1. Corrosion
One general definition of corrosion is the degradation of a material through environmental interaction. This definition encompasses all materials, both naturally occurring and man-made and includes plastics, ceramics, and metals. A significant amount of energy is put into a metal when it is extracted from its ores, placing it in a high-energy state. These ores are typically oxides of the metal such as hematite (Fe2O3) for steel or bauxite (Al2O3·H2O) for aluminum. 1
One principle of thermodynamics is that a material always seeks the lowest energy state. In other words, most metals are thermodynamically unstable and will tend to seek a lower energy state, which is an oxide or some other compound. The process by which metals convert to the lower-energy oxides is called corrosion. Corrosion of most common engineering materials at near-ambient temperatures occurs in aqueous (water-containing) environments and is electrochemical in nature. 1
Electrons (oxidation) of the metal and the consumption of those electrons by some other reduction reaction, such as oxygen or water reduction. The oxidation reaction is commonly called the anodic reaction and the reduction reaction is called the cathodic reaction. Both electrochemical reactions are necessary for corrosion to occur. The oxidation reaction causes the actual metal loss but the reduction reaction must be present to consume the electrons liberated by the oxidation reaction, maintaining charge neutrality. Otherwise, a large negative charge would rapidly develop between the metal and the electrolyte and the corrosion process would cease. 1
The oxidation and reduction reactions are sometimes referred to as half-cell reactions and can occur locally (at the same site on the metal) or can be physically separated. When the electrochemical reactions are physically separated, the process is referred to as a differential corrosion cell. A schematic of a differential corrosion cell is given in Figure 1.1. The site where the metal is being oxidized is referred to as the anode or anodic site. At this site, direct electric current (defined as a positive flow of charge) flows from the metal surface into the electrolyte as the metal ions leave the surface. This current flows in the electrolyte to the site where oxygen, water, or some other species is being reduced. This site is referred to as the cathode or cathodic site. There are four necessary components of a differential corrosion cell. 1
- There must be an anode.
- There must be a cathode.
- There must be a metallic path electrically connecting the anode and cathode. (Normally, This will be the pipeline itself).
- The anode and cathode must be immersed in an electrically conductive electrolyte (normally, moist soil).
Underground corrosion of pipelines and other structures is often the result of differential corrosion cells of which a variety of different types exist. These include differential aeration cells, where different parts of a pipe are exposed to different oxygen concentrations in the soil, and cells created by differences in the nature of the pipe surface or the soil chemistry. Galvanic corrosion is a form of differential cell corrosion in which two different metals are electrically coupled and exposed to a corrosive environment. 1
2. Corrosion Mitigate
The principal methods for mitigating corrosion on underground pipelines are coatings and cathodic protection (CP). Coatings normally are intended to form a continuous film of electrically insulating material over the metallic surface to be protected. The function of such a coating is to isolate the metal from direct contact with the surrounding electrolyte (preventing the electrolyte from contacting the metal) and to interpose such a high electrical resistance that the electrochemical reactions cannot readily occur. In reality, all coatings, regardless of overall quality, contain holes, referred to as holidays, that are formed during the application, or during transport or installation of mill-coated pipe. 1
Holidays in coatings also develop in service as a result of degradation of the coating, soil stresses, or movement of the pipe in the ground. Degradation of the coating in service also can lead to disbanding from the pipe surface, further exposing the metal to the underground environment. A high corrosion rate at a holiday or within a disbonded region can result in a leak or rupture, even where the coating effectively protects a high percentage of the pipe surface. Thus, coatings are rarely used on underground pipelines in the absence of CP. The primary function of a coating on a cathodically protected pipe is to reduce the surface area of exposed metal on the pipeline, thereby reducing the current necessary to cathodically protect the metal. 1
