DCVG rev (d)

(DCVG) above grade direct current
voltage gradient survey.
Parameters affect all of UG structure coating defects detection, sizing and effectiveness of
cathodic protection based on coupon off- potential evaluation
BY Kamal Mohamed :Corrosion Engineer
1/8/2014
Study had carried out on many different parameters which affect all of detection and sizing of UG
structure coating defects that significantly affect all of defect detection ,decision of coating defects
repair , evaluation of cathodic protection system and utilization of coupon off-potential to alternate
structure to electrolyte off-potential .

ABSTRACT
DCVG survey is a method of measuring the electrical voltage gradient in the
soil along and around the pipeline to locate coating holidays, categorize its relative severity once it is
found and indicate the effectiveness of cathodic protection system.
There is quantitative relationship between DCVG indication and the presence of coating defects
Such a relationship depends on many parameters such as soil resistivity , depth of soil above the
defect , DC signal strength , the condition of coating away from the defect ,level of cathodic
protection ,polarization resistance and measuring devices accuracy /sensitivity.
The objective of this study was to indicate the accuracy and realistic of pipeline coating
defects detection , coating defects sizing. and effectiveness of cathodic protection system in case of
using coupon off-potential to alternate structure to electrolyte off-potential during the service.
Results from DCVG techniques as per NACE TM0109-2009 show that soil and coating parameters
are significantly affecting all of DCVG indication ,OL/RE and the calculated IR% which will be
reflected in coating defects detection ,sizing ,evaluation of pipeline coating conditions and
effectiveness of cathodic protection in case of utilization coupon to electrolyte off- potential to
alternate structure to electrolyte off- potential . .
Introduction
Direct Current Voltage Gradient (DCVG)
DCVG survey is a method of measuring the electrical voltage gradient in the
soil along and around the pipeline to locate coating defects, categorize its relative size/severity once it
is found and indicate the effectiveness of cathodic protection.
This is done by using two calculations, the first is in units of mV (DCVG indication) and it is used to
determine the coating defect location as measurements are made along the length of the pipeline.
The second calculation is termed as a percent-IR calculation and it involves measurements moving
away from the pipeline. The measurements of DCVG in mV are completed by detecting a voltage
gradient at the surface of the ground above the pipeline coating defect. This voltage gradient is
detected by the use of two Cu/CuSO4 electrodes. These electrodes are placed at the bottom of
walking stick probes as those used in the CIS survey. One electrode is placed at the ground surface
directly above the pipeline and the other is placed at a location approximately 1.2 meter away from
the pipeline, but also at the ground surface. There is no direct connection made to the pipeline as in
the CIS survey. This measurement takes into account only the voltage gradient in soil surface found
between the two electrodes which is the result of current entering the pipeline at a coating defect

(DCV indication or Maximum DCVG). CP rectifiers are interrupted at a regular cycle which creates a
DC signal detected by the potential difference between the electrodes. The electrodes are electrically
connected to a voltmeter which displays the voltage gradient detected in mV. Measurements along
the pipeline are typically made at 1.2meter intervals. When the survey begins, the surveyor shall
nulls the voltmeter so that the first value or reading is at zero. This means that as the CP current is
interrupted, the voltmeter remains at zero even when the CP current is switched on and off. As a
coating defect is approached, the voltmeter will begin swinging in either the positive direction or the
negative direction from zero depending on the direction of current detected in the soil or
electrolyte(Figure 1).
Figure (1) DCVG and structure / electrolyte current direction
When the CP current is switched off, the voltmeter goes back to zero. However, as the coating defect
approaches, the voltmeter continues to give either the positive or negative reading when the CP
current is switched on that is consistent with the previous measurements. The magnitude of this value
(DCVG indication in mV) increases as the coating defect approaches. The magnitude of the voltage
reading will reach its maximum when the surveyor’s measurement is made directly above the coating
defect. This is evidenced by the voltmeter’s sudden swing from positive to negative or vice versa
when the defect is passed. However, the absolute value of the reading will decrease back towards
zero as movement is made further away from the coating defect. This behavior found by use of the
voltmeter is due to the detection of a direction change in the flow of current in the electrolyte. The
current in the electrolyte or soil is always moving toward the coating defect, therefore when the defect
is passed there is a change in direction of the current. Once a coating defect is found and located, its
size and severity must be determined. This further characterization of the coating defect is done
through the calculation of DCVG in percent-IR. There are two steps that the field surveyor
follows in determining the percent-IR value for a given coating defect . The first is by taking lateral
voltage gradients moving away from the pipeline. These lateral measurements are shown in Figure (2
) Successive lateral measurements are made(OL/RE) in the direction away from the pipeline and the
coating defect until the voltage gradient reaches a value of less than or equal to one mV. The
location where the lateral voltage gradients are not greater than one mV is termed either as remote
earth or IR infinity. these voltage gradients are measured by the 1.2 meter spacing of Cu/CuSO4
electrodes

