corrosion and its mitigation in the oil and gas industry.pdf

10 JAN-MAR 2010 Visit our websites at www.safan.com / www.pm-pipeliner.safan.com

Corrosion and its Mitigation in theOil & Gas Industry – An overviewBy Mr. Nalli. K

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Introduction:The oil and gas industries – mainly concerning the

exploration, production operations, the field operatorsnormally would like to have uninterrupted supply of oiland gas to the export or processing points. The lines and thecomponent fittings of the lines would undergo materialdegradations with the varying conditions of the well due tochanges in fluid compositions, souring of wells over theperiod, changes in operating conditions of the pressuresand temperatures.

This material degradation results in the loss of mechanicalproperties like strength, ductility, impact strength etc; leadsto loss of materials, reduction in thickness and at timesultimate failure.

Hence it is imperative for the field operators, pipelineengineers, designers to have “Corrosion Awareness”concerning the oil and gas industries in their day to dayactivities to combat and mitigate corrosion and to ensuresmooth and uninterrupted flow of oil and gas to the endusers. The present brief is an attempt in this direction.

Wealth of information is available on corrosion and it’smitigation in the oil and gas industry through case histories,technical papers, published literature and corrosioninstitutes, however the present brief is summary andconsolidated information pooled from various sources forthe benefit of the process, operations, maintenance andintegrity engineers involved in the oil and gas activities.

Well head Fluids:The reservoir fluids in oil and gas fields around the world

vary greatly in composition. Some cases the fluid is ingaseous state, in others it is in liquid state. Frequently gasand liquid co-exist in a given reservoir.

The reservoir fluids basically comprise of any one orcombination of the following types of fluids with dissolvedand suspended solids – based on the reservoir formation

and locations.• Liquid hydrocarbons and multiphase systems• Gas and gas condensates• Formation waters,• Sea waters,• Brackish waters

The ageing reservoirs will be souring (increase in the acidgas-Hydrogen Sulfide level) and with the increase in thewater cut the corrosivity of the fluids increases as the wellis being drained of the fluids.

Fig 1 indicates the general process scheme indicating thevarious functions in a typical oil and gas exploration andproduction facilities.

Corrosion and Corrosion morphology:Corrosion is defined as physiochemical reaction between

the metal and the environment resulting in materialdegradation and thus leading to impairment of the intendedfunction of the metal, environment or the integrity of thesystem. This can be general corrosion or regular loss ofmetal on the exposed surface or can be localized corrosionwhere only a limited portion of the surface is in contact.

Corrosion in Oil and Gas industry:Corrosion in oil and gas industry is mainly due to the

reservoir and well fluids tapped during the exploration andproduction operations. Some of the contents present inthe well fluid are represented in Table 1. Theseconstituents few of them or all of them may be present invarying compositions. Each one of the component in thefluid will have influence on the corrorsivity of the fluidand will determine the performance of the materials incontact. Some of the corrosive mechanisms generallyobserved in the oil and gas industry and the impact ofcorrosion agents in the fluid are briefly discussed in thefollowing sections.

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CO2 Corrosion (Sweet Corrosion):CO2 is one of the main corroding agents in the oil and gas

production systems. CO2 will mix with the water formingCarbonic acid making the fluid acidic (reducing the pHvalue).CO2 corrosion is influenced by temperature andincrease in pH value. At elevated temperatures Iron carbide(Siderite) scale will form on the material as a protectivescale and corrosion rate is reduced. The metal startscorroding under these conditions and the forms of CO2

corrosion are ringworm corrosion, Mesa corrosion andpitting corrosion as shown in Fig 2.

CO2 corrosion is enhanced in the presence of Oxygenand organic acids which dissolve the protective Iron carbidescale and prevent further scale formation.

The presence of Bicarbonates in the phase improves thealkalinity of the fluid and thus reduces the corrorsivity ofthe environment.

