Electrocoating studies

In the process of titanium deposition, high adatom surface diffusion rates, low adatom concentrations and low overpotentials favoured the building of existing crystals. Meanwhile, the low surface diffusion rates, high adatom concentrations and high overpotentials increased the formation rate of new nuclei. Clearly, the dominant parameter which affects the nucleation rate is the applied overpotential. At high overpotentials. 

Current form Charge (C) Current density (mAcm ) Ti (%) Grain size (gm) 

In the first series of depositions, toi,/toff = 3/l, distinct differences in the composition and homogeneity were found between the samples obtained by DC and PC. The higher Ti content and smoothness of samples submitted to PC were improved. For ton/toff = 1/3, the Ti content was lower than in the previous series. 

Some experimental results of the Ti electrocoating from the chloride-fluoride system, at 750 °C with DC and PC techniques, are summarized in Table 4.7.1. 

The mechanism of the reduction of Ti in molten NaCl-KCl-NaF-K2TiF6 was studied using CV and CP over a Ti concentration range of 0.1 -0.5 mol%, at temperatures varying from 700 to 800 °C. Two reduction and two oxidation steps were observed, which established that Ti + is reduced to Ti + (soluble) followed by a three-electron reduction of Ti + to Ti (Pt alloy). The first step is the reversible reduction of Ti + to give a soluble product, and the second step is the reduction to Ti metal, and its reversibility may be restricted by chemical/electrochemical complications. 

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Electrochemically polished carbon steel plates (20 x 20 x 2 mm) were used as cathodes for electrocoating studies. 

A generalised model of electrical equivalent circuit for painted surfaces has been considered in many of the recent publications. Googan ( 2) used it to study vinyl coatings free of defects and coatings containing defects. Electrocoatings were also evaluated. Muslanl et al (27) in their investigation of mild steel... 

The electrocoating of Ti from the molten electrolyte at 750 °C was smdied with the DC and PC techniques. The Ti content and surface morphology of Ti/steel layers were studied as a function of current density, electrical charge, current forms and duty cycle. With the increase of the current density, a decrease in crystal grains was noticed in both DC and PC electrodischarge. The PC can be used to improve the quality of the Ti deposited layers. The duty cycle was found to be a key factor to the Ti content as well as to the surface properties of the layers. The high Ti content and smoothness of samples were obtained by the pulse method. Namely, an electrocoated layer with 70.32% Ti content could be obtained by lower PC density with a duty cycle of 75%. 

A practical and effective crossHnking mechanism in cathodic electrocoating is done with polyfunctional blocked isocyanates. The mechanism involves the reaction of an isocyanate group (NCO) with the hydroxy group of the epoxy backbone and liberation of the blocking group. Other crossHnking mechanisms that have been studied are use of addition polymers, Mannich bases, Michael adducts [110] sulfonium stabilized polymers, transesterification reaction of hydroxy, alkoxy, amido and ester systems with hydroxy functional cathodic backbone (Fig. 7-5) [111-114]. 

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