Objective 03: Corrosion Mechanisms Flashcards
Uniform (Generalized) Corrosion
Corrosive action that occurs over an entire surface or over a relatively large section of the surface and may occur at a constant rate over that entire area
How does Uniform Corrosion usually appear?
General, uniform thinning of the metal
What are the electrochemical processes that occur during Uniform Corrosion?
There are many microscopic anode and cathode sites on the surface. These sites continuously alternate between anode and cathode behaviour, creating a uniform attack
How quickly does Uniform Corrosion cause failure?
Meta failure may take longer to occur but if the surface becomes rough other forms of corrosion may be encouraged
What makes Uniform Corrosion easier to deal with?
It can often be predicted and its progression is relatively easy to monitor
Why must one be wary of Uniform Corrosion?
It has the potential for the loss of a large amount of metal and the weakening of a large surface area
Localized Corrosion
Aggresive corrosive activity at very localized sites on a metal surface
What factors may cause the localization of attack by Localized Corrosion?
- Inherent properties of the metal or the design
2. Local breakdown of the corrosion protection
What is the electrochemical activity that ocurs during Localized Corrosion?
The anodic site is much smaller than the cathodic site, which results in a very high concentration of ions leaving the anode.
What makes Localized Corrosion harder to deal with?
It is difficult to predict, monitor, and control
What makes Localized Corrosion dangerous?
Only a relatively small amount of metal loss can cause component failure
What is another name for Galvanic Corrosion?
Dissimilar Metal Corrosion
Galvanic Corrosion
The corrosion that occurs when 2 different metals are immersed in a common electrolyte and there is a connection between the 2 metals (either directly or some other solid connection) which allows an electric current to flow
What 3 conditions must exist for Galvanic Corrosion to occur?
- Difference in electrochemical potential between 2 different metals or between locations within a single metal
- The metals must be immersed in the same electrolyte, which provides a path for soluble metal ions to flow between them
- The metals must be mechanically connected in some way to complete an electric circuit and allow current to flow in a complete conducting path
What causes a difference in electrochemical potential between 2 metals or areas of a metal?
Different metals have different electron structures; therefore, they have different electrical potentials and a potential difference exists between them
Why is a difference in electrochemical potential important to Galvanic Corrosion?
Potential difference (in volts) is the driving force that produces electron flow between the metals
What are the 4 factors that affect the rate of Galvanic Corrosion?
- The difference in electrical potential between the 2 materials (proportional)
- The type and concentration of electrolyte
- The distance between the dissimilar materials (inversely potential)
- The ratio of cathode area to anode area (proportional)
Electrochemical Potential (Voltage)
Unique characteristic of each metal that is determined by the metal’s atomic structure and availability of free electrons
How does Electrochemical Potential affect 2 different metals that are connected in a galvanic cell?
The metal with higher potential acts as the cathode and drives current flow from the other metal, which becomes the anode
Electromotive Series
Tables that rank metals in terms of their electrode potentials, in volts
How does the ranking on the Electromotive Series affect corrosion?
Generally, the higher the electrical potential of a metal, the less likely it is to corrode
What is the relationship of the Galvanic Series to the Electromotive Series?
They are almost synonymous and serve the same purpose
Galvanic Series
Ranking of metals, determined experimentally, which indicates the relative position of each metal in terms of nobility, where the more noble metal is less likely to corrode
How does one determine the rate of corrosion for metals on the Galvanic Series?
Roughly, the further apart 2 metals are, the greater the potential current and therefore the rate of corrosion
What is the Galvanic Series for common industrial metals from anodic to cathodic in which the electrolyte is salt water?
- Magnesium
- Zinc
- Aluminum
- Mild Steel
- Cast Iron
- Lead
- Tin
- Copper
- Bronze
- Bass
- Stainless Steel
- Titanium
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What factor determines the galvanic position for alloyed metals?
The percentage of more noble and less noble metals in the alloy
Concentration Cell Corrosion
Localized and often very aggressive corrosion that occurs when the electrolyte in a corrosion cell becomes more concentrated at one location on a metal surface than in the adjacent environment. The more concentrated cell usually becomes anodic, while the surrounding area becomes cathodic and if the anodic area is small, rapid corrosion in the crevice is promoted.
