The Research Of Semiconductor Behaviors Of Metal/Organic Coating In Electrolytes During Its Degradation

Corrosion of metals is a kind of irreversible phenomena of consumption under action of aggressive medium in use of metals. Covering surface of metals with organic coating was one of the most excellent anti-corrosion methods. Lots of researches about conduction mechanism of coatings during their degradation were carried out by predecessors. The leading research technology was electrochemical impedance spectroscopy (EIS) and relatively perfect theory system had been established. But the complex analysis baffled the development of it. The most important problems were non-unique equivalent circuits and indefinite physical meanings of certain circuit components. Capacitance-potential testing, photo-electrochemistry testing, Fourier transform infrared spectroscopy (FT-IR) and Mott-Schottky analysis, phase angle analysis combined with EIS were utilized to study respective semiconductor behaviors of naked stainless steel (SS), carbon steel (CS), yellow brass (YB) electrodes and coated electrodes (phenolic coating, alkyd coating and epoxy coating were involved in this paper) immersed in electrolytes (sulfuric acid solution, sodium hydroxide solution, sodium sulfate solution, sodium chloride solution) in this paper. It is discovered that the electrochemical behaviors of coated electrodes during its degradation in electrolytes is similar to the electrochemical behaviors of semiconductor oxide film generated on surface of above mentioned metals in electrolytes.Carrier density of oxide films on surface of naked SS, CS and YB electrodes in electrolytes rises with growing immersion time. The carrier density of oxide films on surface of SS, CS and YB immersed for 10 minutes calculated from the data at 1000Hz respectively is 10²⁰cm^-3-10²¹cm^-3, 10²¹cm^-3-10²⁴cm^-3, 10²¹cm^-3-10²³cm^-3, so just as far as carrier density is concerned, descending sort about anti-corrosion property of the 3 kinds of metal in electrolytes is SS, YB, CS. The main cause of SS's corrosion in sodium hydroxide solution is the rising conductibility of chromium-rich layer, while it is due to the ascending conductance of iron-rich layer in sulfuric acid and sodium sulfate solutions. Owing to aggression of chloride ions, deep level donors in passive film of SS are ionized. The corrosion action of sulfuric acid solution to YB is more intensive than the other 3 solutions. The dezincification mechanism of YB is resolution-redeposition theory in sodium sulfate solution and is preferential solution of zinc theory in sodium chloride solution.According to the different electrochemical behaviors of metal/organic coating in electrolytes during its degradation, the corrosion process of coatings can be divided into 4 stages. (1) Insulator stage. In this stage, the Mott-Schottky curves parallel to the potential axis, also the capacitance values keep constant in the scanning potential region and the phase angles closes to -90o. (2) MIS structure (metal-insulator-semiconductor) stage. Surface of organic coating is under semiconductor state but inner layer of coating is still under insulating state, so the semiconductor layer, insulating layer and the metal substrate form a MIS structure. In this stage, Mott-Schottky curves of electrodes start to incline and the carrier density's order of magnitude is generally lower than 1015cm-3. Carrier concentration and thickness of space charge layer irregularly vary with growing immersion time, even, the semiconductor type also transform sometimes in its subsequent stage. (3) Dipole polarization stage. Owing to permeation of outer corrosive medium, rotatable dipoles appear in the coating what make the overall mean of dipole moment being nonzero. Under the action of applied electric field, dipole polarization occurs in the inner of coating. Because of dipole relaxation, the capacitance increases in the negative potential region and the capacitance decreases in the positive region with rising applied potential. Under relative higher frequency (3000Hz, 5000Hz), these phenomenon are obscure. (4) Space charge polarization stage. In this stage, overall coating changes into semiconductor. The semiconductor and metal substrate make up of a metal/semiconductor contact structure. Carrier density of coating gradually increases with rising immersion time and the phase angle tends towards zero degree. Protective performance of organic coating to substrates is weakened quickly and work function of coating is reduced by corrosion in the stage. The carrier's transportation of the electrode was controlled by pore resistance of the coating, space charge layer and kinetics of corrosion reaction of metal substrate.When the order of magnitude of carrier density is below 10¹⁵cm^-3, the organic coating has excellent anti-corrosion performance; and above 10¹⁵cm^-3, organic coating's anticorrosive ability falls drastically, meantime, substrate's corrosion initiating. 10¹⁵cm^-3 is a critical value. Carrier density can be utilized as a parameter to qualitatively or quantitatively evaluate organic coating's protection performance and substrate's corrosion level.Mechanism of semiconductor electrochemical behaviors of metal/organic coating in electrolytes during its degradation can be demonstrated by energy band model. One of the reasons for above-mentioned semiconductor behaviors is organic coating's degradation which results in generation of free radicals according to datas from Fourier transform infrared spectroscopy (FT-IR).