[INFOI] Artikel Korosi (in English)

D.C. Silverman, Aqueous Corrosion, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM
Handbook, ASM International, 2003, p 190–195

CORROSION is a process created by the interaction (reaction) between a material, often a metal or alloy, and its environment that results in degradation of that material. Corrosion is affected by the properties of both the metal or alloy and the environment. In this brief overview focusing on degradation of metals and alloys in aqueous systems, the environment variables shown as follows are addressed:
· pH (acidity)
· Oxidizing power (electrochemical potential)
· Temperature and heat transfer
· Velocity (fluid movement)
· Solution components and their concentration

One point to bear in mind is that corrosion is a process, not a property. That distinction means that the corrosion resistance of a material depends as much on the environmental components and system dynamics to which the material is exposed as it does to the chemical composition and structure of the material itself. The discussion is not meant to be all-inclusive but to provide an overview of the complex effects that environmental variables can have on corrosion and to emphasize some of the more important relationships among them. Often, particular effects can only be deduced from carefully planned experimental testing designed to duplicate the actual system. The more understanding one has of how environmental variables might affect corrosion, the better the chances that the experiment will simulate the actual conditions. The influence of biological organisms on these environmental variables is also an important consideration.
Thermodynamics provides a theoretical framework within which the effects of several environmental variables might be pictured. In aqueous corrosion, the format often used is the potential-pH diagram, or Pourbaix diagram (Ref 1). The expanded portion of the potential-pH diagram of iron at 25 °C (77 °F) shown in Fig. 1 is considered as an example of how this framework might be used (Ref 2). These diagrams are thermodynamic
and show the most stable state of the metal in an aqueous solution. The dependence of iron corrosion on oxidizing power (electromotive force), acidity (pH), temperature, and species concentration.
1. For example, suppose the corrosion potential lies at -0.5 V standard hydrogen electrode (SHE at a pH of 8. The most stable state of iron is Fe2+, indicating that iron dissolution is possible. If the pH is increased to 10 (the acidity is decreased), the most stable state becomes magnetite (Fe3O4), and most likely, the corrosion rate of iron would greatly decrease, its surface becoming oxidized. If the pH is then decreased to approximately 8.5, the most stable state (Fe2+ or Fe3O4) would depend on the concentration of the dissolved iron species. The concentration of dissolved species could influence the corrosion rate. A change in temperature would alter the
entire diagram, changing both the areas of stability and the components within those areas.
The simple example of Fig. 1 shows the dominating role that environmental variables can play in corrosion. Complex interrelationships can exist. The combined values of the variables pH, potential, concentration, and temperature not only affect corrosion but also affect the action of each variable.

Reference :

1. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of
Corrosion Engineers, 1974
2. D.C. Silverman, Presence of Solid Fe(OH)2 in EMF-pH Diagram for Iron, Corrosion, Vol 38 (No. 2),1982, p 453