How can concentration differences occur for Concentration Cell Corrosion?
Any small irregularity in the surface where the electrolyte is stagnant, e.g. in a crevice or under a deposit
What is the most common form of Concentration Cell Corrosion?
Crevice Corrosion
How does Crevice Corrosion work?
Any crevice in the metal interface provides a location for a corrosion cell to concentrate. The micro-environment within the crevice may develop corrosion-enhancing characteristics that differ from the surroundings.
What are 4 corrosion-enhancing characteristics that may develop in a crevice to encourage Crevice Corrosion?
- Loss of Corrosion Inhibitor
- Oxygen Depletion
- Increased Acidity
- Accumulation of more aggressive corrosion products
Why is oxygen depletion corrosion-enhancing?
Since oxygen is an active participant in the cathodic reaction in many environments, the loss of oxygen in a crevice means that the normal cathodic oxygen-reduction reaction of the electrolyte cannot occur.
How does Under-Deposit Corrosion occur?
When a deposit is allowed to accumulate on the surface of metal, the interface between the metal and the electrolyte is interrupted and the surface can develop a concentration cell below the deposit.
How can Under-Deposit Corrosion occur in a boiler tube?
Water in the boiler tube travels through tiny pores in the deposit and reaches the tube surface where it boils. The created steam migrates back out through channels in the deposit but leaves behind concentrated corrosive products in a concentration cell within and beneath the deposit.
What are 2 forms of Concentration Cell Corrosion?
- Crevice Corrosion
2. Under-Deposit Corrosion
Pitting
A form of localized corrosion that is characterized be small cavities in the metal surface, with each cavity a separate, localized corrosion site that tends to propagate into the depth of the metal, rather than spreading out over any width.
How easily visible is pitting?
Pits may be open and easily detected by visual inspection or they may be ‘hidden’ beneath a semi-permeable layer of corrosion products
How does Pitting work?
A tiny irregularity in the metal surface becomes anodic and interacts electrochemically with the surrounding electrolyte. In some cases, the pit becomes covered by a film of corrosion products, which deprives it even more of oxygen, promoting further corrosion. In any case, the pit continues to grow.
Oxygen Pitting
A form of pitting that occurs when free oxygen becomes attached to the metal surface
How does Oxygen Pitting work?
- When a surface comes in contact with attached oxygen, it becomes cathodic while the adjacent surfaces becomes anodic.
- Corrosion products accumulate and eventually enclose the bubble of oxygen, thereby exhausting it
- The area under the corrosion cap reverts to an anode
- Corrosion continues as a concentration cell
Deposition Pitting Corrosion
Pitting that occurs due to the deposition of one type of metal unto another
How does Deposition Corrosion work?
- A fluid passing through a pipe may absorb metal ions from the pipe material
- When the fluid then flows to a location where the metal is different and more anodic, the absorbed ions may be deposited onto the surface at this new location (i.e. plating)
- Microscopic, galvanic action occurs, causing pitting of the anodic material
What are 5 conditions that promote Pitting?
- Small, local damage to the protective oxide film on the metal surface
- Small, local damage to an applied protective coating
- Small irregularities, during manufacture, in the metal itself
- Free oxygen at the metal surface
- Microscopic areas of Galvanic Action
Selective Leaching/Dealloying
Corrosion of one element in an alloy, thereby making the alloy weaker, brittle, and porous
What is a common example of Selective Leaching?
Dezincification of Brass
What are 2 forms of Pitting Corrosion?
- Oxygen Pitting
2. Deposition Corrosion
Hydrogen Induced Corrosion
Corrosion mechanism that is directly related to the presence of hydrogen at the surface of a metal
What are 3 types of Hydrogen Induced Corrosion?
- Hydrogen Embrittlement
- Hydrogen Blistering
- Hydrogen Stress Cracking
How does Hydrogen Embrittlement work?
