corrosion

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Corrosion:

H2S (Sour)

Partial Pressure > 0.3KPa (0.05% PSI) H2S is called sour environment. More than 500 PPMis corrosive.

Fe + H2S ---------- FeS+H2

CO2 (Sweet)

Fe + H2O +CO2 --- FeCO2 + H2 (Above 10 PSI partial pressure corrosive)

O2

4Fe + 3O2 ---- Fe2O3

Flow patterns: 1.0 Oil lines: 7 FPS (foot per second –velocity) and above water remains droplets no

corrosion 2.0 Gas lines: 25 FPS and above water remain droplets no corrosion

Forms of corrosion

1.0General corrosion1.1Galvanic1.2Stray current

1.3Biological1.4Molten salt1.5Liquid metal1.6High temperature

2.0Localized corrosion2.1Filiform2.2Crevice2.3 Pitting2.4Biological

3.0Metallurgical Influenced corrosion3.1Dealloying3.2Intergranular

4.0Mechanically assisted degradation4.1Erosion4.2Fretting4.3Fretting fatigue4.4Cavitation and water drop impingement4.5Corrosion Fatigue

5.0Environmentally Induced Cracking5.1Stress corrosion cracking (SCC)5.2Hydrogen Damage (Hydrogen embrittlemnt)

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5.3Liquid metal embrittlement5.4Solid metal induced embrittlement (SMIE)

Corrosion Monitoring:

1.0 Chemical analysis

2.0 Biological analysis3.0 Inspection records4.0 Inline corrosion detection methods

4.1 Coupons4.2 Linear Polarization4.3 Galvanic4.4 Electrical Resistant Probes (ER)4.5 Biological Probes4.6 Intelligent Pigging

1.0 Corrosion assessment

1.1 Sour gas (H2S) environment – PREDICT 3.01 software - InterCorr 1.2 Sweet gas (CO2) environment – NORSOK M506 software - SHELL

2.0 Pipeline coatings2.1 3L PE – Three layer poly ethylene coatings up to < 80 C (Primer, FBE-Fusion

Bonded Epoxy and EPE)2.2 3L PP – Three layer Poly Propane coatings from 80 C to 120 C2.3 Field joints: Heat shrink fluid : PE or PP banding2.4 Emergency section repair: PLIDCO clamping – 6 months and within 6 months

replacement must be planned.

3.0 Cathodic Protection:

3.1 ICCP Impressed Current Cathodic Protection: On continuous basis- Anodes Fe-14.5 % Si,Platinum plated, Ti and Nb.

3.2 Sacrificial anodes: During installation in case of corrosive soils- Mg, Al and Zn3.3 Protection of steel structures – 0.85 volts

Inhibitors:

4.0 Corrosion Inhibitors:

4.1 QP uses: Servo Cortron CK915 and CK 352 max: 250 PPM.4.2 QP gas line (Sour associated gas 24”line) Petrolite K-143, 1000 PPM weekly batch4.3 The selected inhibitors in QP shall be based on following chemicals (based on high

molecular weight nitrogen molecules) (QP-STD-L-008):4.3.1 Amines/Imidazoline.4.3.2 Salts of amines with fatty acids.4.3.3 Quaternary ammonium salts.

4.4 Corrosion inhibitors shall be suitable for corrosion mitigation of wet CO2 and /or H2S contained in hydrocarbon fluids carried by steel lines or vessels. Usuallycationic type of inhibitors is used. Film forms on cathodic areas. Should reducecorrosion <1MPY

4.5 Specific properties:4.5.1 Water –soluble or oil-soluble/water dispersible.4.5.2 Compatible with production chemicals and construction materials

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4.5.3 Shall be in liquid phase and no foaming4.5.4 Biodegradable4.5.5 Usable up to Max fluid temp. 85 C.4.5.6 Pour point less than -5 C4.5.7 Kinematic Viscosity shall be less than 100cSt at 25 C

4.6 Consideration for injecting corrosion inhibitor:4.6.1 Wet Crude oil lines containing H2S and CO2

Basic sediment and water > 20%Basic sediment and water > 1% and fluid velocities less than 1 m/sMeasured corrosion rates equal or greater than 1 mpy

4.6.2 Wet gas lines –topside piping carrying wet sour gas:Minimum gas velocity 3.5 m/sMax. gas velocity 17-19 m/s

Qualitative guideline for weight loss corrosion of steel (NACE RP14E):

Limiting Values in Brine

Corrosive Gas Solubility*PPM

Non corrosivePPM

CorrosivePPM

Oxygen (O2) 8 <0.005 >0.025

Carbon Dioxide (CO2) 1700 <600 >1200

Hydrogen Sulfide (H2S) 3900 ** **

* Solubility at 68 F in distilled water at 1 Atmosphere partial pressure. Oxygen (O2) is for 1

atmosphere air pressure.

** No limiting valve for weight loss corrosion by hydrogen sulfide (H2S) are shown in thistable because the amount of Carbon Dioxide CO2 and /Or Oxygen O2 greatly influencemetal loss corrosion rate. Hydrogen Sulfide is less corrosive than carbon dioxide due toformation of an insoluble iron sulfide film which tends to reduce metal weight losscorrosion.

Pipe Grades:

1.0 Non Corrosive hydrocarbon service: Most commonly used pipes

1.1 ASTM A 106 Grade “B” Seamless1.2 API 5L Grade “B” Seamless, ERW and SAW welded.

Pigging:

1.0 Pigging during pre commissioning2.0 Swabbing3.0 Scrapping

4.0 Intelligent Pigging5.0 Pigging with batch injection of Corrosion Injection

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DEP 31.38.01.15 Gen-Material selection:

Service Pipingclass

C/NC

Sour service

Temp. CPiping class lim.

