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Resistivity Testing for

C.P. Design

Advanced Corrosion Course 2017

February 21-23, 2017

Rogelio de las Casas DeLasCasas CP, LLC

Romeoville, IL 1

Period #5

February 21-23, 2017 Rogelio de las Casas – DeLasCasas CP, LLC 2

2017 Underground Corrosion Short Course

Advanced CP Course – Period 5

Course Outline

Electrolyte, Resistivity & Ions Water & Soil Resistivity Characteristics Resistivity Testing Calculations




Water & Soil Resistivity Characteristics Properties of Water The resistance to current flow (not electron flow) in any set volume of water is directly proportional to the amount of “dissolved” ions contained in the water. Pure water (de-ionized water) contains little if any dissolved compounds or mixtures. Therefore its resistivity is over 1-million Ω (1-Mohms). (Just because a LIQUID has high resistivity does not mean it is non-corrosive. De-ionized (pure) water is extremely corrosive to bare steel.




Water/Salt Ion Chemistry Dehydrated table salt (NaCl) also has high resistivity. If you take a glass of de-ionized water and add a teaspoon of table salt and stir, the NaCl crystal will “disassociate” into Na “cations” and Cl “anions”. The Na cations will have a “positive” charge and the Cl anions will have a “negative” charge. Some of these “ions” will form “bonds” with water molecules. A salt water mixture has low resistivity because it has free flowing ions.




NaCl Dissolving in H₂O




Ions (Cation or Anion)

For an electrolyte to be corrosive it normally must contain “ample” freely moving ions (anions and cations). Hence the concentration of ions dissolved in an electrolyte correlates directly to the electrolyte’s resistivity.




Water Resistivity

Almost all sea water has less resistivity (less opposition to current flow) then lake or river water.

But some ponds containing contaminates have less resistivity than sea water!

This is because of ion concentration levels. Water resistivity is also effected by

temperature because temperature effects ion dissolution.

As you cool a cup of salt water to 0°F, the Na and Cl ions will start to reform “ionic bonds” and fall to the bottom of the cup. The water at the top of the cup will then start to freeze.

The frozen water at the top of the cup will have high resistivity and the water at the bottom, less.




Soil Resistivity

Soils – For the corrosion professional, this term normally describes the top couple of feet of earth a person is standing on. If a person digs deep enough they will always run out of soil. All soils contain ions but in varying concentrations. Rock – Material compressed under great pressure. Normally contains very few ions and water.




Soil Composition

Over the ages soils gained organic and inorganic material in layers. These layers in turn contain different concentrations of ions. This means each layer can have varying resistivity values. Events such as flood erosion and glaciations upset these layers, especially where glaciers pushed debris into depressions such as ancient river beds. Glaciers also created “moraines” where they melted, leaving large deposits of sand and gravel. And then along came humans who love to dig in the dirt and move it around. This also mixed up nature’s carefully deposited soil layers. The point? Soil composition can vary widely within very small areas.




Soil – Water Content

As was stated with the NaCl crystal, ions cannot move freely if locked in a dehydrated media. This law pertains to soil. During the summer, soil dries out from wind and intense sunlight exposure. These drying effects can reach depths of 10-feet or more during extended periods of no precipitation. At times summer rain showers will not penetrate deep enough to counteract the summer drying effects. Dry soil will have higher resistivity than moist soil. Ions need a solvent (H₂O) to move freely about.




Soil – Water Content, cont.

If you conduct soil resistivity testing during dry seasons, test data will only represent soil resistivity during dry periods. The inverse is true about resistivity readings taken after a heavy rain. Rain soaked soil will have lower resistivity than the same soil when it is dry. Soil “layers” can also have wide differences in resistivity.




Soil – Water Content, cont. If you conduct soil resistivity testing during dry seasons, test data will only represent soil resistivity during dry periods. The inverse is true about resistivity readings taken after a heavy rain. Rain soaked soil will have lower resistivity than the same soil when it is dry. Soil “layers” can also have wide differences in resistivity.




Soil, Water and Ions

Without an understanding of soil/water chemistry, the designer will not comprehend the need to carefully sample the electrolyte at the proposed anode/structure site.




Ion Concentration & Resistivity What Does All This Mean? All “natural” bodies of water contain ions. All soils contain ions, but soil must contain moisture for ions to move around (migrate). Sand/gravel and rock have few free ions. Frozen water or soil will slow the movement of ions to the point where resistivity can be very high. A resistivity measurement taken at one spot will not necessarily represent electrolyte close by. Resistivity measurements can vary widely at different soil or water depths.




Resistivity Data & Weener Calculation 191.5∗R∗s = ρ Lets say we have a soil resistivity meter reading of 4.2Ω (after we multiply the reading by the meter multiplier) and a pin spacing of 20-ft. Using the above formula we “plug and chug”. ρ = 191.5 × 4.2 × 20 ρ = 16,086Ω*cm (mild resistivity)




(Data taken from TABLE-I on page 15 of course booklet)

Test Meter “Ω”

Spacing/ft ρ

P1 24 2.5 11,490

P2 16 5 15,320

P3 10.2 10 19,533

P4 6.2 15 17,801

P5 2.6 20 9.958

P6 0.82 25 3,926

P7

P8




Barnes Calculation (Used to calculate resistivity for each “soil” layer)

191.5 (P2 – P1) 1 _ 1 Rp² Rp¹




Barnes Calculation (Used to calculate resistivity for each “soil” layer)

191.5 (P⁴ - P³) 1 _ 1

ΩP⁴ ΩP³

191.5 x (15 -10) 1 _ 1

6.2 10.2

957.5 0.16 – 0.1

957.5

0.0632

Barnes Layer calculation for layer between P³ & P⁴ ρ= 15,150 Ωcm




(Data taken from TABLE-I on page 15 of course booklet)

Test Meter “Ω” Spacing/ft

Weener ρ Barnes ρ Layer

P1 24 2.5 11,490 N/A N/A

P2 16 5 15,320 23,937 P1 ~ P2

P3 10.2 10 19,533 23,937 P2 ~ P3

P4 6.2 15 17,801 15,958 P3 ~ P4

P5 2.6 20 9.958 4,352 P4 ~ P5

P6 0.82 25 3,926 1,140 P5 ~ P6

P7

P8




Test #1 Meter Ω Spacing/Ft р Barnes р LayerP1 11.2 2.5 N/A P1P2 6.8 5 P1~P2P3 5.2 10 P2~P3P4 4.2 15 P3~P4







Summing Up (or clearing the fog)

Without ions, conventional current will not “flow”. Low moisture content = low ion concentrations. Electrolyte resistivity can vary widely, hence the requirement to do accurate resistivity testing & calculations. Resistivity testing should be accomplished by competent persons. CP designers should accomplish “site” evaluations and check out the electrolyte. Barnes layer calculations are required for accurate data. Resistivity; single most important component of a CP design calculation.

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