Corrosion Basics

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Saudi Aramco Corrosion Basics - A Refresher

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Corrosion Basics - A Refresher

Saudi Aramco DeskTop Standards

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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WHAT IS CORROSION?

Corrosion is the term generally used to describe a detrimental change in the physicalproperties of a metal through chemical or electrochemical reactions.

Corrosion occurs in all phases of oil and gas production, both offshore and onshore, and inrefinery operations. Corrosion is a process through which a metal is returned to a more stablestate resembling the ore from which it was produced. This action, similar to metallurgy inreverse, 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 energythat converts iron ore to iron is the same energy released when the iron converts to rust.

The basic requirements for corrosion are

• A corrosion cell consisting of an anode and a cathode

• An electrolyte to complete the circuit

• Flow of direct current

CORROSION BASICS -- A REFRESHER

FIGURE 1. Metallurgy in Reverse




Figure 2 shows all of these requirements. The anode is the point at which the metal dissolvesby going into solution. At the anode, metal atoms lose electrons and the resulting positiveions 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. Asmall, but measurable, electric current is produced and flows from the anode to the cathodethrough the electrolyte and then passes through the metal from the cathode to the anode. Thequantity of current that passes through the cell is directly proportional to the amount of metalthat corrodes. A current flow of 1 ampere per year will corrode approximately 20 pounds ofsteel.

FIGURE 2. Corrosion Cell(Note that the flow of electrons is opposite to the flow of current.)




Typical reactions at the anode and cathode on a piece of steel are defined in the followingequations.

Anode

Fe → Fe++ + 2e- (Oxidation)

Cathode

2H+ + 2e- → H2 (Reduction)

Anodes and cathodes can form on a single piece of metal because of either local differencesin 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 sothat corrosion is decreased or stopped. Oxygen, if present, combines with the hydrogen inthis insulating blanket to form water and thus removes the hydrogen film. Current flowsagain and corrosion proceeds. This is called cathodic depolarization.

The rate of the destructive attack as a function of time will depend on process or serviceconditions such as pressure, temperature, velocity and impingement, pH and corrodentconcentration, and the presence of oxygen or other depolarizers.




FORMS OF ATTACK

There are many types of corrosion. The most common types include:

• 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 inwater.

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 byshallow areas of localized attack that are free of scale.

Important factors that affect the solubility of CO2 and, therefore, the severity of attackinclude:

• CO2 concentration

• Pressure

• Temperature

• Water composition




The concentration of CO2 is shown by its partial pressure. The partial pressure of CO2 isobtained by multiplying the total pressure by the percentage of CO2. Corrosivity of a sweetsystem is determined by the following criteria:

• 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, highercorrosion rates up to a limit. This action is shown in Figure 3. As temperature increases, thesolubility of CO2 in water decreases. If the pressure is kept constant, an increase intemperature will cause CO2 to be released from the solution. The accompanying rise in pHlowers the corrosion rate as shown in Figure 4. Water composition is another factor in sweetcorrosion. Many dissolved minerals tend to buffer or to prevent the reduction of pH in awater with dissolved CO2, thereby influencing the rate of corrosion.

FIGURE 3. Corrosion Rate Vs. CO2 Partial Pressure




CO2 corrosion is particularly devastating in a high-velocity/high-pressure system where thecombination of corrosion and erosion can lead to extremely high corrosion rates.

FIGURE 4. pH Vs. CO2 Partial Pressure With Varying Temperature




Sour Corrosion

Sour corrosion results from the reaction of hydrogen sulfide and steel in the presence ofwater.

Fe + H2S → FeS + 2Hin H2O

This type of attack produces black iron sulfide scale and numerous pits. A galvanic reactionoccurs between the black iron sulfide scale and the steel. The steel acts as an anode and theiron 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 storagetanks, heater treaters, free water knockouts, and other production equipment where scaleproblems can occur. Disposal water will then carry significant concentrations of oil-wet ironsulfide that will plug filters and lines. Iron sulfide changes to iron oxide with time andexposure 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, anhydritecontamination, etc.)