Figure (2): The percent-IR calculation is shown using the lateral
(OL/RE=V1+V2+V3+V4+V5 -----till remote earth )
and interpolated value of IR drop over the coating flaw.
The voltage gradients are then added up and divided by the IR drop at the soil surface directly above
the coating flaw. However, the IR drop over the soil surface is not measured directly since a direct
connection to the pipeline is not made. Therefore, this value must be interpolated by using the known
values of potentials that are directly connected to the pipeline. This is shown in Figure (3 )as the
closest test posts are used since they are directly connected to the pipeline. At the location of each
test post the IR drop can be measured.
Figure( 3): A diagram showing how an IR measurement is interpolated at the
ground surface above the coating defect.(IR mV at the defect = X mV +X1*(Y mV-X mV)/(X1+X2)
Once a percentage is calculated for a given coating defect , it is categorized based on what range of
values it lies in. There are three categories that are generally used to differentiate the severity or size
of holidays. The first category is for IR drops between zero and 15 percent considered to be safe and
not severe. This is because the CP system is expected to provide adequate protection for a coating
defect with such a low percent-IR value. Therefore, no action is needed. If the percent-IR is between
15 and 35 percent, A sufficient amount of coating damage is expected to be present for a percent-IR
value in this range. However, a field engineer is allowed to make the decision to either take
immediate action and excavate or to schedule another survey .

The third category %IR above 35. For a percent-IR value above 35 percent, immediate action is
necessary. excavation at the site of the coating defect is needed so that the pipeline can be
physically repaired.
Cthodic protection review
Cathodic protection is a scientific approach used to protect a metal structure from degradation. It
involves electrically connecting two metals in an electrolyte. For underground pipelines, the soil can
be considered to be the electrolyte. There are two different types of cathodic protection.
One is through use of a sacrificial anode and the other is by impressed current. CP with the use of a
sacrificial anode involves galvanically coupling the pipeline with a metal more active than the metal of
the pipeline. The metal that is considered to be more active is the metal that has a more negative
standard equilibrium potential. Once they are connected a potential difference develops between the
two metals. The more active metal acts as the anode and the more noble pipeline metal acts as a
cathode. As the more active metal, the anode will give up its electrons much easier than the noble
metal. in case of free corrosion, there are two types of reactions that can occur normally at the
pipeline’s surface. the first type of these reactions is the oxidation of iron
Fe Fe2+ + 2e− (oxidation or anodic reaction)
The other type of reactions is the reduction of oxygen
O2 + 2H2O + 4e− 4OH− (reduction or cathodic reactions)
2H+ + 2e- H2( gas) (reduction or cathodic reactions)
These two types of reactions must be electrically balanced so that they proceed at the same rate.
Since the integrity of the pipeline can be compromised by the iron dissolution reaction, the rate of this
reaction must be reduced. This is done by providing the excess of electrons from the anode. The
metal dissolution reaction of the anode is given as
M Mn+ + ne−
where M represents the sacrificial anode metal being oxidized. By degradation of the anode metal,
electrons are supplied to the pipeline so that the oxygen reduction reaction or hydrogen reduction
reaction can occur which reduces the rate of the iron oxidation reaction. The electrons are delivered
to the pipeline through a low resistance wire as shown in Figure (4)

