Catodic Design

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INTERNATIONAL JOURNAL OfACADEMIC RESEARCH Vol. 3. No. 1. January, 2011, Part I

DESIGN AND INSTALLATION OF A CATHODIC PROTECTIONSYSTEM USING MAGNESIUM ANODE

J. E. O. Ovri*, Michael Iroh

Federal University of Technology, Owerri (NIGERIA)*Corresponding author: [email protected]

ABSTRACT

Design of a Cathodic Protection System for a mild steel pipe using sacrificial magnesium anode wasinvestigated. The soil resistivity was determined using the wenner-four pin method and this was found to be2000cm. The pipe was externally coated with polytape wrapping and a protecting current of 0.019mA corres-ponding to a current density of 0.02mA/m

2above the design requirement of 0.01132mA and 0.01mA/m

2

respectively were obtained. A protecting potential of -1100mV was obtained which is in the range (-850 to -1150)mV for protected mild steel confirmed that the pipe was cathodically protected.

Key words: Cathodic protection system, mild steel, soil resistivity, magnesium anode, polytape wrappings.

1. INTRODUCTION

Cathodic protection employs the principle of delivering electrical current to metal in a corroding medium tosufficiently oppose corrosion current.

1.1SurveyThe application of cathodic protection can be traced back to 1824 when Sir Humphrey Davy in a project

financed by the British Navy succeeded in preventing the corrosion of copper sheating of the wooden hulls ofBritish Navy ships

(1).

The cathodic protection effectiveness at protecting steel in soils has been demonstrated in the early 1940s,when it was applied to an old natural gas piping network that had been developing leaks at a rapidly increasing rateand consequently abandonment was seriously considered. A reduction in the number of leaks was obtainedimmediately a cathodic protection was introduced

(1).

Modern specifications for the cathodic protection of Active Ocean going ships were first described in 1950,since that time progress has been rapid. Considerable advances in cathodic protection technology have beenmade. Better sacrificial anode metals have been developed and currents for the use of controlled applied currentsystems using inert anode have been perfected

(1).

1.2 CoatingOne of the best practices in corrosion technologies involves applying coatings

(2). Coating isolates the pipe

from the surrounding but unfortunately these coatings are almost never perfect. Defects or discontinuities such asholidays and foreign objects are often present. With time and with age these coatings may deteriorate

(3). This is in

line with the argument of NACE standard RP 0169 2002 that coatings proven to be non shielding and allowspassage of protective cathodic protection current should be used for example FBE, polyguard RD 6 ethylene

(4).

The conjoint application of coatings and cathodic protection takes advantage of the most attractive featureof each of them as a method of effective corrosion control.

Thus 90% of the protection is provided by the coating which forms the bulk of the protection whilst 10%protection is provided by cathodic protection. As the coating degrades with time, the activity of the cathodicprotection system develops to protect the deficiency in the coating.

1.3 Principles of application

Cathodic protection is applied by one of two methods. They are impressed current and by sacrificial anode.The former involves the use of an external power supply usually a rectifier which converts ordinary alternatingcurrent (ac) power to direct current (dc). The protected structure is made electrically negative so that it acts as acathode. A second electrode is made electrically positive and completes the circuit as an auxiliary anode.

The second method involves the connection of dissimilar metals (Galvanic) in an electrolyte. Current flowsbetween the two because of dissimilar electrode potentials. The metal with the more electropositive (noble)potential becomes the cathode and is protected from corroding by the more electronegative (active) metal whichcorrodes.

Different metals have different electrical potential and their relative positions in most aqueous environmentsis as follows: magnesium (mg) > zinc (zn) > Aluminium (Al) > Iron (Fe) > Steel (mild) > Tin (sn) > Lead (pb) > Brass(cu-zn) > coppers > Silver (Ag) > Gold (Au).

Considering that magnesium, Aluminium and Zinc are negative to mild steel, hence they are considered foruse (choice) against mild steel.

Theoretically, the value of the soil resistivity determines the final selection process. Since the more resistivethe soil the less its conductivity and hence the need for more negative electrode potential of anode. Magnesium

anode is selected at soil resistivity above 15000cm(5).


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1.4 Protection criteriaThe standard against which the anode potential is related plays important role in determining the actual

potential required to give optimum protection to the pipe. Hence this work made use of cu/cuso4 half cell as thereference electrode.

The protection potential fall within this range.Minimum protection potential = -850mV.Limit prior to over protection = -1150mV.

1.5 Objective of the work

The commonest problems encountered in cathodic protection are:

1. availability of limited voltage which makes it difficult to achieve desired consistent protection current and2. the situation of obtaining higher protection potential which can at a long run disband the coating material,

since most of its application as seen from the previous survey involved large structures. Hence the work is aimed aalleviating this condition, by

1. Ensuring efficient cathodic protection current distribution through proper design for regulating resistancesof the electrical current and avoiding the practice of exposing some part of the pipe in order to reduce theprotection potential.

