The Study On Atmospheric Corrosion Behavior Of Carbon Steel Under Thin Electrolyte Layer In Air Containing SO₂

The effects of temperature and SO₂ on the corrosion behavior of carbon steelwere studied with indoor simulated test. The observation of surface morphology andanalysis of corrosion products by optical microscope (OM), scanning electronmicroscopy (SEM) and X-ray diffraction (XRD) methods were integrated toinvestigate the corrosion performance. Meanwhile the corrosion electrochemicalcharacteristics were discussed by electrochemical measurement under thin electrolytelayer containing SO₂. In the end, the corrosion mechanism and growth model ofcorrosion products of carbon steel were researched in pure air and air containing SO₂with high relative humidity.The results showed that the corrosion rate of carbon steel increased withincreasing temperature. During the corrosion period of 456 hours in pure air withrelative humidity 95%, the mass gain followed exponential growth law and onlycellular corrosion products appeared at 20℃, while the mass gain followedexponential decay law, both cellular and granular corrosion products grewsimultaneously at 30℃and 40℃. The corrosion products were composed ofα-FeOOH andγ-FeOOH, which increased at higher temperature.The presence of SO₂ had an accelerated and catalysis effect on corrosion processof carbon steel. The rust layer thickened with increasing SO₂ concentration. Duringthe corrosion period of 456 hours in air containing SO₂, the mass gain followedexponential growth law at the initial stage, but in the later period, the mass gainfollowed exponential growth law with lower content of SO₂ and exponential decaylaw with higher content of SO₂. The corrosion of carbon steel had filiform corrosioncharacteristics, meanwhile cellular products grew on the filiform corrosion products.The corrosion products mainly includedα-FeOOH,γ-FeOOH, FeSO4, Fe3O4 andγ-Fe2O3. When SO₂ concentration rised and corrosion time prolonged, the content ofα-FeOOH increased.The corrosion potential was the most negative and the corrosion current densityreached the maximum value at 500μm thickness of SO₂ electrolyte layer, whichindicated the easiest corrosion tendency and the biggest corrosion rate. Nyquist curves showed two apparent capacitive arcs under thin electrolyte layer containing 5×10-6volume fraction SO₂. The corrosion rate varied under different thickness electrolytelayer with corrosion time. In the initial stage, the maximum corrosion rate was foundat 500μm thickness. During 36-48 hours, the order of corrosion rate was that:1000μm> 500μm>450μm>300μm, because the diffusion of corrosion products wasrelatively easy under the thicker electrolyte layer, which made the electrolyte contactwith anode and further promoted corrosion process.In air with high relative humidity, the water droplets formed on the surface ofcarbon steel first of all. Fe was dissolved at the center of the electrolyte droplet andoxygen was reduced at the edge, and then Fe(OH)2film formed. In pure air at RH95%,cellular corrosion productsα-FeOOH formed at the center of the water droplets andFe(OH)2 reacted with O2 to produce granular corrosion productsγ-FeOOH aroundcellular corrosion products. The breakage of Fe(OH)2 film resulted in the growth ofthe filiform corrosion products in air containing SO₂. The filiform corrosion productsgrew along with the grain boundaries and ferrite. At the same time, FeSO4nests wereobserved at the area of the breakage of the Fe(OH)2 film, and the cellular corrosionproducts deposited on the nests.