Figure (4) sacrificial anode cathodic protection system
Therefore, through galvanic coupling the more active anode begins to degrade which further protects
the more noble pipeline metal. Most sacrificial anodes are magnesium ,aluminum( sea water) and
zinc since they are all more active as compared to steel pipelines. CP systems with impressed
current involve supplying current from an external source in order to protect pipelines (figure (5)). This
is done by a rectifier or DC generator which can convert alternating current from an external power
source to direct current. This creates a voltage drop between the anode and the pipeline which drives
positive current from the anode to the pipeline through the soil (electrolyte) in other words electrons
drives from the anode to the pipeline through the low resistance wire. by an excess of electrons at
the pipeline’s surface, the potential of the metal is polarized to a more negative potential. The rate of
oxygen reduction reaction is increased and the anodic or oxidation reaction which normally occurs at
the pipeline’s surface becomes unfavorable. If the potential of the pipeline becomes too polarized the
hydrogen evolution reaction can occur.
H2O + 2e− H2 + 2OH−
The evolution of hydrogen can result in hydrogen embrittlement of the pipeline, so it is necessary to
ensure that polarization does not cause the pipeline’s potential to become too negative. In impressed
current. The current is supplied to the pipeline from the anode through the soil. When the current
reaches the pipeline it travels toward the low-resistance wire. This low-resistance wire allows for the
return of current to the anode from the pipeline. The anode in this system is made up of an inert
material so that it will not chemically react with the environment and degrade as a sacrificial anode
would. Sometimes the anode can be more noble than the pipeline. In this case, the rectifier must
overcome both the resistance of the circuit and the back potential created from the more noble anode
in order to flow current in the proper direction. In order to balance the reduction reactions occurring at
the pipeline’s surface, there must be oxidation reactions occurring at the surface of the impressed
current anode. The main reaction occurring is the oxidation of water or
the evolution of oxygen given as
2H2O O2 + 4H+ + 4e−
At more extreme positive potentials the evolution of chlorine can also occur as
2Cl− Cl2 + 2e− (2.6)
Figure ( 5 )(Schematic drawing for impressed current cathodic protection system)

Study done on parameters affecting all of maximum DCVG , OL/RE and the calculated IR%
which affect coating defect detection , sizing and effectiveness of cthodic protection based on
coupon off- potential.
1- soil depth over the coating anomaly
As mentioned in NACE Standard TM0109-2009 (1.2.2) the detection sensitivity of indirect inspection
tools decline as the pipe burial exceeds normal ranges. Although this “normal” range is not clearly
defined, most tools specify ranges beyond which detection capability seizes. This is, of course,
expected because the strength of the gradient fields emanating from a coating defect location (as
shown in Figure 6) diminishes inversely as the distance from the source. increased. the potential
difference between the equipotential lines will decreases and therefore, become undetectable.
Figure (6): Effect of soil depth above coating defect on the measured voltage gradient.
as depth of soil above the defect increased al of mV indication ,OL/RE and IR% will decreased till the
case that the defect can't be detected or sized.
Site survey done on the existing Coupons with bare surface area 3"x3" (9 square inches ) found with deferent % IR due to
deferent depth of soil above the coupon (%IR =0.5 to 2.7) . %IR changed about 6 times by changing depth of soil above the defect
and alsothe maximum DCVG for the first lateral reading (one CSE on the defect and second one 1.2 meters away) decreased by
increasing the soil depth above the coupon.(table 1)
Pipeline
identification:
48 INCH pipeline
Coating type and
conditions
3LPE new coating for pipes , PE heat shrinkable sleeve for fieldjoint and high volume solids
polyurethane coating (100 microns) for cold bends .excellent coating efficiency only minor damages
expected during construction stages
S
No.
Pipeline
identificatio
n
chain age of
coupon Test
station KM
Defect
chain age
S1
mV
D1
KM
S2
mV
D2
km
DX
KM
OL//R
E mV
PIPE/RE
mV
%IR
Drop for
anomaly
Actu
al
size
(Sq-
inch
)
depth
(m)
Remark
s

