A Study On Initial Corrosion Of Low-alloy Steel And Its Protection With New Coating

Low-alloy steel is one of the most important material used in the marineenvironment, much attention has been paid for its corrosion. At present, we researchthe low-alloy steel by analyzing electrochemical properties, corrosion products, theinfluence of alloying elements, the result of real sea test and building its model.However, these methods focus on the whole sample and its later stage of corrosion.Many information of the initial and localized corrosion of low-alloy steel is ignored.To solve above problems, this thesis mainly to carry out the work of thefollowing sections:1. The influence of the concentration of chloride ion and soaking time to the lowalloy steel in simulated seawater corrosion was studied by using electrochemicalimpedance spectrum and polarization curve from a macroscopic perspective. Theresults show that the corrosion rate of low alloy steel in simulated seawater increaseswith the increasing of chloride ion concentration. The change of corrosion ratesynchronizes with the change of cathode and anode rate. The corrosion rate of lowalloy steel varies with the soaking time. On the third day, the corrosion rate of lowalloy steel is the smallest. At last, it becomes relatively stable.2. The early stage of corrosion behavior of low-alloy steel was observed in situ byscanning micro reference electrode and ex situ SEM&EDS. The result shows thatMnS plays an important role in pitting initiation. The mechanism of the early stage ofcorrosion behavior of low-alloy steel was speculated. At the same time, the influenceof chloride ion to the corrosion of low alloy steel was studied by scanningelectrochemical microscope from a microscopic perspective. The result reveals thatthe addition of chlorine ion increase the corrosion rate of low-alloy steel byaccelerating the process of localized cathode and anode of low alloy steel 3. The relationship between the influence factors of early corrosion of low alloysteel and wavelet coefficients which has the function of time-frequency resolutionwas studied by in situ electrochemical noise technique. The results show that thewavelet coefficient of large time scale (d8) is related to the growth of pitting on thelow alloy steel surface; The combination of the wavelet coefficient (d6,d7) isassociated with the formation of passivation film on the low alloy steel surface; thecombination of wavelet coefficient (d5, d6, d7) associated with the dissolution of iron.4. Superhydrophobic Halloysite modified with1H,1H,2H,2H-perfluorooctyltrichlorosilane（HNT-POTS）was prepared and it was characterized by Scanningelectron microscope，Energy dispersive spectroscopy, Fourier transform infraredspectroscopy, X-ray diffraction and thermogravimetric analys. The coating consistingof HNT-POTS was utilized to protect the low-alloy steel. The result is satisfactory.The durability of the HNT-POTS coating was assessed by monitoring the open circuitpotential change in3.5wt.%NaCl aqueous solution. In addition, the failuremechanism of the earlystage of the new coating is discussed...