12. Corrosion Flashcards
What is corrosion?
- Degradation of a metal via an electrochemical reaction with the environment
- Requires the presence of an electrolyte
- Metal loses thickness and its mechanical properties are adversely affected
- Tendency of a metal to return to its natural state.
Corrosion is a chemical or electrochemical process, meaning electrical energy contained within molecules is converted to electrical energy as electrons are released. Can pose environmental and personal safety issues.
Types of corrosion
Generalized
1. Generalized - Atmospheric
2. Generalized - Immersed
Local
1. Galvanic
2. Restricted spaces
3. MIC
4. Intergranular
5. Physical wear
6. De-alloying
Generalized corrosion
Uniformly affects the entire surface of the metal, slow predictable process
Metals are classified by their relative corrosion resistance and their service life is easy to predict based on the thickness of material lost per year. A metal with “good” corrosion resistance loses < 0.5 mm per year so a 20 mm sheet can be expected to last at least 40 years. Rate of corrosion will depend on the environment.
- Generalized - atmospheric
Occurs on superstructures and topside areas of the hull. Tends to be slow than water, mostly the concern is cosmetic in nature.
Some factors that help this corrosion include humidity and airborne pollution.
- Generalized - immersed
Submerged sections of the hull. Can be a general deterioration or a very deep concentration of corrosion of the plating, welds, or items in the area
Local
1. Galvanic
Metals are classified according to the galvanic series. Metals that are known to be fairly immune to corrosion are called “noble”. Metals that corrode quickly are called “base”. When two metals are electrically connected, their positions on the galvanic series will predict which metal will corrode first. The nobler metal will be almost completely protected from corrosion as long as it is connected to the baser metal. Two different metals will corrode when they are electrically connected and immersed in an electrolyte.
Galvanic corrosion in piping can be prevented through the use of insulating flanges. The gasket and insulating sleeves prevent electrical current from moving between one metal and the other. The gaskets must be properly installed and regularly inspected. These are difficult to assemble and expensive.
Stray current corrosion: stray electrical currents can occur within the ship, from a nearby vessel or from power ashore going to ground. It can cause the same type of electrochemical reaction as with galvanic corrosion. Extremely serious, can cause a fitting to be destroyed in just a matter of weeks or ever hours.
Local
2. Restricted spaces
Crevice corrosion: occurs close to the development of crevices/crackers in the metal. A small volume of stagnant oxygen deficient seawater becomes trapped which accelerates corrosion.
Pitting corrosion: stainless steel is susceptible to pitting when immersed in seawater. Causes tiny holes to form in the metal and these holes grow deeper over time while the opening stays very narrow. Creates a restricted space which accelerates corrosion. Can be very difficult to detect until failure occurs as there is little evidence of damage on the surface of the metal.
Under-deposit corrosion: occurs when a restricted space is created underneath a corrosion product, under debris, insultation or lagging or under organic matter.
Poultice corrosion: occurs when a soft, saltwater saturated mass (typically organic) is in contact with metal for long periods of time. Damp insulation (lagging) on pipes, debris in bilges, wood deck sheathing.
Local
3. MIC
Marine biofouling: sessile organisms that attached to all surfaces. Macro can reduce water velocity inside pipes, ship’s speed and fuel consumption. Micro excrete gel-like substance that forms a biofilm on the metal surface: restricts flow of oxygen on surface and contains corrosive metabolites.
Microbial Influence corrosion: occurs due to the microbial by-products and a suitable environment (warm, stagnant). Noted as deep, closely clustered pits in the metal. Corrosion rates increase dramatically and is a significant factor in ships that carry heated cargo. Also accelerates the corrosion in restricted spaces. Heavy pitting can affect structural integrity, may allow leakage from a tank into a cargo area, void spaces, or into the sea. May pose a health risk to crew.
Controlling MIC: suitable isolation of the surface of the metal from the effects of the microbes (thick, epoxy-based coatings applied under strict conditions). Minimize buildup of dirt, sludge and water deposits. Could use biocides but expensive for marine applications. TL;DR regular cleaning, proper coating
Affected areas: ballast tanks, bilges and fuel tanks provide nutrient rich environments; low areas of tanks and piping systems (stagnant water) are particularly susceptible. Stable temperatures in tanks encourage microbial growth. Mostly restricted to the internal surfaces of the hull structures. The majority of the corrosive effects are simply an increase in the rate of galvanic cell corrosion.
Local
4. Intergranular
Weld decay corrosion
Occurs in austenitic steel containing a moderate amount of carbon. When welded, the dissolved carbon in the welded area migrates to the grain boundaries and form chromium carbines. This leaves an area of the weld depleted of chrome which reduces the resistance to corrosion at the boundary.
Attacks grain structure of the steel
Local
5. Physical wear
Erosion corrosion: physical erosion weakens metal, allowing corrosion to set it
Stress corrosion: when under tensile stress, any surface damage that weakens the material can be the initiation site of a crack. Cracks then progress over time as corrosion accelerates inside the crack and tensile stress continues to pry the crack open. Not easily visible.
Corrosion fatigue: when under cyclical load, the same times of vicious cycles of physical damage through stress and chemical damage from corrosion can lead to cracking
Local
6. De-alloying
Self-corrosion or simple electrochemical corrosion involves a single piece of metal (e.g. aluminum, iron) or an alloy (e.g. steel, stainless steel, bronze) in contact with an electrolyte.
