Damage Mechanisms Which Affect Refinery EquipmentPresented By: Thomas D. Farraro Houston, TX Houston, Texas August 24 and 25, 2003STRESS ENGINEERING SERVICES, INC.1

A Damage Mechanism is defined as:

A mechanism or process which results in deterioration of a material or its properties because of a reaction with or in response to the environment to which it is exposed.

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Failure to Understand Damage Mechanisms May Have Dire Consequences

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8 Types of Damage Mechanisms1. Uniform (General) Corrosion Atmospheric Corrosion (external) Process Corrosion (internal )

2. Localized Corrosion Pitting Crevice Corrosion Under Deposit/Coating Corrosion, Thinning, Cracking)

3. Galvanic Corrosion 4. Environmental Cracking (Stress Corrosion Cracking)a) b) c) d) e) f) g) h) Chloride Stress Corrosion Cracking Alkaline Stress Cracking (caustic, amines, carbonates) Ammonia Stress Corrosion Cracking Hydrofluoric Acid Stress Corrosion Cracking Polythionic Acid Stress Corrosion Cracking Sulfide stress Corrosion Cracking Hydrogen Induced Cracking Stress Oriented Hydrogen induced Cracking (ClSCC) (ASCC) (Ammonia SCC) (HFSCC) (PTASCC) (SSC) (HIC) (SOHIC)4

8 Types of Damage Mechanisms (Cont.)5. 6. 7. Intergranular Corrosion Dealloying (Dezincification, Graphitization) High Temperature Corrosiona) b) c) d) e) f) Oxidation Sulfidation Organic Acid Corrosion Carburization Metal Dusting Decarburization

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8 Types of Damage Mechanisms (Cont.)8. Mechanical/Metallurgical Assisted Degradationa) b) c) d) e) Erosion Corrosion Cavitation Fatigue Fretting Metallurgical Effects1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Grain Growth Graphitization Hardening Sensitization Sigma Phase Embrittlement 885 Embrittlement Temper Embrittlement Liquid Metal Embrittlement Brittle Fracture Creep Stress Rupture6

90% of Corrosion Problems Are Caused by One of the Following 12 Chemical Types1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Oxygen Carbon Dioxide Hydrogen Sulfide Sulfur Dioxide Inorganic Acids Inorganic Alkali Halide Salts Organic Acids Organic Chlorides Organic Sulfides Organic Amines Water7

Distribution of Damage Mechanisms Types 30% of all corrosion failures are caused by general thinning (uniform corrosion). 70% of all corrosion failures are caused by local corrosion mechanisms (pitting, stress corrosion cracking, crevice corrosion etc.)

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Wet vs. Dry Corrosion Corrosion can be divided into to distinct processes. Wet Corrosion Electrochemical reactions which require an electrolyte to be present - usually water.

Dry corrosion Chemical reactions which occur in the absence of any electrolyte.9

Why do Materials Corrode? The driving force behind corrosion is entropy; the tendency, in nature , for all things to return to the lowest possible energy state. Iron ore (rust) + Heat Corrosion10

Steel

Wet (Electrolytic) Corrosion4 Essential Elements of an Electrolytic Corrosion Cell

Anode (-) Cathode (+) Electrolyte External circuit

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Anodic Polarization and Passivity Anodic surfaces can be polarized by the formation of a thin impervious layer of corrosion products. When corrosion reactions are completely polarized the metal is said to be Passivated. At this point there is no potential difference between the anodes and cathodes and the corrosion ceases.12

Damage to the Passive Layer When the passive layer is dissolved or disrupted at a given point a very active anodic site is set up which causes accelerated corrosion.Reduction of ions or oxygen Metal Ions Fe++

CathodeElec t Migr ron ation

Cathode Anodetron Elec tion a Migr

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Chemistry of Wet Corrosion Electrolytic (Wet) Corrosion of Steel Oxidation (Rusting)4Fe + 6H2O + 3O 2 4Fe(OH)3 6H2O + 2Fe2O3

Hydrochloric Acid AttackFe + 2HCl FeCl 2 +H0

Sulfur Acid AttackFe + 2HSO3 Fe + H2SO4 Fe(SO3) + H0 FeSO4 + 2H0

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Primary Factors Which Effect Wet Corrosion Rates pH Concentration of Dissolved ionic species (salts, gases) Temperature Pressure Velocity of the fluid

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Other Factors Which Affect Wet Corrosion Heat Transfer Conditions Localized boiling/condensation, uneven heating causing galvanic corrosion

Amount of Suspended Solids coke, catalyst particles, mud, sand, silt etc.

