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The Chemistry in Corrosion (Implications (Rusty car mufflers and the outer…
The Chemistry in Corrosion
Bonding
After this compound is made, it is hydrated with an electrolyte such as water. The product is a reddish- brown hydrated metal oxide that is known as rust.
Iron (III) oxide is an inorganic compound land one of the three main oxides of iron.
Fe2O3 is Iron (III) oxide. It forms when when free oxygen molecules bind to iron. Without this compound, the process of rusting and corrosion could not occur.
This occurs after water and carbon dioxide combine to form carbonic acid, and water is separated into hydrogen and oxygen allowing for the free oxygen to bind to the iron.
The bonding in iron (III) oxide is polar covalent however it does show ionic characteristics as iron is a transition metal bonded to oxygen, a nonmetal. The bonding isn't ionic due to the fact that for it to be ionic, there needs to be a greater electronegativity difference.
this compound has the crystal structure of rhomohedral, and a coordination geometry of octohedral.
Electrochemistry
Equation for Corrosion
4 Fe(s) + 3 O2(g) ⇌ 2 Fe2O3(s)
Fe is being oxidized in the reaction as it goes from an oxidation state of 0 to +3. O2 is being oxidized as it goes from having an oxidation state of 0 to -2. Fe lost 3 electrons whereas O2 gained 2 electrons.
Iron (Fe) has multiple oxidation states as many transition metals do, and can have an oxidation states of +2 or +3. However, in this equation, Fe has a oxidation state of +3.
This is a synthesis reaction where A + B=C
A synthesis reaction is a type of reaction in which multiple reactants combine to form a single product.
When a water droplet comes in contact with the metal, a voltaic cell is created.
This process requires both water and oxygen for rusting to occur.
When iron corrodes it forms a red/brown hydrated metal oxide (Fe2O3•xH2O). The contact between the iron (III) oxide and water created the chemical reaction and the reddish/brown rust to form.
This process occurs when water reacts with iron. First, the water combines with carbon dioxide in order to for a weak carbonic acid. This acid will start to dissolve the iron and while that is occurring, some water is being separated into hydrogen and oxygen. Some of these oxygen molecules attach to the iron creation iron oxide and releasing some electrons. these electrons flow to the cathode which has some metal less reactive than iron. After all this occurs rust is fomed and the flaky, reddish-brown color is present.
Corrosion is a galvanic process where metals deteriorate and corrode until there is nothing left.
A galvanic or voltaic cell relies on electric currents in order to produce a chemical reaction.
When the water and iron come in contact, Fe(OH)2 is formed. After it is oxidized, rust is produced.
The final equation for the rust once the voltaic process and oxidation has occurred is 4Fe(OH)2(s) + O2(g) -> 2Fe2O3 •H2O(s) + 2H2O(l)
The equation at the cathode is:
O2(g)+4H+(aq)+4e−→2H2O(l)E°=1.23V
The cathode is the positive charge as it is being reduced and gaining electrons. Additionally, The E° value of 1.23V proves that it is the reduced cathode.
When a piece of iron is left to sit out, and is exposed to excess moisture, the process of rusting and corrosion will occur. This process can occur much faster in the presence of salt water.
The equation at the anode is:
Fe(s)→Fe2+(aq)+2e− E°=−0.45V
The anode has the negative charge as it is clearly being oxidized with the iron(Fe) going from an oxidation state of 0 to +2. You can also tell this equation is oxidized due to the negative E° value of −0.45V.
The study of the interchange between chemical and electrical energy.
Out of the many metals that can be influenced by corrosion, iron is the one most influenced and affected by it. the way it is oxidized in the voltaic cell is stronger than with the other metals.
Implications
Rusty car mufflers and the outer bodies of the car can develop holes in them due to rust.
iron is a good conductor of electricity, however, rust is an insulator which can hinder the conductivity.
rusting can cause batteries to become unusable due to the fact that current is unable to pass through.
Rust can eat away and corrode structures like bridges and the foundations of buildings.
an estimated 100 billion dollars per year is spent in the United States to replace iron-containing objects that have been damaged or destroyed due to corrosion.
In the US alone, a 2002 study reported annual direct corrosion costs of $8.3 billion. Those costs included $3.8 billion for bridge replacement, $2 billion for bridge decks, $2 billion to repair concrete substructures and $0.5 million for maintenance coating of steel elements.
Kinetics
Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs).
Studies the rate at which a chemical process occurs.
when looking at the rate of reaction for anything, their are always factors that affect the rate. Factors like the concentration of reactants, temperature, pressure, and particle size always affect the rate law.
In the specific case of rusting some of these factors among others play a role in the rate law.
additionally, the type of reactant plays a large role in the rate law. The electrolyte or the liquid reactant like water affects the rate law. For example, salt water causes a faster reaction than fresh water would. The metal would also play a part in the rate, iron and steel react faster and rust quicker than other metals would.
Additionally, the environment plays a role, as it needs to be in an area where water and oxygen are present. if the metal is submerged or in an area without oxygen then corrosion cannot occur.
temperature plays a role as although temperature doesn't play a part in the actual equation, a suitable temperate is needed for moisture to be in the area and allow for the water to react with the metal.
The oxidation rate that iron would have would be a parabolic rate due to the fact that parabolic rates are typical for metals with thick coherent
oxides such as iron.
Solutions
inhibitors
Chromates, silicates, and organic
amines are common inhibitors, they inhibit corrosion and as a result prevent rusting and degradation of metals.
Coatings
a protective coating on the iron or steel being used can protect it from reacting with water and becoming corroded. These coating can be metallic or nonmetallic but both fulfill the job or protecting the metal.
Metallic
similar to cathodic protection, a metallic coating uses a more reactive metal that is stronger and can stand the acidic process more than iron and steel.
These coatings can be replaced and sacrificed more easily than the metal it is protecting, so this way it is safer for structures and allow them to last longer.
Nonmetallic
Like organic coatings, inorganic coatings for corrosion applications serve as barrier coatings
these coatings made from porcelain enamels, chemical-setting silicate cement linings, glass coatings and linings, and other corrosion resistant ceramics allow for protection.
Cathodic Protection
Cathodic protection suppresses corrosion and its effect on the metal and integrity of whatever structure it is used in.
Thier are two forms of cathodic protection: Impressed-current and sacrificial-anode system.
Impressed- current
uses a power source to force current from the anodes to the structure protecting it.
sacrificial-anode system
uses active metal anodes like magnesium or zinc to provide the cathodic-protection current.
metals like magnesium and zinc are more reactive causing these metals to be oxidized instead of iron. Magnesium is a stronger metal and wont corrode as easily, and it is easier to replace a magnesium rod or coating then entire iron or steel foundations.