1 1423 0 1420 1.34 0 12 1423 0.8 9 coupon
2 1420 1.34 1430 2.45
0.4
5
27.2 1424 1.9 21.5 2.5
ANOM
ALY 1
3 1430 2.45 1406 4.96
2.5
1
9.4 1406 0.7 9 coupon
4 1406 4.96 1389 6.26 1.1 6.4 1392 0.5 9 coupon
5 1406 4.96 1389 6.26 1.3 8.2 1389 0.6 9 coupon
6 1509 0 1494 0.25 0 12.3 1509 0.8 9 coupon
7 1509 0 1494 0.25
0.2
5
9 1494 0.6 9 coupon
8 1494 0.25 1506 1
0.0
4
12 1495 0.8
12.3
2
1.5
ANOM
ALY 2
9 0
NO
DEFECT
S
10 1437 3 1465 5 2 19.5 1465 1.3 9 4 coupon
11 1465 5 1421 7
0.9
9
49.5 1443 3.4 24.1 4.5
ANOM
ALY 3
12 0
NO
DEFECT
S
13 1246 9 1353 11 1 35 1300 2.7 9 coupon
14 1353 11 1351 13.5
2.0
5
18 1351 1.3 9 coupon
15 1351 13.1 1194 17
1.9
5
14 1273 1.1 9 coupon
TABLE (1) Coupon with bare surface area 3"x3" (9 sq inch ) showing deferent % IR due to deferent depth of soil above the coupon
(%IR =0.5 to 2.7) . %IR changed about 600% by changing depth of soil above the defect
2- The effect of signal strength (pipe to soil on potential - pipe to soil off potential at coating
defect) on DCVG indication and the calculated IR%
From conventional knowledge as signal strength increased the current density on both the defected
area and through soil will be increased and then all of DCVG indication , above the line OL/RE will be
increased .so detection of coating defect will be improved by increasing the signal strength which will
increase DCVG indication
site test carried out on bare CS coupons with surface area 3"X3" buried at pipeline depth and
measuring /calculating for all of DCVG indication ,OL/RE mV , IR% for deferent signal strength (all
other parameters fixed) we found that DCVG indication and OL/RE mV will be significantly increased
by increasing signal strength
As per NACE TM0109-2009 clause 6.3.1.7 Typical DCVG signal magnitudes measured to remote earth range
between 100 and 1,500 mV in soil environments .
40"pipeline existing test coupon at KM 6+050

increasing signal strength will magnify all of DCVG indication and OL/RE which will improve coating
defect detection sensitivity and also will influence the %IR calculations .
3- The effect of soil resistivity
from conventional , DCVG indications (mV measured between two CSE one on the defect
and second about 1.2 meter perpendicular to pipeline) will increase with increasing defect
size, DCVG indication decreases with soil resistivity increases .This case was not initially
expected. as the identification of defect location will be less clear with increasing soil
resistivity . By site trails it appears that the soil resistivity plays a larger role than coating
defect size in determining DCVG indication in mV. This is supported by the behavior at high
soil resistivities where the DCVG indications show almost no dependence on defect size.
1270 1085 911 591 516 186
IR% 0.91 0.84 0.77 0.52 0.54 0.53
OL/RE 11.5 9.1 7 3.4 2.4 1
DCVG 4.1 3.2 2.8 1.7 1.2 0.4
0
2
4
6
8
10
12
14
Voltage(mV)
SIGNAL STRENGTH (mV)
Effect of signal strength (mV) on DCVG indication ,OL/RE and %IR
IR%
OL/RE
DCVG

in case OL/RE measured and converted to percent-IR the effect of soil resistivity and coating
defect size on. The results show that percent-IR increase with increasing coating defect size
and increasing soil resistivity. soil resistivity can have a greater effect on percent-IR values
than coating defect size. For example, for each soil resistivity, the relative change of %IR
stays the same. This means that as coating defect size changes, the percent-IR indication
changes by the exact same incremental value regardless of what soil resistivity that the
system is at. This result shows that percent-IR can also be better prioritized by taking soil
resistivity into account. In other words, the percent-IR indications obtained could be
misinterpreted causing an inaccurate sizing of the coating defect size .
4-Coating types and conditions of coatings away from the coating defect
%IR is the percent between the voltage gradient caused bythe specific defect to the voltage
gradient made by the remaining surface of the pipe . so as coating with good isolation
properties and less number of defect the %IR for certain defect will be more comparing with
the case if the pipe line with poor coating condition or poor electrical isolation properties
in other words same %IR percent will represent bigger coating defect in poorly coated line
and smaller coating defect in well coated pipeline with high dielectrics voltage properties
From conventional knowledge, as coatings in medium or bad condition it will affect the
aboveground measurement techniques due to the possibility that corroded areas of the
pipeline may be undetected. This can occur because the CP current is so widely distributed
that there is not enough localized current entering the pipeline at a corroded or bare location.
This causes indications from measurement techniques to not be large enough to detect bare
0
5
10
15
20
25
30
35
40
45
0 125 250 375 500 625
DCVGindication(mV)
Coating defect size ( cm2)
Effect of soil resistivity (KΩ-cm) onDCVG indication,OL/RE
(mV) and %IRfor deferenddefectsizes
0.5KΩ-cm
3kΩ-cm
10KΩ-cm

