Chapter 1Corrosion and Other Failures

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Class Exercise

Discuss and summarize expectations/reservations

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Learning Goals

• Examine process units found in refineries

• Describe specific refinery processes

• Identify and examine corrosion and metallurgical problems found in process units

• Examine process monitoring and practices used to control corrosion

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NACE Defines Corrosion as...

……The deterioration of a material, usually a metal, because of a reaction with its environment.

Figure 16.1 Simplified Refinery Flow Diagram

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Refinery Corrosion Categories

• Low-temperature corrosion

– Below 500°F (260°C)

– Water present

• High-temperature corrosion

– Above 500°F (260°C)

– No water present

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Low-Temperature Refinery Corrosion

• Aqueous corrosion

• Wet corrosion

• Electrochemical corrosion

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Corrosion of a Metal

• Oxidation reaction

– Produces electrons

– Occurs at anode, which corrodes

• Reduction reaction

– Consumes electrons from oxidation

– Occurs at cathode, which does not corrode

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Fe Fe+2 + 2e– oxidation of iron

2H+ + 2e– H2 (gas) hydrogen evolution in acid solutions

 O2+ 2H2O + 4e– 4OH– oxygen reduction in neutral or basic

solutions 

2HS– + 2e– H2 (gas) + 2S–2 bisulfide reduction in sour solutions

 

Corrosion Reactions

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2Fe + 2H2O + O2 2Fe+2 + 4OH– 2Fe(OH)2 (ferrous hydroxide)

2Fe(OH)2 + H2O + ½ O2 2Fe(OH)3 (solid ferric hydroxide) = red rust

Corrosion Reactions

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Corrosion Rates

• Determine the suitability of a material for a specific service environment

• Measured as weight loss per unit area

• Expressed in mils or mm of penetration per year (mpy or mm/yr)

• Acceptable for long-term service if below 5 mpy (0.125 mm/yr)

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Polarization

• Activation polarization

– Corrosion controlled by reaction sequence at metal surface

– Controls corrosion in concentrated acids

• Concentration polarization

– Corrosion controlled by diffusion

– Controls corrosion in very dilute acids

– Analogy is wind chill factor

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Passivity

• Increases corrosion resistance in some metals and alloys

• Results from the formation of protective surface films

• Damaged in highly reducing or oxidizing environments or by active ions

• Maintained by dissolved oxygen in water

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Galvanic Corrosion

• Form of wet corrosion

• Two metals are joined electrically

• Requires

– Electrolyte

– Anode

– Cathode

– Metallic pathway

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Galvanic Series Corroded End—Anodic—More Active

MagnesiumZinc

AluminumSteel

3xx, 4xx stainless steel (active state)Copper Alloys

3xx, 4xx stainless steel (passive state)TitaniumGraphite

Protected End—Cathodic—Less Active/More Noble

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Galvanic Corrosion

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Dissimilar Metals in Contact Leading to Galvanic Corrosion• Weld or heat-affected zone may be anodic

to parent metal

• New steel connected to old steel corrodes more rapidly

• Steel pipe connected to copper pipe or tubing

• Steel propeller shaft operating in a bronze bearing

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Galvanic Attack Factors• More severe near junction of two dissimilar

metals

• Severity related to electrical conductivity of solution

• Anode should be large compared to cathode

• Insulate dissimilar metals

• Paint or coat entire assembly or cathode

• Corrosion inhibitors and sacrificial anodesreduce galvanic effects

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Increasing the Corrosion Rate

• Temperature increases

– Can double corrosion rates for activation polarized corrosion

– Can increase water amounts in liquid hydrocarbon and vapor streams

• Concentration increases

– Generally increase corrosion

– Considered in terms of water present

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Causes of Low-Temperature Refinery Corrosion

• Contaminants

– Present in crude oil

– Produced from refinery processes

• Water

• Air (oxygen)

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Corrosives Found in Refining

Processes

Crude OilS

N

Nap Acid

Cl

RefineryNH3

CN-

H2S

HCl

HF

H2SO4

Polythionic Acid

O2

CO2

TABLE 1.1

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High-Temperature Refinery Corrosion