Presence of Microorganisms sulfate reducing bacteria, sulfate oxidizing bacteria

Dissimilar Metals Can lead to galvanic corrosion cells and accelerated corrosion due relative size of anodic and cathodic areas

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Dew/Boiling Point Corrosion The inorganic salts and acids are typically vaporized or absorbed by saturated steam and carried with light hydrocarbons into the overhead systems of fractionation columns and separators. These inorganic salts and acids have very high solubility in water, consequently the first drops of water which condense or the last drops to vaporize will have a high concentration of salts and/or low acid pH which results in rapid localized corrosion, pitting and salt deposition. This is known as Dew Point or Boiling Point Corrosion17

Dew/Boiling Point Corrosion

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Wet or Dry ?Saturated Steam Pressure vs. Temperature3000

2500

Pressure (psig).......

2000

1500

Wet Dry

1000

500

0 200

250

300

350

400

450

500

550

600

650

700

750

Temperature (F)

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Effect of pH on Corrosion Rate for Different Metals

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Effect of pH on Corrosion of Mild Steel

Corrosion Rate in/yr

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Effect of Dissolved Salts on Corrosion Rate The dissolved salts increase the conductivity of water (electrolyte).

Increasing the concentration of dissolved salts accelerates corrosion.22

SALTS

Saltwater vs Carbon Steel and Alloys 90 80Corrosion Rate (mpy)..

70 60 50 40 30 20 10 0 0 50 100 150 Temperature F Adm. Brass 70-30 Cu-Ni 200 250

CS

Titanium23

Effect of Oxygen Concentration & Temperature on Corrosion Rate of Carbon Steel in Tap Water

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High Flow Velocities Can Erode Passive Layers and Accelerate CorrosionFluid Flow

Metal

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Effect of Velocity on Sulfuric Acid Carbon Steel Corrosion Rates98 wt% Sulfuric Acid vs Carbon Steel250

Corrosion Rate (mpy).

200

150

100

50

0 0 1 2 3 4 5 6 7 8 9 10 11 12 Acid Velocity ft/sec 40F 70F 100F 130F

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Corrosion Control Methods Design and Fabrication Methods to Avoid Corrosion Operating Process Parameter Control Use of Corrosion Resistant Materials Metals Non Metals

Create Barriers to Corrosion Inhibitors Coatings and Linings

ElectroChemical Corrosion Control Cathodic Protection27

Uniform (General) Corrosion Atmospheric Corrosion Requires Humidity levels >60% Significant problem for facilities located in coastal zones or heavily industrialized areas. Salt and atmospheric pollutants (SO2, SO3, H2S, etc.) combine with moisture in air to form corrosive salts and acids.28

Atmospheric Corrosion Rates for Carbon and Low Alloy Steels12

Corrosion Rate (mpy)...

10 8 6 4 2 0 0 50 100 150 200 250 300

Metal Temperature Marine / Cooling Tower Drift Area Temperate

Arid / Dry29

Corrosion Under insulation Occurs in carbon and low alloy steel which is insulated if insulation becomes wet. Localized corrosion can occur at penetrations in insulation jacketing at pipe supports, leaking steam tracing where moisture penetrates the insulation. Corrosion rates are similar to atmospheric corrosion rates30

Corrosion Under Insulation Oxidation (Rusting)4Fe + 6H2O + 3O 2 4Fe(OH)3 6H2O + 2Fe2O3

