Chromate reduction, conducting

Although numerous experiments and spectroscopic characterizations have been conducted on the Phillips catalyst, the precise structure of the active site on the silica surface, reduction of the surface chromate species during the induction period, the formation of the first chromium-carbon bond, and the mechanism for ethylene polymerization still need to be further clarified [11]. In order to achieve more specific information, molecular modeling approaches could provide a useful complement to the experiments and enable us to study these obscure mechanistic problems directly at the atomic and molecular level. In the last decade, very precise mechanistic pictures of the Cr-based polymerization catalysts have been obtained using different theoretical methods, especially through a combination of the experimental findings with theoretical calculations. 

Chromate and phosphate-treated metals in a dissimilar metal couple may sometimes obtain a reduction of galvanic corrosion caused by electric current transfer in a conductive medium. 

Given some basic information about the corrosion inhibition mechanisms of chromates, many studies have been conducted for chromate replacements. For effective replacement of hexavalent Cr, however, an inhibitor has to inhibit the oxygen reduction reaction as well as anodic dissolution/pitting, and several studies indicate that hybrid formulations seem to be the best way to do just that. Typically, in these hybrid formulations an organic oxygen reduction reaction inhibitor is included with environmentally benign anodic inhibiting anions. 

Tests for chlorine, bromine, and fluorine content in fluxes are required as per IPC-TM-650, TM 2.3.33 and TM 2.3.35. The presence of these materials cause a reduction in the SIR and can accelerate corrosion of the PCB conductors and assembly solder joints. A test for chlorine and bromine is conducted by placing a droplet of flux on a silver chromate paper, which turns yellow or white if the halides are present. Presence of fluorine is determined by adding a droplet of purple zirconium alizarin to the flux region. A change in color from purple to yellow indicates fluorine is present. 

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