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Step up to the Plate
We don't really think of metals used in a cooling system as soft and vulnerable, but they are. Especially to oxidization, corrosion, and cavitation.
Oxidation occurs when there is a loss of electrons in metal atoms. The oxidation of a metal occurs when the metal loses one or more electrons, so that the atoms of the metal go from the neutral state and become a positively charge ion. This commonly results in the formation of a metal oxide (in the case of iron, that is known as rust). Oxidation in a cooling system is the gradual dissolving of the metals.
What are Oxidation Reduction reactions The exact fate of the cations is important to subsequent
processes, but the primary effect is that atoms leave the metallic state, and
the metal deteriorates. The nature of the reduction step in corrosion sometimes varies with the metal and the environment to which it is exposed. For most metals in an aqueous environment, the important
reduction half-reaction is the reduction of hydronium ions (a hydronium ion is
simply a hydrogen ion attached to a water molecule).
Corrosion is a chemical reaction in which molecules break down due to a chemical reaction with it's surroundings. Most often, corrosion is the electrochemical oxidation of a metal, such as iron "corroding" to form iron oxide. metal corrosion can be defined as the destructive attack of a metal through interaction with its environment. In order for electrochemical reactions to occur, four components must be present and active. Corrosion of metals takes place through the action of electrochemical cells. Although this single mechanism is responsible, the corrosion can take many forms. Through an understanding of the electrochemical cell and how it can act to cause the various forms of corrosion, the natural tendency of metals to suffer corrosion can be overcome and equipment that is resistant to failure by corrosion can be designed. These components are the anode, cathode, electron path, and electrolyte. As in all chemical reactions, corrosion reactions occur through an exchange of electrons. In electrochemical reactions, the electrons are produced by a chemical reaction, the oxidation, in one area, the anode, travel through a metallic path and are consumed through a different chemical reaction in another area, the cathode. In some cases, such as the common dry cell battery, electrochemical reactions can be used to supply useful amounts of electrical current.
Components.
In order for electrochemical reactions to occur, four components must be
present and active. These components are the anode, cathode, electron path,
and electrolyte.
Anode.
In an electrochemical cell, the anode is the site where electrons are
produced through the chemical activity of the metal. The anode is the area
where metal loss occurs. The metal loses electrons and migrates from the
metal surface through the environment. The electrons remain in the metal but
are free to move about in response to voltage gradients.
Cathode.
The cathode in an electrochemical cell is the site where electrons are
consumed. For each electron that is produced at an anodic site, an electron
must be consumed at a cathodic site. No metal loss occurs at sites that are
totally cathodic.
Electron Path.
In order for electrons to flow from the anodic sites to cathodic sites, the electrons migrate through a metallic path. This migration occurs due to a voltage difference between the anodic and cathodic reactions. Electrons can move easily only through metals and some non-metals such as graphite. Electrons from electrochemical reactions cannot move through insulating materials such as most plastics nor can they directly enter water or air. In some cases, the electron path is the corroding metal itself, in other cases, the electron path is through an external electrical path.
Electrolyte.
Electrolytes are solutions that can conduct electrical currents through the movement of charged chemical constituents called ions. Positive and negative ions are present in equal amounts. Positive ions tend to migrate away from anodic areas and toward cathodic areas. Negative ions tend to migrate away from cathodic areas and towards anodic areas.
Anodic Reactions.
Metal loss at anodic sites in an electrochemical cell occurs when the metal
atoms give up one or more electrons and move into the electrolyte as
positively charged ions.
Typical
Reactions.
The generic chemical formula for this metal loss at anodic sites is:
M ---> M+ + e-
where:
M = uncharged metal atom at the metal surface
M+ = positively charged metal ion in the electrolyte
e-
= electron that remains in the metal
This type of chemical reaction is called
oxidation
even though it does not directly involve oxygen but only results in an
increase in positive charge on the atom undergoing oxidation.
More than one electron can be lost in the reaction as in the case for iron
where the most common anodic reaction is:
Fe --->Fe2++ 2e-
where:
Fe = metallic iron
Fe2+= ferrous ion that carries a double positive charge
Correlation
Between Current Flow and Weight Loss.
For each specific anodic reaction a characteristic number of electrons are
produced in the reaction of one metal ions. Thus, all other things being
equal, the metal loss is proportional to the number of electrons that are
produced. As the electrons produced migrate to cathodic areas through the
electron path, the metal loss is proportional to the current flow. In cases
where more positively charged ions are produced, more electrons flow for a
given number of corroding metal atoms but the current flow remains
proportional to the metal loss.
Cathodic
Reactions.
The electrons that are produced at anodic sites are consumed at cathodic
sites. The type of chemical reactions that consume electrons are called
reduction and have the generic chemical formula:
R+ + e- --> Ro
where:
R+ = a positive ion in solution
e- = an electron in the metal
Ro = the reduced chemical
In reduction, the chemical being reduced gains electrons and its charge is
made more negative. In some cases, where the ion in solution has a multiple
positive charge, the total positive charge on the ion may not be
neutralized. In other cases, the chemical which is reduced may not be a
positive ion but is a neutral chemical which then becomes a negatively
charged ion in solution in a reaction such as:
R + e- --> R- Source : "Corrosion Control" NAVFAC MO-307 September 1992
Cavitations is when
At the suction of a pump, especially if operating near the net positive suction head required ((NPSHR). At the discharge of a valve or regulator, especially when operating in a near-closed position. At other geometry-affected flow areas such as pipe elbows and expansions. Also, by processes incurring sudden expansion, which can lead to dramatic pressure drops. This form of corrosion will eat out the volutes and impellers of centrifugal pumps with ultrapure water as the fluid. It will eat valve seats. It will contribute to other forms of erosion corrosion, such as found in elbows and tees. Cavitation should be designed out by reducing hydrodynamic pressure gradients and ing design to avoid pressure drops below the vapor pressure of the liquid and air ingress. The use of resilient s coating and cathodic protection can also be considered as supplementary control methods.
Further In Depth Reading... It's The Water | Too Much Information | Reserve Alkalinity | Noble Metals | Anode Data | Energy | Hungry Water
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Step up to the Plate
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