time period. With coating degrading by the influence of ultraviolet and temperature, a great many defects such as micro-pores and impurities emerge in coating on the structure components, as shown in Figure 2.
The relative humidity has a significant effect on the interface adhesion between coating and metallic substrate. The interface adhesion of coating-metallic substrate interface will reduce with the increase of relative humidity. In the coastal environment, the surface of coating is liable to form a water film layer in virtue of salt fog, rainfall, dew and other natural environment conditions. At the same time, the molecule's bonds such as -CHO-O-C- and -CH2-O-CH2- in coating tend to be damaged by water effect. As pores increase and extend after degradation of coating, the saline matter in water film penetrates into the interface between coating and metallic substrate through the defects in the coating. The metal under the coating defects is the negative polarity zone, making up the corrosion cell. Organic coating generates small molecular products under the co-action of ultraviolet radiation, water, oxygens and chloride ion. Consequently, the volume of coating expands, the porosity increases, and permeability of coating declines. After water evaporation in coating, the coating surface shrinks, and great quantities of stresses in coating occurs. Coating will blister and fall off when the stresses reach the bonding strength between coating and metallic substrate.
The content of chloride ion in the coastal environment is relatively high. After pores, cracks and other defects appear in the coating, the ability of coating to prevent corrosive medium declines, and water, which contains chloride ion, immerges into interface between coating and metallic substrate, resulting in the increase of electrical conductivity and the serious decline of electrochemical impedance performance of coating.
Micro pores emerged after the coating degradation.
Technical scheme of accelerated life test
Service life of 2.5 years in the coastal environment of South China Sea was confirmed in actual service by the manufacturer for the above coating composed of epoxy primer and polyurethane paint on the radar structure components. However, service life of newly developed coating on the radar structure components in the coastal environment should be evaluated and confirmed for the manufacturer. Adopting accelerated failure method in the laboratory and the quantitative relationship analysis between accelerated life test in the laboratory and actual service is necessary since the cost of the fatigue property test and actual service life test in outfield is huge. The same degree of damage can be achieved in less time compared with actual service if the accelerated life test is adopted. Quantitative relationship analysis can be carried out by comparing the degree of coating damage between accelerated life test and actual service. Based on the failure analysis of coating on the radar structure components, accelerated life-test scheme is proposed as shown in Figure 3.
Technical scheme of accelerated life test.
The failure analysis of the coating on the radar structure components shows that the life of the coating is controlled by service environment and working stresses. The main factors leading to failure are temperature, humidity, ultraviolet radiation, temperature difference, salt fog and loads. In order to reasonably reflect the actual influence of the stresses of high humidity, salt fog, ultraviolet radiation, thermal shock and operation loads, an accelerated life-test cycle which includes ultraviolet radiation, damp heat, thermal shock, fatigue and salt spray is proposed in this article as shown in Figure 4. The times of test cycle for the accelerated life test should be determined by the damage degree of the coating. According to service environment of the coating on the radar structure components, the accelerated life-test cycle and test condition are as follows:
Accelerated life test cycle.
Ultraviolet irradiation test: according to the principle that the ultraviolet radiant quantity of coating received in chamber equals the actual quantity, one cycle-test time of ultraviolet radiation should be calculated from the value of the total ultraviolet radiant quantity of each cycle divided by radiation intensity of coating samples in ultraviolet radiation chamber. Moreover, the aging of coating resulted from ultraviolet radiation has the relationship with temperature of sample surface achieved by light. The test condition of ultraviolet radiation is confirmed as follows:
Irradiation time: t = 5 days;
Temperature range of sample surface: T = 55°C ± 5°C.
Heat and moisture test: the temperature and relative humidity of tropical coastal areas in Southern China are very similar to the marine climate and environment of US coastal states (10); thus, this article employs temperature and humidity parts of environmental spectrum of coating accelerated test established by the US Air Force. The test condition is confirmed as follows:
Temperature: T = 43°C ± 2°C;
Relative humidity: RH = 95% ± 3%;
Test duration: 7 days
Thermal shock test: rapid change of temperature will affect the performance of coating and adhesion between coating and metallic substrate. Hence, the thermal shock test should be adopted in the accelerated life-test cycle to simulate the effect caused by rapid temperature change. Extreme temperature of thermal shock test depends on the actual temperature in service. Meanwhile, it should play the role of accelerating the ageing process of the coating. In order to simulate the influence of temperature change on the coating, heating and cooling processes of test should be reasonably controlled. The test condition is confirmed as follows:
Maximum temperature: 100°C;
Minimum temperature: -20°C;
Transfer time: ≤10 seconds;
Shock number: 5 times.
