investigation%20of%20the%20corrosion%20of%20water%20storage%20tank%20in%20al-k

8/8/2019 INVESTIGATION%20OF%20THE%20CORROSION%20OF%20WATER%20STORAGE%20TANK%20IN%20AL-K

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INVESTIGATIONS OF THE CORROSION OF WATER

STORAGE TANK IN AL-KHOBAR PLANT1

Saleh A Al-Fozan, Mohammad Al-Hajri and Anees U. Malik

Research and Development CenterSaline Water Conversion CorporationPO Box 8328, Al-Jubail 31951, KSA

E-mail :

INTRODUCTION

The Manager, Al-Khobar Plant in his letter No. 3100/011/04/T&I dated 14.1.2004

addressed to the Manager, R&D Center, Al-Jubail requested R&D Center to analyze

the cause of corrosion on the bottom plate of product water tank. The R&D Center

decided to take up the necessary investigation.

BACKGROUND

Al-Azizia pumping station is consisted of six water tanks. The external corrosion of

bottom plates was observed during the inspection by Magnetic Flux Leakage (MFL).

These tanks were constructed in 1982 for the storage of product, brackish and blended

water, respectively.

The results of MFL studies indicated metal losses from most of the product water tank

(OXT60B001) bottom plates had exceeded the allowable material loss. Also, pin holes

were found on the bottom plates.

PRODUCT WATER TANK SPECIFICATION

Tank diameter : 49 m

Capacity : 22,500 m3

Design pressure : vacuum - 6 bar

Design temperature range : 40 70oC

Materials

Annular plates : ASTM A 516 gr 70

Bottom plate : ASTM A 285 gr. C

Roof plate : ASTM A 283 gr. C

1Issued as Troubleshooting Technical Report No. 3804/04008 in August 2004.


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Metal thickness

Annular plate : 9.5 mm

Bottom plate : 9.5 mm

Roof Plate : 6.35 mm

Corrosion allowance : 1.5 mm

PHYSICAL EXAMINATION

Product water tank was visually inspected. The following are the general observations:

1. Internal coating :

The internal coating was found deteriorated at different locations in bottom, annular

and roof plates.

2. Pitting:

Pits were observed on the entire surface of the bottom plate. Through holes were

also found on the bottom plate at different locations. The holes were concentrated

on or near annular plate as shown in Figures 1 and 2.

3. Metal sample:

Two corroded pieces were cut from tank bottom plate (Fig. 3) for laboratory

analysis. Also, samples of ground bed were collected for same purpose.

LABORATORY ANALYSIS

The chemical analysis was performed in order to determine the composition of the

bottom plate material and underneath layers of the soil. Table 1 shows the chemical

composition of the bottom plate. Table 2 shows the composition of different soil layers

under tank bottom. Table 3 shows the chloride content in soil under tank bottom.

The Energy dispersive X-ray analysis (EDXA) was carried out to analyze the

composition of corrosion products in the pits and the soil layers. Figure 4 shows the

composition of corrosion products covering the pit area. Figures 5, 6 and 7 show the

EDX analysis results of soil under tank.

GENERAL VIEW

The corrosion on the external surface of the bottom plate appears to initiate by an

electrochemical process where the soil beneath the bottom plate acts as an electrolyte.


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The factors which can affect the corrosion of under ground system are soil corrosivity

and/or stray current.

SOIL CORROSIVITY

There are many factors which affect the soil corrosivity. The factor include: pH,

resistivity, moisture content, chloride ion content, sulfide ion content, sulfate ion

content, oxygen content, bacteria, etc. Figure 8 shows the pitting propagation stage over

steel in soil environment.

STRAY CURRENT CORROSION:

Referring to corrosion damage resulting from current flow other than in the intended

circuit(s), for larger structures, this term usually alludes to corrosion damage caused by

extraneous current(s) flowing through soil and/or water. Stray current corrosion has

been classified into the following types:

1. Direct stray current corrosion: Originating from direct current sources such as

dc rail transit systems, dc welding equipment and cathodic protection systems. In

general, direct stray current corrosion is considered the most severe form of theseproblems.