Salt AttackFe + NaCl +H2O FeCl 2 + NaOH + H0

Sulfur Acid AttackFe + 2HSO3 Fe + H2SO4 Fe(SO3) + H0 FeSO4 + 2H0

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Localized Corrosion

Corrosion limited to a specific relatively small area; while the remaining area is largely unattacked

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Types of Localized Corrosion Pitting Corrosion Crevice Corrosion Under Deposit Corrosion

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PITTING CORROSION - Causes & MechanismsPitting corrosion occurs when a passive film or another protective surface layer breaks down locally. After this initiation (local breakdown of the film) an anode forms where the film has broken, while the unbroken film (or protective layer) acts as a cathode. This will accelerate localized attack and pits will develop at the anodic spots. The electrolyte inside the growing pit may become very aggressive (acidification) which will further accelerate corrosion.ILLUSTRATION '` Chlorides ... Pitting is most commonly induced by chloride ions. Like other halides, these are very potent agents for destroying otherwise protective passive surface films (e.g., on stainless steels, nickel alloys. etc.). In the case of chloride-induced pitting the corrosion mechanism also involves strong chloride concentrating effects in a growing pit. This will accelerate the rate of pit development (autocatalytic growth process).

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Pitting Corrosion Pitting corrosion can be defined as an extreme case of localized attack which results in the development of cavities or pits in the metal surface

This phenomena is common in stainless steels and aluminum alloys which are exposed to chlorides.35

Crevice Corrosion Crevice Corrosion is a form of localized corrosion which occurs in a crevice formed between 2 surfaces at least one of which is a metal.2 types of crevice corrosion: (a) oxygen concentration cell

(b) metal ion concentration cell

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Crevice Corrosion at Pipe Support Point Carbon Steel - High Pressure Propane

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Crevice Corrosion (prevention) Use weld joints without built in crevicesPOOR

GOOD

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Crevice Corrosion (prevention) Dont skip weld !

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Under Deposit Corrosion Similar to crevice corrosion except the crevice is created by foreign material depositing on the metal surface. Corrosion is then caused by difference in oxygen or metal ion concentrations beneath the deposit and the adjacent bare metal. Deposits can be coatings, salts, mud, sand, algae or anything else which adheres or is held against the metal surface.40

Microbiologically Induced Corrosion (MIC)

MIC on Carbon Steel

MIC on Stainless Steel

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Microbiologically Induced Corrosion (MIC) Bacteria can cause corrosion in cooling water systems, firewater systems, heat exchangers, pressure vessels, storage tanks, oil and gas pipelines, and wells. Two common effects of MIC are: MIC directly corrodes structures like steel or concrete, seriously weakening them; MIC also causes thick growths -called tubercles which form on metal surfaces, and cause under deposit corrosion, fouling, and losses in thermal conductivity;

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Microbiologically Induced Corrosion (MIC) Types of BacteriaAnaerobic Sulfate Reducing Bacteria (ASRB)ASRB are responsible for extreme damage to piping and support equipment in many industries. They are probably the most destructive bacteria in the MIC group. ASRB reduce sulfates in the water, soil or oil, to H2S which corrodes the steel under the deposit.

Acid Producing Bacteria (APB)APB is a major player in the MIC corrosion process. APB are capable of producing organic and inorganic acids as well as producing nutrients for ASRB. APB metabolize sulfur in the water, soil or oil, to sulfurous acid which corrodes the steel u nder the deposit.

Iron-related bacteria (IRB)IRB are an important part of the MIC-causing group, because they are able to build tubercles and have many redox (reduction--oxidation) reactions that support SRB and other MIC bacteria. They are also responsible, in many cases, for the destructive corrosive process of iron and steel.

Slime-producing bacteria (SPB)SPB live in conjunction with other MIC-producing bacteria such as APB, SRB, and IRB. They are an important part of the MIC process, often acting as the transient from aerobic to anaerobic conditions and as a support system for the corrosion process. 43

Microbiologically Induced Corrosion (MIC) Damage caused by MIC may appear the same appearance as other corrosion mechanisms. To confirm MIC specific tests for the presence of bacteria must be performed.

