Electrodeposited Fe-p Amorphous Coating Process And Mechanism Study

The application prospect of electrodeposited Fe-P amorphous film is bright because of its good corrosion resistance and high hardness.Its application range,however,has been affected by some deficiency ,such as lower crystallization temperature , the hardness of the film can up to HV1000 only by the ion nitriding after 400℃heat treatment.Fe-P amorphous films with better corrosion resistance,higher hardnes and higher crystallization temperature have been obtained in this paper,the electrodeposition process of Fe-P amorphous film has been studied as well as the mechanism of corrosion and reinforcement.The main results are as follows:The results of thermodynamics analysis show that the Fe atoms can only be deposited in the zone which the pH value is small enough if the potential is lower than line①and the concentration of Cl^- must be exceeded 0.4mol/L.Chloride bath has been chosen because of its higher electrical current efficiency and can form amorphous structure much more easily and promptly if compared with sulfate bath.All of the factors involved have been divided into two sets.those whom made greater influence to the corrosion resistance were named primary factors,and the rest were named secondary factors.The most favorable values of both the primary factors and the secondary factors have been gained after several sets of 267ml Hull cell tests and 160ml rectangular cell tests,the influence of some factors to the corrosion resistance have also been detected.The content of P in Fe-P amorphous film under the best process condition is approximately 21(wt)%.The result of XRD approves that this film is amorphous in structure.Weight loss corrosion measurement has been introduced to determine the corrosion rate of Fe-P films.It is suggests that the value of corrosion rate of Fe-P films in 0.5mol/L H₂SO₄ solution is about 0.08g·m^-2·h^-1.The result of Vickers hardness test show that the hardness of Fe-P films is about HV550 without being heat treatment,but can up to HV1100 after 380℃heat treatment.Furthermore,the oxidation weight gain rate of the film has also been determined when heated,the result demonstrates that Fe-P films can be used under 400℃normally.The electrochemistry workstation has been adopted to study the anode polarization of the Fe-P film.The anode polarization curve with clear passivation section conforms that Fe-P films tend to passivate in 0.5mol/L H₂SO₄ solution.According to the membrane theory and the adsorption theory,it is the extranuclear empty tracks of Fe and P atoms combine with the electron pairs of O^2- that contribute to the formation of the oxide layer or the adsorption layer.thus the corrosion rate of Fe-P films is reduced. The motheds of thermal analysis and X-RAY diffraction were used to study the structural transformation of Fe-P amorphous films after being heated.the relationship between hardness and heat treatment temperature was studied by Vickers hardness test.It is suggests that the Fe-P amorphous films begin to crystallize at 300℃,the hardness of the film increases and maximizes at 380℃in consequence of solid solution reinforcement and disperse reinforcement, and then decreases out of Ostwald coarsening.