Technical information

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

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.

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

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.

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.

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.

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.

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