Biological Activity Reaction Test (BART)

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Galvanic Corrosion Galvanic corrosion is attack associated with the current created by direct contact of dissimilar metals or thermal gradients on the same metal in an electrolyte (wet corrosive environment)

This results in the preferential attack of the more active (anodic) metal, while corrosion on the other passive (cathodic) metal is stopped.

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Galvanic Corrosion

Electrolyte ( saltwater)

Electrolyte ( saltwater)

Brass

Steel

Cold

Hot

Bi-metallic Corrosion

Thermo - Galvanic Corrosion46

Corroded End

Anodic

Protected End

Cathodic47

Stress Corrosion Cracking (SCC) The cracking of a material produced by the combined action of corrosion and tensile stress. This stress can be applied stress or residual stress in the metal (i.e. due to welding , heat treatment etc.

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Environmental Cracking Types Commonly Found in Refineries Chloride Stress Corrosion Cracking Alkaline Stress CrackingCaustic Carbonate Amine

(ClSCC) (ASCC)

Wet H2S CrackingSulfide stress Corrosion Cracking Hydrogen Induced Cracking Stress Oriented Hydrogen induced Cracking (SSC) (HIC) (SOHIC)

Hydrofluoric Acid Stress Corrosion Cracking Polythionic Acid Stress Corrosion Cracking

(HFSCC) (PASCC)51

Stress Corrosion Cracking Prevention

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Chloride Stress Corrosion Cracking Occurs in austenitic and duplex stainless steels when exposed to aqueous environments containing chlorides. Areas of high residual stress welds, cold formed bends, bellows, cold expanded tubes are most susceptible. May occur externally due to chlorides in atmosphere or present in insulating materials.53

Chloride Stress Corrosion CrackingChloride Cracking is presents as surface initiated cracking which propagates perpendicular to the orientation of greatest tensile stress.

Cracking is transgranular passing through the grains of the material.54

Chloride Stress Corrosion Cracking Critical Parameters Limited to austenitic and duplex alloys with 50 ppm Temperature >140F pH 2-8 Tensile stresses >25% of yield strength of the material55

Chloride Stress Corrosion CrackingEffect of Temperature and Concentration

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Chloride Stress Corrosion Cracking Prevention Avoid use of austenitic stainless steels or other susceptible alloys in neutral to acidic environments containing chlorides. Coatings are effective for prevention of external Chloride SCC. PWHT is not effective in preventing Chloride SCC

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Alkaline Stress Corrosion Cracking (ASCC)ASCC is presents as surface initiated deposit filled cracking which propagates perpendicular to the orientation of greatest tensile stress.

Caustic Cracking Carbon Steel

Caustic Cracking 316SS Steel

Cracking is intergranular following the grain boundaries of the material.58

Alkaline Stress Corrosion Cracking (ASCC) Caustic CrackingFe + NaOH Na2FeO 2 + H2

Carbonate CrackingFe + 2(HCO 3) -2 Fe(HCO 3) 2 + H2

Amine CrackingNot caused by pure amine but by carbon dioxide in the amine so it is similar to carbonate cracking.CO2 + H2O 2HCO3-2 Fe(HCO 3) 2 + H259

Fe + 2H+ + 2(HCO 3)-2

Caustic Cracking (Caustic Embrittlement) Caustic cracking is caused by surface initiating cracks which occur primarily in and adjacent to Non-Post Weld Heat Treated Welds or other areas of high tensile stress.

Caustic in boiler carbon steel tubesheet

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Caustic Stress Corrosion Cracking Critical Parameters Most steels and nickel alloys are susceptible to caustic cracking including carbon steels, low alloy steels, stainless and duplex steels and nickel alloys Must have liquid water present Caustic concentration >50 ppm Temperature >120F pH 8-14 Tensile stresses >25% of yield strength of the material. Non PWHTd welds are especially susceptible.61

Caustic Cracking Carbon Steel

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Caustic Cracking of Austenitic Stainless Steels Caustic SCC of austenitic stainless steels occurs between 105 and 205 C (220 and 400 F), depending on caustic concentration. Cracking of austenitic stainless steels is often difficult to distinguish from cracking by chlorides, particularly because common grades of caustic also contain some sodium chloride. As a general rule, however, SCC by chlorides is usually, but not always, in the form of transgranular cracking, while caustic causes intergranular cracking, sometimes accompanied by transgranular cracking due to the presence of chlorides.63

Caustic Cracking of Nickel Alloys

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Caustic Cracking Prevention Most effective method of preventing caustic cracking is Post Weld Heat Treatment Post Cold Working Heat treatment

To relieve residual tensile stresses.

