Resistivity Testing for C.P. Design

Size: px

Start display at page:

Download "Resistivity Testing for C.P. Design"

Shanna Webster
6 years ago
Views:

1 Resistivity Testing for C.P. Design Period #5 Advanced Corrosion 2017 February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC Romeoville, IL 1

2 Advanced CP Period 5 Outline Electrolyte, Resistivity & Ions Water & Soil Resistivity Characteristics Resistivity Testing Calculations February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 2

3 Advanced CP Period 5 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 noncorrosive. De-ionized (pure) water is extremely corrosive to bare steel. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 3

4 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 4

5 Advanced CP Period 5 NaCl Dissolving in H₂O February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 5

6 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 6

7 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 7

8 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 8

9 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 9

10 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 10

11 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 11

12 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 12

13 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 13

14 Advanced CP Period 5 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. February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 14

15 Advanced CP Period 5 Resistivity Data & Weener Calculation 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 = ρ = 16,086Ω*cm (mild resistivity) February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 15

16 Advanced CP Period 5 Test (Data taken from TABLE-I on page 15 of course booklet) Meter Ω Spacing/ft ρ P ,490 P ,320 P ,533 P ,801 P P ,926 P7 P8 February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 16

17 Barnes Calculation (Used to calculate resistivity for each soil layer) Advanced CP Period (P2 P1) 1 _ 1 Rp² Rp¹ February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 17

18 Barnes Calculation (Used to calculate resistivity for each soil layer) (P⁴ - P³) 1 _ 1 ΩP⁴ ΩP³ Advanced CP Period x (15-10) 1 _ Barnes Layer calculation for layer between P³ & P⁴ ρ= 15,150 Ωcm February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 18

19 (Data taken from TABLE-I on page 15 of course booklet) Advanced CP Period 5 Test Meter Ω Spacing/ft Weener ρ Barnes ρ Layer P ,490 N/A N/A P ,320 23,937 P1 ~ P2 P ,533 23,937 P2 ~ P3 P ,801 15,958 P3 ~ P4 P ,352 P4 ~ P5 P ,926 1,140 P5 ~ P6 P7 P8 February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 19

20 Advanced CP Period 5 Test #1 Meter Ω Spacing/Ft р Barnes р Layer P N/A P1 P P1~P2 P P2~P3 P P3~P4 Test #2 Meter Ω Spacing/Ft р Barnes р Layer P N/A P1 P P1~P2 P P2~P3 P P3~P4 Test #3 Meter Ω Spacing/Ft р Barnes р Layer P N/A P1 P P1~P2 P P2~P3 P P3~P4 Test #4 Meter Ω Spacing/Ft р Barnes р Layer P N/A P1 P P1~P2 P P2~P3 P P3~P4 February 21-23, 2017 Rogelio de las Casas DeLasCasas CP, LLC 20

21 Advanced CP Period 5 Summing Up (or clearing the fog) February 21-23, 2017 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. Rogelio de las Casas DeLasCasas CP, LLC 21

anemometer a weather instrument that measures wind speed with wind-catching cups (SRB, IG)

anemometer a weather instrument that measures wind speed with wind-catching cups (SRB, IG) FOSS Weather on Earth Module Glossary 3 rd Edition 2012 absorb to soak in air the mixture of gases surrounding Earth air pressure the force exerted on a surface by the mass of the air above it anemometer