Factors Influencing Corrosion
There are four basic requirements for corrosion to occur. Among them is the anode, where the dissolution of metal occurs, generating metal ions and electrons. These electrons generated at the anode travel to the cathode via an electronic path through the metal, and eventually they are used up at the cathode for the reduction of positively charged ions. These positively charged ions move from the anode to the cathode by an ionic current path. Thus, the current flows from the anode to the cathode by an ionic current path and from the cathode to the anode by an electronic path, thereby completing the associated electrical circuit. Anode and cathode reactions occur simultaneously and at the same rate for this electrical circuit to function. The rate of anode and cathode reactions (that is the corrosion rate), is defined by American Society for Testing and Materials as material loss per area unit and time unit.
In addition to the four essentials for corrosion to occur, there are secondary factors affecting the outcome of the corrosion reaction. Among them there are temperature, pH, associated fluid dynamics, concentrations of dissolved oxygen and dissolved salt. Based on the pH of the media, for instance, several different cathodic reactions are possible. The most common ones are:
Hydrogen evolution in acid solutions,
Oxygen reduction in acid solutions,
Hydrogen evolution in neutral or basic solutions,
Oxygen reduction in neutral or basic solutions,
Metal oxidation is also a complex process and includes the hydration of resulted metal cations among other subsequent reactions.
In terms of pH conditions, this book has emphasized near neutral conditions such as the media leading to less emphasis on hydrogen evolution and oxygen reduction reactions since both hydrogen evolution and oxygen reduction reactions that take place in acidic conditions are less common.
Among cathode reactions in neutral or basic solutions, oxygen reduction is the primary cathodic reaction due to the difference in electrode potentials. Thus, oxygen supply to the system, in which corrosion takes place, is of the utmost importance for the outcome of corrosion reaction. Inhibitors are commonly tested in stagnant solutions for the purpose of weight-loss tests, thus ruling out the effects of varying fluid dynamics on corrosion. Weight-loss tests are performed at ambient conditions, thus effects of temperature and dissolved oxygen amounts are not tested as well, while for salt fog chamber tests, temperature is varied for accelerated corrosion testing. For both weight loss tests and salt fog chamber tests, however, dissolved salt concentrations are commonly kept high for accelerated testing to be possible.
When corrosion products such as hydroxides are deposited on a metal surface, a reduction in oxygen supply occurs since the oxygen has to diffuse through deposits. Since the rate of metal dissolution is equal to the rate of oxygen reduction, a limited supply and limited reduction rate of oxygen will also reduce the corrosion rate. In this case the corrosion is said to be under cathodic control. In other cases corrosion products form a dense and continuous surface film of oxide closely related to the crystalline structure of metal. Films of this type prevent primarily the conduction of metal ions from metal-oxide interface to the oxide-liquid interface, resulting in a corrosion reaction that is under anodic control. When this happens, passivation occurs and metal is referred as a passivated metal. Passivation is typical for stainless steels and aluminum. Corrosion of a metal surface mainly depends on nature of metal and the nature of the corroding environment.
3.1 Nature of the Metal
Certain characteristics that make up the nat