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Carbonate Cracking Caustic cracking is by surface initiating cracks which occur primarily in and adjacent to Non-Post Weld Heat Treated Welds or other areas of high tensile stress. Only carbon and low alloy steels are susceptible. High strength steels used in underground pipelines are susceptible to external carbonate cracking when excessive cathodic protection current is applied. Primary cause of cracking in amine systems

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Carbonate CrackingCritical Parameters Only carbon or low alloy steels are susceptible Liquid Water must be present CO2> 2% Temperature >120F pH 8-10 Tensile stresses >25% of yield strength of the material. Not Post Weld Heat Treated

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Carbonate Cracking Prevention Most cost effective method of preventing carbonate cracking is: Post Weld Heat Treatment Post Cold Working Heat treatment Austenitic Stainless Steels are resistant to carbonate cracking

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Alkaline Stress Corrosion Cracking Special Precautions Be careful with external welded attachments to equipment and piping in alkaline service (caustic, carbonate or amine). Residual stresses from welding external attachments without PWHT can result in internal ASCC due to residual stresses at the ID surface of the metal. All welds, pressure containing, internal attachment and external attachment welds must all be PWHTd to avoid ASCC.

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Wet H2S Cracking

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Chemistry of Wet H2S CrackingChloride and Bisulfide Corrosion Most refinery process streams contain both chlorides and bisulfides.Alkaline Conditions Blistering/Cracking

Cl

-

Acidic Conditions -Corrosion

2H0 +FeS

-S-Fe-SHHS-

-S-Fe-Cl +SH-

FeCl2 + H2S

In acidic (low pH) conditions the right hand reaction dominates and will lead to formation of FeCl2 and H2S Increased Corrosion Rates In alkaline (high pH) conditions the left hand reaction dominates and will lead to formation of FeS and H2 Hydrogen Blistering and Wet H2S Cracking71

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Chemistry of Wet H 2S CrackingEffect of Cyanides and pH

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Chemistry of Wet H 2S CrackingEffect of Cyanides

Atomic Hydrogen is formed as a result of corrosionFe++ +2HSFeS + S= + 2H+ Fe(CN)6-4 +6NH4 + S= + 6H+

FeS + 6NH4CN + 6H2O

When alkaline pH conditions are present, cyanide dissolves the protective iron sulfide layer on the surface of the metal and causes a 3 fold increase in atomic hydrogen generation.

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Where do the Cyanides and Ammonia Come From? Organic Nitrogen compounds present in crude oil are converted to ammonia and cyanide during catalytic cracking or desulfurization. Cyanides will be found down stream of fluid catalytic cracking, hydrodesulfurization and hydrocracking units. The amount of ammonia and cyanides formed depends on the amount and type of nitrogen compounds found in the feed to these units.75

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Forms of Wet H2S CrackingTensile StressTensile Stress

Sulfide Stress Corrosion Cracking (SSC) Hard Weld Cracking

Stress Oriented Hydrogen Cracking (SOHIC)-

Hydrogen Blistering and (HIC)

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Hydrogen Blistering and Hydrogen Induced Cracking (HIC)Hydrogen blisters may form as surface bulges on the ID, OD, or within the wall thickness of the steel. The blister results from hydrogen atoms that form on the surface of the steel during the wet sulfide corrosion process. The hydrogen atoms collect at discontinuities in the steel, laminations, inclusions etc.) where they recombine into molecular hydrogen which is to large a molecule to diffuse out of the steel.

HIC occurs when cracks form at the end of the blisters and grow to connect other blisters.

