What is galvanic corrosion?
Galvanic corrosion, also known as bimetallic corrosion. This type of corrosion is a common form of corrosion degradation that in most cases with proper design, corrosion is completely preventable.
Definitions of some organizations of galvanic corrosion:
International Committee ASTM G01, defines galvanic corrosion as accelerated corrosion of metal due to electrical contact with a more noble metal or non-metallic conductor in a corrosive environment.
international organization NACE describes galvanic corrosion as current-related corrosion caused by the electrical connection of different electrodes in an electrolyte (ionic conductor).
International Organization for Standardization (ISO) Explains galvanic corrosion: the corrosion caused by salt corrosion performance(A combination of short circuits of different electrodes connected in series with an ionic conductor).
Other organizations have specific definitions for corrosion. These organizations include the Electrochemical Association, the American Water Association, and the American Chemical Society.
What these definitions have in common is that galvanic corrosion involves the electrical interaction of at least two different metals or non-metallic conductors in the environment. Which accelerates the corrosion of at least one of them.
Galvanic corrosion more information:
Common agents include dissimilar metals, electrical contact, and conductive electrolytes in contact with them. . In the absence of any of these factors, galvanic corrosion will not occur.
Galvanic corrosion accelerates the natural corrosion of a metal in an electrolyte. . Even without galvanic corrosion, metals may undergo uniform corrosion, groove corrosion, cavitation, or other types of corrosion.
Galvanic corrosion has an accelerating effect on other forms of corrosion and in some cases causes some kind of corrosion. Which would not be seen otherwise.
Galvanic corrosion detector:
Two hundred years ago, Luigi Galvani and Alessandro Volta discovered a phenomenon known as galvanic corrosion.
Where there is electrical contact between different metals that are also in contact with the same conductive medium (electrolyte), the less noble metal (anode) is attacked. The other metal, the cathode, is protected by this. This is the principle of cathodic protection of metals.
In practice, the rules for the use of unconventional metals are routinely ignored when selecting materials for installations, so galvanic corrosion is a common corrosion phenomenon.
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Conditions for galvanic corrosion
Galvanic corrosion may occur if the following conditions are met:
The electrolyte is in contact with both metals
This electrolyte does not always have to be aggressive to any metal. It may be in the form of some liquid in which both metals are immersed, but just as dense a Layer or a wet solid, such as soil, salt deposits, or corrosion products , can act as an electrolyte.
There is an electrical connection between the two metals
In general, this does not mean direct physical contact between metals. This is unnecessary as long as the current passes from one metal to another. The junction itself does not have to be immersed in the electrolyte.
There is a potentially sufficient difference between the two metals
There is a potentially sufficient difference between the two metals, so that a galvanic current of a certain intensity occurs.
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What factors can affect the severity of galvanic corrosion?
The corrosion behavior of a bimetallic pair is determined by the corrosion potential of each metal separately in the existing electrolyte. Accordingly, galvanic assemblies have been collected in which metals and metal alloys are generally classified as seawater from noble to uneven in a given environment. However, the interpretation of this galvanic series requires a bit of caution.
The amount of possible difference does not affect the speed of reactions in any way because it depends on other factors.
- Polarization effect
- Electrolyte composition
- Aeration and flow rate
- Level ratio
It is known by the polarization of any phenomenon that prevents or completely prevents the external movement of metal ions from the anode or electrons from the cathode.
For example, the deposition of zinc hydroxide on a zinc anode and the formation of a passive oxide shell on stainless steel or titanium that acts as a cathode. Hydrogen adsorption causes the cathode to polarize.
The most common form of polarization is concentration polarization, which results from an increase in the concentration of metal ions at the anode or an increase in the concentration of –OH (increase in pH) at the cathode. The recent increase can lead to hardening deposition, as a result, corrosion is inhibited by the formation of a protective Layer of gypsum rust.
In the case of galvanic bonding, polarization of the reduction reaction at the cathode is predominant.
Electrolyte composition has a great effect on the rate of galvanic corrosion. High conductivity tends to promote it, while low pH causes hydrogen to form at the cathode.
In pure water (distilled or mineral), corrosion of bimetals rarely causes problems. Also, the size of the affected area depends on the conductivity of the solution.
Adjacent to the joint surface of two metals, the effect of galvanic corrosion is generally most severe. As the distance from this point increases, the intensity of the attack decreases. In low-conductivity solutions, corrosion is limited to sharp grooves on the joint surface.
Aeration rate and fluid flow rate
In galvanic corrosion, the cathodic reaction generally involves the reduction of dissolved oxygen. Just like the corrosion of a metal. Galvanic corrosion depends in part on the amount of oxygen reaching the metal surface from the electrolyte surface.
This release (and thus corrosion) by increasing the flow rate accelerates, which increases the intensity of electrolyte containing oxygen.
In neutral electrolytes, single metal corrosion and bimetallic corrosion can often be suppressed by removing oxygen. Under such anaerobic conditions, however, cathodic depolarization, resulting in corrosion, can be caused by sulfate-reducing bacteria (SRBs).
In certain metals and alloys, flow velocity has another effect that is clearly visible in seawater. This effect can act in two ways: As the flow rate increases, copper and capronickel become more active and more corrosive. While materials such as stainless steels are more noble and therefore less corrosive.
