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Saudi Aramco Corrosion Basics - A Refresher
Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s
employees. Any material contained in this document which is not
already in the public domain may not be copied, reproduced, sold, given,
or disclosed to third parties, or otherwise used in whole, or in part,
without the written permission of the Vice President, Engineering
Services, Saudi Aramco.
Chapter : Inspection
File Reference: COE10300
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CONTENTS
PAGES
WHAT IS CORROSION? ...................................................................................... 1
FORMS OF ATTACK............................................................................................ 4
Sweet Corrosion .......................................................................................... 4
Sour Corrosion ............................................................................................ 7
Stress Corrosion Cracking........................................................................... 7
Oxygen Corrosion ....................................................................................... 9
Concentration Cell Attack ......................................................................... 12
Galvanic Corrosion.................................................................................... 12
Bacterial Corrosion.................................................................................... 13
CORROSION MONITORING AND CONTROL METHODS ............................ 15
GLOSSARY ......................................................................................................... 16
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CORROSION BASICS -- A REFRESHER
WHAT IS CORROSION?
Corrosion is the term generally used to describe a detrimental change in the physical
properties of a metal through chemical or electrochemical reactions.
Corrosion occurs in all phases of oil and gas production, both offshore and onshore, and in
refinery operations. Corrosion is a process through which a metal is returned to a more stable
state resembling the ore from which it was produced. This action, similar to metallurgy in
reverse, is illustrated in Figure 1. Most metals are found in nature as metallic oxides or salts,
as in the case of iron, and have the same chemical composition as rust, Fe2O3. The energy
that converts iron ore to iron is the same energy released when the iron converts to rust.
FIGURE 1. Metallurgy in Reverse
The basic requirements for corrosion are
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A corrosion cell consisting of an anode and a cathode
An electrolyte to complete the circuit
Flow of direct current
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Figure 2 shows all of these requirements. The anode is the point at which the metal dissolves
by going into solution. At the anode, metal atoms lose electrons and the resulting positive
ions go into solution. The electrons migrate through the metal to the cathode. At the cathode,
various ion species in solution remove these electrons to complete the corrosion reaction. A
small, but measurable, electric current is produced and flows from the anode to the cathode
through the electrolyte and then passes through the metal from the cathode to the anode. The
quantity of current that passes through the cell is directly proportional to the amount of metal
that corrodes. A current flow of 1 ampere per year will corrode approximately 20 pounds of
steel.
FIGURE 2. Corrosion Cell
(Note that the flow of electrons is opposite to the flow of current.)
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Typical reactions at the anode and cathode on a piece of steel are defined in the following
equations.
Anode
Fe → Fe++ + 2e-
(Oxidation)
2H+ + 2e- → H2
(Reduction)
Cathode
Anodes and cathodes can form on a single piece of metal because of either local differences
in the metal or in the environment.
If the hydrogen produced adheres to the cathode and forms an insulating blanket,
polarization results. Polarization introduces a resistance and interferes with current flow so
that corrosion is decreased or stopped. Oxygen, if present, combines with the hydrogen in
this insulating blanket to form water and thus removes the hydrogen film. Current flows
again and corrosion proceeds. This is called cathodic depolarization.
The rate of the destructive attack as a function of time will depend on process or service
conditions such as pressure, temperature, velocity and impingement, pH and corrodent
concentration, and the presence of oxygen or other depolarizers.
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FORMS OF ATTACK
There are many types of corrosion. The most common types include:
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Sweet corrosion
Sour corrosion
Stress corrosion cracking
Oxygen corrosion
Concentration cell attack
Galvanic corrosion
Bacterial corrosion
Sweet Corrosion
Sweet corrosion results from CO2 gas or low molecular weight organic acids dissolving in
water.
CO2 + H2O → H2CO3
(Carbonic acid)
Fe + H2CO3 → FeCO3 + H2
This type of attack is characterized by a general loss of metal over the entire surface or by
shallow areas of localized attack that are free of scale.
