30581 - IJAER ok 325-344 author Frank Byron

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International Journal of Applied Engineering Research
ISSN 0973-4562 Volume 10, Number 1 (2015) pp. 325-344
© Research India Publications
http://www.ripublication.com
Investigation of The Failure of A Drive Rod on an Offshore
Platform Due To Microbial Induced Corrosion (MIC)
Frank Byron
The University of Trinidad and Tobago, Point Lisas Industrial Estate, Point Lisas,
Trinidad & Tobago, W. I.
E-mail: [email protected] Tel.: 1 868 642 8888
Nazim Mohamed*
The University of Trinidad and Tobago, Point Lisas Industrial Estate, Point Lisas,
Trinidad & Tobago, W. I.
E-mail: [email protected] Tel.: 1 868 642 8888
Clément Imbert
The University of the West Indies, Faculty of Engineering, St. Augustine Campus,
Trinidad & Tobago, W.I.
E-mail: [email protected] Tel: 1 868 662 6267
Abstract
This paper investigates the premature failure of a drive rod on an offshore
platform. Pitting occurred along the length of the three sections of the rod at
localized points and at regions where the rod was coupled to other components
including the extension supports. Several analysis were undertaken to find the
root cause of this failure. Based on the investigation conducted and the results
obtained it was concluded that the failure of the drive rod was initiated from
the pitting process of microbial induced corrosion (MIC). The microorganisms
included the aerobic manganese and iron oxidizing bacteria; the anaerobic
sulphur reducing bacteria (SRB) and the facultative aerobic acid producing
bacteria.
Key words: Drive rod, pitting process, microbial induced corrosion (MIC),
aerobic manganese and iron oxidizing bacteria, the anaerobic sulphur reducing
bacteria (SRB) and the facultative aerobic acid producing bacteria.
Introduction
The drive rod (comprising three rods) for a fire pump on an offshore platform failed
prematurely two years after installation. Failure occurred while performing a routine
maintenance operation and occurred at a region that showed severe pitting close to the
coupled area. The type of fire pump is a vertical shaft turbine type having multiple
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Frank Byron, Nazim Mohamed and Clément Imbert
impellers that are submerged in the sea water. It is suspended from a discharge head
by sections of column pipes which also support and guide the pump’s vertical drive
shaft and bearing. The pump is driven by a diesel engine. A schematic diagram of the
pump is shown in Appendix 1.
Material and Method
The investigation undertaken comprised the following [1]:
1. Visual Inspection. The sample was visually inspected and photographs were
taken.
2. Dimensional Measurements. Dimensional measurements were taken using a
Mitutoyo Digital Caliper Model CD-6” C.
3. Macrography A macroscope with digital imaging capability, model Omano
OMVV-GX4 (T), was used to observe the sample at macro level and to record
features of interest.
4. Micrography and Microprobe Analysis. A JEOL Scanning Electron
Microscope (SEM) Model JSM-6490LV, with an attached Energy Dispersive
Spectrometer (EDS), Model Oxford 7574 was used in the analysis.
5. Microstructure Analysis. A Nikon Optical Microscope Model Epiphot 200
was used in the analysis and photomicrographs were taken at points of
interest.
6. Elemental Analysis. The analysis was carried out using a JEOL scanning
electron microscope (SEM), Model JSM-35CF, with an attached wavelength
dispersive spectrometer (WDS) Model JSM-35FCS. The carbon and sulphur
contents were determined using an ELTRA carbon/sulphur determinator
Model CS2000.
7. Compound Analysis. The analysis was carried out using a Bruker-Axs X-Ray
Diffractometer (XRD) Model D8 Advance.
8. Hardness Testing. An Indentec Vickers Macro and Micro Hardness Tester
Model ZHV30 was used to determine the hardness profile. Testing was carried
out using guidelines given in ASTM E384 Standard Test Method for knoop
and Vickers Hardness of Materials. A hardness scale HV 10 was selected.
9. Electrochemical Test Methods. Studies were carried out using GAMRY
Instruments Potentiostat/Galvanostat/ZRA Reference 600.
10. Microbiologically influenced corrosion testing. Samples of seawater within
the steel casing of the failed drive rod were sent to the microbiology lab for a
total count of microbiological activities.
Results obtained from items (6) and (8) above were compared with the
specifications of wrought martensitic stainless steel type AISI 416.
Results and Discussion
Visual Inspection
The visual inspection of the rod revealed the following:
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
327
Three rods were coupled by means of threading to form one rod, and were all
severely pitted and corroded especially at the joints where final failure
occurred. (Figure 1)
Figure 1: The drive rod comprised three rods that were coupled together and were all
severely pitted and corroded (red arrows). Also note reduction in diameter at failed
ends (white arrows).
