Carboxylate - Urea Pulse Electrodeposited Chromium And Chromium Alloy And Iron - Nickel - Chromium Coating Black

Chromium and chromium alloys electrodeposition from trivalent chromium bath have recently attracted significant interest because of its low energy consumption, low toxicity and low pollution. While thick deposits (chromium and chromium alloys) and deposits of high content chromium cannot be easily obtained from trivalent chromium electrolyte. It is difficult to obtain functional chromium and chromium alloys from trivalent chromium electrolyte. In order to resolve these problems, the technology and correlative theories of pulse electrodepositing nanocrystalline chromium and chromium alloys (Ni-Cr, Fe-Cr and Fe-Ni-Cr) from trivalent chromium bath were investigated, and the coloring for Fe-Ni-Cr stainless steel plating layer was studied as well.Three suitable carboxylate complexing agents of Cr³⁺ have been chosen. They were mixed with urea to obtain the complex system of compound carboxylate-urea, which work as complexing agents of Cr³⁺. The optimum bath and operating conditions for pulse electrodepositing nanocrystalline chromium from compound carboxylate-urea complex system were obtained through orthogonal experiment. The influences of the bath and the operating conditions on the thickness of chromium coatings and the electrodepositing velocity were investigated systemically. Under the optimum conditions, the nanocrystalline chromium coating with thickness up to 11.2μm and crystal grain size less than 100 nm Was obtained. Adopting the complex system, the current efficiecy can be up to 25.32% and mirror-like chromium coatings can be obtained without using brightener.On the basis of the above optimum nanocrystalline chromium process, the Ni-Cr alloy and Fe-Cr alloy pulse electrodepositing processes from compound carboxylate-urea complex system have been investigated systemically. Through the investigation of the alloys electrodepositing solution ingredients, the process variables and their relationship with the Cr content, thickness, surface appearance and grain size, the optimum processes have been obtained respectively. Under the optimum conditions, the nanocrystalline Ni-Cr alloy coating with 20.05μm thickness, 27.917% Cr and 80.2HR30T hardness, and the nanocrystalline Fe-Cr alloy coating with 15.12μm thickness, 39.73% Cr and 78.8HR30T hardness were obtained respectively.On the basis of the above optimum nanocrystalline chromium and chromium alloys(Ni-Cr and Fe-Cr) processes, the pulse electrodepositing process of Fe-Ni-Cr alloy from compound carboxylate-urea complex system has also been investigated systemically. The influences of the bath and the operating conditions on the Fe-Ni-Cr alloy composition, thickness, current efficiency, surface appearance and grain size have been discussed. Under the optimum conditions, the nanocrystalline Fe-Ni-Cr alloy coating with 23.38μm thickness, 25.56% Cr and 82.0HR30T hardness was obtained. The Cr content of Fe-Ni-Cr alloy coating reaches the standard of general stainless steel.The studies by scanning electron micrograph(SEM) on the surface appearance and microstructure of chromium and chromium alloys reveal that the deposits have delicate grain, and the metallic appearances are bright and smooth, which exhibit free-pitting and cracking. It has been confirmed that the structures of the Cr, Ni-Cr, Fe-Cr and Fe-Ni-Cr are crystals by the X-ray diffraction (XRD). The the grain size of Fe-Ni-Cr calculated by of Scherrer formula was less than 50nm, which was consistent with that of SEM analysis. The corrosion electrochemical behavior of nanocrystalline Cr, nanocrystalline Fe-Cr, nanocrystalline Ni-Cr, and nanocrystalline Fe-Ni-Cr stainless steel coatings in 1mol/L H₂SO₄ solution, 3.5% NaCl solution and 10% NaOH solution have been studied respectively, The results exhibit that the deposits above have excellent corrosion resistance.The electrochemical behaviors of chromium and chromium alloys electrodeposition from compound carboxylate-urea complex system have been investigated by modern electrochemical methods. The results indicated that both single complexing agent and compound complexing agent with suitable concentrations can increase