spots on the coating. For example, voltage gradients in the ground will not arise if there is not
a large enough coating defect of localized current entering the pipeline at a given location.
from site survey on in service pipeline with medium or bad coating condition we got that ,
Conclusions
Based on the above study and conventional knowledge we found that the sensitivity of the
aboveground DCVG survey techniques for coating detect detection and sizing will vary due to many
parameters as following
I- detection of coating defects ,
-DCVG indications increases with increasing coating defect size.
-DCVG indication decreases with increased soil resistivity.
it was found that soil resistivity has an even larger effect on DCVG indications than the size of the
coating defect. Ifthe soil resistance is very low thenthe DCVG may be not reach the ground level and then
the defectcan't be detected
-DCVG indications increase by increasing signal strength This is explained by increasing of current
entering the pipeline at the coating defect. in some cases when signal strength is weak we can detect
the defect.
-DCVG indications decrease with increasing depth of soil cover above coating defect. (pipe diameter
playing role in this parameter) and if the depth of soil increased above certain limit may be defect cant
be detected
II- Sizing of coating defect , classification of coating defect severityand decision for coating
defect repair .
-Percent-IR values increased with both increasing coating defect size and soil resistivity. From these
results it was found that the evaluation of percent-IR values should not exclude consideration of soil
resistivity. For example, a high soil resistivity could cause a small defect to yield a large percent-IR
value leading to unnecessary excavation. Also, most of the large coating defects had low percent-IR
values that under normal evaluations would be as minimal threat to potential corrosion failure. This
further proves that percent-IR values are not enough to make a decision on whether a defect is
severe or not. The level of soil resistivity among other factors must also be incorporated into this
assessment.
DCVG indications do not give enough information alone to make assessments about coating defect
size. No conclusion or assessment about the size of the coating defect can be made using the %IR .
this study showed that by knowing more about other parameters of the system (such as soil
resistivity, depth of soil cover, and pipe diameter), a quantitative relationship can be made between
DCVG indications and coating defect size. Overall
III- Effectiveness of cathodic protection system based on coupon off- potential evaluation
based on the coupon off- potential conventional study ,level of CP which can protect the coupon will
protect all other defects with same or less area

.
Figure X. coupon performance diagram for coupons placed in the same soil environment as the
pipeline coating defect that expose the bare steel . the soil resistivity was 100,000 ohms-cm black
symbols correspond to calculations where a protected coating holiday .open symbols corresponded
to pipeline with coating holidays that were seriously under protected gray symbols correspond to
cases where the coupon reading was not conservative ,but the error was less than 15 mV(5)
so if we going to depend on coupon off- potential we must assure that the maximum coating defect
size must be equal or less than coupon area which can't be identified by DCVG coating defect sizing
calculation as the calculations error vary more than the coupon size.
REFERENCES
1. NACE TM0109-2009,“Standard Practice for Aboveground Survey technique for the Evaluation
of Underground Pipeline Coating Condition” (Houston,TX: NACE, 2009).23
2- EVALUATION OF ABOVE-GROUND POTENTIAL MEASUREMENTS FOR ASSESSING PIPELINE INTEGRITY By
JAMES PATRICK MCKINNEY UNIVERSITY OF FLORIDA 2006
3- J. Wagner, Cathodic Protection Design I, NACE International,Houston,TX, 1994.
4-NACE CP4 course manual
5-Application of boundaryelementmodels to predicteffectiveness ofcoupons for accessing cathodic protection ofburied
structure (DP Riemer and M.E Orazem)

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