Failure of Above Ground Storage Tanks Nace Corrosion 2007

FAILURE OF ABOVE GROUND STORAGE TANKS

A STUDY BY

Mohammed. Javeed Shaikh and

Zain. Khadir Muhideen

[email protected] Saline Water Conversion Corporation.

ABSTRACT Corrosion is one of the main causes of the failure of the storage tanks. In any plant, a number of tanks are used to store and deliver a variety of liquids and gases. The biggest problem is the corrosion of the bottom plate. The complete network of plant consists of more than 100 tanks. These are used for storing and distributing potable water, petrol, diesel and H.F.O. About 70 numbers of Large Above Ground Storage Tanks (AST) are used for the storage of Heavy Fuel Oil and water. During internal inspection of one of the tanks, bottom plates shown signs of pitting corrosion of varying degrees and roof had numerous holes of varying sizes. But they mainly centered on the locations where roof was touching rafters. Magnetic flux leakage test (MFL) test indicated underside corrosion of bottom plates in number of tanks. The paper presents and discusses the aspects involved with the corrosion of tank roof and the bottom plates. An attempt has been made to discuss the inspection methodology, the causes for corrosion, and methods of repair and refurbishment of the tanks. It also describes the suitable Cathodic Protection system for the underside corrosion protection of tank bottom plates. Keywords: magnetic flux leakage, rafter, pitting, soil consolidation, bottom plate.

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INTRODUCTION Scope of this study is to analyze the corrosion behavior of the tanks and to find out various causes promoting corrosion of the tank material and to recommend a suitable remedial action.

CONSTRUCTION DETAIL OF THE TANK:

Tank Diameter : 46.0 meter Tank Height : 20.5 meter Roof type : Self supporting, Dome type Roof thickness : 4.76 mm thick Bottom type : Flat Material of bottom plate : ASTM A- 283 – grade C / FE 52 Painting system : Zinc primer 30 microns for transport protection Code of design : API 650 G – K Medium of storage : Heavy Fuel Oil Operating Pressure : Atmospheric Corrosion allowance : 0 mm Shell to roof joint : Fracture joint Annular plates / Inboard plate : 9 mm / 6.35 mm Shell plate Thickness : Bottom 20 mm top most 9 mm Bottom and roof plate : lap weld construction. Year of commission : 1982

Tank is installed over a concrete ring wall foundation. Bottom plate is resting on fine grade asphalt concrete 50 mm thick followed by compact sand, then lean concrete followed by oil and sea water proof foil (membrane). In theory all these layers form a perfect insulating material which in normal condition doesn’t allow CP current to pass in order to protect the exposed bottom surface. Likewise it doesn’t allow corrosion current to leave the tank bottom to enter soil and corrode the bottom plate ref fig # 1.

CORROSION PROTECTION PROVIDED IN THE DESIGN: Tank internal surface areas are painted and no other corrosion prevention method is applied, the tank is not insulated. Tank external surface areas are painted for corrosion protection. But bottom plate soil side has no painting. From a corrosion engineer’s point of view Bottom plate and Roof are the main areas of concern. Among many, Cathodic Protection (CP) is a proven method for controlling corrosion. However cathodic protection is not provided for these H.F.O. tanks.

INSPECTION & OBSERVATION:

Complete tank was drained. About 300 mm thick sludge and slurry were cleaned. The tank was hydro cleaned and gas freed for internal inspection. Visual inspection before sand blasting revealed numerous water filled blisters in the existing paint. After sand

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blasting the floor was inspected again and a number of through holes were observed with corrosion pitting up to a depth of one mm ref fig # 2. But no generalized corrosion was noticed. Thickness measurement using Ultrasonic gauges was done to assess the thickness reduction due to corrosion2, result showed severe metal loss around the holes. To get an accurate profile of tank bottom condition, Magnetic Flux Leakage (MFL) floor scanning was done. On analysis of the result it was confirmed that bottom plate was severely corroded due to under side corrosion ref fig # 3. Tank was found to have several bulges outward and this was prominent at the annular plates, index plates compared to the in board plates. MFL test result shows maximum metal loss in the area of bulge ref fig # 4a and 4b. The Paint on the floor plates has blisters of various sizes from 5 mm to 15 mm in diameter. The roof plate has hundreds of holes ref fig # 5 & 6.

