Filming, chromate

The films contain soluble chromates that act as corrosion inhibitors that provide a modest improvement in corrosion resistance. As with phosphatizing, the film produced serves as an excellent coating base. Indeed, it can be difficult to make most polymeric coatings adhere to the nonferrous alloys without such a treatment. Epoxy primer, for example, which does not adhere well to bare aluminum, adheres very well to chemical conversion coatings. Recent concerns with the toxicity of the hexavalent chromium used in chromating have, however, limited its usage. 

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Chromating Chromating is considered in Section IS.3. The chromate film on zinc is adherent and can be drab, yellow-green or colourless in appearance the colour varies considerably with the method of application. It retards white rust , the white deposit which sometimes forms on fresh zinc surfaces which are kept under humid conditions (see Section 4.7). A chromate film is damaged by heat and if used as a basis for paint adhesion, should preferably not be heated above 10°C, nor for longer than 1 h. 

Chromate treatments can be applied to a wide range of industrial metals. They are of two broad types (a) those which are complete in themselves and deposit substantial chromate films on the bare metal and (b) those which are used to seal or supplement protective coatings of other types, e.g. oxide and phosphate coatings. Types of treatment for various metals are summarised in Table 15.16. 

In order that a chromate film may be deposited, the passivity which develops in a solution of chromate anions alone must be broken down in solution in a controlled way. This is achieved by adding other anions, e.g. sulphate, nitrate, chloride, fluoride, as activators which attack the metal, or by electrolysis. When attack occurs, some metal is dissolved, the resulting hydrogen reduces some of the chromate ion, and a slightly soluble golden-brown or black chromium chromate (CtjOs CrOs xHjO) is formed. 

The Stability of the natural oxide film reinforced by the chromate ion determines the conditions of pH, ratio of activating anion to chromate, and temperature at which the oxide is broken down and a chromate film deposited. Thus magnesium alloys can be chromate-treated in nearly neutral solutions, whereas aluminium alloys can be treated only in solutions of appreciable acidity or alkalinity. 

The same principle tends to apply to the protective efficiency of the chromate film, i.e. the greater the intrinsic corrosion resistance of the metal, the greater the protection conferred by the soluble chromate in the chromate film. 

Several immersion treatments using solutions containing chromates have been developed for aluminium. It is not always clear to what extent the films formed can properly be called chromate films, i.e. films containing a substantial amount of a slightly soluble chromium chromate, but even if the film consists largely of aluminium oxide or hydroxide or other salt with chromate physically absorbed, it will still provide a reservoir of soluble chromate at the metal surface. Treatments fall into two classes alkaline and acid. The latter are of more recent development. 

The chromate film deposited by the Cronak process on zinc consists largely of a hydrated chromium chromate and contains some 10% by weight... 

Copper and its alloys can be cleaned and brightened by immersion in solutions of substantial quantities of dichromate with a little acid (see, for instance method Q of DEF STD 03-2/1). Such solutions impart some resistance to tarnishing, ascribed to the formation of very thin chromate films. 

In spite of the effectiveness of chromates in stopping the rusting of steel in aqueous solutions, no successful chromate filming process has been developed for this purpose. 

Medium acid baths, pH 4-5 At this acidity a dichromate solution plus sulphate ion as activator is sufficient to deposit chromate films in 30 min or so at room temperature or in a few minutes at boiling point. Unfortunately, a solution of alkali dichromate and alkali sulphate is quite unbuffered, and other substances must be added to give the bath a useful life over the working pH range. Acetates have been used successfully, but salts of aluminium, chromium, manganese and zinc have been more commonly employed. The pH of the solution rises slowly during use until basic chromates or sulphates begin to precipitate. The solution can then be rejuvenated by the addition of chromic or sulphuric acid or acid salts. 

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. 

Although replacing the contacts with silver-free connectors was the long-range solution in submarines and in all other circuit board applications, some short-term preventative measures were desirable in the effort to avoid expensive retrofits. A modified chromate coating process was found to extend useful life of the connectors. Because the nature of the chromate film growth is influenced by the electrochemical behavior of the contacts and impurities, the exact nature of the films formed on real and model contacts was examined as well as effects of impurities. 

The specific goals of the experiment were to characterize the passive film and determine if it was primarily protective with chromium in the +3 state or if some chromium was in the +6 state indicating the possibility of a self-healing film. The effects of various impurities on chromate film growth were also examined. 

Consideration of Surface Analysis Concerns. The researchers in this study used a wide range of surface and other tools, taking appropriate advantage of the strengths of the various methods. Previous work had shown that AES and XPS could be used to study chromate films without unreasonable problems and provided a basis for the current study. XPS was used to obtain specific chemical information while AES was used whenever spatial resolution and electron imaging were desired. RBS and electron microprobe work was used to analyze composition structures of thicker layers. 

Experimental. Much of the XPS and AES work was done on coupons made of a BeCu alloy which had been electroplated with Ag or Au or both. The chromate films were grown in a manner consistent with specifications for refurbishing contacts on submarine navigational computers. 

Results. XPS spectra of the chromate films on the bimetallic strips, on pure silver and on pure gold showed no indication of a Cr+6 state (Fig. 11) suggesting that flaws in the chromate film would not heal and therefore would be susceptible to corrosive attack. This non-healing aspect of the chromate films on the Ag/Au contacts differs from the effect of chromate films on A1 (51) and was confirmed in field studies. Therefore, the chromate films formed on the Au/Ag/Be-Cu contact are protective, but will not heal over scrapes and other damage caused by usage. 

Report NAEC-AML 2065, Effect of Hexava-lent Chromium Content on Performance of Chromate Films, Aeronautical Materials Lab, Naval Engineering Center, Philadelphia, Pa., 1964. 

The layers have a more or less blue color depending on the thickness of the film. The thickness of the layer should be 50 to 90 nm, however exact control of the layer thickness is difficult. The corrosion protection of a blue chromating film is very limited. 

Formation of chromium films containing only chromium(lll) A first example was the blue chromating films without chromium(VI). But these films are too thin and reveal only limited corrosion protection. 

Recent publications report on chromium films containing only chromium(III)oxi-des and with thicknesses similar to the yellow chromating films [72, 73]. The thicker film is formed if a bath contains a more efficient catalyst like silicofluoro anions at elevated temperatures (80-90 C). The film is not self-healing. The process is called chromiting. The high process temperature and the lack of self-healing properties of the film demand further... 

Olive chromating These films are thicker than the yellow chromated films and contain even more chromium (VI). They do not show much better corrosion-protective properties than the yellow film. They are used for military apphcations and where the special color is demanded. 

In practice, the chromate film is not the simple chromium hydroxide, but gel-like composite hydroxide film composed of chromium acid, sulfate, chloride, fluoride. 

Magnesium is a relatively reactive metal, and can be chromated in nearly neutral solutions as well as in acid solutions. The range of treatments possible illustrates well the rdle of pH, activating anion, temperature and duration of treatment in promoting the breakdown of passivity in the chromate solution and the consequent formation of a chromate film. 

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