Immersion coatings

The immersion deposit is necessarily somewhat defective, for the reasons already mentioned, though immersion deposits from complex ions are finer grained and more satisfactory than those reduced from aquocations. The zinc coating is, under the best conditions, an acceptable basis for a copper undercoat from the cuprocyanide bath, on which other coatings can be plated, but there is usually a fair proportion of rejects in commercial operation. Other processes similar in principle use tin or bronze immersion coatings. 

Figure 9.14 Composite image (missing a small part) of a fracture surface of a polysulfone dry-wet phase immersion coating after 4s forced drying, 14s free drying, 16s immersion, and quick withdrawal before cryo-immobilization...

Immersion coating is a classic process for applying zinc onto steel. The object made of steel is briefly immersed into a bath of liquid zinc (boiling point 420 °C) and a zinc deposit forms whose thickness depends on the immersion time. At the Fe-Zn interface, a interdiffusion zone is created that ensures good adhesion. Other metals with low melting points, in particular aluminum, can be deposited in the same fashion. 

Mayne deduced the importance of electrolytic resistance as a protection mechanism from the high rates of water and oxygen transport through coatings. Specifically, Mayne and coworkers [7, 24-27] found that the resistance of immersed coatings could change over time. From their studies, they concluded that at least two processes control the ionic resistance of immersed coatings ... 

Magnalux 304 is a chemical resistant immersion coating comprised of a vinyl ester resin containing mica flakes that reduce permeability and improve edge retention for longer service life. Offering maximum flexibility, the coating is formulated with an extended pot life and can be applied with conventional or airless spray equipment on steel and concrete surfaces. 

Table 1 Parts immersion coated with manganese phosphate for wear resistance...

In electronic applications, the corrosion of the surface leads to reduced performance and device lifetime. " In biomedical applications, corrosion leads to Ni ion release into the body and nickel toxicity. Therefore, forming an organic thin film on nickel oxide may serve as an effective barrier to conosioa This is in large part due to the flexibihty that SAMs offer they form stracturally well-defined films on the solid surface and they can be deposited by a nnmber of easy techniques such as immersing the substrate in solution, aerosol spraying and vapour deposition. Currently, immersion coating is the most popular and widely studied method for monolayer formation. " ... 

As an example of SAMs, eorrosion inhibition by self-assembled films formed by adipic acid (AA) molecules on caibon steel surfaces is discussed below. SAMs of AA were formed on iron oxide/caibon steel surfaces by the immersion coating method. The metal was immersed in an aqueous solution containing 60 ppm of Cl (to initiate the corrosion process and the formation of iron oxide) in the absence and presenee of adipic acid. The formation, uniformity, ordering and bonding of the monolayers accompUshed by the immersion method have been evaluated by FTIR and AFM. The electrochemical properties of the unmodified and modified caibon steel surfaces were characterized by polarization study and EIS analysis to test the abiUty of the monolayer to reduce the corrosion of the surface. 

With immersion coating, the three dimensional part (or web) to be coated is completely submerged into a low viscosity coating or etchant, as can be seen in Figure 9.5. 

There are lead finishing problems associated with the Sn-Cu system similar to Sn-Ag solution instability, composition control issues, and immersion coatings. 

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