DISCUSSION

Corrosion of bottom plate at HFO side largely depends upon:

Design Of bottom plate The bottom plates are lap welded. This construction leaves a small gap at the under side of the lap joint. The bottom plates are not sitting firmly on soil and an uneven gap was noticed between the soil and the plate. Material Bottom plate is made of structural steel material (ASTM A 283 Gr C) a generally used material in tank construction Medium stored Medium stored is Heavy fuel oil (HFO) which will not create any corrosion on its own. Corrosion protection system in the design Internal painting is the only corrosion protection provided for the tank bottom. External soil side of the bottom plate is not painted or coated. No Cathodic Protection system was (CPS) provided for the protection of bottom plates. Corrosion of bottom plate interior surface depends upon the property of stored material. As the stored material is heavy fuel oil, the medium will not create corrosion on the bottom plate on its own. However numbers of holes were observed on the roof causing rain water to enter the tank. Due to higher density than HFO it settled on the floor bottom and enters in the holiday in the paint making paint blister and initiating pitting type corrosion on the floor plate at the HFO side.

Corrosion of bottom plate at soil side could be due to various actions such as: Stray current Galvanic Corrosion Corrosivity of soil Soil consolidation Pitting Differential aeration

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Stray current Corrosion damage to an under ground structure caused by a cathodic protection system installed on another structure is commonly termed as interference. It is the result of a form of stray current electrolysis. This form of stray current damage is most commonly associated with impressed current cathodic protection system. Stray current corrosion has been classified into the following types:

Direct stray current corrosion The effect of stray current which could be causing the corrosion was studied. As a part of expansion project a new HFO tank was added to tank farm. Impressed Current Cathodic Protection (ICCP) system was installed for this tank. With this CP system in ON Position, Potential of the tank in question, reading was 865 mV w.r.t. to Cu-CuS04. When the CP system was kept in OFF Position for 20 hours. The Potential of the tank in question, reading goes down to 480 mV w.r.t. to Cu-CuS04. This is approximately the natural potential of carbon steel in soil. It can very well be concluded that there is interference from the new HFO tank CP system. Here however the return path is through common suction, discharge header, heating metallic piping circuit. Hence no corrosion / untoward effect

Alternating stray current corrosion: The only source of alternating current to the tank was analyzed. A high tension 230 KV transmission line is passing in the vicinity of the tank. Since the distance between line and tank is 257 meters, no significant effect due to the line was observed.

Galvanic Corrosion The term galvanic corrosion is generally restricted to the change in normal corrosion behavior that results from the current generated when one metal is in contact with different metal and both the metals are in a corrosive solution. In this case the 2 different metals are copper grounding / earthing and carbon steel tank bottom and the soil is the corrosive medium. Tank bottom acts as an anode, soil as an electrolyte and copper as a cathode. Thus tank bottom plate ends up supplying CP to copper earthing and in the process it is consumed (metal loss) Since the Anode to cathode ratio is very high the corrosion due to earthing system is minimal. It is preferred that earthing metal should always end up being anode to the tank bottom. The annular plate and the inboard sketch plate are made of same material with different thickness. Even though the design specification of material is same, in practice due to various material structure imperfections the potential of annular plates and the sketch plates vary at site of imperfection and create a galvanic cell at these sites causing corrosion.

Corrosivity of soil The soil underneath of the bottom plate plays a vital role in corrosion of the tank. The important factors affect the soil Corrosivity are : pH, resistivity, moisture content, chloride ion content, sulphate ion content, oxygen content and bacteria etc…The soil underneath the tank was analyzed in the laboratory and high concentration of chloride content was observed. Refer to

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EDX result and laboratory analysis report. The chloride ions in the soil had migrated through the first asphalt layer and initiated localized corrosion.

