External Corrosion
Contents
Fundamentals of Cathodic Protection
Carbon steel structures exposed to natural waters generally corrode at an unacceptably high rate unless preventive measures are taken. Corrosion can be reduced or prevented by providing a direct current through the electrolyte to the structure. This method is called cathodic protection The basic concept of cathodic protection is that the electric al potential of the subject metal is reduced below its corrosion potential, such that it will then be incapable of corroding. Cathodic protection results from cathodic polarization of a corroding metal surface to reduce the corrosion rate. The anodic and cathodic reactions for iron corroding in an aerated near neutral electrolyte are: respectively. As a consequence of Equation,the pH of the seawater immediate close to a metal surface increases. This is beneficial because of the precipitation of solid compounds (calcareous deposits) by the reactions:
These deposits decrease the oxygen flux to the steel and, hence, the current necessary for cathodic protection. As a result, the service life of the entire cathodic protection system is extended. Offshore pipelines can be protected as a cathode by achieving a potential of –0.80 VAg/AgCl or more negative, which is accepted as the protective potential (Ec o) for carbon steel and low-alloy steel in aerated water.
- Wasted resources;
- Possible damage to any coatings;
- The possibility of hydrogen embrittlement.
Cathodic protection systems are of two types: impressed current and galvanic anode. The latter has been widely used in the oil and gas industry for offshore platforms and marine pipeline in the past 40 years because of its reliability and relatively low cost of installation and operation. The effectiveness of cathodic protection systems allows carbon steel, which has little natural corrosion resistance, to be used in such corrosive environments as seawater, acid soils, and salt-laden concrete.
External Coatings
Oil and gas pipelines are protected by the combined use of coatings and cathodic protection. The coating systems are the primary barrier against corrosion and, therefore, are highly efficient at reducing the current demand for cathodic protection. However, they are not feasible for supplying sufficient electrical current to protect a bare pipeline. Cathodic
protection prevents corrosion at areas of coating breakdown by supplying electrons.
Thick coatings are often applied to offshore pipelines to minimize the holidays and defects and to resist damage by handling during transport and installation. High electrical resistivity retained over long periods is a special requirement, because cathodic protection is universally used in conjunction with coatings for corrosion control. Coatings must have good adhesion to the pipe surface to resist disbondment and degradation by biological organisms, which abound in seawater. Pipe coating should be inspected both visually and by a holiday detector set at the proper voltage before the pipe is lowered into the water. Periodic inspection of the pipeline cathodic protection potential is used to identify the coating breakdown areas. Coatings are selected based on the design temperature and cost. The principal coatings, in rough order of cost, are:
- Tape wrap;
- Asphalt;
- Coal tar enamel;
- Fusion bonded epoxy (FBE);
- Cigarette wrap polyethylene (PE);
- Extruded thermoplastic PE and polypropylene (PP).
The most commonly used external coating for offshore pipeline is the fusion bonded epoxy (FBE) coating. FBE coatings are thin-film coatings, 0.5 to 0.6 mm thick. They consist of thermosetting powders that are applied to a white metal blast-cleaned surface by electrostatic spray. The powder will melt on the preheated pipe (around 230 C), flow, and subsequently cure to form thicknesses of between 250 and 650 mm. The FBE coating can be used in conjunction with a concrete weight coating. The other coating that can be used with a concrete coating is coal tar enamel, which is used with lower product temperatures.
The external coating can be dual layer or triple layer. Dual-layer FBE coatings are used when additional protection is required for the outer layer, such as protection from high temperatures or abrasion resistance. For deepwater flowlines the high temperature of the internal fluid dissipates rapidly, reaching ambient within a few miles. Therefore, the need for such
coatings is limited for steel catenary riser (SCRS) at the touchdown area where abrasion is high and an additional coating with high abrasion resistance is used. Triple-layer PP coating consists of an epoxy or FBE, a thermoplastic adhesive coating, and a PP top coat. The PE and PP coatings are extruded coatings. These coatings are used for additional
protection against corrosion and are commonly used for dynamic systems such as steel catenary risers and where the temperature of the internal fluid is high. These pipe coatings are also frequently used in reel pipelines.
