Galvanic corrosion of aluminum
It should be emphasized, that the resistance of aluminum and aluminum alloys to normal environmental conditions is very high. The main source of protection against corrosion is strong, self-healing oxide film, which is always present on aluminum in an ambient air atmosphere (Figure 1).
Figure 1 – Natural aluminum corrosion protection – surface oxide film [4]
The main types of aluminum corrosion
For aluminum corrosion the following main types are characteristic [4]:
- general corrosion
- crevice corrosion
- Frettinig-corrosion
- Stress corrosion
- Galvanic corrosion
- Pitting (pitting) corrosion
- intergranular corrosion
- subsurface corrosion
Figure 2 – General corrosion of aluminum: dissolution of natural oxide film
solutions of strong alkalis and some acids [4]
Figure 3 – Crevice corrosion of aluminum [4]
Figure 4 – Fretting corrosion of aluminum: mutual friction of two aluminum components
in conditions of rough contact [4]
Figure 5 – Corrosion of aluminum alloys under stress: under certain conditions
alloys Al-Cu, Al-Mg, Al-Zn-Mg [4]
Figure 6 – Galvanic corrosion of aluminum alloy
It occurs under conditions of wet or wet contact
with another, more “honorable” metal, such as copper [4]
Figure 7 – Pitting (pitting) corrosion of aluminum
under the influence of chloride ions [4]
Figure 8 – Intergranular corrosion and subsurface corrosion [4]
Depending on the environmental conditions, loading and functionality details of any of the types of corrosion can cause premature failure. Moreover, improper use of aluminum parts and products can exacerbate corrosion processes.
Galvanic corrosion of aluminum
The most frequent errors aluminum structural design associated with galvanic corrosion. Galvanic or electrochemical corrosion occurs, when two dissimilar metals form an electrical circuit, closes the liquid electrolyte or a film or a corrosive environment. Under these conditions, the potential difference between dissimilar metals produces electric current, passing through the electrolyte, which (current) leads to corrosion primarily of the anode or less noble metal from this pair.
SUMMARY galvanic corrosion
When two different metals come in direct contact with the electroconductive liquid, experience shows, that one of them can corrode, ie corrode. This is called galvanic corrosion.
Other metal will not corrode, conversely, it will be protected from this type of corrosion.
This type of corrosion is different from the types of corrosion, that might arise, if both of these metals were placed separately into the same liquid. Galvanic corrosion may occur with any metal, as soon as the two different metals are in contact in an electrically conductive liquid.
Appearance galvanic corrosion
The appearance of galvanic corrosion is very characteristic. This corrosion is not to scatter all over the surface of the product, as is the case with the point - pitting - corrosion. Galvanic corrosion is localized in dense aluminum contact area with another metal. Corrosive to aluminum has a uniform character, it develops in the depths of craters, which have a more or less rounded shape [3 [.
All aluminum alloys are subject to identical galvanic corrosion [3].
battery principle
Galvanic corrosion works like a battery, which consists of two electrodes:
- cathode, where the reduction reaction
- anode, where the oxidation reaction.
These two electrodes are immersed in the conductive liquid, which is called an electrolyte. Electrolyte - is usually a dilute acid solution, for example, sulfuric acid, or saline, for example, copper sulfate. These two electrodes connected to an electrical circuit outside, electrons which circulates. Inside the fluid transfer occurs by electric current the ions move. Liquid, thus, provides an ionic electrical connection (Figure 9).
Figure 9 - Principle of a galvanic cell [3]
Figure 1 shows cell, wherein the electrolyte is a sulfuric acid solution. Sulfuric acid is completely dissociated in water (since it is a strong acid) by the formation of H ions+, which define the acidity of the medium. The following electrochemical reaction occurs [3]:
- the zinc anode is oxidized:
Zn → Zn2+ + 2e−
on a copper cathode recovered protons H+:
2n + + 2e− → n2
The complete reaction is:
Zn + H2O → Zn (OH)2 + H2
This cell produces electricity due to the consumption of zinc, which is released as zinc hydroxide Zn (OH)2.
For the cell to work, three conditions must be met simultaneously:
- two different metals, which form two electrodes;
- presence of electrolyte;
- electrical continuity throughout the chain.
If at least one of these conditions is not met, for example, if electrical contact is broken, then the cell will not produce electricity, and oxidation at the anode will not occur (as well as reduction at the cathode).
Conditions for galvanic corrosion
Galvanic corrosion is based on the same principle and in order, for it to occur, the following three conditions must be met simultaneously [3]:
- various types of metals;
- presence of electrolyte;
- electrical contact between the two metals.
Various types of metals
For all metals, which relate to different types of, Galvanic corrosion is possible. Metal with electronegative potential (or more electronegative metal, if they are both electronegative) acts as an anode.
The tendency of various metals to form galvanic vapors and the direction of electrochemical action in various corrosive environments (sea water, tropical climates, industrial atmosphere, etc.) are shown in the so-called galvanic series. The farther removed from each other metals in these rows, the more serious galvanic corrosion can be. In different corrosive environments, these sequences of metals can be different (Figure 10).
