Underground Corrosion: Metallic Utilities vs Steel Piling

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

Underground Corrosion: Metallic Utilities vs Steel Piling

So what’s the difference between underground corrosion of metallic utilities and that of buried steel piling?

Interestingly, steel piles driven vertically into the ground typically experience little corrosion. Factors that cause severe corrosion of metals placed in the ground in horizontal trenches are much less impactful for vertical steel piles. The main reason? The presence or absence of oxygen. The presence of aerated material is fundamentally necessary to the corrosion process (with some exceptions).

In soil backfill, the soil is aerated and therefore corrosion occurs. However for steel piling, the natural soil (below any fill) normally contains limited amounts of oxygen resulting in only limited corrosion. Furthermore, the normally small portion of the steel pile which is located near the surface in aerated soil becomes cathodic to the larger part of the steel pile contained within the deaerated soil (the anode). This large anode-small cathode results in low rates of corrosion.

Special care should still be taken where steel piles are driven through fill. On occasion, fills are hostile to metal, particularly where the pH is lower than 4.5 and/or where organic matter or cinders exist within the fill. Also, if the length of piling below the fill is short, the large anode-small cathode equation changes resulting in more corrosion. And further, if there is groundwater with a significant gradient (flowing groundwater), the water could contain oxygen and could cause corrosion issues.

Steel piles driven into “alkali” soils, or where the ground water is salty or brackish, should be considered on a separate basis since soil containing sodium and chloride ions is hostile to buried ferrous metal.

Dissimilar metals such as copper grounding wires should not be connected to steel piles, unless the ratio of surface area of the copper is small compared to the steel. Also, the reinforcing steel in the pile cap should not touch the steel pile to avoid an electrical connection between them resulting in galvanic corrosion. Steel imbedded in concrete has a higher electrical potential than steel imbedded in soil, resulting in corrosion of the soil-embedded steel if electrically connected.

Mr. Rabeler was the lead investigator for an evaluation of underground corrosion of helical anchors buried in soil that would normally be corrosive to buried shallow utilities. Corrosion rates were measured for both bare steel and for galvanized (zinc coating) steel using weight loss measurements (comparison of weights before and after burial), field polarization testing (instantaneous corrosion rates), and measurement of the zinc thickness (before and after burial). Corrosion rates were significantly lower than would be predicted for shallow metallic utilities. Also, the zinc corrosion rate decreased significantly with time. Details of this study were published in a technical paper, which can be downloaded here.

You can also learn more about corrosion in Mr. Rabeler’s previous blog posts on galvanic corrosion, why metals placed below the ground corrode, how pH affects soil corrosion, and mitigating the risks.

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