The field joint coating for the three-layer systems is more difficult to apply and takes a longer time. However, in Europe, PE and PP coatings are preferred because of their high dielectric strength, water tightness, thickness, and very low CP current requirement.
Cathodic Protection
Cathodic protection is a method by which corrosion of the parent metal is prevented. For offshore pipelines, the galvanic anode system is generally used. This section specifies parameters to be applied in the design of cathodic protection systems based on sacrificial anodes.
Design Life
The design life tr of the pipeline cathodic protection system is to be specified by the operator and shall cover the period from installation to the end of pipeline operation. It is normal practice to apply the same anode design life as for the offshore structures and submarine pipelines to be protected because maintenance and repair of CP systems are very costly.
Current Density
Current density refers to the cathodic protection current per unit of bare metal surface area of the pipeline. The initial and final current densities, ic (initial) and ic (final), give a measure of the anticipated cathodic current density demands to achieve cathodic protection of bare metal surfaces. They are used to calculate the initial and final current demands that determine the number and sizing of an anode. The initial design current density is necessarily higher than the average final current density because the calcareous deposits developed during the initial phase reduce the current demand. In the final phase, the developed marine growth and calcareous layers on the metal surface will reduce the current demand.
However, the final design current density should take into account the additional current demand to repolarize the structure if such layers are damaged. The final design current density is lower than the initial density. The average (or maintenance) design current density is a measure of the anticipated cathodic current density, once the cathodic protection system has attained its steady-state protection potential. This will simply imply a lower driving voltage, and the average design current density is therefore lower than both the initial and final design value.
Coating Breakdown Factor
The coating breakdown factor describes the extent of current density reduction due to the application of a coating. The value fc ¼ 0 means the coating is 100% electrically insulating, whereas a value of fc ¼ 1 implies that the coating cannot provide any protection.
The coating breakdown factor is a function of coating properties, operational parameters and time. The coating breakdown factor fc can be described as follows:
where t is the coating lifetime, and k1 and k2 are constants that are dependent on the coating properties. Four paint coating categories have been defined for practical use based on the coating properties in DNV:
Category I: one layer of primer coat, about 50-mm nominal dry film thickness (DFT);
Category II: one layer of primer coat, plus minimum one layer of intermediate top coat, 150- to 250-mm nominal DFT;
Category III: one layer of primer coat, plus minimum two layers of intermediate/top coats, minimum 300-mm nominal DFT;
Category IV: one layer of primer coat, plus minimum three layers of intermediate top coats, minimum 450-mm nominal DFT.
For cathodic protection design purposes, the average and final coating breakdown factors are calculated by introducing the design life tr:
Anode Material Performance
The performance of a sacrificial anode material is dependent on its actual chemical composition. The most commonly used anode materials are Al and Zn.
Resistivity
The salinity and temperature of seawater have an influence on its resistivity. In the open sea, salinity does not vary significantly, so temperature becomes the main factor. The resistivities of 0.3 and 1.5 U$m are recommended to calculate anode resistance in seawater and marine sediments, respectively, when the temperature of surface water is between 7 and 12 C.
Anode Utilization Factor
The anode utilization factor indicates the fraction of anode material that is assumed to provide a cathodic protection current. Performance becomes unpredictable when the anode is consumed beyond a mass indicated by the utilization factor. The utilization factor of an anode is dependent on the detailed anode design, in particular, the dimensions and location of anode cores.
Galvanic Anode System Design
The bracelets are clamped or welded to the pipe joints after application of the corrosion coating. Stranded connector cables are be used for clamped half-shell anodes. For the anodes mounted on the pipeline with concrete, measures should be taken to avoid the electrical contact between the anode and the concrete reinforcement.
Selection of Anode Type
Normally, bracelet anodes are distributed at equal spacing along the pipeline. Adequate design calculations should demonstrate that anodes can provide the necessary current to the pipeline to meet the current density requirement for the entire design life. Because the installation expense is the main part of CP design, larger anode spacing can reduce the overall cost. However, the potential is not evenly distributed along the pipeline. The pipeline close to the anode has a more negative potential. The potential at the middle point on the pipeline between two anodes is more positive and must be changed or negative in order to achieve cathodic protection for the whole pipeline.
