"Aluminum corrosion/dissolution"

Question:

"I have an automotive application where polyester film which has been vacuum metalized with aluminum is adhered to a variety of substrates with a pressure sensitive adhesive. On some substrates the aluminum, which is between the polyester film and the pressure sensitive adhesive, corrodes away.

I can create this failure in the lab environment on some substrates.

The samples are set up as follows:
Samples are about 9 inches long and submerged about 4.5 inches into a 4% NaCl solution in a glass jar. This jar and it's contents are then put into a Ziploc plastic bag to avoid evaporation of the water. This package is then put into an oven at 150 deg. Fahrenheit.

1. When the aluminum metalized film is attached to plain steel, there is aluminum corrosion under the film starting from the edge of the film, particularly at the water level and just above. The steel rust considerably. This occurs with in 24 hours.

2. When the aluminum metalized film is attached to stainless steel, there is no aluminum corrosion under the film and there is no rust due to the steel being stainless.

3. When the aluminum metalized film is attached to stainless steel, as in #2 above and the stainless steel is screwed to plain steel, there is aluminum corrosion under the film starting from the edge of the film, particularly at the water level and just above. The plain steel rust considerably. This occurs with in 24 hours.

Measurements indicate that even though the aluminum should be insulated from the substrate by the adhesive, there is some continuity.

Since the aluminum does not corrode when the film is adhered to stainless steel, example #2 above, it does not seem to be simple crevice corrosion or galvanic corrosion.

In examples #1 and #3 above the plain steel is rusting, so it is the anode. Yet the aluminum is corroding. Is the aluminum corroding cathodically? I know aluminum is more active than steel but the surface area differences are huge. Is the greater steel surface area driving the aluminum to corrode cathodically?

A couple of additional experiments were done to explore the possibility of the impact of this galvanic couple.

Samples were made with a double layer of the pressure sensitive adhesive over the aluminized film and attached to plain steel. These samples were tested in the hot salt water as described above. There was almost no aluminum corrosion.

Another set of samples were made with this double layer of adhesive but they were made so that at a point above the water line the aluminum was in direct contact to the steel substrate. These samples were tested in the hot salt water as described above and there was significant aluminum corrosion all around the edges of the film that were below the water line. This suggest to me that a sort of galvanic cell is at work. Yet another sample, however, with the same construction was put into a jar, completely covered with salt water and sealed tight and no corrosion of the aluminum occurred. Indeed the plain steel did not rust much as the oxygen was consumed. Clearly oxygen plays a key roll here.

You can probably tell that I have some knowledge but am not an electrochemist. I think I am actually setting up several different cells with these experiments. In any case can you recommend how I could avoid this problem in real use? Or suggest where I can find information? The simplest idea would be to add something more active into the adhesive to protect the aluminum. Can you recomend what that might be?"

Answer:

Your experiments, plus knowledge of aluminum corrosion, suggest one explanation for the aluminum corrosion you are experiencing. Certainly, aluminum will act as the anode in a galvanic cell with either carbon steel or stainless. And yes, oxygen does play a role in the corrosion in acting as a cathodic depolarizer. What may be occurring with the aluminum/carbon steel couple, is this: when carbon steel corrodes, hydroxyl ions will concentrate at the cathodic site of the corroding area, unless swept away by liquid movement or further converted to iron oxide. As aluminum is amphoteric, and possesses limited resistance to alkaline environments, I suspect that the aluminum may be corroding due to a localized alkaline environment resulting from the corrosion process. The reason that the aluminum does not corrode with the stainless steel, is that the stainless steel is not corroding, thus not generating the hydroxyl ions.

 

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