Sucker Rod Corrosion

8/17/2019 Sucker Rod Corrosion

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BASIC CONCEPTS OF SUCKER ROD CORROSION

© 1993-2016 Weatherford. All rights reserved. i

Table of Contents

INTRODUCTION ................................................................................ 1

CORROSION ...................................................................................... 2

Electrochemical Corrosion .............................................................................. 2

Chemical Corrosion ......................................................................................... 3

Oxidation ......................................................................................................... 4

Galvanic Corrosion .......................................................................................... 4

Erosion-Corrosion ........................................................................................... 5

Hydrogen Embrittlement ................................................................................ 5

Corrosion Fatigue ............................................................................................ 5

FIELD SERVICE PROBLEMS ................................................................ 6

Conditions that Cause Sucker Rod and Coupling Failures ............................... 6

Types of Sucker Rod and Coupling Failures .................................................... 6

DESIGNING TO REDUCE FAILURES ................................................... 16

Mill Defects ................................................................................................... 16

Manufacturing Problems .............................................................................. 16

Handling Problems ........................................................................................ 17

Mechanical Damage ...................................................................................... 17

Improper Joint Makeup ................................................................................ 17

Bent Rods ...................................................................................................... 18

Guided Sucker Rods ...................................................................................... 19 Poor Pumping Conditions ............................................................................. 19

Corrosion Problems ....................................................................................... 20

Oxygen Corrosion .......................................................................................... 21

Problems Caused by a Stuck Pump ............................................................... 22

Hammering the Surface of Couplings ........................................................... 23

Thread Galling ............................................................................................... 23

Collection of Background Data and Selection ............................................... 25

Non -Destructive Testing (NDT) .................................................................... 25

Mechanical Testing ....................................................................................... 25 Selection and Preservation of Fracture Surfaces .......................................... 26

Macroscopic Examination of Fracture Surfaces ............................................ 26

Fracture Classifications ................................................................................. 27

REFERENCES ................................................................................... 29




© 1993-2016 Weatherford. All rights reserved. 1

Introduction

Historically, the first recorded incidence of corrosion was a

problem encountered by British ships operating in the

Mediterranean early in the nineteenth century. Worms living

in those waters would enter the wooden hulls and eat the

timbers until ships required dry-docking for replacement of

these wooden structures. The British decided to cover the

ship hulls with thin sheets of copper and the worm problem

was solved, or so it appeared. Soon the copper sheets began

falling off the hulls, and the worm problem returned.

The steel nails holding the copper had disintegrated where

the copper and steel were in contact, and no one could

explain the reason. We now know that a galvanic action

occurs between dissimilar metals in sea water. Using copper

nails resolved the problem.

Last century, scientists began to recognize the tremendous

scope of corrosion and the cost associated with failed

equipment. Of note was the realization that stray current

from street car railways was damaging and even destroying

underground metal structures and communication cables.

Corrosion is a natural phenomenon that is not necessarily

limited to metals. The effects of corrosion can be observed

every day, everywhere in the world; and the cost of its

damage to metal objects amounts to billions of dollars each

year. Consider the following costs to an operator if a single

rod string fails as a result of corrosive action:

1. Downtime: lost production because the string can no

longer activate the pump

2. Workover: cost of a workover rig and the labor to pull

the tubing and rod string

3.

Replacement: cost of a new string of rods and possibly

the cost of replacing tubing and the downhole pump

4. Work: labor of company personnel who could be doing

other things

Multiply those costs by the number of corrosion failures in

rod strings in one year and the total cost to operators

becomes enormous.

Today the mechanics of corrosion are understood and its

behavior can be controlled. In some cases it can be

completely eliminated by following proper procedures. This

guide focuses on the types of corrosion related to sucker

rods, the causes, and ways to reduce sucker rod failures.





Corrosion

Corrosion can be defined as the deterioration of a

substance or its properties because of a reaction with its

environment. The substance we will consider is steel. Steel

is an iron compound with alloying elements such as carbon

and manganese.

The driving force that makes metals corrode is a natural

phenomenon. Iron, from which steel is produced, is found in

combination with other elements, and these are called ores.

Any iron-based product in a usable form is in only a

temporary condition and eventually, if let unhampered, will

return to its original form (an ore).

Left unprotected in an atmosphere, a metal will release

energy as it combines with other elements and returns to

its natural state as an ore. The release of energy and its

attendant combination with other elements to form ore

is corrosion.

One of the most obvious examples is a steel object left

exposed to weather. It will begin to rust and, if left

undisturbed, will completely deteriorate. Rust is a

combination of iron and oxygen, in which the iron gives up

its energy and returns to its natural state.

There are multiple types of corrosion:

•

Electrochemical corrosion

• Chemical corrosion

– Sour

– Sweet

• Oxidation

• Galvanic

• Corrosion-erosion

• Hydrogen embrittlement

•

Corrosion fatigueIn most instances more than one type of corrosion

contributes to a failure.

Electrochemical Corrosion

Most corrosion in the oil field is attributable to the presence

of water in large or small amounts. Corrosion in the

presence of water is an electrochemical process. Please keep

this in mind as we discuss other types of corrosion.

Figure 1 shows that steel is not homogeneous. It is a mixture

of iron, carbon, and other alloying elements; but for

purposes of illustration, let’s consider only the iron and

carbon elements. Some of the carbon is dissolved in the iron,

and the balance exists as iron carbide.

Figure 1: Photomicrograph showing a pearlite – ferrite

microstructure. Pearlite is a mixture of cementite (Fe3C); Ferrite is

almost pure iron.

Iron carbide (Fe3C) has a lower tendency to corrode than

pure iron (Fe), thus with an electrode and in the presence of

an electrolyte (such as water or salt water), the two different

compositions will complete an electrical circuit and current

will flow (Figure 2).





Figure 2: Schematic showing current flow between iron and iron

carbide grains with resulting corrosion of the iron

Some definitions are necessary to describe the

electrical circuit:

1. Anode. The anode is that portion of the metal surface

that is corroded.

2. Cathode. The cathode is the opposite side of the cell

that is unaffected by the current flow and therefore doesnot corrode.

3. Electrolyte. The fluid that transmits the current between

anode and cathode.

When iron corrodes, the iron dissolves, goes into solution,

and gives up two electrons. Since an atom of iron contains

an equal number of protons (positively charged particles)

and electrons (negatively charged particles), the iron in

solution is called an ion. The iron ion is positively charged.

Fe → Fe2+

+ 2e –

Iron Atom Iron Atom Electrons

Although the cathode remains unaffected, another chemical

reaction takes place at the cathode. The electrons left

behind at the anode when iron went into solution travel to

the cathode through the solid metal. Hydrogen in water has

an affinity for those electrons and consumes them.

2H+ + +2e –

→ H2

Hydrogen Ions Electrons Hydrogen Gas

Figure 3: Schematic showing basic current and electron flow in

localized corrosion cell at the surface of ferrous metals immersed in

an electrolyte

The area corroded is always the anode. The cathode

remains unaffected.

The affinity of iron in solution for oxygen is greater than the

affinity of hydrogen for oxygen. Hydrogen gives up one

electron when it goes into its ionic state. Oxygen gains two

electrons and becomes negatively charged.

In a simplified form, the reaction is as follows:

Fe + H2O → FeO + H2↑

Iron Atom Water Iron Oxide Hydrogen Gas

Fe+ + H

+H

+O → FeO + H2↑

Positive Iron Negative Oxygen

Positive Hydrogen

Iron Oxide Hydrogen Gas

The positively charged iron ion and the negatively charged

oxygen ion combine to form iron oxide. The two positively

charged hydrogen ions then form a molecule and escape at

the cathode as a gas.

Chemical Corrosion

SourHydrogen sulfide is very soluble in water. When both

hydrogen sulfide and water are present in well fluids, sulfuric

acid (H2SO4,) is formed. Although other reactions may be

involved, simply stated, the final reaction is the formation of

iron sulfide (FeS).

H2S + Fe → FeS + H2↑

Hydrogen Sulfide Iron Atom Iron Sulfide Hydrogen Gas

Iron sulfide is a black scale that clings to the surface of the

metal and is cathodic to the iron. It accelerates the corrosion

in the locality of the scale and causes deep pitting.

Sweet

The term sweet corrosion refers to corrosion caused by the

presence of carbon dioxide (CO2) in a producing well. Carbon

dioxide easily dissolves in water to form carbonic acid.

CO2 + H2O → H2CO3

Carbon Dioxide Water Carbonic Acid

An electrochemical reaction occurs and the iron replaces

hydrogen to form iron carbonate.

Fe + H2CO3 → Fe2CO3 + H2

Iron Carbonic Iron Carbonate Gas

The end result is pitting. The severity of the pitting is

determined by many factors, such as pressure, temperature,

and the amount of CO2 present.

Both sour and sweet corrosion can occur in the same well.





Oxidation

Corrosion of metal by oxidation is the ordinary rust observed

on any unprotected piece of steel. It can only occur in the

presence of water, and then only if dissolved oxygen is

present. However, it will also occur in sea water and salt

solutions. We will limit our discussion to salt water, a fluidthat is prevalent in pumping wells.

