Corrosion Protection

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.

                   
  Atmosphere

  Industrial
  Urban Nonindustrial
  Suburban
  Rural
  Indoors 
                   
 (Source: ASTM B695-85)

Mean Corrosion Rate

5.6 micrometers (0.22 mils) per year
1.5 micrometers (0.06 mils) per year
1.3 micrometers (0.05 mils) per year
0.8 micrometers (0.03 mils) per year
considerably less than 0.5 micrometers
(0.01 mils) per year

How Long Do Mechanically Plated Deposits Last In The Salt Spray Test ?

  Zinc Thickness                                                     Salt Spray Protection

  0.00012 inches (3 micrometers)                                        24 hours
  0.00024 inches (6 micrometers)                                        48 hours
  0.00035 inches (9 micrometers)                                        72 hours
  0.00047 inches (12 micrometers)                                      96 hours
  0.00059 inches (15 micrometers)                                     120 hours
  0.00098 inches (25 micrometers)                                     192 hours
  0.00157 inches (40 micrometers)                                     250 hours
  0.00197 inches (50 micrometers)                                     300 hours

 (Sources: American Society for Testing and Materials (ASTM B695-85)
     and General Motors Corporation (GM4345M and GM4344M)).

In general, chromate conversion  coatings  will  add corrosion 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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