characterization corrosion

7/30/2019 Characterization Corrosion

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ABSTRACT

Boiler tubes have occasionally failed in refineries and gas

plants. The Research and Development Center (R&DC) has

helped plant engineers overcome problems by identifying the

nature and source of compounds leading to failure (e.g.,

corrosion products, formation materials and scale deposits).

The boiler consists of a furnace, boiler tubes, steam drum,

mud drum and boiler. The furnace provides heat to the boiler,changing water into steam. The presence of certain

compounds in sample deposits from boiler tubes can indicate

why they fail. The presence of vanadium and sodium

compounds in the sample indicates the quality of burning

fuels is poor. If large quantities (>15%) of hematite, Fe2O3,

appear in the deposits, it indicates the presence of dissolved

oxygen in the boiler feed water. If iron carbonate appears in

the samples, it indicates the presence of dissolved carbon

dioxide (CO2) in the system. If the metallic copper is present

in the deposits, it indicates erosion in the boiler. In the last

case, special precautions to prevent the plating out of copperduring cleaning operations are required. The procedures used

to identity corrosion products, formation materials and scale

deposit materials will be described.

INTRODUCTION

The furnace, boiler tubes, steam drum, mud drum and boiler

are all parts of the boiler. When the furnace provides heat to

the boiler, the water changes into steam. The failure of boiler

tubes has been observed in refineries and gas plants for several

years. The failures occurred mainly due to corrosion erosion,

deposition and scale formation. Due to the failure of boiler

tubes, the refinery and gas plants were shut down, ultimately

causing a financial loss. Therefore, the Research and

Development Center (R&DC) provides support to plant

engineers in identifying the nature and source of compounds

leading to failure (e.g., corrosion products, formation materials

and scale deposits), using X-ray powder diffraction (XRD) and

X-ray fluorescence (XRF) techniques. The findings will help

engineers to take proper action to prevent future occurrences,

avoiding plant slowdowns that result in loss of production.

ADVANTAGES OF THE XRD TECHNIQUE

XRD is an excellent analytical technique used for the phase

identification of a crystalline material in the form of a solid or

powder, such as a catalyst, scale deposit, chemical, core, shell,

clay mineral and cement1-10. XRD identifies compounds,

whereas XRF, induced coupled plasma and atomic absorption

techniques only identify elements. A sample XRD and XRF

analysis is given in Table 1.

The XRD method1-10 differentiates between different forms

of a compound with the same chemical formula. If the sample

is calcium carbonate (CaCO3) either it is calcite (scale

formation materials), aragonite (scale), or vaterite. Additionally,

XRD identifies the forms of a compound, such as the iron

sulfides (FeS), that have different chemical formulae; e.g., pyrite

(FeS2), marcasite (FeS2), mackinawite (FeS0.9), pyrrhotite (Fe7S8)

and greigite (Fe3S4). It is very important to know the form of

(FeS), because some of these iron sulfides are pyrophoric.

Furthermore, the XRD technique can differentiate the

hydration state of compounds, e.g., gypsum (CaSO4.2H2O),

bassanite (CaSO4.0.5H2O) and anhydrite (CaSO4).

XRD APPLICATIONS IN SAUDI ARAMCO

The X-ray Group of the Analytical Service Division fully

supports the R&DC Downstream and Strategic and Upstream

research projects, on topics such as scale mitigation, oil to

hydrogen, pipeline integrity, black powder and catalysts. We

support the Engineering Service Agreement projects, including

mineralogical determination for the Qusaiba shale in the

northwest region (ERAD), analytical services support for the

Operation Service Division of the EXPEC ARC, and

hydrajetting impacts in the filtration system. We support theTechnical Service Projects (TSPs) in facilities, such as the

64 SPRING 2011 SAUDI ARAMCO JOURNAL OF TECHNOLOGY

Characterization of Corrosion Products inSaudi Aramcos Oil and Gas Facilities Usingthe X-ray Powder Diffraction Method

Authors: Dr. Syed Rehan Zaidi, Dr. Husin Sitepu and Ahmed A. Al-Shehry

Table 1. XRD phase identification and XRF elemental analysis of corrosion products

Weight Percentage (Wt%)

XRF XRD

Element Wt% Element Wt% Compound Wt%

Fe 60.4 Ca 0.2 Magnetite-Fe304 44

S 28.8 Si 0.1 Hematite-Fe2O3 28

Na 0.5 P 0.1 Pyrite-FeS2 7

Al 0.2 - - Sulfur-S 21


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Rabigh refinery, Jiddah refinery and Ras Tanura refinery

(RTR), Berri gas plant (BGP), Juaymah gas plant and Abqaiq

gas plant, and the Consulting Services Department (CSD).