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Hydrogen Blistering

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Hydrogen Blistering

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Stress Oriented Hydrogen Induced Cracking (SOHIC) SOHIC is HIC cracking which occurs in areas of high tensile stress. The tensile stresses cause the Hic cracking to orient itself perpendicular to the direction of the tensile stress. SOHIC can lead to through wall cracking.

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Sulfide Stress Corrosion Cracking (SSC) SSC is also known as hard weld cracking Cracking typically initiates at the surface the weld metal or Heat Affected Zone (HAZ) base metal adjacent to the weld. Cracking can also occur in cold worked bends or other areas where high hardness exists. Cracking will be oriented perpendicular to the direction of the tensile stresses.

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Wet H2S CrackingCritical Factors for Hydrogen Blistering, HIC, and SOHIC Susceptibility is limited to carbon and low alloy steels Wet H2S Cracking requires the formation of atomic hydrogen as a result of a corrosion reaction. >50ppm H2S pH < 5 or >8 Liquid water must be present >20 ppm CN- will significantly increase the rate and severity of damage PWHT will not prevent Blistering or HIC but may improve resistance to SOHIC83

Sulfide Stress Corrosion Cracking (SSC) Carbon steels, stainless steels, duplex steels, and nickel alloys are all susceptible to SSC. SSC requires the formation of atomic hydrogen as a result of a corrosion reaction. >50ppm H2S pH < 5 or >8 Liquid water must be present >20 ppm CN- will significantly increase the rate and severity of damage Sulfide Stress Cracking requires hardness of steel to be >237HB PWHT is effective in preventing SSC84

Hydrogen Embrittlement Hydrogen embrittlement occurs when atomic hydrogen diffuses into the steel in sufficient quantity to cause a decrease in the fracture toughness (impact strength) of the steel without causing any cracking or blistering. The source of the hydrogen may be corrosion or atomic hydrogen created during a welding process. Hydrogen embrittlement is reversible by performing a Hydrogen Bake Out at 400-600F for 1-4 hours which allows trapped atomic hydrogen to diffuse out of the steel.85

Wet H2S Cracking Special Precautions Steel that has suffered hydrogen blistering, HIC or SOHIC is irreversibly damaged. If repairs are to be made to hydrogen damaged steel remember that the steel is still saturated with atomic hydrogen and has possibly suffered hydrogen embrittlement. Prior to making any repairs on hydrogen damaged steel a Hydrogen Bake Out should be performed to avoid causing additional damage to the steel due to trapped atomic hydrogen.86

Hydrofluoric Acid HF - Hydrogen Cracking Similar to SSC, Cracking typically initiates at the surface the weld metal or Heat Affected Zone (HAZ) base metal adjacent to the weld as a result of exposure to HF acid.

Monel 400 HF Cracking

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HF Hydrogen Cracking (HFSCC) Carbon steels, stainless steels, duplex steels, and nickel alloys are all susceptible to HFSCC. SSC requires the formation of atomic hydrogen as a result of HF acid corrosion. Liquid water must be present Steels with Carbon Equivalent (CE)>.43 are highly susceptible. CE=%C+%Mn/6+%(Cr + Mo +V)/5+%(Cu+Ni)/15 HFSCC susceptibility increases with hardness of steel. Steels harder than 237HB are highly susceptible. PWHT is effective in preventing HFSCC

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Polythionic Acid Stress Corrosion Cracking (PTASCC) Polythionic acids are formed when iron sulfide FeS on the ID surface of an austenitic stainless steel in sour service is exposed to moisture and oxygen.Polythionic acid

FeS + O2 + H2O

HSxOy + FeOH

The polythionic acid attacks the sensitized grain boundaries of the austenitic stainless steel in the presence of residual tensile stresses which causes an intergranular stress corrosion crack to form.

