The Effect of Crystalline phase of Sodium Molybdate on the Corrosion Inhibition for Carbon Steel

鉬酸鈉晶相對碳鋼緩蝕的影響

黃靖謙(Ching-Chieh Huang) ； 莫智傑(Chih-Chieh Mo) ； 歐信宏(Hsin-Hung Ou)

《中國機械工程學刊》 43卷1期 (2022/02) Pp. 69-73

摘要

This study is aiming at the effect of crystalline phase of sodium molybdate (Na_2MoO_4) on the corrosion inhibition of carbon steel. Our TGA result indicated three endothermic events, two of which corresponds to the transition of crystalline phase (i.e. the phase transformation from cubic to orthorrombic phase and from orthorrombic tohexagonal phase) and the last event is owing to the melting of Na_2MoO_4. Based on the weight-loss measurements, the efficiency of corrosion inhibition performed by Na_2MoO_4 with different crystalline phase differed as a result of different stacking orientation of MoO_4^(2-) on the test coupon. For example, the corrosion rate of carbon steel in the presence of MoO_4^(2-) with orthormobic phase was found at 0.01MPY which was 2.1 times better than that of commercial Na_2MoO_4 with cubic phase. This result was further supported by the PP (Polarization potential) and EIS (Electrochemical Impedance Spectroscopy) measurements in terms of the thermodynamic and kinetic viewpoints. Therefore, the corrosion inhibition capability of Na_2MoO_4 could be enhanced after appropriate thermal treatment. Further investigations by XPS are underway for examining the microstructure and chemical composition of MoO_4^(2-)Fe^(2+) complex film possibly formed on the test coupon.

關鍵字

corrosion inhibition ； sodium molybdate ； electrochemical analysis

並列摘要

本研究旨在研究鉬酸鈉（Na_2MoO_4）之晶相對於碳鋼緩蝕作用的影響。實驗之TGA結果表明三個吸熱狀態，其中兩個對應於晶相的轉變（即從立方相到正交相和從正交相到六方相的相變化），最後一個狀態是由於Na_2MoO_4的熔化。根據失重量測，不同晶相的Na_2MoO_4的緩蝕效率因MoO_4^(2-)在試樣上的堆積方向不同而異。例如，碳鋼在具有正交相的MoO_4^(2-)存在下的腐蝕速率為0.01MPY，是具有立方相的商用Na_2MoO_4的2.1倍。從熱力學和動力學的角度來看，PP（極化電位）和EIS（電化學阻抗譜）測量進一步支持了這一結果。因此，經過適當的熱處理，Na_2MoO_4的緩蝕能力可以增強。本研究中另亦透過XPS進行更進一步的探討，可以檢查可能在試樣上所形成的MoO_4^(2-)Fe^(2+)複合膜的微觀結構和化學成分。