What should a "new" AL tank look like inside?

[RiverRat]

OK here goes. I bought a 30cf pony at the LDS last night. Nice and shiny. How old could it be right? Well like an idiot I blindly trusted them and was a little annoyed when I got home and saw the date stamp of 07@02. So basically 2.5 years and I need a hydro. It was also empty, no air but with valve attached. So I start to thinking, not really good to sit around for 3 years without air in it. I stick my flashlight in it just now and, I know aluminum won't rust, but it looks like black rust on the bottom, like pitted type spots. And on the sides 1-2 inches up. Is this Aluminum oxide "rust". Now I'm concerned it'll fail hydro in 2 years.........Did I get burned here????? I'm going back tomorrow as I paid new price on this. Never ends..............

 

[android]

aluminium oxide is typically a grey/white powder or hazy coating on the surface of the metal.

 

[RiverRat]

Yeah that's what I thought, with a flashlight sure looks dark though???
Even if the LDS passes the VIP it's the hydro in a few years I'm concerned about.
Especially if something got in there.

 

[FredT]

Look not at the color, but at the finish. The inside should have a reflective finish, as it should match the finish on the punch that formed it. Some tightly adhering ALO2 will form as the blank cools, but the surface finish will still be bright.

BTW Use a rifle/shotgun bore light or other light pipe system to get light inside the tank to inspect it. The surface reflection off the neck and threads when using a flashlight masks the internal info as the "excess" light causes the pupils to contract. It's sort of like trying to get a clean picture on film with an F22 aperature when the light meter says to use an F4 opening.

FT

 

[DA Aquamaster]

A mini mag light works well as a tank inspection light in a pinch. Just screw the reflector end off and the rest of the light can be lowered through the neck on a piece of string.

Many dive shops will discount a tank with a hydro date older than a year as the owner is not going to get a full 5 years out of the tank before it has to be hydro'd. A tank with a 2.5 year old hydro should come with a pretty healthy discount.

 

[RiverRat]

Thanks guys! You're all the best! I'll modify a light to get a better look then go hammer the dive shop as I paid $120.00 for this sucker.

 

[RiverRat]

OK, I modified a small light. Yes it looks much better to the eye lowered in. What looked like rust pitting (I used to do autobody so familier with it) looks more like "staining" now. Dark stained areas. There seems to be a little particulate matter (looks like solder spatter?) in the center and a few little "dings" or nicks. Not sure if that was from someone dropping something in there doing an inspection. I guess my question now would be just what would fail a tank in VIP? Nicks? Here's a pic that came out ok showing "staining" etc.

tank

 

[FredT]

Looks like a new tank. The staining is probably caused by oxidation on the blank before punching to extrude the side walls. Notice the flow lines up the side of the tank. These indicate it was pre-existing at manufacture. The shiny finish on the stain also indictes it was there when the tank was punch formed.

 

[RiverRat]

Looks like a new tank. The staining is probalby caused by oxidation on the blank before punching to extrude the side walls. Notice the flow lines up the side of the tank. These indicate it was pre-existing at manufacture. The shiny finish on the stain also indictes it was there when the tank was punch formed.

Thanks for the crash course in VIP Fred! You the man.

 

[Nemrod]

Aluminum does not rust, only ferrous metals rust. The scuba tanks are made from 6061 alloy I believe which is extremely resistant to corrosion and oxidation. Aluminum will oxidize, some alloys more than others, but unless other factors are involves such as the presence of corrosive chemicals, salts, water etc, it is self limiting. As soon as a oxide layer forms it shields the underlying alloy from the air and virtually no more oxidation occurs. Since several have stated that the shiney surface on the "stained" areas indicates the stain was there prior to forming I surely agree. Your only beef is the age of the hydro, other than that the tank is a new tank and will serve you well. N