Corrosion

What is Corrosion

 

In general, corrosion can be defined as "exposure of a material to chemical, electrochemical, or wear and tear through metallurgical interaction between the material and the environment." Incoming, this occurs in a slow but ongoing character. In some cases, the corrosion effects occur only by staining the metal in the form of a thin adhesive film and can have a subsequent corrosion retarding effect. In most cases, the corroded product has a large and porous surface character and is unprotected.

 

Corrosion, which is one of the most important problems of the industry, causes billions of dollars of damage in the industry every year. Despite having a great deal of knowledge about the subject, corrosion is a complicated problem and there is still a lot to learn despite extensive research. In some cases, such as direct chemical effects, corrosion can become highly inevitable.

 

The main reason for corrosion is that metals are unstable in their purified pure forms. Metals always tend to return to their forms in nature.

 

 Electro-Chemical Principles

Corrosion is an electrochemical process caused by the conversion of some or all of the metal from metallic to ionic. Corrosion requires a current between a particular surface of metal and an electrolyte side. Electrolyte is any solution containing ions. For example, pure water contains equal amounts of positively charged hydrogen ions (H +) and negatively charged hydroxyl (OH-) ions.

 

In this case, the electrolyte can be ordinary water, brine or acid or any condensed alkaline solutions. To complete the electric current, there is always a need for two electrodes, one anode and the other cathode, and these must be connected. Electrodes can be two different metals or two different areas on the same metal. The junction between the anode and cathode is achieved through a metallic bridge and generally with a simple contact. In order for electric current to occur, there must be a potential difference between the electrodes.

If we put a simple piece of iron in a hydrochloric acid solution, we observe the formation of dense hydrogen gas. There are numerous fine anode and cathode areas on the metal surface caused by residues called minor inclusions, surface irregularities, different stress occurrences, areas and different cell orientations or changes in environmental conditions. These conditions are shown schematically in Figure-1.

 

On the anode side, positively charged iron atoms emerge from the surface and mix into the solution as positively charged ions, while those in the form of negatively charged electrons remain in the metal. Electrons meet and neutralize positively charged hydrogen ions that come to the surface through the electrolyte solution on the cathode side. Positive ions that have lost their charge become neutral atoms and interact with hydrogen gas.

 

Thus, as the process continues, the oxidation and corrosion of the iron occurs on the anode side and hydrogen output on the cathode side. The dissolved metal ratio is directly proportional to the number of electrons formed depending on the strength and potential of the metal.

Figure 1:

Ions in the anode in small regional cell battery formation

and schematic representation of the formation of hydrogen in the cathode

 



Figure 2:

Polarization of the regional cathode with a layer of hydrogen

For the corrosion to continue, it is necessary to remove the corrosion layers from the anode and cathode. In some cases, hydrogen gas formation at the cathode is very slow and the formation of the hydrogen layer on the surface of the metal slows down the reaction.

 

This situation shown in Figure-2 is called cathotic polarization. However, the level of dissolved oxygen in the electrolyte solution reacts with hydrogen and reaches the level to form water, causing corrosion to progress.

 

Removal of this layer for iron and water depends on the effective dissolved oxygen density adhering to the cathode in the water.

 

This effective density depends on ventilation, amount of movement, temperature, amount of dissolved salt and other factors, respectively. The products of the anode and cathode processes often react with each other in solution and cause the formation of many known and visible corrosion products.

 

The products of the anode and cathode processes often react with each other in solution and cause the formation of many known and visible corrosion products.

 

For example, they encounter the movement of hydroxyl ions in water from the cathodic reactions of iron to the anode through electrolyte and the opposite movement of iron ions. These ions form iron hydroxide as it is degraded in Figure-3.

 

This then reacts with the oxygen in the solution to form the ferric-hydroxide, which will precipitate and form the rust layer of iron. Depending on the alkalinity, the amount of oxygen and agitation of the solution, this rust is formed on the surface away from the iron surface or in a way that further affects and increases the corrosion process.

Figure 3:

The formation of iron hydroxide in the rusting of iron

 

 

 

 

 


Factors Affecting Corrosion

One of the most important factors affecting corrosion is the potential electrical difference created by different metals when connected together or inserted into an electrolyte solution. This potential is due to the chemical nature of the anotic and cathotic regions. Indicator values ​​that some metals can be anodic when compared to hydrogen are given in the standard "Electro-motion Power Series" table in Table-1. By taking the value of the standard hydrogen cell to zero, the electrode potential values ​​of the metals are seen comparatively. These values ​​are given in descending order. More active metals show a stronger tendency to dissolve in solution than the metals at the top of the list, at the bottom of the list.

The Electro-Motion Series contains only metals under conditions where the series are defined. Metals worked in especially electrolyte solutions containing salt solution are included. Under real operating conditions, their behavior may differ in other electrolyte solutions. Instead of Electro-Motion Series, similar "Galvanic Series" are used in an experiment on the composition of metals in a wide range of environmental conditions. Here, "galvanic" is the meaning that produces and uses electric current. Table-2 gives examples of some metals and alloys moving at high speed in sea water. The materials at the top of this list are anodic and subject to corrosion, while the materials at the bottom of the list are cathodic and galvanically protected. The electrical potential difference between the two metals is related to the difference between them in the galvanic series. Combining materials close together in the galvanic list causes corrosion to occur more slowly than those that are far apart.

