Galvanic Series

GALVANIC SERIES

SANGEETH P.S.

1

Galvanic Series

SANGEETH P.S.

2

Galvanic Series

Introduction

Metals and their alloys are the backbone of all engineering projects and

products. Various metals are used in one or the other form. In all the jobs, ranging

from primitive type agricultural implements to advanced aircrafts, automobiles,

railway, heavy machinery equipments, shipping and transportation, and in all such

fields, metals occupy a place of prime importance.

The major problem that we are facing while using metals is corrosion and

decay. It is important to understand that there are several different types of corrosion

including galvanic corrosion, concentration-cell corrosion, stress corrosion, fretting

corrosion, pitting and oxidation. Probably the most common form of corrosion is rust

associated with steel structures. Most metals (except noble metals such as Au, Pt, etc)

undergo corrosion. Various methods are used to control the corrosion of metals such

as proper designing, cathodic protection, Galvanizing, applying protective coating etc.

Galvanic series give real and useful information for studying the corrosion of metals

and alloys. It determines the nobility of metals and alloys.

Galvanic Series

In electrochemical series (reduction electrode potential arranged down in an

increasing order), a metal high in the series is more anodic and undergoes corrosion

faster than the metal below it. For example, Li corrodes faster than Mg; Zn corrodes

faster than Fe; Fe corrodes faster than Sn; Cu corrodes faster than Ag and so on.

However, some exceptions to this generalization are known. For example, Ti (above

Ag in the electrochemical series) is less reactive than Ag. In Zn-Al couple, Zn (below

Al in the electrochemical series) is corroded; while Al acts cathodic and is protected.

These observations, exactly opposite to that of predicted by emf series, are due to the

fact that metals like Ti and Al develop, strongly adhering oxide layers on their

surfaces, thereby making their effective electrode potential more positive (or less

negative)

From above, it is clear that electrochemical seris does not account for the

corrosion of all metals and alloys, and it gives no information regarding the position

of alloys. Consequently, a more practical series, called galvanic series have been

prepared by studying the corrosion of metals and alloys in a given environment. There

will be separate galvanic series for different external environment. Hence the relative

position of metal can change in other environments.

SANGEETH P.S.

3

Table 1

Galvanic Series

Galvanic Series in Seawater

A galvanic series has been drawn up for metals and alloys in seawater, which

shows their relative nobility. The series is based on corrosion potential measurements

in seawater. The relative position of the materials can change in other environments.

The further apart the materials are in this series, the higher the risk of galvanic

corrosion.

Galvanic series relationships are useful as a guide for selecting metals to be

joined, will help the selection of metals having minimal tendency to interact

galvanically, or will indicate the need or degree of protection to be applied to lessen

the expected potential interactions. In general, the further apart the materials are in the

galvanic series, the higher the risk of galvanic corrosion, which should be prevented

by design. Conversely, the farther one metal is from another, the greater the corrosion

will be. However, the series does not provide any information on the rate of galvanic

corrosion and thus serves as a basic qualitative guide only.

Active(or anodic or easy

to corrode)

Noble(or cathodic or

resistant to corrosion)

1. Magnesium and magnesium alloys2. Zinc3. Aluminum 11004. Cadmium5. Aluminum 2024-T46. Iron and steel7. 304 Stainless steel (active)8. 316 Stainless steel (active)9. Lead10. 10.Tin11. Nickel (active)12. Inconel nickel-chromium alloy (active)13. Hastelloy Alloy C (active)14. Brasses (Cu-Zn alloys)15. Copper16. Bronzes (Cu-Sn alloys)17. Copper-nickel alloys18. Monel (70Ni-30Cu)19. Nickel (passive)20. Inconel (80Ni-13Cr-7Fe) (passive)21. 304 Stainless steel (passive)22. 316 Stainless steel (passive)23. Hastelloy Alloy C (passive)24. Silver25. Titanium26. Graphite27. Gold28. Platinum

SANGEETH P.S.

4

Galvanic Series

Table 1 represents the relative reactivity of a number of metals and

commercial alloys in seawater. Galvanic series begins with the more active (anodic)

metal and proceeds down to the least active (cathodic) metal. The various metals

within grouped together are reasonably compatible when used together; those in

different groups may cause a corrosion problem. Some metals, especially those with

significant contents of nickel and chromium, are included in the table in both their

active and passive conditions. Passivation, surface cleaning and sealing, lowers the

metal’s electrical potential and improves its corrosion behavior.

As the series suggests, steel and aluminum are relatively compatible, but if

brass and steel contact, the steel, as the anode, will corrode. The following figure

illustrates the effects of the galvanic series. A brass plate is connected to an aluminum

plate using a passivated stainless steel. If no protection is used over the contacting

surface, galvanic corrosion will occur. The brass and aluminum plates will both

corrode where they touch the stainless steel. The aluminum plate will corrode more

heavily due it being more anodic than the brass. The aluminum plate will corrode

where its exposed surface is in contact with the brass plate.

Figure 1

A "galvanic series" applies to a particular electrolyte solution; hence for each

specific solution which is expected to be encountered for actual use, a different order

or series will ensue. In a galvanic couple, the metal higher in the series represents the

anode, and will corrode preferentially in the environment. However, for any

combination of dissimilar metals, the metal with the lower number (or higher in

series) will act as an anode and will corrode preferentially.

SANGEETH P.S.

5

Galvanic Series

Reference

1. “Engineering Chemistry”, Author-Jain & Jain,

Dhanpat Rai Publishing Company.

2. http://en.wikipedia.org/wiki/Galvanic_series

3. http://www.corrosion-doctors.org/Definitions/galvanic-series.htm

4. http://www.corrosionsource.com/technicallibrary/corrdoctors/Modules/

Aircraft/galvseri-sea.htm

SANGEETH P.S.

6