• Other names

– Dry corrosion

– Direct chemical combination

• Associated with high temperatures

• Occurs above the dew point

• Typically caused by gases and liquids

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Metal Oxide Formation

• Oxidation reaction

– Metal exposed to air

– Oxidized to an ion at metal/scale interface

• Reduction reaction

– Oxygen is reduced at scale surface

• Corrosion reaction combines oxidation and reduction to form metal oxide

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High-Temperature Oxidation Limits (Table 1.4)

Alloy Temperature

Carbon steel 1050F (565C)

5 Cr-1/2 Mo 1200F (648C)

9 Cr-1 Mo 1300F (704C)

410 SS 1300F (704C)

304/316/321/347 SS 1600F (871C)

309 SS (HT) 2000F (1093C)

310 SS (HK) 2100F (1149C)

Alloy 625 2000F (1093C)

Alloy 825 2000F (1093C)

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Factors Influencing Diffusion of Metal and Oxygen Ions

• Temperature

• Temperature fluctuations

• Integrity of the oxide layer

• Presence of other gases in the atmosphere

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Factors Influencing Scale Formation

• Dissolution of oxygen atoms in some metals

• Low melting points and high volatility of some oxides

• Existence of grain boundaries in the metal and the scale

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High-TemperatureCorrosion Rate Laws

• Linear Rate Law

– Nonprotective oxide layer permits oxygen access to metal

– Oxide layer thickness increases with time

• Parabolic Rate Law

– Oxide layer forms protective barrier

– Protection relates to oxide layer thickness

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High-TemperatureCorrosion Conditions

• High pressures

• High flow velocities

• Sulfur compounds

• Naphthenic acids

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Refinery Damage and Damage Mechanisms

• Metal loss due to general and/or localized corrosion

• Stress corrosion cracking (SCC)

• High-temperature hydrogen attack (HTHA)

• Metallurgical effects

• Mechanical failures

• Other forms of corrosion

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Summary

• Low-temperature corrosion

• High-temperature corrosion

• Major classifications of damage and damage mechanisms

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Additional Damage Mechanisms

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Pitting

• Highly localized corrosion in form of small holes or pits

• Occurs in stagnant flow conditions in presence of chloride ions

• A problem with martensitic, ferritic, and austenitic stainless steels

• Alloying with molybdenum reduces pitting in stainless steels

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Crevice Corrosion• Localized corrosion

• Promoted by stagnant solutions in crevices a few mils wide

• Most severe in high chloride environments

• Also called under-deposit corrosion

• Mitigation includes

– Designing for proper drainage

– Welding instead of bolting or flanging

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Crevice Corrosion

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Atmospheric Corrosion

• Mostly a problem in coastal refineries

• Accelerated by the presence of

– Sulfur compounds (hydrogen sulfide)

– Chlorides

– Fly ash

– Chemical dusts

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Intergranular Attack• Localized attack at and adjacent to grain

boundaries

• Little corrosion on grains, resulting in grain separation

• Caused by

– Corrosive action of a chemical

– Enrichment or depletion of alloying element

• 300 series Austenitic SS most vulnerable

• Additional details during PTA discussion

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Soil Corrosion

• Caused by differential concentration cells in the soil

– Oxygen

– Water

– Chemicals

• Problem with underground piping and tank bottoms

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Stress Corrosion Cracking (SCC)

• Spontaneous cracking of materials

• Combination of corrosion and tensile stresses

• Two types of stresses

– Residual

– Applied

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Stress Corrosion Cracking

• Susceptible Materials

• Environment

• Tensile Stress

– Residual

– Applied

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Stress Corrosion Cracking (Table 1.5)

Cl 3xx SS

NaOH (and other caustics) CS, 3xx SSNH4 Cu-base (especially brass)

Amines CS, Cu-base

H2S CS (High Strength), 4xx SS

HCO3 = (carbonates) CS

NO3 = (nitrates) CS

PTA (polythionic acids) Sensitized 3xx and 800

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Chloride Stress Corrosion Cracking (ClSCC)

• Chloride ions (only traces are required)

• Temperature above 130°F to 175°F (54°C to 79°C)

• Either a low pH or the presence of dissolved oxygen

• Tensile stress

• Water

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Chloride SCC

Crack started at high-stress area at ring joint corner

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Alkaline Stress Corrosion Cracking (ASCC)

• Tensile stress

• Alkaline cracking from exposure to caustic, amine, or carbonate solutions

• Temperatures above 150°F (66°C), 75°F (23.9°C), or 100°F (37.8°C), respectively

• Concentrations of 50 ppm to 100 ppm caustic cause cracking

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Caustic Soda

Service Graph

From: NACE Corrosion Data Survey

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Cracking from caustic carryover into boiler superheater tube

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Caustic Cracking in Dead Leg Bypass Piping at Crude Furnace Inlet

A cross-section view of the flange and pipe. The arrow marks the ID crack.

