Studies On Pre-plating Processes Of NaNO₃Anodic Activation For Steel And Mechanism Of Activation

The most important problem is corrosion in the process of using steel.According to the statistics, the loss due to corrosion of the steel materials accountsfor1/5of the total yield in the world each year. It seriously damages to the modernindustry and even threats to the safety and life of people. Thus people have paidmore and more attention for the anti-corrosion of steel. In the processes of researchon the anticorrosive of steel materials, people gradually realize that coatings arebetter than other technologies, which are the most important, widely used to protectthem. In the total of steel products in the world, coating products account for about1/10. In the processes of plating, the pretreatment is particularly important, for itwill directly impact on the performance of coatings. Therefore, there is an urgentneed for a simple and high effective process of the activation pretreatment for thesteel materials.NaNO3anodic activation can quickly remove the oxide film and resultant on thesubstrate surface, which makes the substrate surface slightly etched, the metal latticeis fully bared and the substrate surface is in the activated state. In the appropriateconditions, it can improve the bonding force between the coatings and substrates.Also it has many advantages that chemical activation does not have, such as simpledevice, fast speed of activation, low cost and no pollution etc. This work studied theprocesses of NaNO3anodic activation on the activation quality of annealed,quenched45#steel and35CrMo alloy steel. We evaluated the effect of activation bybending and interfaces method. Based on those and combined with the changes of pH,components of the substrate surface and becomes of the solution after activated, weinvestigated the mechanism of activation. The specific works are as follow:1. Studied on pre-plating processes and mechanism of NaNO3anodic activationfor annealed45#steelThe structure of the annealed45#steel consists of ferrite (F) and pearlite phase(P)，the pearlite phase is the mechanical mixture of ferrite and cementite. For theferrite is more easily dissolved than cementite, that in the difference of concentration,temperature, activation time and current density, the area ratio of different phase,component and morphologies of the substrate are different after activated, whichaffect the binding force between the coatings and substrates. Through the analysis of experimental results, the optimum processes of NaNO3activated annealed45#steelwere the concentration200~400g·dm-3, temperature30~40℃, activation time5~10min, the current density of anodic activation5~10A·dm-2, pH6.00~10.00. In theconditions, the lattice on the substrate surface is bared, which achieve an optimalactivation state and the substrate surface also has certain roughness that enhances thecontact area and hinge of the coatings and substrates. Both of those increase theadhesion strength. According to the experiment results, we put forward a newmethod to determine the adhesion of the hard coatings electroplated on the othersubstrates. From the changes of pH, activation effect and components on theactivated annealed45#steel, we investigated the activation mechanism of ferrite andpearlite.2. Studied on pre-plating processes and mechanism of NaNO3anodic activationfor quenched45#steelThe microstructure of quenched45#steel are composed mainly lath andacicular martensite and a small amount of residual austenite. The experiment foundthat martensite phase in NaNO3solution is more prone to activate. Based on theactivation conditions of annealed45#steel, we selected the concentration of theNaNO3solution300g·dm-3, studied on the different solution temperature, activationtime, current density and initial pH affected on the components and roughness of theactivated substrate surface. By observing the interface of coatings to evaluate theactivation effect in each process condition, the optimum conditions for pH is6.00~9.00, activation time is5min, current density of activation is5A·dm-2,temperature of the solution is30℃. We also analyzed the reasons influence theadhesion between tungsten alloy and quenched45#steel substrate and the activationmechanism of martensite.3. Studied on the pre-plating processes and mechanism of NaNO3anodicactivation for35CrMo alloy steelThe most commonly way of heat treatment for35CrMo is quenched at850℃firstly, then temperated at580℃. Microstructure of35CrMo is the uniform sorbiteafter dealing with in this way (the fine-grain cementite uniformly distributes in theferrite matrix). We studied the electrochemical reactions of sorbite and alloyelements in the processes of NaNO3anodic activation. The activation effect wasevaluating by the bending test in different process conditions. The optimumconditions are that the concentration of NaNO3300~400g·dm-3, temperature30℃,activation time5min, the current density of anodic activation5~10A·dm-2, pH values6.00~10.00. The activation mechanism of35CrMo alloy steel wasinvestigated by the changes of the color of the solution, the pH value and theadhesion between the coatings and substrates.