Table 1 Soil analysis for chloride content 1

Element Unit Sample – 1 Sample – 2

Chloride (cl) Mg/k.g. 520 400

EDX result of soil layer below plate number 43

Soil consolidation Over a period of time the sand underneath the tank bottom is expected to consolidate in varying degrees. This results in air gap between tank bottom and soil, (compact concrete) as per our observation in the HFO tank consolidation is maximum at annular ring plates and at in board plates it would be minimal. It is but natural for corrosion to occur where consolidation exists. In practice concrete can never be flat on which tank bottom will be firmly in place there by providing a solid contact area. By nature concrete will have its own contour plus welding pattern (lap weld). These two configuration leaves sufficient area of bottom plate elevated from concrete. Because of moisture entrapment natural corrosion commences and if continuous supply of oxygen is available bottom of tank is likely to fail at a certain stage of time.

Soil consolidation can be taken care by Grouting below the tanks to fill up the gap between soil and the bottom plates which could be beneficial in two ways viz…ref fig # 6

1. Prevents stress on bottom plates due to sand consolidation

Element Element % Atomic O 49.55 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

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2. Provide a passive film over steel.

Grouting: Applying properly compounded concrete can provide a corrosion resistant product, provided concrete doesn’t contain any salts or other material that could prevent polarization.

Differential aeration The oxygen content is one of the important factors causing corrosion. The non metallic sealant between ring wall foundation and tank failed, thus causing air ingress in to the tank bottom. The corrosion is maximum at the annular plate due to air entrapment in the bulged bottom plate, as more oxygen is available nearer to the ingress location i.e. annular plates and hence more corrosion.

Pitting

Pitting at medium side Pitting is a localized corrosion that occurs on badly exposed surface of passive metal under relatively stagnant condition. With the passive film broken, the pit is initiated and propagates. Figure shows a pitting propagation stage over steel in soil environment. Holes in the roof led to entry of rain water in the tank which then settled at the bottom as a pocket due to higher specific gravity and initiated pitting process on the bottom floor, resulting in the coating damage on the tank bottom. Rate of attack is much greater at certain areas on a surface than at others. This is a dangerous form of corrosion as this causes failure of tank by perforation.

Chemistry of pitting corrosion at medium side

Intense corrosion takes place at the small anodic area underneath the deposited water 4

Pitting at soil side5 At the soil side of the bottom plate the asphalt layer forms the passive coating .Due to bulging, imperfection with fabrication at localized areas, the pit initiated where the asphalt is not sticking to the plate due to presence of anions such as chloride or the presence of highly oxidizing strength of environment (Soil)

H2O -- H+ + OH– Fe – Fe2+ + 2e Fe2+ + 2OH- -- Fe(OH)2

Area of vessel in contact with oil becomes most effective cathode. the oil has sufficient dissolved oxygen to cause a rapid cathodic reaction 4H+ + 4e + O2 --- 2H2o

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Chemistry of pitting corrosion at soil side

FAILURE OF BOTTOM PLATE – CONCLUSION

Based on the investigation, the under side corrosion of the Heavy fuel oil Tank is attributed to a combination of corrosion mechanisms, such as stray current, soil Corrosivity, soil consolidation, galvanic action and pitting. The fluid side corrosion is due to the damage of the painting system and water accumulation due to the holes in the roof causing rain water/condensation of moisture to enter the tank. Bottom plate internal Painting system has failed due to aging over a period of time.

CORROSION OF ROOF PLATE

A large number of holes were noticed in the roof. However the external surface of the roof plates shows no sign of corrosion. Through inspection of the inner side of the tank roof revealed severe corrosion at the following areas: The roof plate just above the rafter (support angles) at the inner side. The roof plate, at the area where welded one over other (lap welded area)

Other areas are found to be normal without corrosion. The support angles are not corroded because this has a corrosion protection painting ref fig # 7a and 7b.

Corrosion of roof largely depends upon

Design The design is Cone roof umbrella type construction. Roof plates are directly laid on top of rafters (support). The area between the lap weld and the area above the rafter are the locations of internal corrosion. The roof plates are welded only at external side and left without welding internally thus leaving a gap at inner side of the lap joint. These areas serve as reservoirs for corrosive fluids.

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Material Roof is made of structural steel material (ASTM - A 283 Grade C) generally used material for the tank. Medium stored Medium stored is HFO. This HFO produces Hydrogen Sulfide (H2S) which fills the vapor space. Corrosion protection system in the design Painting is the only corrosion protection available for the tank roof, due to inaccessibility to the plate area above, the rafter is not painted as the roof is constructed over the support rafter.

