Study On The Preparation And Performance Of Mechanically Deposited Cadmium Coatings

Cadmium plating is still the main protection means for some steel-based parts of ships and ships,especially for the surface protection of fasteners and connectors used in naval equipment.The current cadmium coatings are basically electroplated,including 300 M,30CrMnSiNi2A,A-100 and other high-strength steel bolts and surface protection of connectors.However,when high-strength steel is treated with an electroplating process for corrosion protection,hydrogen embrittlement and cadmium brittleness are easily generated.In order to solve the above problems,a cadmium plating layer is prepared on the surface of the steel by a mechanical deposition process,and the above-mentioned problems caused by the electroplating process are avoided while achieving surface anticorrosion.In view of this,this paper will carry out research on mechanical deposition of cadmium and cadmium plating.The paper uses orthogonal experimental design to optimize the composition and ratio of activator for mechanical deposition of cadmium plating.The thickness of the cadmium coating,the corrosion area of the 600 h salt spray experiment,the self-corrosion potential,the corrosion current density,the relative density and other characteristics were analyzed,and the composition and ratio of the activator were determined.At the same time,based on the determination of the activator formulation of mechanical deposition of cadmium plating,the single factor variable was set up,and the influence of process parameters on the characteristics of cadmium plating was studied and the optimum plating parameters were determined.The paper successfully prepared the mechanical deposition of cadmium coating,and analyzed the microstructure and deposition mechanism of the coating by XRD,SEM and EDS.The bonding strength,porosity and corrosion resistance of cadmium coatings were analyzed by cross-cut experiment,porosity experiment,electrochemical polarization,neutral salt spray test and full immersion test.The research of the thesis draws the following conclusions:(1)The activator group distribution ratio for mechanical deposition of cadmium plating was: polyethylene oxide(PEO)(10%);polyvinyl alcohol(11%);bentonite(2.2%);sodium citrate(8%);butynediol(2%);diatomaceous earth(0.9%);the stannous salt was added in an amount of 50%,and the balance was water;the cadmium plating prepared by the activator formulation had the best comprehensive characteristics.(2)Based on the determination of the activator formulation,the effect of the preparation process of mechanically deposited cadmium coating on the coating characteristics was studied: the preparation of the coating requires a sufficient pre-treatment process(heating degreasing,dilute hydrochloric acid rust removal),and the pH of the plating solution is 1~ 2,the establishment of the base layer should use moderate speed(about 40 r / min),the coating cycle thickening stage should use a higher speed(about 70 r / min),after 10 min of strengthening process can make the surface of the cadmium coating smooth Uniform,with a bright metallic luster,the best coating performance.(3)The XRD scan results of the coating show that the main body of the coating is cadmium,and there is no cadmium compound.Among them,there is a small amount of tin,the diffraction peak of cadmium is sharp,and the crystallinity of cadmium is high,at the same time,there is a phenomenon of grain growth in the plating layer.The EDS scan results show that tin is present between the cadmium powder particles and acts as a "bonding".The cadmium powder particles are inlaid into layers by physical biting.The cadmium powder particles constituting the coating body do not undergo atomic layer reorganization.Most of the cadmium powder particles are plastically deformed under the collision of glass beads,and some cadmium The powder particles are pressed against each other to form a massive structure,and the cadmium powder particles(cadmium powder clusters)undergo the following processes: aggregation ? adsorption ? compaction ?translation,rotation ? deformation ? densification,into a dense coating,cadmium powder The mutual extrusion and stacking prolongs the path of corrosive medium intrusion into the substrate,and improves the corrosion resistance of the plating layer.The scribing and cross-cut test shows that the plating layer has not fallen off,and the bonding strength meets the requirements of the anticorrosive coating.(4)In the 5% sodium chloride solution,the cadmium plating layer(-740 mV)is an anodic protective coating compared to the iron matrix(-600 mV).Theself-corrosion potential of the cadmium plating layer is increased by 560 mV compared with the mechanical galvanized layer(-1300 mV),and the corrosion tendency is greatly reduced.The corrosion current density is reduced by about 70 times compared with the mechanical galvanized layer,and the corrosion rate is greatly reduced.Under the condition of neutral salt spray test,the time of 5% red rust area of cadmium coating at 20 micron is about 600 hours,and the corrosion resistance of cadmium coating is greatly improved than that of zinc coating.