- When the concentration of hydrogen, as by-product of corrosion, increases at the surface of steel, some of the hydrogen atoms may diffuse into the steel.
- The hydrogen collects between the metal atoms and distorts their natural structure
What is the effect of Hydrogen Embrittlement on steel?
The steel has a reduced ability to deform elastically, causing a loss of ductility and an increase in the brittleness of the steel, which can lead to fracture of the metal
How does Hydrogen Blistering work?
- Hydrogen atoms diffuse into the interior of steel
- Atomic hydrogen collects at voids inside the steel
- The atoms bond together to form molecular hydrogen
- The molecules become too large to migrate out of the steel and become trapped
- More hydrogen molecules accumulate to the point where internal pressure increases
What is the effect of Hydrogen Blistering on steel?
As the internal hydrogen pressure increases, it can reach a point where metal is displaced outwards, causing fissures or blisters on the surface
How does Hydrogen Stress Cracking work?
- Atomic hydrogen diffuses into the steel
2. The hydrogen interacts with the atomic structure of the steel, at or near the grain boundaries, forming a crack
Hydrogen Stress Corrosion Cracking
Where corrosion occurs on the surface of a metal undergoing Hydrogen Stress Cracking and migrates into the formed cracks and widens them
Mechanically Assisted Corrosion
Group of corrosion mechanisms, each of which is enhanced by some form of mechanical action
What are the 2 most common mechanical conditions that contribute to corrosion?
- Stress
2. Fatigue
What combination of 3 factors lead to Stress Corrosion Cracking?
- Tensile Stress (imposed by operating conditions)
- Susceptibility of the material to corrosion (i.e. composition)
- Corrosive Substances to which the metal is exposed, (i.e. environmental conditions)
Stress Corrosion Cracking
Delayed failure process in which cracks start slowly and propagate slowly. Eventually the stresses in the remaining metal exceed the breaking strength and the metal suddenly fails
How does the tensile stress required for Stress Corrosion Cracking usually compare to Yield Stress of the metal?
It’s much less
What can cause tensile stress that can contribute to Stress Corrosion Cracking?
- External Load
- Internal Pressure
- Residual Stresses from the manufacturing process
What are the 3 common forms of Stress Corrosion Cracking?
- Chloride Stress Corrosion Cracking
- Sulphide Stress Corrosion Cracking
- Caustic Embrittlement
Which metal is susceptible to Chloride Stress Corrosion Cracking?
Stainless Steels containing less than 30% nickel
How does Chloride Stress Corrosion Cracking work?
Chloride attacks the metal along grain boundaries, causing corrosion in the form of cracks
What 3 conditions are necessary for Chloride Stress Corrosion Cracking to occur?
- Chloride ions must be present
- Oxygen must be present
- The metal must be under tensile stress
What happens if any of the 3 conditions necessary for Chloride Stress Corrosion Cracking to occur is increased?
The probability and severity of corrosion increases
What 2 factors increase the rate of Chloride Stress Corrosion Cracking?
- Temperature increases
2. pH of the environment is less than 7
What 3 conditions are necessary for Sulphide Stress Corrosion Cracking?
- Hydrogen sulphide must be present
- Water must be present
- The metal must be high strength alloy steel under applied or residual tensile stress
What is the ultimate corrosion mechanism caused by Sulphide Stress Corrosion Cracking?
Hydrogen Induced Cracking
How does Sulphide Stress Corrosion Cracking ultimately cause Hydrogen Induced Cracking?
- Sulphide ions in the aqueous hydrogen sulphide atmosphere inhibit the recombination of hydrogen atoms on the metal
- The smaller hydrogen atoms then diffuse into the metal’s crystal structure, causing additional hydrogen induced cracking
What is another term for Caustic Embrittlement?
Caustic Stress Corrosion Cracking
What mechanism is Caustic Embrittlement similar to?
Chloride Stress Corrosion, except oxygen doesn’t have to be present
Caustic Embrittlement
Under very high caustic environment, mild steels and stainless steels under tensile stress crack
When does a material experience Fatigue?
