Research On Preparation And Forming Mechanism Of Trivalent Chromate And Chromate-Free Passive Films On Electroless Ni-P Coating

The electroless Ni-P coating (ENPC) has been widely applied in field of petrochemical, aerospace, national defense, energy source and micro-electron because of its high throwing power, excellent corrosion and wear resistance as well as solderability. Despite its excellent properties, the strong tendency of self-passivation of Ni in coating makes the outermost layer of the coating be easily oxidized and thus lose its brightness even discolor, which will seriously affect the appearance of the coating. And this Ni oxide film is susceptible to breakage, and its presence will deteriorate the solderability and weaken the corrosion resistance of the Ni-P coating. Thus it is necessary that the coating must be treated by some appropriate post treatments to improve its corrosion performance and anti-discoloration (anti-oxidization) ability. The traditional chromate (â…¥) treatment can meet the demand of corrosion resistance and anti-discoloration, but chromate (â…¥) will bring very serious damage to both human beings and environment. Since the importance of eco-enviroment has been awared, the urgent action should be taken to research chromate(â…¥)-free passivation technics for ENPC.In this work, chromate(â…¥)-free methods were used to solve the mentioned problems on Ni-P coating. To avoid pollution, the preparation technics for environmental friendly chromate(â…¢) and chromium-free passive films were systematically researched. The forming mechanisms of the passive films were also proposed. At the same time, for the purpose of comparison, the forming process of chromate(â…¥) passive film was also discussed.A series of experiments found that the ENPC is apt to discolor and difficult to be effectively passivated at acid condition, especially when the pH value of passive bath is lower than 2.0, the coating is likely to darken. By a alkaline bath, a chromate(â…¢) passive film was obtained on ENPC. The content of main salt, working temperature and pH value for the bath were determined by single-factor experiments. Taking nitric acid (50vol.%) exposure test and Tafel test in 3.5%NaCl solution as fast-checking methods, the corrosion resistance and anti-discoloration ability of the chromate(â…¢) passive film were checked. The test results indicated that the chromate(â…¢) passive film can improve the anti-oxidation ability of the coating and obviously enhance the corrosion performance in Cl--containing solution.At alkaline condition, with dodecanoic acid (DA) as film-forming agent, a chromium-free passive film was prepared on ENPC, the content of DA and working pH value were determined by single-factor experiments. Fast-checking tests showed that although the improvement of anti-oxidation ability of the DA film in nitric acid exposure test was almost negligible, this chromium-free passive film could dramatically impove the corrosion resistance of ENPC in 3.5%NaCl solution.. A sealing agent for ENPC, whose storage time is more than one year, can be obtained by hydrolyzing SCA at an appropriate proportion between SCA, ethanol and water. A compact self-assembled film would form on ENPC by treated in the sealing agent, and this compact film could improve the corrosion resistance of EPC in 3.5%NaCl solution by two orders of magnitude, while the anti-oxidation in nitric acid was only impoved by one order of magnitude.The nitric acid exposure test, Tafel test and electrochemical impedance spectroscopy (EIS) were employed to analyze the anti-oxidation ability and corrosion resistance of the researched passive films, including chromate(VI) passive film. The scanning electronic microscopy (SEM) was utilized to observe the effect of Cr3+- and Cr6+-treatment on the appearance of ENPC, the elemental composition of the Cr3+ and Cr6+ passive films were studied by X-ray photoelectron spectroscopy (XPS) and the chemical states of these elements were analyzed by high resolution XPS spectra. The research on the semiconductor properties of the films was conducted by Mott-Schottky (M-S) method. The electrochemical tests indicated that the result of Tafel test was in good agreement with that of EIS, and the order of the anti-oxidation ability for the various films was: chromate(â…¥) passive film > SCA self-assembled film > chromate(â…¢) passive film > DA passive film. The SEM result showed that no obvious difference could be observed between the blank Ni-P coating and passivated coating (both Cr3+- and Cr6+-treated coatings). The XPS analysis on the Cr6+-treated coating illustrated that the passive film comprised O,Cr and Ni, while the chromate(â…¢) passive film was made up of C, N, O, Cr, Ni and P. The high resolution XPS analysis indicated that the Cr in chromate(â…¥) passive film was only in the state of trivalence, its existence form was Cr₂O₃ and Cr(OH)₃, 87.0at% of the total Cr was in the form of Cr₂O₃. The element Cr in chromate(â…¢) passive film was also in the form of Cr₂O₃ and Cr(OH)₃, but only 21.4at% of the Cr was Cr₂O₃. The M-S test results illustrated that Cr⁶⁺ passive film, Cr³⁺ passive film, and DC passive film exhibit n-type characteristic at lower potential and p-type at higher potential at various frequencies; while the SCA film shows a contrary p/n characteristic, namely p-type at lower potential and n-type at higher potential.Based on the XPS analysis of Cr6+ passive film formed on the ENPC, the function of Ni and P in the forming process of passive film was discussed. It could be inferred that both Ni and P in coating will react during the chromate(â…¥) passivation process, but the reaction products of Ni and P will not involve in the formation of passive film. In the light of the XPS analysis on the Cr³⁺ passive film, the aromatic compound in chromate(â…¢) passive bath would participate in the formation of Cr³⁺ passive film by chemisorption on Ni-P coating. Because the consumption of OH- during the adsorption leads to decrease of pH value in the solution layer very clear to Ni-P coating, Cr(OH)₄^-particles will change into Cr(OH)₃ and deposit on the surface of Ni-P coating and eventually form Cr³⁺ passive film. The research on the DA passive film found that the DA molecules adsorb on Ni-P coating by chemisorption, its adsorption process meet the Langmuir isothermal, namely monolayer adsorption film. At cure temperature, the SCA film was formed through a self-assembled process by condensation reaction between hydroxyl in silanol molecule and hydroxyl on Ni-P coating, at the same time the condensation reactions will also take place between the hydroxyl groups of silanol molecules and form a compact SCA film.