A COMPARISON OF THE CORROSION PRODUCTS AND MECHANISMS OF VARIOUS FORMS
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Transcript of The Forms of Corrosion-Part2.ppt
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Module 2“The Forms of Corrosion”
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Corrosion Forms on Metals
• General
• Localized (pitting and crevice)
• Galvanic-
• Stress Corrosion Cracking
• Flow-assisted
• Intergranular
• Dealloying
• Fretting
• Corrosion Fatigue
• High-temperature
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General Corrosion Definition
• Characterized by chemical reactions, oxidation and
reduction, that proceed uniformly over the entire
wetted surface.
• Uniform attack, general corrosion, represents the
greatest destruction of metal on a tonnage basis.
• This form of corrosion is easily monitored and the
life of equipment accurately estimated.
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General Corrosion
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General Corrosion due to Dissimilar
Surfaces
• Mill Scale = Cathode
• New (Anode) –Old Pipe (Cathode)
• Stressed Surfaces= Anode• Welded Sections (Heat Affected Zone)=
Anode
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New — Old Pipe Cell
Old Pipe(cathode)
New Pipe(anode)
Old Pipe(cathode)
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Different Stress
The portion of the metal under higher stress,
(eg. cold worked) will be anodic to the portion
of the metal at a lower stress level.
High Stress =Anode
Low Stress = Cathode
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General Corrosion due to Dissimilar
Electrolytes
In most cases, the metal immersed in the
more conductive solution will be the anode,
and the metal immersed in the less
conductive solution will be the cathode.
More Conductive = Anode
Less Conductive = Cathode
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General Corrosion due to Oxygen
Concentration
The metal immersed in the oxygen lean
electrolyte will be the anode, and the metal
immersed in the oxygen rich electrolyte will
be the cathode.
Oxygen lean = Anode
Oxygen rich = Cathode
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Metal Ion Concentration
• Cathode –in saturated solution
• Anode –in unsaturated solution
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General Corrosion Remedies
• Coatings
• Linings
• Alloy selection
• Change the chemistry
• Corrosion inhibitors
• Cathodic or anodic protection
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General Corrosion Spray Tower
• Severe corrosion of the inner
carbon steel walls at the mid-section of a tower between thetwo spray levels was found.
• More than half of thickness wasgone from the original 1/4 platein 8 years.
• Notice support beams fromupper level to the lower level.
• Cause high temperature andcondensation water fromsprayed detergent slurry.
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Neutralization Mixers
Reactors that see various
concentrations of sulfuric
acid should use Alloy 20
UNS No- N08020.
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Heating Coils
• Heat transfer surfaces are
very susceptible to
general and localized
pitting corrosion due tothe higher temperature.
• Material selection for heat
transfer surfaces is
therefore important.
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Atmospheric Corrosion Example
• Electrical Panels are an area
where corrosion can play
havoc. For example where
the surrounding air is high
in salt or acidity.
• Electrical fires have been
associated with corroded
contacts.
• Sealing these areas from the
outside air and moisture isimportant.
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Handouts Reference General Corrosion
Carbon Steel- pp. 373-403 “Carbon and Low Alloy Steels”
Stainless Steel- pp. 553-559 “Corrosion of Stainless Steels”
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Localized Corrosion Definitions • Pitting
A deep, narrow corrosive attack in a localized region which oftencauses rapid penetration of the substrate thickness.
• Crevice Corrosion
A form of localized attack in which the site of the attack is an area
where free access to the surrounding environment is restricted..
• Filiform Corrosion
A special form of oxygen cell corrosion occurring beneath organic or
metallic coatings on materials. The attack results in a fine network of
random “threads” of corrosion product developed beneath the coatingmaterial.
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Pitting on Stainless Steel 304 SS
• Micro view of pitting
at 50x magnification
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Copper Pitting
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Pitting Remedies
• Use more alloyed material ( molybdenum iskey alloying element)
• Reduce eliminate the pitting agents
• Lower the process temperature• Coatings
• Cathodic protection
• In the case of SS increase process velocity
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Electrochemical Lab Pitting Dat
• First graph showing the
effect of pH and Cl- at 35C
• Second graph showing the
effect of increasing temp.
with 4 diff. Cl- conc.
• The third graph showing
pitting potentials with
different alloys with
increasing temperatures.
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Heat Exchangers
(above 140F with salt)
• 316L Stainless Steel dryer heat exchangers aresusceptible to localized pitting of heat exchanger
tubes . Past inspections showed shallow pits on the
inner tube surfaces that eventually led to failure.
• Dryer heat exchangers should be fabricated out of a
material with at least 4% molybdenum to avoid
pitting. 316 has only about 2% molybdenum.
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Handouts Reference Pitting
• Carbon Steel p. 385,389 “ Carbon and Low Alloy Steels”
• Stainless Steel p.113-114 “Localized Corrosion”
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Exercise 2 Soft-soap Business Issue”
Instructions:• Form into groups of three with people from other functional
groups if possible.
