Sacrificial Protection Explained
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.
Corrosion Rates
The corrosion protection offered by zinc deposits is dependent upon three factors: coating thickness, post-plate treatments, and environmental exposure. Zinc plating (without chromates or passivates) 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 ASTM G87 Moist SO2 Test (Kesternich Test) and the ASTM B117 Salt Fog (Salt Spray) Test. In this latter test (which is much more common), a fog is generated from a 5 percent 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.
| Atmosphere |
Mean Corrosion Rate |
| Industrial |
5.6 Microns (0.22 mils) per Year |
| Urban Nonindustrial |
1.5 Microns (0.06 mils) per Year |
| Suburban |
1.3 Microns (0.05 mils) per Year |
| Rural |
0.8 Microns (0.03 mils) per Year |
| Indoors |
Considerably Less Than 0.5 Microns (0.01 mil) per Year |
(Source: ASTM B695)
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How Long Do Mechanically Plated Deposits Last in the Salt Spray Test?
| Zinc Thickness |
Salt Spray Protection |
| 0.00012" (3 Microns) |
24 Hours |
| 0.00024" (6 Microns) |
48 Hours |
| 0.00035" (9 Microns) |
72 Hours |
| 0.00047" (12 Microns) |
96 Hours |
| 0.00059" (15 Microns) |
120 Hours |
| 0.00098" (25 Microns) |
192 Hours |
| 0.00157" (40 Microns) |
250 Hours |
| 0.00197" (50 Microns) |
300 Hours |
Sources: American Society for Testing and Materials (ASTM B695)
Post-Treatment Coatings
In general, post-treatments will add corrosion protection as follows:
| Clear Chromates (Hex) |
12-24 Hours |
| Trivalent Passivates |
12-96 Hours |
| Yellow Chromates |
96-168 Hours |
| Olive Drab Chromates |
96-192 Hours |
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No Rinse Hyperguard
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96-192 Hours
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Hyperguard™ 326™
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240+ Hours
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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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Increase Your Protection with Sealants
Proprietary sealants (such as PS&T's Hyperseal®) can be applied over chromates and passivates to significantly enhance the protection of the chromate or passivate. The additional protection offered by such products is typically 100 to 200 additional hours, although exceptionally lengthy results have been reported.
The best and longest results are achieved over hexavalent chromates or those trivalent passivates that generate the most hexavalent chromium (e.g. Hyperguard™ 326™).
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How Hexavalent Chromates and Trivalent Passivates Work
Chromates protect the underlying deposit by delaying the onset of the white corrosion products. The chromating processing is technically called a "conversion 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 inhibitors and the process binds it tightly to the surface of the zinc. It is important to recognize that the application of the chromate conversion coating is the most cost effective corrosion protection one can buy. Trivalent passivates, as reported in Plating and Surface Finishing in October 2007, function by generating hexavalent chromium during the corrosion process. They are applied in the same process of dipping the parts, but there is no conversion at the topmost part of the zinc coating. In essence, it is just another layer over the top of the zinc coating. However, the thicker this layer is, the more hexavalent chromium will be produced and thus the more corrosion protection the underlying deposit will have. Some common thin build trivalent passivates are: Tridescent™ (Luster-On Products), Chromiting™ (Sur-Tech International) or HyproBlue™ (Pavco). These will add about 100 hours of corrosion protection whereas the Hyperguard 326 (a thick build passivate) can add over 240 hours.
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