The mode of corrosion showed extensive localized pitting along the length of
the rod and uniform corrosion with minute pits that gave a sponge like
appearance at areas that were covered by coupled components. (Figure 2)
Figure 2: The surface of the rod showed severe localized pitting corrosion (red
arrows) and an adjacent area showed severe uniform corrosion with minute pits giving
a sponge like appearance (white arrows).
The corrosion product in the pits and crevices appeared black in colour while
the corrosion product on the exposed surface appeared reddish brown in
colour. (Figure 3)
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Frank Byron, Nazim Mohamed and Clément Imbert
Figure 3: Corrosion product in the pits and crevices appeared black in colour (red
arrows) while the product on the exposed surface appeared reddish brown in colour
(white arrows).
Dimensional Measurements
Table 1: shows the physical dimensions of the drive rod
Table 1: Dimensional Measurements
Rod
Dimensions
(mm)
3,000.00
Total length of rod
60.00
Diameter of rod
30.00
Diameter of rod at failure
8.80
Average depth of irregular pits
Average diameter of irregular 10.00
pits
0.52
Average depth of minute pits
Average diameter of minute pits 0.77
Macrograph
Macrograph of the rod revealed the following:
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
329
The localized pits were irregular in shape and comprised clusters of minute
pits closely spaced on the surface. (Figure 4)
Figure 4: The localized pits are irregular in shape and they comprise a cluster of
minute pits closely spaced on the surface (red arrows).
Minute tubercle deposits are shown close to the threaded section of the rod;
this region showed the greatest metal lost because of the pitting. (Figure 5)
Figure 5: Minute tubercles are shown close to the threaded section of the rod that had
the greatest metal lost due to pitting corrosion (red arrows).
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Frank Byron, Nazim Mohamed and Clément Imbert
A cross-section of the rod showed the morphology of the surface pits as
elliptical in shape and they appear as a collection of sharp-edged pits. Also
note the circular pit below the surface pits. (Figure 6)
Figure 6: A cross-section of the rod showed the pits as elliptical in shape and they
appear as a collection of sharp-edged pits (red arrows). Also note the circular pit
below the surface (white arrows).
The transverse section of an elliptical pit was polished and showed a
subsurface pit approximately 3 mm below the surface pit. (Figure 7)
Figure 7: The cross-section of an elliptical pit was polished and showed a subsurface
pit approximately 3mm below the surface pit. Mag. 30X.
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
331
Microstructure
Sections of the drive rod were ground, polished and etched and revealed the
following:
The structure showed a typical quenched and tempered martensitic stainless
steel, with a fine martensite structure. (Figure 8)
Figure 8: Typical quenched and tempered martensitic stainless steel. Also noted is
the fine martensite structure (red arrows) Mag. 500X.
A typical cross-sectional profile of one type of pit showed the pit opening
being much smaller in diameter at the surface than below the surface; the pit
also formed an undercut. (Figure 9)
Figure 9: A cross-sectional profile of one type of pit showed the pit opening being
much smaller in diameter at the surface than below the surface; the pit also formed an
undercut (white arrows) Mag. 500X.
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Frank Byron, Nazim Mohamed and Clément Imbert
A typical cross-sectional profile of another type of pit showed the elliptical
shape with the depth and width dimensions. (Figure 10)
Figure 10: A typical cross-section of a pit showed the elliptical shape with the depth
and width dimensions (red arrows) Mag.100X.
Micrograph and Microprobe Analysis
SEM micrograph and microprobe analysis of the sample revealed the following:
Typical deposit products in the tubercle on the surface of the rod. (Figure 11)
Figure 11: SEM micrograph of the tubercle deposit material on surface of rod.
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
333
Microprobe analysis of the tubercle deposit on the surface of the rod revealed
the elements present (Figure 12).
Figure 12a: Microprobe analysis of the tubercle deposit on the surface of the rod.
Element
O
Mg
Al
Si
Cl
K
Ca
Cr
Mn
Fe
Ni
S
Zn
Totals
Weight %
30.12
3.95
2.00
2.09
0.36
0.15
8.21
4.20
22.78
20.56
0.70
3.14
1.73
100.00
Figure 12b: Analysis of Spectrum
Microprobe analysis of the corrosion product at the upper layer of the surface
pit revealed the elements present (Figure 13).
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Frank Byron, Nazim Mohamed and Clément Imbert
Figure 13a: Microprobe analysis of the corrosion product at the upper surface of the
pit.
