OCEN 201 Introduction to Ocean & Coastal Engineering

OCEN 201 Introduction to Ocean & Coastal Engineering

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

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