Book 2: Chapter 7

Corrosion Control on Oil Pipelines

Objectives

• After reading the chapter and reviewing the materials presented the students will be able to:

• Understand the nature of corrosion• Identify the corrosion cell• Describe types of corrosion cells• Explain methods of corrosion control• Classify internal corrosion of pipe • Examine corrosion of structures other than line

pipe

Introduction

• Corrosion of the exterior surfaces of line pipe is of major concern.

• Corrosion of the internal pipe surfaces can be extremely serious in some cases.

• Effective control of pipeline corrosion is essential for economic success and maintenance of public confidence in the safety and other socially desirable features of this transportation method.

The Corrosion Cell

• Each of the following conditions is required for corrosion to occur:

• 1. The metal must be in contact with an electrolyte including some water molecules that are broken down into positively charged hydrogen ions (H+) and negatively charged hydroxyl ions (OH-). Usually the soil moisture or water surrounding pipelines is the electrolyte.

• 2. A portion of the wetted surface must be anodic and another portion cathodic.

• 3. There must be an electric potential between the anode and cathode ( very small and usually measured in fractions of a volt).

• 4. The anode and cathode must be connected by a solid conductor such as a metal. The pipe itself is normally this conductor.

• 5. There must be a continuous electrolytic path between anode and cathode. The soil moisture or water in which the pipeline is immersed provides this path.

• When these conditions exist, electric current flows and metal is ionized or dissolved at the anodes.

The Corrosion Cell

• The amount of metal corroded at the anode is directly proportional to the amount of current.

• Each metal has a characteristic electrochemical equivalent. In the case of iron , this equivalent is 20 pounds per ampere per year which means that one ampere of current discharged for one year will dissolve 20 pounds of iron.

• With proper instruments, current flow in both metallic and electrolytic portions of the circuit can be measured and the extent and locations of corrosion can be inferred.

• Anode areas are very small and cathode areas are large. The concentration of metal loss in small areas often results in the formation of deep pits that may penetrate the pipe wall.

Types of Corrosion Cells

• Dissimilar Metal: Dissimilar metal corrosion cells are the simplest kind and are so well known that they have been almost eliminated in modern pipelines.

• They are called galvanic cells from Lugi Galvani who discovered that such cells could be used as batteries to produce useful amounts of electric current.

• A galvanic cell where iron is the cathode and magnesium is the anode immediately suggests a method of preventing corrosion of iron (steel) structures such as pipelines.

Types of Corrosion Cells

• Dissimilar Electrolyte: Differences in the electrolyte in contact with different parts of a single metal can result in formation of a corrosion cell.

• Areas that are anodic because of oxygen deficiencies are likely to have higher concentrations of dissolved salts in the soil-water electrolyte.

• A characteristic of most soils that cause anodic areas to develop on pipelines is lower electrical resistivity than soils that cause development of cathodic areas.

Types of Corrosion Cells

• Dissimilar Surface Condition of Metal: Different parts of a single metal can behave like different metals because of their surface condition.

• The rolling of hot steel into plates for pipe making results in the formation of thin, dense, tightly adhering, iron oxide scale, commonly known as mill scale.

• Upon exposure to the weather or underground, the scale disintegrates slowly in an uneven pattern.

Types of Corrosion Cells

• Stray Current: Cathodic protection of underground structures results in the intentional introduction of direct currents in the ground.

• Following the laws of electricity, these currents choose paths of lowest resistance and sometimes interfere with intended cathodic protection or with corrosion control of other structures.

• Pipelines, especially those joined by welding, offer paths of much lower resistance than the earth.

• Pipe areas where current enters become cathodic, and areas where the current leaves become anodic, and the cell is complete.

• Unlike naturally occurring corrosion cells, voltages of such cells can be quite high and anodes correspondingly more active.

Results of Corrosive Cell Action

• Most clean metals corrode at exceedingly high rates when first exposed to electrolytes.

• The rate decreases with passage of time and in many cases becomes inconsequential before serious or even detectable damage occurs.

Methods of Corrosion Control

• The nature of corrosion immediately suggests two means of its control.

• One is to break the electrical circuit of existing or potential corrosion cells.

• The other is to make all parts of the structure cathodic since metal is not damaged at cathodes.

• Protective Coatings: Covering metal with waterproof, electrically nonconductive coating is an age old method of corrosion control.

• Coatings are a major cost item in pipeline construction usually ranging from 5 to 10 percent of total construction costs.

Line Pipe Exposure

• Coatings are exposed to many elements during the service of the line pipe.• 1. Water: Moisture is nearly always present on buried pipelines.• 2. Uneven pressures: As a result of normal soil action and pipe movement. • 3. Bacteria, molds, fungi: May attack and cause deterioration to coatings.• 4. Capillary action of water: Water intrudes into crevices and causes

separation or disbonding of the coating from the steel under some conditions.• 5. Temperature changes: Corrosion accelerates with increase in temperature.