In the present study, we identified the compounds of

deposits formed in the boilers of refineries and gas plants by

using the XRD technique, as requested by the TSP Program

Director, and determined the nature, source and formation

mechanism of the deposits. Once all the phases for each of the

deposits in the XRD data had been identified using HighScore

Plus software, then the quantitative phase analysis, i.e., weight

percentage (Wt%) for each phase, was completed using the

Rietveld method7-9. The findings will help refinery and gas

plant employees to take proper action to prevent future

occurrences.

EXPERIMENTAL PROCEDURE

Sample

The starting powders were collected from boiler tubes F-3001

in the Jiddah refinery, the RTR boiler #8 tubes, screen tubes

of a boiler at the Yanbu gas plant and at boilers at the Berri

gas plant. The deposits were considered to be excellent

canidates for this study because these deposit materials caused

the failure of the refineries and gas plants boiler tubes. The

untreated deposit samples were manually ground with an

agate mortar and a pestle, and homogenized. Subsequently,

5 grams of the homogenized deposit sample were further

manually ground for several minutes to achieve a fine particle

size. This grinding was conducted to achieve adequate

intensity reproducibility7-8. The fine powder was then

mounted into the XRD sample holder by back pressing.

XRF Spectra Measurements

XRF is one of the powerful analytical tools used to determine

the elemental composition of all kinds of materials. In this

study, we used the SPECTRO energy dispersive X-ray

fluorescence (EDXRF) spectrometry to analyze the elemental

compositions of the corrosion product samples. This

spectrometry simultaneously detects the element from sodium

(Na) to uranium (U). The concentration range goes from the

part per million level to 100%. Due to the X-ray interaction

with electrons, the elements with high atomic numbers

provide better accuracy than the lighter elements.

Four grams of the fine powder were mixed well and

homogenized with 0.9 grams of Licowax C micropowder PM

binder (Hoechstwax). The homogenized mixtures were

pressed at a pressure of 20 tons to form pellet samples with a

diameter of 31 mm. The pellet samples were then irradiated

with X-ray photons from a molybdenum X-ray tube. The

energies of the X-rays emitted by the sample were measured

using a silicon semiconductor and were processed by a

multichannel analyzer. Subsequent processing of the data

provided spectral information that identified the elements

present in the sample and the intensities of the

X-rays emitted by these elements. The intensity of the rays

was processed by the instruments software to determine the

elemental concentrations.

XRD Data Measurements

Step-scanned patterns were measured with a PANalytical

XPERT XRD diffractometer. The XRD data were measured

from 4 to 90 in a 2 Bragg angle. A position sensitive

detector (PSD) XCelerator was used with a scanning step time

of 10 seconds, ensuring reasonable intensity counting

statistics. PSD length in 2 was 2.12, with irradiated and

specimen lengths of 1 mm and 10 mm, respectively. A detector

step size of 0.01 was employed to provide adequate sampling

of the peaks (full width at the half maximum (FWHM)

approximately 0.11 to 0.34). All of the samples were spun

during the data collection to improve particle counting

statistics, so the intensities are not dominated by a small

number of crystallites. Figure 1 shows the goniometer of the

XRD instrument used in this study.

XRD Phase Identification

For the qualitative analysis (i.e., phase identification), the

software package PANalytical HighScore Plus, combined with

the International Center for Diffraction Data (ICDD) and

powder diffraction file (PDF) database of the standard

reference materials, was used. Elemental composition data

obtained from the EDXRF analysis was used as additional

information for the phase identification.

