Transcript OCEN 201 Introduction to Ocean & Coastal Engineering Materials & Corrosion

OCEN 201
Introduction to Ocean &
Coastal Engineering
Materials & Corrosion
Jun Zhang
[email protected]
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
• Metals:
-Steels & special steels (Stainless steels),
-
-Aluminum,
-Titanium,
-Non-ferrous Alloys (Copper-nickel)
• Nonmetallic
Materials
-Thermoplastics
-Composites*
-Cement & Concrete* (cheap and usually anticorrosion)
-Wood
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.
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
•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.
•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,
lap joints and surface
deposits.
•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
•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.
Preventing Corrosions
•Design
•Coating
•Cathodic Protection
-Impressed cathodic protection (requires to
use of external electric power.
- Galvanic cathodic protection