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Polythionic Acid Stress Corrosion Cracking (PASCC) Critical Factors Susceptibility is limited to sensitized austenitic stainless steels and nickel alloys A sensitized stainless steel is one that has been exposed to temperatures in the range of 750F 1500F for a duration sufficient to result in grain boundary segregation of carbides. PASCC requires the presence of a iron sulfide scale on the ID surface of the steel. The sulfide scale must be exposed to moisture and oxygen to form polythionic acids. This usually occurs during equipment outages and turnarounds. Liquid water must be present Tensile stresses >25% of the yield strength of the material. PWHT will not prevent PASCC

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Polythionic Acid Stress Corrosion Cracking (PASCC) Prevention Use of stabilized grades of stainless steel ( 321SS, 347SS) which are not susceptible to sensitization at the operating temperatures. Prevent moisture or oxygen from entering the equipment during outages by utilizing positive pressure purge of inert gas (nitrogen). Wash the equipment with a dilute alkaline solution (soda ash) immediately upon opening to neutralize any polythionic acid which may form. Alkaline film must be maintained on the surface until equipment is returned to service. Note: Soda ash solutions always contain some chlorides. Ensure that all alkaline solution is completely drained from equipment before returning to service to avoid concentration of chlorides and subsequent chloride SCC.

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Intergranular Corrosion Intergranular corrosion is a highly localized corrosion which occurs a and adjacent to grain boundaries. Typically intergranular attack is caused by the actions of a specific chemical environment. For example, the corrosion which initiates PASCC in stainless steels is intergranular corrosion by polythionic acid. Sulfuric acid and Hydrofluoric acid are other refinery services that can cause intergranular corrosion.

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Dealloying Dealloying is a corrosion process in which specific elements are corroded away leaving the remainder of the alloy intact.

In refineries, dealloying corrosion is usually associated with Cast iron (graphitization, actually deironification) Brass (dezincification)

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Liquid Metal Cracking (LMC) LME is a form of catastrophic brittle failure of a normally ductile metal caused when it is exposed to another liquid metal and is stressed in tension. In refineries, LMC has occurred when Copper alloys were exposed to liquid mercury (mercury is a common contaminate in produced gas and crude oil) Austenitic stainless steels were exposed to molten zinc or aluminum (usually as a result of a fire).

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High Temperature (Dry) Corrosion Oxidation Sulfidation Carburization/Metal Dusting Decarburization98

High Temperature (Dry)Corrosion Oxidation

3Fe + 2O2 Fe + H2O Fe + H2S

Fe3O4 Fe3O4 + H2 FeS + H2 Fe(COOH)2 +C6 H16

Sulfidation

Organic Acid (Napthenic)

Fe +C6H15COOH 2Fe + 2CO

Carburization (Metal Dusting)

2Fe3C + O2

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Oxidation The reaction of a metal with oxygen at dry high temperature > 700F conditions. Uniform metal loss, corrosion rate increases with metal temperature. Cr addition to steels is effective in reducing high-temperature corrosion100

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High Temperature Oxidation Rates of Various Alloys

80Corrosion Rate (mpy)

60

40

20

0 800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

2100

2200

2300

2400

2500

Temperature F CS 309 SS 1-3% Cr 310 SS/HK 5% Cr Incoloy 9Cr/410SS Inconel/Hastelloy 304/316SS

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Carburization Carburization is caused by the carbon diffusion into the steel at elevated temperatures >1100F in a carburizing environment. Carburizing environments are those which are chemically reducing (deficient in oxygen) and contain carbon compounds ( CO, Coke etc.) The rate of carburization increases with temperature. Carbon steels, stainless steels, and nickel alloys are all susceptible to carburization. Carburization produces a carbon-rich surface layer on the material which is hard and brittle. Some alloys are intentionally carburized to improve hardness and wear resistance.

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Carburization In refineries, typical areas to subject to carburization are: The ID of furnace tubes which are in services which produce coke deposits on the ID of the tubes. Cyclones and other internals of FCC unit regenerators which have uneven burning patterns can result in localized carburization in some areas.

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Metal Dusting Metal dusting is a catastrophic form of carburization which occurs at temperatures > 1250F. Metal dusting can result in rapid localized loss of wall thickness do to intergranular carburization and grain drop out. The damage takes the form of severe localized metal loss with no surface deposits.