Water solutions rapidly dissolve oxygen from the air, and

this is the source for the required oxygen in the corrosion

process. Normally oxygen is not found in well fluids; but if it

is introduced by any method, rusting will continue as long as

oxygen is present. The simplified chemical reaction is:

4Fe + 3O2 4Fe2O3

Iron Oxygen Iron Oxide

For an example of the effect of oxygen on steel, consider thephotos taken of the ocean liner Titanic that lies at the

bottom of the Atlantic Ocean. Why has the hull not corroded

away? Because at those depths, no oxygen exists.

Galvanic Corrosion

Galvanic corrosion is very rare in the oil field. Three

conditions are necessary for galvanic corrosion to occur:

• Electrochemically dissimilar metals must be present.

• The metals must be in electrical contact.

• The metals must be exposed to an electrolyte.

Electrochemical dissimilarity refers to the amount of

energy stored by the metal when it is removed from its

natural condition.

An example of metals in electrical contact is the steel nails

that held copper sheeting on the hulls of British ships in salt

water. A potential can be created between any two

dissimilar metals and even between different types of steels.

The third condition for galvanic corrosion relates to the

strength and type of electrolyte and the presence or absence

of oxygen.

When the three conditions listed above are met, the anodic

(most active) metal will deteriorate while the cathodic (less

active) metal will be relatively unaffected. Figure 4 indicates

the relative anodic and cathodic properties of some

commercial metals and alloys in sea water. From their

positions on the scale in Figure 4, it becomes apparent that

steel nails will be destroyed by the more noble copper. Thescale also explains why gold is usually not found in chemical

combination with other elements.

Active

or

Anodic

Magnesium

Magnesium Alloys

Zinc

Galvanized Steel

Aluminum 1100

Aluminum 2024 (4.5 Cu, 1.5 Mg, 0.6 Mn)

Mild Steel

Wrought Iron

Cast Iron

13% Chromium Stainless SteelType 410 (active)

18-8 Stainless Steel

Type–Tin Solders

Lead

Tin

Muntz Metal

Manganese Bronze

Naval Brass

Nickel (active)

76 Ni, 16 Cr, 7 Fe Alloy (active)

60 Ni, 30 Mo, 6 Fe, 1 Mn

Yellow Brass

Admiralty Brass

Red Brass

Copper

Silicon Brass

70:30 Cupro Nickel

G-Bronze

Silver Solder

Nickel (passive)

76 Ni, 16 Cr, 7 Fe

Alloy (passive)

13% Chromium Stainless Steel

Type 410 (passive)

SilverGraphite

Gold

Platinum

Noble

or

Cathodic

Figure 4: Galvanic series of some commercial metals and alloys in

sea water

H2O





Erosion-Corrosion

This type of corrosion is caused by corrosive fluid, impinging

flow, or turbulence of the fluid upon metal surfaces.

Impingement can be caused by solid material or gas bubbles

entrapped in the fluid being pumped. These materials,

moving against a metal surface, tend to abrade or erode thesteel. The erosion takes the form of elongated pitting or a

deep groove. The rapid movement of corrosive fluid

removes a protective scale and exposes the underlying

metal, which accelerates corrosion.

Turbulence is a major factor in this type of attack. When a

liquid flows over a metal, there is usually a critical velocity

below which impingement does not occur, but above which

impingement rapidly increases. To illustrate this

phenomenon, consider a curve in a lazy stream. As long as

the stream contains little water and the flow is gentle, the

banks remain unchanged. Increase the water volume and

the stream becomes more turbulent, thereby erodingthe bank.

The break out of gases in low-pressure zones can

impinge on the surface of a sucker rod and remove any

protective coating.

Hydrogen Embrittlement

We have discussed chemical and electrochemical reactions

in which positively charged hydrogen ions in an electrolyte

are expelled from the system as hydrogen gas. However, all

hydrogen may not leave. It is possible, even most probable

in a sour well environment, that some of the hydrogen will

enter the steel as either atomic or molecular hydrogen anddiffuse into its structure. Once absorbed, the hydrogen

molecules build pressure at crystallographic vacancies or

discontinuities, such as voids, which generate microscopic

cracks. This results in a brittle failure of the steel at stress

levels considerably below the yield strength of the steel.

The more acidic the well fluid, the more susceptible the steel

is to hydrogen embrittlement. Since hydrogen

embrittlement is related to the chemical and

electrochemical reactions we described in sour wells,

the term sulfide stress cracking is also used to describe

this phenomenon.

Corrosion Fatigue

Thus far we have primarily considered the mechanics of

corrosion alone. However, corrosion and fatigue, acting in

concert, are the cause of many sucker rod failures.

Cyclic stressing of sucker rods does not typically cause a

failure of the sucker rod string if the stresses are limited to alevel below the endurance limit of the steel. Environmental

conditions are extremely important in corrosion fatigue. Salt

water is a corrosive medium. When dissolved gases such as

hydrogen sulfide (H2S), carbon dioxide (C02), and free oxygen

are present in salt water, corrosivity increases and fatigue

life decreases.

Usually fatigue begins at the rod surface as a pit in the

metal, a nonmetallic inclusion, or some other steel

defect. However, it may begin with some kind of

mechanical damage.

Though the stresses may be uniform over the balance of therod, the stresses induced at the bottom of a pit will be

considerably higher; and as corrosion continues to attack,

the cross section is reduced. As stresses across that area

increase, a fatigue failure results.

This will be discussed later in this guide.





Field Service Problems

In this section we will discuss field problems that you will

encounter with sucker rods and sucker rod couplings. We

will also show examples to illustrate each mode of failure.

Failure prevention of any component starts with a

knowledge of failure mechanisms. To prevent failures, it is

necessary to understand the many ways a product can fail.

Each failure can be considered a valuable specimen from

which to extract as much information as possible. The

information can be used to improve product quality and

skills in the application of sucker rods. Understanding

product failures and being able to extend field service life is

important to you, the salesman or the field engineer, and to

the user who will benefit by receiving longer service from

the equipment and by paying less for lifting.

Conditions that Cause Sucker Rod and Coupling

Failures

Examination of sucker rod failures reveals several enemies to

dependable rod string operation. S.M. Bucaran, et aI,

presented the paper (No. 55) NACE/72 "Proper Selection,

Handling, and Protection of Downhole Materials—A Practical

and Economical Approach" in which they stated that rod and

coupling failures can be classified simply as caused by wear,

corrosion, mechanical causes, or mishandling; and plenty of

cases involve two or more of the basic causes1.

Here is another list of causes for sucker rod and

coupling failures:

• Overload condition

• Application issues

– Rods in compression

– Rod wear

– Pumping speed

– Stuck pump

–

Incomplete pump fillage

• Corrosion problems

– Water floods

– Sour gas

– CO2 injection

• Handling problems

– Improper makeup procedures

– Bent rods

– Mechanical damage

• Inhibitors

– Lack of inhibition

– Poor inhibition program

• Manufacturing defects

–

Mill defects – Forging defects

– Threading defects

Types of Sucker Rod and Coupling Failures

Metallurgical investigations of failed sucker rods typically

reveal a fatigue pattern. The fatigue crack originates at the

point of highest stress, and the stress peaks occur at the

surface. Therefore, the fatigue failures are surface failures

and the damage, or intrusion, is a sharp notch or roughness

and a stress-raiser that starts the crack.

Breaks in sucker rod pins have a similar appearance to

fatigue failure, but they usually occur if the joint is not made

up properly, a condition referred to as loss of displacement.

A loss of displacement occurs when a connection loosens

downhole. This can be caused by improper makeup

procedures, applications issues, or mechanical damage.

Coupling breaks will occur as a result of wear, corrosion, or

loss of displacement. Hydrogen sulfide embrittlement

failures have occurred in couplings with hardness greater

than 23 Rockwell C.

We will define and discuss four modes of sucker rod and

coupling failures:

•

Fatigue: the effects of stress concentration and corrosion

• Corrosion: pitting versus uniform attack, and the

damaging effects of hydrogen sulfide embrittlement

• Wear: its effects on service life

• Tensile failure: not a service failure but an overload

condition caused by carelessness





Fatigue

The word fatigue may suggest that metals become tired

from supporting a load for a long time. Certainly metals do

not tire in a biological sense; nor do they deteriorate as a

direct result of supporting a constant load. However, they do

fracture in a brittle manner when subjected to cyclical

loading that varies sufficiently in intensity, even though onesuch cycle produces no detectable effect.

Fatigue is the progressive, localized, permanent structural

change that occurs in a material subjected to repeated or

fluctuating stresses that have a maximum value less than the

tensile strength of the material. Fatigue fractures are caused

by the simultaneous action of cyclic stress, tensile stress, and

plastic strain, all three of which must be present.

