How to Solve Filiform Corrosion Issues When Spraying on Aluminum Alloy
Q. We are spraying on an aluminum alloy and are having problems with aluminum filiform corrosion. Can you help me understand what is causing this?
A. Aluminum alloys used in the industry are composed predominantly of aluminum mixed with alloy metals such as copper, zinc, tin, iron, magnesium, manganese, chrome, titanium and silicon, as well as a few other metals and in various combinations.
The alloy compositions are registered with the Aluminum Association and the wrought alloys are defined by a series number given by the International Alloy Designation.
For example, the 6000 series aluminum are alloys with magnesium and silicon, and these are quite common. The magnesium improves the strength and the silicon improves the casting. The alloy is selected based on the engineering needs for tensile strength, workability, formability and corrosion resistance.
Aluminum is a very thermodynamically reactive metal, much more reactive than iron, in fact. However, aluminum owes its corrosion resistance to a thick oxide film that passivates the metal with strong adhesion properties and protects the underlying aluminum metal. This is very unlike iron, which has an oxide layer (Fe2O3) that will flake off of the iron and will continue the corrosion cycle.
However, aluminum is not immune to corrosion. Furthermore, the various alloy metals that are typically used to enhance the forming and end use properties can participate in the corrosion cycle. There are many types of corrosion failures, such as pitting, intergranular corrosion and crevice corrosion, just to name a few.
Of great interest is the corrosion type known as filiform corrosion, as it can be easily observed on coated aluminum substrates.
Filiform corrosion is most common in environments with a relative humidity between 75 and 90 percent and temperatures ranging from 70 to 110°F. The humidity is the most critical variable for the corrosion to propagate, because this is necessary to dissolve the salt ions.
The corrosion typically starts where there is an imperfection in the substrate and coating layer. The imperfection can be introduced from a scratch or a stone chip that weakens the adhesive bond between the substrate and the coating.
The corrosion starts at this locus, which forms the head of the corrosion defect. The corrosion normally appears as a distinct thread-like filament, like a worm track, that appears under the coating surface.
The worm track often appears white and is visually striking. The width of the track can be from about 0.1 mm to 4 mm and can be, for example, 2 cm long. The corrosion typically penetrates about 15 microns deep into the aluminum.
This effect can be seen with many types of coatings, usually in the range of 100 microns dry film thickness used over aluminum substrates.
The corrosion mechanism is an anode-cathode electrochemical cell. The head of the worm-like filament is the anode, and the cathode is the longer tail segment. Typically, ions like Cl- (and others) and H+ are present in the aqueous solution in the head, which propagates the corrosion. The pH values in the head can be less than 2. In the tail, the cathode reduction produces hydroxyl ions. The key reaction in the anode “head” is the oxidation of aluminum to the Al3+ ion, which then reacts with the hydroxide ions produced in the tail to form insoluble precipitates laying down additional tail material. The precipitates lift the coating above the substrate to create the track. The tracks can grow at a rate of 0.1 mm/day. Thus, the filiform worm track continues to progress on the substrate.
The damage is not extensive to the aluminum but is cosmetically objectionable, especially when the track is long and white in color.
This type of filiform corrosion can damage all types of aluminum products such as wheels, automobile bodies and aircrafts. To repair the effect requires sanding and the application of a new layer of coating. To prevent filiform corrosion, proper surface pretreatment is required.
The most important factor for the coating is to have very strong adhesion to the substrate to prevent the corrosion from starting.
Tim December is a technical expert for BASF Automotive Coatings Solutions. Visit basf.com.
Originally published in the May 2017 issue.
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