The presence of an electrolyte
The contact area must be wetted by an aqueous solution of, to provide ionic conductivity. Otherwise, there is no opportunity for galvanic corrosion.
Electrical contact between the metals
Electrical contact between the metals can take place either by direct contact between the two metals, or via fasteners, for example, bolt.
Figure 10 [1]
As seen from the figure diagrams 10 aluminum and its alloys become anodes in electrochemical cells with most metals, and aluminum corrodes, as they say, sacrificially protect against corrosion and other metal galvanic couple.
Only magnesium, and zinc, including galvanized steel, They are more anodic and therefore, themselves exposed to corrosion, protect it from aluminum.
Aluminum and cadmium at all have almost the same electrode potentials and thus no aluminum, or cadmium are not subject to galvanic corrosion. Unfortunately, cadmium is found to be very toxic, and used less and less, and in many countries simply prohibited, as an anti-corrosion protection.
galvanic couples
The relative location of the two metals or alloys in the galvanic series indicates only the possibility of galvanic corrosion, if the difference of their electrochemical potential is large enough. More than this number does not say anything, and especially nothing - about the speed or intensity of galvanic corrosion. It can be zero or insignificant or even invisible. Its intensity depends on the types of metals, which come into contact - galvanic couple.
Pair: aluminum – unalloyed steel
In building construction aluminum parts, are open to the effects of climate and weather effects, They can be connected with screws of conventional steel. experience shows, that the aluminum in contact with the steel bolts subjected to only very superficial corrosion. The resulting rust, which has no effect on aluminum, completely impregnates the layer of aluminum oxide and forms a spot on the surface. Actually, for aluminum structures in contact with the exposed steel is important influence on its appearance and decorative qualities, rather than the ability to resist corrosion.
This phenomenon has the following explanation:
- are formed on the contact surfaces of the film with the products of corrosion - rust on steel and aluminum oxide on aluminum, which and slow electrochemical reaction.
Pair: aluminum – Cink Steel
Judging by the galvanic series, zinc is more electronegative, than aluminum. Galvanized steel fasteners may, so, used for the connection and assembly of structures of aluminum alloys. It must be remembered, that when the zinc coating becomes too worn, to protect steel and aluminum, comes the previous scenario of contact between aluminum and bare steel [3] .
Pair: aluminum – stainless steel
Although there is a large potential difference between the stainless steel and aluminum alloys - about 650 mV, very rare to see galvanic corrosion on aluminum in contact with stainless steel. Therefore, aluminum structures are very often assembled using stainless steel bolts and screws [3].
Pair: aluminum – copper
Contact between the aluminum alloys and copper, as well as copper alloys (bronze, brass) leads to very little galvanic corrosion of aluminum when exposed to atmospheric conditions. Nonetheless, It is recommended to provide electrical insulation between the two metals, to localize the corrosion of aluminum.
It should be noted, that copper corrosion product is, so-called, times a. This patina - a bluish-green patina on copper, which consists essentially of copper carbonate. This chemically patina effect on aluminum and may be reduced to form small particles of copper. These copper particles, in turn, can cause local pitting corrosion of aluminum [3].
Closer to the touch – More corrosion
Accelerated galvanic corrosion is usually most intense near the junctions of two metals; with distance from the junction, its intensity decreases. Significant effects on the rate of corrosion has a value of cathode surface area ratio, contacting the electrolyte, to the area exposed anode surface. It is desirable to have a small ratio of cathode area to anode area.
How to avoid galvanic corrosion
- Select a pair of aluminum or a metal alloy, which is as close as possible to it in the galvanic series for the corrosive environment under consideration (see. Figure 10).
- Apply the "cathode" fasteners. Avoid combinations with an unfavorable (large) cathode-to-anode area ratio (figure 3).
- Provide complete electrical isolation of the two metals to be joined. This can be accomplished by insulating spacers, bushings, washers, etc.. (drawing 12).
- If you use coloring, you should always paint the cathode. If you paint only the anode, any scratch on it will give an adverse ratio of cathode surface to the anode lead to corrosion and scratches.
- Increasing the thickness of the anode, or set the connection of replaceable massive anode metal gasket.
- If possible, place the galvanic contact is corrosive environment.
- Avoid threaded joints of metal, forming a galvanic couple. Replace them soldered or welded connections.
- If possible, apply corrosion inhibitors, for example, a recirculating fluid systems, which may play a role in electrolyte galvanic corrosion.
- In cases, when metals should remain in electrical contact through external electrical circuit, you need to space them as far apart as possible to increase the resistance of the liquid circuit (electrolyte).
- If necessary, and there, where possible, apply cathodic protection with zinc or magnesium sacrificial anodes.
- In the most aggressive environments only zinc, cadmium and magnesium can be in contact with aluminum without the occurrence of galvanic corrosion. Note, that the use of cadmium coatings is largely limited due to their ecological insecurity.
Figure 11 [1]
Figure 12 [1]
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