Acetates in the well fluids observed to affect the corrosionrate and rate of corrosion is observed to be low whenacetate concentration in the fluid is low.

Hydrogen Sulfide Corrosion (HydrogenAttack and Sour Corrosion)

Though the presence of Hydrogen Sulfide corrosion isless significant, the primary concern with Hydrogen Sulfideis Hydrogen attack on the metal and causing the metalembrittlement. Fluids with high levels of H2S are termed asSOUR and NACE defines the H2S levels above 0.05 psi ofpartial pressure as sour condition.

The Hydrogen attack mechanism is complex and iscaused by absorption of atomic Hydrogen in steel dependingupon certain pressure, temperature and the pH value of thefluid. The forms of sour corrosion are uniform, pitting, andstep wise cracking as shown in Fig 3.

Table 1Component Symbol Unit

Carbon Dioxide CO2 Mole/mole %

Hydrogen Sulfide H2S Mole/mole %

Oxygen ( Dissolved ) O2 Mg/lit or ppb

Chlorides Cl- Mg/lit (ppm)

Bicarbonates HCO3- Mg/lit (ppm)

Iron Fe2+ Mg/lit (ppm)

Magnesium Mg2+ Mg/lit (ppm)

Potassium K+ Mg/lit (ppm)

Sodium Na+ Mg/lit (ppm)

Calcium Ca2+ Mg/lit (ppm)

Sulfates SO4- Mg/lit (ppm)

Sulfur S Mg/lit (ppm)

Mercury Hg Mg/lit (ppm)

Lead Pb Mg/lit (ppm)

Zinc Zn Mg/lit (ppm)

Chromium Cr Mg/lit (ppm)

Barium Ba Mg/lit (ppm)

Total Hardness As CaCO3 Mg/lit (ppm)

Total Suspended Solids TSS Mg/lit (ppm)

Total Dissolved Solids TDS Mg/lit (ppm)

pH value pH Number

Typical well fluid componentsMultiphase – Liquid Hydrocarbons

Corrosion due to Oxygen:Oxygen is strong oxidant and reacts with the metal very

Fig 1.Typical Processing Scheme for Oil and Gas Facility


quickly. The dissolved Oxygen in the formation / producedwater is one of the primary causes of corrosion in theproduction equipment. Although Oxygen is not present inthe well fluids, Oxygen ingress takes place in the well fluidsthrough leaking pump seals, casing and process vents,open hatches.

The forms of corrosion associated with Oxygen is mainlyuniform corrosion, pitting type corrosion as shown in Fig 4

Chlorides – Stress Corrosion cracking:The presence of Chlorides in the well fluids attack the

material through the de-passivation effect induced byChloride ions and is quite aggressive on 300 series ofAustenitic Stainless steels. High Nickel alloys are practicallyimmune to this attack and resist chloride corrosion. Thechloride stress corrosion is influenced greatly by thetemperature, chloride concentration and residual stressesin the metal.

The presence of oxygen and low pH value acceleratesthe attack on the metal. This chloride corrosion is normallyis indicated as pitting type on the metal surface as shownin Fig 4

Elemental Sulfur Corrosion:Elemental Sulfur will be present in some reservoir fluids

and is very strong oxidant. It mixes with the water in thefluid and forms Sulfuric acid and reacts with Carbon andlow alloy steels to form Sulfides. Corrosion due to elementalsulfur increases with temperature. Low alloy materials likeCRA are quite susceptible to elemental Sulfur attach howeverhighly alloyed Nickel CRA’s like Inconel / Incoloy areresistant to Sulfur attack.

The Sulfur attack is by and large is localized and similarto pitting type of corrosion.

Galvanic Corrosion:This type of corrosion occurs when two metallic materials

with different nobility (electrochemical potential) are incontact and are exposed to an electrolytic environment. Insuch situation the metal with less or most negative potentialbecomes anode and starts corroding. The most prominenttype of galvanic corrosion occurs in a coupling betweenCRAs (Stainless steel or Nickel alloys) with Carbon or lowalloy steels in a deaerated surroundings.