Although these three conditions are sufficient to cause

fatigue, a host of other variables—such as temperature,

crystal system of the metal, grain size, environment

(corrosive or otherwise), metallurgical structure, and stress

system—alter the conditions for fatigue. Figure 5 illustrates

the three general types of fluctuating stresses that can

cause fatigue.

Figure 5: Typical fatigue stress cycles

Case 1 represents an idealized situation wherein stress

fluctuates in a sinusoidal fashion from tensile to

compression and the net resultant stress is zero. This is the

most common form used to study fatigue in a laboratory,

but it is also approached in service by a rotating shaftoperating at a constant speed without overload.

In Case 2, we have the sinusoidal stress form but the

resultant, or mean, stress is not zero.

Case 3 shows an irregular or random stress cycle.

The process of fatigue consists of three stages (Figure 6):

1. Initial fatigue damage leading to crack initiation

2. Crack propagation until the remaining uncracked

cross-section of a part becomes too weak to carry the

imposed loads

3. Final, sudden fracture of the remaining cross-section

Figure 6: Relative lengths of the various stages of a sucker rod

body failure

Consequently, fatigue cracks are classified as brittle cracks,

as identified by little or no evidence of ductility in the area

adjacent to the second stage cracking. There is no necking or

shear lips on the fracture as shown in the final stage of

failure. Fatigue failures are always normal (90°) to the

applied stress. Chevron marks, or a herringbone pattern,

point to the fracture origin.

Fatigue beach marks, clamshell marks, or families of

striations may be observed with the unaided eye

(Figure 7). The familiar beach marks can be observed

macroscopically with a light microscope or under the

electron microscope (Figure 8).

Figure 7: The three stages of fatigue of a sucker rod body failure





Figure 8: Field failure with visible beach marks

Fatigue beach marks are clearly evident with little or no

magnification for lower strength, more ductile materials and

for a variety of strength ranges under conditions of low load,

high-cycle stresses. The distinguishable concoidal markings

usually represent points of variation in the load

environment. For higher strength materials and for high-load

fatigue, the use of an optical or electron microscope is often

required. With the close examination of a fatigue fracture

face, much or all of the following information regarding the

crack can be determined:

1. Point(s) of crack nucleation

2. Direction of crack growth

3. Size of prior crack

4. Relative magnitude of stress

5. Direction of loading (axial, bending, reverse bending, etc.)

Fatigue failures occur at apparently safe values of

predetermined, calculated average stress over the cross-

section of a sucker rod or coupling, and failure is caused by

concentrated loads at a surface discontinuity. Such

concentrated stresses produce local plastic strain (stress

exceeds yield strength) that is critical in cyclic loading.

In practice, prediction of the fatigue life of a material is

complicated because the fatigue life is very sensitive to small

changes in loading conditions, local stresses, service

environment, and local characteristics of the material.

However, the endurance limit of any material under cyclic

stress is lower than its strength under static load. As load

decreases, the number of cycles to failure increases until at

some load, the number of cycles to failure becomes so large

that we need not fear failure. Normally the endurance limit

for sucker rods is defined as the maximum stress that the

string will withstand without failure after ten million or more

cycles of stress.

Generally, when loads are low, only one crack is generated.

Conversely, multiple cracking is a sign of high loads. In addition,

the ratio of prior crack area to total cross-sectional area gives

information about the magnitude of stress at final rupture. With

the use of an electron microscope, direct measurements of

striation spacing offers good insight into the stress environment

during crack growth.

The preparation of a metallographic specimen containing a

crack is often helpful in identifying the crack mechanism.

Fatigue cracks are invariably transgranular or transcrystalline

and sometimes are branched. Fatigue striations are not

evident, however, on profile.

Bending fatigue failures can be divided into three

classifications: one-way, two-way, and rotary. The fatigue

crack formations associated with the type of bending loadare shown in Figure 9.

Case

Stress Condition

No Stress Concentration Mild Stress Concentration High Stress Concentration

Low Overstress A High Overstress B Low Overstress C High Overstress D Low Overstress E High Overstress F

1

One-way bending load

2

Two-way bending load

3

Reversed-bending load-

rotation load

Figure 9: Fracture appearances of fatigue failures in bending by Dr. Charles Upson, " Why Machine Parts Fail," Penton, Cleveland 13, Ohio





API Bulletin RP11BR "Recommended Practice for Care and

Handling of Sucker Rods" recommends using a modified

Goodman diagram (Figure 10) for determining the allowable

range of stress for a string of sucker rods in noncorrosive service.

Figure 10: Modified Goodman diagram for allowable stress and

range of stress for sucker rods in noncorrosive service

Figure 11 shows a number of variables that influence the

fatigue properties. Note that surface notches and corrosion

reduce fatigue strength. Figure 12 shows stress

concentration factors for the various surface conditions.

Figure 11: Generalized relation of ultimate tensile strength (Su ) and

fatigue strength (Sn )

Figure 12: Stress concentration factors for various surface

conditions. This graph is a derating factor for influence of surface

conditions on fatigue.

Sn = 0.5(Su)(C1)(C0)(CS)

Sn = Endurance limit

Su = Ultimate strength

Sn /Su = 0.5

C 1 = 1.0, 0.9, or 0.58 depending on whether the load is

by bending, axial, or torsion, respectively

C 0 = A size factor, usually taken at 1.0 for diameters

less than 0.4 inches and at 0.9 for diameters

between 0.4 and 3.0 inches

C S = Surface factor, which varies for the surface

conditions shown in Figure 12

TMinimum Tensile Strength

T

2

T

4

+T

3

– T

3

0 0

T

1.75

S M I N = M i n i m

u m S t r e

s s , P S I

S A = A

l l o w a

b l e S t r e s

s, P S

I

S A

= (T + M S

MIN) SF 1

S A

= (0.25T + 0.5625 SMIN) SF 2

S A

= S A – S

MIN3

When:

S A

= Maximum available stress, PSI

S A

= Maximum allowable range of stress, PSI

M = Slope of SA curve = 0.5625

SMIN

= Minimum stress, PSI (calculated or measured)

SF = Service factor

T = Minimum tensile strength, PSI

45°

4

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

060 80 100 120 140 160 180 200 220 240 260

120 160 200 240 280 320 360 400 440 480 520

S u r f a c e f a c t o r C

S

Tensile strength Su, ksi

Hardness, Bhn

Corroded in

tap water

Corroded in salt water

Fine-ground or

commercially polished

Mirror-polished

M ac hi ne d

H o t - r o l l e d

As f o r g e d

140

130

120

110

100

90

80

70

60

50

40

30

20

1040 60 80 100 120 140 160 180 200 220 240 260

E n d u r a n c e l i m i t , 1 , 0

0 0 p s

i , S

n

Tensile strength, 1,000 psi, Su

Severely notched specimens

Corroding specimens

Normal for

polished specimens

50% ratio

Rare

cases





Figure 13 shows the importance of limiting the system to

low and intermediate hardness and indicates the importance

of residual stress in fatigue; that is, materials with a greater

endurance limit are higher in carbon and thus have a

higher temperability.

Figure 13: Relation of hardness and fatigue strength for

several steels

Sucker rods have a hot-rolled surface (note derating factor

for hot-rolled materials in Figure 12). For a sucker rod with

100,000-psi tensile strength, the endurance limit for

complete stress reversals would be approximately 50,000 psi

for polished test specimens and 30,000 psi for test

specimens with a hot-rolled surface. A salt water

environment further reduces fatigue strength.

The API modified Goodman diagram (Figure 10) has been

adjusted to indicate a derating for a hot-rolled, shot-blasted

surface and a substantial safety factor of 2. The adjustment

or derating for corrosion (service factor) is selected by each

user as his experience indicates. Figure 14 indicates typical

service factors for various environments.

Environment

Corrosiveness/Derated Service Factor

None

1.0

Mild

0.9

Moderate

0.8

Severe

0.7

H2S 0% X < 10 ppm (0.001%) 10 ppm < X < 100 ppm (0.001% - 0.01%) 100 ppm < X (0.01% and greater)

CO2 0% X < 250 ppm (0.025%) 250 ppm < 1500 ppm (0.025% - 0.15%) 1500 ppm < X (0.15% and greater)

Figure 14: Table of derating factors (NACE Standard MR0176-2000)

150

160

140

130

120

110

100

90

80

70

60

50

20 30 40 50 60

E n d u

r a n c e L i m i t – 1 , 0

0 0 p s i

Rockwell “C” Hardness

B

C

E, F

D

A

A

H-11 Austempered

H-11

Conventional

A = SAE 4063

B = SAE 5150

C = SAE 4052

D = SAE 4140

E = SAE 4340

F = SAE 2340





The API modified Goodman diagram cuts off at neutral (zero)

rather than at reversed stress because the rod string is

susceptible to column buckling and should not be put

into compression.

To avoid exceeding the yield strength (plastic range), the

upper boundary of the shaded portion in Figure 10

represents approximately 58% of the ultimate tensile

strength, which is less than the yield strength.

