Conversion coatings coating effectiveness

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... 

Chromate conversion coatings are effective treatments for magnesium and are applied before the adhesive bonding or the paint, but the chemistry used is harmful and restricted by regulations. The alternatives to the chromate treatment and the treatment process for magnesium are discussed below in further detail (Skar and Albright 2002). 

As with chemical etches, developing optimum conversion coatings requires assessment of the microstructure of the steel. Correlations have been found between the microstructure of the substrate material and the nature of the phosphate films formed. Aloru et al. demonstrated that the type of phosphate crystal formed varies with the orientation of the underlying steel crystal lattice [154]. Fig. 32 illustrates the different phosphate crystal morphologies that formed on two heat-treated surfaces. The fine flake structure formed on the tempered martensite surface promotes adhesion more effectively than the knobby protrusions formed on the cold-rolled steel. 

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... 

With regards to the conversion coated substrate, Wittel (25) observed that at temperatures greater than 140°C, tetrahydrate zinc phosphates lose part of their water of hydration. It is likely that the water of hydration liberated in the phosphate recrystallization process has a negative effect on the adhesion of the polymer matrix to the B40 panel. 

Table 3 - Effect of chemical conversion coatings on corrosion potentials of 7075-T6 A1 alloy.

Figure 1. Effect of 5% NaCl/S02 fog exposure on corrosion resis properties of chemical conversion coatings on 7075-T6 aluminum a...

FIGURE 16.2 Effect of surface treatment on the durability of epoxyaluminum joints exposed to room temperature water immersion. (1) Anodized, (2) grit-blasted plus vapor degrease, (3) vapor degrease, (4) chromate conversion coating.18... 

In view of its practical importance, it is surprising that there has been relatively little detailed study of this form of transfer, even to the extent of defining the rate of formation of a transfer film, or the effects of such factors as counterface material, hardness and surface finish. The general design assumptions tend to be based on the requirements for transfer from composites, namely a surface roughness of 0.2 fjm C.L.A., and the possible use of a soft plating or a chemical conversion coating on the counterface. 

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 toxic effects of arsenic can be counteracted with (1) saline purgatives, (2) various demulcents that coat irritated gastrointestinal mucous membranes, (3) sodium thiosulfate, and (4) mono- and dithiol-containing compounds and 2,3-dimercaptopropanol. Arsenic uptake in rabbit intestine is inhibited by phosphate, casein, and various metal-chelating agents. Mice and rabbits are significantly protected against sodium arsenite intoxication by A-(2,3-dimercaptopropyl)phthalamidic acid. Conversely, the toxic effects of arsenite are potentiated by excess dithiols, cadmium, and lead, as evidenced by reduced food efficiency and disrupted blood chemistry in rodents. 

Conversion Coatings Conversion layers have been described in detail in chapter 5.3. In this section, the effects of the pretreatment on the kinetics of the coating delamination are discussed with focus on the electrochemical principles. 

Conversion coatings. Some metals can be treated by oxidizing agents, preferentially chromate solutions, to prepare an oxide film saturated by chromate ions. This is one of the most effective corrosion preventing treatments of metal surfaces and is applied to aluminum, copper, and zinc. Chromate ions are known for toxic properties. Pohtical decisions have banned this surface treatment for the future. So far no alternative corrosion protecting treatment of similar effectiveness has been developed. 

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