以無電鍍鎳磷/奈米二氧化鈦複合鍍層提昇70Cu-30Zn合金在氟鹽溶液中之應力腐蝕性質

本研究以無電鍍法，於無電鍍鎳磷浴中添加不同濃度1g/L、5g/L、10g/L、15g/L的奈米二氧化鈦使其共同析鍍在70Cu-30Zn黃銅合金基材表面上，以探討無電鍍複合鍍層在0.1M NaF水溶液中的耐腐蝕性，並以慢應變速率拉伸試驗法(Slow strain-rate test, SSRT)評估其應力腐蝕(Stress corrosion cracking, SCC)的抵抗性。並利用掃瞄式電子顯微鏡（SEM）、X光能量散射分析儀(EDS)、維克氏微硬度試驗機等，觀察鍍膜表面形態、成份分析、量測表面硬度(Hv)。研究結果顯示，無電鍍鎳磷雖能增加黃銅合金的硬度、耐蝕性與耐應力腐蝕性，但添加奈米二氧化鈦後更有促進強化的效應，無電鍍鎳磷奈米複合鍍膜隨著添加奈米二氧化鈦濃度的增加，析出與附著量均有增加，並可增進耐腐蝕性與應力腐蝕抵抗性。在添加奈米二氧化鈦濃度達10g/L時，無電鍍鎳磷奈米複合鍍膜呈現最佳的耐腐蝕性與應力腐蝕抵抗性。此結果主要歸諸於無電鍍鎳磷奈米複合鍍膜除可提升腐蝕電位、降低腐蝕電流外，可阻止黃銅合金基材在氟化鈉水溶液中形成不穩定的鈍化膜Cu2O，因而不會在拉伸時產生局部拉伸使Cu2O膜破裂(Film rupture)造成脫鋅(Dezincification)溶解，形成沿晶應力腐蝕脆斷(Intergranular stress corrosion cracking, IGSCC)。

關鍵字

70Cu-30Zn ；無電鍍；慢應變速率；應力腐蝕；脫鋅；沿晶

並列摘要

The present study used electroless plating techniques by adding nano-TiO2 in the plating solution with various concentrations of 1 g/L, 5 g/L, 10 g/L and 15 g/L to form Ni-P/nano TiO2 composite coatings on the 70Cu-30Zn brass alloy substrate. The corrosion resistance behavior was evaluated by potentiodynamic measurement and the susceptibility of the stress corrosion cracking (SCC) was evaluated by slow strain rate test (SSRT) in 0.1 M NaF solution. The surface morphology, element analysis and surface microhardness of the electroless Ni-P composite coatings were observed and analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and vicker’s microhardness tester. Experimental results indicated that the electroless Ni-P plating coating had a beneficial effect to enhance the hardness as well as corrosion and SCC resistance of brass alloy in 0.1 M NaF solution. When the nano-TiO2 particles added this beneficial effect was significantly raised with increasing the concentrations of nano TiO2 up to 15 g/L. The improved SCC resistance was attributed to the electroless Ni-P/nanoTiO2 composite coatings preventing the formation of an unstable Cu2O passive film for brass alloy in fluoride solution. This unstable passive film could be destroyed by strain and F- attack during slow strain rate tensile test, and resulting in dezincification dissolution and an intergranular stress corrosion cracking (IGSCC) of the brass alloy.