Coatings chromate filming

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. 

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. 

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. 

Nowadays, the use of the reflection electron microscope (REM) or, recently, the tunnel electron microscope, as well as secondary ion mass spectrometry (SIMS), AES, electron-dispersive X-ray spectrometry, impedance spectroscopy, and so on, are yielding substantial increases in the knowledge of corrosion reactions in coatings and at their interface with metal or other substrates. As far as zinc or zinc-coated surfaces are concerned, problems of interfacial and intercoat adhesion, differential diffusion phenomena and electrolytic cell behavior on the substrate, and interreactions of zinc with conversion coatings (chromates, phosphates, silanes, silanols, etc.) have been analyzed, leading toward spectacular improvements in, for example, paint adhesion, absorption of conversion coatings and, in general, the protective action inside films as well as on their substrates. 

Accelerated tests such as ASTM B 201 are very useful in evaluating the quality of chromate films on zinc or zinc-coated surfaces. The test proceeds until white corrosion products have formed and should not be allowed to continue until red rust forms. Only the time to formation of "white rust is relevant. The effects of impurities on the corrosion rate of zinc can be determined by humidity testing. 

The inhibiting effect of the soluble hexavalent compounds in the film provides another protective action. Chromate films commonly exhibit an iridescent color. When a treated metal is exposed in a natural or salt spray atmosphere the color of the surface changes from yellow to green, or becomes clear. This indicates the reduction of soluble hexavalent chromium to insoluble trivalent chromium. When in contact with moisture the hexavalent chromium slowly leaches, providing a self-healing action for defects in the coatings. The leached hexavalent chromium acts as an anodic inhibitor. Dissolved hexavalent chromiums are adsorbed by the defects and form passive films that are composed of mixtures of insoluble trivalent chromium compounds and oxides of the base metal. 

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. 

A chromate film is normally very thin and exhibits poor resistance to abrasion. Phosphate conversion coatings do not provide adequate corrosion protection. The phosphate-chromate coatings were developed to improve the quality of both chromate and phosphate coatings. Combining the two different conversion coatings increases corrosion and abrasion resistance. 

Concerning the Z modulus curves, at high frequencies, chromated and ppHMDSO coatings display the same characteristics. At low frequencies, slightly better corrosion resistance is observed for the chromated film (300 kO) than the polymer one (100 kO). On the other hand, the resistance of the untreated substrate was lower by one order of magnitude. Therefore the results obtained from Tafel plots and impedance spectra are consistent. 

The high corrosion resistance offered by chromate films is attributed to the presence of both hexavalent and trivalent chromium in the coating. Analyses of coatings by wet chemical methods and with surface-sensitive techniques have shown that both hexavalent chromium, Cr or CifVl), and trivalent chromium, Cr or CifllO. are present in the films. The trivalent chromium is believed to be present as an insoluble hydrated oxide, whereas the hexavalent chromium imparts a self-healing character to the film during oxidative (corrosive) attack by species such as chloride... 

Filiform corrosion is characterised by the formation of a network of threadlike filaments of corrosion products on the surface of a metal coated with a transparent lacquer or a paint him, as a result of exposure to a humid atmosphere. This phenomenon first attracted attention because of its formation on lacquered steel, and for this reason it is sometimes referred to as underfilm corrosion, but although it is most readily observed under a transparent lacquer it can also occur under an opaque paint film or on a bare metal surface. Filiform corrosion has been observed on steel, zinc, magnesium and aluminium coated with lacquers and paints, and with aluminium foil coated with paper. Surface treatment of the metal by phosphating or chromating lessens the tendency for filiform corrosion to occur, but it is not completely... 

Acid treatments The principal acid processes were developed in the USA under the name Alodine, and are marketed in the UK as Alocrom and under other names. The original solutions were based on acid solutions containing phosphate, chromate and fluoride ions. Immersion for up to 5 min in the cold or warm solution leads to the deposition of a greenish film containing the phosphates of chromium and aluminium, and possibly some hexavalent chromate. The more recent Alocrom 1 200 process uses an acid solution containing chromate, fluoride and nitrate. Room-temperature immersion for 15 s to 3 min deposits golden-brown coatings which contain chromate as a major constituent. 

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