Corrosionist  The Website of Corrosion Protection and Corrosion Prevention


You are here >>> Home - Fundamentals of Corrosion Chemistry    
Bookmark and Share

Fundamental of Corrosion Chemistry

 
   
                                               

 

DEFINITION OF METAL CORROSION.

 

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

             

              

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 suffer corrosion 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, 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. 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 --->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

see also:

corrosion types

galvanic corrosion chart

metal rust remover

what is rust

                                  
 
 
 
 
 
  Useful Documents on Corrosion Chemistry and Fundamental of Corrosion  
    
 
 
  The Nernst Equation and Pourbaix Diagrams

This teaching and learning package (TLP) investigates the Nernst equation and Pourbaix diagrams, which are both important parts of electrochemistry and corrosion science.

 
 
 
  Kinetics of Aqueous Corrosion This teaching and learning package (TLP) introduces the mechanism of aqueous corrosion and the associated kinetics  
 
 
 
 
     Corrosion Theory and Corrosion Protection  The annual cost of corrosion and corrosion protection in the United States is estimated by the National Association of Corrosion Engineers (NACE) to be in excess of 10 billion dollars. This figure is perhaps less intimidating considering that corrosion occurs, with varying degrees and types of degradation, whenever metallics are used. b. Corrosion can be mitigated by five basic methods: coatings, cathodic protection, materials selection, chemical inhibitors, and environmental change. A basic understanding of corrosion will enable USACE personnel to comprehend how these methods help prevent corrosion, and it will establish an overall introduction to the purpose for the entire engineer manual on painting   
 
 
      Basic principles of Corrosion 

In this section, the basic principles of corrosion are introduced. In principle this applies for all metals, including aluminium. It consists of two main parts. In the first part the thermodynamics of corrosion reactions will be explained. The thermodynamics determine whether or not reaction can occur. In the second part the kinetics of the corrosion reactions will be explained. The kinetics of corrosion reactions determine the corrosion rate

 
 
 
      An introduction to Corrosion Phenomena The objective of this talk is to provide broad-brush comments on a wide range of corrosion phenomena in order to provide a measure of common ground for the more detailed talks which follow, and also to discuss in slightly greater depth a few topics for which time does not allow individual coverage. The topics discussed are restricted to those included in the area commonly called “wet” corrosion; atmospheric corrosion and corrosion at high temperatures will be treated separately. At this stage, at least, the treatment will not be either deeply chemical or deeply mechanistic, but will be restricted to factual discussion of the ways in which corrosion may lead to materials problems in practice  
 
 
      Beginners Guide to Corrosion What follows is a simple explanation of how corrosion occurs, what the different types are how problems can be solved. It is intended to be used by the non-expert to gain an initial appreciation of the subject before exploring further.  
 
 
     Basic of Corrosion Control A SHORT INTRODUCTION TO CORROSION AND ITS CONTROL CORROSION OF METALS AND ITS PREVENTION  
 
 
     The electrochemistry of Corrosion The surfaces of all metals (except for gold) in air are covered with oxide films. When such a metal is immersed in an aqueous solution, the oxide film tends to dissolve. If the solution is acidic, the oxide film may dissolve completely leaving a bare metal surface, which is said to be in the active state.  
 
 
     Basic of Corrosionwebpage is the first in a series of modules for the chemical process industry that have been written by Bud Ross for the Nickel Development Institute. It introduces the various forms of corrosion: Uniform, galvanic, pitting, crevice, erosion-corrosion, intergranular, corrosion fatigue, envrionmentally-assisted cracking and stress corrosion cracking.  
 
 
     Electrochemistry of Corrosion    
 
 
     Electrochemistry Encyclopedia This site contains popular-science style articles describing many aspects of electrochemistry, crosslinked with the Electrochemistry Dictionary for definitions. New articles are added at irregular intervals, and all articles are periodically updated as recent developments occur in the field.  
 
 
     Fundamentals of Corrosion and Corrosion Control