Quantitative Phase Analysis (QPA) Using the

Rietveld Method

Quantitative Phase Analysis (QPA) of multicomponent

mixtures10-14 using XRD data10 has been used worldwide to

determine the Wt% for a given phase. This QPA method does

not require measurement of calibration data or the use of an

internal standard; however, knowing the approximate crystal

structure of each phase of interest in a mixture is necessary.

The use of an internal standard10 will allow the determination

of total amorphous phase content in a mixture. Analysis of

synthetic mixtures has yielded high-precision results, with

SAUDI ARAMCO JOURNAL OF TECHNOLOGY SPRING 2011 65

Fig. 1. The parts of the XRD goniometer and optics.


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to take corrective measures, preventing scale buildup andavoiding future tube failure. Table 2 shows the elementalcomposition obtained from the EDXRF spectrometry and thechemical compounds obtained from the XRD technique.

Vanadium, sulfur and sodium are present in fuel oil. Whenfuel oil is burned, vanadium and sodium compounds presentin the fuel in high quantities react with oxygen to form V 2O5and Na2O in the furnace, and they stick to the metal surface.The V

2O

5and Na

2O react on the metal surface to form a low

melting point phase of the conpound, such as sodium vanadate.Under optimum conditions, they can form a liquid that fluxesthe protective oxide scale, exposing the underlying metal tooxidation. Therefore, these ash deposits pose potentialcorrosion problems.

The presence of sodium oxide and vanadium oxide in ashdeposits suggests the occurrence of fuel ash corrosion. Tomitigate this corrosion, fuel oils that contain low quantitiesof vanadium, sodium and sulfur are used. If the aboveoption is not possible, a fuel additive treatment can beadopted to prevent the formation of a low melting point

phase of the sodium vanadate complexes. Additivescontaining magnesium and aluminum oxide have beensuccessful in controlling fuel ash corrosion.

Identification of Scale Removed from RTR Boiler Tubes

Scale deposits were observed in the high-pressure boilertubes at RTR. Figure 2 shows two samples of the affectedtube sections submitted to the R&DC TSP ProgramDirector: (1) A section cutout from wall and screen, and (2)A large tube. For the tube in Fig. 2a, an analysis wasrequired to identify the deposits. The identification of the

deposits led to a procedure to chemically clean the boilertubes without damaging them.

errors generally less than 1.0% absolute. Since this techniquefits the complete diffraction pattern, it is less susceptible toprimary extinction effects and minor amounts of preferredorientation6-9. Additional benefits of this technique overtraditional quantitative analysis methods1-5 include: (1) Thedetermination of precise cell parameters and approximatechemical compositions, and (2) The potential for thecorrection of preferred orientation6-9 and microabsorptioneffects15.

The weight of a phase in a mixture is proportional to theproduct of the scale factor, as derived in a multicomponentRietveld analysis of the powder diffraction pattern, with themass and volume of the unit cell. If all phases are identifiedand crystalline, the weight fraction Wof phasep is given by:

where s, Z, M and Vare the Rietveld scale factor, the numberof formula units per unit cell, the mass of the formula unitand the unit-cell volume (in 3), respectively. This equation is

the basis of a method providing accurate phase analyseswithout the need for standards or for laborious experimentalcalibration procedures. It is noted that the Rietveld methodfor quantification of a mixture of 20 phases using theHighScore Plus software (with a total of 6,000 reflections) is30,000 times more powerful than the reference intensity ratiomethod1-5 for the quantification of just two phases of themixture.

RESULTS AND DISCUSSIONS

Analysis of External Deposits from Boiler Tubes in the

Jiddah Refinery

The Jiddah refinery boiler is an oil-fired boiler. A hugeaccumulation of ash deposit was observed on the externalsurface of the tubes. The engineers asked the R&DC TSPProgram Director to identify the deposits to determine thesource and formation mechanism, so they could use the results

Table 2. Summary of elemental and chemical compositions of the Jiddah boiler

deposits


XRF Elemental XRD Chemical Composition

Composition

V 31.5 Vanadium oxide - V2O5 70

S 5.9 Sodium vanadium oxide - 15

NaV2O5Ni 5.4 Sodium vanadium sulfate 13

hydrate - Na(SO4)2. H2O

Na 4.6 Mackinawite - FeS 2

Si 1.3 Calcite - CaCO3 Trace

Fe 1.2 - -

Al 0.9 - -

Ca 0.2 - -

Figs. 2a and 2b. Scale deposits accumulated in high-pressure boiler tubes at RTR.

