Conversion coatings compounds

The wide range of soHd lubricants can generally be classified as either inorganic compounds or organic polymers, both commonly used in a bonded coating on a matching substrate, plus chemical conversion coatings and metal films. Since solid-film lubricants often suffer from poor wear resistance and inabihty to self-heal any breaks in the film, search continues for improved compositions. 

The bath components for a nitrite—nitrate accelerated bath basic to this conversion coating process are (/) 2inc metal or 2inc oxide dissolved in acid (2) phosphate ions added as phosphoric acid (J) addition of an oxidant such as sodium nitrite and (4) addition of nitric acid. Other oxidants such as peroxide, chlorate, chlorate in combination with nitrate, or an organic nitro compound may also be used. 

Chromate conversion coatings are thin, noncrystalline, adherent surface layers of low solubiHty phosphoms and/or chromium compounds produced by the reaction of suitable reagents with the metal surface (2,3). The two classes of chromate coatings are chromium phosphates (green chromates) and chromium chromates (gold chromates). 

Entries on new materials, including re-cyclate plastics, fullerenes, hard-surfaced polymers, dendrimers, transflective materials, rapid prototyping materials, silicone nitride, supercritical fluids, bulk molding compounds, conversion coatings, folic acid, replacements for chloro-fluorocarbons ... 

When the coating compound incorporates some of the metal being treated (as above), it is often referred to as a conversion coating. 

The term conversion coating is used to describe coatings in which the substrate metal provides ions which become part of the protective coating. The coating layers are composed of inorganic compounds that are chemically inert. These inert compounds on the surface reduce both anodic and cathodic areas and delay the transit of reactive species to the base metal. This results in increases in the slopes of anodic and cathodic polarization curves, thereby decreasing the rate of corrosion of the substrate. 

A greenish layer results that consists primarily of hydrated chromium phosphate with hydrated chromium oxide, concentrated toward the metal. Aluminum oxides and other aluminum salts are present at the conversion coating-aluminum substrate interface. Since these coatings do not contain any hexavalent chromium compounds they do not provide self-healing action for defects. The thinner chromic acid-phosphoric acid conversion coatings are an excellent base for paint layers and thicker coatings are often applied unpainted. 

Chromate conversion coatings consist of soluble hexavalent chromium compounds and insoluble trivalent chromium compounds, and in some instances an oxide of the base metal. Most coatings are amorphous gel-like precipitates providing excellent continuity. 

In this case the phosphate film consists of phosphophyllite and hopeite. The solubility and continuity of the conversion coating determines the effectiveness of the barrier action. Phosphate films are nonelectric conductive compounds and are stable in neutral atmospheres, their solubilities being the lowest at pH range 6-8. Since the phosphate films deposit on the cathodic areas and not on anodic sites, their continuity is not as good as those of anodic oxides and chromate films. The anodic sites remain as pinholes. 

Chromate conversion coatings consist of soluble hexavalent chromium compounds and insoluble trivalent chromium compounds forming a film. When exposed to the atmosphere hexavalent chromium slowly leaches from the film, changing the surface appearance from irridescent yellow to either a green color or to clear. Eventually the structure of the film consists of more of the insoluble trivalent chromium compounds. The leached hexavalent chromium provides passivation for any damaged areas on the surface. In a natural atmosphere the film lasts for months or even years. 

Ilevbare GO, Scully JR. Oxygen reduction reaction kinetics on chromate conversion coated Al-Cu, Al-Cu-Mg, and Al-Cu-Mn-Fe intermetallic compounds. J Electrochem... 

Inadequate conversion coating (A coating produced by chemical or electrochemical treatment of a metallic surface that gives a superficial layer containing a compound of the metal),... 

This chapter will consider five techniques (1) electroplating metals, alloys and composites (2) electroless plating of metals, alloys and composites (3)immcfston plating of metals (4) chemical conversion coatings based on metal compounds and (5) electrophoretic painting. 

Phosphate and chromate coatings are examples of conversion coatings. Conversion coatings are so-called because the surface metal is converted into a compound having the desired porosity to act as a good base for a paint. If iron phosphate is used, the following reaction takes place ... 

Conversion coatings refer to the types of coating which on application convert the substrate into a compound with desirable properties. The surface so prepared provides a high degree of adhesion and corrosion resistance. Some important conversion coatings are described below. 

Chromate conversion coatings are formed by a chemical or an electrochemical treatment of metals or metallic coatings in solutions containing hexavalent chromium (Cr " ") and, usually, other components. The process results in the formation of an amorphous protective coating composed of the substrate, complex chromium compounds, and other components of the processing bath. 

Variously designed weight-loss coupon, electrochemical and surface analytical techniques have been utilized in REM-based corrosion inhibitors and conversion coatings research. In particular, electrochemical techniques including EIS and polarization measurements have been widely used to evaluate corrosion inhibition by REM compounds under various environmental conditions. Relatively less attention has been paid to the evaluation of localized corrosion inhibition by REM-based compounds, probably because of methodological difficulties and complexities in making accurate localized corrosion rate measurements. Recently developed techniques such as the scanning probe techniques, electrochemical noise analysis and the wire beam electrode are expected to be useful tools in further REM inhibitor research. 

X. Yang, G. Wang, G. Dong, F. Gong andM. Zhang, Rare earth conversion coating on Mg-8.5 Li alloys , J. Alloys and Compounds, 487,64 (2009). 

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