• Look at slide pictures
• Read Background
• Answer Questions
• Be prepared to share your answers with the rest of the class
B k d “S ft P S i ”
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Background “Soft-soap Pump Springs”
• A Soft-soap product
experienced corrosion of 302
SS pump springs that could beseen by the consumer on the
store shelf
• The corrosion occurred mainly
on spring parts that were in
contact with the plastic.Shown here is corrosion on
last two winds
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Crevice Corrosion Background
M+Cl- + H2O = MOH + H+Cl-
Corrosion Inside Crevice (Anode)
Chemical Reaction with Salt NaCl
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Electron Micrograph of 302 Spring
Showing corrosion also
occurring between
spring coils
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Results of Statistical Analysis
• Conditions: 120F in Pump after 49 days
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Results of Statistical Analysis
Another Example of Crevice
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pCorrosion
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Exercise 2-1 Soft-Soap Pump Springs
• What type of corrosion occurred on the 302
SS spring?• What would be two possible solution based
on the statistical data and remedies?
Questions to answer
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Crevice Corrosion Remedies
• Use continuous complete penetration welds
• Laps joints should be seal welded
• Remove deposits frequently
• Use proper packing materials in joints
• Avoid crevices in heat exchanger tube sheet• Use higher alloyed material or plastic
• Use a corrosion inhibitor such as Sodium sulfate
• Alter the chemistry by adjusting the pH or loweringthe salt concentration
Forms of Cracking Corrosion Definitions
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Forms of Cracking Corrosion Definitions
• Stress Corrosion Cracking
Refers to metal cracking caused by the simultaneous presence of tensilestress and a corrosive chemistry.
• Corrosion Fatigue
Results from the combined action of a cyclic tensile stress and and a
corrosive environment.
• Hydrogen-Induced Cracking (HIC)
Results from the combined action of a tensile stress and hydrogen in
metal.
Stress Corrosion Cracking Susceptibility
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Stress Corrosion Cracking Susceptibility
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Chloride Stress Cracking Temperature
Above 140 F Stress Corrosion Cracking
occurs. Therefore if possible avoid
continuous process operations above this
temperature with 304/316 SS
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Cracking - On A Microscopic Level
• Cracks that run across grains is called
transgranular
• Cracks that follow grain boundaries are
called intergranular
Stainless vs. Other Alloys Stress Cracking
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Lab
Data
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Lye Separators
• After about 9 yearsstress corrosioncracking was pervasivein a continuos lyeseperation plant. Atypical crack is shownhere.
• The material of construction was 304
SS. Crack
Stress Corrosion
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Cracking 100x mag.
Carbon Steel Caustic Soda Service Chart
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Area A
Area B
Area C
Carbon steel
Carbon steelstress relieve
Nickel basedalloys
Carbon Steel Caustic Soda Service Chart
Sodium hydroxide concentration 0 100
180 F
Stress cracking line
120 F
A i Chl id
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Ammonium Chloride
This is an example
where a tank fluid isnot compatible with
316 stainless steel.
Pitting and stress
corrosion cracking with quaternary
ammonium chloride
fabric softener.
St C ki R di
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Stress Cracking Remedies
• Evaluate a Duplex alloy- 2205 or 2507
• Increase nickel concentration in the alloy
• Lower hardness of alloy through heat treat
• Lower process temperature (60C max)• Cathodic Protection
• Remove the bad actor ions such as chloride
L E t
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Lye Evaporators
During the evaporation step, lyes can be
corrosive to 316 SS alloy heat exchangers
by pitting and stress corrosion cracking. In
addition lyes can contain NaClO3. NaClO3,Sodium Chlorate, is corrosive to stainless
steels. If this is the case, an upgrade in heat
exchanger metallurgy is necessary to Inconel
625 for tubing.
F ti C ki
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Fatigue Cracking
Expansion joints thatundergo high stress
and strain due to
heat/mechanical forces
are susceptible to
corrosion fatiguecracking. In general
the higher the nickel
concentration in the
alloy the better the
fatigue properties.
Corrosion Fatigue Remedies
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Corrosion Fatigue Remedies
• Reduce the grain size
• Improve the surface finish
• Increase alloying elements ( Ni, Cr, Mo)
Surface Preparation and Corrosion Fatigue
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Surface Preparation and Corrosion Fatigue
• In general the fatigue life increases as themagnitude of surface roughness decreases.
• Best- A Metallographic finish, free of
machining grooves and grinding scratches
Hydrogen Induced Cracking
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Hydrogen Induced Cracking
Galvanic Corrosion Definition
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Galvanic Corrosion Definition
Corrosion due to the electrochemical
potential difference between different
metals. However, potential differencescan be caused by differences in the
homogeneity of the same metal.
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Corrosion Galvanic
Galvanic Series
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More
Reactive
LessReactive
PotassiumMagnesium
BerylliumAluminumZincChromiumIron
NickelTinCopperSilverPlatinum
Gold
Galvanic Example in Bottle Making
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• Equipment that is used to make plastic bottles often
times uses different metals to conduct heat at
different rates. For example beryllium copper andaluminum. This alloy combination allows bottle
thicknesses to differ for design purposes.
