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The corrosion process involves the deterioration of a substance, usually a metal, or its properties because of the reaction with its environment.
All corrosion of iron at normal ambient conditions is what process?
The formation of the surface layer, while relatively thin can form an effective barrier against further attack, and thus slow the rate of the corrosion process.
What is the most common product of corrosion?
The anode is the part of the metal that corrodes (i.e dissolves in the electrolyte).
What four elements are necessary for the corrosion cell to form?
The greater the current flow in the corrosion circuit,
The greater the metal loss
What is Ohm's law?
- E is the driving voltage of the circuit.
- I is the current magnitude
- R is the resistance of the circuit.
Where the metal surface does oxidation take place?
The It is the more noble region on the electrode (metal surface) where the electrons are consumed.
While oxidation occurs at the anode, what occurs at the cathode?
Reduction of material
What is a driving force for the corrosion process?
The potential differences
Define Return Path (metallic pathway)
The return path connects the anode and the cathode and allows passage of electrons, generated the anode to the, when corrosion takes place on a metal surface there is always a metal pathway joining the anode to the cathode
And electrolyte is a medium that conducts Ionic (rather than electronic) current.
Define Corrosion Control
Anode, cathode, return Path, and electrolyte must be present for corrosion to occur. The effort of removing one or more of them is Corrosion Control
What are the four elements of the corrosion cell?
- Return path
What occurs when chemical salts are dissolved in the electrolyte?
It increases the efficiency rate of the corrosion reaction
Define Pitting Corrosion?
If the anodes and cathodes to remain in the same place for a period of time, but corrosion is localized, and referred to is pitting corrosion.
The fine Mill Scale
Mill scale is found on new iron and steel in the form of blue-black layers, of iron oxide, some of which are harder than the parent metal. The nanoscale is electrically positive reactive to the iron or steel, so it is Product to the parent metal.
Mill Scale is?
A blue-black layer of iron/iron oxide
Cathodic relative to substrate
Generally removed prior to painting
What is the Galvanic Series?
Galvanic Series is a list of metals in order of their corrosion potentials, with the most easily corroded, or most active, at the top, and the least easily corroded, or least active, at the bottom
List the Galvanic Series
- Carbon steel
- Stainless Steel
What are the general rules of Galvanic (dissimilar metal) corrosion?
When dissimilar metals are connected, the most active (or anodic) metal corrodes more rapidly, while the more noble is better protected corrodes less rapidly.
As the potential difference between these two dissimilar metals increases, the rate of Galvanic corrosion increases.
Corrosion rates increase as potential differences between metal increases.
What are the four factors that affect the rate of corrosion?
How does temperature affect corrosion reaction?
Corrosion reactions are electrochemical in nature and usually accelerated with increasing temperature, therefore corrosion proceeds faster in warmer environments than colder ones.
Hiding chemical salts affect the corrosion process?
Chemical salts increased the rate of corrosion by increasing the efficiency (connectivity) of the electrolyte.
It also acts as a hygroscopic material extracting moisture from the air which increases the corrosion and non-immersed areas.
Define Time of Wetness
Time of wetness refers to the length of time atmospherically exposed substrate has sufficient moisture to support the corrosion process.
What are the six Corrosion Environments?
- 1 Chemical/Marine
- 2 Chemical with high humidity
- 3 Marine with high humidity
- 4 Chemical with low humidity
- 5 Rural with low humidity
- 6 Rural
Define Chemical/Marine Environment?
A very severe environment causes rapid rusting. Airborne salts and chemical pollutants may serve to stimulate corrosion. Humidity and seawater provide electrolytes, which also hasten the process.
Define Chemical with High Humidity Environment?
This environment is highly corrosive, because of its gases, chemicals, and high humidity.
Define General Corrosion
General corrosion results in a relatively uniform loss of material over the entire surface.
Thinning over the affected surface.
Define localized corrosion
Localized corrosion occurs at discrete sites on the metal surface. The areas immediately adjacent to the localized corrosion normally corrode too much lesser extent.