Low temperatures can cause coating brittleness and spontaneous cracking of coating.

• 6. Solvents: Certain components may be soluble and be leached out during long service periods.

• 7. Absorbent soils: Besides being absorbent to water, soils may be absorbent to other materials that can destroy coatings.

• 8. Mechanical damage: Removing earth around the pipeline may cut or gouge its coating.

Combating Exposure• Pipelines are expected to serve for 20 to 50 years or longer.• 1. Resistance to water: Absorption of water reduces the electrical resistance and hence

the effectiveness of pipe coatings.• 2. Resistance to uneven pressures: Pressure from pipe resting on hard ditch bottoms,

rocks pressing against the pipe may completely displace or penetrate the coating.• 3. Resistance to bacteria, molds, and fungi: Materials that deteriorate by rotting or attack

by bacteria, molds, or fungi are not generally suitable as components of protective coatings.

• 4. Resistance to capillary action of water: Pinholes resulting from application procedures and foreign matter can provide such paths.

• 5. Suitability for service temperature: Knowledge of temperatures to which the pipe coating will be exposed to is necessary to select the proper type of coatings.

• 6. Resistance to solvents: The likelihood of exposure to solvents and rating the property of solvent resistance when selecting coating method must be considered.

• 7. Resistance to absorbent soils: Wrappings of plastic film are recommended for some soils.

• 8. Resistance to mechanical damage: Embedding fibrous material in the moisture barrier increases resistance to mechanical stresses.

Developments in the Coating Field

• Coatings most closely approaching the ideal are the thermosetting and thermoplastic resins applied to new pipe under carefully controlled conditions at coating plants.

• It is accomplished by chemical treatment or pickling to remove scale and shot-blast cleaning.

• Slight roughening of the pipe surface to provide an anchor pattern is required.

• The pipe is heated to appropriate temperature, usually in the 300 to 500 degree F range, and the thermosetting coating is applied by spray or other means.

• The heat causes polymerization to occur within the coating film.

• In the case of thermoplastic resins the coating material is usually applied as a fine powder that melts on contact with the hot pipe and forms a homogenous film.

Test of Coatings in Service

• There are above ground means of determining average electrical resistance of coatings that are a good indicator of coating condition.

• Individual coating breaks and imperfections can also be located and their size estimated.

• Changes in the amount of current required for cathodic protection is the principal and usually sufficient means of evaluating coating condition.

Cathodic Protection

• Corrosion control can be achieved by making all parts of the pipeline cathodic.

• These cells are formed by connecting the negative terminals of direct current power sources to the pipeline and connecting the positive terminals to expendable solid conductors buried in the ground.

• Corrosion control requires continuous expenditures for operation, maintenance, and monitoring of its effects.

Factors Affecting Protection

• Some of the conditions affecting cathodic protection that need to be determined are current requirements, power requirements, and power sources.

• Current Requirements: The amount of current needed varies widely depending upon the type of soil or water in which it is buried. A commonly used requirement is 1 milliampere for each square foot of buried steel in soil having an average resistivity of 1,000 ohm-cm. Bare steel in sea water would require 4 milliamperes or more per square foot. Coatings, even poor ones, make a tremendous difference in current requirements.

• Power Requirements: The voltage of power sources in use ranges from less than 1 volt to 100 volts and more. New pipelines with excellent coatings usually require negligible power. Large currents are needed in the case of some existing, older, poorly coated pipelines.

• Power Sources: Rectifiers that convert alternating current to direct current are used as energy sources for cathodic protection. Direct current voltages are adjustable to maintain current output. Magnesium or Zinc metal are used as anodes and connected by insulated copper wire.

Limitations on Use

• 1. Circuit Resistance: Unit resistance of soil varies from about 20 ohm-cm for seawater saturated soil to more than 1,000,000 ohm-cm for dry sand. High soil resistivity makes good coating necessary as it may make cathodic protection impossible.

• 2. High Voltage: Large negative values could cause coating damage and waste of power.

• 3. Shielding: Other metallic objects between the pipeline and the source of cathodic protection may intercept the current and keep it from reaching the pipeline. Corrosion that causes significant damage to pipe has occurred because of such shielding.

• 4. Requirements for Cooperation: Some existing poorly coated pipelines require large amounts of current for cathodic protection. Supplying the current may be an economical means of extending the useful life of the facility. Otherwise recoating or replacing might be required.

Effectiveness of Protection

• Many pipelines with increasing frequency of corrosion leaks have been cathodically protected with rapid decline in leak frequency and often with their complete cessation.

• Oil storage tanks have large areas of steel in contact with the soil, and cathodic protection has been used effectively to control soil side erosion.

• Cathodic protection has been used to limited extent to control corrosion inside of oil tanks where corrosive water and brine settle out of the oil.

