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Galvanic Corrosion and Series Table in Sea Water

Ship's Log - Materials
01/03/2002

Galvanic TableCorrosion is either chemical or electrochemical in nature. The distinction between chemical and electrochemical corrosion is based on the corrosion causing mechanism. Chemical corrosion is the direct result of exposure of a material to a chemical and is governed by the kinetics of chemical reactions.

Electrochemical corrosion is the dissolution of a metal through the oxidation process and can be defined as "degradation of a metal by an  electrochemical reaction with its environment". This phomena also known as galvanic corrision.

The basic unit in which electrochemical reactions occur is the electrolytic cell. An electrolytic cell consists of three components;

  1. Anode
  2. Cathode
  3. Electrolyte

The anode is the site of oxidation and therefore the site of corrosion. The anode and cathode are electrically connected. The medium surrounding the anode and cathode is the electrolyte. In the absence of any one of these components, the electrochemical corrosion reaction will stop.

Any metal when immersed in an electrically conductive fluid, has a specific electrical potential that is measurable as a voltage and each metal has a different electrical potential when immersed in the same electrolyte such as sea water.

According to the potential difference of these two metals, the current flows from higher voltage metal to the lower one. This action raises the voltage of the lower-voltage metal above its natural potential. To establish the equilibrium, the lower-voltage metal discharges a current in to the electrolyte.


Table of Galvanic Series in Sea Water Range of Corrosion Potential
anodic or active Magnesium and Magnesium Alloys -1,60V to -1,63V
Zinc -0,98V to -1,03V
Aluminum Alloys -0,76V to -1,00V
Mild Steel -0,60V to -0,71V
Wrought Iron -0,60V to -0,71V
Cast Iron -0,60V to -0,71V
Aisi 410 Stainless Steel - Active in Water 0,46V to -0,58V
Aisi 304 Stainless Steel - Active in Water 0,46V to -0,58V
Aisi 316 Stainless Steel - Active in Water -0,43V to -0,54V
Inconel - Active in Water -0,35V to -0,46V
Aluminum Bronze (92%Cu-8%Al) -0,31V to -0,42V
Naval Brass (60%Cu-39%Zn) -0,30V to -0,40V
Yellow Brass (65%Cu-35%Zn) -0,30V to -0,40V
Red Brass (85%Cu-15%Zn) -0,30V to -0,40V
Tin -0,31V to -0,33V
galvanic or cathodic Copper -0,30V to -0,57V
Lead-Tin Solder (50%-50%) -0,28V to -0,37V
Admiralty Brass (76%Cu-28%Zn-1%Sn) -0,28V to -0,36V
Aluminum Brass (76%Cu-22%Zn-2%Al) -0,28V to -0,36V
Manganese Bronze -0,27V to -0,34V
Silicon Bronze
-0,26V to -0,29V
Aisi410 Stainless Steel - Passive in Water -0,26V to -0,35V
Lead -0,19V to -0,25V
Inconel (78%Ni-13,5%Cr-6%Fe) - Passive in Wayer -0,14V to -0,17V
Nickel 200 -0,10V to -0,20V
Aisi304 Stainless Steel - Passive in Water -0,05V to -0,10V
Monel 400 (70%Ni-305Cu) -0,04V to -0,14V
Aisi316 Stainless Steel - Passive in Water 0,00v to -0,10V
Titanium -0,05V to +0,06V
Platinium +0,19V to +0,25V
Graphite +0,20V to +0,30V

Table.1) Galvanic series for some metals and alloys in sea water. There is a greater likelihood for galvanic corrosion between the two connected metals when the difference of the electrical potential is greater between them. [3]

 

The current passes through the electrolyte back to the higher-voltage metal and completes the electrical circuit between the two pieces. The current flowing through the electrolyte is generated by an electrochemical reaction that steadily consumes the lower-voltage metal a process. This is the galvanic corrosion.

References

1. Corrosion Basics and Computer Modeling, 1986, V.G. DeGiorgi
2. Corrosion for Students of Science and Engineering, 1988, K. R. Trethewey - J. Chamberlain
3. Corrosion in Sea Water - Professional Boat Builder - December/January, 1995, Nigel Calder
 

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