PipeMaterial

Remarks

Medium Min Max

Air

Maintenance air 141 NC - 0 120 CSPlant air 141 NC - 0 120 CS

Instrument air 841 NC - 0 60 Gal. CS

Antifoam/Coagulant 341 C - 0 120 SS316

Chemicals

General service 341 C - 0 120 SS316 Chem.Inj/dosing Max.tempdoes not takeinto account

Methanol 141 NC - 0 120 CS

147 NC - -50 100 CS

Oxygen scavenger 341 C - 0 120 SS316

Scale and corr inhibitor 341 C - 0 120 SS316 Presence of chlorides

Sodium Hypochlorite 641 0 50 TitaniumWax inhibitor 341 C - 0 120 SS316

De-Emulsifier 341 C - 0 120 SS316

Diesel oil 141 NC - 0 120 CS

Foam fire fighting 341 NC - 0 120 SS316

Fuel gas 145 NC - 0 200 CS For norm.dutiesCS may beused

146 NC yes 0 200 CS

343 NC - 0 200 SS316

Gas 147 NC - -50 100 CS

148 NC yes -50 100 CS

149 C - -50 100 CS150 C Yes -50 100 CS

345 NC - -200 50 SS316

346 C Yes -200 50 SS316

Gas (Class 150-600) 145 NC - 0 200 CS

146 NC Yes 0 200 CS

(Class 900-2500) 153 C Yes 0 200 CS

Glycol 141 NC - 0 120 CS

142 NC Yes 0 120 CS

143 C - 0 120 CS

144 C Yes 0 120 CS145 NC - 0 200 CS

146 NC Yes 0 200 CS

Hydrocarbonscondensate

141 NC - 0 120 CS

142 NC Yes 0 120 CS

143 C - 0 120 CS

144 C Yes 0 120 CS

Hydraulic oil 341 NC - 0 120 SS316

Jet fuel 341 NC - 0 120 SS316

Luboil 141 NC - 0 120 CS

142 NC Yes 0 120 CS

143 C - 0 120 CS

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144 C Yes 0 120 CS

341 NC - 0 120 SS316

Mud bulk dry 141 NC - 0 120 CS

Oil based 141 NC - 0 120 CS

Mud circulation 141 NC - 0 120 CS

142 NC Yes 0 120 CS

143 C - 0 120 CS144 C Yes 0 120 CS

Oil

Crude 145 NC - 0 200 CS

146 NC Yes 0 200 CS

Recovered crude 145 NC - 0 200 CS

146 NC Yes 0 200 CS

Multiphase 145 NC - 0 200 CS

146 NC Yes 0 200 CS

Seal oil 141 NC - 0 120 CS

142 NC Yes 0 120 CS143 C - 0 120 CS

144 C Yes 0 120 CS

314 NC - 0 120 SS316

Water

Produced water for injection

141 NC - 0 120 CS

142 NC Yes 0 120 CS

143 C - 0 120 CS

144 C Yes 0 120 CS

Sea water cooling 541 0 66 Cu/Ni

Seawater fire fighting 541 0 66 Cu/Ni

Fresh cooling water 141 NC - 0 120 CS

Fresh heating water 145 NC - 0 200 CS

Potable water 841 0 60 Galv.CS

Washdown water 841 0 60 Galv.CS

Waste

Sewage 842 0 60 Galv.CS

Waste treatment 842 0 60 Galv.CS

C = CorrosiveNC = Non-corrosive

Failure mode grouping (ASME B31.8S): Total 22 reasons for failure of pipe lines1.0 Time- Depended

1.1 External corrosion1.2 Internal corrosion1.3 Stress corrosion cracking

2.0 Stable2.1 Manufacturing related defects

2.1.1 Defective pipe seam2.1.2 Defective pipe

2.2 Welding/fabrication related

2.2.1 Defective pipe girth weld2.2.2 Defective fabrication weld2.2.3 Wrinkle bend or buckle

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2.2.4 Stripped thread/broken pipe/coupling failure2.3 Equipment

2.3.1 Gasket O ring failure2.3.2 Control /relief equipment mal function2.3.3 Seal/pump packing failure2.3.4 Miscellaneous

3.0 Time dependent3.1 Third party Mechanical damage

3.1.1 Damage influenced by first, second and third parties(Instantaneous/immediate failure)

3.1.2 Previously damaged pipe (delayed failure mode)3.1.3 Vandalism

3.2 Incorrect operational procedure3.3 Weather related and outside force

3.3.1 Cold weather 3.3.2 Lightning3.3.3 Heavy rains or floods

3.3.4 Earth movements4.0 Unknown

Controlling Pitting & Crevice corrosion in Stainless Steel:1.0 Controlling factors

1.1 pH1.2 Chloride concentration1.3 Presence of Oxygen or oxidizers

Improvement in pitting & crevice resistance in SS can be attained by increasing Cr, Moand Nitrogen content in the steel. Frequently used empirical formula is known asPitting Index or Pitting Resistance Equivalent (PRE) which for various types of stainless steels are represented as:

Ferritic - PRE= %Cr + (3.3 x %Mo) Austenitic - PRE= %Cr + (3.3 x %Mo) + (30 x %N)Duplex - PRE= %Cr + (3.3 x %Mo) + (16 x %N)

Pitting resistance increases with PRE value. For sea water pH 7.8 with an index of above 32 have adequate resistance to pitting. At 40 Index have highly resistant topitting and crevice corrosion at ambient temperature.

Pitting and crevice tendency increases with temperature.Heat tint film formed on ss after welding should be removed by pickling.Deposit on surface should be avoided.

corrosion

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