Barrier coatings organic reactions

Recent applications of e-beam and HF-plasma SNMS have been published in the following areas aerosol particles [3.77], X-ray mirrors [3.78, 3.79], ceramics and hard coatings [3.80-3.84], glasses [3.85], interface reactions [3.86], ion implantations [3.87], molecular beam epitaxy (MBE) layers [3.88], multilayer systems [3.89], ohmic contacts [3.90], organic additives [3.91], perovskite-type and superconducting layers [3.92], steel [3.93, 3.94], surface deposition [3.95], sub-surface diffusion [3.96], sensors [3.97-3.99], soil [3.100], and thermal barrier coatings [3.101]. 

Electrochemical impedance spectroscopy (EIS) offers an advanced method of evaluating the performance of metallic coatings (passive film forming or otherwise) and organic barrier coatings. The method does not accelerate the corrosion reaction and is nondestructive. The technique is... 

The solution coating technique was used in the preparation of the cellulose triacetate membrane discussed above. A solution of cellulose triacetate in chloroform was deposited on the porous support and the solvent was then evaporated leaving a thin film on the porous support. Thin film polymerization was used to prepare a polyfuran membrane barrier layer on polysulfone. In this case, the monomer furfuryl alcohol is polymerized in situ by adjustment of pH and temperature. This membrane proved to be highly susceptible to oxidizing agents and is of limited value. By far the most valuable technique in the formation of membrane barrier layers is interfacial polycondensation. In this method, a polymer is formed on the porous support surface at the interface of organic and aqueous phases by reaction of specific molecules dissolved in each phase. It is by this method that a number of polyamides and polyurea membrane barrier layers have been formed on polysulfone. Elements containing these membranes are available commercially. 

While barrier-effect pigments solely increase the diffusion path of corrosive species and the coating practical adhesion of the coating, active pigments such as chromates additionally inhibit electrochemical reactions at the metal-polymer interface as well in defects of organic coatings. 

The mechanism of inhibition action by organic compounds is adsorption. There are two types of adsorption, namely, physiosorption and chemisorption. For physiosorption, corrosion inhibition may be a result of the formation of mechanical barriers by inhibitor species. This will hinder the movement of ionic species from and to the surface of the corroding metal. On the other hand, for chemisorption, the chemical reaction takes place on the surface of the corroding metal, with subsequent formation of a thin coating. This hinders movement of species to and from the corroding surface as well. The characteristics of physiosorption are as follows ... 

Additional protection may be supplied by resistance inhibition, which is also a part of the barrier mechanism. Retardation of the corrosion action is accomplished by inhibiting the charge transport between cathodic and anodic sites. The reaction rate may be reduced by an increase in the electrical resistance and/or the ionic resistance in the corrosion cycle. Applying an organic coating on a metal surface increases the ionic resistance. The electronic resistance may be increased by the formation of an oxide film on the metal. This is the case for aluminum substrates. 

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