Research Progress of Zn-Ni Corrosion Resistant Coating Prepared by Electro-plating on 30CrMnSiA Surface
Aviation high strength structural steel 30CrMnSiA has strong hydrogen embrittlement sensitivity because of its strong me-chanical properties.In the service process,the surface coating of structural steel is damaged faster and accelerated corrosion is more serious due to long-term exposure to rain forest humidity.Therefore,in practical engineering,high strength structural steel is required to meet the requirements of long time in the Marine humid environment,so as to reduce the loss caused by corrosion.Although the tra-ditional Cd plating technology has excellent corrosion resistance,especially in the marine environment corrosion resistance is more ex-cellent,but Cd is highly toxic will cause harm to the human body and the environment,and does not conform to the principle of nation-al environmental protection.Zn plating technology has excellent corrosion resistance and low hydrogen embrittlement sensitivity in gen-eral industrial atmosphere,but it is not suitable for Marine environment and has poor corrosion resistance.Zn-Ni alloy coating is the rise of a kind of important protective coating for steel materials,general nickel content is below 20％,this kind of alloy coating corro-sion resistance not only 7～10 times higher than galvanized layer,and have a good painting,weldability and formability,and therefore is widely used in automobile,aviation,light industry,household appliances,such as steel protective.In addition,Zn-Ni alloy also has the advantage of low hydrogen embrittlement,can be used as a substitute for cadmium coating in the aviation industry,is currently one of the most potential to replace the traditional Cd plating technology.Zn-Ni alloy electroplating is mainly divided into two systems:acidic Zn-Ni alloy electroplating and alkaline Zn-Ni alloy electroplating.Among them,acid electrogalvanizing nickel alloy was studied earlier,mainly including sulfate system and chloride system,technology is relatively mature,with high current efficiency,rapid depo-sition rate,high brightness,but its bath dispersion ability is poor,and waste liquid treatment is difficult.Alkaline electrogalvanizing nickel alloy is studied late,mainly including cyanide system,pyrophosphate system,zincate system.Compared with acidic electro-plating Zn-Ni alloy,it has the advantages of good bath dispersion ability,strong deep and uniform plating ability,simple bath compo-sition,easy treatment of waste liquid and so on.The co-deposition of Zn-Ni alloy is abnormal co-deposition.In the electroplating pro-cess,hydrogen evolution on the cathode surface leads to a decrease in the concentration of H+around the cathode surface,resulting in a rise in pH of the cathode surface.At this time,Zn2+and OH-will react to form Zn(OH)2 colloidal film.Its attachment to the cathode surface makes Ni2+need to pass through the Zn(OH)2 colloid film and consume more energy during deposition,thus inhibiting the de-position of nickel,that is,zinc deposition is superior to nickel deposition,showing abnormal co-deposition.In this paper,Zn-Ni alloy electroplating technology was discussed from three aspects:Zn-Ni alloy electroplating process,Zn-Ni alloy codeposition mechanism and Zn-Ni alloy coating corrosion resistance mechanism,and the future was prospected.
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