Materials Performance

AUG 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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56 AUGUST 2018 W W W.MATERIALSPERFORMANCE.COM 1. 4 h corrosion/10 S1 waveform cycles 2. 8 h corrosion/10 S1 waveform cycles 3. 12 h corrosion/10 S1 waveform cycles 4. 16 h corrosion/10 S1 waveform cycles 5. 20 h corrosion/10 S1 waveform cycles 6. 24 h corrosion/10 S1 waveform cycles For example, during the exposure time frame for 4 h corrosion/10 S1 waveform cycles, corrosion was carried out in 20% exfoliation corrosion (EXCO) solution 7 for 4 h and subjected to 10 S1 waveform fatigue cycles, then another 4 h in 20% EXCO solu- tion and 10 S1 waveform fatigue cycles, and then another 4 h in 20% EXCO solution and 10 S1 waveform fatigue cycles, etc. until the specimen experienced fatigue fracture, which determined the sample's corrosion/ fatigue alternating cycle life. Results and Discussion Fatigue Life Under Corrosion/ Fatigue Alternation The fatigue lives of the six sample groups, obtained by changing the corrosion time during the corrosion/fatigue alterna- tion, are shown in Table 2. Using Group 1 as an example, three samples were alternately exposed to corrosion in 20% EXCO solution for 4 h and 51,920 S1 waveform fatigue cycles until they fractured (Table 2). The average fatigue life for Group 1 was 249,216 cycles. Table 2 illustrates that the fatigue life of the specimens gradually decreased and the numb er of tim e s th e sp e cim ens were exposed to corrosion decreased as the cor- rosion time (h) increased in the alternation process. This is consistent with actual use. That i s, if th e aircraft 's p arkin g tim e increases, the environment will corrode the body components, the body 's fatigue per- formance will weaken, and the fatigue life will be reduced accordingly. Fatigue Fracture Under Corrosion/ Fatigue Alternation Crack Sources The fracture surface of each sample was observed by scanning electron microscope. It was found that the number of fatigue sources, and the area where they were located, increased with increasing corro- sion time during the alternation process. As shown in Figure 1, the white circles in the figures are the sources of fatigue cracks. Formation of slip bands in the material normally requires high stress. Due to the presence of larger second-phase materials or inclusions in the crack initia- tion sites, however, the presence of inclu- sions created stress concentration around the inclusions. This led to local plastic deformation, so that cracks initiated on the interface between the inclusions and the matrix. Fracture of inclusions also led to the initiation of cracks. Secondly, when exposed to the corro- sive solution, pits formed on the surface of the samples during immersion. The exis- t en c e of a c tiv e pit s cau s ed th e lo cal mechanical properties of the material to decline, which resulted in local deforma- tion at a low stress level and the initiation of cracks. Therefore, the number and area containing fatigue crack sources increased with the increase of corrosion time in the alternating process. Corrosion Pits In the alternating corrosion/fatigue interaction, greater pit density was found inside the material as the corrosion time increased in the alternating process. As the corrosion time increased from 8 to 24 h, the number and density of pits increased. Since the oxide film on the material's surface was destroyed by the medium in the etching solution, the electrochemical reaction took place at anodic areas of th e mat erial . Because of the presence of anodic phases in grain boundaries, corrosion initially tended to spread to the grain boundaries, leading to intergranular corrosion. Chloride ions in FIGURE 1 The number of the crack sources related to hours of corrosion time. FIGURE 2 Corrosion pits after corrosion exposure for 24 h. MATERIALS SELECTION & DESIGN

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