the hydrogen evolution potential, which can enhance the current efficiency, and can increase the Cr³⁺ two-step electrodeposition potential, which can obtain bright and compact chromium and chromium alloys coatings. Single complexing agent can increase nickel electrodeposition potential, but their influences on Fe²⁺ discharge is very complicated. The compound complexing agent makes the electrodeposition potential of Cr³⁺, Ni²⁺, Fe²⁺ approach each other, which benefit the co-electrodeposition of Fe-Cr, Ni-Cr and Fe-Ni-Cr. The compound complexing agent can increase apparent activation energy of Cr³⁺, Ni²⁺, Fe²⁺ electrodeposition respectively, which is consistent with the result that it can enhance their electrodeposition potential. The activation energy of Cr³⁺ electrodepositing in alloy was lower than that of Cr³⁺ alone, and activation energy of Cr³⁺ electrodepositing in Fe-Ni-Cr alloy was lower than that of in Fe-Cr and Ni-Cr alloys. So the electrodeposition of iron and nickel can catalyze the electrodeposition of chromium. The studies of alternating current (A.C.) indicate that the A.C.impedance spectra of Ni-Cr, Fe-Cr and Fe-Ni-Cr co-electrodeposition were similar, and that Ni-Cr, Fe-Cr and Fe-Ni-Cr co-electrodeposition were all electrochemical controlled. The middle product of trivalent chromium, which is electrochemically active and adsorbed on the electrode, has been detected by A.C. impedance testing. Amongst the three, the impedance of Fe-Ni-Cr is the smallest.For the first time, the mechanism of trivalent chromium electrodeposition and electrocrystallization from compound carboxylate-urea complex system has been explored by using several kinds of electrochemical methods. [Cr(H₂O)₅L]²⁺ reduction process is controlled by electrochemical reaction. Firstly, complex ions [Cr(H₂O)₅L]²⁺ undergoes chemical transformation reaction before being reduced to [CrL]²⁺ in the cathode, and this transformation reacts very quickly. Subsequently, [CrL]²⁺ gets three electrons by two steps, the first reducing is rate control step. The middle product, which is electrochemically active and adsorptive, is formed in the first step. Therefore the reducing reaction mechanism of Cr is as follows The kinetics equation and the A.C.impedance of the Cr electrodeposition reaction are derived from the mechanism and the kinetic parameters are obtained from the experiments. The kinetics equation is determined in theory: The theoretical values of the kinetic parameters are in agreement with experimental ones very well, which shows the hypothesized mechanism of chromium is correct. The study of potentiostatic step indicated that the electrocrystallization of chromium follows the mechanism of progressive and three-dimensional growth.On the basis of the coloring process for 1Crl8Ni9Ti stainless steel, self-made Fe-Ni-Cr stainless steel alloy coating worked as coloring matrix. The optimum chemical coloring solution and operating conditions and the optimum electrochemical coloring solution and operating conditions were adopted to color for self-made Fe-Ni-Cr stainless steel alloy coating. The black colored film with excellent reproduction, brightness and uniform properties and the stability to bear color-change can also be obtained under the same conditions. The results of energy dispersive spectra show that the main elements of black colored film are Fe, Mn and Cr. The analysis of coloring reaction for Fe-Ni-Cr stainless steel alloy coating indicated that the process of film-formed mechanism mainly is as follows: (a) the surface metal atoms of Fe-Ni-Cr alloy coating dissolving actively and diffusing constantly to the interface of Fe-Ni-Cr alloy coating and coloring solution; (b) Cr³⁺, Mn²⁺ and OH^-in the solution absorbing on the surface of the matrix; (c) the ions diffusing to the interface and the ions of Cr³⁺ and Mn²⁺ absorbed on the matrix surface and OH^- or H₂O molecule absorbed on the matrix surface form hydroxide, which is absorbed on the matrix surface. And then the hydroxide hydrolyzed to form metal oxide. According to the micro-structure and component of the colored film and relevant theory, the relation between coloring potential and thickness of coloring film has been obtained as follows: Ez=2.07＋26.96χ.