Conversion coating processes

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. 

The coil coating sequence, regardless of basis material or conversion coating process used, consists of three operational steps ... 

The basic objective of the conversion coating process is to provide a corrosion-resistant film that is integrally bonded chemically and physically to the base metal and that provides a smooth and chemically inert surface for subsequent application of a variety of paint films. The conversion coating processes effectively render the surface of the basis material electrically neutral and immune to galvanic corrosion. Conversion coating on basis material coils does not involve the use of applied electric current to coat the basis material. The coating mechanisms are chemical reactions that occur between solution and basis material.1-4... 

A large segment of the metal parts produced by industry are painted for both decorative purposes as well as to increase the corrosion resistance and extend the useful life of the product. To obtain maximum quality from painted metal articles, it is of paramount importance to pretreat the metal parts with a conversion coating process. ( 1,2) Pretreatment processes contribute a significant improvement in corrosion protection and durability to metal articles by ... 

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. 

This more involved multi-step process can be used for both iron phosphate and zinc phosphate conversion coating processes. 

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. 

The range of structural and chemical properties provided by the many conversion coating processes is reflected in the functionality of conversion coated surfaces. In broad terms, conversion coatings offer surface passivation increased adhesion to paints or structural materials and improved tribological performance. Commonly found substrate-coating combinations are given in Table 1. ... 

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 initial treatment used in Europe was a chromium conversion coating process using a proprietary product called Iridite. This was a two-component system which contained a blend of highly toxic materials such as chromium salts and ferro- and ferricyanides. 

Song and Mansfeld further developed a new conversion coating process for EG steel based on a Ce(N03)j solution with some additives that reduced the total treatment time to 70 seconds, which would make this process more attractive for industrial applications. The corrosion current densities of untreated and treated... 

The first conversion coating process using rare earths was described in a provisional patent application in 1988 by Hinton and Wilson. " This application described a... 

For conversion coating processes, Wang et demonstrated that a resistant coating could be developed on stainless steel using a sulphuric acid-based process with added permanganate and cerium nitrate where the permanganate oxidized the Ce to Ce. The reaction proceeds via ... 

The first chemical conversion coating process was developed by O. Bauer and O. Vogel in 1915. It consists of treating aluminium in a solution of potassium carbonate, sodium bicarbonate and potassium dichromate for 2 h at 90 °C. A dark grey layer is formed by this process. 

After treatment, the black ferric-phosphate coating can be scrubbed off, leaving a fresh metal surface. Multiple applications of phosphoric acid may be required to remove all rust. The black phosphate coating can also be left in place, where it will provide moderate further corrosion resistance. (Such protection is also provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes.)... 

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