Chromate conversion coating process

The chromate conversion coating process is aided by fluoride, which prevents rapid passivation of the A1 surface, thus allowing Cr " to Cr " reduction and is also aided by ferricyanide, which functions as a mediator between A1 oxidation and chromate reduction and accelerates the redox reaction. ... 

There are essentially three main steps in a conversion coating process cleaning, conversion coating, and post-treating. These three different, but equally important, steps in the pretreatment of metal articles will be discussed in more detail for the purpose of providing a background for the main emphasis of this paper, the post-treatment part of the conversion coating process, and more specifically chromium-free polymeric post-treatments which have been developed in recent years to replace the environmentally unacceptable chromate systems. 

Figure 28.7 Schematic of corrosion process on chromated primer coated on chromate conversion coated A1 alloys surfaces.

A nonchromated, water-borne primer applied to [2B] alloy samples, with the appropriate surface preparation and plasma deposition of an ultrathin plasma polymer, was also compared to controls prepared by depositing a chromated primer on chromate conversion-coated A1 substrate. The same comparison was also performed for IVD Al-coated 2024-T6 substrates (pure aluminum is deposited by ion vapor deposition process on aluminum alloy 2024-T6). In the latter case, the primer could not be removed from the IVD Al-coated panels that were treated with the plasma polymer prior to spray primer application. It is interpreted that the water-borne spray paint penetrates into the column structure of the top surface of the IVD Al-coated substrates when the surface energy was modified by the application of a plasma polymer. This effect could be viewed as interactive coating with a porous surface. 

The largest cost that can be eliminated is the chromate conversion coating itself and the cost for treatment of spent solution and rinse water. In comparison to these costs, the cost for trimethylsilane (LCVD gas) to be used in the closed system LCVD mode is almost negligible. Therefore, the addition of LCVD process is economically favored, if one considers the cost for overall processing for corrosion protection of IVD-processed metallic objects. 

Oxide and chromate conversion coatings (used primarily for zinc, aluminum, tin, and magnesium) improve paint adhesion relative to that which is observed on untreated metal. These coatings are applied by a variety of proprietary processes (9, 57. 58). 

Chromate conversion coating can be defined as the process where works are surface finished in baths containing hexavalent chromium [9]. Zinc plated steels, zinc, magnesium, aluminum, etc. in a chromate bath dissolve into the solution as ions and react with hexavalent chromium to form trivalent chromium. When the case of zinc plated steel is taken as an example, zinc on the surface dissolves as follows. 

The surface finishing process where hexavalent chromium ion would be issued (used) is mostly for chromate conversion treatment. Chromate is a chemical conversion coating process. This means that the surface finishing is by chemical reactions between chemical agents and materials. The chromate conversion treatment uses a chromate bath composed of hexavalent chromium ions. Currently trivalent chromate and some topcoats are used. 

Where the corrosion resistance of a coating depends upon its passivity, it is common to follow plating with a conversion coating process to strengthen the passive film. Zinc, cadmium and tin in particular are treated with chromate solutions which thicken their protective oxides and also incorporate in it complex chromates (see Section 1S.3). There are many proprietary processes, especially for zinc and cadmium. Simple immersion processes are used for all three coatings, while electrolytic passivation is us on tinplate lines. Chromate immersion processes are known to benefit copper, brass and silver electrodeposits, and electrolytic chromate treatments improve the performance of nickel and chromium coatings, but they are not used to the extent common for the three first named. 

Chromate conversion coatings are produced by chemical treatments which— as the name indicates—convert the zinc surface to form a complex surface layer usually O.S-3 p,m thick, containing chromium hydroxide, zinc hydroxy-chromate, and zinc chromate. Films range from thin with a clear and bright appearance to thicker, yellow-iridescent, brown, or drab appearances. Appearance varies with bath formulation, process parameters, film thickness, and substrates. Corrosion resistance increases mainly with thickness. Film characteristics are given in Tables 1.6A and B. 

The chromate pretreatment layer, which is also called the chromate conversion coating (CCC) varies in thickness depending on the chemistry of the process and the application method used. The CCC layer is, however, usually not thicker than a few microns, which in coating weight is somewhere between 5 and 25 mg/m, expressed as Cr [19], This CCC layer improves the adhesion between the metal and the primer, it aids in the protection of scratches and defects and it also protects cut edges of the metal to some extent [20]. The hexavalent chromate in the CCC layer is known for its low solubility and the self-healing effect, which means lliat it only leaches out on demand when the base metal has been scratched [21]. 

The barrier action of a chromate coating increases with its thickness. Chromate conversion coatings can be used as a base for paint or alone for corrosion protection. Previously it was described how the leached hexavalent chromium acts as an anodic inhibitor, by forming passive films over defects in the coating. Since the films formed on aluminum by the chromic acid-phosphoric acid process contain no hexavalent chromium, they do not provide self-healing from defects. 

There are two types of processes whereby chromate conversion coatings may be produced chromic acid processes (just discussed) and chromic acid-phosphoric acid processes. The overall reaction for the formation of a phosphoric acid process coating is... 

One of the better surface treatments for copper, utilizing a commercial product named Ebonol C (Enthane, Inc. New Haven, CT), does not remove the oxide layer but creates a deeper and stronger oxide formation. This process, called black oxide, is commonly used when bonding requires elevated temperatures for example, laminating copper foil. Chromate conversion coatings are also nsed for high strength copper joints. 

Conversion coating processes produce a thin film of predominantly chromium oxide on metal surfaces. The colour of this film depends on the substrate metal, and may vary in colour, from pale-yellow to gold to dark-brown or black. Today, the most commonly used CCC process for aluminium, zinc and cadmium (Biestek and Weber 1976) is an acid treatment (pH 1—2), based on a two-part solution containing a source of hexavalent chromium ion, e.g. chromate, dichromate or chromic acid. The solution for treating aluminium alloys, generally contains fluoride ion, which assists in the dissolution of the original oxide film, and an accelerator, e.g. ferricyanide, to facilitate the formation of the chromium oxide (Biestek and Weber 1976). 

The results obtained by both Mansfeld et al. and Hinton et al. are encouraging and suggest that a conversion coating based on a Ce oxide could be an attractive alternative to the chromate conversion coatings widely used to protect aluminium and zinc. However, as a practical commercial process, treatment times would need to be of the order of minutes, and the corrosion protection performance would need to be similar to that provided by the chromate conversion coating. 

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. 

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