5456-H116船用鋁合金腐蝕破損機制之研究

A Study on Mechanism of Corrosion and Fracture of 5456-H116 Marine Aluminum Alloys

本研究旨在分析探討5456-H116船用鋁合金板材發生腐蝕裂損之原因，同時取一新板作為對照組，藉由外觀觀察、板材成分分析、微硬度量測、拉伸試驗、金相組織分析、斷面SEM觀察、EDS元素成分分析、極化試驗及ASTM-G67 NAMLT（nitric acid mass loss test）抗蝕性能測試等材料破損分析實驗，進行船用鋁合金腐蝕裂損機制之探討與比較。研究結果發現，合金內之β相（Mg2Al3）在晶界上呈現連續析出之敏化（sensitization）現象，導致板材腐蝕電位降低，並造成硬度及抗拉強度降低，且金相組織觀察有晶間腐蝕（intergranular corrosion，IGC）及應力腐蝕開裂（stress corrosion cracking，SCC）成長現象，從拉伸破斷面觀察可知其破壞主要為劈裂或準劈裂脆斷形態。研判肇因主要為船體長期在高溫日曬下，β相逐漸沿晶界析出形成敏化，同時受海洋腐蝕環境及波浪反覆應力作用影響，導致IGC及SCC現象發生。本研究期能確切判斷船用鋁合金之破損原因，並藉研究結果提供相關產業於船舶構件設計及材質選用之參考。 In this study, we investigate the causes of successive failures of 5456-H116 aluminum alloy by a series or research including appearance examinations, material composition analysis, micro-hardness test, tensile test, metallographical and micrographical analysis on fractured surfaces, electrochemical measurements, and Nitric Acid Mass Loss Test (NAMLT). The results indicate that the continuous network of β-phase precipitates at the grain boundaries lead to intergranular corrosion (lGC), stress corrosion cracking (SCC), and exfoliation on the internal surfaces which propagates along the rolled direction of alloys. In addition, the sensitization resulted from the β-phase precipitates also cause the decrease of their corrosion potential and strength. From the fracture morphology analysis, it is discovered that the fracture sections are cleavage and quasi-cleavage types. We recommend that our finding can serve as a reference for manufacturers regarding the construction and maintenance of aluminum alloy vessels.