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Weldment Corrosion

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Joins two metals together using heat and/or pressure. Creates a phase change ... Oxygen/Acetylene flame used to melt ... weld bead. Corrosion Fatigue ... – PowerPoint PPT presentation

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Title: Weldment Corrosion


1
Weldment Corrosion
  • April 10th, 2001
  • Ricardo Fernandez
  • Evan Rege
  • Mike Beatty

2
Outline
  • Introduction to Welding
  • Weld Corrosion
  • Mechanisms
  • Metal Selection
  • Weldment Corrosion Cases
  • Prevention

3
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

4
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

5
Gas Welding
6
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

7
Arc Welding
8
Welding Concerns
  • Oxidation
  • Flux
  • Inert environment
  • Stress Build-up
  • Choose appropriate material for welding rod to
    reduce mechanical stress

9
Welding Concerns
  • Weld Decay
  • De-zincification
  • Grain Boundary Chromium Depletion
  • Knifeline Attack
  • Weldment Corrosion

10
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)

11
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

12
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

13
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

14
Corrosion Fatigue Cracking
15
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)

16
Hydrogen Induced Cracking from Welding
  • Arc Welding

H2O
WELD
17
Hydrogen Induced Cracking from Welding
  • Heat breaks down Water

H2O
O2
H
WELD
18
Hydrogen Induced Cracking from Welding
  • Hydrogen causes cracking
  • Oxygen creates crevice

H
O2
WELD
19
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

20
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

21
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

22
Summary of Weld Corrosion Mechanisms
  • Stress Corrosion Cracking
  • Hydrogen Induced Cracking
  • Crevice corrosion
  • Intergranular Cracking
  • Sensitization from welding heat stress
  • Carbide precipitation

23
Case Study Examples
  • Offshore Structure Weld Decay
  • Solar Power Plant Chloride Corrosion
  • Gas Pipeline SCC
  • Pipes Crevice Corrosion
  • Chloride Stress Corrosion Cracking

24
Offshore Structure
  • Welded Steels
  • Stress from sea and Corrosive Environment
  • Problems
  • Weld decay
  • Weld cracking
  • Pitting

25
Offshore Structure - Prevention
  • Painting Structure
  • Better choice of weld material
  • Cathodic Protection Zinc bar
  • Better welding of steels
  • Better weld geometry

26
Offshore Structure - Prevention
  • Better Weld Geometry
  • Butt Weld

27
Weldment Corrosion In Solar Power Plants

28
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

29
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

30
Gas Pipelines - SCC
  • Stress Corrosion Cracking in weld
  • Stress cracks under operation.
  • Cracking is longitudinal along the weld

31
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

32
Pipes Crevice Corrosion
33
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

34
Chloride Stress Corrosion Cracking
  • Intergranular crack propagation in welded region
  • Stressed region
  • Unprotected region
  • to environment.
  • Chlorine
  • accumulation

35
Chloride Stress Corrosion Cracking
  • High chloride concentration found
  • If compared to other parts of the sample there is
    no cracking
  • Chloride enhances
  • cracking mechanisms

36
Summary of Prevention Mechanisms
  • Materials Selection
  • Weld Geometry
  • Effective welding mechanisms (heat input, cooling
    rate)
  • Cathodic Protection
  • Annealing
  • De-sensitizing stainless steel

37
THE END
  • Questions?
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