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


Why do metals placed in the ground corrode?
Are there ways to test for soil corrosivity?
What can be done to mitigate underground corrosion?

These questions and more will be answered in SME’s blog series, “Underground Corrosion – The Answers Uncovered!”

Why do metals placed below the ground corrode?
First, let’s discuss aeration issues, a significant cause of underground corrosion. Aeration is the access of oxygen to buried metal. Oxygen stimulates corrosion by combining with metal ions. Differing amounts of oxygen can result in corrosion cells. Areas of little oxygen become anodes (where corrosion occurs) and areas of more oxygen become cathodes (protected from corrosion). The porosity of the soil is an important parameter affecting aeration.

For iron, in the presence of oxygen, oxidation with resulting precipitation of iron as ferric hydroxide will occur close to the metal surface forming a protective blanket. However, if the soil has large concentrations of soluble salts, precipitation of a protective blanket is prevented and corrosion occurs.

In well-aerated soils, the rate of corrosion generally decreases rapidly since a protective membrane forms rapidly. The oxide layer is cathodic to most metals, so increasing the oxygen supply is akin to increasing the cathode area, thereby protecting the metal. However, in poorly-aerated soils, the protective membrane is slow to form, resulting in more aggressive corrosion. Moreover, when you have an underground pipeline transitioning between low oxygen soil and high-oxygen soil, a corrosion cell develops. The portion of the pipeline in the low oxygen clay soil becomes the anode and corrodes.

What happens if the invert of a pipe is laid on relatively deaerated natural clay soil while the pipe is surrounded by aerated sand backfill?
The backfill around the buried metal has plenty of oxygen, but the clay soil in contact with the bottom of the pipe has little oxygen. As a result, a potential difference develops between the small anode where the metal is in contact with deaerated clay soil, and the large cathode where aerated soil is in contact with the balance of the pipe’s surface. This results in corrosion problems along the bottom of the pipe. This corrosion cell can be eliminated by placing aerated sand below the pipe, keeping the clay soil away from the bottom of the pipe.

There are other situations where corrosion problems develop due to differences in the oxygen concentration. 
For example, bringing a utility pipe underground below the floor slab into a building. The available oxygen beneath the building may be lower than outside of the building, resulting in development of an anodic area on the pipe that is located under the building where the oxygen level is lower than the oxygen concentration outside of the building (cathode area). Corrosion occurs at the anodic area under the building. The same type of corrosion cell could develop below pavements.

Differences in the oxygen concentration in groundwater can also cause soil corrosion issues.
Groundwater in undisturbed soil that is stagnant may be relatively oxygen-free. Rainwater percolating into soil contains high concentrations of dissolved oxygen. Differences in the oxygen concentration in the ground water may lead to corrosion at the anode (stagnant water locations). In addition, if groundwater is present such that part of the pipe is located below and part is above the water level, a differential aeration condition may result and corrosion may occur.

For more information regarding underground corrosion, contact Bob Rabeler. And, watch for the second post in this series coming soon!

TAGS: Geotechnical

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