One definition of CP is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell. This is accomplished by shifting the potential of the metal in the negative direction by the use of an external power source (referred to as impressed current CP) or by utilizing a sacrificial anode. In the case of an impressed current system, a current is impressed on the structure by means of a power supply, referred to as a rectifier, and an anode buried in the ground. In the case of a sacrificial anode system, the galvanic relationship between a sacrificial anode material, such as zinc or magnesium, and the pipe steel is used to supply the required CP current. 1
3. Impressed Current Cathodic Protection
CP is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell. In each case, anodic areas and cathodic areas are present on the pipe surface. At the anodic areas, current flows from the pipeline steel into the surrounding electrolyte (soil or water) and the pipeline corrodes. At the cathodic areas, current flows from the electrolyte onto the pipe surface and the rate of corrosion is reduced. In light of the above, it becomes obvious that the rate of corrosion could be reduced if every bit of exposed metal on the surface of a pipeline could be made to collect current. 1
4. Criteria for Cathodic Protection
Although the basic theory of CP is simple (impressing DC on a structure to reduce the corrosion rate), the obvious question that arises is: How do we know when we have attained adequate protection on a buried structure? The answer to this question is that various criteria have been developed over the years that permit a determination of whether adequate protection is being achieved. Those criteria in more common usage involve measuring the potential between the pipeline and earth. The measurement permits a rapid and reliable determination of the degree of protection attained. Basically, potential criteria are used to evaluate the changes in structure potential with respect to the environment that are caused by CP current flowing to the structure from the surrounding soil or water. potential of a pipeline at a given location is commonly referred to as the pipeto-soil potential. The pipe-to-soil potential can be measured by measuring the voltage between the pipeline and a reference electrode placed in the soil directly over the pipeline. 1
The most common reference electrode used for this purpose is a copper-copper sulfate reference electrode, which is commonly given the acronym CSE. The potential is referred to as an on potential if the measurement is made with the CP system energized. The off or instant off potential estimates the polarized potential when the measurement is made within one second after simultaneously interrupting the current output from all CP current sources and any other current sources affecting that portion of the pipeline. 1
5. Selection of Type, Size and Spacing of Cathodic Protection System
Some of the questions to be resolved when planning a pipeline CP system include the following:
- Shall galvanic anodes be used or would an impressed current system be a betterchoice?
- How much total current will be required to attain adequate CP?
- What should be the spacing between installations, and what will be the current output
required from each installation? - What provisions should be made to permit testing the completed installation?
- Are there special conditions at certain locations that will require modifications in the general plan for CP?
These questions cannot be answered using only material covered up to this point. The needed information that will influence the decision includes such items as:
- The corrosivity of the environment;
- The soil structure and resistivity;
- Whether the pipeline is bare or coated;
- If coated, the quality and electrical strength of the coating and the presence of environmental conditions that may cause the coating to deteriorate;
- The metal or alloy used in the pipeline;
- The size of the pipeline and its ability to conduct CP current;
- The presence of metallic structures from other resources (usually termed foreign structures) crossing or close to the pipeline to be protected;
- The presence of stray current from man-made or natural sources. 1
7. Example Cathodic Protection Calculation for Pipeline
There is a long underground pipeline that shall be protected by cathodic protection in this project, therefore the calculation cathodic protection will be divided into two (2) section area, south and north section area. The north section area will be calculated by two (2) segment and south area will be divided into six (6) segment covered protection area of pipelines. Based on the availability for electrical power source and the segment 7 of pipeline covered area protection, the Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection will be calculated.