Element
O
Mg
Al
Si
Cl
Ca
Cr
Mn
Fe
Ni
S
Zn
Totals
Weight %
39.83
1.96
1.11
0.86
0.58
6.75
11.32
12.60
20.93
0.20
3.41
0.53
100.00
Figure 13b: Analysis of Spectrum.
Microprobe analysis of the corrosion product at the lower layer of the
subsurface pit revealed the elements present. (Figure 14)
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
335
Figure 14a: Microprobe analysis of the corrosion product at the lower surface of the
pit.
Element
O
Na
Mg
Al
Si
S
Cl
Ca
V
Cr
Mn
Fe
Ni
Totals
Weight %
39.12
0.51
0.61
0.65
0.74
8.35
0.11
20.86
0.14
16.18
0.95
10.92
0.86
100.00
Figure 14b: Analysis of Spectrum.
Elemental Analysis
A Quantitative analysis of the drive rod is compared with AISI 416 specifications and
is presented in Table 2.
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Frank Byron, Nazim Mohamed and Clément Imbert
Table 2: Quantitative Analysis of Drive Rod.
Elements
Carbon
Percentage composition by weight %
Rod
AISI 416
0.129
0.150 max
Sulphur
0.329
0.150 min
Phosphorus
0.020
0.060 max
Manganese
1.06
1.25 max
Silicon
0.56
1.00 max
Chromium
12.45
12.00-14.00
Nickel
0.21
0.15-0.25
Molybdenum 0.04
0.60 max
Iron
Remainder
Remainder
The sample satisfied the chemical requirements of an AISI 416 martensitic
stainless steel [2].
Compound Analysis
X-Ray diffraction analysis was performed on the two coloured corrosion deposits
observed on the sample. The black deposit was extracted from the pits and crevices of
the corroded surface of the rod and the reddish brown deposit was extracted from the
exposed corroded surface of the rod. The results are presented in Table 3.
Table 3: Compound Analysis Result of Two Coloured Corrosion Deposit.
Corrosion
Compound Name
deposits
Reddish Pyrrhotite
Black
brown
Calcium Iron Oxide
Hematite
Lepidocrocite
Goethite
Chromium Iron Carbide
Iron Manganese Carbide
Northupite
Groutite
Formula
Fe0.95S1.05
CaFe3O5/CaO.Fe.Fe2O3
Fe2O3
Fe3+O(OH)
Fe2O3.H2O
Cr15.58Fe7.42C6
FeO.4Mn3.6C
Na3Mg(CO3)2Cl
Mn3+O(OH)
Similar compounds were identified in both coloured deposits. The difference in
colour is as a result of a predominant compound in the iron oxide phase when exposed
to the atmosphere. The transformation from lepidocrocite to goethite gives a reddish
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
337
brown colour and is usually formed in the early stages of atmospheric corrosion. The
black colour is influenced by the manganese oxide, and iron sulphide compounds. The
X-Ray Diffractograms are presented in Charts 1 and 2 below.
Chart 1: X-Ray Diffraction pattern of black corrosion products.
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Frank Byron, Nazim Mohamed and Clément Imbert
Chart 2: X-Ray Diffraction pattern of reddish brown corrosion products.
Hardness Testing
A Vickers hardness profile (HV10) on a polished section of the rod reveals hardness
reading shown in Table 4. An equivalent Rockwell Hardness C (HRC) number is also
recorded.
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
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Table 4: Vickers Hardness Test (HV10).
Test No
Reading # 1
Reading # 2
Reading # 3
Average HRC
T121933
248
243
250
247
23
The sample satisfies the hardness requirements as a tempered AISI 416 martensitic
stainless steel.
Electrochemical Test Methods
The corrosion rate of the drive rod was determined from the Potentiostat by using a
linear polarization technique in the simulated environment. The applied potential is
plotted versus the current within ±20mv which is often linear. The slope produced is
proportional to the corrosion rate of the metal in the environment so the corrosion rate
is then calculated. The corrosion potential of the drive rod was also measured using
the polarization resistance method. The results are shown in Table 5 and the
polarization Chart 3 is shown below.
Table 5: Polarization Test.
Item
Environment
Temperature pH
°C
416
Stainless
Steel
Sea water
23.6
7.69
Corrosion
Rate
(mm/y)
0.07
Corrosion
Potential
(mV)
-519
Following the guidelines given in the comparative chart described by M.G.
Fontana, the uniform corrosion rate determined can be considered excellent under the
simulated conditions.
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Frank Byron, Nazim Mohamed and Clément Imbert
Chart 3: Polarization resistance chart showing corrosion rate of drive rod in sea
water.