2a 2b

Fig. 3. XRD histogram of deposits from boiler tubes at RTR (inside screen tube)

along with the reference patterns of identified phases.


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The XRD results, Table 3 and Figs. 3 to 6, showed that the

scale deposits scraped from the insides of the tubes mainly

consisted of iron oxide corrosion products with calcium

phosphate hydroxide (apatite) and magnesium phosphate

hydroxide. The deposits removed from the outside of the large

tube consisted of calcium sulfate and iron oxide corrosion

products. Additionally, the high percentage of hematite might

indicate the presence of dissolved oxygen in the boiler water.

Identification of Deposits from Screen Tubes of a Boiler at

Yanbu Gas Plant

Scale deposits were observed in a high-pressure boiler at the

Yanbu gas plant. The boiler tube failed due to these deposits.

The deposits were removed from the boiler tube andsubmitted by the CSD to R&DC to support failure analysis

work. The XRD results, Table 4 and Fig. 7, showed a high

percentage of hematite and metallic copper, which indicates

the presence of dissolved oxygen in the boiler feed water and

also erosion in the boiler tubes.

Identification of Scale Deposits Removed from BGP

An unknown material produced with a sulfur product

was found in a condenser at a plant in the BGP sulfur

recovery unit. The plant engineers concern was that the

unknown material might be from the super claus catalyst

(alumina and silica). If this was the case, it meant that the

mesh holding the catalyst has a pinhole, causing a catalyst

leak, which would require total plant shutdown and catalyst

removal to repair or replace the mesh.

The XRD results, Table 5 and Figs. 8 to 10, showed no

alumina or silica, as expected by the BGP engineers. It meant that

the mesh holding the catalyst is good. Ammonium hydrogen

sulfate can be formed in the boiler feed water due to treatment

with the chemical compound, which contains ammonia. The

formation of ammonium hydrogen sulfate can be avoided by

increasing the furnace temperature to burn the ammonia.


Table 3. Summary of chemical compositions of deposits from boiler tubes at RTR


Screen Side Large Large

Compounds Tube Wall Tube Tube

Inside Inside Inside Outside

Magnetite-Fe304 41 46 60 47

Hematite-Fe2O3 34 28 14 12

Hydroxylapatite -

Ca5(PO4)3(OH) 13 13 11 -

Magnesiumphosphate

hydroxide

Mg2(PO4)OH 12 13 15 -

Anhydrite - CaSO4 - - - 13

Talc - Mg3Si4O10(OH)2 - - - 8

Table 4. Summary of results of deposits from a boiler at Yanbu gas plant

Compound Weight Percentage (Wt%)

Magnetite - Fe3O4 39

Hematite - Fe2O3 35

Copper - Cu 26

Fig. 4. XRD histogram of deposits from boiler tubes at RTR (inside wall tube)


Fig. 5. XRD histogram of deposits from boiler tubes at RTR (inside large tube)


Fig. 6. XRD histogram of deposits from boiler tubes at RTR (outside large tube)


Fig. 7. XRD histogram of deposits from a boiler at Yanbu gas plant along with

the reference patterns of identified phases.


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oil is not good; if hematite is present in the boiler deposits, it

means that the boiler feed water contains dissolved oxygen; and if

the metallic copper is present in the deposits, it indicates erosion

in the boiler tubes. Special precautions must then be taken to

prevent the plating out of copper during cleaning operations.

ACKNOWLEDGMENTS

The authors would like to thank the management of Saudi

Aramco for permission to publish the results in this article.Awad M. Al-Mofleh, Yazeed Al-Dukhayyil and Abdulelah Al-

Naser are acknowledged for their encouragement and support

for this study. Thanks are also due to Fahad Al-Khaldi for his

help in preparing the XRF samples.