When it is subjected to repeated loading and unloading stresses
When does a material experience Corrosion Fatigue?
When it is subjected to cycling stresses in a corrosive environment
What is the effect of fatigue on material subject to fluctuating load?
If the load stresses are higher than a certain threshold, usually below yield strength, microscopic cracks will begin to form at the surface
What can cause excesses stresses on boiler tubes?
- Load changes during cold start, forced cooling, or during shutdown
- Boiler restart where thermal stratification of water occurs along the tube length
How does Corrosion Fatigue work?
- Cycling stress causes the corrosion protection layer on the metal surface to be broken
- Corrosion is accelerated and causes either general weakening of the surface or localized pits
- This weakening reduces the stress loads at which fatigue cracks may begin
- When a primary corrosion fatigue crack does begin, the corrosion will propagate into the crack and widen it
What effect does a corrosive environment have on fatigue failures?
It causes them to occur sooner
What are 2 deteriorating effects caused by the flow of electrolyte?
- Mechanical Erosion
2. Electrochemical corrosion
What are 2 common forms of Flow-Induced Corrosion?
- Erosion-Corrosion
2. Flow Accelerated Corrosion
Erosion-Corrosion
The repetitive creation and destruction of the protective surface film on a metal due to flow that exceeds a critical velocity surface friction and turbulence cause shear stresses which ultimately remove the film and give space for corrosion rates to increase. When flow returns to normal, the protective film is re-established.
What characterizes surface corrosion due to Erosion-Corrosion?
Shallow pits or striations in the direction of flow
What characterizes Flow Accelerated Corrosion?
Gradual, generalized thinning of a pipe wall within a specific area that may lead to sudden failure when the pipe is no longer able to withstand the internal pressure
Which areas in piping are most susceptible to Flow Accelerated Corrosion?
Where there is direction changes or other disturbances
What characterizes the areas in piping that are most susceptible to Flow Accelerated Corrosion?
- Pipe is continuously contacted by the feedwater
2. Feedwater is deaerated
Why does dearation contribute to Flow Accelerated Corrosion?
The protective magnetite layer on the internal pipe surface is not well established, because protective magnetite formation relies on a reducing reaction with dissolved oxygen in the water.
How does flow contribute to Flow Accelerated Corrosion?
At areas of high flow and turbulence, the water tends to dissolve the iron from the magnetite faster than it can be replenished, leaving an iron-deficient magnetite layer. In an attempt to maintain the magnetite layer, iron is continuously taken from the metal, causing continuous, general thinning of the area.
Microbiologically Influenced Corrosion (MIC)
Corrosion that is directly caused, influenced, or accelerated by bacteria
What are 4 conditions necessary for bacteria to grow and influence corrosion?
- A metal surface to host the bacteria
- Nutrients to feed the bacteria
- Water
- Oxygen*
What are 5 ways that bacteria can affect corrosion?
- Directly affect anodic and cathodic reactions
- React with protective surface layers
- Create corrosive conditions
- Produce deposits
- Secrete acidic fluids
What are 3 types of Bacteria that are of interest in corrosion?
- Aerobic Bacteria
- Anaerobic Bacteria
- Facultative Bacteria
Aerobic Bacteria
Bacteria that require oxygen to live and grow
Anaerobic Bacteria
Bacteria that do not directly require oxygen to grow and some may even die from it
Facultative Bacteria
Bacteria that can grow with or without oxygen
What type of bacteria are Iron bacteria?
Aerobic
How do Iron Bacteria affect Corrosion?
- They form a sheath of ferric hydroxide around themselves as they grow, using soluble iron ions found in the water
- The sheath can provide a site for the formation of an oxygen concentration cell
- This sheath serves as a site for anaerobic and facultative bacteria to act
What type of bacteria are Sulphate-Reducing Bacteria (SRB)?
Anaerobic
What do Sulphate-Reducing Bacteria do?
They reduce sulphates in the water to produce sulphides and subsequently produce hydrogen sulphide
What are 4 corrosion problems caused by Sulphate-Reducing Bacteria?
