The Electrochemical Behaviors And The Electronic Properties Of Passive Film On Iron In PH8.4 Borate Buffer Solution

The electrochemical behavior of iron in 0. 136mol ?T'borate buffer solution(pH=8. 4)was investigated by using polarization curve, potential decay curve, anodic charge curve and alternate current impedance measurements. The electronic properties of passive film on iron in borate buffer solution formed by anodic oxidation and inorganic inhibitors were investigated by Mott-Schottky analyses with Auger Electron Spectroscopy, Scanning Electron Microscopy with Energy Dispersive Spectrometry analyses. Different mechanisms of corrosion inhibition of iron by inhibitors were classified by polarization curve measurements.1. When there no oxidants exist or no potentials are applied in pH8. 4 borate buffer solution, iron is under the condition of active corrosion. While polarized anodically, it is characteristic of an active-passive metal by five regions: cathodic region less than -0. 77V(vs. SCE, the same in the potentials mentioned below), active dissolution region from -0. 77V to -0. 60V, active-passive region from-0.60Vto-0.50V, passive region from -0. SOVto 0.90V and transpassive region higher than 0. 9V.2. At room temperature, the reactions involved during the formation, growth and dissolution of the passive film on iron in pH8. 4borate buffer solution are as follows:(a) Cathodic region: In the solution dearated with N2 gas, the cathodic process of the corrosion of Fe is a depolarization by hydrogen.2H20+2e H2+20H- (controlled by charge transfer step) (1)(b) Active-dissolution region: Fe dissolves to Fe2" ion and then deposited Fe(OH)2 which is less protective, accompanied by the formation of small quantity of ferric compounds.Fe-Fe2++2e (main) (2)Fe2++20H- Fe(OH),(less protective) (3)Fe(OH)2+OH- Fe(OH)3+e (secondary) (4)(c) Active-passive region: Reactions involved in this region are similar to those in active-dissolution region. The protective deposited layer is formed as the ratio of ferric ions to ferrous ions in the deposited increases to an extent. In this region, minus charge transfer resistance appears in the alternate current impedance Nyquist plots. When the potential is at - 0. 50V, ferric ion concentration in the deposited is so high that passive film of some protection is formed and Fe enters into passive region. At the same time, reaction (4) plays the leading role, controlled by the diffusion of Fe in the deposited layer.(d) Passive region: In the initial stage of the passive region (-0. 50-OV), the reaction is mainly for the transformation of Fe in the deposited to Fe, controlled by the charge-transfer and diffusion steps. In the later stage of passive region (0-0. 9V), reaction(5) is mainly responsible for thickening the passive film, accompanied with the oxidation of small amount of Fe" to Fe'1".Fe(OH)2+OH Fe(OH),+e (secondary) 2Fe+60H~+6e Fe203+3H20 (main) (5)(e) Transpassive region: Passive film dissolves and oxygen evolves.2Fe+60H-+6e Fe203+3H20 (secondary) Fe(III)(in FeAfilm) Fe(VI)+3e(film dissolved) (6) 2H,0+e?02+20H- (7)3. In pH8. 4 borate buffer solution, the inhibition mechanisms of Fe corrosion by inorganic inhibitors are different.(a) The dissolved oxygen in the solution is an oxidative inhibitor for it can passivate Fe itself. The passive film formed by oxygen is similar to that by potentiostatic method and will dissolve in pH8. 4borate buffer solution without oxygen.(b) Cr can be detected by the AES, EDS and anodic polarization in the passive film formed on Fe by chromate in pH8. 4borate buffer solution. Chromate is an oxidative-precipitation inhibitor.(c) As dissolved oxygen in the solution behaves, nitrite is an oxidative inhibitor.(d) Different from chromate and nitrite, molybdate is a precipitation inhibitor.(e) Vanadate is an oxidative inhibitor.4. The passive films forme...