Corroded aluminum water tank

[Flesh]

I have an aluminum storage tank used for tap water. It is corroding from the inside out in a very strange manner. I have included some photos at the link below. Does anyone recognize this type of corrosion and possible cause? I think I may have two types of corrosion occurring: #1 pitting and #2 exfoliation - I don't know how to explain it any other way.

Other notes - There appears to be hard water scale building up on the inside walls. The tank material is 5052H32. The tank was found in this condition after someone noticed a pinhole leak. EDS analysis of the scale found calcium and silicon. Chlorine and sulfur was found in the corrosion "pits".

http://www.imageviper.com/pages/show_album.php?aid=595

Thanks.

 

[metengr]

Flesh;
Your photos of the corroded areas look to be similar to biological attack (or MIC). I have seen this some time ago. The local areas in your photos appear like tubercles, but I can't be sure. Have you ruled our MIC?

 

[Flesh]

metengr,

I had an independant lab look at a sample and they had stated that tubercles (from MIC) were not present. I would describe the pits in fig03 as "flowering", where the center portion appears to be drilling through the material and "leaves" surround the pit. I have not yet opted to perform a culture of a water sample. Is it expensive to perform such a test?

 

[MechanicalChef]

This is just me thinking out loud, but the first thing that came to mind was an issue of precipitating out alloying elements. (It was an issue I encountered w/ anodizing 2024). But based one the conditions and the alloying elements of 5052, that isn't a possible correlation.

Out of curiosity, what is the source of the water entering the tank? Is it water pumped from the ground? Logically, if you lived somewhere such as Florida with large amounts of limestone in the soil, that could explain the deposited minerals (calcium, silicon, sulphur).

Why the pattern formed the way it did is far beyond my expertise. Lastly have you tested the water to see if it's pH at level that would be particularly harmful to Al?

 

[Flesh]

MechanicalChef,

Water source could be city or well water. Still trying to confirm which one.
pH was measured to be 7, neutral.

 

[metengr]

Flesh;
I would have a sample of the water tested from the tank. One additional question, is the corrosion just on the tank bottom or random (sides and bottom)?

 

[metengr]

Flesh;
One last question, was the corroded sample subjected to a detailed metallographic examination to confirm corrosion pitting or other form of penetration (intergranular corrosion)?

 

[Flesh]

metengr,
Corrosion occurs on sides and bottom, but most severe attack is on the bottom. Figures 1 and 3 represent the corrosion found on the bottom.

The corrosion is intergranular, which follows the grain direction for 1 to 2 cm. The second phase particles, along the rolling direction, remain unharmed.

Slight correction on the material used: the bottom(shell) is made of 5052H32 and the heads are made of 3103(temper unknown). I don't think this should cause any corrosion problems though.

 

[metengr]

Flesh;
If the 5052 was not properly solution annealed and has been cold worked, exposure to elevated temperature (even 100 deg C) either during fabrication or in service can result in sensitization from Al-Mg precipitates occurring along the grain boundaries. You probably need to replace the material because corrosion will continue.