Corrosion, Construction Materials, Class societies and Maintenance Flashcards
Advantages and disadvantages of Wood
Advantages:
Beautiful
Light
Corrosion resistant
beautiful
Disadvantages:
Fire
Rot
Expensive
Subject to insect attacks
Advantages and disadvantages of Steel
Advantages:
Strong
Not easily damaged/easy to repair
Comparatively easy to build
Cheap
Good fire resistance
Disadvantages:
Corrodes heavily
Heavy
Magnetic
Requires insulation against condensation
Advantages and disadvantages of FRP
(Fibreglass reinforced plastic)
Advantages:
Lightweight
Comparatively easily repaired
Cheap, especially when mass produced
Corrosion free
Disadvantages:
Comparatively easily damaged
Very poor fire resistance
Osmosis
Skilled construction required
Advantages and disadvantages of Aluminium
Advantages:
Light
Does not rust
Easily maintained
Easily repaired
Disadvantages:
Poor fire resistance
Expensive
Galvanic corrosion
requires specialist welding
Chemical corrosion
The gradual destruction of a metals surface due to a reaction between it and a substance surrounding it in its external environment
Acids or alkaline solutions can chemically combine with another substance and cause corrosion.
This may be deliberate with instances such as etching primers etc
Rust
(Oxidation)
Occurs when iron, Oxygen and water combine, iron then loses electrons to oxygen atoms, creating Iron Oxide.
As the oxide is not firmly attached to the plate, it allows further oxidation to occur.
Describe the process of Osmosis
Occurs when water gets inside the gel membrane of a GRP vessel
Allows water to come into contact with the resins
Water hydrolyses and causes blisters, as the laminate breaks apart
Cheaper resins such as polyester intensify hydrolysation
How do we correct Osmosis while out of the water?
Grind back and cut out the damaged area, and the surrounding area
Ensure the area is dry, and clear of contaminants
Repair area with EPOXY resin
Cover with Gel coat
Finally paint on antifouling
Electrochemical (Galvanic) Corrosion
Corrosion between two dissimilar metals when bonded together, via the creation of an electrical current.
The metals have different electrical potential and create a small cell similar to a battery
Electrons flow from the Anode to the Cathode through the electrolyte (Salt water in our case)
Explain the Anodic Index, and the terms Anode/ Cathode
The anodic index determines the electrical potential of metals. It will state in any specific instance which metal will become:
The weakened Anode
The strengthened Cathode
Describe the process of Jet Bonding
Explosive bonding, whereby one strip of the two dissimilar metals is made by crushing them together explosively, forming a molecular bond.
This strip is then welded to the steel and aluminium section of the superstructure to form a ‘bridge’
Must be painted in a non conductive paint to ensure they do not become connected via an electrolyte
4 ways of testing the watertight integrity of a weathertight door
Hose test
Chalk test
Ultrasonic test
Light test
Describe the function of Impressed Current Cathodic Protection
Creates an impressed current that counteracts any Galvanic current created beneath the vessel
Protects the Antifouling system and the Running gear
Reference electrodes send signals to a control unit, which sends current through the current rectifier, out through Inert anodes.
What does IACS stand for?
International Association of Class Societies
They all govern themselves under this body
Role of Class societies?
Lay down standards for construction
Lay down standards for materials used in construction
Supervise the construction until build is finished
Carry out annual surveys of structures and equipment to ensure ship stays compliant international standards
Maintain a register of all vessels of >100GT built under their survey
Provides surveyors at most major ports in conjunction with flag state authorities