• Thermal degradation (above 350-375 °F) of certain sulfur containing drilling fluidadditives (lignosulfonates)

• 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 analloy. If either stress or corrosion is absent, then no cracking will occur. The best example ofstress corrosion cracking in the oil field is hydrogen embrittlement. The embrittling action iscaused 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 steelsurface and migrates into the grain boundaries of the metal. After going into the steel, theliberated hydrogen combines either with itself to become molecular hydrogen or with carboncompounds in the steel. These compounds and the molecular hydrogen have larger moleculesthan 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 splitsand blisters and may crack. This action is shown in Figure 5.




FIGURE 5. Hydrogen Blistering




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-blanketedstorage tanks, systems operating in a vacuum, and so forth. For example, the three mostlikely points where oxygen enters oil field water injection systems are at the wells, at thetanks, and through pump seals.

The annulus of a well must be kept sealed or blanketed to exclude O2. This is particularlytroublesome in water source wells equipped with electrical submersible pumps. Maintainingan effective seal around electrical cables or rotating shafts is extremely difficult. Gasblanketing 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 noteffective. Pump seals are another point of oxygen entry. If water is leaking from a seal, O2can enter this site by diffusion against pressure.

The presence of trace amounts (1 ppm or less) of oxygen greatly increases the effects of othercorrodents. Corrosion caused by trace amounts of oxygen typically results in extreme attackin crevices, behind obstructions in the fluid flow, and in other shielded areas. Acceleratedcorrosion of this type takes place where oxygen is either absent or in small amounts. Theremoval of hydrogen by oxygen increases the cathodic reaction. Therefore, locations lackingoxygen tend to become anodic. The more oxygen in the system, the greater the weight loss asshown in Figure 6.



Saudi Aramco DeskTop Standards1

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FIGURE 6. Influence of Dissolved Oxygen on Corrosion of Steel




1

The effect of NaCl content on the solubility of oxygen in water at various temperatures isshown in Figure 6A.

FIGURE 6A. Solubility of Oxygen in Water




2

Concentration Cell Attack

Concentration cell attack is an intense localized corrosion that occurs within crevices andother shielded areas. This attack is usually associated with small volumes of stagnant solutioncaused by holes, gasket surfaces, lap joints, surface deposits, and crevices under bolts andrivet heads. This type of attack is shown in Figure 7.

Galvanic Corrosion

Galvanic corrosion is produced when current flows between two different metals or betweenareas of the same metal having different characteristics. This corrosion occurs becausedifferent metals have different tendencies to corrode and because metals are rarelyhomogeneous. 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 ofthe steel. Galvanic corrosion is a very rapid and localized attack.

FIGURE 7. Concentration Cell




3

Bacterial Corrosion

There are three major groups of bacteria in the oil field.

• Slime formers

• Sulfate-reducing bacteria

• Iron bacteria

Bacteria may cause plugging and corrosion problems.

• Plugging

• Bacteria cells and “slime”

• Chemical precipitates

• FeS (sulfate-reducing bacteria)• Fe(OH)3 (iron bacteria)

• Corrosion

• Production of corrosive substances (H2S)

• Deposits accelerate corrosion

• Depolarization of cathode of corrosion cell by sulfate-reducing bacteria

Indicators of bacterial activity include:

• Pressure increase in injection wells

• Black water

• Slime accumulations on filters

• Increase in sulfide




4

Bacteria growth tends to concentrate in the following locations:

• In films on pipe surfaces or walls of tanks

• Filter beds

• Stagnant areas in a system

• Tank bottoms• Gauge settings, bull plugs sample connections• Wellbore below perforations




5

CORROSION MONITORING AND CONTROL METHODS

Corrosion problems should be anticipated and the need for corrosion monitoring recognizedbefore a pressure vessel is fabricated or a pipeline laid. This permits the best possiblematerial selection and design features to be incorporated in new construction.

Whether in the plant or in the field, corrosion monitoring and control efforts should includeexamination of

• 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 obtainingcorrosion monitoring information and in preserving valuable evidence.

The prevention or control of corrosion commonly uses one of the following methods:

• 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




6

GLOSSARY

anode Area where corrosion occurs and where current leaves themetal and enters the solution

cathode Area where no corrosion occurs and where current enters themetal from solution

corrosion Detrimental change in the physical properties of a metalthrough 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 orstopping 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 organicacids dissolved in water

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Corrosion Basics