Important factors that affect the solubility of CO2 and, therefore, the severity of attack
include:
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CO2 concentration
Pressure
Temperature
Water composition
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The concentration of CO2 is shown by its partial pressure. The partial pressure of CO2 is
obtained by multiplying the total pressure by the percentage of CO2. Corrosivity of a sweet
system is determined by the following criteria:
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Partial pressure above 30 psi usually indicates attack.
Partial pressure between 7 and 30 psi may indicate corrosion.
Partial pressure less than 7 psi is considered noncorrosive.
Increased pressure results in the increased solubility of CO2 in water and, therefore, higher
corrosion rates up to a limit. This action is shown in Figure 3. As temperature increases, the
solubility of CO2 in water decreases. If the pressure is kept constant, an increase in
temperature will cause CO2 to be released from the solution. The accompanying rise in pH
lowers the corrosion rate as shown in Figure 4. Water composition is another factor in sweet
corrosion. Many dissolved minerals tend to buffer or to prevent the reduction of pH in a
water with dissolved CO2, thereby influencing the rate of corrosion.
FIGURE 3. Corrosion Rate Vs. CO2 Partial Pressure
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CO2 corrosion is particularly devastating in a high-velocity/high-pressure system where the
combination of corrosion and erosion can lead to extremely high corrosion rates.
FIGURE 4. pH Vs. CO2 Partial Pressure With Varying Temperature
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Sour Corrosion
Sour corrosion results from the reaction of hydrogen sulfide and steel in the presence of
water.
Fe + H2S → FeS + 2H
in H2O
This type of attack produces black iron sulfide scale and numerous pits. A galvanic reaction
occurs between the black iron sulfide scale and the steel. The steel acts as an anode and the
iron sulfide scale as a cathode. Additional problems occur with the precipitated iron sulfide.
This insoluble precipitate is preferentially oil-wet and is commonly carried over into storage
tanks, heater treaters, free water knockouts, and other production equipment where scale
problems can occur. Disposal water will then carry significant concentrations of oil-wet iron
sulfide that will plug filters and lines. Iron sulfide changes to iron oxide with time and
exposure to air.
There are four major sources of hydrogen sulfide in the oil field.
• Inflow of “sour” formation fluids
Bacterial decomposition of sulfates in drilling fluid (gyp muds, anhydrite
•contamination,
etc.)
Thermal degradation (above 350-375 °F) of certain sulfur containing drilling fluid
•additives
(lignosulfonates)
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Make-up water in mud system containing sulfur compounds
Stress Corrosion Cracking
Stress corrosion cracking is caused by the combined forces of stress and corrosion on an
alloy. If either stress or corrosion is absent, then no cracking will occur. The best example of
stress corrosion cracking in the oil field is hydrogen embrittlement. The embrittling action is
caused by the liberation of hydrogen from hydrogen sulfide and from the corrosion process.
The liberated hydrogen, frequently referred to as atomic hydrogen, is absorbed on the steel
surface and migrates into the grain boundaries of the metal. After going into the steel, the
liberated hydrogen combines either with itself to become molecular hydrogen or with carbon
compounds in the steel. These compounds and the molecular hydrogen have larger molecules
than the liberated hydrogen (atomic hydrogen) and are trapped in the steel. These large,
trapped molecules then cause excessive pressure within the steel. As a result, the steel splits
and blisters and may crack. This action is shown in Figure 5.
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FIGURE 5. Hydrogen Blistering
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Oxygen Corrosion
In oil production, the removal of corrosives is primarily directed at the exclusion of oxygen.
Oxygen is normally absent in oil producing systems or, at most, present in trace amounts.
However, oxygen can enter a supposedly “closed” system through pumps, nongas-blanketed
storage tanks, systems operating in a vacuum, and so forth. For example, the three most
likely points where oxygen enters oil field water injection systems are at the wells, at the
tanks, and through pump seals.
The annulus of a well must be kept sealed or blanketed to exclude O2. This is particularly
troublesome in water source wells equipped with electrical submersible pumps. Maintaining
an effective seal around electrical cables or rotating shafts is extremely difficult. Gas
blanketing is usually the only effective method of excluding oxygen from wells of this type.