Hydrogen ebrittlement is also possible on the more noblemetal if it is susceptible. The presence of H2S and lowtemperatures encourages the galvanic type of corrosion.(Fig 5).

Erosion Corrosion:Erosion corrosion is a form of corrosion with mechanical

removal of metal due to high flow rates of the media and iscommon in all metallic materials. The rate of corrosionincreases with sand or solid suspended particles in the fluidand is dependent on fluid flow rate, the density andmorphology of solids present in the fluid.

Crevice corrosion:Crevice corrosion is normally localized corrosion taking

place in the narrow clearances or cervices in the metal andthe fluid getting stagnant in the gap. This is normally in theform of pitting formed on the surface of the metal andgradually extending into the metal itself.

Microbial Induced Corrosion (MIC-Sulfatereducing Bacteria):

This form of corrosion is induced due to the presence ofsulfate reducing bacterium which grows in anaerobicconditions. This bacterium is normally present in reservoirfluids, formations waters and soils. The microbes tend toform colonies in a hospitable environment and allowenhanced corrosion under colony. The formation of thesecolonies is promoted by neutral water especially whenstagnant. The form of microbial induced corrosion is pittingand the products of corrosion involve iron sulfates, slime,

Uniform Mesa Corrosion Corrosion

Fig 2: Corrosion due to CO2(Sweet Corrosion)

Pitting due to H2S H2S corrosion

Fig 3: Corrosion due toH2S (Sour Corrosion)

Oxygen Corrosion Pitting due to Chlorides

Fig 4:Corrosion dueto Oxygen andChlorides

Galvanic corrosion MIC corrosion

Fig 5: Galvanicand MICCorrosion

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plugging and also bacterial growth. (Fig 5)

Methods to combat corrosion in Oil & Gasindustry

Fighting corrosion to improve the equipment and plantlife continues to be a nightmare for many of the plantoperating staff. While many methods have been advised toarrest these events, these methods can be broadly classedinto three (3) main categories:• Change the material of construction for the specific

application• Reduce the intensity of corrosive attack by modifications

in corrosive media• Create a barrier layer between the material and media

to avoid the direct contact.

Change in Materials of Construction:When it is observed that the existing material of

construction is prone for corrosive attack, it is normallydecided to change the materials of construction and select

alternate material to suit the specific need. Generally thematerials used in the hydrocarbon applications can bebroadly grouped as metals, non metals. Each variety ofthese materials has its specific applications and limitations.

At times the change of material may have additional cost.However it is worthwhile to think in terms of life cyclecosting, which may show a longer equipment life andlower maintenance cost in spite of high initial cost. Adetailed study of process and operating conditions has tobe carried out before selection of a new material.

The new generation of Stainless Steels - Duplex stainlesssteels / Super Duplex Stainless steels by and large resistalmost all types of corrosion. The steel mills all over theworld have been continuously developing new materialswith different metallurgies to resist almost any type ofcorrosion in the oil and gas industry.

While exotic materials like Titanium, Zirconium etc willoperate in almost all corrosive and high temperatureenvironments, the initial cost is prohibitive to select materialsunless the equipment downtime is critical to the process

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and the facility.Non ferrous materials like Copper, Nickel and Copper

Nickel alloys also have found good use in the sea waterenvironments when normal materials like carbon steel andstainless steels generally perform badly and fail.

Selecting suitable grades of Stainless Steels like lowcarbon variety and stabilized grades will avert intergranularcorrosion or weld decays if welding operations are involved.

