Compositional Design Of Sulfuric Acid Dew Point Corrosion-resistant Steel And Corresponding Anti-corrosive Mechanism

Because of its low cost and excellent mechanical properties, mild carbon steel is animportant alloy used in store tanks, petroleum refneries, and so on. However, mild steel isvery susceptible to corrosion in acidic media, restraining its utility. It is well known thatbetter sulfuric acid dewpoint corrosion resistance can be improved by carefully controllingalloy composition of mild carbon steels.In this study, orthogonal experimental design （OAD） method with four parametersand three levels, i.e. L9（34）, was adopted in immersing and electrochemical tests tooptimize the composition of C, S, Cu, and Sb which affect the corrosion rate remarkbalely.Considering the actual working condition of chimneys and fue ducts, the test temperatureswere chosen at20and70°C. The corresponding H₂SO₄concentration at the sulphuric aciddewpoint are²0and50wt-%in accordance with Taylor vapour–liquid equilibriumdiagram, respectively. Immersing and polarization tests are carried out to measure thecorrosion rate. The SEM, EDX, XRD, EMPA, potentiodynamic polarization tests of thesamples which were tested in differernt corrosion environments was detected and theanti-corrosion mechanism is also discussed.Immersing tests results revealed that the optimal chemical composition with the bestcorrosion resistance is0.05C,0.007S,0.4Cu,0.02Sb （wt.%） in20%H₂SO₄solutionwhile that is0.11C,0.02S,0.4Cu,0.08Sb （wt.%） in50%H₂SO₄solution. The influenceof elements to the relative corrosion rate （RCR） was ranked in the following order:Cu>Sb>C>S at20°C while that was Sb＞C＞S＞Cu at70°C. The RCR of nineexperimental trials of OAD in20%and50%H₂SO₄solution were range from4.78%to10.28%and16.41%to26.00%, respectively. In poteniodynamic polarization test, theRCRs in20%and50%H₂SO₄solution were range from3.55%to9.21%and18.21%to28.82%, respectively. The RCRs of refined carbon steel according to the optimal resultwere3.54%and14.89%immersing in20%and50%H₂SO₄solution, respectively.Poteniodynamic tests results revealed that the optimal chemical composition is0.05C,0.013S,0.4Cu,0.02Sb （wt.%） in20%H₂SO₄solution and the optimal chemicalcomposition is0.11C,0.02S,0.4Cu,0.08Sb （wt.%） at in50%H₂SO₄solution. The influence of elements to the RCR was ranked in the following order: C>Sb>Cu>S at20°Cwhile that was Sb>C>S>Cu at70°C. Potentiodynamic polarization result showed that theRCRs of refined carbon steel were4.06%and14.48%. The RCR of the optimized steel in50%H₂SO₄solution is the smallest though that in20%H₂SO₄solution is not the thesmallest, proving the validity of the orthogonal array test result. Potentiodynamicpolarization curves of the plain steel and the optimized steel showed that the corrosionpotential （Ecorr） of the optimized steel is nobler than that of the plain steel and thecorrosion current （Icorr） of the optimized steel is much lower than that of the plain steel.The Ecorrand Icorrof the plain steel are-442.5mV and2.79mA while that of the optimizedsteel are-403.5mV and0.15mA in20%H₂SO₄solution. Corrosion potential shifted nobleand corrosion current increased when testing in50%H₂SO₄solution. The Ecorrof the plainsteel and the optimized steel shifted to-349.7mV and-346mV, while the Icorrincreased to43.53mA and7.89mA.EDX, XRD and EPMA indicate that the contents of five elements （Si, S, Sb, Cr, Cu）on the corroded optimized steel are much higher than substrate while only two elements（Si and S） are higher that the plain steel substrate. X-ray diffraction patterns of corrosionproducts on the PLCS and the SADPS further reveals that corrosion products on the plainseel and the optimized steel are Fe₂（SO₄）₃+FeSO₄and （Cu, Fe）SO₄+FeSbO₄+FeSb₂O₄+CuSbO₄+CuO, respectively. Thermodynamic analysis indicates thatCu₂Sand CuS probably existed in the corrosion products.The analysis of SEM revealed that the plain steel corroded seriously while theoptimized steel showing good corrosion resistance in20%and50%H₂SO₄solution. Afterimmersing and poteniodynamic testings in20%H₂SO₄slution, there is nearly nocorrosion products on the corroded surface of the plain steel while the corroded PLCSsurface is covered by a loose, cracking and discontinuous layer in50%H₂SO₄slution. Bycontrast, the corroded surface of optimized steel is characterized by dense and continuouslayer, which supports that the SADPS has superior corrosion than that of the plain steel.