Microbiologically Influenced Corrosion Testing
One litre of seawater was sampled from the ocean at the offshore platform in the steel
casing that contained the failed drive rod; it was then submitted to the microbiology
lab for a total count of microbiological activities. The sample was taken two months
after the failure. The results are presented in Table 6.
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
341
Table 6: Microbiological Count.
Laboratory Sample No. Sample Label
1
Platform water pump
Heterotrophic Plate count
(CFU/ml)
1.16 x 102
Heterotrophic Plate Count (HPC) measured in colony forming units per millilitre
(CFU/ml) is a non-specific term for the growth of viable, naturally occurring bacteria
in water. The plate counts showed that there was a presence of the bacteria in the
water; however the type of bacteria could not be determined with this method.
Discussion
Visual examination of drive rod showed that the drive rod failed as a result of a
reduction in diameter due to the pitting corrosion process. Pitting occurred along the
length of the three sections of the rod at localized points and at regions where the rod
was coupled to other components including the extension supports (see Figures 1, 2,
3, and 5). Uniform corrosion features were also observed at areas on the rod that were
enclosed by the appropriate bearings and bushings. (Figure 2)
Macrographs of the pits revealed that the localized pits were irregular in shape as
they comprised a cluster of minute pits closely spaced to form larger pits on the
surface. (Figure 4) The minute pits were formed under deposits that appear as
tubercles. (Figure 5) A cross-section of the rod that contained the pits showed the
morphology of the pits as trough pits that were elliptical in shape and having sharp
edges. (Figures 6 and 10) Some pits were also observed below the surface (subsurface
pits) with deep undercuts that can be classified as sideways pits. (Figures 6, 7 and 9)
The microstructure showed a typical quenched and tempered martensitic stainless
steel structure with fine martensite. (Figure 8) SEM microprobe analysis of the
tubercle deposits on the rod surface identified the deposit to be by-products of the
metabolism of the micro-organisms that include trapped detrital materials, and
corrosion products. (Figures 12, 13 and 14) Analysis of the product in the pit close to
the surface showed a large concentration of manganese and a lesser amount of sulphur
whereas analysis of the pit below the surface plane showed a larger concentration of
sulphur with lesser manganese. (Figures 13 and 14)
X-Ray diffraction analysis of both black and reddish brown corrosion products
showed similar compounds. The iron oxide phases were present together with other
compounds including iron sulphide and manganese oxide which were predominant in
the corrosion products. The electrochemical polarization test using the polarization
resistance technique showed a uniform corrosion rate of 0.07mm/y and a corrosion
potential of the material in the seawater as -519 mV. The stainless steel can be
considered active as it will corrode by pitting. The bacterial count at the time of
collection can be considered acceptable (<500 CFU/ml) as the sample was taken two
months after the removal of the drive rod from the sea water.
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Frank Byron, Nazim Mohamed and Clément Imbert
Microbiological influenced corrosion (MIC) is defined as corrosion that is
influenced by the presence and activities of micro organisms, including bacteria and
fungi. The by-products from the organisms promote several forms of corrosion
including pitting, crevice corrosion and under deposit corrosion, they initiate and
accelerate the corrosion process by providing oxidation or reduction reactions [3]. The
bacteria are broadly classified under three categories; (a) aerobic, that is one that
survives in the presence of oxygen and are metal-oxidizing bacteria which include
manganese oxidizing and iron oxidizing bacteria; (b) anaerobic, that is one that do not
survive in oxygen and these include the sulphur reducing bacteria (SRB); (c)
facultative aerobic, that is one that can live under both conditions and these include
the acid-producing bacteria. Aggressive corrosion takes place when all three types are
colonized together. The anaerobic bacteria can thrive in aerobic environments when
they are present beneath biofilms/ deposits in which aerobic bacteria consume the
oxygen [4].
For metabolism of the anaerobic SRB, oxygen is extracted from the sulphate ions,
and this reaction converts the soluble sulphates to hydrogen sulphide which attacks
the metal surface forming pits that are larger in diameter below the surface than at the
surface, the pits can be described as sideways pits with deep undercuts. The corrosion
product produced includes iron sulphide. The formed sulphides also lower the
repassivation and pitting potential, which increases the metal susceptibility to pitting.