7.1 General Parameter
The general parameter to calculate the cathodic protection system shall be based on the following parameters:
| Design life time | : | 20 years for permanent system and 2 years for temporary system |
| Current density for bare steel (at 30° C) | : | 20 mA/sq.m (for 30° C operating temperature) |
| Pipe coating breakdown | : | 0.7% (for 20 years coating type 3LPE)0.16% (for 2 years/temporary)100% (for 20 years bare pipe casing) |
| Soil resistivity | : | 2,473 ohm.cm (Groundbed 1) |
| Minimum protective potential | : | – 0.85 volt against Cu/CuSO4 |
| Anode material | : | Titanium/Mixed, Metal/Oxide Anode (Ti/MMO) for ICCP and High Potential Magnesium Anode for SACP |
| Maximum anode current output | : | 8 Ampere/ea |
| Design anode current output | : | 4 Ampere/ea |
| Maximum current density | : | 100 A/m² ( at soil + coke breeze) |
| Anode size | : | 25.4 (dia) mm x 500 (length) mm/ea |
| Anode type | : | string of 5 tubular anode |
| Groundbed configuration | : | Semi deep well, diameter 10 inch x overall depth 29.5m. |
| Transformer rectifier safety factor | : | 1.25 |
| Max DC voltage | : | 60 V DC |
| Pipeline Length | : | 47,322 m |
7.2 Design Calculation of Impressed Current Cathodic Protection
7.2.1 Surface Area
Bangko – Batang pipeline area to be protected by ICCP #1 is calculated as follow:
Ap = π x OD x Lp (underground pipeline Area)
Where :
Ap1 = Area to be protected of Segment 7 Bangko – Batang (m²)
Ap2 = Area to be protected of Segment 8 Balam – Bangko (m²)
Ap3 = Area to be protected of Segment 9 Benar- Bangko (m²)
OD = outside diameter of pipe (m), conversion inch to meter (OD x 0.0254)
Lp = Length of pipe (m)
7.2.2 Required Protective Current
The current requirement for protecting the object is calculated as follow :
CDC = CBx CDB x (1 + IT x (T-30°)/10)
Where:
Ip1 = Protective current requirement of Segment 7 (Amp)
Ip2 = Protective current requirement of Segment 8 (Amp)
Ip3 = Protective current requirement of Segment 9 (Amp)
A = Area to protected (m²)
iT = Bare steel current density at >30⁰ C (25% per 10 ⁰C)
(ISO15589-1) (Amp/m2)
T1 = Normal pipe Operating Temperature of Segment 7 (°C)
T2 = Normal pipe Operating Temperature of Segment 8 (°C)
T3 = Normal pipe Operating Temperature of Segment 9 (°C)
CDC1 = Coated steel current density at Tp ⁰ C of Segment 7 (mA/ m²)
CDC2 = Coated steel current density at Tp ⁰ C of Segment 8 (mA/ m²)
CDC3 = Coated steel current density at Tp ⁰ C of Segment 9 (mA/ m²)
CDB = Bare steel current density at 30° (mA/ m²)
CB = Estimate final Coating Break down (0.7%)
7.2.3 Number of Anode
The quantity of anode is calculated based on total required protective current and the designed individual anode current output as per-following formula:
N = IP x CSf / Ia
Where :
N = Number of anode (pcs)
IP = Total protective current required (A)
CSf = Current safety factor (1.15)
Ia = Anode current output (A)
7.2.4 Groundbed Resistance
Anode ground bed is planned to be installed in deep well arrangement. The single vertical ground bed resistance can be calculated by the following formula:
7.2.5 Cable Resistance
The wiring resistance of cable shall be calculated by the following formula :
Where:
Rc = Wiring resistance of cable (Ω)
17.2 = Copper (annealed) resistance (μΩ.mm)
Lc = Length of cable (mm)
Ac = Cable cross section (mm)
p-n = 1) for cable rectifier (-) to Anode Junction Box (Main Positive) and cable from rectifier (-) to Negative Junction Box (Main Negative)
7.2.6 Voltage Drop
Cable voltage drop is calculated based on cable resistance and designed current flowing through the cable circuit as per following:
Where:
Vg = Total voltage drop (volt)
Rg = Single groundbed resistance (Ω)
IP = Total protective current required (A)
Ng = Groundbed quantity (ea)
7.2.7 Required Voltage for Rectifier
The required DC voltage for rectifier panel is calculated as follows:
V = VT x SF
VT = Vg + Vcp + Vcn + Vb
Vcp = Ip x Rcp / Ncp
Vcn = Ip x Rcn / Ncn
Where:
V = Required DC voltage rectifier (V)
SF = TR safety factor
VT = Total voltage drop (V)
Vcp = Positive cable voltage drop (V)
Vcn = Negative cable voltage drop (V)
IP = Total protective current required (A)
Emf = Back electro motive force (V)
Rcp = Positive cable resistance (Ω)
Rcn = Negative cable resistance (Ω)
Ncp = Number of main positive cable (Ea)
Ncn = Number of main negative cable (Ea)
7.2.8 Required Current for Rectifier
The required DC Current for rectifier panel is calculated as follows:
I = Ip x SF
Where:
I = Required DC current rectifier (A)
Ip = Total protective current required (A)
SF = Safety factor (%)
7.2.9 Attenuation Calculation (Potential Distribution)
For example calculation will be taken for Segment 7 (Bangko – Batang) only.
