利用三點彎曲試驗探討Mg-5wt.%Sn合金在不同NaCl濃度下蝕孔與應力腐蝕裂縫關係之研究

Abstract

本實驗採用的三點彎曲試驗法，有別於一般常用之慢應變速率拉伸試驗的定量量測，乃是一種觀察應力腐蝕裂縫的定性分析。本實驗主要藉由三點彎曲試驗來分析Mg-5wt.%Sn合金在含不同Cl-濃度(1wt.%、3.5wt.%、7wt.%)環境下蝕孔與裂縫之間的關係以及裂縫的傳播路徑。 實驗結果發現Mg-5wt.%Sn合金在各環境下應力腐蝕裂縫為很明顯的沿晶走向，主要原因是該合金經過熱處理後Mg2Sn第二相以5~10μm細長狀分布在晶界上，而在Mg2Sn的伽凡尼腐蝕效應以及晶界高化學活性的作用下，晶界成了此合金應力腐蝕裂縫的優先活化路徑。 另外，透過掃描式電子顯微鏡(SEM)觀察到Mg-5wt.%Sn合金浸置於越高Cl-濃度環境下，蝕孔的數目也隨之增加，且應力腐蝕效應越大，亦促使蝕孔成長及加速裂縫的生成。Mg-5wt.%Sn合金在7wt.%NaCl中浸泡約3小時會產生裂縫，在3.5wt.%NaCl則需浸泡大約5小時始出現裂縫，然而在1wt.%NaCl中浸泡8小時還未觀察到裂縫的生成，明顯表達在濃度低時裂縫出現的潛伏期相較濃度高時為長。而裂縫的傳遞速率在高濃度區段會較低濃度區段為快。 由以上結果可以觀察Mg-5wt.%Sn蝕孔數目、深度、裂縫出現時間以及傳遞速率對於Cl-濃度的敏感性，證實Cl-對於表層鈍化膜(MgO/Mg(OH)2)的影響。

In this study, we investigated the qualitative analysis of stress corrosion cracks for Mg-5wt.%Sn alloy by employing 3-point bending test which was different from slow strain rate test regarded as quantitative measurement. We focused on analyzing the correlation between pittings and stress corrosion cracks by using the method mentioned above as well as the discussion of the crack propagation path of Mg-5wt.%Sn under different concentration of Cl- (1,3.5,7wt.%). The result indicated that the route of stress corrosion crack of Mg-5wt.%Sn was obvious intergranular type. We suspected that this behavior could be attributed to the second phase Mg2Sn which was 5~10μm and distributed along the grain boundary after heat treatment. As a result, with the galvanic effect of Mg2Sn and the high chemical activity of grain boundary , we suggested that the prior stress corrosion path of Mg-5wt.%Sn could be grain boundary. The phenomenon that number of pittings for Mg-5wt.%Sn increased with increasing Cl- concentration could be observed through Scanning Electron Microscope (SEM). Besides, Cl- would promote the growth of pittings and accelerate the generation of cracks. The cracks of Mg-5wt.%Sn alloy would appeared with immersion in 7wt.%,3.5wt.% and 1wt.% NaCl solution for 3 hours,5hours and more than 8 hours, respectively. This outcome expressed that the incubation period of crack initiation with immersion in higher Cl- concentration could be sooner than that with immersion in lower Cl- concentration. Furthermore, the propagation rate of cracks under higher Cl- concentration was faster than that under lower Cl- concentration. It could be concluded that the number and depth of pittings, the incubation time of cracks and the propagation rate of cracks were highly sensitive to the concentration of Cl-. These results confirmed the influence of Cl- concentration on the surface passive film which was mainly composed of MgO and Mg(OH)2.