Effect of nitrate on exfoliation corrosion of 2A12-T4 aluminum alloy under full-immersion corrosion condition
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摘要:
针对沿海暴露20年的2A12-T4铝合金试验件，采用球差校正透射电镜，在剥蚀区域纵截面腐蚀最深处的腐蚀产物中，首次测到了N元素的存在，说明铝合金沿海大气剥蚀过程中存在${\text{NO}}_x^{{ - }}$。开展2A12-T4铝合金在4种不同${\text{NO}}_3^{{ - }}$浓度腐蚀溶液中的全浸腐蚀试验，发现随着溶液中${\text{NO}}_3^{{ - }}$浓度降低，试验件的剥蚀严重程度明显减弱，且在含有${\text{Cl}}_{}^{{ - }}$和${\text{SO}}_4^{{{2 - }}}$，而不含${\text{NO}}_3^{{ - }}$的溶液中，试验件未能发生剥蚀。结合对产生气体的测试和离子色谱仪分析，表面和截面腐蚀行为扫描电子显微镜（SEM）、能谱仪（EDS）分析等，讨论${\text{NO}}_3^{{ - }}$对铝合金剥蚀的影响机理。建议在开展飞机铝合金结构沿海大气剥蚀模拟试验时，不能忽视腐蚀环境中${\text{NO}}_3^{{ - }}$的作用。
Abstract:The 2A12-T4 aluminum alloy specimen exposed to the coastal atmosphere for 20 years was analyzed with spherical aberration corrected transmission electron microscope. The presence of nitrogen was first determined in the corrosion products at the deepest corrosion part of the longitudinal section of the exfoliation corrosion area. This indicated the presence of ${\text{NO}}_x^{{ - }}$ in the coastal atmospheric exfoliation corrosion process of aluminum alloy. Then the full immersion corrosion tests of 2A12-T4 aluminum alloy in four different nitrate concentration corrosion solutions were carried out. It was found that the exfoliation corrosion severity of the specimens decreased significantly with the decrease of nitrate concentration in the solution; while in the solution containing ${\text{Cl}}_{}^{{ - }}$ and ${\text{SO}}_4^{{{2 - }}}$ rather than ${\text{NO}}_3^{{ - }}$, the exfoliation corrosion of the specimens did not occur. At the same time, combined with the test of the generated gas, ion chromatograph analysis, surface and sectional corrosion behavior SEM and EDS analysis, etc., the influence mechanism of nitrate on the exfoliation corrosion of aluminum alloy was analyzed and discussed. It is suggested that the role of nitrate in the corrosive environment cannot be ignored when carrying out the coastal atmospheric exfoliation corrosion simulation tests of aircraft aluminum alloy structures.
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Key words:
- nitrogen element /
- exfoliation corrosion /
- aluminum alloy /
- nitrate /
- atmospheric exfoliation corrosion
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图 2 暴露20年后试验件剥蚀区域纵截面（S-L）的晶间腐蚀形貌和腐蚀产物EDS分析
Figure 2. S-L sectional SEM morphology and results of EDS of products of intergranular corrosion inside exfoliation corrosion area of 2A12-T4 aluminum alloy exposed for 20 years
图 3 试验件分别在1#、2#、3#、4#溶液中腐蚀192 h后的宏观表面腐蚀形貌（1-8#为对照组）
Figure 3. Macroscopic surface corrosion morphology of specimens after 192 h corrosion in 1#, 2#, 3# and 4# solutions (1-8# is control group)
图 4 试验件分别在1#、2#、3#、4#溶液中腐蚀192 h后的微观表面腐蚀形貌
Figure 4. Micro surface corrosion morphology of specimens after corrosion in 1#, 2#, 3#, 4# solution for 192 h
图 5 不同试验件的纵截面（S-L）腐蚀形貌
Figure 5. Microscopic corrosion morphologies of S-L sections of different specimens
图 6 腐蚀192 h后1#溶液的离子色谱分析结果
Figure 6. Results of ion chromatographic analysis of solution 1# after 192 h corrosion
图 7 纵截面（S-L）腐蚀形貌和能谱分析结果（腐蚀5 h）
Figure 7. Corrosion morphology and EDS results of longitudinal S-L sections (corrosion for 5 h)
图 8 近似平行于L向的连续晶界发生严重晶间腐蚀的过程示意图
Figure 8. Schematic diagram of process of severe intergranular corrosion occurring at the continuous grain boundary approximately parallel to L direction
图 9 2-8#试验件的纵截面（S-L）腐蚀形貌
Figure 9. Corrosion morphology of longitudinal S-L section of 2-8# specimen
表 1 2类试验件2A12-T4铝合金材料的化学成分
Table 1. Chemical composition of two types of specimens (2A12-T4 aluminum alloy)
% 类型 化学成分的质量分数 Si Fe Cu Mn Mg Ni Zn Ti Al 大气
暴露0.10 0.22 4.62 0.54 1.60 0.99 0.22 0.13 余量 实验室腐蚀 0.40 0.35 4.30 0.70 1.40 0.08 0.20 0.13 余量 
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表 2 万宁市自然环境试验站大气环境数据
Table 2. Environmental data of Wanning test site
环境参数 年平均值 温度/°C 23.9 相对湿度/% 87.6 降水量/mm 198.156 风速/（m·s−1） 2.431 SO2/（mg·m−3） 0.045275 NO2/（mg·m−3） 0.002025 Cl− 沉积速率/（mg·（dm2·d）−1） 14.5875 降雨pH 5.125
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表 3 不同腐蚀溶液腐蚀介质成分及浓度
Table 3. Components and concentrations of corrosion media in different corrosion solutions
腐蚀溶液编号 腐蚀介质成分 浓度/ (mol·L−1) PH 1#（EXCO溶液） NaCl 4.00 0.4 KNO3 0.50 HNO3 0.10 2# NaCl 4.00 0.4 K2SO4 0.25 H2SO4 0.05 3# NaCl 4.00 0.4 K2SO4 0.25 HNO3 0.10 4# NaCl 4.00 0.4 KNO3 0.50 H2SO4 0.05
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表 4 试验件编号示例
Table 4. Example of specimen number
腐蚀
时间/h试验件编号 1#
（EXCO溶液）2#
（EXCO溶液）3#
（EXCO溶液）4#
（EXCO溶液）5 1-1# 2-1# 3-1# 4-1# 24 1-2# 2-2# 3-2# 4-2# 48 1-3# 2-3# 3-3# 4-3# 96 1-4# 2-4# 3-4# 4-4# 120 1-5# 2-5# 3-5# 4-5# 144 1-6# 2-6# 3-6# 4-6# 168 1-7# 2-7# 3-7# 4-7# 192 1-8# 2-8# 3-8# 4-8#
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表 5 2-8#试验件的纵截面（S-L）EDS数据
Table 5. EDS results of longitudinal S-L section of 2-8# specimen
% 点 化学成分质量分数 O Na Mg Al S Cl K Cu 1 20.34 1.08 68.55 10.03 2 24.45 1.82 64.44 9.30 3 17.29 1.49 61.86 19.36 4 29.67 1.21 59.70 9.42 5 33.84 0.95 0.92 58.57 1.82 0.37 3.53 6 47.75 1.03 45.36 2.45 3.41 7 40.88 1.07 0.62 51.66 1.60 0.61 0.41 3.14 8 45.10 0.87 47.94 2.17 0.67 0.32 2.93 9 60.46 32.09 3.08 0.66 3.71
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