OCEN 201 Introduction to Ocean

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OCEN 201Introduction to Ocean Coastal
Engineering

• Materials Corrosion
• Jun Zhang
• Jun-zhang_at_tamu.edu

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• Materials Used in Offshore Coastal Water
•      
• Must withstand very harsh marine environment.
•    
• Structures under the severe impact of wind,
  waves currents, ice.
• (hurricane, typhoon, storms,
  earthquake (tsunami) etc)
•  
• Ocean water is highly corrosive (electrolyte)
• Bio-fouling
• Should be non-toxic to marine organism (paint).
• High pressure in deep water 

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• Metals
• -Steels special steels (Stainless steels),
  - -Aluminum,
• -Titanium,
• -Non-ferrous Alloys (Copper-nickel)
• Nonmetallic Materials 
• -Thermoplastics
• -Composites
• -Cement Concrete (cheap and usually
  anti-corrosion)
• -Wood

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• Marine Corrosion is the deterioration of metals
  in the marine environment due to electro-chemical
  reaction. (see pp161 (old E. 129-130) regarding
  electrochemical reactions)
• Ships, marinas, pipelines, offshore structures,
  desalination plants, ocean energy conversion
  device heat exchangers are some examples of
  systems that experience marine corrosion.
• Exposure of components to sea water can be
  continuous or intermittent.
• Maintenance costs for ships, offshore structures
  and related equipment are dependent on how marine
  corrosion issues and failures are managed.

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Typical Types of Marine Corrosions

• Uniform Corrosion Corrosion over entire surface
  area
• Galvanic Corrosion occurs when two dissimilar
  metals are connected directly or by a metallic
  path and are immersed in seawater that acts as an
  electrolyte.
• Noble metals erode slower and the other (Active)
  metals erode much faster. Table (5-15) at p162
  (old E. p131)
• Galvanic Corrosion can be used to protect the
  metals by bolting Zinc or Aluminum (anode) to it
  (cathode).

Stainless screw cadmium steel washer
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• Intergranular Corrosion.
• The microstructure of metals and alloys is made
  up of grains, separated by grain boundaries.
  Intergranular corrosion is localized attack along
  the grain boundaries, or immediately adjacent to
  grain boundaries, while the bulk of the grains
  remain largely unaffected.
• A classic example is the sensitization of
  stainless steels or weld decay.

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lap joints and surface deposits.

• Crevice Corrosion Pitting.
• occurs in narrow metal to metal or non-metal to
  metal gaps where the convection of water is
  hampered. Aggressive ions like chlorides must be
  present in the electrolyte. Crevice corrosion
  develops quite similar to pitting corrosion after
  the initiation stage. Examples of such geometries
  include flanges, gaskets, disbonded
  linings/coatings, fasteners,

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• Erosion Corrosion arises from a combination of
  chemical attack and the physical abrasion as a
  consequence of the fluid motion. The best way to
  limit erosion-corrosion is to design systems that
  will maintain a low fluid velocity and to
  minimize sudden line size changes and elbows. 
  The photo shows erosion-corrosion of a
  copper-nickel tube in a seawater surface.  An
  imperfection
• on the tube surface
• probably cause an
• eddy current which
• provided a perfect
• location for
• erosion-corrosion

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• Stress Corrosion results from the combination of
  an applied tensile stress a corrosive
  environment.
•  Some materials only become susceptible to
  corrosion in a given environment once a tensile
  stress is applied. Once the stress cracks begin,
  they easily propagate throughout the material,
  which in turn allows additional corrosion and
  cracking to take place. The tensile stress is
  usually the result of expansions and contractions
  
• that are caused by
• violent temperature
• changes or
• thermal cycles. 

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Preventing Corrosions

• Design
• Coating
• Cathodic Protection
• -Impressed cathodic protection (requires to use
  of external electric power.
• - Galvanic cathodic protection