pitting-acceptance or rejection - cait · astm a6-13 • conditioning: – shall be well faired...
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Pitting-Acceptance or RejectionPitting-Acceptance or Rejection
Presented by:Mike Reeder
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Pitting Corrosion
• Localized corrosion of a metal surface that is confined to a small area
• Damages the deep structure of the metal
• Can help to initiate stress corrosion cracking
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Localized Corrosion
• Causes the need for unplanned maintenance• Often hidden in crevices or under multiple coats
of maintenance coatings– Disguises the extent of the damage
• Can lead to serious consequences unless dealt with properly and promptly
• Produce stress risers at the leading edge of the pit that create initiation points for failure
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Corrosion
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Corrosion Cell
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Corrosion Cell
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Pitting Corrosion
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Steel Surface
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Possible Causes
• Scratch• Local change in
composition• Damage to
protective coating• Chlorides• Mill Scale
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Mill Scale
• Corrosion may be encouraged by the presence of mill scale.
• Mill scale is cathodicto the substrate
• Mill scale promotes corrosion at the anodic bare steel when moisture is present
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Coating Failure
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Soluble Salts
• Promote the breakdown of the oxide layer
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Soluble Salt Remediation
• Water Jetting-What are the acceptable levels?– PennDOT Specifies NV-2 unless the paint manufacture
has more stringent levels.
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ASTM A6-13
• Conditioning:– Shall be well faired without abrupt changes in contour– Plate- Grinding can not reduce the thickness of the plate
by:• More than 7% under the nominal thickness, but in no case
more than 1/8” for material ordered to weight per square foot
• Below the permissible minimum thickness for plates ordered to thickness
– Structural Size Shapes:• 1/32” for material less than 3/8” in thickness• 1/16” for material 3/8” to 2” thick• 1/8” for material over 2” in thickness
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Depth/Thickness Measurments
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Severe Pitting-Is This Acceptable?
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ASTM A6-13
• Note 4-Non-injurious surface or internal imperfections, or both, may be present in the structural product as delivered and the structural products may require conditioning by the purchaser to improve its appearance or in preparation for welding, coating, or other further operations.
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Coating Concerns
• Cleanliness• Surface Profile• Sharp Edges• Contamination• Aesthetics
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Sharp Edges
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Aesthetics
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Aesthetics
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Osmotic Blistering
• Salts draw moisture and form blisters that build up internal pressure
• Forms a mild hydrochloric acid causing corrosion and coating failure
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Service Life
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Questions?