The API version of the Goodman diagram helps in visualizing

the failure strength of a sucker rod string with a variety of

loads. The relationship is shown in the shaded area of

Figure 10: The top line on the shaded area represents the

maximum stress and the bottom line represents the

minimum stress with which the sucker rod can be loaded

without fatigue failure in a noncorrosive environment. As

the stress range (maximum stress minus the minimum

stress) is reduced, the maximum stress and minimum stress

can be increased. When the stress range is reduced to zero,

the load is static. To operate in the safe range, it is necessary

to determine the maximum allowable loading on the basis of

calculated or measured minimum allowable loading.

Example

(Refer to notes covering calculations and Goodman diagram

in Figure 10).

Example Allowable Sucker Rod Stress Determination Using

Range of Stress

Calculating maximum allowable stress:

= �4 +

= �4+ 0.5625

Where:

SA = Maximum available stress, psi

M = Slope of SA curve = 0.5625

T = Minimum tensile strength, psi

Per API 11B T = :

Grade K = 90,000 psi

Grade C = 90,000 psi

Grade D = 115,000 psi

Smin = Minimum stress, psi (calculated or measured)

SF = Service factor (noncorrosive = 1, H2S = 0.80)

Calculating minimum allowable stress Smin:

= − 4÷ 0.5625

Exercise #1

Assume a string of API Grade C sucker rods with a minimum

tensile strength of 90,000 psi (T) is being used at a minimum

downstroke stress of 10,000 psi (Smin). At what peak polished

rod stress (SA) can we operate this string in noncorrosive

service (SF = 1)?

= �4+

SA = (90,000/4 + 10,000 x 0.5625) 1

SA = (22,500 + 5625) 1

SA = 28,125 PSI in noncorrosive service

Converting to load for different size top rods:

L = Load

SA = Maximum available stress, psi

Arod = Cross-sectional area of sucker rod body

L = SA (Arod )

5/8 in. — 28,125 psi × .307 in2 = 8,634 lb

3/4 in. — 28,125 psi × .442 in2 = 12,431 lb

7/8 in. — 28,125 psi × .601 in2= 16,903 lb

1 in. — 28,125 psi × .785 in2 = 22,078 Ib

Exercise #2

What is the peak polished rod stress at which we can

operate in H2S environments (remember SF for H2S is 0.80)?

= �4 +

= (90,000/4 + 10,000 × 0.5625) 0.80

= (22,500 + 5625) 0.80

SA = 22,500 lb stress in H2S service

Exercise # 3

For API Grade C, given a peak polished rod stress of 35,000

psi, calculate minimum allowable stress (Smin).

= �− 4÷ 0.5625

= (35,000 – 0.25 × 90,000) ÷ 0.5625

= (35,000 – 22,500) ÷ 0.5625

= 22,222 psi in noncorrosive service





To convert for different size top rods in corrosive service:

5/8 in. — 28,125 psi × 0.80 × 0.307 in.2 = 6,908 lb

3/4 in. — 28,125 psi × 0.80 × 0.442 in.2 = 9,945 lb

7/8 in. — 28,125 psi × 0.80 × 0.601 in.2 = 13,523 Ib

1 in. — 28,125 psi × 0.80 × 0.785 in.2

= 17,663 lb

Corrosion-Fatigue

We have defined and discussed metal fatigue. We know that

metals under cyclic loading have a limited useful strength in

a noncorrosive environment and that the endurance limit

(fatigue strength) is primarily dependent on tensile strength.

Sucker rods that operate below the endurance limit in a

noncorrosive environment will withstand an infinite number

of pump strokes (stress reversals) before failure. These

factors affect sucker rod fatigue:

• The range between minimum and maximum tensile

stress: the wider the range, the lower the number of

cycles to produce a fatigue failure

• Pitting, stress cracking, and severe uniform corrosion

• The environment in which the sucker rod is used

The environment has a very pronounced effect on fatigue

strength. In laboratory tests, samples tested in air and in tap

and salt water showed progressively lower fatigue strength.

The more corrosive the environment, the lower the fatigue

strength becomes. Refer to Figure 11 for derating factors for

various surface conditions and environments.

Corrosion combined with cyclic stress is more damaging than

either corrosion or fatigue alone. The part played bycorrosion in this type of degradation is extremely important.

Without corrosion, fatigue failures would be greatly

reduced, or very likely there would be no fatigue cracking

except in the few cases in which fatigue failures were caused

by mechanical notches. However, the presence of corrosion

lowers the endurance limit of steel. There is actually no real

endurance limit in a corrosive environment; whether the

endurance will be higher or lower is dependent on the salt

content of the water and the presence of oxygen, carbon

dioxide, and hydrogen sulfide gases.

Corrosion-fatigue failures can be considered similar to notch

fatigue failures. The difference is that the corrosion-fatiguecrack starts at the root of the corrosion pit, but notch fatigue

starts at the root of mechanical defect.

Corrosion-fatigue can be considered as a type of notch

fatigue in which a point of stress concentration has been

formed by a corrosion pit, and the fatigue progress is

accelerated by the action of corrosion. Not all corrosion pits

produce cracks at the root. General-type pitting typically

does not produce sharp notches, but rather flat or rounded-

bottom pits. This type of pitting is less likely to produce

cracking, but small pits with sharp roots are logical locations

for crack initiation.

There are several types of downhole corrosion and they

affect sucker rods differently. However, all pumping wells

produce fluids that are corrosive to some degree. Figures 15

and 16 show fatigue cracks in corrosion pits, one from a sour

well (H2S) and the other from a sweet well (CO2).

Figure 15: Start of fatigue cracks in corrosion pits in a sour

well (H2S)

Figure 16: Start of fatigue cracks in corrosion pits of a well

containing CO2 (sweet well)





Development of a Modified Goodman Diagram for

Sucker Rods

A fatigue test consists of placing a specimen in the fatigue

machine, subjecting the specimen to a defined load (typically

reverse bending by rotation), and running the machine until

the specimen fails or until it has run in excess of ten

million revolutions.

The process for developing a modified Goodman diagram for

sucker rods includes running a series of fatigue tests. When the

test of the first specimen is complete and the number of stress

reversals required to cause failure has been determined, an

identical specimen is placed in the machine and tested to failure

under a different load. This procedure is repeated several times

using identical specimens and a different load each time. The

results of these tests are plotted in terms of stress and number

of cycles to fracture. A curve (called an S-N curve) similar to the

one in Figure 17 is then drawn.

Figure 17: Effect of decarburization on fatigue strength of rotating

beam specimens of SAE 4140 steel, tempered for normal hardness

of Ro-48 [8].

The maximum stress that will not produce failure in the

material after ten million (107) cycles is referred to as the

endurance limit . It has been determined that when steel is

tested under normal reverse bending stress conditions, its

fatigue strength (endurance limit) is approximately 50% of

the tensile strength. The fatigue strength of steel with rough

surface, as in hot-rolled sucker rods with no corrosion

effects, is approximately 1/3 of the tensile strength when

tested under reverse bending.

The fatigue strengths under these various test conditions can

be related by a Goodman diagram to show the maximum

usable stress for sucker rod materials. It is important to note

that Goodman diagrams are based on a linear relationship

with the tensile strength, not on yield strength.

Figure 18: Variations in completely reversed cyclic stress

Fluctuating stress about a mean stress is shown:

S A = Stress Amplitude

SR = Stress Range

SE = Fatigue Strength

SM = Mean Stress

Smax = Maximum Stress

Smin = Minimum Stress

SU = Tensile Strength

Figure 19: The Goodman diagram, based on polished samples and

reverse bending stress, helps to visualize the change of fatigue

strength of a material under a variety of loads.

In Figure 19, the line AB represents the maximum stress with

which a material can be loaded without fatigue failure. This

stress level is at a minimum when the stress range, SR, is

Fatigue life, cycles

M a x i m u m s

t r e s s ,

k s i

Control (no decarburization)

Decarburized

104 105 106 107

20

40

60

80

120

100

SR

Smax

Smin

S t r e s s

0

+

–

SR

Smax

SM

Smin

S A

S t r e s s

0

+

0

100

50

50

B

10050

SM × 103

S E

× 1 0 3 p

s i

SU

SU

S m a x

S

S m i n

M e a

n

SR S

A

A

Tensile strength = 100,000 psi

Endurance limit in reverse bending = 50,000 psi





maximum (and is a complete reversal from tension to

compression; the average stress is zero). As the stress range

is reduced (left to right on diagram), the maximum stress can

be increased and the minimum stress can be increased

proportionately. When the stress range is reduced to zero,

the maximum stress is increased to equal the tensile

strength, SU, and this is a static load.

The modified Goodman diagram in Figure 20 is based on

as-produced sucker rods, and the endurance limit in reverse

bending is T/3. Because sucker rods do not operate in

compression, the diagram is shifted to the right. Point T/4

was selected as a safety factor because it is not practical to

operate at T/2, which is a statistical value, and scatter in test

results is to be expected. Point T/1.75 is an arbitrary value

that represents about 57% of the tensile strength and is

always below the yield strength.

