One of the three is galvanic corrosion which I have talked more about specifically in another post.
Today will be a more general explanation of the three corrosion types, which are all localized corrosion formed on a passive aluminum surface.
- Pitting Corrosion
- Crevice Corrosion
- Galvanic Corrosion
A passive aluminum surface is found in the pH range of 4 - 9. The passive layer could be the natural formed aluminum oxide layer or the anodic aluminum oxide formed by anodizing.
There are two main reasons for localized corrosion
- the type and aggressiveness of the environment
- the chemical and metallurgical structure of the aluminum
In environments in which the anodic film is insoluble, corrosion is localized at weak spots in the oxide film around the intermetallic phases, and takes one of the forms described in the following sections, if it occurs at all. Local corrosion can only be found when aluminium is passive, covered by the oxide layer.
Pitting Corrosion is the removal of metal at localized sites on the anodized surface, resulting in the development of cavities. For pitting to occur an electrolyte must be present. The weakness in the oxide around the intermetallic particles will cause a spot where two different potentials are found, corresponding to the pure aluminum and the intermetallic phase. This leads to the destabilization of the oxide protecting the matrix around the particle and, thereby initiating corrosion in the form of a pit.
Crevice corrosion occurs in the presence of a crevice formed between two aluminum surfaces, or between an aluminum surface and a metallic or non-metallic material. Localized corrosion in the form of pits or etch patches may occur. An important variable in crevice corrosion is the width of the crevice opening. The aluminum-silicon alloys corrode many times faster than the aluminium-magnesium alloy when crevice corrosion has started.
Galvanic corrosion is due to an electrical contact with a more noble metal or a non-metallic conductor in a conductive environment. The galvanic corrosion is very dependent on the cathode reaction. The efficiency of this reaction will determine the corrosion rate. The most common examples of galvanic corrosion of aluminum alloys are when they are joined to steel or copper and exposed to a wet saline environment.
So how will you as an end user find these three corrosion types on your anodized surface.
Architects will probably see all three but galvanic corrosion will be the biggest issue for the architect as when mounting aluminum fronts to a steel frame with the wrong steel bolts. This could cause structural, safety problems whereas pitting corrosion and crevice corrosion often are seen as more decorative corrosion problems.
Designers will definitely see all three too. The pitting corrosion is found on for example screen doors and crevice corrosion can be seen in corners of window frames. The reason for the latter are that the profiles are anodized in long lengths and then cut, leaving the cut ends of the profiles unprotected. These unprotected ends will together with humidity form a superb environment for crevice corrosion.
This photo shows a severe attack by crevice corrosion on electrolytic colored, anodized aluminum. The owner has tried to stop the corrosion attack by applying silicone but that can actually make the corrosion attack even worse.
Engineers have to keep all three corrosion mechanisms in mind when using aluminum in all kinds of applications.Especially when considering which environment the application is to be used and what kind of alloys are specified. The anodizing can be a superior choice but also a surface treatment which makes it all worse.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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