What is Rust?

How does Rust form?

DEFINITION OF METAL CORROSION.

Metal Corrosion can be d efined as the destructive attack of a metal through interaction with its environment.

FORMS AND MECHANISMS OF CORROSION

DRIVING FORCE FOR CORROSION.

Most metals used in the construction of facilities are subject to corrosion. This is due to the high energy content of the elements in metallic form. In nature, most metals are found in chemical combination with other elements. These metallic ores are refined by man and formed into metals and alloys. As the energy content of the metals and alloys is higher than that of their ores, chemical re-combination of the metals to form ore like compounds is a natural process.

FUNDAMENTAL MECHANISM OF ATTACK.

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 corrode can be overcome and equipment that is resistant to failure by corrosion can be designed.

The Electrochemical Cell.

As in all chemical reactions, corrosion reactions occur through an exchange of electrons. In electrochemical reactions, the electrons are produced by a chemical reaction 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. In marine corrosion, however, the most common result is the transformation of complex and expensive equipment to useless junk.

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 ---> Fe⁺ + 2e^-

where:

Fe = metallic iron

Fe⁺ = ferrous ion that carries a double negative 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- --> R

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, the 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}