Resources

 

UNDERGROUND CORROSION – Understanding Galvanic Corrosion

03.14.18   Robert C. Rabeler, PE | More by this Author

UNDERGROUND CORROSION – Understanding Galvanic Corrosion

To understand galvanic corrosion, you must first understand what is known as the electromotive series. Different metals buried in soil have different typical potential (voltage) values:

Metal                   Potential or Voltage
Gold +1.60
Silver  +0.80
Cabon +0.30
Copper/Brass  -0.20
Lead  -0.50
Cast Iron   -0.50
Steel/Mild   -0.50 to -0.80
Aluminum  -1.05
Zinc   -1.10
Magnesium   -1.75

Batteries take advantage of these differences in potential. For example, some batteries pair zinc with carbon. As you can see from the table above, the potential difference (or driving voltage) for connecting zinc with carbon is 1.4 volts (1.1 + 0.3 = 1.4).

Galvanized pipes (steel pipes with zinc coating) take advantage of the electromotive series as well. Because zinc is more negative than steel (-1.10 V vs -0.5 to -0.8 V) it protects the underlying steel – at least until the zinc corrodes away.

SO WHY DOES GALVANIC CORROSION OCCUR?
Galvanic corrosion occurs where two metals having different positions in the electromotive series are in contact within a soil-water electrolyte. The metal with the more negative potential becomes the anode (the metal that corrodes) and the metal with the less negative potential becomes the cathode (the metal that is protected).

Take the example of a copper tube connected to a cast iron pipe shown below. The copper has a higher potential, and therefore becomes the cathode. The cast iron pipe has the lower potential, and therefore becomes the anode. The cast iron pipe would corrode but the copper tube would be protected. This is one reason why many buried copper lines last so long.

The relative size of the anode and cathode areas is a major factor in determining the amount of corrosion damage. For a given potential difference, if the anode area is large and the cathode area is small, the total current and thus the rate of corrosion may be negligible. On the other hand, if the anode area is small with respect to the cathode area, severe damage may occur at the anode.

For example, it is common practice to screw brass service cocks into steel (or cast-iron) water mains with little or no corrosion as a result. A possible explanation of this is that the anodic area (the steel) is so large in comparison to the cathodic area (the brass). The corrosion is widely distributed and is likely unimportant.

However, what if a small galvanized pipe cap is placed on a steel pipe? The galvanized pipe cap (that is zinc coating) becomes the anode (where corrosion occurs), and the steel pipe becomes the cathode. In this case, the surface area of the anode is small compared to the large steel pipe – resulting in aggressive corrosion of the galvanized pipe cap.

For more information regarding underground corrosion and how you can test for it and mitigate it, contact Bob Rabeler...and, watch for the third post in this series coming soon.


TAGS: Geotechnical

Meet our People

The SME team is made up of great people with lots of expertise in the geotechnical, environmental, building materials, pavements, and construction materials fields and related engineering and applied science disciplines. While our skill sets vary, we share a passion for solving challenges and delivering practical solutions.

Meet our People

Subscribe For Updates

Sign up to receive the latest SME insights, news, and project updates.

Subscribe For Updates

Sign up to receive the latest SME insights, news, and project updates.



Leave A Comment