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I got a water tank with a mechanical assembly in.

The assembly consist of parts of anodized aluminium mounted on stainless steel plates (AISI 304). This is submerged down in fresh water. The tank is open to air, and there is very little movement/current in the water.

After a few weeks there has developed some "fuzzy" dots on the surface of the anodized aluminium parts. See image.

Question is what this is?

My first thought was that this is organic material from the water that fastened easier to the rougher alu surface than the steel surface?

However, now I think it is more a chemical reaction due to galvanic currents due to material difference in aluminium and stainless steel. This should dissolve the aluminium and transfer the electrones to the stainless steel plate. But would this make it look like the fuzzy growth on the alu-part?

Or could it be something totally different I haven't though off?

One thing I wonder about in this connection is that the fuzz is not more concentrated close to where it is connected to the AISI 304 plate?

! Note: The alupart is anodized, which should make it electrical isolated on all surfaces. However, the Alu-part and the steel plate is connected with a few screws, and through this has electrical connection.

Photo of the "fuzzy" anodized alu part (Eloksert means anodized)

Caption: Photo of the "fuzzy" anodized alu part ("Eloksert" means anodized)

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  • $\begingroup$ Why did you reject the "organic material" hypothesis? That fuzz sure looks like bacterial growth (no water movement). Can you add a biocide? O3? $\endgroup$ – James Gaidis Jun 11 at 12:55
  • $\begingroup$ I thought it be ut of the question since it only adhered to the aluminium and not any other parts in the pool. (Stainless steel and POM plastic and a rubber band is also in the pool). $\endgroup$ – Jared Hansen Jun 12 at 15:05
  • $\begingroup$ Although there is definitely a biofilm all over. I guess I could add a biocide, but not sure if ozone would be the best. Pool is for testing equipment, and the water should be as "dead" as possible. $\endgroup$ – Jared Hansen Jun 12 at 15:11

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