REFERENCES

1. Chung, F.H.: Quantitative Interpretation of X-ray

Diffraction Patterns of Mixtures I. Matrix-flushing Method

for Quantitative Multicomponent Analysis,Journal of

Applied Crystallography, Vol. 7, 1974a, pp. 519-525.


Diffraction Patterns of Mixtures II. Adiabatic Principle of

X-ray Diffraction Analysis of Mixtures,Journal of

Applied Crystallography, Vol. 7, 1974b, pp. 526-531.


Diffraction Patterns of Mixtures III. Simultaneous

Determination of a Set of Reference Intensities, Journal of

Applied Crystallography, Vol. 8, 1975, pp. 17-19.

4. Klug, H.P. and Alexander, L.E.: X-Ray Diffraction

Procedures for Polycrystalline and Amorphous Materials,

2nd

edition, New York: John Wiley & Sons Inc., 1974.5. Jenkins, R. and Snyder, R.L.: Introduction to X-ray

Powder Diffractometry, New York: John Wiley & Sons

Inc., 1996.

6. Sitepu, H., Sherik, A.M., Zaidi, S.R. and Shen, S.:

Comparative Evaluation of Cobalt and Copper Tubes

using X-ray Diffraction Data for Black Powder in Sales

Gas Transport System, paper 10100, presented at the 13th

Middle East Corrosion Conference, Manama, Bahrain,

February 14-17, 2010.

7. OConnor, B.H., Li, D.Y. and Sitepu, H.: Strategies for

Preferred Orientation Corrections in X-ray PowderDiffraction Using Line Intensity Ratios, Advances in

X-ray Analysis, Vol. 34, 1991, pp. 409-415.

8. Sitepu, H., OConnor, B.H. and Li, D.Y.: Comparative

Evaluation of the March and Generalized Spherical

Harmonic Preferred Orientation Models Using X-ray

Diffraction Data for Molybdite and Calcite Powders,

Journal of Applied Crystallography, Vol. 38, 2005, pp.

158-167.

9. Sitepu, H.: Texture and Structural Refinement of Neutron

Diffraction Data of Molybdite (MoO3) and Calcite

CONCLUSIONS

XRD is an excellent tool to determine the nature, source and

formation mechanism of deposits formed by the processes in the

various units of refineries and gas plants. The XRD results can

guide the engineers at the affected refinery and gas plant to

overcome the problems by devising the right corrective

procedures. For example, if sodium and vanadium compounds

appear in the samples (ash deposits) examined, it indicates the fuel

Table 5. Summary of chemical compositions of deposits from BGP


Plant Plant Plant

Compounds Deposit as White Yellow

Received Part Part

Ammonium hydrogen

sulfate - (NH4)3H(SO4)2 84.8 100 92.2

Sulfur - S 15.2 - 7.8

Fig. 8. XRD histogram of the as-received deposits from BGP along with the

reference patterns of identified phases.

Fig. 9. XRD histogram of deposits (white part) from BGP along with the

reference patterns of identified phases.

Fig. 10. XRD histogram of deposits (yellow part) from BGP along with thereference patterns of identified phases.


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(CaCO3) Powders and Ni50.7Ti49.30 Alloy, Powder

Diffraction Journal, Vol. 24, No. 4, 2009, pp. 315-326.

10. Bish, D.L. and Howard, S.A.: Quantitative Phase

Analysis Using the Rietveld Method, Journal of Applied

Crystallography, Vol. 21, Part 2, April 1988, pp. 86-91.

11. Madsen, I.C., Scarlett, N.V.Y., Cranswick, L.M.D. and

Lwin, T.: Outcomes of the International Union of

Crystallography Commission on Powder Diffraction

Round Robin on Quantitative Phase Analysis: Samples 1ato 1h,Journal of Applied Crystallography, Vol. 34,

2001, pp. 409-426.

12. Scarlett, N.V.Y., Madsen, J.C., Cranswick, L.M.D., et al.:

Outcomes of the International Union of Crystallography

Commission on Powder Diffraction Round Robin on

Quantitative Phase Analysis: Samples 2, 3, 4, Synthetic

Bauxite, Natural Granodiorite and Pharmaceuticals,

Journal of Applied Crystallography, Vol. 35, 2002,

pp. 383-400.