2. Designing a means of applying cathodic protection to high resistive electrolyte as experienced in thiswork.

2. EXPERIMENTALS

Available parameters, environmental conditions and engineering factors (safety, utilization) etc were put intoproper consideration.

Procedure: The pipe was polished to silver mirror finish prior to the start of test to remove all traces ofexisting corrosion products and specifically make it more prone to corrosion if not adequately protected. Coatingwas manually applied in a manner that ensures effective adhesion to the pipe and devoid of voids, wrinkles,holidays and gas entrapment in line with ASME code for pressure piping

(6). Connecting cables were attached to

the pipe surface tightly and well insulated for effective current supply as well as for pipe to soil potentialmeasurement. The soil was excavated to the required depth of 1 meter and the pipe was installed along with anodeand are connected with the cable. The cu/cuso4 cell was wetted and put in place on wet ground. The designs andset are described in the following sections:

2.1Pipe geometry

i. Length (L) = 3 meters (m)ii. External Diameter (D) = 100mm = 0.1m

iii. Total surface Area Sa = DL

where = 3.142 D = 0.1mL = 3m

sa = 3.142 x 0.1 x 3 = 0.943m2

iv. current density (i). for all coated steel in soil, current required for adequate cathodic protection based onsuperficial area (MA/m

2) ranges from 0.01 to 0.2 (mA/M

2)

(5). Current density chosen for this investigation =

0.01MA/m2.

v. Total current required to protect the pipe

I = Sa x i x fWhere Sa = 0.943f = safety factor = 0.4i = current density = 0.01mA/m

2

hence I = 0.943 x 0.01 x 0.4I = 0.01132MA

vi. Cross Sectional area of pipe

A = r2

= (R t)2

Where R = external Radius = 0.5mt = pipe thickness = 0.01mA = 3.42 (0.05 x 0.01)

2

= 0.005m2

vii. Linear resistance of pipeRs = es x L

AA = 0.005m

2


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es Resistivity of steel = 1.8 x 10-7m

L = 3mRs = 1.8 x 10

-7x 3

0.005

= 1.08 x 10-4

viii. Coating Leakage Resistance of pipeRL = RC

DL whereRC = Coating resistance = 15,000m

2

RL = 15,000

3.142 x 3 x 0.1 = 15.9 x 103

ix. Attenuation factor

Af= RS0.5

where RS = Linear Resistance of pipe = 1.0 x 10-4 and RL =

RL Coating Leakage Resistance of pipe = 15.9 x 103

Af= 1.08 x 10-4 0.5

15.9 x10

3

Af= 1.06 x 10-4

x. Pipe characteristics resistanceZ = (RS x RL)0.5

Z = (1.06 x 10-4

) x (15.9 x 103)

= 1.3

xi. Designed life of pipe Y = 40 years

2.2 Anode design

i. The total current (Ia) required by the anodeIa = Sa x i x SfWhere Sa = surface area of pipe = 0.943m

2

i = Current density = 0.01mA/m2Sf= Safety factor = 1.5 henceIa = 0.943 x 0.01 x 1.5

= 0.014MA.

ii. Required anode composition shown in Table 1 below.

Table 1. Anode composition

Element Composition

Cu 0.02 max

Al 0.01 max

Si 0.05 max

Fe 0.03

Mn 0.5 1.3

Ni 0.001Others 0.05

Mg remainder

(iv) Calculation of desired anode weight

= Where = total anode current = 0.014 mA = anode consumption rate = 7.9 kg/Ayr = designed life of pipe = 40 years = utilization factor = 0.8

= 0.014 7.9 400.8 = 5.3(v) Minimum anode package weight = 7.5kg hence number of required anode = 1.


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(vi) Anode efficiency = 50%(vii) Resistant of anode to earth

= 2 2.38 1 + = Soil resistivity from Table 4 = 2000 = 20mL = Length of anode = 0.25m

d = Equivalent diameter of anode = 2 where a and b are anode dimensionsa = 0.075m

b = 0.25m

= 20.750.253.142 = 0.167 = Coating Resistance = 15,000 = 200023.1420.25 2.380.250.167 1+ 15000 = 12.73(2.3log(11.98) 1) +15000 = 18.846 + 15000 = 15018.852.3 CableThe cables used were single core stranded copper cable of size 6 with consistent size to avoid voltage

drops.

2.4 InstrumentsThe instruments were calibrated to an accuracy of 0.5% of full scale detection. They were portable and

sufficiently robustly constructed to withstand rough handling and to be capable of operating reliably under a widerange of climatic conditions. The instruments are half cell, multimeter for current, voltage and resistancereading, Rheostat for varying resistances, and Resistivity meter for testing soil resistivity.

