鐵銹和銹安定化處理膜在碳鋼和耐候鋼上之行為研究

A Study of the Performance of Rust Films and Stabilizers on Carbon and Weathering Steels

耐候鋼在大氣中經過一段時間的曝露，會生成美觀而有保護性的銹層；然而於使用初期，生成的銹是不安定的，以致會污染四週的環境及結構物。為了改善此一缺點，本文研究了五種銹安定化處理劑用於耐候鋼和碳鋼後，在台灣三個不同環境下之銹安定化效果及其腐蝕行為。經過四至四年半的曝露，顯示耐候鋼在臨海區，僅離子型樹脂塗料可被使用，而新離子型者僅適用於臨海工業區，致於BAS-60和磷酸鹽處理劑適用於臨海工業區和多風都市鄉村區，BMU-605均無法適用。交流阻抗結果顯示耐候鋼銹層之頻譜有兩個半圓，等效電路分析出兩個RC迴路，表示銹層具有保護底材之能力；反之，碳鋼銹層不具保護性，鈍化膜電阻趨近於零，等效電路僅有一個RC迴路。經銹安定化處理之試片，最初階段，Nyquist圖均有一個高Rpf值的RC迴路，但處理膜失去作用時，即與裸露者相似。開路電位量測顯示不同處理劑之一般電化學特性，並可用Evans圖來解釋其機構，但對抗局部腐蝕效應無法有效顯現。另外本研究亦證實以耐候鋼為底材之處理膜之抗大氣腐蝕性遠較碳鋼者為優。

It is well known that weathering steel can form a beautiful and protective rust layer after certain period exposure. However, in the initial stage, its rust is not stable and pollutes surrounding structures and environments. In order to improve this shortcoming, this research was carried out to evaluate and investigate the rust stabilizing performance of five rust stabilizers on a weathering steel and a carbon steel in three different environments in Taiwan. After four to four and a half years exposure, it was found that ionic type resin and new ionic type resin only can be used in coastal and coastal-industrial areas respectively. The urethane modified alkyd undercoat (BAS-60) and the phosphated treatment were able to form stabilized rust, except in the coastal environment, while the moisture-curing urethane varnish (BMU-605) did not perform satisfactorily in all three areas. Equivalent-circuit analysis of the AC impedance data for the rust films developed on weathering steel revealed two RC circuits, consistent with good protection of the base metal. However, the corresponding analyses of the data for the rust film on carbon steel revealed an RC circiut with a low Rpf value, indicating poor protection of the base metal. The Nyquist plots obtained in the presence of rust stabilizers on the both steels revealed an RC circuit with only a high Rpf value, but they became similar to the corresponding plots in the absence of stabilizers when stabilizers lost their function. Open-circuit potential measurements showed that the electrochemical characteristics of the various stabilizers were different, but all the protection mechanisms could be explained by Evans diagrams. In addition, the atmospheric corrosion resistance of stabilizers on weathering steel was better than that on carbon steel.