In the case of tanks, gas blanketing with natural gas or nitrogen is best. Oil blankets are not
effective. Pump seals are another point of oxygen entry. If water is leaking from a seal, O2
can enter this site by diffusion against pressure.
The presence of trace amounts (1 ppm or less) of oxygen greatly increases the effects of other
corrodents. Corrosion caused by trace amounts of oxygen typically results in extreme attack
in crevices, behind obstructions in the fluid flow, and in other shielded areas. Accelerated
corrosion of this type takes place where oxygen is either absent or in small amounts. The
removal of hydrogen by oxygen increases the cathodic reaction. Therefore, locations lacking
oxygen tend to become anodic. The more oxygen in the system, the greater the weight loss as
shown in Figure 6.
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FIGURE 6. Influence of Dissolved Oxygen on Corrosion of Steel
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The effect of NaCl content on the solubility of oxygen in water at various temperatures is
shown in Figure 6A.
FIGURE 6A. Solubility of Oxygen in Water
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Concentration Cell Attack
Concentration cell attack is an intense localized corrosion that occurs within crevices and
other shielded areas. This attack is usually associated with small volumes of stagnant solution
caused by holes, gasket surfaces, lap joints, surface deposits, and crevices under bolts and
rivet heads. This type of attack is shown in Figure 7.
FIGURE 7. Concentration Cell
Galvanic Corrosion
Galvanic corrosion is produced when current flows between two different metals or between
areas of the same metal having different characteristics. This corrosion occurs because
different metals have different tendencies to corrode and because metals are rarely
homogeneous. It really is not necessary to have two different metals for galvanic corrosion.
Iron sulfide scale acts as a cathode when covering steel and leads to accelerated corrosion of
the steel. Galvanic corrosion is a very rapid and localized attack.
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Bacterial Corrosion
There are three major groups of bacteria in the oil field.
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Slime formers
Sulfate-reducing bacteria
Iron bacteria
Bacteria may cause plugging and corrosion problems.
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Plugging
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Bacteria cells and “slime”
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Chemical precipitates
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FeS (sulfate-reducing bacteria)
Fe(OH)3 (iron bacteria)
Corrosion
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Production of corrosive substances (H2S)
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Deposits accelerate corrosion
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Depolarization of cathode of corrosion cell by sulfate-reducing bacteria
Indicators of bacterial activity include:
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Pressure increase in injection wells
Black water
Slime accumulations on filters
Increase in sulfide
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Bacteria growth tends to concentrate in the following locations:
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In films on pipe surfaces or walls of tanks
Filter beds
Stagnant areas in a system
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Tank bottoms
Gauge settings, bull plugs sample connections
Wellbore below perforations
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CORROSION MONITORING AND CONTROL METHODS
Corrosion problems should be anticipated and the need for corrosion monitoring recognized
before a pressure vessel is fabricated or a pipeline laid. This permits the best possible
material selection and design features to be incorporated in new construction.
Whether in the plant or in the field, corrosion monitoring and control efforts should include
examination of
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The area of corrosion
Reports of maintenance men and operating personnel
Materials of construction, process streams, and corrosion products
Awareness of corrosion as a specific problem has resulted in more care taken in obtaining
corrosion monitoring information and in preserving valuable evidence.
The prevention or control of corrosion commonly uses one of the following methods:
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Change in the environmental conditions
Corrosion-resistant materials
Separation of the metal from the corrosive environment by the use of coatings
Use of inhibitors
Cathodic/anodic protection
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GLOSSARY
anode
Area where corrosion occurs and where current leaves the
metal and enters the solution
cathode
Area where no corrosion occurs and where current enters the
metal from solution
corrosion
Detrimental change in the physical properties of a metal
through chemical or electrochemical reactions
depolarization
Removal of the hydrogen film at the cathode
oxidation
Removal of electrons from an atom
polarization
Result of hydrogen adhering to a metal and reducing or
stopping the current flow
reduction
Gain of electrons by an atom
sour corrosion
Corrosion caused by H2S dissolved in water
sweet corrosion
Corrosion caused by CO2 and/or low molecular weight organic
acids dissolved in water
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