 

If metallic ions are removed by the method of forming an insoluble composite layer deposited on the anode, this layer completely covers the surface, isolating the metal and completely preventing corrosion. This type of oxide layer formation occurs in aluminum and chrome. The effect of dissolved oxygen on corrosion is twofold, it shows the effect on cathodic depolarization. If oxide formation removes metallic ions from metal, corrosion will increase. If oxygen gets hydrogen around the cathode, corrosion will increase. The amount of cathode area affects the effectiveness of oxygen in removing hydrogen. It will be easier for hydrogen to react with oxygen in a large cathode area.

 

Rinsing or mixing process has an effect on increasing the corrosion rate as it contacts the fresh corrosion solution with the metal.

 


Tablo -2: Metal ve Alaşımların Deniz Suyu İçindeki Galvanik Serileri


Anodik (Korozyona Uğrayan) Uç

Magnezyum
Çinko
Alüminyum
Kadmiyum
Alüminyum Alaşımları
Karbon Çeliği
Düşük Alaşımlı çelik
Dökme Demir
Paslanmaz Çelik (aktif)
Sarı Pirinç
Alüminyum Pirinç
Kırmızı Pirinç
Bakır
Alüminyum Bronz
Bakır-Nikel Alaşımı
Nikel (pasif)
Gümüş
Paslanmaz Çelik (pasif)
Titanyum
Altın
Platinyum
Katodik (korunan) Uç

 

 

Table - 1

Electro - Motion Force Series

Electrode Reaction

Standard Electrode Potential, E º
Volt, 25 ºC

Electrode Reaction

Standard Electrode Potential, E º
Volt, 25 ºC

K = K+  + e-

-2,922

Co = Co++ + 2e-

-0,277

Ca = Ca++ + 2e-

-2,87

Ni = Ni++ + 2e-

-0,250

Na = Na+ + e-

-2,712

Sn = Sn++ + 2e-

-0,136

Mg = Mg++ + 2e-

-2,34

Pb = Pb++ + 2e-

-0,126

Be = Be++ + 2e-

-1,70

1/2H2 = H+ + e-

-0,000

Al = Al3+ + 3e-

-1,67

Cu = Cu++ + 2e-

0,345

Mn = Mn++ + 2e-

-1,05

Cu = Cu+ + e-

0,522

Zn = Zn++ + 2e-

-0,762

Ag = Ag+ + e-

0,800

Cr = Cr3+ + 3e-

-0,71

Pd = Pd++ + 2e-

0,83

Ga = Ga3+ + 3e-

-0,52

Hg = Hg++ + 2e-

0,854

Fe = Fe++ + 2e-

-0,440

Pt = Pt++ + 2e-

1,2

Cd = Cd++ + 2e-

-0,402

Au = Au3+ + 3e-

1,42

In = In3+ + 3e-

 

Au = Au+ + e-

1,68

     

 

Specific Types of Corrosion 

Specific Corrosion definition is generally used for specific types of corrosion in industrial applications. When the entire surface of the metal is rusted at the same rate, this is called uniform corrosion. This type of corrosion is not very common in metals because very rarely metals are found in a homogeneous state.

 

Nucleation (Pitting) Corrosion

 


Inclusions are seen in inhomogeneous sections of metals due to open areas that are not visible to the eyes. These inhomogeneous sections create potential differences in metal and cause the formation of deeply insulated holes. Starting in a small area caused by small irregularities on the surface that cannot be seen, the pit grows rapidly and takes the form of a large indentation. Following this recess, the pit expands under the metal surface. Then, the formed metal shell collapses and a pit caused by corrosion on the surface appears. Figure-4 shows the nucleation corrosion that occurs on a metal surface immersed in sea water. A small opening in the protective layer causes nucleation corrosion to start. For example, when the chrome coating (if not) on old car bumpers cracks, nucleation corrosion begins.

 

 

 

 

Figure-4: Nucleation corrosion of a piece of metal immersed in sea water

 

 

 

 

Cowing Corrosion

As seen in Figure-5, it is formed by the bubbles and cavities collapsing in the liquid. Like repeated loads applied to the surface, the vibration movement between a surface and liquid causes great stresses when these bubbles form regularly and collapse or burst. These depressions or explosions gradually create high stress pulses that cause particles to break off the surface and eventually pits and indentations. Stainless steels have a very good resistance to pitting corrosion. However, the resistance of cast iron, bronze and steel castings to this type of corrosion is low.

 


Figure-5: Corrosion corrosion caused by the collapse or explosion of air bubbles formed at the points where the regional pressure is lower or equal to the vapor pressure of the liquid.


