Chromic acid conversion coatings

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 for aluminum are carried out in acidic solutions. These solutions usually contain one chromium salt, such as sodium chromate or chromic acid and a strong oxidizing agent such as hydrofluoric acid or nitric acid. The final film usually contains both products and reactants and water of hydration. Chromate films are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of accelerators such as cyanides, acetates, formates, sulfates, chlorides, fluorides, nitrates, phosphates, and sulfamates. 

Hexavalent chromium-bearing wastewaters are produced in the metal finishing industry in chromium electroplating, in chromate conversion coatings, in etching with chromic acid, and in metal finishing operations carried out on chromium as a basis material. 

In order to obtain maximum corrosion protection for painted metal articles, the metal parts are pretreated with an inorganic conversion coating prior to the painting operation. These zinc or iron phosphate coatings greatly increase both paint adhesion and corrosion protection. Traditionally, a chromic acid post-treatment has been applied to these phosphatized metal surfaces to further enhance corrosion protection. 

Zeolite coatings have been investigated as corrosion-resistant coatings for aluminum alloys in aerospace applications. The currently used chromic acid anodization and chromate conversion coatings are effective but release hexavalent chromium, which is... 

Conversion coatings are formed by a reaction of the metal on the surface of the substrate with a solution. For example, chromate coatings are formed by the reaction of water solutions of chromic acid or chromium... 

The chromic acid-based conversion coatings and resistant barriers for decorative applications are formed according to Eq. (13.7) ... 

Chemical Acid etching, e.g. chromic, nitric or mixed acids Alkaline etching, usually proprietary solutions based on sodium hydroxide. Conversion coatings... 

Aqueous deposition methods are used to apply conversion coatings for decorative and protective reasons. These conversion coatings are formed by immersing the substrate metal in an aqueous solution of chromic acid, chromate or dichromate salts. For the deposition to occur activating ions such as sulfate, nitrate, chloride, or fluoride must be added. Then as hydrogen is generated by the attack of the activating ions some of the chromium ions are reduced to form a hydrated chromium chromate which is deposited on the substrate surface. 

Chrome baths always contain a source of hexavalent chromium ion (e.g., chromate, dichromate, or chromic acid) and an acid to produce a low pH which usually is in the range of 0-3. A source of fluoride ions is also usually present. These fluoride ions will attack the original (natural) aluminum oxide film, exposing the base metal substrate to the bath solution. Fluoride also prevents the aluminum ions (which are released by the dissolution of the oxide layer) from precipitating by forming complex ions. The fluoride concenfration is critical. If the concentration is too low, a conversion layer will not form because of the failure of the fluoride to attack the natural oxide layer, while too high a concentfa-tion results in poor adherence of the coating due to reaction of the fluoride with the aluminum metal substrate. 

There are two types of processes by which conversion coatings can be produced chromic acid processes and chromic acid-phosphoric acid processes. In the formation of the chromic acid based conversion coating the following overall equation governs ... 

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. 

While it is possible to bond to a freshly abraded or cleaned metal surface, chemical treatments are preferred for rendering the metal surface inactive to corrosion over time. For low carbon steel, phosphatising is the recommended pre-bond surface preparation treatment. Stainless steel should be passivated or acid etched, while titanium is usually treated with a hydrofluoric acid pickle. Almninium or magnesium are best treated with a chromate conversion coating. Zinc and cadmium are generally prepared mechanically but a phosphate or chromic acid treatment may be used. Brass and copper may be treated with an ammonium persulphate etch or an acid-ferric chloride etch. 

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). 

Chromate conversion solution consists of chromic acid H2Cr04 or H2Cr207, chromate salts and certain activator ions such as sulfates, chlorides, fluorides, phosphates and complex cyanides with pH around 1 2. As the solutions for chromate conversion treatment are acidic which cause the dissolution of Mg into the solutions as Mg, there is a local rise in pH in the immediate vicinity of the metal-solution interface. Mg ions combine with chromate ions to form a compound that is insoluble at the locally higher pH region. This compound precipitates on the metal surface as an adherent coating. Chromate conversion treatment is a very fast process (30-60 s) and can... 

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