Galvanic Corrosion2

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Galvanic Corrosion


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Galvanic corrosion is an electrochemical process in which one metal corrodespreferentially when in electrical contact with a different type of metal and bothmetals are immersed in an electrolyte.

When two or more different sorts of metal come into contact in the presence of an

electrolyte a galvanic couple is set up as different metals have different electrodepotentials.

electrolyte provides a means for ion migration whereby metallic ions can movefrom the anode to the cathode ,This leads to the anodic metal corroding morequickly than it otherwise would;

Conversely, a galvanic reaction is exploited in primary batteries to generatea voltage. A common example is the carbon-zinc cell where the zinc

corrodes preferentially to produce a current.


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Galvanic Series

Metals (including alloys) can be arranged in a galvanic series representing

the potential they develop in a given electrolyte against a standard

reference electrode.

The relative position of two metals on such a series gives a good indication

of which metal is more likely to corrode more quickly.

However, other factors such as water aeration and flow rate can influence

the process markedly.


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Galvanic Series

Platinum

Gold

Graphite

Titanium

Silver

/ Chlorimet 3

\ Hastelloy C

/ 18-8 Mo stainless steel (passive)

| 18-8 stainless steel (passive) 5

\ Chromium steel >11 % Cr (passive) / Inconel (passive)

\ Nickel (passive)

/ Silver solder

| Monel

| Bronzes

| Copper

\ Brasses

/ Chlorimet 2

\ Hastelloy B

/ Inconel (active)

\ Nickel (active)

Tin

Lead

Lead-tin solders

/ 18-8 Mo stainless steel (active)

\ 18-8 stainless steel (active)

Ni-resist

Chromium steel >11 % Cr (active)

/ Cast iron

\ Steel or iron

2024 aluminum Cadmium

Commercially pure aluminium
http://www.stoprust.com/sacrificial-anodes.htmhttp://www.stoprust.com/sacrificial-anodes.htm


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Factors Effecting Galvanic

Corrosion

Environmental Effects:

The nature and aggressiveness of the environment determine to a large

extent the degree of galvanic corrosion.

Usually the metal with lesser resistance to the given environment becomesthe anodic member of the couple.

Galvanic corrosion also occurs in the atmosphere, the severity depends

largely on the type and amount of moisture present.


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Distance Effect. Accelerated corrosion due to galvanic effect is usually greatest near the

junction, with attack decreasing with increasing distance from the point.

The distance effect depends on the conductivity of the solution .

Two metal corrosion is readily recognized by the localized attack near the

junction.


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Area Effect.

Another important factor in galvanic corrosion is the area effect.

The ratio of the cathodic to the anodic areas is called the area effect.

An unfavorable area ratio consists of a large cathode and a small anode

Corrosion of the anodic area may be 100 to 1000 times greater than if the

anodic and cathodic areas were equal in size


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Combating galvanic Corrosion

1, select combinations of metals as close together as possible in the galvanic

series .

2, Avoid unfavorable are effect of small anode and large cathode .

3, Insulate dissimilar metals whenever practicable.

4, Apply paints and coatings with caution.5, Add inhibitors if possible to decrease the aggressiveness of the

environment.

6, Avoid threaded joints for materials far apart in the series.

7, Design for the use of readily replaceable anodic parts or make them thicker

for longer life.

8, Install a third metal which is anodic to both metals in the galvanic contact.


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Galvanic corrosion


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