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Decarburization Decarburization is caused by the removal of carbon from steel at elevated temperatures >1100F in a low carbon activity gas environment (i.e. hydrogen). The carbon reacts with the hydrogen to form methane and is removed from the surface of the steel. Surface decarburization is the first sign of high temperature hydrogen attack. The rate of decarburization increases with increasing temperature. Carbon steels, low alloy steels are susceptible Surface decarburization produces a carbon deficient pure iron surface layer on the material which is very soft and ductile. Internal decarburization can lead to the formation of micro fissures which result in loss of fracture toughness and intergranular cracking. (High Temperature Hydrogen Attack)105

High Temperature Hydrogen Attack (HTHA) Hydrogen gas in contact with steel at high temperatures can result in decarburization and the subsequent formation of hydrocarbonsC(Fe) + 4H0 CH4

The formation of methane bubbles in the steel causes a loss of fracture toughness and can result in intergranular fissures and cracking.

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High Temperature Hydrogen Attack (HTHA)

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High Temperature Hydrogen Attack

Hydrogen Attack Formation of Microfissures 108

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API 941 Limits for Hydrogen Service

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High Temperature Hydrogen Attack (HTHA)Prevention Addition of Cr and Mo to steel improves resistance to HTHA. New equipment should be fabricated from materials known to be resistant to HTHA at their operating pressure and temperature using API 941 guidelines. Existing equipment which does not meet the current guidelines should be inspected if HTHA has occurred and to what extent. HTHA causes a loss in strength and fracture toughness of the base metal and can result in brittle fracture. If HTHA is found the equipment should be evaluated to determine fitness for continued service.

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Metallurgical Damage Mechanisms Embrittlement Embrittlement occurs when an alloy suffers a loss of fracture toughness or ductility. The embrittlement may be limited to specific ranges of temperature with ductility returning when the material temperature is moved out of the embrittlement range. Embrittlement is usually measured by means of tensile tests ( reduction of area and elongation values) or impact tests which measure the force required to fracture the material.112

Metallurgical Damage MechanismsEmbrittlement

Sigma Phase Embrittlement 885 Embrittlement Temper Embrittlement

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Sigma Phase Embrittlement Sigma Phase formation occurs when austenitic stainless steels with more than 17% Cr are held in the temperature range of 1000F 1500F. Sigma is a hard, brittle, non-magnetic phase which is formed by transformation of ferrite present in the austenitic stainless steel. Upon embrittlement there is an increase in the alloys room temperature tensile strength and hardness and severe loss of ductility. As a result cracking is likely to occur during cooling from operating temperatures, during handling, and during weld repair.

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Sigma Phase Prevention High nickel alloys are immune to sigma phase formation. Ductility may be restored to an embrittled material by solution annealing the material @ 1800F 2000F followed by rapid cooling. Since sigma phase is formed by transformation of ferrite, specifying a maximum ferrite content of 10% will the prevent the austenitic stainless steel from becoming embrittled.115

885F Embrittlement 885F Embrittlement occurs after aging of duplex and ferritic stainless steels (i.e. 430, 446, 2205) at 650F 1000F and produces a loss of ambient temperature ductility. The loss of ductility involves the ferrite phase it self and is not related to sigma phase embrittlement Ductility can be restored by heating the embrittled component to 1200F followed by rapid cooling.116

Temper Embrittlement Temper embrittlement occurs in 2-1/4 Cr-1 Mo and 3 Cr -1 Mo steels when operating between 700F-1050F for long periods of time. In refineries these steels are typically utilized for Hydrotreating and Hydrocracking reactors which typically operate in this temperature range. Temper embrittlement is not apparent at operating temperature but rather results in a loss of ambient temperature ductility which can lead to brittle fracture during shut down or start up of the equipment. Temper Embrittlement is normally accommodated by limiting the allowable pressure on the equipment to 25% normal until the metal is above 250F. The susceptibility to temper embrittlement for new equipment can be reduced by limiting the J and X factor to 100 and 15 respectively using the formulas below :J=(Si + Mn) x (P + Sn) x 104 =