How to do atmospheric exposure test for thermal spray coating

       The atmospheric exposure corrosion test of the coating is mainly aimed at large-area sprayed zinc, sprayed aluminum or zinc-aluminum alloy long-term protective composite coatings. my country has a vast land area, thousands of kilometers of coastline, and the atmospheric environment is very different. The atmospheric corrosion and corrosive effects of different regions are very different. The corrosiveness of the atmosphere in the rural environment is much lower than that of the urban, industrial, and marine atmospheres. In urban and industrial atmospheres, there will be sulfur-containing fuel combustion products, so that the atmosphere contains a large amount of sulfur trioxide. When the gaseous sulfur dioxide is dissolved in dew, the pH value is reduced, and the corrosion process and supporting electrolyte are provided; coastlines fly in the marine environment. Chloride generated by fog, in the form of solid or fine concentrated droplets, is the main cause of atmospheric corrosion.



The salt exposure test is carried out to obtain data on the corrosion resistance of coatings in various atmospheric environments, and to evaluate the relationship between the test results under given test conditions and atmospheric environments. The rate of corrosion depends on the relationship between the environmental and atmospheric variables of atmospheric corrosion environmental testing, and is so complex that field exposure test results cannot fully and accurately predict service performance, but can provide guidance on the service performance of coatings.



The atmospheric exposure test method specifies the technical requirements for the type of test sample, sample preparation, sample handling, sample marking, test quantity, comparison and reference samples, sample storage, sample data recording, etc.



The technical requirements and relevant regulations for the atmospheric corrosion test site are put forward in terms of atmospheric exposure types, test site requirements, test site location, test site safety, test site environmental factors, etc., as well as sample placement, test procedures, test reports, etc. technical requirements or regulations.



1. Sample requirements



150mm x 100mmx (1~ 3) mm rectangular flat sample, according to the national standard of GB/T9793 "Metal and Other Inorganic Covering Thermal Spraying Zinc, Aluminum and Its Alloys", the coating is prepared on 6 surfaces, and the coating thickness δ= 150-20um, select the corresponding sealing agent for sealing treatment; the number of samples is not less than 3, and there are obvious marks, which should be clear and durable during the entire exposure test. The markings on the test specimens shall be in areas that do not interfere with the inspection and evaluation of the test results.



2. Sample data record



Record the substrate material, surface treatment method, coating material (spraying material, sealer material), spraying process method and process parameters, coating thickness (metal coating thickness, sealing layer thickness), coating physical properties (porosity) of the sample rate, etc.).



3. Atmospheric corrosion test site



Select a site that can be normally exposed to the sufficient action of the climatic environment as the test site, and should be set in a representative site similar to the environment where the coating material is used. The sample is directly exposed to the open air exposure in the entire atmospheric conditions and air pollution environment; or a sheltered or partially enclosed space, such as a louver box, etc.; if conditions permit, it can also be exposed to the test on site.



4. Exposure Rack



The exposure rack is used to hold the specimen. To ensure that the sample is not damaged during the test, the corrosion of the sample is not affected by other materials, and the height of the sample exposure test from the ground must be greater than 0.75m. The sample is placed on the exposure rack and preferably kept at a 45% inclination from the ground plane.



5. Test procedure



The atmospheric corrosion time is long and the process is slow. According to the corrosion resistance of the coating material, the exposure test time can be arranged for 1 year, 2 years, 5 years, 10 years and 20 years. According to the length of the test period, regularly check and observe the corrosion status. The inspection and observation interval is generally 6 months or 12 months, and the interval for the test period less than 6 months is 1 month, 3 months, and 6 months. Records include color, condition, uniformity, and spalling tendency of corrosion products.



6. Evaluation of results



The preform is evaluated by visual inspection, metallographic inspection, and changes in mass loss.