97-520 datasheet(61/64 Pages) LSTD

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CORROSION OF EMI GASKETS

5.1

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GALVANIC CORROSION OF METALS

The galvanic series is a common means of ranking the relative activity of

metallic couples. The galvanic series does not provide a measure of the

corrosion current which will flow in a dissimilar metal couple, but it does

indicate which couples are likely to incur significant corrosion damage. In

galvanic couples consisting of two very incompatible metals, it is possible to

reduce the corrosion rate through good design practice. In this situation, it is

best to maximize the ratio of anode to cathode surface areas. For a particular

current density, the corrosion rate on the more anodic metal will be lower

the larger this ratio, because the corrosion reaction is spread over a larger

surface area. In addition, if corrosion products build up on the cathode, they

will build at a faster rate the smaller the surface area of the cathode relative

to the anode. In this situation, the deposits could create a barrier that slows

down corrosion.

The above technique is used in reverse to enhance the performance of electrical

contacts. It is desirable that electrical contacts remain clean to provide a low

resistance connection. When corrosion occurs, metal is removed from the

more anodic metal. This process cleans the more anodic metal. By making

the electrical contact out of one metal and surrounding it by a larger surface

area of a more cathodic metal, the contact will be cleaned by the corrosion

reaction. The contact may corrode away eventually, but it will function more

reliably during its shortened life.

The probability that two dissimilar metals will corrode when coupled together

can be predicted from their difference in the electrochemical potentials. This

information is tabulated in the Metals Galvanic Compatibility Chart on page

5-2. The common metals and their anodic index are listed along the left side

of the chart. The metals are grouped in 0.05 volt increments, with some of

the groups containing no common metals. Group Number One (left hand

column) contains the most cathodic metals and has an anodic index of zero.

The anodic index increases as metals become more anodic. The arrow (on the

far right hand side of the chart) points in the direction of increasingly anodic

metals. On the right side of the chart, typical finishes available on metallic parts

manufactured by Laird Technologies are listed along the top. The colored bars

indicate the galvanic compatibility of these common finishes to the metals listed

on the left. The color code is based on both the electrochemical differences

between the metals and the finishes and the corrosiveness of the environment.

The chart shows that the force that drives the corrosion reactions is directly

related to the electrochemical difference between any two metals. By reducing

this difference, the corrosion rate is decreased.

BASIC CORROSION PREVENTION

Whenever possible, avoid the use of dissimilar metals. The following five steps

may be taken to prevent, or at least minimize corrosion potential in the

event that it is necessary to use dissimilar metals in intimate contact with one

another:

1. Limit contact between metals with widely different electrochemical

potentials. The Metals Galvanic Compatibility Chart, on page 5-2, indicates

which metal pairs have large differences in electrochemical potential.

2. Insert a third metal between the two dissimilar metals which reduces the

potential difference of the galvanic couple. For example, nickel or tin plated

copper is suitable for use with aluminum and silver combinations.

3. Design the flange interface so that the surface area of the anodic metal

is significantly larger than the cathodic metal. The electromotive force

(EMF) difference remains the same. However, the current density is

decreased, so the corrosive attack on the cathodic metal is reduced.

4. Eliminate moisture, salts and other electrolytes from entering the

joint interface by improved flange design or, if not possible, use an

environmental seal outboard of the conductive element in a dual

EMI shield/environmental seal.

LT-3034 Metals_catalog

10/8/04

10:31 AM

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