Mechanism of corrosion in Roof

Crevice corrosion Crevice corrosion generally originates due to differential aeration. Initially corrosion starts as crevice and the material is thinned progressively at the above areas due to corrosion. This occurs in crack or crevices formed between mating surfaces of metal assemblies. Over lapping of roof plate’s construction, gap between the support rafter and the roof plate provides a reservoir for corrosive solutions H2S/moisture /condensation.

Galvanic Corrosion The unpainted portion of the roof above the rafter and paint protected area together forms the galvanic cell, Secondly the corroded area due to initial crevice and the non-corroded area together accelerate the galvanic action to corrode the under side of the roof. The combination of Crevice and Galvanic corrosion causes the thinning and holes in the roof plate.

Poor Ventilation There are a number of roof vents with flame arrestors. The flame trap pack is completely clogged with corrosion products and this caused the poor performance of the system and hence H2S was not fully vented out from the tank.

FAILURE OF ROOF PLATE – CONCLUSION

From the above investigations, it can be concluded that the corrosion of the roof plate is mainly due to galvanic and crevice for which the important causes are the gap between the rafter and roof plate. No sealing at the Lap welded areas at internal side and poor ventilation of H2S.

REPAIR OF THE TANK BOTTOM AND APPLICATION OF C.P TECHNIQUES FOR EXISTING TANKS

The damaged tank bottom plate can be repaired in many ways based on the severity of damage. The repair work has to be carried out as per API-653 Standard. It recommends repair/replace the bottom plate if the calculated min. thickness is expected to be below 2.5 mm at the next inspection.3 Repair can be done by over laying, coating or by welding patch plates. For large areas of significant corrosion the entire bottom plate has to be renewed.

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Bottom plate renewal can be done in two ways: 1. Replacing the existing plate and installing new plate 2. Installing new plate over the existing plate (bottom over bottom)

Grouting by suitable method, to fill the gap between the plate and soil, where appreciable soil consolidation is observed. In our case it was decided to replace the bottom plate due to poor condition of the plate. Bottom over bottom method was chosen for the ease of erection and provision of CPS system.

TYPE OF CATHODIC PROTECTION It was also decided to introduce Cathodic Protection system to improve corrosion protection for the new bottom plate. The various C.P systems for the tank are:

ICCP shallow ground bed Installation of anode around the Periphery. From different case studies it was known that the CP system with anode at peripheral of the tank is not suitable for the bottom plate mainly due to the non uniform current distribution. Secondly due to shielding effect of the ring wall concrete foundation reducing the protection current is available to bottom plate. Beside this system would invariably result in stray current with neighboring structures H.F.O Tanks, Water Tanks, utility piping, earthing, etc… ICCP Deep well ground bed Installation of anode at remote area of tank farm provide common cathodic protection system for all the tanks of the farm, in this case sufficient structures polarization may be difficult to achieve due to shielding effect as tank farm has complex structure of under ground piping network, road culvert, building foundation, beside soil resistivity is low and water table at this location is approximately – 1.5 meters. In this method, tank farm Earthing grid / pit may consume bulk of CP current.

ICCP Ribbon type Ribbon type anode to be installed under individual tank, covering all the areas of the bottom plate.

CP System Adhered For the repaired tank with bottom over bottom ICCP ribbon type anode is most suitable considering factors of corrosion and C.P. This will be laid directly between two bottoms. Old bottom plate should be electrically isolated from new one. Other data will remain same as in installation of CP system for new tank.

REPAIR WORK CARRIED OUT AND OUR EXPERIENCE

It was decided to carry out temporary repair work on this tank until plate replacement. Based on the MFL result, the corroded and thinned area were marked and repaired by patch welding of plates. The welded areas of the patch were checked by using a Vacuum box tester. It was noticed that 50 mm area close to the original lap welding of the plates were having holes. It is to be noted that many of the MFL testing equipments do not have the facility to inspect 100% of the surface, since the probe is ~ 50 mm away

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from the end of the plate joint. Manual Ultrasonic Thickness measurements were carried out at all lap welded areas and the patches have been extended accordingly.

