高電流效率鋁陽極錠的開發研究

A Study of the Development of Aluminium Anodes with High Galvanic Current Efficiency

鋁陽極錠早已廣泛被用來保護鋼製的海上設備及海邊結構物，而開路電位和電位效率為此陽極錠必須考慮的主要特性。本研究利用田口法L18表來探討合金成份對此電氣特性的效應，尤其是電流效率；並利用金屬模和砂模澆鑄成不同尺寸的陽極錠來觀察其電氣特性的差異。結果顯示田口法可有效尋找出較佳合金組成，並顯示添加銦或減少鐵含量均可明顯提高電流效率。電流效率高之陽極錠於試驗後之表面較亮白且平滑，為一均勻腐蝕，反之，則表面較暗且粗糙，且於晶界存有較多含鐵析出物，產生沿晶腐蝕，此主要受鐵含量高，銦和鎂添加量不足所致。另外，鑄錠增大或使用砂模均會使凝固速率降低，鑄造組織也會由共存之柱狀晶和等軸晶變成均為等軸晶，同時促使鋅和鎂在晶粒凝固時產生偏析，此導致初期的腐蝕電位降低，腐蝕得以均匀平滑，並提高了電流效率。經由合金設計和凝固條件控制，電流效率達96%以上，開路電位－1100mV以下之高品級鋁陽極錠已成功的被開發出。

Aluminium anodes have been widely used to protect marine equipments and offshore structures. Both open circuit potential and galvanic current efficiency are the main requirements for this anode material. In this study, the orthogonal array L18 of Taguchi method was employed to study the effects of alloy elements on both of these characteristics of aluminium anodes, especially galvanic current efficiency. Additionally the differences in both characteristics of those anodes casted by metal and sand molds into different section sizes also have been investigated. The results indicate that the Taguchi method can effectively search the better alloy design for the anodes, and the galvanic current efficiency can be improved by addition of indium or decrease of iron content. The surface of the anode with high galvanic current efficiency is bright and smooth after test because of the occurrence of uniform corrosion. On the other hand, because more iron-containing precipitates developed along the grain boundaries resulting from the high content of iron on deficit of indium and magnesium the surface of the anode with low galvanic current efficiency is dark and rough. In addition, a decrease of solidifcation rate by increasing cast size or using sand mold makes the casted structure change from a coexistance of columnar and equiaxed into equiaxed. This slow solidification rate results in microsegregation of zinc and magnesium in cellular solidification which induces decrease of initial corrosion potential, uniform corrosion and increase of galvanic current efficincy. A high grade aluminium anode with over 96% galvanic current efficiency and － 1100mV open circuit potential has been developed by the alloy design and control of solidification condition.