Coatings conversion

The presence of a suitable passive film on a metal can confer a high degree of corrosion resistance (Chapter 10), increased electrical insulation, an aesthetically pleasing surface finish, enhanced ability to key a subsequent paint film or the facility to absorb a dye or an oil. 

A number of electrochemical treatments are used widely to produce inorganic conversion coatings on a metal, the surface layers including oxides, chromates, phosphates and their mixtures. Depending upon the process, these treatments may be carried out under open circuit or they may be driven by an impressed current. 

Anodizing is the process of forming a surface oxide film by electrochemical oxidation. It is used as a surface-finishing technique particularly for aluminium 

For these applications, aluminium is anodized in an acid electrolyte, usually 10% sulphuric acid although chromic acid and sulphuric acid/oxalic acid mixtures are also employed. Oxalic acid at low current density gives excellent results and is used for aluminium which is to be coloured. Chromic acid is preferred for the anodizing of complex shapes where thorough rinsing of the surface after oxidation is a problem aluminium will corrode in sulphuric acid but not in chromic acid which passivates the surface. 

At the inert cathode, hydrogen evolution is the main reaction  

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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 appHcation of this type of conversion coating can be by spray or immersion and is easily tailored to the needs of the user (see Coating processes). The number of stages may vary from two, ie, clean and phosphate then rinse, to as many as five, ie, clean, rinse, phosphate, rinse, and post-treat. The performance requirements dictate the number of operation stages as well as the need for a post-treatment. 

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. 

Testing of Painted Products. The enhancement of paint adhesion is one of the principal functions of conversion coating (20—22). A group of tests based on product deformation is used to test the painted product. The appHance and cod-coating industries use the mandrel bend, the cross-hatch adhesion test, and the direct and reverse impact tests. Adhesion after a water soak is judged using a cross-hatch test performed on the exposed surface. 

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). 

Chromium Phosphate. Chromium phosphate treatment baths are strongly acidic and comprise sources of hexavalent chromium, phosphate, and fluoride ions. Conversion coating on aluminum precedes by the foUowing reactions (24) ... 

Operation and Control. Control of a chromium phosphate conversion coating bath requires monitoring chromium and aluminum concentrations, active fluoride level, and temperature. Coating weight is very sensitive to active, ie, uncomplexed, fluoride. An innovative electrochemical method using a siHcon electrode (25) is employed for measuring active fluoride. A special precaution in chromium phosphate bath operation is the... 

Product Utilization. The principal appHcation for chromium phosphate coatings is as a paint base for painted aluminum extmsions and aluminum beverage can stock. In these appHcations, extremely demanding performance criteria are met by the chromium phosphate conversion coatings. As an example, the Architectural Aluminum Manufacturer s Association Voluntary Specification 605.2-92 requires humidity and salt spray testing for 3000 hours and allows only minimal incidence of paint failure after testing (26). 

Chromium Chromate. Chromium chromate treatment baths are acidic and made up from sources of hexavalent chromium and complex fluoride, fluorosiHcate, fluorozirconate, fluorotitanate, and siHcofluorides. Optional additional components added to accelerate coating rate are free fluoride, ferricyanide, and other metal salts such as barium nitrate. Conversion coating on aluminum precedes by the following reactions (2,3,17) ... 

Operation nd Control. Control of chromium chromate conversion coating baths is accompHshed by controlling chromium concentration and pH. The quaHty of the conversion coating is sensitive to aluminum accumulations in the coating bath as well as to rinse water purity. Sulfate contamination is a particular problem. 

Finishes for aluminum products can be both decorative and useful. Processes in use include anodic oxidation, chemical conversion coating, electrochemical graining, electroplating (qv), thin film deposition, porcelain enameling, and painting. Some alloys respond better than others to such treatments. 

Porcelain enameling requires the use of frits and melting temperatures of 550 °C or below. Enamels are appHed over chemical conversion coatings that are compatible with the frit. AHoy selection is important to obtain good spall resistance. Alloys 1100, 3003, and 6061 are employed most extensively among wrought products and alloy 356 for castings. 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

In Section 4.3 several recently developed and experimental surface preparation methods for steel such as conversion coatings, plasma spray and sol-gel that attempt to simultaneously improve durability and bond strength over grit blasting will be discussed. 

Recent efforts to improve the performance of conversion coatings have led... 

Fig. 31. Electron micrographs that compare crystal size of (top) a grain-refined microcrystalline coating and (bottom) a conventional zinc phosphate conversion coating [54].

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