Factors affecting the rate of corrosion
- Coating condition and type
- Location on the vessel
- Temperature of the seawater
- Salinity of the seawater
- Oxygen content in the seawater
- pollutants in the air
- Difference in potential between the anodic and cathodic zones
- Surface area of anodic and cathodic zones
- Amount of stay current output from shipboard machines
Corrosion prevention and control
3 general approaches:
1. Material selection (appropriate material for the application
2. Insulation (prevent an electrical circuit from forming)
3. Cathodic protection
Best practice: combo of all 3 where appropriate
- Material Selection
Copper alloys (copper nickle, bronze and brass) are used for applications where the part is immersed in seawater and paint is not suitable
Steel is suitable only with paints/coatings and/or cathodic protection
Aluminum alloys are suitable and can be painted or strengthened to resist corrosion
Titanium is sometimes used in heat exchangers b/c highly resistant to corrosion but v expensive
- Cathodic protection: sacrificial anodes
Made of a metal found on a lower position of the galvanic series (more base), will protect the more noble metal from corrosion. Anodes are designed to waste away. Fabricated in various sizes and shapes to suit the area. For ship’s hull, cast around a steel strap that is welded to the hull to provide the necessary electrical connection of the anode to the hull plating.
It is necessary to calculate the estimated wastage of the anode that will occur over time. But as the anode deteriorates, it becomes smaller and will protect an ever-decreasing area throughout the life cycle. Therefore, any calculation of the number and size of anodes must include an amount in excess of the nominal protection required.
Factors affects the sacrificial anodes’ ability to protect
- Possible damage to the hull coatings (increased demand on anodes)
- Possible physical damage to one or more anodes
- Deterioration of the electrical contact between hull and anode (reduced effectiveness)
- Loss of an anode (remaining anodes in higher demand, waste away more quickly)
- Possibility of anodes being completely or partially covered eliminates or reduced their effectiveness
- Increase in stray electrical currents (excessive wastage, shortened life space).
Anodes must be replaced periodically. During refit, sacrificial anodes should be checked to see if they have performed correctly. Excessive wastage of the anodes between refit periods over and above their normal amount will dictate an investigation of the cause.
Sacrificial Anodes: Advantages
- No power source required
- Simple installation
- Economic for small schemes
- Overprotection not problematic
- Good overall current distribution
- Low maintenance costs
Sacrificial Anodes: Disadvantages
- Decreased output with time and condition of hull coating
- Large number of anodes required for high risk areas
- No feedback to adjust protection rate
- Possibility of physical damage to anodes
- Possibility of insufficient corrosion protection
- Added weight
Impressed Current Cathodic Protection (ICCP)
Incorporates non-sacrificial anodes (usually titanium) recessed into the hull in designated locations. The anodes are supplied with a specific amount of DC current from a control system. This current provides the same electrochemical reaction as the wastage of an anode, which then protects the surrounding metal surface.
Many ICCP systems continuously monitor the potential difference between the anodes and the protected surface by the use of reference cells (silver chloride) placed on the submerged steel surface. This measurement is fed back to the system’s control panel that automatically calculates and dispenses the correct amount of compensating current to be sent to the anodes. It is desirable to provide a proper amount of protection since under protection will allow corrosion to occur while overprotection may cause the paint in the immediate area to blister and peel. As the coating deteriorates on the hull, current will need to be increased to compensate and better protect the hull.
Components include a number of anodes, reference electrodes, a control panel and a power supply. Anodes are surrounded by a dielectric shield coating of epoxy to protect the hull region around the anode.
ICCP produces a chlorine gas and is therefore not used in the protection of tanks due to spart hazards as well as the possibility of hazards to personnel entering the tank.
Certain items (propeller, shafting and rudder) are electrically insulated from the ship’s hull. This can cause stray current and sparking so proper grounding is required. For rotating shafts, this is done by using a slip ring and graphite brushes. Rudder and other appendages are supplied with earth straps. Once connected to the hull, they are also protected by ICCP.
ICCP advantages
- Can be used on large, complicated schemes
- Demand responsive to condition of hull (self-adjusting)
- Long life for system components
- Continued protection even with high coating damaged (extra system capacity)
ICCP Disadvantages
- Initial installation and replacement parts are expensive
- Power source required
- Routine electrical maintenance
- Loss of protection with power failure
- Local anode area susceptible to damage due to current output
- Overprotection is possible, damaging paintwork and leaving chalky deposit on areas of bare metal.
Sea Inlet protection
Corrosion protection of sea bays may be accomplished by the use of ICCP but the system is different than typical systems. These systems can also offer a degree of antifouling protection by producing an environment unsuitable for marine growth (marine growth doesn’t like electrical current).
MGPS (Cuproban Marine Growth Prevention System) consists of copper and aluminum (or soft iron) anodes strategically located in sea chests or sometimes inboard, but as close to the sea-water intake as possible. One set of anodes is recommended for each sea water service. The anodes are connected to a control panel that feeds a current to the anodes. The resultant ions and floc produced by the anodes is carried by the sea water, spreads through the pipe work, and creates an environment that is distinctly unfriendly to the marine life
MGPS advantages
- Simple installation
- Reliable, automatic anti-fouling with minimal attention from the crew
- Complete protection against biofouling
- Reduced corrosion
- Minimal power requirements and modular electronic control panel using one module per flow line
- Easy system expansion by adding modules
- Unique anode wear indicator feature in control panel tells when anodes need replacement
- Unique anode save feature controls current to the anode base on there being or not being flow in the pipeline. Considerably increases anode life when flow is intermittent.
