Corrosion Flashcards
Explain fretting corrosion?
When two heavy loaded surfaces micro move between each other. (one being metallic). The rubbing from vibration removes the natural protective layer allowing metal particles to oxidise and form hard abrasive compounds that is a self penetrating corrosion.
It can be found on hydraulic pumps and can cause fatigue cracking. Also can be speed up by lack or proper lubrication.
Explain Erosion damage?
The wearing away of a material by the abrasion of fluids.
Explain Fatigue Corrosion?
Caused by a combined effect of corrosion and stress applied in cycles of components.
For instance a reciprocating rod on the piston in a hydraulic double acting actuating cylinder. The extension stroke, compression load is applied. Retracting stroke, tensile load is applied.
This then causes a part to fracture and fail at stress well below the rated fatigue limit.
Explain Stress Corrosion?
This is a combination of tensile stress and corrosion. It can be caused by unequal cooling from high temperatures.
Explain Stress Corrosion Cracking?
Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material, and tensile stress. All three conditions must be met simultaneously.
The failure can happen without significant deformation or obvious signs of deterioration.
Aluminium and Stainless steel are common for this.
Explain Crevice Corrosion?
This is a localised form of corrosion found in crevices such as under gaskets and washers.
It occurs when a stagnant solution is between the two. This can happen from a deletion of oxygen or inhibitor in the crevice.
What are 4 methods to prevent/ reduce corrosion?
- Coatings
- Inhibitors
- Cathodic Protection
- Proper Material Selection
Why is a marine atmosphere considered to be more aggressive corrosion environment?
- Humidity
- Wind
- Temperature
- Airborne contaminants
- Location
- Biological organisms
What methods are used to stop Marine corrosion?
- Cathodic protection - either impressed current system or sacrificial anode system.
- Organic coatings, like antifouling
- Correct alloy selection
What are the 4 fundamental parts of an ICCP System?
- Titanium anodes
- Zinc or Silver electrodes
- Shaft Bonding Slip Ring
- Automated control panel
Explain how ICCP Anodes work?
They conduct a DC protective current to prevent corrosion in areas where paint is missing on the hull.
Due to the different alloys and the rotation of the propeller, this will produce a current between the two so it also tackles this.
This is why you will find a shaft earthing strip.
Explain Galvanic Erosion?
This is the corrosion of metals which is greatly spread up by the influence of an electrical current, also known as a stray current.
This is caused by a current having to use the hull as the return path instead of the wire return from the central earth point.
It also can be known as Hot Dock when in port.
Explain the term Hot Dock?
The term arises from boats seemingly experiencing more severe corrosion when on the dock. The cause is small DC voltages present in the green neutral wire at the shore power point.
When two or more boats are connected to shore power, one side of the necessary circuit required to form a galvanic couple is provided by the AC green grounding wire, which is
also connected to the boat ground system, engine, and underwater hardware. The seawater electrolyte provides the other side of the circuit. Galvanic current flowing
around the circuit will corrode the least noble metal between the two (or more) boats.
A common way is a bronze propellor to a Aluminium sterndrive.
What two options can you have to stop Hot Docking?
Galvanic Isolater
Isolation Transformer
difference between a galvanic isolator and isolation transformer?
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