The explanation is that the latter materials become passive in good aeration and lubrication solutions, making them resistant to corrosion. Aluminum, stainless steel and titanium have stable oxide skin and tend to polarize.
In aerated solutions, the oxide film thickens, so that the galvanic corrosion of the coupled metal continues to be further reduced.
In copper and noble metals such as platinum and silver, the natural oxide layer formed is much thinner. It rapidly converts to metal, where it acts as an efficient cathode without polarization, resulting in galvanic corrosion.
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5 Ways to prevent galvanic corrosion
Measures used to prevent galvanic corrosionare to remove the elements that are responsible for its formation. However, these countermeasures are very effective when implemented in the early stages of project design.
Galvanic corrosion (also called bimetallic corrosion or corrosion of dissimilar metals) is a process of electrochemical degradation. This corrosion occurs when two different metals come in contact with each other in the presence of an electrolyte.
This type of corrosion is characterized by rapid corrosion of one metal, while the other metal is largely unaffected. In other words, a metal becomes an anode and preferably corrodes. It therefore sacrifices itself while protecting itself from another metal, the cathode.
Galvanic corrosion is relatively aggressive and causes millions of dollars in damage annually. Reciprocal measures to prevent its formation should be considered in the conceptual or early stages of the design process.
(The concepts of corrosion control in the equipment design process are discussed in detail.) The causes are very different and can involve several different types of chemical reactions. Therefore, it is necessary to have an understanding of the underlying mechanisms and factors affecting the development of this type of corrosion.
Causes of galvanic corrosion
The main driving force in galvanic corrosion is a property known as potential difference.
When a metal is immersed in an electrolyte, it uses the potential of the electrode. The amount of electrode potential for different metals is shown in a table known as the galvanic series. Therefore, the potential difference between the two metals is the difference between the potentials of their respective electrodes as defined in the galvanic series.
When two metals are in contact with each other while in the presence of an electrolyte, the potential difference between them causes electrons to be transferred from the anode (metal with more electronegativity) to the cathode (metal with more electro positivity).
This transfer of electrons leads to a series of oxidation and reduction reactions, which cause galvanic corrosionof the anode.
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Ways to prevent galvanic corrosion
Measures to prevent galvanic corrosion are generally based on the removal of the underlying element. Which mostly include blocking the electrical path in the metal or electrolyte parts of the system, removing oxygen from the electrolyte and introducing corrosion inhibitors.
One of the most effective ways to cut off the electrical path in an electrochemical cell is to place a non-conductive material between the points of contact of the metals connected to each other.
Galvanic corrosion occurs because electrons move from the anode to the cathode, creating a galvanic electric current in the system.
Insulation blocks the flow of electrons, thus preventing oxidation and reduction reactions. In practice, insulation is usually done using bushes, washers, and polymer or elastomer-based coatings.
For example, in the oil and gas industry, epoxy washers reinforced with non-conductive glass (GRE) They are usually placed between the flanges of the connecting pipes to disrupt the electrical conductivity between adjacent pipelines.
(How to use gaskets has been discussed in how new methods of extracting and transporting oil and gas affect the corrosion of pipelines.)
One of the main elements needed to cause galvanic corrosion is the electrolyte. The electrolyte contains ions that facilitate oxidation and reduction reactions in the galvanic cell. Therefore, measures including separation of bonded metals and electrolyte can be effective in preventing galvanic corrosion.
This is achieved by using water-repellent compounds that act as a barrier between the metal substrate and the electrolyte solution. colors, coatings, oils and greases have all been used extensively in a variety of industries.
Select the appropriate connection metals
Galvanic corrosion can also be prevented by minimizing potential differences between metals. As mentioned earlier, electrons flow from the anode to the cathode due to the potential difference, which acts as the driving force. The greater the potential difference, the higher the induced galvanic current and the more severe the corrosion rate.
Conversely, there are metals with little potential difference between them, and the goal is to select bonded metals with the potential of similar electrodes, ie metals that are close together in the galvanic series, to reduce the possibility of galvanic corrosion.
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Corrosion inhibitors are compounds (usually liquid) that are added to electrolytes to suppress the chemical reactions that cause galvanic corrosion.
Inhibitors work in a variety of ways, mostly involving complex chemical processes. However, the inhibitors that are most effective against galvanic corrosion are those that remove dissolved oxygen from the electrolyte solution.
Oxygen depletion reduces the likelihood of a reduction reaction occurring at the anode. Because cathodic reactions depend on anodic reactions, the galvanic process stops.
Minimize contact levels
Several studies have shown that the amount and intensity of galvanic corrosion is affected by the ratio of the cathode region to the anode region.
The higher the level of the cathode relative to the anode (That is, the ratio of cathode to anode is higher), the greater the reduction rate at the anode. Therefore, galvanic corrosion will be more severe In contrast, the smaller the cathode relative to the anode region, the less harmful it is consequently.
In the early stages of design, it must be ensured that the anodic metal surface is as high as possible relative to the cathode. For example, steel fasteners on an aluminum plate will perform better than aluminum fasteners on a steel plate.
Preventing galvanic corrosion usually involves taking countermeasures in the early stages of project design. Understanding the mechanisms involved in this electrochemical reaction is key to selecting the most appropriate and effective preventive measures for a given situation.
It should also be noted that several protection methods can be implemented simultaneously to increase protection and higher levels of performance.
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