1. Test the radiator
The experimental objects are six kinds of radiators with different materials, and their numbers are shown in Table 1 to table 3 (including flat tube, fin and collector tube). A. The flat tubes of B and C are non composite materials. A is 1xxx pure aluminum, the other two are AA3003 high frequency welded pipe and extruded pipe. The three kinds of composite flat tubes are AA3003, fa7805 and fa7872. Table 2 shows the strength, total thickness and other physical parameters of various flat tubes after brazing. Because of the strength advantage, the composite tube material, especially the long-life material fa7872, can be 20-40% thinner than the three non composite tube materials. The application of this kind of high strength alloy meets the market demand for light products.
The fin chemical composition of each radiator is shown in Table 3. The combination of fin and flat tube is also very important to the corrosion resistance. In the subsequent experiments, we can see the influence of different collocations on the external corrosion resistance of radiator.
2. Test and results
In order to compare and evaluate the corrosion resistance of these flat tubes and their combinations, accelerated corrosion tests in laboratory, including salt spray corrosion test, immersion test and oy solution corrosion test, were used to evaluate the external and internal corrosion resistance of different flat tubes and their fin combinations.
2.1 external corrosion test:
a. Salt spray corrosion test (accelerated atmospheric corrosion test of American Standard ASTM g85-2a3) was used to test the corrosion resistance of brazed side. This is also the most widely used laboratory accelerated corrosion test, and it is also the most widely used laboratory accelerated corrosion test. The experimental sample is placed in Ascott (s1000xp) salt spray chamber, and the main plane is 15 o angle to the vertical direction. The simulated seawater solution with pH of 2.6-3.0 and [Cl -] = 0.56mol/l was used as the experimental solution. During the experiment, the temperature in the chamber was kept at 49 ± 1 ℃, and the half-hour solution spraying and 1.5-hour residence were conducted alternately. The relative humidity in the box was ≥ 98%. In the process of exposure test, the sample is taken out regularly for leak detection, and the failure condition is judged.
The perforation failure time of each radiator sample in salt spray corrosion test is shown in Fig. 1. The service life of three kinds of non composite 1xxx pure aluminum and AA3003 flat tube combination is less than two days, and the service life of composite flat tube, especially long-life materials fa7805 and fa7872 composite flat tube combination, is more than 40 days in salt spray test. The corrosion resistance of flat tube material is very important. At the same time, the sacrificial protection effect of fins on flat tubes can further extend the service life of radiators.
The pitting of the flat tube during the 2-day experiment was caused by the rapid corrosion of the flat tube, as shown in Fig. However, the fins welded with the non composite flat tube had only slight corrosion, and the fins did not play a sacrificial protection role for the flat tube. The fa82a4 composite flat tube with AA3003 as the core material has a higher sacrificial protection effect due to its combination with fins with higher zinc content, thus prolonging its life in salt spray experiment.
b. Immersion test: the surface of flat tube brazed side was etched with caustic soda solution to remove part of the composite layer, and then immersed in the corrosion solution [Cl -] = 0.56 mol / L. After immersion for 5 hours, the sample surface was treated with concentrated nitric acid, and the surface morphology and cross-sectional morphology were observed, as shown in Fig. 3. The longitudinal deep pitting corrosion occurred in the non composite and composite flat tubes of pure aluminum and 3003, while the two long-life composite tubes only showed transverse corrosion in the surface layer. This is because the diffusion precipitation zone formed between the brazing layer and the core material plays a good sacrificial protection role in brazing of long-life materials.
2.2 internal corrosion test
Oy aqueous solution circulation experiment. The salt solution containing Cu2, Fe, Cl -, SO42 – ions and pH = 3 circulates in the flat tube at a constant speed. The temperature of the reaction system is periodically switched between 88 ℃ and room temperature. In the experiment, the corrosion morphology of each sample was observed regularly, and leakage detection was carried out. As shown in Table 4, the composite flat tube D, e and f all have the water contact side of aa7072 or fa6815 alloy. The composite layer on the water contact side has a lower electrode potential compared with the core material due to the presence of certain zinc alloy element, thus providing sacrificial protection for the core material. No perforation failure occurred in the three kinds of composite flat tubes during 24 days of cyclic test.
In the laboratory accelerated corrosion test, the non composite materials of pure aluminum and AA3003 have premature perforation failure due to the poor performance of brazing side and water contact side. In practical use, it is very easy to fail due to corrosion perforation, which can not guarantee one year life. Similar materials should not be used as flat tubes in radiators.
High strength and long-life composite materials fa7805 and fa7872, as flat tubes of radiators, show excellent corrosion resistance and provide possibility for further thinning of materials and continuous reduction of product cost.
At the same time, the proper fin collocation can provide sacrificial corrosion protection to the flat tube and further prolong the service life of the radiator.