The Study On Corrosion Behavior Of Carbon Steel Under Thin Electrolyte Layer

The corrosion rate, as well as the morphology variation, formation and growth ofthe corrosion products were investigated in atmosphere containing3.5%NaCl and5×10^-6SO₂by means of three-dimensional video microscope, scanning electronmicroscopy（SEM）, energy disperse spectroscopy（EDS）, X-ray diffraction（XRD）. Thethin electrolyte layer thickness controlling and electrochemical measurement systemwas utilized to investigate the corrosion electrochemical behavior and mechanism ofthe20#carbon steel. Meanwhile, the accelerating effect of Cl^-on atmosphericcorrosion of carbon steel was dicussed.Mass gain curve of carbon steel corroded for504h in atmosphere containing5×10^-6SO₂at95%RH and25℃, is divided into two stages which respectively followexponential growth law and exponential decay law; while in atmosphere containing3.5%NaCl, the mass gain is significantly faster than that in the SO₂-pollutedatmosphere, indicating the accelerating effect of Cl^-is obviously higher than that ofSO₂,the mass gain curve is divided into two stages both of which follow linear growthlaw, and the rate is relatively slower in the second stage. During the period of2160h,the second stage continued rapid trend of corrosion to1032h, and for the third stageof1080-2160h, the corrosion rate relatively slows down which follow exponentialdecay law.In atmosphere containing NaCl and SO₂, corrosion preferential occurred at theactive spots, around which the corrosion products gradually expanded and showedsmall cellular feature, then it grow up with the steady accumulation of corrosionproducts. As corrosion continues, the cellular products are broken, causing the outflowof the electrolyte. After that, a new anodic area may become activated, andcontinuously develops to produce filament consisting of many new small cellularproducts. Accordingly, the filament and cellular corrosion products are simultaneouslyfound on the surface. Compared with SO42-, the Cl^-which has a smaller ionic radiusshows a strong corrosive, thus the formation of cellular corrosion products is mucheasier and the filament products grow faster than that in the SO₂-polluted environment. After the long-term corrosion, the sample surface forms a continuous rust layer. Dueto the catalytic effect of Cl^-on the corrosion reaction, the wedge-shaped pits and thepits in which diameter is greater than the depth are found. The corrosion products aremainly of iron oxides which show massive, cellular and flake characteristics. The rustlayer is mainly composed of-FeOOH, γ-FeOOH, Fe₃O₄, γ-Fe₂O₃, and in theintermediate stage rod products γ-FeOOH appear, then convert to-FeOOH that hasflake feature.The surface potential of different areas for different time was investigatedthrough the scanning kelvin probe （SKP） technique. The results showed that thesurface of carbon steel presents localized corrosion pattern, at the initial period,corrosion micro cell forms, where at the anodic regions the corrosion dissolutionoccurs and the cellular products are generated, leading to a large difference of thesurface potentials. As corrosion continues, the surface potential difference decreasesfor the increasing number of local corrosion dissolution areas and corrosion products,which contribute to positive shift of surface potential. The corrosion current densityand limiting diffusion current density reach the maximum value at350μm thicknessof the layer, which reveals the easiest corrosion tendency and the biggest corrosionrate. The diffusion of oxygen and corrosion products is relatively easy under the thinelectrolyte layer， which promotes corrosion process. The corrosion process iscontrolled by charge transfer and diffusion. The order of corrosion rate under350μmelectrolyte layer for96h was as follows:96h>48h>24h>12h>6h.