The aerobic bacteria biofilms are able to accumulate ions such as the heavy metals
manganese and iron in large concentrations from the surrounding water where they
reoxidizes manganese species; they deposit the manganese oxide and tubercles of iron
oxide on the surface of metals that alter the electrochemical properties on the surface
to induce corrosion. Whenever microbial sulphur reduction and manganese deposition
occurs simultaneously within biofilms, a differential potential cell is likely to form on
the metal surface. The area under the sulphide species becomes anodic while the area
under the manganese species become cathodic and localized corrosion occurs.
Crevice and galvanic corrosion is also accelerated when copper alloys and steel is
coupled on to stainless steel4 as seen the Figures 1, 2, 3, and 5.
Crevice corrosion is a major problem in marine environment because of the low
resistivity and high salinity of seawater. Threats always exist at threaded joints;
crevice corrosion is exacerbated in warm natural sea water where biofilms form
rapidly, and pits form in crevices where there is slowly flowing seawater. The salinity
recorded in the sea water was 25.43 parts per thousand (ppt) that is total dissolved
salts that attacks the stainless steel under certain conditions. The current required for
protecting the stainless steel requires a couple of mA/m2, and needs approximately
100 mV to keep the potential below the pitting potential in the safe passive region [5].
The nature of failure occurred as the AISI 416 martensitic stainless steel was used
as the material of choice for the drive rod. The rod remained submerged in the almost
stagnant water for a long period. During stagnation naturally occurring
microorganisms formed biofilms, and aerobic microorganisms used the dissolved
oxygen during their metabolism. A heterogeneous colonization of micro organisms
occurred; pits developed rapidly especially at the crevices of the coupled material.
Investigation of The Failure of A Drive Rod on an Offshore Platform Due To…
343
Uniform corrosion developed on the rod that was covered by the bearing and
bushing material when there was an overall breakdown of the passive film. The
rubbing against the rod removed the protective oxide layer and the heat generated
increased the corrosion rate. The corrosion under the tubercle deposits supported the
micro organisms including manganese and iron oxidizing bacteria with their corrosion
products predominately identified at the surface of the pit and the SRB and their iron
sulphide corrosion products predominately identified at the lower surfaces. The
pitting process continued and reduced the rod’s diameter at the coupled areas to
approximately half the original diameter. The rod could no longer withstand the
strength required for the drive torque and subsequently failed.
Conclusion
Based on the investigation conducted and the results obtained the following
conclusions are drawn. Failure of drive rod initiated from the pitting process of
microbial induced corrosion (MIC). The microorganisms included the aerobic
manganese and iron oxidizing bacteria; the anaerobic sulphur reducing bacteria (SRB)
and the facultative aerobic acid producing bacteria. The organisms formed colonies
embedded in a matrix of heterogeneous biofilm and induced pits in an environment of
high salinity. The type of corrosion includes uniform corrosion, crevice corrosion and
pitting corrosion. Uniform corrosion occurred under the bearings and bushings of the
rod while pitting corrosion occurred along the length of the rod and intensified at the
crevices of the coupled joints. The morphology of the pits included trough elliptical
pits and sideways subsurface pits that initiated from microbial activities and
intensified with the salinity of the sea water. The drive rod was made from AISI 416
martensitic stainless steel. Final failure occurred when the rod became so pitted that it
could no longer accept the drive torque required to push the pump.
Recommendations
Martensitic stainless steels are typically not suitable for continuous immersion in
slowly flowing seawater. In slowly flowing or stagnant seawater, cathodic protection
must be used to prevent pitting and crevice corrosion. Austenitic stainless steel and
duplex steels have a greater resistance for salt water environment and MIC corrosion.
The fire water pump should be allowed to operate more frequently to minimize the
chance of microbial colonization on the rod. Sea water chlorination is the most
effective way of controlling biological colonization on steel members; when chlorine
is dissolved in water it hydrolyzes rapidly and creates a hostile environment for living
organisms3.
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Frank Byron, Nazim Mohamed and Clément Imbert
Bibliography
[1] ASM Handbook, Failure analysis and prevention, Vol. 6 and Vol. 13.
[2] SAE HS-1086/2004, ASTM DS-561 Metals & Alloys in the Unified
Numbering System 10th Edition.
[3] Thomas R. Jack, NOVA Chemicals Ltd. Biological Corrosion Failures.
ASM Handbook Volume 11: Failure Analysis and Prevention (#06072G).
[4] Dillon. C. P. (Ed.), (1982). Forms of Corrosion – Recognition and
Prevention: (NACE International Handbook 1, Vol. 1) Houston.
[5] Leena Carpén. (2008). Corrosion of Stainless Steel in Fire Protection
Systems: Technical Research Centre of Finland (VTT).
Appendix 1: Schematic diagram of vertical turbine fire pump with drive rod assemble
Drive rod in casing
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