Weldment Corrosion

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Weldment Corrosion

• April 10th, 2001
• Ricardo Fernandez
• Evan Rege
• Mike Beatty

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Outline

• Introduction to Welding
• Weld Corrosion
• Mechanisms
• Metal Selection
• Weldment Corrosion Cases
• Prevention

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Introduction to Welding

• Joins two metals together using heat and/or
  pressure
• Creates a phase change along the weld joint
• Common welding techniques Gas welding and Arc
  welding

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Gas Welding

• Oxygen/Acetylene flame used to melt metal at weld
  joint
• Weld rod or filler wire fed into weld area to
  provide additional material to seal the joint

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Gas Welding
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Arc Welding

• Metal electrode held above weld joint, another
  attached to the base metal
• Current of 100-200 Amps applied between the
  electrodes creates an electric arc
• Welding rod added to help seal the joint

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Arc Welding
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Welding Concerns

• Oxidation
• Flux
• Inert environment
• Stress Build-up
• Choose appropriate material for welding rod to
  reduce mechanical stress

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Welding Concerns

• Weld Decay
• De-zincification
• Grain Boundary Chromium Depletion
• Knifeline Attack
• Weldment Corrosion

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Welding and Environmentally Induced Corrosion

• Weldements have a heightened sensitivity to
  environmentally induced corrosion
• Residual stresses can cause Stress Corrosion
  Cracking (SCC)
• Stress Risers and inclusions due to welding can
  increase the sensitivity to Corrosion Fatigue
  Cracking (CFC)
• Welding in the presence of water or organic
  molecules can trap hydrogen in the weld, leading
  to Hydrogen Induced Cracking (HIC)

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Welding Defects

• Susceptibility to environmentally induced
  corrosion is increased by improper welding
  technique
• Defects include
• Stress risers
• Inclusions
• Incomplete welding
• Improper filler metal or flux used

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Stress Corrosion Cracking

• Causes
• Residual tensile stresses
• Stress risers caused by improper welding
• Prevention
• Use a metal that is resistant to SCC in
  environment
• Specify post welding heat treatment
• Ensure smooth weld bead

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Corrosion Fatigue Cracking

• Possible when weld is subjected to variable loads
• Stress risers at the weld will increase the
  sensitivity to CFC
• Prevention
• Smooth weld area
• Minimization of inclusions

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Corrosion Fatigue Cracking
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Hydrogen Induced Cracking

• Hydrogen present in water or organic molecules is
  released at high temperatures
• Monatomic Hydrogen is infused in the weld pool
• Hydrogen combines within metal and causes HIC
• Prevention
• Store welding rods in dry environment
• Use low-hydrogen electrodes (non-organic binders
  and flux)

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Hydrogen Induced Cracking from Welding

• Arc Welding

H2O
WELD
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Hydrogen Induced Cracking from Welding

• Heat breaks down Water

H2O
O2
H
WELD
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Hydrogen Induced Cracking from Welding

• Hydrogen causes cracking
• Oxygen creates crevice

H
O2
WELD
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Chromium Depletion and Intergranular Corrosion

• In the Heat Affected Zone of the weld, chromium
  may precipitate out of solution as a carbide
• Prevention
• Pre or post heat treatments
• Use low carbon content alloys (304L Stainless)
• Use Niobium or Titanium stabilized alloys

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Filler Metal Selection

• The filler metal is usually of a different
  composition than the base metal
• Making the filler metal noble to the base metal
  will cathodically protect the weld
• Oxidation and diffusion will lower the alloy
  concentration in weld, so filler metal should
  also have higher amounts of alloying elements
  than the base metal

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Weldment Metallurgy

• In the Unmixed Zone, non-equilibrium cooling can
  cause potential problems
• Grain boundary segregation
• Primary phase structures
• Precipitation of alloying elements
• Prevention
• Pre and post heating avoids unequilibrium cooling

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Summary of Weld Corrosion Mechanisms

• Stress Corrosion Cracking
• Hydrogen Induced Cracking
• Crevice corrosion
• Intergranular Cracking
• Sensitization from welding heat stress
• Carbide precipitation

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Case Study Examples

• Offshore Structure Weld Decay
• Solar Power Plant Chloride Corrosion
• Gas Pipeline SCC
• Pipes Crevice Corrosion
• Chloride Stress Corrosion Cracking

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Offshore Structure

• Welded Steels
• Stress from sea and Corrosive Environment
• Problems
• Weld decay
• Weld cracking
• Pitting

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Offshore Structure - Prevention

• Painting Structure
• Better choice of weld material
• Cathodic Protection Zinc bar
• Better welding of steels
• Better weld geometry

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Offshore Structure - Prevention

• Better Weld Geometry
• Butt Weld

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Weldment Corrosion In Solar Power Plants

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Weldment Corrosion In Solar Power Plants

• Material Selection Ni Superalloy vs. Stainless
  Steel
• Aeration during
• Maintenance
• Corrosion along pipe walls at welds due to
  Chromium depletion and De-zincification

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Gas Pipelines - SCC

• Stress Corrosion Cracking
• Caused by tough environment
• Operation of load and stresses
• Stress are either mechanical or chemical
• Mechanical Stresses scratches, rivets, residual
  stresses
• Chemical Stresses cracks, corrosion in the media

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Gas Pipelines - SCC

• Stress Corrosion Cracking in weld
• Stress cracks under operation.
• Cracking is longitudinal along the weld

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Pipes Crevice Corrosion

• Crevice Corrosion in Duplex Steel
• Corrosion due to Heating
• Internal oxide layer created Crevice
• Caused pipe to corrode and burst
• Aggressive media increased corrosion

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Pipes Crevice Corrosion
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Pipes Crevice CorrosionPrevention

• Use an oxygen-free backing gas to prevent scale
  formation during welding
• Annealing
• Use specific filler metals
• Increase in Ni increases resistance to SCC, but
  also becomes brittle
• Find stability between heat and nickel
  concentration

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Chloride Stress Corrosion Cracking

• Intergranular crack propagation in welded region
• Stressed region
• Unprotected region
• to environment.
• Chlorine
• accumulation

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Chloride Stress Corrosion Cracking

• High chloride concentration found
• If compared to other parts of the sample there is
  no cracking
• Chloride enhances
• cracking mechanisms

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Summary of Prevention Mechanisms

• Materials Selection
• Weld Geometry
• Effective welding mechanisms (heat input, cooling
  rate)
• Cathodic Protection
• Annealing
• De-sensitizing stainless steel

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THE END

• Questions?