表面腐蝕生成物對熱處理AA7050鋁合金在鹼性3.5% NaCl水溶液中應力腐蝕之影響

Effect of Pre-immersion on the SCC of Heat-treated AA7050 in an Alkaline 3.5% NaCl

本文研究AA7050鋁合金經過不同熱處理T6、T73、RRA、OP1以及OP2後，預浸泡在鹼性（pH=12） 3.5% NaCl水溶液中240小時，對其應力腐蝕破裂（SCC）之影響。使用慢應變速率拉伸試驗（Slow Strain Rate Test, SSRT）、電化學試驗（Electrochemical Test）、霍氏光譜分析（FTIR）及化學分析電子光譜儀（ESCA）等方法，探討合金生成表面膜與應力腐蝕破裂（SCC）性質關係。實驗結果顯示，AA7050鋁合金試片若預先浸泡在鹼性氯化鈉腐蝕液中240小時，則因表面生成腐蝕生成物，會提高其抗SCC能力，此種影響以T6熱處理試片最大，其次為RRA熱處理，即T6 （43%） > RRA （27%） > OP1 （18%） > OP2 （14%） > T73 （11%）。透過EDS分析AA7050鋁合金表面腐蝕生成物，其主要成份為Al、Zn、Mg及Cu之氫氧化物；在FTIR的結果中顯示，T6熱處理試片，其表面腐蝕生成物中，Cu的氫氧化物較OP1熱處理者多，此時抗SCC能力最低。AA7050試片在不同熱處理後，浸泡於鹼性3.5% NaCl溶液中240小時，其表面腐蝕生成物經ESCA分析顯示，T6表面生成膜主要為CuO組成，OP1與RRA則為CuO與CuCl2混合生成物。

Aluminum alloy AA7050 heat-treated variously (i.e., T6, T73, RRA, OP1, and OP2) was pre-immersed in an alkaline (pH = 12) 3.5% NaCl for 240 hours, and the effect of the pre-immersion on its stress corrosion cracking (SCC) in the same environment was of interest. This effect was investigated using slow strain rate test (SSRT) under electrochemical monitoring. The corrosion products on the specimens were analyzed through Fourier-transform infrared spectrometer (FTIR) and electron spectroscopy for chemical analysis (ESCA).
For the specimens experienced the same heat treatment, the pre-immersed ones revealed higher SCC-resistance than those without pre-immersion. The enhancement of SCC-resistance varied with the type of heat treatments, i.e., T6 (43%) > RRA (27%) > OP1 (18%) > OP2 (14%) > T73 (11%). Analysis of the corrosion products benefited to delineate the enhancement in SCC-resistance. Examination of through scanning electron microscopy (SEM) equipped with energy dispersive spectrophotometer (EDS) demonstrated that the corrosion products consist of elements such as Al, Zn, Mg, Cu and oxygen. Resulting from FTIR spectra, the corrosion product on T6 specimen enriched more copper hydroxide than on OP1 and RRA specimens. Analysis of ESCA exhibited that pre-immersion leads to the formation of CuO in the corrosion products of T6 and a mixture of CuO and CuCl2 in the corrosion products of OP1 and RRA..