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Corrosion Fundamentals

The Nature of Matter

Atoms

Ions

Molecules

Acids and Bases

Atoms:

Model of an AtomAll matter is made of atoms composed of protons, neutrons, and electrons. The center, or nucleus, of the atom is composed of positively charge protons and neutral neutrons. The outside of the atom has negatively charged electrons in various orbits. This is shown schematically in the picture to the right where the electrons are shown orbiting the center, or nucleus, of the atom in much the same way that the planets orbit the sun in our solar system.

All atoms have the same number of protons (positively charged) and electrons (negatively charged). Therefore all atoms have a neutral charge (the positive and negative charges cancel each other). Most atoms have approximately the same number of neutrons as they do protons or electrons, although this is not necessary, and the number of neutrons does not affect the identity of the element.

The number of protons (atomic number) in an atom determines which kind of atom we have, and the atomic mass (weight) of the atom is determined by the number of protons and neutrons in the nucleus (the electrons are so small as to be almost weightless).

There are over 100 different elements that have been discovered. These are shown in the Periodic Table of the Elements below. The letter symbols for the elements come from their Latin names, so for example, H stands for hydrogen, C for Carbon, O for oxygen, while Fe stands for iron and Cu stands for copper.

Periodic Table of Elements

Only a few of the elements are common, and most corrosion occurs due to only a dozen or so metallic elements (iron, aluminum, copper, zinc, etc.) reacting with common nonmetallic elements (oxygen, chlorine, sulfur, etc.).

Ions:

Ions are formed when atoms, or groups of atoms, lose or gain electrons.

Metals lose some of their electrons to form positively charged ions, e.g.

Fe+2, Al+3, Cu+2, etc.

Nonmetals gain electrons and form negatively charged ions, e.g.

Cl-, O-2, S-2, etc.

Molecules:

Compounds are groups of metals and nonmetals that form distinct chemicals. Most of us are familiar with the formula H2O, which indicates that each water molecule is made of two hydrogen atoms and one oxygen atom. Many molecules are formed by sharing electrons between adjacent atoms. A water molecule has adjacent hydrogen and oxygen atoms sharing some of their electrons.

Acids and bases:

Water is the most common chemical on the face of the earth. It is made of three different constituents, hydrogen ions, hydroxide ions, and covalently bonded (shared electron) water molecules. Most of water is composed of water molecules, but it also has low concentrations of H+ ions and OH- ions.

Neutral water has an equal number of H+ ions and OH- ions. When water has an excess of H+ ions, we call the resultant liquid an acid. If water has more OH- ions, then we call it a base.

We measure the strength of an acid or a base on the pH scale. pH is defined by the following equation:

pH = -log [H+]

A detailed explanation of pH and acids and bases is beyond the scope of this web site. It is sufficient to note that some metals (e.g. zinc and aluminum) will corrode at faster rates in acids or bases than in neutral environments. Other metals, e.g. steel, will corrode at relatively high rates in acids but have lower corrosion rates in most neutral and basic environments.

Even a strong acid, with a pH of 0, will be less than 1/1000th by weight hydrogen ions. Neutral water, at a pH of 7, is less than 1 part H+ in 10 million parts covalently bonded water molecules.

Mnemonic device: Many people have a hard time remembering whether an acid or a base has a high pH number. Just remember that acid comes before base in the alphabet and that low numbers come before high numbers. Acids have low numbers (less than 7), bases have high numbers (greater than 7). Neutral waters have pH's near 7 and tend to be relatively noncorrosive to many materials.

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