2.5 Test postThis is the point along the pipe where potential measurements are taken. The pipe is exposed at this point.

The test post is of length = 1.5 = 150 = 15 = 0.4 Height above buried pipe = 50 cm. A bolt passes through the test pole at a distance of 0.25m from the top.

= 7The bolt is supported by insulating shelve of length 30mm and two washers. This will stop the bolt frommaking contact with post wall. Also a nut is added to secure the internal cable. The general design set up is shownin Fig. 1.

Fig. 1. Set up of work

3. RESULTS

Table 2. Soil physical properties result

Properties Unit of measurement ValueMoisture content % 52.5

Specific gravity - 2.61


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Table 3. Soil chemical properties result

Property Unit of measurement Value

pH - 4.4

Sulphate wt% 18.65Carbonate wt% 0.22Chloride wt% 1.82

3.1 Soil resistivity result

Using Wenner four pin method the ohmic relationship was employed to obtain the resistance of eachmeasurement of current and voltage =

= where V = Voltage = Current, R = ResistanceThe resistance multiplied by the factor of 191.5 and depth distance gives the resistivity result in ().

Table 4. Resistivity result

S/N Spacing (ft) R() Factor Depth Resistivity ()1 4 2.61 191.5 0 4 2000

2 6 2.82 191.5 0 6 3240

3 8 2.00 191.5 0 8 4596

Plot of Resistivity against Depth is shown in Fig. 2

Fig. 2. Plot of Resistivity versus Depth

Table 5. Potential along the pipe

Distance (mm) Value (-mv)

0 1000

1000 1100

2000 1100

3000 950

Current density obtained = 0.02mA/m2Pipe protection current = 0.019mA


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Plot potential variation along the pipe is shown in Fig 3

Fig. 3. Plot of Potential along the pipe

4.DISCUSSION

The soil resistivity result falls in line with the argument of Francis(7)

that resistivity increases with depthshowing low corrositivity. The corrosivity varies inversely to the resistivity of the electrolyte hence satisfying thesuitability of application of cathodic protection by sacrificial anode method particularly using magnesium anodewhich is a highly reactive anode.

4.1Potential distributionThe result shows that the soil is homogenous and this enhanced uniform potential distribution as can be

seen from the result in table 7 and figure 3 showing that the variation of potential is within the limit of optimumprotection.

The protection potential obtained (-1100mv) being a little above the optimum (-850mv) for optimumprotection is desirable considering the fact that though the resistivity of the soil is high (low corrosivity), there is thetendency of sulphate induced bacteria corrosion as can be seen from the result in (table 3). This higher percentage

of sulphate can instigate local corrosion and cause coating deterioration through some microbial actions.Furthermore, table 2 shows high moisture content of the solid which can also decrease the resistance of

the coating to allow for high demand for current according to ohmic law. This can lead to drop in the protectionpotential but nonetheless will be within the limit of optimum protection.

4.2 Protection currentThe demand for optimum protection current according to design (sec 2.1) is 0.01132MA while the value

obtained exceeds the value to reach up to 0.019MA. The current density designed with was 0.01MA/m2

and thevalue obtained was 0.02MA/m

2. This was as a result of the introduction of safety and utilization factors to give room

for safety and utilization since some reaction in the soil can induce increase in corrosion rate, which can cause thevalue to fall below optimum value prior to the end of design life of pipe. Again coating may degrade and expose thepipe leading to simultaneous increase in current demand.

5. CONCLUSION

The cathodic protection of mild steel pipe as investigated in this work gave the results -1100m (protectionpotential). 0.019MA pipe protection current and current density of 0.02MA/M

2above values of (-850mv),

0.01132MA and 0.01MA/m2

optimum protecting voltage, desired current and current density respectively. Thissatisfies the objective of protecting small steel pipe effectively at optimum cost.

REFERENCES

1. Proceedings of the Royal Society Corrosion of Copper Sheating by seawater and on methods ofpreventing this effect 114 (1824) PP 151 246 and 115 (1825) PP 328 316.

2. Cuvan Edward Corrosion Control in Gas Pipeline; Coating protection pipeline and gas Journal (Oct2007)

3. Okorafor Chigbu: Cathodic protection as a means of saving National asset; Journal of CorrosionScience and Technology 1.1, 2004.

4. NAEC Standard RP 0169 2002 Control of External Corrosion on Underground or Submergedmetallic piping systems.

5. Sherir L. L. Corrosion: Vol. 2 Newness Butterworth.6. ASME Code for pressure piping gas transmission and distribution piping system.


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7. Francis P. E. (1981) Cathodic protection Arnold Publishers (London).8. Moore J. J. (1990) Chemical Metallurgy (2Ed.) Elsevier Science and Technology book.9. Bushman J. B. Galvanic Anode Cathodic Protection system design: Bushman and associates

incorporated (Ohio).

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