Resistivity has to be one of my top ten favorite words used in the engineering community, and today I would like to dive a little deeper into how important this factor is in the longevity and durability of a pipe’s life cycle – hence the reciprocity or “give back” of resistivity.
I was thinking about the questions I often have to field with regard to the levels of acidity (or pH) in the native soils around the pipe and how they play in to the potential corrosion and expected service life a pipe might yield. As critical as the soil pH is, resistivity is another key component when considering durability and is often overlooked when considering service life. There are many factors that affect the durability of steel pipe which include soil resistivity and acidity (pH) as mentioned, as well as moisture content, soluble salts, oxygen content (aeration), and bacterial activity. Note that all underground corrosion processes involve the flow of current from one location to another and the “resistance” to current flow through a material is measured by the resistivity of that material. It is essentially the opposite of conductivity, and more specifically – it is an intrinsic property that quantifies how strongly a given material opposes the flow of electric current. So, in other words, the higher the resistivity and/or lower the soil moisture content, the greater the durability. Very small values of resistivity can greatly affect the potential for corrosion.
The moisture content should not be overlooked either. Granular materials with a higher permeability rate allow the water to pass through the backfill quicker and will enhance the durability of the pipe. Conversely, backfill with a high clay content will tend to have much lower permeability rates and hold the water longer, thus possessing a much greater opportunity for corrosion and decreased durability and service life. (See Table 9.2 in the NCSPA book on p. 478 - available online at www.ncspa.org)
So, how does one check and guard against this? Well, I highly encourage you to make sure a water analysis is performed which can be done for a minimal cost (usually around $100 or less) especially in those areas of unknown soils and new construction. You can install a standard galvanized corrugated metal pipe expecting a minimum of a 50-year service life, only to be sorely disappointed when an inspection twelve years later results in the discovery of an invert that is completely deteriorated and a reline or a costly replacement is required. One of the first questions the Developer/Owner/Engineer needs to ask is how long do we want the pipe to last? or what is the Service Life? For example – in the right conditions, an aluminum pipe can almost always provide the desired service life down to a resistivity of 500 ohm-cm and a galvanized pipe can be effective down to around 2000 ohm-cm and up to 8000 ohm-cm. In addition, an Aluminized Steel Type 2 material will do the same down to around 1500 ohm-cm. (check out the Contech CMP Design Guide - p.4) There are also additional items that can be provided to enhance the service life such as increasing the gage or thickness of the material, paving the invert, or enhancing the pipe with a polymer coating.
All that to say, resistance is a good thing not only in life but also in the life span of a pipe. In the long run it strengthens us and provides us with increased endurance and durability.