• Galvanic corrosion can and did occur at these
joints. Replacement of the expensive parts wereneeded.
• In this case the water for cooling needs to be
buffered to a pH of 8 and eliminate as much salt as
possible.
Galvanic Corrosion Remedies
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Galvanic Corrosion Remedies
• Choose alloys of similar chemical potential
• Avoid bad area ratios: large cathode/small
anode surface areas
• Insulate dissimilar alloys
• Coatings apply with caution ( pinhole free)
• Change the environment ( chemistry)
Flow-Assisted Corrosion Definitions
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Flow-Assisted Corrosion Definitions
• Erosion-corrosionOccurs when the velocity of the fluid is sufficient to remove protective
films from the metal surface.
• Impingement Corrosion
Localized erosion-corrosion caused by turbulence or impinging flow.
• Cavitation Corrosion
Mechanical damage process caused by collapsing bubbles in a flowing
liquid.
Impingement- Erosion
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Plodder Worms- Mixers
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Typically are silicon
aluminum castings .
These alloys are
susceptible toerosion and pitting
corrosion with time.
Erosion Corrosion Remedies
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Erosion Corrosion Remedies
• Change the metallurgy ( adding chrome and moly 316 SS)• Alter the design ( provide for large sweeps, eliminate 90 elbows)
• Lower the velocity ( less than 2 meters per second)
• Raise the pH ( pH above 9 helps)
• Lower the temperature• Deaerate
• Filter
• Weld overlays or metallic coatings
• Rubber linings
Intergranular Corrosion Definition
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Intergranular Corrosion Definition
• Corrosion due to the preferential attackat, or adjacent to, the grain boundaries of a metal.
• Most often associated with welds in lowpH fluids.
• Caused by chromium being tied up bycarbon.
Intergranular Corrosion 500 x mag.
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Dark areas are corroded
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Fretting Corrosion Definition
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Fretting Corrosion Definition
• Fretting corrosion is defined as metaldeterioration caused by repetitive slip atthe interface between two surfaces incontact.
• It is not corrosion due to rotation or erosion.
• More exactly.. motion from vibration andcorrosion effects during that time.
Fretting Corrosion Examples
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g p
• Axles
• Housings and shafts
• Spline connections
• Pin joints
• Riveted lap joints
Resistance to Fretting Corrosion
Under dry conditions
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Under dry conditions
Steel on steel Low
Nickel on steel Low
Aluminum on Aluminum Low
Zinc on steel Medium
Copper on steel Medium
Silver plate on steel High
High-temperature Corrosion
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g p
• A form of material degradation classifiedabove ( 750+ F).
• General or pitting corrosion occurs.
High Temperature Corrosion
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g p
• Oxidation reactions include water vapor, hydrogen,
hydrogen sulfide, ammonia, sulfur dioxide, chlorine,
sulfur, carbon dioxide and oxygen
• Development of scales
• Corrosion products- solids
- liquids
- gases
High Temperature Remedies
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g p
• Determine possible mechanism of corrosion
- sulfidation, carburization, hydrogen effect
• Select alloy most resistant to above
• Carbon steel maximum temperature range is
800- 850 F in air.
Summary of Scaling Temperatures
in Air
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in Air
• 1010 Steel ... 900F
• Alloy Steel, 9Cr, 1.0 Mo...1200 F
• 410 SS, 12 Cr ...1400 F
• 304, 321, 347 18 Cr, 8 Ni... 1650 F
• 310 SS, 25 Cr, 20 Ni... 2100 F
Exercise 2-2 Sulfur Burner Exhaust Pipin
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• High temperature
corrosion of 321 SS at
1300 F with SO2
• The thickness of theplate was 1/8 inch and
lasted less than two
years.
Handout on High Temperature
Corrosion
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Corrosion
See Corrosion by “Hot Gases and MoltenCompounds”
-- Specific to this case pp. 243-248 .
Corrosion of Stainless Steel in Sulfur
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Corrosion of Stainless Steel in Sulfur
vapor 1300 F Lab Data
314 16.9 mpy mils per year
310 18.9
309 22.3
304 27.0302B 29.8
316 31.1
321 54.8
Haynes alloy 556 6
Exercise 2-2 Sulfur Burner Exhaust
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• Review the previous data and the HighTemperature Corrosion handout.
• What is the term used to describe this type of
corrosion attack?• What should be the recommended material
of construction?
What did we learn?
The most common forms of corrosion attack on metals
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• The most common forms of corrosion attack on metals.
• Metals and non-metals can become brittle and crack when misapplied.
• Carbon steel is attacked in a general fashion, making it easy to monitor andcorrect.
• Stainless steels on the other hand, are attacked in a localized fashion,making it difficult to monitor and correct.
• The critical operating temperature for stress corrosion is above 140Fahrenheit for stainless steels 304 and 316.
• Stainless steels need to be selected carefully.
• There are different types of high temperature corrosion.
• That corrosion issues can be dealt with in different ways.
• Engineering processes and structures need to be inspected and protectedfrom corrosion.