2. The flow of direct stray current is not necessarily steady with time, in terms of

magnitude and current path(s). This has led to a further distinction between

dynamic stray currents (unsteady state) and static stray currents (steady state).

3. Alternating stray current corrosion: Originating from alternating current

sources such as overhead ac power lines.

4. Telluric effects: A "natural" form of dynamic stray currents induced by

transient geomagnetic activity.

Stray current which imparts corrosion problem in tanks, is difficult to identify and it is

painstaking to find means to rectify the effects. Electromagnetic or inductive

interference on pipelines or tanks occurs when there is extended and close parallel


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routing with three-phase high voltage alternating current (HVAC) overhead

transmission lines. The induced AC interference voltage is due to any phase imbalance

in the linesas shown inFigure 9. The likelihood of interference increases with rising

operating currents in the overhead lines, with increasing quality of the coating on the

pipeline, and with the length of pipeline parallel to and close to the HVAC transmission

lines. Voltages are induced in the pipeline by magnetic coupling with the high-voltage

lines, and results in currents flowing in the pipeline. These currents result in a voltage

difference between the pipeline and the surrounding soil and lead to localized

corrosion.

RESULT AND DISCUSSION

From the metal loss of tank plate at some locations (9.5 mm) and the operation time

(approximately 20 year), the corrosion rate of bottom plate can calculated. The

corrosion rate at some location is higher than 0.43 mm/year. The design corrosion rate

of tank bottom plate is 0.06 mm/year as general corrosion. This fact indicates that the

corrosivity of the soil under the tank bottom plate was not properly assessed during the

commissioning of the tanks. Figure 10 shows the severity of corrosion under the tank

bottom plate. From the soil analysis data the concentration of chloride ions appear to be

high as shown in table 3 and EDX results. From bottom plate sample and floor scan

results it appears that the corrosion is localized as pitting on bottom plate. Figure 8

shows the pitting corrosion propagation stage on steel surface. The external surface of

the bottom plate is coated by asphalt layer to separate the tank bottom from the outside

environment. The chloride ions in soil had migrated through asphalt layer and initiated

localized attack. Table 3 shows the chloride contents in soil layer beneath asphalt layer

under plate # 43 plate # 54.

The presence of stray current source near the tank also contributed in aggravating the

pitting corrosion bottom plate. Figure 11 shows the transmission lines in the vicinity of

the tanks area. The approximate distance from the line and tanks is 77 m. Figure 12

through Figure 14 shows the results of floor scanning. The results show the severity of

corrosion of tank # OXT 60 B001 and tank # OXT 30 B001 compared with tank OXT

50 B001. The tanks # 30 and 60 are closer to transmission lines as compared with tank


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# 50. This result is an indication of the influence of stray current in accelerating the

corrosion of tank bottom of tanks # 30 and 60.

SOLUTION OF STRAY CURRENT

Grounding the tank bottom to earth discharges the induced AC current, and reduces the

potential on the tank bottom. The tank bottom can be grounded to earth by use of zinc

anodes or galvanized steel grounding rods installed at periodic intervals.

The use of zinc anodes for grounding is effective, provided enough anodes or

grounding rods are installed, and the system is not required to dissipate fault currents

and lightning. Either extruded zinc ribbon can be laid in the ditch with the tank, or deeprods can be driven into the ground. When the ground resistance decreases with depth,

deep rods are preferred. Deep rods also are more convenient to install after

construction.

If grounding systems are used, they add load to the tanks cathodic protection (CP)

system. However, the load on CP is small due to 2 factors: (i) the potential of zinc rods

is close to CP potential of the tank and (ii) the area of the ground rods is smaller thanthe CP system. AC grounding, however, must be decoupled from the cathodic

protection system. Otherwise, the cathodic protection will not be maintained on the

tank. De-coupling is achieved using polarization cells or new solid-state devices which

pass AC over a pre-set threshold voltage, but which block DC current.

CONCLUSIONS

1. The mechanism of corrosion of tank bottom plate is localized corrosion in the

form of pitting.