December (Crack) Corrosion

 

The bonding of the two metals exposed to the corrosion environment is the formation of corrosion in the holding areas. We know that the small gaps in the joints usually contain more solution and this wetness or solution drying takes longer than other regions. Likewise, these intervals cause corrosion under normal operating conditions, even in liquid or solution. As can be seen in Figure 6, corrosion potential conditions are created due to a potential difference in oxygen concentration.

 

Oxygen is easily accessible to the outside of the junction, which will act as a cathodic tip in the circuit. The part inside the junction point will serve as anode. Due to the gradually decreasing oxygen density in the part between the junction point, the electric potential difference will increase.

 

If this situation persists, corrosion will begin over time at the junction due to the decrease in oxygen density. Corrosion always begins where the oxygen concentration is low. Gaps and cracks can also lead to differences in metal-ion density.

 

For example, the metal-ion density in the range may be higher than outside. As can be seen in Figure 7, this can cause corrosion in the region where the metal-ion density is low, that is, in the outer part of the junction region. The most effective way to prevent such corrosion is to make a design that will completely eliminate the gaps at the design stage, or to fill all gaps and joints that are thought to cause problems with filler metal.

 

Figure - 6: Gap corrosion due to the difference in oxygen density.

Figure - 7: Gap corrosion due to the difference in metal-ion density

 


Wear Corrosion

 

It is a general type of surface wear caused by vibration that occurs due to impact, impact or friction on high connection surfaces, high load surfaces. Such corrosion occurs in parts that are pressed or clamped tightly and that operate under instantaneous variable loads. Abrasion corrosion damages the gears, impairs their dimensions and reduces their fatigue strength.

 

If two metals rub, the applied forces will cause small particles to be welded on the surface. With the ongoing movement, these small particles move, scratch the surface and react with the atmosphere to form particles in the form of dust at the junction. In Figure 8, you can see the wear corrosion of a shaft in the oil pump gear during the fatigue test. There are many ways to prevent wear corrosion.

 

The most convenient way is to eliminate the vibration caused by tight clamp or to mount the part in a way that will not cause more vibration. Some of the other methods are; In order to increase the hardness of the joining surfaces, to put a rubber-type piece on the joints that will absorb the movement, to lubricate in a dry environment, to close the entire surface with materials such as rubber filling in order to disconnect the material from the atmosphere, etc. 'Dr.

 

Figure - 8: Wear corrosion of the shaft in an oil pump drive gear during fatigue test

 

Intergranular Corrosion

 

It is a corrosion that occurs when alloys (or cells) have a potential difference in alloys. Such corrosion usually occurs with the formation of another phase that precipitates in a solid solution. Since the formation of sediments at the grain boundaries is much faster, the regions around the grain boundaries are filled by the dissolved element and the cell boundaries disappear to a certain extent. (See Figure -9) When viewed superficially, the damage may not be fully visible and in most cases there will be a noticeable decrease in mechanical properties.

Stress (or Stress) Corrosion

 

It is a type of corrosion that occurs under some environmental conditions due to the external stresses of metals or internal stresses left from cold rolling. The resulting cracking may be in the form of intergranular or over grains or a combination of both. The magnitude of the stress that causes cracking depends on the environment causing the corrosion and the structure of the base metal.

 

Tension corrosion is a very important type of corrosion as it can occur in many metals. Although almost all metals are affected in terms of stress corrosion, a condition that creates cracks or fractures for one metal may not naturally affect the other metal in the same way. It is therefore difficult to predict exactly where it will occur. The nitrogen contained in the steel makes the steel prone to stress corrosion cracking in some nitrate solutions.



Steels containing aluminum are more resistant to stress corrosion cracking, as aluminum reacts with nitrogen to form aluminum nitrate. Some stainless steels are more sensitive to stress corrosion cracking in chlorite containing environments such as sodium chlorite, such as calcium chlorite. The stress corrosion crack problem is a problem that occurs when austenitic steels are used in chlorite-containing environments. Figure - 10 shows the tension corrosion crack of a 304 quality austenitic stainless steel.

 

Ferritic stainless steels are more resistant to stress corrosion crack problems than austenitic or martensitic stainless steels. Stress corrosion crack significantly reduces the mechanical properties of the material. The effect of some types of corrosion on the mechanical properties of the material can be seen in Table-3.

 

Table - 3: Effect of Some Corrosion Types on the Mechanical Properties of the Material

Corrosion Type

Weight Loss (%)

Penetration Depth (%)

Losses in Properties (%)

Breaking Muk.

Flowing Muk.

Elongation

Smooth

1

1

1

1

1

nucleation

0,7

5

7

5

15

Intergranular

0,2

15

25

20

80

Voltage

0,1

100

100

100

100

 



Figure - 9: Intergranular corrosion (500x) after 316 quality stainless steel is kept in boiling sulfate-sulfuric acid solution for 27 hours.



Figure - 10: Stress corrosion cracking caused by exposure of 304 quality stainless steel to chlorite containing water

 


Galvanic Corrosion  

 

As a result of the contact of two metals with each other in an environment prone to forming corrosion, it is formed on the interface where they come into contact.