CP Design Practice
Offshore pipeline CP design includes the determination of the current demand Ic, required anode mass M, and number and current output per anode Ia. Because the initial polarization period preceding steady-state conditions is normally quite short compared to the design life, the mean (time-averaged) design current density i comes very close to the steady-state current density. Therefore, it is used to calculate the minimum mass of anode material necessary to maintain cathodic protection throughout the design life.
Anode Spacing Determination
Bethune and Hartt have proposed a new attenuation equation to modify the existing design protocol interrelating the determination of the anode spacing Las, which can be expressed.
This approach makes certain assumptions:
- Total circuit resistance is equal to anode resistance.
- All current enters the pipe at holidays in the coating (bare areas).
- The values of fc and fa are constant with both time and position. The ISO standards recommend that the distance between bracelet anodes not exceed 300 m.
Commonly Used Galvanic Anodes
The major types of galvanic anodes for offshore applications are slender stand-off, elongated flush mounted, and bracelet. The type of anode design to be applied is normally specified by the operator and should take into account various factors, such as anode utilization factor and current output, costs for manufacturing and installation, weight, and drag forces exerted by ocean current. The slender stand-off anode has the highest current output and utilization factor among these commonly used anodes.
Pipeline CP System Retrofit
Cathodic protection system retrofits become necessary as pipeline systems age. An important aspect of such retrofitting is determination of when such action should take place. Assessment of cathodic protection systems is normally performed based on potential measurements. As galvanic anodes waste, their size decreases; this causes a resistance increase and a corresponding decrease in polarization. Models have been constructed for potential change that occurs for a pipeline protected by galvanic bracelet anodes as these deplete. Anode depletion is time dependent in the model.
Bracelet anodes have been used for cathodic protection of marine pipelines, especially during the “early period” when many oil companies had construction activities in the Gulf of Mexico. According to recent survey data, many of these early anode systems have been depleted or are now being depleted. Anodes can be designed as multiples or grouped together to form an anode array (anode sled). Anode arrays typically afford a good spread of protection on a marine structure. They are a good solution for retrofitting old cathodic protection systems.
References
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[2] Norwegian Technology Standards Institution, CO2 Corrosion Rate Calculation Model, NORSOK Standard No. M-506, 2005.
[3] R. Nyborg, Controlling Internal Corrosion in Oil and Gas Pipelines, Oil and Gas Review(Issue 2) (2005).
[4] A. Dugstad, L. Lunde, K. Videm, Parametric Study of CO2 Corrosion of Carbon Steel, Corrosion/94, NACE International, paper no.14, Houston, TX, 1994.
[5] National Association of Corrosion Engineers, Metals for Sulfide Stress Cracking and Stress Corrosion Cracking Resistance in Soul Oilfield Environments, NACE MR 0175 (December 2003).
[6] www.corrosion-doctors.org/Inhibitors/lesson11.htm. [7] D.A. Jones, Principles and Prevention of Corrosion, first ed., McMillan, New York, 1992, pp. 437–445.
[8] E.D. Sunde, Earth Conduction Effects in Transaction Systems, Dover Publishing, New York, 1968, pp. 70–73.
[9] Det Norsk Veritas, Cathodic Protection Design, DNV RP B401, 1993.
[10] W.H. Hartt, X. Zhang,W. Chu, Issues Associated with Expiration of Galvanic Anodes on Marine Structures, Paper 04093, Corrosion (2004).
[11] K. Bethune, W.H. Hartt, A Novel Approach to Cathodic Protection Design for Marine Pipelines: Part II -Applicability of the Slope Parameter Method, presented at Corrosion, paper no.00674, 2000.
[12] International Organization for Standardization, Pipeline Cathodic Protection-Part 2:Cathodic Protection of Offshore Pipelines, ISO/TC 67/SC 2 NP 14489, Washington, D.C, 1993.
[13] DeepStar, Flow Assurance Design Guideline, 2001.