Figure 20: Modified Goodman diagram based on as-produced

sucker rods for which the endurance limit in reversed bending is T/3

Wear

The Handbook on Failure Analysis and Prevention from the

American Society for Metal describes wear as a surface

phenomenon that occurs by displacement and detachment

of material. Because wear usually implies a progressive loss

of weight and alteration of dimensions over a period of time,

wear problems generally differ from those entailing

outright breakage.3

Although worn parts may break, it is more likely that a worn

part will be removed from service because it no longer

performs satisfactorily or because its performance is

marginal. Although the replacement of a broken part is not

questionable, the replacement of a worn part may be,

particularly in the absence of established standards.

In general, wear may be defined as damage to a solid surface

caused by the removal or displacement of material by the

mechanical action of a contacting solid, liquid, or gas. When

a failure is caused by one type of wear, analysis may be

relatively simple. However, many wear failures are caused by

combined modes of wear.

Sucker rods and couplings exhibit wear by one or a

combination of the following modes:

• Abrasion: Displacement of material from a surface by

contact with hard projections on a mating surface (metal-

to-metal contact) or by hard particles, such as sand and

corrosion products, trapped between two sliding surfaces

(Figure 21)

• Adhesion wear: Wear occurs when two metallic surfaces

slide against each other under pressure (also described as

scoring, galling, seizing and scuffing)

• Erosive wear: Abrasive wear involving loss of surface

material by contact with a fluid that contains foreign

matter or particles

• Corrosive wear: A mode of wear in which chemical or

electrochemical reaction contributes to the wear rate

(Figure 22); for example, the pitting caused by

CO2 corrosion.

• Erosion-Corrosion: A type of wear in which there is

relative movement between a surface and a corrosive

fluid (Figure 23). The fluid may or may not contain

abrasive particles. In cavitation erosion, the repeated

formation and collapse of vapor bubbles at the surface

imposes contact stresses that may cause pitting

or spalling.

Figure 21: Extreme abrasion wear on a coupling

T

0

T

S A = A

l l o w a b l e

S t r e s

s ( p s i )

S m i n

T

4

T

2

T

1.75

T

3+

T

3 –

T = SU = Tensile





Figure 22: Corrosive wear of a rod body. The rod has rubbed against the tubing, exposed clean metal, and initiated

localized corrosion attack and subsequent pitting and grooving.

Figure 23: A rod and guide exhibiting erosion-corrosion of the rod from well fluids passing through the gap in the guide.

Tensile FailuresTypically, tensile failure of a sucker rod is not a service-oriented

failure. Such failures typically have one of these causes:

• Tensile stress overload of the string while trying to free a

stuck pump

• Pulling the pin off the rod upset while over-tightening the

joint with uncalibrated power tongs

•

Torsional overload in a PCP application

When load exceeds the tensile strength of the rod or pin, the

failure is identifiable by the necked-down area (shown in

Figure 24) and cup and cone (Figures 25 and 26) with the 45°

shear lip.

Figure

24: A photo of a sucker rod pin overtightened using power tongs.

Note the necking down of the pin undercut.

Figure 25: Tensile failure showing the cone breakface sheared at

45°. This was a lab failure created during routine testing.

Figure 26: Sucker rod pulled in two when a stuck sucker rod string

was overpulled.





Designing to Reduce Failures

We have discussed sucker rod and sucker rod coupling

failure mechanisms and presented examples of each failure

mode. Understanding of the factors contributing to service

failures is necessary to control them. Corrective action can

improve the service life of the equipment.

This chapter will describe various conditions that cause

sucker rod and coupling failures and steps that are available

for preventing future failures. In most cases, illustrations

will be shown that represent the primary cause of each

failure mechanism.

Mill DefectsWhen a sucker rod fails prematurely, you will often hear the

customer say "bad steel," "faulty material," or "that string

was in service only a few months, and the string it replaced

lasted 5 to 6 years—must be a bad heat of steel."

These opinions are rarely justified because failures caused by

faulty material almost never happen. However, if a customer

does experience a failure related to a mill defect, the cause

will most likely be a surface defect such as a scab or sliver.

Figure 27 shows a scab (sliver), a loose or torn segment of

material or debris rolled into the surface of the bar. One end

of these particles of metal is metallurgically bonded to thebody of the rod. The remaining section is rolled into the bar

surface but only attached physically. If the particle is

dislodged, a deep surface pit remains, normally with a sharp

root. The scabs and slivers act as notches in the surface of

the metal, reducing the fatigue-endurance limit by a ratio of

perhaps two or three to one. The condition then develops

into a notch fatigue failure.

Figure 27: Surface damage caused by a sliver

For mill defects, the obvious corrective measure is to inspect

the affected string, preferably by flux-leakage magnetic

equipment, and to discard rods with surface defects.

Inspection service companies provide this service. Also most

manufacturers of sucker rods have inspection equipment in

house for inspection of bars before processing.

Manufacturing Problems

In rare cases, sucker rods are shipped with manufacturing

defects. The most common defects are forging laps in the

bead, undersize pin threads, oversize coupling threads,

forged-in scale pits in the rod body adjacent to the bead,

forging laps on the square, and deep steel stamp marks in

the flats of the upset square.

Please bear in mind that when we talk about manufacturing

defects, we are including all sucker rod manufacturers. No

manufacturer is exempt. You will find, however, that failures

caused by manufacturing defects are very uncommon.

A deep stamp mark in the steel can result in a fatigue crack

that progresses to ultimate failure. The stamp mark is asharp notch that raises local stress and increases surface

stresses in the notch to the point of exceeding the

endurance limit. An example can be seen in Figure 28. This

type of problem can be controlled by using steel stamps with

less sharpness to reduce penetration in the square and

reduce root sharpness at the bottom of the stamp mark.

Sometimes early failures in surface notches in the upset are

caused by a combination of high loading and the notch

effect. In such cases, redesigning the string to a lower





operating stress level will eliminate the failure in the square

and improve service life.

Figure 28: Classic example of a notch fatigue failure

Handling Problems

Mishandling is a factor in rod and coupling failures. Examples

include mechanical damage to the rod surface, improper

joint makeup, bending or kinking the rods, and hammering

the surface of couplings.

In most cases, damage by mishandling could have been

avoided or the damaged part could have been discarded toeliminate the possibility of an early failure. Training of field

personnel will reduce handling-related problems.

Field personnel can support users by keeping them informed

of any handling-related failures. Field personnel might also

suggest a course of action that could be effective in

correcting handling problems.

Mechanical Damage

Usually, mechanical damage is the result of a permanent

deformation in the surface of the rod. Figure 29 shows an

example in which fatigue started at the root of the damage

and progressed to a depth at which the rod cross-sectioncould not support the operating load.

Figure 29: Deformation in the surface of a rod and the resulting

fatigue crack that led to tensile failure.

Improper Joint Makeup

For proper makeup, the API sucker rod joint is designed so

that the pin is in tension. The important factor is that the

joint must be tightened sufficiently to induce a preload in

the pin—a preload high enough to prevent the contact faces

from separating when the string is under its maximum

tensile load. If the joint has insufficient torque, the first full

thread root will not only be subjected to a high range of

stress, but will also be exposed to bending, as seen in

Figure 30. Early failure occurs if the rod string carries any

appreciable load.

Figure 30: Results of improper makeup of coupling to rod. The photo

on the top shows damage to the first full thread root; the photo on

the bottom shows damage to the coupling. Both failures were

caused by under-torquing.





Applying the proper circumferential displacement to the

joint during makeup is highly recommended. When power

tongs are out of calibration, too much torque can be applied

during makeup, and the pin can break under tensile

overload. Loose joints not only cause pin fatigue failures, but

the separation of the pin-coupling faces also allows corrosive

fluids to enter the coupling and initiate corrosion fatiguefailures of the coupling or pin. Refer to API Publication

RPIIBR, "Recommended Practice for Care and Handling of

Sucker Rods," for proper methods to determine correct joint

makeup and to control connection failures.

Separation of joint faces has also been attributed to

unscrewing of the joint. Without the drag of friction on the

mating surfaces and with the smooth finish or rolled

threads, each stroke permits a little rotation until the

rod string separates with no apparent damage to either

pin or coupling.

Bent RodsStraightness is important on heavily loaded rods. A bent rod

(Figure 31) will produce a high order of cyclic stress

variations and cause an early failure. Under load, any degree

of bend imposes higher tensile stresses on the inside, or

concave side, of the bend compared to the same load on a

straight rod. A string operating at its maximum loading will

be over stressed at the concave side of the bend. When the

endurance limit is exceeded, the string will fail from fatigue

that started on the inside of the bend.

Figure 31: A load of rods, some bent

API Specification 11B Twenty-seventh Edition, November 1,

2011, for sucker rods, Page 24, Section A.6.1, “Straightnessand Surface Finishes” specifies body and end straightness for

5/8-in. to 1 1/8-in.-diameter rods:

• Body straightness: Within any 12 inches, the maximum

allowable bend is 0.065 inch (0.130 TIR)*.

*Total indicator run out (TIR) is the total dial gauge deflection measured at

the rod surface as the rod is rotated 360°. The bend of TIR valves is twice

the amount measured by straight edge.

• End straightness: As measured by supporting the rod body

at a distance of 6 inches from the rod pin shoulder and

measured by dial indicator riding on the machined pin

shoulder, the maximum allowance is 0.130 TIR.

Most bent rods are caused by rough handling in shipment,

by improper handling while running the string in and out of

the hole, or by dropping the string (Figure 32). Bending can

also be caused by fluid pound, gas pound, or tagging bottom.

Failure caused by bending is identified by these features:

• All fatigue cracks are on one side of the bar.

• Corrosion pits may or may not be present.

• The bar is visibly bent and failure started on the

concave side.

• The break face is not perpendicular to the rod body.

Figure 32: Bent sucker rods most likely damaged by dropping the

rod string

Rods that are bent to the maximum API recommended

straightness, when under load, are stressed about 10% more

on the concave side (Figure 33).





Figure 33: Calculation of stress in concave area

Guided Sucker Rods

Sucker rod guides have evolved from simple metal scrapers

to a highly engineered thermoplastic product. With the large

increase in directional and horizontal wells, rod guides are

used not only to remove paraffin from tubing and sucker

rods, but also to protect and stabilize the sucker rod string.

Rod guides can significantly increase the life of a sucker rod

string by eliminating rod and tubing wear. However, there

are some disadvantages to using rod guides.

Application IssuesDuring the pump cycle, all rod guides disrupt the fluid flow,

some much worse than others. This causes an increase influid velocity and a low pressure zone. Because of this

disruption, a low pressure zone is created on the upper side

(closest to the surface) of the rod guide, as seen in Figure 34.

The increase in fluid velocity can cause fluid erosion, and the

low-pressure zone can cause erosion-corrosion and CO2

breakout.

Figure 34: Rod guides can increase the corrosion rates on

production tubing.

As the rod guide contacts the tubing surface, it can remove

corrosion inhibitor or any protective scale that has formed.

This will increase the corrosion rate at the contact location.

When this occurs, it typically appears as a groove the width

of the rod guide vane. It can also appear as though the rod

guide has worn through the tubing.

Rod guides will add weight to the rod string and increase the

contact friction on the tubing. This should be taken into

consideration when using guides. Proper rod guide design and

placement are critical. An improperly designed rod and guide

system will shorten the run life of the sucker rod string.

Poor Pumping Conditions

Sucker-rod body wear and coupling wear can be indications

of poor pumping conditions:

• Poor pumping speeds can induce compression loads on

the pump downstroke and cause buckling and rod wear

(Figures 35 and 36). This condition can be correctedby proper design of the rod string and reducing the

pumping speed.

• Deviated wells cause coupling and rod wear (Figure 37).

• Wear is also an indication of cork-screwed tubing from

improper tension on the anchor or packer.

• Fluid and gas pounding can cause rod buckling. Reduce

pump size or reduce pumping speed to correct fluid

pound. Gas interface can be reduced with a pressure

regulator or application of a specific downhole pump and

gas separator.

•

Coupling wear can occur in a rod string with a rod rotator

installed but no rod guide protection (Figure 38).

Figure 35: An example of rod wear.

Figure 36: Cracking along one side of the rod (flexing) caused by

improper pumping conditions

L

P

P

X

y

∆d

L = Length in inches

P = Load in pounds

E = Mod. of elas. 29×10 6

∆ = Out-of-straignt (in inches)

d = Diameter (inches) of sucker rod

A = Square area of sucker rod

[Stress at Concave Area = ] (1)

Stress =4∆Ed

+P

L2 A

[Stress Concentration Factor = ] (2)

SCF = 1 +1.414 • 108 • ∆ • d3

PL2 + 1.767 • 107 (d4)





Figure 37: Extreme coupling and rod wear that most likely was

caused by running through a deviated well without rod guide protection. A spray metal coupling can be used when wear or

corrosion is a problem. Caution should be taken when using spray

metal couplings.

Figure 38: Coupling wear in a rod string with a rod rotator installed

but no rod guide protection

Corrosion Problems

As discussed earlier, the word corrosion denotes destruction

of metal by chemical or electrochemical action. Chemical

corrosion, although starting rapidly, often slows as soon as

an obstructive layer of corrosion products forms upon the

metal surface. If, however, this corrosion product is

continuously being cracked by bending or being removed byrubbing or other mechanical action, corrosion will continue

unchecked at its original rapid rate. Familiar examples of this

conjoint action on sucker rods are corrosion fatigue (in

which cyclic stress ruptures the corrosion by-product layer),

down-hole wear, and impingement attack by gas and fluid.

The major corrosives encountered in oil wells are carbon

dioxide, hydrogen sulfide, and oxygen dissolved in water.

Practically all well fluids produced by sub-surface pumps and

rods are corrosive to some degree. The corrosivity varies not

only from field to field, but also from well to well in the same

field. It also varies with time in any well.4

The different types of corrosion are generally characterizedby pit shape and scale formation.

Carbon Dioxide Corrosion

Pits created by carbon dioxide corrosion are normally

deep with sharp edges and round bottoms as in Figure 39.

The scale is iron carbonate, which is hard and grey to

black in color. Pits may connect or channel in high fluid-

flow environments.

Figure 39: Corrosion damage to sucker rods by a carbon dioxide

(sweet corrosion) environment

Hydrogen Sulfide Corrosion

The iron sulfide produced by the action of hydrogen sulfide

and water on steel typically adheres to the steel surface as a

black powder or scale. The scale tends to cause a local

acceleration of corrosion because the iron sulfide is cathodic

to the steel. The pits are usually scattered on the metal

surface and are saucer-shaped with round edges (Figure 40).

Cracks will form in the root of the pit. When placed in dilute

hydrochloric acid, the corrosion by-product (iron sulfide) will

release an odor like that of rotten eggs. The hydrogen

released in the reaction enters the steel to cause

embrittlement or to form molecular hydrogen, which leadsto blisters and cracks.

Figure 40: Corrosion damage by a hydrogen sulfide environment

(sour corrosion)





MIC (Microbiologically Influenced Corrosion)

MIC is a form of corrosion that originates from a large

growth of bacteria that can live in oxygen-rich (aerobic) or

oxygen-free (anaerobic) environments. Downhole

environments tend to be oxygen free, so most MIC found in

the wells are anaerobic. MIC tends to be localized, so large

isolated pits can form on sucker rods. Two types of MIC aretypically found downhole: sulfate reducing bacteria (SRB)

and acid producing bacteria (APB). Both forms can cause

significant damage if left untreated.

SRB generate hydrogen sulfide and contribute to localized

corrosion by their ability to grow in the absence of oxygen.

The hydrogen sulfide reacts with iron in solution to form iron

sulfide (FeS) precipitate and scale. The FeS scale is cathodic to

steel (Figure 41), SRB corrosion pits tend to be isolated and to

have shallow bottoms with soft edges. Many times SRB

corrosion creates worm-like pits surrounding the larger pit.

Figure 41: The FeS scale is cathodic to steel, resulting in corrosion of

scale-free areas which, in turn, adds more iron to the solution.

Acid-producing bacteria produce organic acids as a by-

product that reduces the pH, which can dissolve the sucker

rod. APB corrosion pits tend to interconnect with sharp-

edged, flat-bottomed pits. Figures 42 and 43 show examples

of both types of MIC corrosion.

Figure 42: Example of SRB corrosion

Figure 43: Example of APB corrosion

Oxygen Corrosion

Subsurface equipment in oil wells is subject to oxygen

corrosion only if oxygen from the air is introduced into the

well. The presence of carbon dioxide or hydrogen sulfide

increases the rate of oxygen corrosion. Corrosion of

downhole equipment in the oxygen environment usually

shows a general form of attack, sometimes producing large,shallow, flat-bottom pits (Figure 44).

Figure 44: A typical form of oxygen corrosion. Localized attack may

result in deep pitting, and the corrosion by-product is ferric oxide.

Remedial measuresImproving corrosion-fatigue life of sucker rods requires

inhibiting corrosive wells, which also gives added protection

to the tubing string and the well casing. Because corrosion is

a surface reaction, any modification of the steel-corrosive

media interface will affect the rate of corrosion. When

added to a corrosive system, specific chemicals, called

inhibitors, modify the interface to reduce the corrosion rate.

All of the major inhibitor suppliers can furnish effective

inhibitors and proper application for reducing corrosion in

most fields. Even under the best conditions, however,

inhibitors will not be 100% effective.

Protective coatings, such as epoxy, have been used withsome success on sucker rods. The difficulty is achieving a

coating free from pin holes and handling damage. Spray

metal couplings have been used successfully for many years

to reduce corrosion rates.

Oxygen corrosion in oil wells is best controlled by the

exclusion of oxygen. The casing valve should always be

closed to the atmosphere. If production is reduced by closing

the casing, then a small check valve or ball and seat should

be installed on the casing. This will allow gas to vent to the





atmosphere by holding only an ounce or two of pressure to

exclude oxygen from the annulus.

The beneficial effects of lowering maximum stress levels in a

corrosive environment have been discussed previously.

Using alloy rods in place of carbon or carbon manganese

rods has been successful as a means of improving rod life ina corrosive environment, but no sucker rod is impervious to

corrosion.

The most reliable way to avoid sulfide stress cracking is to

use non-susceptible materials. If high-strength rods are

necessary, then an effective inhibition program should

be used.

Problems Caused by a Stuck Pump

Pulling on the rod string to unseat a stuck pump can cause

accidental overload, excessive rod stretch, or tensile

breakage. Excessive stretch occurs when the load exceeds

the yield strength. Breakage occurs as the load increasesfrom beyond the yield strength to the tensile strength. If

sucker rods are permanently stretched by overload, there

will most likely be localized, external, and perhaps internal

damage that may lead to early failures. Consequently, the

affected rods should be removed from service.

Calculating the maximum allowable pull can prevent

stretching and tensile failure (Table 1).

Rod Type Size (in.)

Load

(lb) (DaN)

MD

5/8 23,400 10,400

3/4 33,800 15,000

7/8 45,900 20,4001 60,000 26,600

D

5/8 27,600 12,200

3/4 39,700 17,600

7/8 54,100 24,000

1 70,600 31,400

1-1/8 89,400 39,700

KD

3/4 37,700 16,800

7/8 51,400 22,800

1 67,100 29,800

1-1/8 84,900 37,700

Grade HD

T66/XD

3/4 45,700 20,300

7/8 62,200 27,600

1 81,200 36,100

1-1/8 102,800 45,700

S67 67D

3/4 43,700 19,400

7/8 59,500 26,400

1 77,700 34,500

1-1/8 98,400 43,700

S873/4 45,700 20,3007/8 62,200 27,600

1 81,200 36,100

S88

3/4 51,600 22,900

7/8 70,300 31,200

1 91,800 40,800

1-1/8 116,200 51,700

EL® rod

5/8 35,900 15,900

3/4 51,600 22,900

7/8 70,300 31,200

1 91,800 40,800

1-1/8 116,200 51,700

Table 1: Maximum weight indicator pull (load) that can be applied

to a stuck sucker-rod string





Size (in.)

Weight

(lb/ft) (kg/m)

5/8 1.114 1.657

3/4 1.634 2.432

7/8 2.224 3.310

1 2.904 4.322

1-1/8 3.676 5.471

Table 2: Weight of sucker rods per foot

Note: The ratings are based on 90% of the minimum yield strength for a sucker-rodstring in “like new” condition. The maximum pull should be reached with a steadypull and not a shock load. For a tapered string, calculate the weight of the suckerrod above the smallest and lowest section, and add the calculated weight to thevalue tabulated here for the type and size of the lower section. For a single-tapersucker-rod string, the values tabulated here are the maximum pull.

CalculationsSF – safety factor

Sa – allowable stress (psi)

Sy – yield strength of sucker rod (psi)

A – cross-sectional area of sucker rod (in2)

L – load in lbs

Sa = Sy x SF

L = Sa x A

weight of rods = weight per foot (see Table 2) x length of rodsection

Sy – sucker rod maximum yield strength (psi)

Type of Rod API C

Minimum yield strength 60,000 PSI

Smallest rod 3/4 in.

500 feet of 7/8-in. rods above the top 3/4-in. rod

1. 60,000 psi × 0.90 = 54,000 psi

2. 54,000 psi × 0.442 sq. in. = 23,868 lb

3. Weight of 7/8-in. rods above the top 3/4-in. rod

500 ft × 2.224 lb/ft = 1,112 lb

4. Maximum allowable pull in pounds

25,194 + 1,112 = 24,980 lb

Rods that have been in service for a length of time may be

damaged by corrosion or corrosion-fatigue to a degree that

breakage will occur when the maximum calculated loads are

applied. Under these conditions, the string should be

thoroughly inspected, and affected rods should be discarded

before rerunning. If the pump cannot be unseated, the

tubing should be pulled and rods backed off.

Hammering the Surface of Couplings

When pulling a well or when a coupling needs to be

removed after installation, many rig crews “warm up” the

coupling by hammering. Hammer blows cause mechanicaldamage to the coupling and can induce cracking. The cracks

can be stress raisers and sites for corrosion fatigue. Damage

from hammering can become points where localized

corrosion will start its attack. Hammering the faces of any

coupling may result in an improper joint makeup that can

cause pin fatigue.

Hammering on spray metal couplings causes surface cracks

in the hard surfacing (Figure 45) and will cause localized

corrosion and fatigue. Any coupling that has been

hammered on should be discarded.

Figure 45: Spray metal coupling with damage caused by hammer

blows to the surface

Thread Galling

Thread galling can occur because of damaged or dirty

threads (Figure 46). Joints seldom cross-thread because the

pin must be aligned in the coupling recess before the first

threads engage. Cross-threading, however, may be possible

when power tongs are used in field assembly and the

threads are damaged during stubbing.

Coupling and pin threads must be clean and lubricated

before assembly; if power tongs are used, the API RP11BR

recommendation for sucker rod joint makeup should

be followed.





Figure 46: Thread galling during makeup. This rod has been

overtightened, which stripped the threads and deformed the

makeup face.

Failure Analysis

In discussing failure analysis work, you must consider failure

prevention. Engineering teamwork is required to create the

kind of reliability that will give a product a superior

advantage over its competitors. However, a vital ingredient

in attaining such reliability is an adequate method for

analyzing the inevitable failures that occur duringengineering tests or during service. You must also divide

your products into two categories: (1) downhole equipment

with no threat of injury or life and (2) surface equipment for

which failure could be a threat to life or could cause injury.

All parts have a finite life. Acceptable limits are mostly

established by the user, economic situation, environment,

and competition. Remember that when a part fails, there is a

reason for failure and there are corrective measures. This

chapter is concerned primarily with general procedures,

techniques, and precautions used in the investigation and

analysis of metallurgical failures that occur in service.

Figure 47 is a schematic of the stages of failure analysis.

Figure 47: Stages of a failure analysis





Collection of Background Data and Selection

Failure investigation should be directed first at gaining a

good understanding of the conditions under which the part

was operating. The investigator must know as much as

possible about the manufacturing processes and service

histories of the failed component and must reconstructinsofar as possible the sequence of events leading to the

failure. Unfortunately, in the majority of instances, the

investigator will receive a failed part with little information

about its history and operating conditions. In such cases, the

physical evidence must be the sole basis for the analysis.

Service history depends on how detailed and thorough the

recordkeeping was before the failure. Service history should

include environmental details, such as normal or abnormal

loading, accidental overloads, cyclic loads, temperature,

temperature gradients, and corrosive environment. When

service data is sparse, the analyst must deduce service

conditions, and much depends on his skill and judgmentbecause misleading deductions can be more harmful than

the absence of information.

Preliminary Examination

The failed sample must be visually examined and documented

photographically to create a permanent record of the evidence

for later reference in light of new information that may become

available. The failed parts should be examined before any

surface cleaning to document the status of the surface

condition; for example, rust and scale that indicate the type of

environment in which the part was operating. The visual

examination typically will allow the investigator to identify the

mode of fracture (brittle, ductile, fatigue, etc.), points ofinitiation, and direction of propagation. Visual examination can

be aided by low magnification tools such as a stereomicroscope.

Photographic documentation should place particular

importance on fracture surfaces and surface defects. The

use of normal shadows can give depth to a surface, which

makes it easier to photograph and to direct attention to

important details.

Study of the FractureWhere fractures are involved, the next step in preliminary

examination should be general photography of the entire

fractured part, including broken pieces, to record their size

and condition and to show how the fracture is related to thecomponents. Next should be a careful examination of the

fracture faces to determine the areas of prime interest and

what magnification is needed to bring out fine details.

Photograph the fracture face at the magnification

determined to give the most revealing details.

Non -Destructive Testing (NDT)

NDT Method Capabilities

Radiography

• Measures differences in radiation

absorption

•

Inclusions, porosity, cracks

Ultrasonic

• Uses high-frequency sonar to find surface

and subsurface defects

• Inclusions, porosity, thickness of material,

position of defects

Dye

penetrate

• Uses a die to penetrate open defects

• Surface cracks and porosity

Magnetic

particle

• Uses a magnetic field and iron powder to

locate surface and near-surface defects

• Surface cracks and defects

Eddy current

• Based on magnetic induction

• Measures conductivity, magnetic

permeability, physical dimensions, cracks,

porosity, and inclusions

Table 3: Nondestructive Tests

Mechanical Testing

Hardness testing, the simplest of the mechanical tests, is

often the most versatile tool available to the failure analyst:

• Assists in evaluating heat treatment (hardness

requirements)

• Enables an approximation of the tensile strength of steel

• Detects work hardening

• Detects softening or hardening caused by overheating, by

decarburization, or by carbon or nitrogen pickup.

Two other useful mechanical tests are tensile and impact

tests and ductile-to-brittle transition tests.

Remember that the effects of size in fatigue stress-corrosionand hydrogen embrittlement testing are not well

understood. However, on the basis of the limited evidence

available, it appears that resistance to these failure

processes decreases as specimen size increases.





Selection and Preservation of Fracture Surfaces

Preservation of fracture surfaces is important to prevent

important evidence from being destroyed. Usually

protection during shipment will include wrapping samples in

cloth, cotton covering, or other suitable soft material. Avoid

contact of the fracture with chemicals, including water, thatcould result in corrosion damage.

Sectioning

When sectioning specimens, use these techniques to protect

the fracture area:

• Flame cut far away from fracture

• Dry saw

• Carbide cutoff with liquid coolant

Secondary Cracks

When a primary fracture has been damaged or corroded

such that most of the evidence is obliterated, it is desirableto open any secondary cracks to expose their fracture

surfaces for examination. Secondary cracks may provide

more information than the primary fracture.

Macroscopic Examination of Fracture Surfaces

Examination of fracture surfaces at 1× to 100× diameter can

be done with the unaided eye, a hand lens, or a low-power

stereoscopic microscope. Occasionally it may be

advantageous to use a scanning electron microscope at

low magnifications.

Extensive information can be obtained from examining a

fracture surface at low-power magnification. Fracturesurfaces may give an indication of the stress system that

produced failure. Failure in monotonic tension produces a

flat (square) fracture normal to the maximum tensile stress

under plane-strain conditions and to a slant (shear) fracture

at about 45° if plane-stress conditions prevail. Because pure

plane-strain and pure plane-stress conditions are ideal

situations that seldom occur in service, many fractures are

flat at the center but surrounded by a picture frame of slant

fractures. The slant fracturing occurs because conditions

approximating plane strain operate at the center of the

specimen but relax toward plane-stress near the surfaces.

An example of this behavior is the familiar cup-and-conetensile fracture. In thin sheets or small-diameter rods, full

slant fractures may occur because axial stresses are relaxed

by plastic deformation that impedes plain-strain from

developing, making the crack propagation easier in a

slant direction.

Macroscopic examination can usually determine the

direction of crack growth and hence the origin of failure.

With brittle, flat fractures, determination depends largely on

the fracture surface showing "chevron marks" of the type

shown in Figure 48. The direction of crack growth is almost

always away from the tips of the chevrons. Chevron marks

occur because nearly all cracks are stopped at an early stage

in their development; as the crack front expands, the traces

of the steps form chevron marks. The photograph in

Figure 48 shows the fracture surface of a 5-in.-diameter, seal

stem, tubular section. As indicated by the arrows, thefracture reveals multiple crack propagation paths depicted

by the chevron marks.

Figure 48: Chevron patterns point to the multiple origins of the

fracture.

Where fracture surfaces show both flat and slant structures,

it may be generally concluded that the flat fracture occurred

first. Conversely, if a fracture has begun at a free surface, the

fracture-origin area is usually characterized by a total

absence of slant fracture or shear lip.

Low-power examination of fracture surfaces often reveals

regions that have a different texture from the region of final

failure. Fatigue, stress corrosion, and hydrogen

embrittlement fractures may all show these differences.

Figure 49, which shows the fracture surface at the sucker rod

body, is an excellent example of the type of information that

can be obtained by macroscopic examination. The chevron

marks clearly indicate that the fracture origin is at the point

marked by the arrow. This region, unlike the rest of the

fracture, has no shear lip. The flat surface suggests that the

stress causing the failure was tension parallel to the lengthof the rod. Note the difference in texture.





Figure 49: Fracture surface of a sucker rod at approximately actual

size, showing point of initiation (at arrow) Chevron marks, and

development of shear lips.

Metallographic ExaminationMetallographic examination of polished and etched sections

by optical microscopy is a vital part of failure investigation

to determine:

• Class of material and structure

• Whether abnormalities are present

• Heat treatment

• Corrosion, oxidation, work hardening of surfaces

• Characteristics of any cracks and their mode of

propagation.

•

Location of sample in respect to fracture origin.

Fracture Classifications

Although a satisfactory classification of failures involving

fractures does not exist, fractures will be classified in terms

of their growth mechanism. Crack initiation will not be

considered. Table 4 lists different modes of fractures and

characteristics of each.

Mode of Fracture Typical Fracture Surface Characteristics

Ductile

Cup and cone

Dimples

Dull surface

Inclusion at the bottom of the dimple

Brittle

intergranular

Shiny

Grain boundary cracking

Brittle

transgranular

Shiny

Cleavage fractures

Flat

Fatigue

Benchmarks

Striations (SEM)

Initiation sites

Propagation area

Zone of final fracture

Table 4: Modes of fractures and their characteristics

Ductile FractureDuctile fractures are characterized by tearing of metal and

gross plastic deformation. Ductile tensile fractures in most

materials have a gray, fibrous appearance and are classified

on a macroscopic scale as flat-face (square) or shearface

(slant) fractures.

Flat-face tensile fractures in ductile materials are produced

under plane-strain conditions (that is, in thick sections) with

necking. These fractures typically occur normal to the

direction of loading, and some shear lip is formed at the

junction of the fracture surface and the part surface.

Microscopic examination at >100× of flat-face tensile

fractures in ductile materials will reveal equiaxed dimples in

the flat-face region.

Brittle fracturesThere are two types of brittle fractures: transgranular

clearage and intergranular. The transgranular facetsobserved on brittle fractures are produced by clearage along

numerous parallel crystallographic planes, thus creating a

terraced fracture surface. The intergranular fractures are

grain surfaces that have been exposed by crack propagation

along grain boundaries (a rock candy fracture surface).





Fatigue Fracture

Fatigue fractures result from repeated or cyclic stresses,

each of which may be substantially below the nominal yield

strength. The most noticeable macroscopic features of

classic fatigue-fracture surfaces are the progression marks

(also known as bench marks, clamshell marks, or tide marks).

Little macroscopic ductility is associated with fatiguefracture. Microscopically, surfaces of fatigue fractures are

characterized by presence of striations, each of which is

produced by a single cycle of stress; however, every cycle

does not produce a striation.

Stress-Corrosion Cracking

Stress-corrosion cracking is a mechanical-environmental

failure process in which mechanical stress and chemical

attack combine in the initiation and propagation of fracture

in a metal part. Stress-corrosion cracks may be intergranular,

transgranular, or a combination of both. Transgranular

fractures that show branching are typical of stress-corrosion

cracking of austenitic stainless steels of an 18cr-8 nitype. Figures 50 and 51 show examples of stress-

corrosion cracking.

Figure 50: Intercrystalline stress corrosion cracks in precipitation

hardened stainless steel UNS S17400 alloy at 300X magnification;

solution etched using Vilella’s reagent

Figure 51: Transcrystalline stress corrosion cracking in a Type 316L

stainless steel gas lift valve assembly at 50X magnification; solution

etched using Fry’s reagent

Embrittlement by Liquid Metals

Embrittlement can occur as a result of liquid metal around

grain boundaries, that is, penetration of copper alloys by

mercury and penetration of steel by molten tine and

cadmium. Polished samples or polished and etched sections

can be examined by microscope. Positive identification is

possible by electron-microprobe analysis.

Hydrogen embrittlement (sulfide cracking)

Positive identification of hydrogen embrittlement is often

difficult, and it is frequently impossible to differentiate

between hydrogen-induced, delayed fracture and stress-

corrosion-cracking fracture. Analysis should includethe following:

• Check hardness

• Check for H2S

• Run metallographic examination

Chemical Analysis

Perform chemical analysis to confirm analysis of materials.

Unreported gaseous elements (or interstitial) have profound

effects on mechanical properties of some metals. In steel the

effects of oxygen, nitrogen, and hydrogen are of major

importance. Oxygen and nitrogen may cause strain agingand quench aging. Hydrogen may induce embrittlement.

Failures Resulting From Corrosion Damage

Various forms of corrosion attack render parts inoperable.

These modes were discussed earlier in this document.





References

1.

Bucaram, S.M., Byors, H.G., and Kaplan, M. “Proper Selection, Handling, and Protection of Downhole Materials – APractical and Economical Approach.” Paper No. 55. Corrosion/72

2. Madayag. 1969. “Metal Fatigue, Theory and Design.” Wiley. New York. 2─126.

3. “Failure Analysis and Prevention.” Metals Handbook. Vol. 10. American Society for Metals. Metals Park, Ohio.

4. Corrosion of Oil-and-Gas-Well Equipment. Book 2 of the Vocational Training Series. API.

5. Dvoracek, L. M. "Corrosion Fatigue Testing of Oil Well Sucker Rods Steels." Union Oil Company of California. Union

Research Center, Brea, California 92621.

6. Mehdizadeh, P., McGlasson, R. L., and Landers, J. E. 1963. "Corrosion Fatigue Performance of a Carbon Steel in

Production Environments." Paper presented to the South Central Region Conference of NACE. New Orleans, Louisiana.

October 18─22.



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