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z Yes, if all the grounding and the pipes are connected between them. It does not matter where you con- nect the electrode of the structure; the potential value depends on where you install the half-cell.
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The test method is common in power plants. Since the ground- ing cables are connected to the structures that you are protect-
ing, they too are under CP. Connecting the voltmeter to the grounding cable is okay. The important item is where you place your CSE—as you move it around the area, you will get an average reading of the underground structures. You might also place the reference cell in a remote location to determine long-line corrosion potentials. The only cautionary note is that the bare grounding cable will absorb most of your CP current and might also shield some pipes.
Relationship between soil resistivity and CP
problem and CP may not be needed, particularly if the line is bare. If there are differences in soil resistivity along the length of the line, then corrosion cells can form and CP will be needed. If the line is coated, CP will be needed to prevent cor- rosion at coating holidays that might be exposed to spots of different resistivity and particularly if the line is not electri- cally isolated from other structures.
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NACE International, Vol. 51, No. 4 A
I once installed a 500-ft (152-m)- long groundbed in similar soil us- ing scrap steel for the anode with [MI [IT\ JIKSÅTT IVL Q\ _WZSML _MTT
April 2012 MATERIALS PERFORMANCE 33
What is the maximum range of soil resistivity for a cathodic protec- tion (CP) system? In soil
_Q\P ZM[Q[\Q^Q\a & ¤ KU KIV 1 UISM CP work and is it needed for steel pipes?
If the soil resistivity is 100,000 ¤ KU \PZW]OPW]\ \PM TMVO\P WN the line (which is very improba- ble), corrosion may not be a
some experience years ago in Saudi Ara- bia, where in the dry season we measured such high resistivity. But when it rained, the high chloride content of the sand caused the resistivity to plummet and the soil became extremely corrosive. You can determine if this is the case by
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measuring the minimum resistivity using California Test Method (CTM) 643. This method (and all the CTMs) can be found at www.dot.ca.gov/hq/esc/ctms/index. html. They also have a test method for water-soluble sulfate (CTM 417) and one for water-soluble chloride (CTM 422).
If this is a regulated pipeline, you will need CP. Impressed current will be necessary.
In soils of this high resistivity, CP may not be required because the corrosion rate would be expected to be minimal. However, I had