Table 2 shows some of the commonly used materials inthe hydrocarbon and oil and gas industries. The table is forguidance only. Detailed study of flow conditions, corrosionmechanisms involved and the expected life of material isimportant before selecting a specific metal for the application

It is all the time more important to understand that nosingle material is cure for all the corrosion evils. A materialwhich is good for stress corrosion cracking may fail due tofatigue. Another material which can resist high temperaturecorrosion may fail due to pitting

Change of Corrosive media or theenvironment:

At times it is necessary to reduce the intensity of corrosiveattack of the environment by adding certain chemicalsknown as ‘Inhibitors’ to reduce the aggressiveness of themedia. These chemicals are injected into the media atpoint–to point in the process. The chemicals, theconcentration and the frequency of injection depend uponthe process media and normally recommendations of theinhibitor manufacturer since most of the cases thesechemical though generic in nature are proprietary items.

The inhibitors are normally chromates, phosphates andsilicates added as per the recommendations of themanufacturer.

Also removal of the Oxygen from the fluid media improvesthe chances of corrosion resistance of materials in contact.Controlling and stabilizing the pH value of the media isanother method of combating corrosion. There are manyvarieties of corrosion inhibitors available in the market, butjudicious approach and recommendations of the


manufacturer have to be followed while injecting thesechemical in the well fluid streams.

Intermediate barrier to avoid direct contactwith media:

A protective layer or barrier on the material to avoid thedirect contact with the process media will enhance thematerial and equipment life. The barrier layer can be paint,a coating or a lining, or a metallic lining or metallic sheets.There are also non metallic linings like fiber glass, glassflake, epoxy, rubber etc which are normally carried out onthe equipment like separators, KO drums, storage tanks etc.Nickel, Zinc and Cadmium coatings are also preferred attimes on certain components like flanges, bolting etc.

Again it should be clearly understood that abovearrangement is not permanent cure and only will extendthe life of the bare materials underneath the barrier to someextent. The paints over prolonged exposure to atmosphereheat and sunlight may flake off. The tape or lining on apipeline may get physically damaged, crack and delaminateexposing the bare material beneath to corrosive media.However this method of combating corrosion is cheap andless expensive than opting for a costly material ofconstruction.

Corrosion Monitoring and ManagementThe best way to check corrosion is by visual inspection

and checking up the material degradation periodically.However it may be possible to check the material conditionexternally and it is impractical if not impossible to checkthe internal surfaces now and then.

One of the methods is to carry out the on-stream-Inspection by doing the wall thickness measurementsperiodically on fixed and vulnerable locations on theequipment, piping and pipelines to assess the materialconditions and corrosion rates. However this method hasits own limitations since this check point under investigationmay show lower corrosion which may not be true at someother point that might have corroded heavily and goneundetected.

Also corrosion is monitored by placing electronic probesin the pipelines and by measuring the change in the electricresistance in the probe coil. However this method isindicative more of the process fluid than the wetted materialscondition.

The cross country pipelines are normally checked withintelligent pigging operations like magnetic flux or ultrasonicpigs. These pigs will detect the internal conditions of thepipeline, corrosion conditions on the pipe wall thicknessand also indicate the wall thickness available on the pipewall.

Most of the equipment like separators, drums, heaters etcare checked for corrosion during annual shutdown andturnaround operations. Based on the physical assessmentof the material conditions corrective action is initiated tochange the material or replace the equipment or at times dotemporary repair work before replacement is carried out. Inpractice it is observed that physical inspection is the bestmethod of monitoring corrosion and assessing the materialconditions.

ConclusionUndoubtedly understanding the corrosion mechanism is

very important before considering various material optionsfor the applications. However it should be clearly understoodthat no particular material is the cure for all the evils ofcorrosion. Each and every case has to be considered in it’stotality before a decision is made on the proper materials.Consultations with process, operations, materials andcorrosion engineers are necessary in the fitness of things tosave millions to fight the corrosion menace.

PetroMin PipeLiner would like to showappreciation to Mr. Nalli. K from Mott MacDonaldCo LLC, Oman for this independent article.

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