7.2.9.1 Pipe Resistance
Calculation of pipe’s potential distribution (attenuation) are carried out by the following formulas:
T (meter) = T (inch) x 0.0254
OD (meter) = OB (inch) x 0.0254
Where:
Rp = Pipe resistance (Ω/m)
ρ = Linier steel pipe resistance (Ω.m)
T = Pipe wall thickness (m)
OD = Pipe outside diameter (m)
7.2.9.2 Coating Conductance
Where:
G = Coating conductance (Ω-1.m-1)
ODP = Pipe outside diameter (m)
ω = Specific coating resistance (Ω.m2)
7.2.9.3 Attenuation Constant
Where:
α = Attenuation constant (m-1)
RP = Pipe resistance (Ω.m-1)
G = Coating conductance (Ω-1.m-1)
7.2.9.4 Pipe Characteristic Resistance
Where:
RPK = Attenuation constant (m-1)
RP = Pipe resistance (Ω.m-1)
g = Coating conductance (Ω-1.m-1)
7.2.9.5 Pipe Potential Shifting
Where:
En = Pipe’s natural potential (V)
Ea = Pipe’s potential nearest to groundbed (V)
Eb = Pipe’s potential at end point (V)
α = Attenuation constant (m-1)
L = Pipe’s length (m)
7.2.9.6 Required Protective Current
Where:
I0 = Required Protection Current (A)
En = Pipe’s natural potential (V)
Ea = Pipe’s potential nearest to groundbed (V)
rp = Pipe’s characteristic resistance (Ω)
α = Attenuation constant (m-1)
LP = Pipe’s length (m)
7.3 Design Calculation of Temporary Sacrificial Anode Cathodic Protection System
7.3.1 Surface Area
Pipeline Bangko – Batang area to be protected by SACP is calculated as follow:
Ap = π x OD x Lp (underground pipeline Area)
Where :
Ap = Area to be protected (m²)
OD = outside diameter of pipe (m), conversion inch to meter (OD x 0.0254)
Lp = Length of pipe (m). As per 2000 m length
7.3.2 Required Protective Current
The current requirement for protecting the object is calculated as follow:
CDC = CBx CDB x (1 + IT x (T-30°)/10)
Where:
Ip = Protective current requirement (Ampere)
A = Area to protected (m²)
iT = Bare steel current density at >30⁰ C(25% per 10 ⁰C) (ISO15589-1) (Amp/m2)
T = Normal pipe Operating Temperature (°C)
CDC = Coated steel current density at Tp ⁰ C (mA/ m²)
CDB = Bare steel current density at 30° (mA/ m²)
CB = Final Coating Break down (0.0016)
7.3.3 Weight of Anode
Where:
Wt = required anode net weight (kgs)
Ip = required protective current (ampere)
Y = anode design life (2 years)
8760 = conversion factor for year to hour
U = anode utilization factor (0.80)
C = anode current capacity
7.3.4 Number of Anode (By Weight Calculation)
The quantity of anode is calculated based on total weight required and individual anode net weight as per following formula:
Nd = Wt/Wa
Where:
Nag = Number of anodes based on weight calculation (pcs)
Nd = Number of anode (pcs)
Wt = Required weight anode (kgs)
Wa = Unit net weight anode (kgs)
7.3.5 Anode Resistance
Anode resistance (for example KP 2+000) as per DNV RP-B401 page 31 table 10-7 can be calculated by the following formula:
Where:
Ra = Anodes to soil resistance (W)
r = Soil resistivity (9,650 W-cm by soil resistivity survey data)
La = Length of anode with backfill (80 cm)
Da = Diameter of anode with backfill (cm)
7.3.6 Current Output Capacity of Anode
The current output capacity of anode is calculated from the driving voltage between anode and protected pipe, and the anode ground bed resistance as follow:
Ia = Delta V/Ra
Where:
Ia = current output capacity of anode(Amps)
Delta V = driving voltage (-0.85) – (-1.75) (0.85 V)
Ra = anode ground bed resistance (W)
7.3.7 Number of Anode (By Current Output Capacity)
The quantity of anode is also calculated based on total current required and individual anode current output capacity as per following formula:
Nd = Wt / N
Nsf = Nd x SF
Nai = Ip / Ia
Nfs = Nai x SF
Where:
Nd = Quantity of anode based on mass method (pcs)
Wt = Required weight anode (kg)
N = Anode nett weight per unit (kg)
Nsf = Quantity of anode based on mass method with 15% (pcs)
Nag = Proposed quantity of anode by mass method
Nai = Proposed quantity of anode by current output method (pcs)
Ip = Total current required (A)
Ia = Individual anode current output capacity (A)
No = Quantity of anode based on current method (pcs)
Nfs = Quantity of anode based on current method with 15% (pcs)
SF = Safety Factor (15 %)
Based on the above calculation formula, the cathodic protection system required can be seen as attachment below:
