1

Failure analysis of a steel elbow pipe from a gas well

María GARCÍA-MARTÍNEZ1; Mª Pilar VALLES GONZÁLEZ1; Alejandro

GONZÁLEZ MEIJE1; Ana PASTOR MURO1

1National Institute of Aerospace Technology (INTA). Materials and Structures Department.

Ctra. Torrejón-Ajalvir 28850, Torrejón de Ardoz, Spain

Abstract

A steel elbow pipe from a gas well, through which circulated water pressurized at 4 kg/cm2 and at 10-

30°C temperature, was perforated originating a water leakage. The pipe was fabricated with carbon steel

type ASTM A234 WPB which is widely used in pressurized pipeline systems for services at moderate

and high temperature. The elbow presented a weld bead, due to its union with the pipe system was made

by TIG welding. The exterior surface of the elbow was painted. Several tests were carried out in order

to determine the cause of the perforation.

Visual and radiographic inspections have been carried out in order to detect possible defects in the weld

bead. A chemical analysis of the elbow material was performed. Mechanical characterization of the

material was carried out by a hardness test. Microscopy and fractographic studies were executed using

a field emission scanning electron microscope (FE-SEM) equipped EDX. A semicuantitative analysis

of weld bead was carried out by EDX. To characterize the steel microstructure, metallographic samples

were prepared to be observed using an optical microscope.

Corrosion processes were originated in the inner pipe area. The presence of CO2 in the water might be

very important for the corrosion phenomena in the welding area as a result of the presence of a galvanic

couple between the weld bead and the pipeline steel. However the CO2 was not the only cause of the

pipe damage. The fractographic study of the perforations and the flow analysis indicated that cavities

have been originated as a consequence of a cavitation-corrosion phenomenon which is a particular

erosion process caused by the implosion of gas bubbles on the metal surface.

Keywords

Failure analysis, Cavitation-corrosion, Tribo-corrosion, Steel elbow pipe, Gas well

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Introduction

Pipelines failures represent a very serious and expensive problem in the oil and gas industry.

These industries expend the equivalent of millions of dollars every year to repair damages [1-

4]. In the case of oil and gas wells, when CO2, which accompanying natural gas as impurity,

dissolves in formation water, carbonic acid corrosive to the carbon steel is formed [5-8].

Pipelines and component fittings of the lines would undergo material degradations due to

corrosion. This material degradation results in the loss of mechanical properties like strength,

ductility, impact strength, and so on. This leads to loss of materials, reduction in thickness, and

at times ultimate failure. Serious consequences of the corrosion process have become a problem

of worldwide significance.

Besides corrosion, erosion has been identified as a potential damage and failure mechanism in

pipelines and elbows employed in oil and gas wells. Elbows are the weak parts of gathering and

transferring pipelines [9, 12]. The term “erosion” applies to deterioration due to mechanical

force. When the factors contributing to erosion accelerate the rate of corrosion of a metal, the

attack is called “erosion-corrosion”. Erosion-corrosion is affected by velocity, turbulence,

impingement, presence of suspended solids, temperature, and prevailing cavitation conditions.

Cavitation-corrosion is a particular form of erosion-corrosion caused by the "implosion" of gas

bubbles on a metal surface [13, 14]. Cavitation is not itself a type of surface deterioration, but

a hydrodynamic phenomenon that can result in a certain type of surface deterioration fairly

common. Its impact on the behaviour of water conduction pipes, hydraulic pumps, marine

propellers, etc, is crucial and must be considered both the design of these devices and the

characteristics of the materials used [15]. Cavitation occurs on metal surfaces in contact with a

liquid. Pressure differentials in the fluid generate gas or vapor bubbles in the fluid. When these

bubbles encounter a high-pressure zone, they collapse and cause explosive shocks to the

surface. These surface shocks produce localized deformation and pitting. Cavitation pits

eventually link up and cause a general roughening of the surface and material removal.

Cavitation is similar to particle erosion in its damage. However, surface characteristics formed

by cavitation are different from those formed by particle erosion. Cavitation produces rounded

microcraters in the surface, while particle erosion produces imprints of the impacting particles.

Crater formation moves surface material to the edges of the craters, and these extrusions

eventually break off, causing loss of material from the surface [16].

In this work, a tube section of a failed elbow pipe from a gas well was studied to understand

the root cause of failure.

Background

A gas production company reported a failure of a steel elbow pipe from a gas well. The 45º

elbow, through which circulated water pressurized at 4 kg/cm2 (400 kPa) and at 10-30 °C

temperature, was perforated originating a water leakage in the inner part (see Fig. 1a where

flow direction advancement is indicated). The tube had an inner diameter of 10” and a wall

thickness of 0,25”. The pipe was fabricated with carbon steel type ASTM A234 WPB and the

exterior surface was painted. The perforation was originated next to the weld bead of the elbow,

which was made by TIG welding. The company cut the failure pipe section with an angle

grinder for its inspection. The mentioned cut was performed approximately in the middle of the

weld bead cross section in order to facilitate the replacement of the damaged pipeline (Fig. 1b).

Furthermore, the elbow part was exposed to outdoor conditions and could suffer atmospheric

corrosion.

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a) b)

Fig. 1. a) Failed elbow pipe in the field of the gas well. b) Cut tube section of the failed elbow

pipe

Typical analysis result of water analyzed by the company during the period of reported failure

is given below:

pH 6,9 Total suspended solids (mg/l) 44 Chlorides (mg/l) >5.000 Sulphides (mg/l) <0,40 Iron (mg/l) 43

Materials and method

Firstly, visual observation of the fractured elbow and through a stereo microscope Leica Wild

model M10 was performed. Then radiographic inspection has been carried out in order to detect

possible defects in the weld bead of the elbow. The weld bead was cut in four section for its

inspection.

The elbow pipe material was chemical analyzed using an X-Ray Fluorescence Spectrometer

(XRF) Panalytical PW2404 and fusion and combustion techniques with LECO equipment. The

weld bead was semi-quantitative analyzed by X-ray dispersive energy (EDX) Oxford Inca

coupled to a field emission scanning electron microscope (FE-SEM) Jeol model 6500F owing

to there wasn´t enough material to analyze by XRF.

Corrosion deposits were collected and analyzed with a Panalytical X´Pert Pro X-ray diffraction

equipment (XRD) equipped with an X-ray tube of copper.

Three metallographic samples from three different areas, on the longitudinal section, were

prepared in a conductive resin. One of them containing an area diametrically opposite to that of

the cavities and the other two containing each of the cavities. In the case of cavity 2, the area

chosen was that which contained the perforated section originating a water leakage. The

mounted samples were polished and examined using a stereo microscope Leica Wild model

M10, an optical microscope (OM) Leica model MEF4M and a field emission scanning electron

microscope (FE-SEM) Jeol model 6500F. Then, the samples were chemical etched with Vilella

etching (1 g of picric acid, 5 ml of HCl in 100 ml of ethanol) to reveal the microstructure in the

OM. The grain size was obtained using the standard ASTM E112 planimetric method.

Two more metallographic samples from weld bead were prepared in a conductive resin in order

to inspect the weld bead for possible defects.

Mechanical characterization of the material was carried out by a hardness test using a durometer

Galileo, according to UNE–EN ISO 6506-1 standard.

LEAKAGE

FLOW DIRECTION

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Fractographic studies of the area which contained cavity 3 were carried out using a scanning

electron microscope (SEM) Jeol model JSM-840 with a Bruker EDX microanalysis system.

Previously, the area was softly brushed with water and soap and an ultrasonic bath in order to

remove corrosion products.

Results and discussion

Visual observation

Inspections, both visual and using a stereoscopic microscope, revealed three principal cavities

next to the weld bead on the inside radius of the elbow, typical of a cavitation-corrosion

phenomenon. In addition, yellow and grey deposits were observed covering most of the inner

pipe area (Fig. 2a and b). Moreover, blistering of the deposits in the area next to the cavities

were detected. Such blistering may correspond to one of the signs of damage of cavitation-

corrosion failure mechanism. The areas not covered with deposits had a brown coloration

similar to iron oxide.

a) b)

b) d)

e)

Fig. 2. a) Failed elbow pipe as received. b) Yellow and grey deposits and blistering. Brown

areas not covered by deposits in the inner of the pipe. Cavities in the weld bead. c) Front view

of the inner elbow failure pipe with three cavities. d) Front view of the cavity 2 with the

perforation and scratches. e) Front view of the outer wall of the elbow.

In Fig. 2c the three cavities are shown at further magnifications. Cavity 1 was out of the weld

bead and cavities 2 and 3 into the weld bead, originating a step between both areas. Cavity 2

presented the perforation through which the water leakage occurred (Fig. 2d). Superficial

PERFORATION SCRATCHES

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scratches were observed, maybe done during the dismantled of the failure section pipeline.

Brown corrosion products were observed in the three cavities area but no corrosion deposits

mentioned were noted, may be due to the dismantled of the failure section pipeline for its

replacement. The outer wall of the elbow was painted, although in the cavities area there were

areas in which the paint has disappeared (Fig. 2e).

Radiographic inspection

Nondestructive radiographic inspection of the weld bead was carried out as a result of the

detection by the company of possible defects, such as pores and melting lack in some weld

beads of the water line.

The inspection revealed the presence of three important cavities (indicated by yellow arrows in

figure 3), as has already observed in the visual inspection, and two more cavities smaller (red

arrows in figure 3). The green arrows indicated in Fig. 3b were scratches observed in the outer

wall.

a) b)

Fig. 3. a) Cut samples containing the cavities that were radiographically inspected. b)

Radiographic inner view of the cut samples from figure 3a.

Multiple corrosion indications were noted with circular geometry that were generally located

in the base material (marked with a yellow circle in Fig. 4a). The radiographic image where the

corroded areas were visualized is shown in Fig. 4b (blue arrows) as areas more radio-opaque

(darker) and therefore more thickness loss. Although no discontinuities of the weld bead were

detected, some suspicious deficient areas were detected and consequently needed a further

inspection with metallographic techniques (areas marked with blue and white rectangles in Figs.

4a and b). Red arrows in Figs. 4a and b point the areas in the weld bead with little bites that

correspond to more radio-opaque (darker) areas in the radiographic image.

a) b)

Fig. 4. a) Cut sample subjected to radiographic inspection. Corroded areas are indicated with

the yellow circles. Suspicious deficient areas of the weld bead marked with blue and with

rectangles. b) Radiographic inner view of sample from figure 4a. Corroded areas (blue arrows)

and suspicious deficient areas of the weld bead are more radio-opaque (darker).

Chemical and microstructural characterization

The elbow pipe chemical composition obtained by XRF and by fusion and combustion

techniques is shown in Table 1, concluding the material is a carbon steel type ASTM A234

WPB in accordance with the company specifications.

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Fe C Si Mn P S

Balance 0,20 0,21 0,90 0,02 0,07

Table 1. Steel elbow pipe chemical composition in weight percent

The semi-quantitative chemical composition of the welding bead, determined by EDX coupled

to a FE-SEM due to there wasn´t enough material to analyze by XRF, is shown in Table 2.

Regarding the values of Table 2 and considering the limitations of this technique for not

determining the carbon composition, which is a fundamental element to classify the weld bead

steel, it may be concluded that the weld bead material corresponds also to a carbon steel, whose

composition seems to be suitable for welding with carbon steel pipe.

Fe Si Mn

Balance 0,37 1,37

Table 2. Weld bead chemical composition in weight percent

Microstructure consisted on ferrite-perlite grains slightly deformed along the rolling direction

(Fig.5a and b). Grain size analysis resulted in a very small grain corresponding to 9-10 size,

according to the ASTM E112 standard three circles method. According to the information of

the company, an outer layer of paint was detected (Fig. 5c).

a) b) c)

Fig. 5. a) Grains deformed along the rolling direction. b) Ferrite-perlite microstructure. c)

Paint layer of the outer wall.

Mechanical characterization

The Brinell hardness value resulted from the hardness test was 143 HB, which corresponds to

strength level of 480 MPa, which is within its operating limitations according to ASTM A 234

standard [17].

Weld bead inspection

Weld bead from different areas of the elbow pipe was inspected by stereo microscope and by

OM resulting in a good appearance of the welding with no detectable defects. Areas examined

were those marked on the radiographic inspection as well as weld bead next to the three

principal cavities. For example, the acquired images for the welding area of cavity 2 and far

away from the cavities are shown in Figs. 6 and 7, respectively. In Fig. 7 the perforation through

which the water leakage occurred can be observed.

PAINT LAYER

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a) b) c)

Fig. 6. a) Welding far from cavities area. b) At higher magnifications, image in OM of transition

area between weld bead and steel. c) Normal affected area by heat during welding in steel.

a) b) c)

Fig. 7. a) Welding in cavity 2 area and perforation of the elbow pipe. b) At higher

magnifications, image in OM of transition area between weld bead and steel. c) Normal affected

area by heat during welding in steel.

Considering the visual and weld bead inspection it may be concluded that there is absolutely

no relation between the failure and the welding, owing to some cavities were originated outside

weld bead and welding was properly applied.

Chemical analysis of the corrosion deposits

Chemical analysis by XRD of the inner wall and collected corrosion deposits were performed

(Fig. 8). Onto the inner wall, ferrous-ferric oxide (FeO, Fe3O4), silica, calcium silicate and iron

sulphide were detected as majority compounds, in accordance with the analysis result of water

analyzed by the company. Regarding the deposits, they were differentiated according to their

coloration, being referred to as yellowish deposit and gray deposit. In the first one particles of

calcium silicate, ferrous-ferric oxide, iron carbonate and calcium sulphate were detected. In the

second one particles of silica, iron oxide and iron carbonate and calcium sulphate were detected.

Sulphide, carbonate and iron oxides detected are consequence of the corrosion phenomenon of

the steel pipe. Iron carbonate had been able to be formed as consequence of the presence of

CO2 dissolved in the circulating water. The presence of this gas in the water might be very

important for the corrosion phenomena in the welding area as a result of the presence of a

galvanic couple between the weld bead and the pipeline steel [5-8]. However, due to the loss of

material in the cavities not only occurred in the weld bead, but also far from it and did not

extend to the entire weld bead, the CO2 was not the only cause of the pipe damage.

1

2

1

2

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a)

b)

c)

Fig 8. XRD analysis of: a) corrosion deposits directly onto the inner wall, b) yellowish

corrosion deposits and c) grey corrosion deposit.

Microscopic studies

The study by optical microscopy of the longitudinal section of cavity 2, which was composed

by a small cavity with the water leakage perforation located in the weld bead and a larger cavity

located in the steel, showed a material fold and grains deformation at the top and at the bottom

of the perforation pipe (Fig. 9). This deformation may be occurred due to the pressurized water

flowing through the perforation.

Also oxides layers were observed on both sides of the pipe wall, on the inner and on the outer

wall. The composition of the oxides was determined by EDX in FE-SEM, verifying that the

ones located in the inner wall were mainly iron oxides, sulphur and silicon, which may

correspond to sulphide or sulphate and silicate or silica, which is in accordance with the XRD

analysis in the previous section (Fig. 10a). On the other hand, the ones located in the outer wall

were mainly iron oxides and silicon which may correspond to silicate or silica (Fig. 10b).

SiO2 CaSiO3 FeS Fe3O4 FeO

SiO2 Ca2SiO4 FeCO3 Fe3O4 Fe2O3

Fe3O4 SiO2 CaSiO4 FeCO3

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a) b) c)

Fig. 9. Images of etched cavity 2: a) Upper area of the cavity with oxides layer on the inner

and outer wall. b) Material fold and grains deformation. c) Lower area of the cavity with oxides

layer on the inner and outer wall and grains deformation.

a) b)

Fig. 10. EDX spectrum of oxides layer in cavity 2 where a) iron oxide, sulphur and silicon and

were detected in the inner wall and b) iron oxide and silicon in the outer wall

a)

b) c)

d) e)

Fig. 11. a) Coating SEM image with four layers. EDX spectrum of: b) Layer 1 next to material

base where iron oxide, silicon and aluminium were identified. c) Layer 2 rich in zinc. d) Layer

3 rich in silicon, magnesium and aluminium. e) Layer 4 of paint coating rich in carbon, oxygen,

titanium, sulphur and iron.

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In addition, the coating which protected the outer wall was disappeared in some points. FE-

SEM images shows four layers. The first layer and adhered to the steel, corresponded to an iron

oxide, the second one to some type of primer rich in zinc, the third one to an organic layer

composed by carbon, oxygen, silicon, magnesium and aluminium, and the fourth one to the

outer paint layer, rich in titanium, sulphur, carbon and oxygen (Fig. 11).

Fractographic analysis

Fractographic studies of cavity 3 showed cavities and protuberances in the weld bead area (Fig.

12a). In addition, corrosion attack associated with microstructure was observed due to the

stacking of crystals in Fig. 12b. In this sense, corrosion processes were originated in the inner

pipe area, according with the visual observation and the microcopy studies.

a) b)

c) d)

Fig. 12. Microfractographic studies of cavity 3:a) cavities and protuberances in the weld bead

area. b) Stacking of crystals. c) Microcavities outside the weld bead and d) Microcavities at

higher magnification.

One of the characteristics of cavitation pitting is that the pitting occurs primarily in a pattern on

the surface, with most of the pits concentrated in specific areas, depending on the local vibration

or flow characteristics. In the case of pipelines these areas are downstream. According to this

and taking into account the visual observation and the flow analysis, cavities have been

originated as a consequence of a cavitation-corrosion phenomenon which is a particular erosion

process caused by the implosion of gas bubbles on the metal surface. Another characteristic of

cavitation damage is that this type of phenomena usually takes place in inner area closest to the

curvature center of the elbow, on both sides of the longitudinal pipe axis and in the region where

the elbow ends following the direction of fluid advancement, just where the cavities have been

detected in this case of study. As fluid flows through an elbow (see the scheme in Fig. 13), the

centrifugal forces are inversely proportional to the radius of the elbow’s curvature. A vacuum

is created on the inside radius of the elbow, originating cavitation phenomenon and the damage

will be jagged (Fig. 12c and d). On the other hand, a high stagnation pressure appears on the

outside pipe wall, resulting in erosion damage.

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Fig 13. Diagram of the flow of the fluid in the elbow

In the light of the results obtained, there are several possible factors involved in this cavitation

process:

Liquid composition, density, surface tension, and viscosity

Dissolved gas content, like CO2

Flow velocity

Pipeline design

Material

Vibrations

Though, there is not enough information to determine which of the previous factors could take

part in the cavitation process and regarding the cavitation-corrosion phenomenon is progressive

and hence time is needed to loss of material of inner surface of the pipelines for the perforation

of the wall and the water leakage takes place, it was recommended to carry out an ultrasonic

nondestructive inspection of other elbows in the gas well facility. In addition, others ways to

avoid cavitation is a reduction the flow velocity, streamlined fluid flow by design changes to

avoid sudden pressure drops, stiffen the part to change vibration characteristics, reduce surface

roughness and materials changes, i.e. using a metal with a higher fatigue strength can slow but

not eliminate the damage. If none of these possible solutions work, it would be necessary to

simply replace the parts during schedule maintenance.

Conclusions

Chemical analysis and microstructure study conclude that the elbow material presented a

microstructure that consisted on ferrite-perlite grains, slightly deformed along the rolling

direction, and was a carbon steel type ASTM A234 WPB with a Brinell hardness value of 143

HB, in accordance with the material standard and the company specifications. Regarding the

weld bead material it may be concluded that corresponded also to a carbon steel, whose

composition seems to be suitable for welding with carbon steel pipe.

Visual and weld bead inspection resulted in there was absolutely no relation between the failure

and the welding, owing to some cavities were originated outside weld bead and the welding

was properly applied.

Corrosion processes were originated in the inner pipe area where sulphides, carbonates and iron

oxides deposits from the steel corrosion were detected. Iron carbonate had been able to be

formed as consequence of the presence of CO2 dissolved in the circulating water. However, due

to the loss of material in the cavities not only occurred in the weld bead, but also far from it and

did not extend to the entire weld bead, the CO2 was not the only cause of the pipe damage.

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Besides, the fractographic study of the perforations and the flow analysis indicated that the

cavities have been originated as a consequence of a cavitation-corrosion phenomenon.

Regarding the cavitation-corrosion phenomenon is progressive and hence time is needed to loss

of material of inner surface of the pipelines for the perforation of the wall and the water leakage

takes place, it was recommended to carry out an ultrasonic nondestructive inspection of other

elbows in the gas well facility. In addition, others ways to avoid the cavitation were also

recommended.

Acknowledgements

Authors want to thank all Metallic Materials Area staff, especially to the Microanalytical and

Microstructural Characterization Laboratory.

This research did not received any specific grant from funding agencies in the public,

commercial or not-for-profit sectors.

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