CONCLUSION The under side corrosion of the HFO tank is due to the combined actions of Stray Current, Pitting, Soil Corrosivity and soil Consolidation, The fluid side corrosion is due to damage of the painting system and water accumulation at the bottom due to the holes in the roof causing rain water to enter the tank. The roof plate corrosion is due to the crevice corrosion occurred between the rafter support and the plate and the gap between the lap welded joints of the roof plate and the accumulation of hydrogen sulfide at the crevice and the galvanic corrosion between the unpainted and corroded area with painted and un corroded area.

RECOMMENDATIONS

Repair the tank bottom installing new bottom plate over the existing bottom plate. Provide ICCP with Ribbon type anode, insulation pad / liner. Seal all openings between annular plate and concrete ring foundation to prevent moisture / oxygen from entering the space. The painting procedure on the bottom plate must include surface preparation involving grit blasting conforming to international standards, application of zinc rich epoxy primer or a compatible inorganic zinc silicate primer of minimum fifty microns dry film thickness, followed by at least four coats of high build solvent free epoxy paint. Determine Concrete ring wall Rebar status w.r.t. to shell plate. If isolation is found, install at least a continuity bond to avoid rapid corrosion rates of reinforcing rod due to stray current. Prior to design and installation of an alternate CP system, stray current effect on neighboring structures shall be studied and suitable remedial action being taken to nullify this effect. One of the remedial actions may be bonding together the ground utilities such as potable water line, Fire Fighting water lines, H.F.O lines, and other piping including earthing and grounding system. When liquid level is low in the tank all the bottom plates may not be in contact with the soil. This situation is similar to sand consolidation therefore CP is ineffective. Hence periodic inspection of soil consolidation be done and the gap should be filled by grouting with suitable filling compound (cement + sand mixer). The damaged roof has to be replaced. To avoid further corrosion, the crevice between the plates at lap joints, and the plate to rafter support should be minimized or eliminated with suitable design. Venting system is to be improved to avoid the accumulation of H2s.

REFERENCES:

1. Technical report ref # TSR 3804/01008 by research and development center, SWCC Al-Jubail. 2. External cathodic protection of on grade metallic storage tank bottoms RP 0193 - 93 3. API standard RP 653 – Tanks inspection, repair, alteration and reconstruction.

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4. Prevention of corrosion R.M.E. Diamant 5. Petroleum storage principle – oil tank corrosion – Alex Mars

AUTHORS:

Mohammed Javeed Shaikh: Cathodic protection specialist with 16 years of hands on experience in Large A.S.T., Water and Gas Transmission lines, Off shore structures. Member of NACE international, Member of SWCC Corrosion committee. Zain Khadir Muhideen: Sr. corrosion engineer with 35 years of experience in power plant maintenance, testing and inspection. Life member of Indian institute of metals. Associated with life assessment of boiler, tanks, vessels and gas transmission lines.

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Tank bottom plate

Fine graded asphalt concrete T = 50 mm with grain size not exceeding 5 mm

Soil

Non rotting Oil and Sea water proof foil

Shell plate

Leveling concrete T =50 mm

Fig # 1 Construction details of tank bottom

Fig # 2 Blisters in the bottom plate paint at HFO side - before sand blasting

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Fig # 3 Pitting corrosion in the floor plate at HFO side - after sand blasting and cleaning

Fig # 4a -Tank floor MFL inspection result

MFL Test result HFO side internal corrosion is minimum. Corrosion (pitting) originated from the soil side of the plate. most severe was the index plate and annular plates as we move toward center plate metal loss decreases.

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Fig # 4b Bulging out of first index plate

Fig # 5 Holes in the Tank roof

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Analysis shows corrosion has originated from inside of the tank and is restricted only to

• Lap welded area

• Roof plate just above the support angle (rafter)

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Fig # 6 Filling the gap between floor plate and soil by grouting

Fig # 7a Corrosion of roof plate (Interior side of a roof plate sample -showing corrosion slightly wider than the rafter)

Conc. mixer

Pump

Hose

Man-hole

Red indicate area where soil consolidation has occurred in varying degree

Grout Inj. points

Tank bottom

Ring wall

Bitumen impregnated board

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Fig # 7b View of the roof plate sample (exterior side-showing no corrosion)

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Documents

Failure of Above Ground Storage Tanks Nace Corrosion 2007