Pitting on Al Tank Walls - Anthracite? 1

[kmer]

The interior tank walls (5058H116 Aluminum) of a conventional filter are deeply pitted. The pitting seems to be mainly just above or right at the anthracite (top) free surface.

The pitting is generally conical in shape. Diameter 0 to 12mm and 0 to 2 or 3 mm deep.

The tanks are 10 years old and go through backwash cycles every 30 to 40 hours, which include air scouring.

I've just started searching for answers.

 

[sbush]

We see this in many filters (both aluminum and painted steel). During backwash the top of the filter bed expands upward and the media granules are moving. This action abrades the surface of the tank wall. Aluminum (even marine grade Series 5000) developes a protective layer of oxidation on the surface (like rust on carbon steel). When this protective layer is removed, the oxidation process starts all over again only to be removed again and again with subsequent filter cleanings. Over time, the metal becomes thinner in the effected area and pitting will occur.

You could add a barrier of protection by applying an ANSI/NSF-61 approved coating, like an amine cured epoxy (with a specific primer that is suitable for use on aluminum surfaces). If you have the capability or can find someone to do it, metalizing (a.k.a. thermal spraying) with pure aluminum wire would be an excellent and inexpensive solution. We do it in our factory but not in the field. We purchase aluminum wire from Metco. Metco might be able to recommend an applicator in your area.

Another option is to simply wait until the metal erodes to the point where it leaks, then weld an aluminum plate over the entire effected band above the media. The new plate can even be a little thicker than the wall material.

S. Bush

http://www.water-eg.com

 

[sbush]

Here is link to Metco's Web site:

http://www.sulzermetco.com/en/DesktopDefault.aspx/tabid-1740//3392_read-5304

S. Bush

http://www.water-eg.com

 

[kmer]

I didn't know for sure, however, did speculated that it may be abrasion from the air scour. It was also suggested to me to consider chemical reaction with impurities in the aluminum.

I'm inclined to go with the former based on your information. The link was also very informative.

Thanks,

Kmer