Chapter 3

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,

(Eq. 36)

Oxygen reduction in acid solutions,

(Eq. 37)

Hydrogen evolution in neutral or basic solutions,

(Eq. 38)

Oxygen reduction in neutral or basic solutions,

(Eq. 39)

Metal oxidation is also a complex process and includes the hydration of resulted metal cations among other subsequent reactions.

(Eq. 40)

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 nature of a metal determine its susceptibility to corrosion. Among them is metal’s position in the galvanic series, the relative areas of the anode and cathode, the purity of the metal, physical state of the metal, its passivity, as well as nature of its corrosion products and its oxide film formed on the substrate surface.

3.1.1 Position in Galvanic Series

When two metals are in electrical contact, the more active metal higher up in the galvanic series that has the greater oxidation potential constitutes the anode in the presence of an electrolyte, and suffers corrosion. The rate and severity of corrosion depends on the difference in their positions in the galvanic series. The greater the difference, the faster is the corrosion of anodic metal.

3.1.2 Relative Areas of the Anode and Cathode

The rate of corrosion is greater when the area of the cathode is larger. When the cathodic area is larger, the demand for electrons will be greater, and this results in an increased rate of dissolution of metals at anodic regions. The corrosion of the anode is directly proportional to the ratio of the cathodic area to anodic area. Rapid and severe corrosion is observed if the anodic area is small due to heavy current density at the anodic area.

3.1.3 Purity of Metal

The presence of impurities leads to the formation of local electrochemical cells. In other words, the impurities present in a metal create heterogeneity, and thus galvanic cells are set up with distinct anodic and cathodic areas in the metal. The higher the percentages of impurity present in a metal, the faster the rate of corrosion of the anodic metal. For instance, impurities such as lead and iron in zinc result in the formation of tiny electrochemical cells at the exposed part of the impurity, and the corrosion of zinc around the impurity takes place due to local action. Corrosion resistance of a metal may be improved by increasing its purity.

3.1.4 Physical State of the Metal

Metal components subjected to unevenly distributed stresses are easily corroded. Even in a pure metal, the areas under stress tend to be anodic and suffer corrosion. As an example, caustic embrittlement corrosion in a metal takes place in stressed parts such as bends, joints, and rivets in boilers.

3.1.5 Passivity or Passivation

The phenomenon in which a metal or an alloy exhibits much higher corrosion resistance than expected from its position in the electrochemical series is known as passivity or passivation. The formation of a very thin protective and invisible film around 0.0004 mm thick on the surface of the metal or an alloy makes it noble. One example is steel containing Ni and Cr. Chromium (Cr) forms a protective layer of Cr2O3 on the steel, making it passive in oxidizing environments. Gold (Au) and platinum (Pt) are chemically very inert and hence show superior corrosion resistance properties. The elements or alloys can be formatted in a series with decreasing tendency of anode formation or nobility, as shown below:

Na > Mg & Mg alloys > Zn > Al > Cd > Fe > steel and cast iron > Pb > Sn > Cu > Ni > Cr > stainless steel > Ag > Ti > Au > Pt

3.1.6 Nature of the Corrosion Product

If the corrosion product is soluble or volatile in the corroding medium, then the underlying metal surface will be exposed readily, and corrosion occurs at a faster rate; however, if the corrosion product is insoluble in the corroding medium, forming a film at the surface, then the protective film formed tends to suppress further corrosion. If the corrosion product is oxide, the rate of corrosion mostly depends on the specific volume ratio; the greater the specific volume ratio, the lesser is the oxide corrosion rate.

3.1.7 Nature of the Oxide Film

Metals such as Mg, Ca, and Ba form oxides with volumes less than the volume of the metal. Hence, the oxide film formed is porous, through which oxygen can diffuse and bring about further corrosion. On the other hand, metals like Al, Cr, and Ni form oxides with volumes greater than that of metal, and the non-porous oxide film so formed protects the metal from further corrosion.

3.2 Nature of the Corroding Environment

The nature of the corroding environment is the other main factor affecting a metal’s susceptibility to corrosion. The effect of the temperature, humidity level, pH, dissolved oxygen concentration and the formation of oxygen concentration cells, nature of the electrolyte, flow rate, presence of corrosive ions and presence of impurities are a few important factors to cite under this category.

3.2.1 Effect of Temperature

The rate of corrosion increases with increasing temperature since the rate of chemical and electrochemical reactions and the rate of ions increase, which is why stress corrosion and intergranular corrosion are usually observed at high temperatures. Additionally, a passive metal may become active at a higher temperature.

3.2.2 Dissolved Oxygen Concentration and Formation of Oxygen Concentration Cells

The rate of corrosion increases with an increased supply of oxygen, which is the reason why the corrosivity of water decreases with temperature since dissolved oxygen content decreases with temperature. The regions where oxygen concentration is lesser become anodic and suffer corrosion. Corrosion often takes place under metal washers, where oxygen cannot diffuse readily. Similarly, buried pipelines and cables passing from one type of soil to another suffer corrosion due to differential aeration such as lead pipeline passing through clay and then through sand. The part of the lead pipeline that passes through clay gets corroded since clay is less aerated than sand.

3.2.3 Nature of the Electrolyte

The nature...