Solid-State Control-Low Voltage Heating of Motors

In recent years, industrial plants have been utilizing more electric motors as drivers because of their high efficiency and reliability and lower maintenance costs. Electrified continuous process plants are using electric motors for both primary and standby drives. When these standby motors are not operating during normal plant runs, their windings collect moisture. Moisture attacks insulation and causes insulation failure. A breakdown of motor insulation may occur when a standby motor is suddenly energized to take the place of the primary motor. When it is not in operation, directly heating the motor winding with a low voltage system, coupled with automatic control, will guard the motor from moisture attack and extend its service life. The experience with low voltage heating of motor windings as described by M. H. Yuen in his article [1] has proved its effectiveness. However, conventional relays, timers, contactors, etc., were utilized in the circuits.     

Characteristics of Zinc Corrosion and Formation of Conversion Films on the Zinc Surface in Acidic Solutions of Cr(III) Compounds

The influence of the components of an acidic solution: Cr(III) nitrate-malonic acid-Co(II) salt and treatment conditions on zinc dissolution and formation of Chromate films as well as on their decorative and protective properties have been studied using the analytical, XPS, structural and accelerated corrosion test methods. An organic acid is the main component, which has an essential influence on zinc dissolution and formation of Chromate films as well as their decorative and protective properties. The influence of organic acid is directly related with the state of the Cr³⁺ ions in chromating solution. When the Cr³⁺ ions are in the form of hexaaquaions, the organic acid increases the quantities of the zinc dissolved and the Cr(III) deposited on the zinc surface (especially at 60°C). It also predetermines the formation of a thick, porous Chromate film with large cracks at 60°C. Its decorative and protective properties are rather poor. When Cr³⁺ ions are in the form of a complex with organic acid, the quantities of the zinc dissolved and the Cr(III) deposited on the zinc surface significantly decrease and thinner Chromate films with an even surface, good decorative appearance and high corrosion resistance are formed Decorative blue-bright Cr(VI)-free films with a slight iridescent tint, obtained in solution, containing Cr(III) nitrate (0.2), malonic acid (0.3) and Co(II) nitrate (0.02) mol dm^?3, at p H 1.6-2.0 at room temperature over 30–60 s, possess corrosion resistance (192–240 h in a salt spray chamber) similar to that of iridescent Chromate films, obtained in acidic Cr(VI) solution.     

Kinetic Regularities of Zinc Corrosion in Chromating Baths

Electrochemical modeling of zinc corrosion in chromate solutions showed that its rate (which is controlled by the limiting current density of Cr(VI) reduction to Cr(III)) is determined by the limiting diffusion current of hydrogen ions to the zinc electrode surface partially covered with the chromate coating. The sizes of active and passive surface sites are comparable with the thickness of the diffusion layer of the solution, which means that the zinc surface corroding in a chromate solution is macroscopically heterogeneous. Investigating the kinetics of the electrode reactions showed that, in the absence of a noticeable film formation, the steady-state cathodic polarization curve of zinc in sulfate–chromate solutions can be described by a kinetic equation that takes into account the pH _s dependence on the dissolution rate of zinc.     

Distinctions between the Activating Effects of Sulfuric and Phosphoric Acids on the Process of Zinc Chromating

Activating effect of anions of sulfuric and phosphoric acids (both in the range from 0.025 to 0.200 mol/l) on the process of zinc chromating in acid (pH 1.1) 0.2 M CrO₃ solution is studied by analytical methods. A general balance of oxidized zinc (Zn²⁺) and reduced chromium (Cr³⁺) and their distribution between the solution and chromate film for different solution compositions, as well as the elemental composition throughout the film's depth, are determined. It is found that the zinc oxidation and Cr⁶⁺ reduction reactions do not proceed in the absence of SO^2– ₄ ions (that is, when only PO^3– ₄ ions are present), so that the chromate film cannot form. However, the PO^3– ₄ ions combined with SO^2– ₄ ions increase the Zn²⁺ and Cr³⁺ ions concentration in the solution significantly, while their concentrations in the film correspondingly decrease. SO^2– ₄ ions activate the zinc surface because they form soluble complex compounds with Zn²⁺ and Cr³⁺ ions and increase the part of the surface, on which the cathodic reduction of Cr⁶⁺ to Cr³⁺ occurs. The activating action of phosphoric acid is caused by the increase in the total (analytical) concentration of H⁺ ion in the solution; hence, the deposition of the Cr³⁺ and Zn²⁺ hydroxide compounds onto zinc is retarded, due to the increased near-surface concentration of H⁺ ion.     

Calculation of Limiting Diffusion Currents in Cr(VI) Solutions

The diffusion currents in Cr(VI) solutions are shown to be proportional to the difference between the bulk and surface concentrations of proton donors. In the range of the proton discharge limiting current, the corresponding discharge from water molecules is not observed. The concentration profiles of proton-donating particles within the diffusion layer are close to straight lines, which makes it possible to calculate the limiting diffusion current of hydrogen from an apparent (mean) diffusion coefficient and an apparent thickness of the diffusion layer according to the model of additive diffusion currents. The calculated currents well agree with the experimental data obtained with a rotating disc graphite electrode.     

Influence of Corrosion on Microstructure of Chromated Zinc Surfaces. Part 1. Microstructure changes of electrodeposited zinc in the process of chromating

The microstructure of zinc surfaces has been investigated using x-ray photoeiectron spectroscopy and scanning electron microscopy. The inclusion of a 0.5% HNO₃ bright dip stage into the technological process of zinc chromating stimulates equal-rate generation of anode-active spots on zinc surfaces and at the same time influences the formation of more uniform conversion coating. Intensive zinc surface etching is observed under the forming conversion coating.