Study On Corrosion Behaviour And Corrosion Control With MAP/Nano-Pd Coating Of Welded 304 Stainless Steel In 3.5%NaCl Solution

Stainless steel is widely used in nuclear power plants,satellite launch bases,power station and many other fields because of its excellent processability and corrosion resistance.And welding is the most popular process for manufacture of various facilities and equipments of stainless steel,because of the high temperature input and different cooling rate during the welding process,there exist always detrimental effects resulted in change of composition,structure and stress state on the welding zone,which will cause various localized corrosions happened selectively in the welding zone of stainless steel.This thesis is mainly to study the corrosion behavior of welded 304 stainless steel by argon arc technique,and to develop some new techniques for efficiently control of corrosion at welding zones.Palladium as a precious metal,it has excellent corrosion resistance,palladium film can be deposited on the metal surface through electroplating and electroless plating for corrosion protection purpose.Mussel adhesive protein is a natural,environmentally friendly material that can be extracted from mussel foot silk with outstanding adhesive properties,film forming properties,non-toxicity,and excellent corrosion resistance.This work focuses on the use of low-price industrial grade mussel adhesion proteins to prepare mussel adhesion protein/nano-palladium membrane on welding zone of stainless steel.And the anticorrosion property of the film on the welded stainless steel was investigated by means of electrochemical methods,and the mechanism of corrosion control was also discussed.The main progress and achievements are listed as follows:(1)The corrosion behavior of argon-welded 304 stainless steel was investigated by using scanning electrochemical reference electrode technology,scanning Kelvin probe technology,array electrode,and the changes in microstructure before and after welding were characterized by metallographic microscopy,XRD diffraction and Electron Back-Scattered Diffraction experiments.Through the methods of scanning electrochemical reference electrode technology,scanning Kelvin probe technology,and array electrode technology,it was found that the stainless steel welds are more prone to start pitting corrosion than the base metal,which mainly exhibit the local corrosion.The characterization result shows that the conversion of austenite phase to the ferrite phase has occurred at the welding zone and the conversion is about 3%,which may be the cause of the corrosion in the welding area.(2)Palladium film was deposited on the surface of welded stainless steel by electroplating method and hydrothermal method.Through the characterization of SEM test,it was found that palladium film could be prepared on the sample surface by two methods,and the palladium in the film showed nanometer particle size.The electrochemical tests show that the palladium film has excellent corrosion resistance.(3)The mussel adhesion protein/nano-palladium film was deposited on the welded stainless steel by "one-step immersion" and "two-step immersion" The composite film was characterized by SEM,infrared spectroscopy,Raman spectroscopy,and XRD.Electrochemical tests show that the composite film is much better than the palladium film to protect the sample from corrosion.