Crude Oil Pipelines

• Most crude oils are classified as sweet, which means that they contain no hydrogen sulfide.

• The small quantities of brine in these oils would be corrosive in contact with steel, but the oil forms a water repellant film and the droplets of brine that settle out are carried along by the flowing oil.

• Sour crude oils contain hydrogen sulfide gas in solution. • Unlike products of some other types of corrosion, which tend to be

protective and reduce further corrosion, iron sulfide tends to accelerate it. The iron sulfide absorbs brine and holds it in contact with the pipe.

• Trunk pipelines are rarely subject to severe internal corrosion even when they transport only sour crude oils. When oil is in turbulent flow in pipelines, any loose material is swept with the oil stream. The result is that the corrosive agents in the oil are kept out of contact with the steel.

Product Pipelines

• Some refined petroleum products are corrosive to the inside of pipelines.

• Light bodied products such as automotive and aviation fuels which do not wet the steel with a water repellant film are more likely to be corrosive.

• Some of the water comes out as minute droplets and is then deposited on the pipe surface.

• Small corrosion cells are formed and some rusting occurs.• The corrosion products are eventually carried along by the liquid

and contaminate it.• Internal corrosion of product pipelines can be effectively

controlled by corrosion inhibitors added to products.

Corrosion of Structures other than Line Pipe

• Paints and coatings have been the principal means of control of structures such as oil and product storage tanks, above ground piping, machinery, buildings, and docks.

• Paints also serve other functions such as improving appearance, contributing to safety, reflecting heat, and retarding the spread of fire.

• The National Association of Corrosion Engineers and various coating societies have developed recommended practices, standards, and other helpful technical data in the field of surface preparation and coating application.

Summary• Effective control of pipeline corrosion is essential for economic success and maintenance of public confidence in the safety and other socially

desirable features of this transportation method.• The amount of metal corroded at the anode is directly proportional to the amount of current. Each metal has a characteristic electrochemical

equivalent. In the case of iron , this equivalent is 20 pounds per ampere per year which means that one ampere of current discharged for one year will dissolve 20 pounds of iron.

• Dissimilar Metal: Dissimilar metal corrosion cells are the simplest kind and are so well known that they have been almost eliminated in modern pipelines.

• Dissimilar Electrolyte: Differences in the electrolyte in contact with different parts of a single metal can result in formation of a corrosion cell.• Dissimilar Surface Condition of Metal: Different parts of a single metal can behave like different metals because of their surface condition.• Stray Current: Cathodic protection of underground structures results in the intentional introduction of direct currents in the ground.• The nature of corrosion immediately suggests two means of its control. One is to break the electrical circuit of existing or potential corrosion cells.

The other is to make all parts of the structure cathodic since metal is not damaged at cathodes.• Protective Coatings: Covering metal with waterproof, electrically nonconductive coating is an age old method of corrosion control. Coatings are a

major cost item in pipeline construction usually ranging from 5 to 10 percent of total construction costs.• Pipelines are expected to serve for 20 to 50 years or longer.• Coatings most closely approaching the ideal are the thermosetting and thermoplastic resins applied to new pipe under carefully controlled

conditions at coating plants.• There are above ground means of determining average electrical resistance of coatings that are a good indicator of coating condition.

• Corrosion control can be achieved by making all parts of the pipeline cathodic.

• Some of the conditions affecting cathodic protection that need to be determined are current requirements, power requirements, and power sources.

• Most crude oils are classified as sweet, which means that they contain no hydrogen sulfide.

• Sour crude oils contain hydrogen sulfide gas in solution. Unlike products of some other types of corrosion, which tend to be protective and reduce further corrosion, iron sulfide tends to accelerate it. The iron sulfide absorbs brine and holds it in contact with the pipe.

• Trunk pipelines are rarely subject to severe internal corrosion even when they transport only sour crude oils. When oil is in turbulent flow in pipelines, any loose material is swept with the oil stream. The result is that the corrosive agents in the oil are kept out of contact with the steel.

• Internal corrosion of product pipelines can be effectively controlled by corrosion inhibitors added to products.

• Paints and coatings have been the principal means of control of structures such as oil and product storage tanks, above ground piping, machinery, buildings, and docks.

• The National Association of Corrosion Engineers and various coating societies have developed recommended practices, standards, and other helpful technical data in the field of surface preparation and coating application.

Home Work

• 1. What is the electrochemical equivalent of iron and what does it mean?

• 2. What are the two means of corrosion control?• 3. What is an age old method of corrosion control?• 4. What is an above ground means of determining coating

condition?• 5. What is the disadvantage of sour crude with respect to

corrosion?• 6. Why are trunk pipelines rarely subject to internal corrosion

even when they transport sour crude?• 7. How can internal corrosion of product pipelines be

effectively controlled?