LZ91鎂鋰合金之二次相對過錳酸鹽化成處理的影響

Abstract

LZ91雙相鎂鋰合金由HCP結構的α-Mg與BCC結構的β-Li所組成，兩者分別提供材料之強度與延展性，使其成為極具潛力的輕量化材料。然而，LZ91合金在大尺度上有雙相微結構誘發的伽凡尼效應，在小尺度上同樣有晶出相與析出相的微小電池對，使局部腐蝕更加劇烈。LZ91較差的抗蝕性限制了其在工業上的應用。本研究首先針對LZ91鎂鋰合金進行基材與二次相的分析，再使用過錳酸鹽化成處理使底材表面生長一層化成膜，再透過腐蝕測試比較化成前後LZ91合金之抗蝕性並同時探討二次相對化成處理的影響。 為了解LZ91鎂鋰合金之基材與二次相，首先使用XRD、EBSD等進行相鑑定與微結構形貌觀察。接著，將40 ℃ 0.1 M KMnO4化成液以濃硫酸調整至pH=1.5，再對LZ91底材進行20與50 s化成處理。化成膜之形貌與組成分別由SEM與EDS等分析方法觀測。最後，透過OCP、PDP、即時攝影、浸泡試驗與析氫等腐蝕測試方法了解腐蝕產物層與化成膜的抗蝕性與腐蝕行為。 研究結果顯示，LZ91鎂鋰合金的二次相除了有雙相隨機分布的微米級晶出Fe-Mn相，還有析出於β-Li晶界與晶粒內部、α/β相界之次微米級MgLi2Zn相。在過錳酸鹽化成處理20 s後，LZ91合金均勻披覆一層完整連續的化成膜；50 s化成處理則因化成膜過厚而產生脫水裂紋。此外，化成處理使較陰極的MgLi2Zn上方長出較厚的富錳顆粒狀膜層，Fe-Mn相則無此沉積現象。 在0.05 M NaCl腐蝕測試溶液中，LZ91合金化成20 s樣品在PDP與析氫試驗等腐蝕測試結果中顯示最佳的抗蝕性。然而，其Fe-Mn相周圍的膜層不均使化成膜抗蝕效果受限。化成50 s樣品則因化成膜裂紋導致不均勻的腐蝕產物層、嚴重的絲狀腐蝕與化成膜剝離，使腐蝕程度較未處理樣品來得嚴重。此外，未處理、化成20與50 s樣品的腐蝕行為可區分為First Stage與Second Stage，兩階段的腐蝕擴展皆主要發生於β-Li。

The LZ91 dual-phase Mg–Li alloy, consisting of α-Mg with an HCP structure and β-Li with a BCC structure, exhibits a promising balance of strength and ductility, making it an attractive candidate for lightweight structural applications. However, its poor corrosion resistance, resulting from macroscopic galvanic coupling between the dual phases as well as localized micro-galvanic cells formed by constituent particles and precipitates, significantly limits its industrial applications. In this study, the matrix and secondary phases of the LZ91 alloy were first systematically analyzed. Subsequently, a permanganate conversion treatment was applied to form a conversion coating on the substrate surface. Corrosion tests were then performed to compare the corrosion resistance of the alloy before and after the treatment, while the influence of secondary phases on the conversion process was also investigated. To study the matrix and secondary phases in detail, phase identification and microstructural analysis were conducted using XRD and EBSD. The 0.1 M KMnO₄ solution, acidified to pH 1.5 with H₂SO₄ and maintained at 40 °C, was used to form conversion coatings on the LZ91 alloy for immersion durations of 20 and 50 s. The morphology and composition of the conversion coatings were characterized using SEM and EDS. Finally, the corrosion performance was evaluated by OCP, PDP, real-time monitoring, immersion tests, and hydrogen evolution tests. The results indicate that the secondary phases in the LZ91 Mg–Li alloy include micron-sized dual-phase randomly distributed Fe–Mn constituent particles, as well as submicron MgLi₂Zn precipitates at the β-Li grain boundaries, within β-Li grains, and along the α/β phase interfaces. After 20 s of conversion treatment, the LZ91 alloy was uniformly coated with a continuous and intact film. However, the 50 s treatment resulted in dehydration cracks due to the excessive thickness of the coating. Notably, the conversion process formed a thicker Mn-rich particulate film on the more cathodic MgLi₂Zn precipitates, whereas no such deposition was observed on the Fe–Mn particles. In 0.05 M NaCl test solution, the 20 s conversion-treated LZ91 sample exhibited the best corrosion resistance in both PDP and hydrogen evolution tests. However, the non-uniform coating surrounding the Fe–Mn particles limited the overall protective effectiveness of the conversion coating. In contrast, the 50 s conversion-treated sample showed more severe corrosion than the untreated sample, attributed to coating cracks that caused an uneven corrosion product layer, pronounced filiform corrosion, and delamination of the conversion coating. Furthermore, the corrosion behavior of untreated, 20 s, and 50 s conversion-coated samples could be distinguished into a first stage and a second stage, both of which exhibit corrosion propagation primarily within the β-Li phase.