Fatigue test: the adhesion between coating and metallic substrate gradually declines with various long-term loads, and then protective performance declines severely. The fatigue loads of the radar structure components approximately periodically alter, and the maximum load of fatigue test is the peak load in stress cycle. The test condition is confirmed as follows:
Maximum load: 78 MPa;
Minimum load: 25 MPa;
Cycle number: 360 times.
Salt spray test: salt fog is a crucial factor of corrosion for coating and metallic substrate on the radar structure components. The acid rain sometimes occurs in south tropical marine climate. Therefore, acetic acid-salt spray test with solution of pH 4.5 is feasible, and the condition is confirmed as follows:
Temperature: T = 35°C ± 2°C;
pH: 4.5;
Salt spray deposition: (1∼2) mL · (80 cm2 · h)−1;
Test time: 7 days.
The quantitative analysis of accelerated life test and practical service under coating degradation
Service life of 2.5 years in the coastal environment was confirmed in actual service by the manufacturer for the above coating composed of epoxy primer and polyurethane paint on the radar structure components. The coating samples after actual service for one year, two years and three years are prepared and collected by the manufacturer. At present, there is no preferable quantitative evaluation method for measuring the degree of coating degradation. The quantitative relationship between accelerated life test in the laboratory and actual service is concluded in this work based on the comparison of gloss loss, discoloration, chalking, blistering, cracking, and the analysis of the electrochemical impedance spectroscopy of the coating, and the actual service time equivalent to one accelerated life-test cycle is determined finally.
Accelerated life test was conducted for coating samples composed of epoxy primer and polyurethane paint on the unused radar structure components. The phenomenon of gloss loss, discoloration and chalking of the coating slightly occurred after the first cycle of accelerated life test. The gloss loss, discoloration and chalking became obvious after the second cycle. After the third cycle, coating around rivets on the samples was blistered, the micro-cracks appeared, and the degree of gloss loss, discoloration and chalking became serious, as shown in Figure 5. After the fourth cycle, the crack of coating and substrate corrosion were observed through macroscopic inspection.
Appearance of the coating after the third test cycle.
Gloss loss, discoloration, chalking, blistering and cracking are the important indicators to measure the failure of the coating. The measurement of electrochemical impedance spectroscopy is also a significant method to evaluate the protective performance of organic coating. Through applying the technique to investigate the change of low-frequency impedance of organic coating, the change of organic coating protective property can be quantitatively characterized (11). According to GB/T 1766-2008 “the rating method of aging of paints and varnish coating”, the damage degree of the coating after one cycle, two cycles and three cycles of the accelerated life test in the laboratory and the coating after actual service for one year, two years and three years provided by the manufacturer was evaluated, and the indexes of gloss loss, discoloration, chalking, blistering and cracking were classified. The results were shown in Table I, and the electrochemical impedance spectrums between 10−2 Hz and 105 Hz of the above several coatings were measured in Figure 6.
Evaluation of the organic coatings aging according to GB/T 1766-2008
Electrochemical impedance spectrums of the coating.
It is known that the grade of the coating after one cycle, two cycles and three cycles of the accelerated life test in the laboratory approximately equals the grade of the coating after actual service for one year, two years and three years provided by the manufacturer, and the electrochemical impedance spectrums of the two conditions are basically the same as shown in Table I and Figure 6. It is concluded that the quantitative relationship between accelerated life test in the laboratory and actual service is that one cycle of the accelerated life test equals actual service for one year. The actual service life of the newly developed coating can be predicted after the accelerated life test in the laboratory for the manufacturer based on the above quantitative relationship.
Conclusion
The failure of coating on the radar structure components has been studied in the present work, and the main conclusions are as follows:
The main factors leading to the coating failure are ultraviolet radiation, temperature, moisture, salt fog, and loads through analyzing the failure of coating on the radar structure components served in the South China Sea coastal environment.
An accelerated life-test technical scheme has been developed for coating on the radar structure components served in coastal environment, and it includes ultraviolet radiation, damp heat, thermal shock, fatigue and salt spray. After the third cycle of the accelerated life test, coating samples were blistered and cracked, and the coating was failed.
The quantitative relationship between accelerated life test in the laboratory and actual service is concluded in this work based on the comparison of gloss loss, discoloration, chalking, blistering, cracking and the analysis of the electrochemical impedance spectroscopy of the coating. It is that one cycle of the accelerated life test equals actual service for one year. The actual service life of the newly developed coating can be predicted after the accelerated life test in the laboratory for the manufacturer based on this quantitative relationship.
Footnotes
Financial support: No grants or funding have been received for this study.
Conflict of interest: None of the authors has financial interest related to this study to disclose.