亞硝酸鈉在氯化鈉水溶液中對碳鋼之腐蝕與抑制行為

The Corrosive and Inhibitive Behavior of Sodium Nitrite for Carbon Steel in Sodium Chloride Aqueous Solutions

本研究以動電位掃描極化方法探討亞硝酸鈉在氯化鈉水溶液中及在pH = 3－11對碳鋼抗蝕性之影響。其結果歸納如下：1. 碳鋼在單純氯化鈉水溶液中：腐蝕速率均隨氯化鈉濃度之增加而增加，且隨pH之增加而降低；在pH = 7和9之系統，有些情況反較在pH=5之系統者為大，其原因可能係氯離子之侵蝕性與氫氧根離子之抑制性兩者妥協和產生孔蝕綜合作用之結果。2. 碳鋼在同時含氯化鈉與亞硝酸鈉之水溶液系統中；在pH = 3之系統，腐蝕速率隨亞硝酸鈉濃度增加而增加，此顯示亞硝酸鈉具有腐蝕性，而在pH = 7，9和11者，則具相反之趨勢，此顯示亞硝酸鈉具有抑制性；在pH= 5者，則呈先降後升之趨勢，此可能係亞硝酸鈉腐蝕與抑制之雙重性相互影響之結果。3. 亞硝酸鈉之腐蝕性可能係由於亞硝酸分子之氧化性而將鐵氧化作Fe2+；其抑制性可能係由於亞硝酸根離子之高氧化力使鐵表面形成氧氣之吸附膜，而後再生成Fe(OH)3或Fe2O3。4. 異丙醇具有減緩腐蝕或抑制之作用，此可能係由於降低導電度與溶氧量。

The study was performed by scanning potentiodynamic polarization method to investigate the effects of sodium nitrite tor carbon steel in sodium chloride solutions. The results are summarized as follows: 1. In pure sodium chloride aqueous soltuions: The corrosion rate increases with increasing concentration of sodium chliride, and decreases with increasing of PH. the corrosion rates for some cases at PH = 7,9 are higher than those at pH = 5. This may be caused by the mixed effects of the corrosive effect of Cl- and the inhibitive effect of OH- and the formation of pitting. 2. In the sodium chloride and sodium nitrite mixed aqueous solutions: The corrosion rate increases with increasing concentration of sodium nitrite at pH = 3, which may be explained by the corrosive behavior of NaNO2, however, the opposite trend is found for the cases at PH = 7,9 and 11, which may be explained by the inhibitive behavior of NaNO2. for the solution at PH = 5, the corrosion rate first increases with increasing nitrite concentration, followed by decreasing with an increase in nitrite concentration; the behavior may be explained by its dual behaviors of corrosive and inhibition. 3. The corrosive behavior of sodium nitrite is attributable to that sodium ntrite is reduced to NO, NH4+, and N2 while iron is oidized to Fe2+. Its inhibition behavior may be caused by that sodium nitrite has a very marked oxidizing power to form oxygen adsorped film on the carbon steel surface, followed by the formation of Fe(OH)3 and Fe2O3. 4. The presence of isopropyl alcohol decreases the corrosion rate of carborm steel. This may be due to the decrease in solution conductivity and the reduction in corrosive concentrations on the carbon steel surface by isopropyl alcohol.