13. Hill, R.J. and Howard, C.J.: Quantitative Phase Analysis

from Neutron Powder Diffraction Data Using the

Rietveld Method,Journal of Applied Crystallography,

Vol. 20, 1987, pp. 467-474.

14. OConnor, B.H. and Raven, M.D.: Application of the

Rietveld Refinement Procedure in Assaying Powdered

Mixtures, Powder Diffraction Journal, Vol. 3, No. 4,

1988, pp. 2-6.

15. Hermann, H. and Ermrich, M.: Microabsorption of

X-ray Intensity in Randomly Packed Powder Specimens,

Acta Crystallographica Section A, Vol. A43, No. 3, 1987,

pp. 401-405.


Dr. Husin Sitepu joined Saudi Aramcos

Research and Development Center

(R&DC), Analytical Services Division,

in 2008. Currently, he is contributing

to several research projects under both

the Downstream and Strategic and

Upstream R&DC programs, in

providing crystallographic information on developed

materials including nano-materials and catalysts. Before

joining Saudi Aramco, Husin worked at NIST Center forNeutron Research in Gaithersburg, MD; Virginia Tech

University in Blacksburg, VA; Ruhr University Bochum

Universitt in Bochum, Germany; the Institute Laue-

Langevin, Neutrons for Science, in Grenoble, France; the

University of British Columbia in Vancouver, Canada; and

the Curtin University of Technology in Perth, Australia.

He has authored and coauthored 32 papers in several

peer-reviewed journals, including the International Union of

CrystallographysJournal of Applied Crystallography.

Husin has extensive experience in Rietveld refinement of

polycrystalline structures using X-ray, synchrotron and

neutron powder diffraction data.

He received his Postgraduate Diploma, M.S. and Ph.D.degrees in Physics from the Curtin University of Technology,

Perth, Western Australia, in 1989, 1991 and 1998,

respectively.

Husin is a member of the International Center for

Diffraction Data (ICDD), the International Union of

Crystallography (IUCr), and the Neutron Scattering Society

of America (NSSA).

Ahmed A. Al-Shehry has worked at

Saudi Aramco since 1981. He started

his career working in the Chemistry

Analysis Unit of the Laboratory

Department. He now works in the

Elemental Analytical Unit, part of the

Analytical Services Division of R&DC.

Ahmed is a Senior Digital System Technician and an

expert in several analytical techniques, including atomic

absorption spectromery (AAS), which is an analytical

procedure for the qualitative and quantitative determination

of chemical elements employing the absorption of optical

radiation (light) by free atoms in the gaseous state; flame

atomic absorption spectrometry, which is a very common

technique for detecting metals and metalloids in

environmental samples; inductively coupled plasma optical

emission spectrometry (ICP-OES), which is an analyticaltechnique used for the detection of trace metals; and XRD

and XRF used to determine the chemical compositions of

corrosion products, carbonate rocks, cements and catalysts.

He has successfully conducted a series of Technical Service

Projects (TSPs) to support refineries and gas plants. Also,

Ahmed has contributed to the Downstream and Strategic, and

Upstream R&DC programs by providing chemical

compositions of scale mitigation and catalysts projects.

BIOGRAPHIES

Dr. Syed Rehan Zaidi has been with

Saudi Aramco since 1992. His

specialized area of research is the

mineralogical characterization of

geological samples (clay and bulk rock)

by using the XRD technique. Syed is

also responsible for the XRD method

development and research work. He is also familiar with the

other analytical techniques, such as: XRF, SEM, FTIR,

TGA, DSC and ICP instruments.Syed received his B.S. (Honors) and M.S. degrees in

Chemistry from Aligarh Muslim University, Aligarh, India,

in 1977 and 1980, respectively. In 1986, he received his

Ph.D. degree in Inorganic Chemistry from Aligarh Muslim

University, Aligarh, India.

Syed has published more than 20 papers in peer review

journals. He is a member of the American Chemical Society

(ACS) and the Society of Petroleum Engineers (SPE).

characterization corrosion

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