Resistivity vs conductivity of soils & corrosion

[darthsoilsguy2]

Hello Folks,

I'm looking at a geotechnical study done in 1990 for a buried pipeline where they measure soil conductivity of samples and it appears they inversed it to get resistivity. We perform resistivity testing along with other corrosion-related soil testing for pipeline and mse wall geotech studies regularly, but i am not familiar with this conductivity test. What i find most concerning is that some of the reported resistivities are 100X more resistive than i've seen with our field data with same units.

Out of 25 samples, here are the lowest and highest resistivities

Boring B-62, Depth 8.5-10, Soil Type CL, pH 7.3, Conductivity uMHO/cm 114.0, Resistivity 8,772 OHM-cm
Boring B-19, Depth 5-10, Soil Type MH, pH 5.4, Conductivity uMHO/cm 0.6, Resistivity 1,666,667 OHM-cm

the purpose of why i'm looking at this data is to find a few good spots to examine the buried ductile iron pipe for corrosion and pitting. i plan on doing some additional resistivity tests to confirm, but i was wondering if anybody has had a similar experience or has any insights to this. perhaps there is a limits of the test issue here that i don't know about.

thanks
-DSG

 

[darthsoilsguy2]

Please note this thread is also posted in soils testing engineering as
thread261-337997

 

[SJones]

If the pipeline is coated, would it not be best to perform a coating survey to find locations where corrosion could be occurring? NACE TM0109 for example. Probably a bit more direct than playing around with soil resistivities at pipe depth.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[BigInch]

Check the units. Are they really Ohms/m, or Ohms/cm? If you are used to working with Ohms/cm, then the Ohms/m value will be 100 x what you are used to using.

Independent events are seldomly independent.

 

[darthsoilsguy2]

it is buried and uncoated ductile iron pipe without cathodic protection. really the soils are about the only indicator for aggressive exterior side pitting besides unearthing the whole line.

units don't help me out.... if i bring the big one down from 1,666,666 to 16,666, then i have to bring the 8,772 down to 87 which would be an extremely corrosive soil that i would have trouble believing

 

[BigInch]

Correct.
< 10 is noncorrosive.
10-50 is lightly corrosive.
50 to 100 [&Omega;]/cm is moderate soil.
> 100 [&Omega;]/m is corrosive.

Independent events are seldomly independent.