A micrograph of the ID cracking. Note that the cracking is intergranular and there is a gray scale in the crack. This is typical of caustic stress-corrosion cracking in carbon steel. Nital etch, magnification 500x.

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Boiler Feed Water

• Can result in low-temperature corrosion problems

• Causes of corrosion

– Dissolved oxygen

– Low pH

• Treatment includes mechanical deaeration and chemical treatments

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Boiler Feed Water Corrosion

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Steam Condensate Corrosion

• Caused by dissolved oxygen and carbon dioxide

• Shows up as pitting (oxygen) or uniform metal loss (carbon dioxide)

• Controlled by

– Deaeration to remove dissolved gases

– Chemical treatments to reduce alkalinity of makeup water

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Carbon Dioxide (CO2) is found in...

…Steam condensate systems

…Hydrogen plants

…Vapor recovery section of catalytic cracking units

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Carbon Dioxide Corrosion

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Cooling Water Corrosion

• Can be very costly to refiners

• Caused by

– Airborne contaminants

– Concentrated dissolved minerals

– High temperatures

• Controlled with corrosion inhibitors and proper material selection

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Cooling Water Corrosion in Carbon Steel Heat Exchanger Tubes

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Mechanical Failures

• Operational changes in process temperature or pressure/upsets

• Overfiring of furnaces to increase throughput

• Control instrument failures

• Exposure to fire

• Overloading of structural members

• Over-tightening of bolts

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Incorrect or Defective Materials

• Material mix-ups by suppliers

• Vendor substitutions

• Substitution of castings for wrought or forged shapes

• Discontinuities in wrought materials

• Material substitutions during shutdowns

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Mechanical Fatigue

• Failure caused by cracking after continued application of cyclic stress

• Promoted by stresses higher than endurance limit (in CS)

• Risk increased by deep scratches, sharp corners, and weld intersections

• Minimized by eliminating stress raisers

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Fatigue Failure

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Corrosion Fatigue

• Corrosion (pitting) promotes mechanical fatigue

• Two-stage process

– Stage 1: Formation of corrosion pits

– Stage 2: Development of cracks

• Mitigated by

– Stress relieving

– Corrosion inhibitors/protective coatings

• No endurance limit

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Cavitation Damage

• Caused by rapid formation and collapse of vapor bubbles in liquid at a metal surface, resulting from pressure variations

• Accelerated by corrosion and turbulence or vibration

• Reduced by

– Use of resistant alloys

– Proper design to avoid turbulence/cavitation

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Mechanical Damage

• Misuse of tools or other equipment

• Wind damage

• Careless handling of equipment when moved or erected

• Wear or mechanical abrasion

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Abrasive Service Alloys

• Tungsten carbide/sintered carbide compacts

• High-chromium cast irons/hardfacing alloys

• Martensitic irons/hardfacing alloys

• Austenitic cast irons/hardfacing alloys

• Pearlite steels

• Ferritic steels

• Austenitic steels, i.e., 13% manganese

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Overloading• Hydrostatic testing of vessels, which applies

excess weight

• Excessive bending stresses from attachment of pipe support brackets to vessel shells

• Addition of piping to existing pipe supports, or piping left overhanging on supports

• Weakened metal members from corrosion, fire, or change in shape or position

• Thermal expansion and contraction

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Overpressuring

• Application of pressure in excess of maximum allowable working pressure

• Causes

– Buckling

– Bulging

– Ruptures

– Splits

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Brittle Fracture

• Loss of ductility, resulting in poor impact strength

• Occurs at low temperatures

• Impact loading

• Constant stress

• Catastrophic failure accompanied by loud noise

• Characteristic chevron or herringbone pattern

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Figure 1-Aerial View

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300 Feet

FractureOrigin

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Origin

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450 Feet

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Crack initiated at backing bar weld on vessel ID

Chapter 1Corrosion and Other Failures