The Corrosion And Passive Behavior Of Carbon Steel In Concrete Environment Under Stress

The corrosion of reinforcing steel bar is one of the main factors that leadto the degradation of concrete structure. Many literatures showed that thepassive film protects reinforcing steel bar from corrosion in concrete.However, once the passive film was degradated by chloride ions, low pHvalue, or other environment factors, the corrosion of reinforcing steel bar inthe concrete would begin. In this research work, the effects of stress onpassive behavior of reinforcing steel bar were studied. The presented resultsmay provide some references for the load designing of concrete structures.Also, they may help for repairing the degradated concrete structure.The corrosion behavior of reinforcing steel bar in concrete structurenearby the solution level was studied firstly. The results showed that thecorrosion of rebar immersed in solution is more serious than the rebars abovethe solution surface. Furthermore, as the depth of the rebar in solutionincreased, the corrosion of rebar decreased. For the rebars above the solution,corrosion became slighter as the height increased. The effects of cerium nitrate on corrosion behaviors of carbon steel insimulated concrete pore solutions were studied. In pore solutions withCe(NO₃)₃added, the corrosion potential, polarization resistance andimpedance of carbon steel obviously increased contrast to the situationwithout cerium salts. Additionally, the results indicated that a small amount ofCe(NO₃)-3can effectively promote passivation of the steel and reduce thethreshold [NO₂^-]/[Cl^-] ratio for corrosion control. The surface layer formed incerium salt contained pore solutions were more compact and smooth, showingbetter corrosion resistance than the films formed in solution without ceriumsalt.The effects of plastic deformation on passive behavior of carbon steel inconcrete extract were studied, and the results showed that the passive rate ofcarbon steel increased obviously with the strain magnitude. However, thestability of the formed passive films decreased with the strain magnitude. Thedegradation of passive film under tensile and compressive stress wasresearched respectively. Both the potential and EIS results showed that thedegradation degree of passive film increased with the strain magnitude.Additionally, for the same strain magnitude, the passive films sufferedcompressive stress was more severe damaged than that suffered tensile stress.The surface analysis indicated that there were many micro-cracks in passivefilm under tensile stress; while many buckles formed in the passive filmssuffered compressive stress. Finally, according to the results, the degradation modes of passive film under tensile stress and compressive stress weresuggested. The repassivation process of carbon steel in cement extract aftersuffered tensile stress was also studied. The electrochemical results indicatedthat the repassivation was more difficult as the stress magnitude increased,especially for the stress lead to substrate plastic deformation. On the otherhand, for the samples subjected to elastic deformation, the degradation ofpassive film is determined by the amplitude of the load, and smaller repetitiveload repetitive would easier repassivation than that suffered once higherloadhas no obvious effect on the passive bahavior.Finally, the influence of stress on corrosion behavior of carbon steel inconcrete structure was studied in sodium chloride solution. The results showedthat the number of micro-cracks in concrete was obviously increased with thestress magnitude. Furthermore, the corrosion degree of rebar in concretewould increase as the stress magnitude increased. Additionally, the influenceof stress on corrosion behavior of rebar in concrete would be divided to twoprocesses. The first process was that the stress accelerates the diffuse ofchloride in concrete. Then the stress would affect the surface of reinforcingsteel bar, and lead to increased corrosion rate of rebar in the chloridecontaining concrete.