ie-Chemistry P15
Corrosion

chemistry with electron transfers

in early development

rule

Electricity is one of the forms of energy.  Energy is the powerful concept that we use to unify our understanding of the operation of the universe.  Other forms of energy are heat, light, sound, and motion.  We find that energy can be stored in forces such as gravity (hydroelectric dams), chemical bonds (electric batteries), and nuclear forces (as produces starlight).  Energy can be converted from one form to another.  This can spontaneously occur when the system's degree of randomness (we calculate as entropy) increases.  We can reverse this process by constructing a greater system in which the total entropy still increases, but energy is transferred so that the part of our choice becomes more organized.

Corrosion is an effect of this spontaneous process where energy stored in chemical bonds is shared with their environment as the molecules take on arrangements containing less stored energy (enthalpy).  In the process electrons are transferred.  The recipient molecules can be some distance from the donor molecules.  The process is often considered undesirable and efforts made to slow or prevent it.  But if carefully engineered, the process can be used as in the case of dry cells and fuel cells as a portable source of energy for release when and where needed.

Iron and steel

bloomeryIron is not the easiest metal to utilize.  So it was not the first metal used by humans.  But because of its abundance and the relatively low cost to obtain desirable properties, Iron is the metal most used today.

  1. The Iron is mined from concentrated ore deposits in the ground primarily as iron oxides, Fe3O4 and Fe2O3, but often accompanied by other minerals such as silicates (quartz = SiO2), Aluminum (clays), Titanium and Phosphorus ores.

  2. The Iron in the ore is separated in a blast furnace in a smelting operation.  The Oxygen is removed from the crushed ore by Carbon (coke, coal cooked to remove volatile gases, oils and water) which has a greater affinity for Oxygen at high temperature.  Blasts of air are added to form carbon monoxide which serves as the reducing agent providing electrons for the Iron and in exchange bonding with a second Oxygen. Limestone is used as a flux to help lower the mixture's melting temperature and to remove impurities.  Upon heating it produces the more reactive lime by:
    The half reactions showing the transfer of electrons and the overall reaction are
    blast furnace
  3. Steel is made from the liquid iron by alloying with other elements, primarily Carbon which, by providing an impurity in the metal lattice, hardens and strengthens the steel, but makes it more brittle.

  4. The steel is rolled, forged, or cast into usable shapes for use by society.

  5. Exposure to Oxygen and water promote the spontaneous corrosion (rusting) of the Iron back to some form of iron oxide.

Long before the chemical changes were understood, it was noted that the iron ore seemed reduced in mass as it was smelted to Iron.  We now generalize that term to describe processes where electrons are gained.  Since electrons and electric charge are conserved, the electrons must be equally produced by another substance in a process we call oxidation.    (Note the o mnemonic to help remember what oxidation means.) Understanding the chemical reactions and balancing the reactions can help optimize the amounts of materials added to avoid leaving excesses to impair the quality of the product and the profits.

Investigation

There are several ways to try to prevent or retard corrosion of the iron.  They include adding another element to the alloy, coating the structure to prevent contact with oxygen and water, and electrically attaching another metal which will sacrificially provide electrons on behalf of the iron.  The two procedures which follow investigate the latter two types of protection.

Procedure 1

  1. Obtain a handful of shiny bright iron nails.  If they are not already shiny, they may be polished with steel wall.

  2. Coat at least one, each with one of as many kinds of coatings as you can find available. Coatings might include various oils and grease, lacquer such as fingernail polish, hand lotion, latex and oil-based paints, tape and plastic wrap.  It might also be valuable to investigate if a partial covering or multiple coats of the material provide better or worse protection.

  3. Either place each nail in a separate container or place them as far apart as possible and a broad flat container.  Cover the nails with a gelatin solution prepared with water and a small amount of table salt.  Some packages of prepared gelatin contain colored dyes which may change color depending upon the acidity.  Wearing protective eye goggles and being prepared to flush away any spell of corrosive material with water, test any potential colored material with the few drops of the acid vinegar and the base ammonia to determine if they may be used as a suitable indicator.  If you can't find any gelatin containing a suitable acid-base indicator, you might investigate if adding crushed berries to the gelatin would work.

  4. Observe the nails daily for at least several weeks nothing any color or other changes.  At the conclusion of the experiment, try to remove some of the protective coating to determine if there is been any change in the nails underneath.

  5. Record your procedure and observations in your journal along with any analysis and conclusions which seemed appropriate.

Procedure 2

  1. Obtain a handful of shiny bright iron nails.  If they are not already shiny, they may be polished with steel wall.  Obtain is many other kinds of metal wire or strips as possible.  For example, you may be able to obtain Copper wire by removing the protective coating from a short section of spare electrical cord.  Aluminum is common as kitchen foil.  If you cannot find Zinc, several galvanized nails, nuts or bolts may substitute.

  2. Wrap it least one nail each with a unique kind of metal.  There may be value in investigating how much of the nail needs to be covered, or if the Iron only needs to touch a second kind of metal.  If you choose to use a galvanized nail as one pre-coated in Zinc, you might file or scrape off some of the Zinc to determine if there is any protection for the bare Iron.

  3. Either place each nail in a separate container or place them as far apart as possible and a broad flat container.  Cover the nails with a gelatin solution prepared with water and a small amount of table salt.  As in the first procedure, having an acid-base indicator in the gelatin might provide additional useful information about the corrosion.

  4. Observe the nails daily for at least several weeks nothing any color or other changes.  At the conclusion of the experiment, try to remove some of the protective coating to determine if there is been any change in the nails underneath.

  5. Record your procedure and observations in your journal along with any analysis and conclusions which seemed appropriate.

Reference

Gary Drigel, Arlyne Sarquis, Mike D'Agostino, Corrosion in the Classroom, The Science Teacher, April/May 2008

rule

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created 10 April 2008
revised 10 April 2008
by D Trapp
Mac made