Corrosion Behavior And Protective Methods Of Stainless Steel In Marine Environment

In metal marine corrosion,the corrosion resistance of the metal itself is important.The corrosion mechanism and protection method of marine steel has become an important subject in marine corrosion science.This article studies the initial behavior of stainless steel pitting in a laboratory simulated seawater environment.Through the measurement of polarization curve and electrochemical impedance spectroscopy,it is found that the passivation film on the surface of the stainless steel substrate has a good repairing ability in the initial period of immersion(0~36h).This time is the induction stage of pitting corrosion.When the time period is The impedance spectrum measured in the interval(36h~48h)exhibits inductive reactance characteristics at low frequencies and the polarization curve fluctuates,indicating that this is the metastable stage of pitting corrosion.When the immersion time is 60 h,a second capacitive reactance arc appears in the high-frequency part of the impedance spectrum.At this time,the pitting has changed from a metastable state to a stable pitting stage.The temperature and NaCl concentration that affect the pitting resistance of this material were studied.It was found that temperature has a greater effect on its pitting resistance.As the temperature increases,the pitting resistance of the material decreases,and the higher the NaCl concentration of the solution,the The worse the pitting resistance.Nickel-phosphorus and nickel-tungsten-phosphorus alloy coatings were prepared on the surface of stainless steel.The nickel-phosphorus coating is an amorphous coating,which has a good bonding force with the substrate,and significantly improves corrosion resistance and hardness.After adding sodium tungstate to the plating solution,it was found that as the amount of sodium tungstate increased,the content of tungsten in the plating layer also increased.Increasing the content of tungstate will increase the cell size on the surface of the coating.Therefore,the quality of the coating obtained when the content of sodium tungstate is 0.4 g/L is the best.Phosphating films were prepared on the surface of stainless steel by chemical and electrochemical methods.After electrochemical phosphating treatment,the corrosion resistance improved significantly,the self-corrosion potential moved forward,and the self-corrosion current decreased by an order of magnitude.The bonding strength of the phosphate coating was tested.The bonding strength of the electrochemical phosphate coating was 98,and the bonding strength of the chemical phosphate coating was 96.Copper sulfate resistance drip test results: 315 S for chemical phosphating and 520 S drip resistance for electrochemical phosphating film.Scanning electron microscopy shows that the microstructure is different.The chemical phosphating film is composed of a small number of rods and a large number of block crystals.The electrochemical phosphating film is composed of a large number of blocks and a small number of rod-shaped crystals.The phosphating film is dense.XRD results showed that the composition of the two phosphating films was the same,the components were zinc calcium phosphate,calcium phosphate,and zinc oxide.Among them,zinc oxide may be obtained by oxidizing the zinc element in the phosphating film.