2. The pitting corrosion of tank bottom plate is due to the higher corrosivity of

underneath soil which is further accelerated by the presence of stray current

source in the form of high voltage transmission lines in the close proximity of the

tanks.


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RECOMMENDATIONS

1. Application of the cathodic protection system to tank bottom plate will minimize

the corrosion attack due to the soil corrosivity.

2. Check grounding system (rods and connection).

3. Increase the grounding rods numbers.

REFERENCES

1. Test & Inspection Report "corrosion of water storage tanks", Al-Khobarplant, 2004.

2. J.H. Fitzgerald III, "Stray Current Analysis", in Uhlig's CorrosionHandbook, Second Edition, R.W. Revie Editor, Wiley, 2000.

3. Electricity Safety (Stray Current Corrosion) Regulations 1999 , S.R. No.50/1999 Version as at 3 May 1999.

4. Boat US Marine Insurance Report, "Back to Basics" , Seaworthy July2001.

5. Melvin Romanoff, "Underground Corrosion", NACE publication, 1989.


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Table 1. Bottom plate material analysis

Element Fe Si Al Mn C S

% 97 0.1 --- 0.6 NA NA

Table 2. Under tank soil analysis

Element S # 2 S # 3 S # 4 S # 5 S # 6

%Fe 48 54 49 0.2


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Figure 1. Photograph showing the pitting over the tank bottom plate

Figure 2. Close view of pits over the tank bottom plate

Pits over the tank bottom plate


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Figure 3. Tank bottom plate as in received condition internal side

Through pits on plate


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Figure 4. EDX result of pit composition of plate number 43

0 5 10 15 20Energy (keV)

0

5

10

15

cps

O

Si ClCa

Fe

Fe


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Element Element% Atomic%O 43.44 61.53

Mg 2.31 2.15

Al 2.72 2.28

Si 20.69 16.69

S 2.59 1.83

Cl 0.99 0.63

K 1.06 0.62

Ca 22.80 12.89

Fe 3.40 1.38

Figure 5. The EDX result of asphalt layer below plate number 43

0 5 10 15 20Energy (keV)

0

2

4

6

8cps

O

Mg

Al

Si

S

Cl K

Ca

Ca

Fe


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Element Element% Atomic%O 49.45 68.40

Mg 3.71 3.37

Al 1.65 1.35

Si 9.64 7.60

S 0.49 0.34

Cl 1.37 0.85

K 1.09 0.62

Ca 29.21 16.13

Fe 3.39 1.34

Figure 6.The EDX result of soil layer below plate number 43

0 5 10 15 20Energy (keV)

0

10

20

30

40

cps

O

Mg

Al

Si

SCl

K

Ca

Ca

Fe


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Element Element% Atomic%

O 48.35 67.46

Mg 3.08 2.83

Al 1.29 1.07

Si 10.83 8.61

S 0.61 0.43

Cl 0.43 0.27

K 1.31 0.75

Ca 31.59 17.59

Fe 2.50 1.00

Figure 7. The EDX result of soil layer around tank (outside tank)

0 5 10 15 20Energy (keV)

0

10

20

30

40

50

cps

O

MgAl

Si

SCl K

Ca

Ca

Fe


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Figure 8. Pitting corrosion propagation stage over steel in soil

Figure 9. Different distances between the under ground structures and each phase

transmission line, along with phase imbalance, lead to induced AC

interference on the pipeline

Fe(OH)3

OH- OH-

O2

O2 O2

O2

Soil Surface

OH-

Fe2+ Fe

2+

Oxide FilmFe(OH)2

Fe3O4

Metal


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Figure 10. Photograph showing sever corrosion on external side of tank bottom

plate

Figure 11. Photograph showing the tank location and over head high voltage

transmission line A and B

Corrosion underneath bottom plate


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Figure 12. Floor scanning result of tank # OXT60B001


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Figure 13. Floor scanning result of tank # OXT30B001


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Figure 14. Floor scanning result of tank # OXT 50 B 001

investigation%20of%20the%20corrosion%20of%20water%20storage%20tank%20in%20al-k

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