- Pitting, directly under a colony of bacteria
- The generation of hydrogen sulphide can make the water more acidic, raising the level of general corrosion
- Sulphide cracking and sulphide blistering
- Sour corrosion that forms insoluble iron sulphide, which itself forms more sites for further pitting
Atmospheric Corrosion
Degradation of a metal surface when exposed to air and its pollutants
How significant is Atmospheric Corrosion?
It is a leading cause of metal failure and aesthetic damage and is an important factor in the service life of equipment and the durability of structural materials
What are the 3 categories of Atmospheric Corrosion?
- Dry
- Damp
- Wet
How fast is the process of Dry Atmospheric Corrosion?
Very slow
What characterizes Dry Atmospheric Corrosion?
A stable film that forms on the surface of a metal in the presence of oxygen
What characterizes the film created by Dry Atmospheric Corrosion?
It’s a passivating film that creates a protective barrier which prevents or slows further corrosion
What characterizes a desirable protective film?
- Free of defects
2. Self-repairing when damaged
What is the effect of the presence of sulphur on passivating films?
It increases the possibility of defects and destroys the film’s ability to protect the metal
What relative humidity causes Damp Atmospheric Corrosion?
70% or higher
How does Damp Atmospheric Corrosion work?
- A very thing, invisible film of moisture forms on metal surfaces
- The film acts as an electrolyte for the transfer of electrical current, thus leading to galvanic corrosion of the metal
What causes the corrosion rate to increase for Damp Atmospheric Corrosion?
- Increased temperature
2. Increased relative humidity
What is the effect of salt on Damp Atmospheric Corrosion?
It increases the corrosion rate
Why does salt increase the corrosion rate of Damp Atmospheric Corrosion?
- Salts increase the current-carrying ability of the electrolyte
- Salts absorb moisture and thus create the corrosive film even if relative humidity is lower
When does Wet Atmospheric Corrosion occur?
When visible pockets or layers of water exist on a metal surface or if water becomes trapped in tiny imperfections in the surface
How does Wet Atmospheric Corrosion work?
The water acts as an electrolyte and iron is lost from the metal surface
What are 2 ways that Wet Atmospheric Corrosion can be sustained/increased?
- If corrosion products dissolve in the water, it can become more conductive and thus increase corrosion rate
- Even if the water dries, the corrosion products may remain wet and sit on the surface, causing corrosion to continue
What pollutant can cause the atmosphere to become more corrosive?
Sulphur
What causes Stray Current Corrosion?
External source of current that causes stray currents to follow unintended electrical paths
What are 3 things that can cause electrical current to tray from its intended path?
- Poor electrical conductors
- Damaged insulation
- Dirty or loose electrical connections
Which is the most destructive source of stray current: DC or AC?
DC
How is Galvanic Corrosion distinguished from Stray Current Corrosion?
Lab analysis
What is an indication of Stray Current Corrosion?
A concentration of pits in an area where they are not normally found
How can Stray Currents be located?
Measuring voltage drops and current flows along the buried structure
What are found in the intergranular boundaries of metals and metal alloys?
Atoms that have been displaced from their original crystals and other impurities
Intergranular Corrosion
Corrosion at grain boundaries in metals and metal alloys due to them being less corrosion resistant than the grains themselves
What is the ultimate effect of Intergranular Corrosion?
The metal structure is weakened and the entire boundary around the grains can be lost, causing the grains to loosen and dislodge
Where is Intergranular Corrosion predominant?
Metal alloys
What causes some metals to be sensitive to corrosion?
Improper heat treatment or welding which weakens the grain boundaries and adjacent grain areas
What is the most common prevention for intergranular corrosion?
Properly controlled heat treatment of the metal (after manufacture or welding) which helps to stabilize the grains and their boundaries
What are 13 general categories of Corrosion Mechanisms?
- Uniform
- Localized
- Galvanic
- Concentration Cell
- Pitting
- Selective Leaching
- Hydrogen Induced
- Mechanically Assisted
- Flow Induced
- Microbiological
- Atmospheric
- Stray Current
- Intergranular
