The primary purpose of mechanical plating processes is to enhance the corrosion protection of the parts plated.
Zinc (and other active metals such as cadmium and aluminum) protect the underlying ferrous substrates by a process called sacrificial protection.
This is sometimes called cathodic protection.
In this method of corrosion protection, the metals that are more active (chemically speaking, with more negative potentials or higher in the electromotive series), such as zinc, protect those that are less active or more noble by "sacrificing" themselves to protect the underlying more noble base metal. This process works effectively even if the sacrificial metal coating is slightly damaged.
The corrosion protection offered by zinc deposits is dependent upon three factors:
n Coating thickness
n Post-plate treatment(s)
n Environmental exposure
Zinc plating (without chromates) corrodes at rates which are dependent upon the severity of the environment, as shown in the summary presented below. There is a significant body of data on local corrosion rates currently available.
Because the corrosion protection offered by sacrificial deposits is so lengthy, accelerated tests are routinely used to predict the long-term effectiveness of the deposits. The two most common tests used are the Kesternich Test and the ASTM B-117 Salt Fog (Salt Spray) Test. In this latter test (which is much more
common) a fog is generated from a 5% neutral (i.e., a pH of 7) salt (sodium chloride) solution. The parts are then evaluated for the first appearance of white corrosion products ("white rust" or oxides of zinc) and (later in the test) the formation of red rust, or base metal corrosion.
How Long Do Mechanically Plated Deposits Last In The Salt Spray Test ?
In general, chromate conversion
protection as follows:
Clear Chromates (Hex) 12 - 24 hours
Yellow Chromates 96 - 168 hours
Olive Drab Chromates 96 - 192 hours
No Rinse Hyperguard 96 - 192 hours
Proprietary sealants (such as PS&T Brand Hyperseal) can be applied over chromates to significantly enhance the protection of the chromates. The additional protection offered by such products is typically 100 to 200 additional hours, although exceptionally lengthy results have been reported.
Chromates protect the underlying deposit by delaying the onset of the white corrosion products. The chromating process is technically called a "conversion coating process"; in this process the parts are dipped into a solution containing chromates and the solution "converts" the topmost part of the zinc into zinc chromate; this zinc chromate is a highly effective corrosion inhibitor and the process binds it tightly to the surface of the zinc. It is important to recognize that the application of chromate conversion coatings is the most cost effective corrosion protection one can buy.
Mechanical plating and mechanical galvanizing offer a proven means of protecting ferrous substrates from environmental corrosion. The nature of the deposit can be adjusted to provide protection from even the most severe environmental conditions.
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