Acceptable amount of rust?

[Flycaster]

i recently bought a steel 100 and inside is a light film of rust in streaks which trail down from the area of the valve.
I was told they are going to tumble with media to clean it up.
Does anyone have illustrations of what is / isn't acceptable?
Thanks.

 

[tech3324]

VIP - Interior Pictures?

One of my tanks had streaks like yours, lds did a VIP and said it was fine.

 

[Doby45]

It will tumble and look new. It really is hard to kill a steel tank. You need to actively and intentionally neglect it. An annual VIP will catch 99% of the issues with a steel tank before it gets out of hand.

 

[2airishuman]

Opinions vary, photos are hard to take.

The way I look at it, if there are pieces of corrosion that could fall off and pose a hazard of clogging or jamming the 1st stage, the cylinder is due for a shot blast (or tumble). If there is an accumulation of dust or loose corrosion in the bottom of the cylinder then it must be removed and a shot blast or tumble strongly considered. Flash rust that only forms a film with no accumulation of dust in the bottom of the cylinder, even if uneven or streaked, isn't a hazard, though if I'm taking a pile of cylinders in for shot blasting I may add that one to the pile.

I've shown cylinders to experienced PSI inspectors, that concerned me because small crystals were starting to form on the walls, and was told that the cylinder was safe to use but it might not hurt to tumble it next time I get a chance, so that the corrosion would not worsen.

A cylinder would be unsafe for further use only when pitting due to corrosion reaches a certain threshold depth. I'm unsure of the exact rules and when in doubt I have a PSI inspector take a look. I believe it's 1/3 of the wall thickness unless the pitting forms a line or is widespread enough, like a honeycomb, that it would weaken the overall wall. In those cases the remaining material has to be the minimum thickness specified by the manufacturer, which is hard to determine, but usually the cylinders are manufactured close enough to the minimum that these sorts of defects will be cause to condemn a cylinder.

 

[OkByMe]

I have tumbled tanks, washed them out with hot soapy water and before I could get them dry... A slight film of rust was re-forming on the inside. This is completely acceptable and unless you coat the inside with a water soluble chemical which binds to the the surface, you are going to get his. On the ones I bought with the coating, I have abrasively tumbled to remove it. Eventually that coating will start to peel and might enter the valve, the rust film will not.

 

[Jack Hammer]

i recently bought a steel 100 and inside is a light film of rust in streaks which trail down from the area of the valve.
I was told they are going to tumble with media to clean it up.
Does anyone have illustrations of what is / isn't acceptable?
Thanks.

That's sounds like pretty normal flash rust. Not a big deal unless you are getting partial pressure fills. It should tumble out fairly easily.

 

[fmerkel]

Trace surface rust is not an issue. Pitting > rust eating into the metal to the point of weakening it, that is an issue.

Re - partial pressure blending: Outside of being scrupulous about cleanliness, not even sure where trace surface rust is a problem with blending. Rust is iron oxide...that is > already 'burned'. It cannot oxidize even more. Before being 'found' during an annual VIP I would guess LOTS of steel tanks with trace rust get PP blending all the time. Then they get tumbled, and start all over again, especially if the tank owner or fill shop is not good about making sure no water is in the valve or whip.

 

[2airishuman]

I have tumbled tanks, washed them out with hot soapy water and before I could get them dry... A slight film of rust was re-forming on the inside.

There are various other ways to prevent that.

Some people dry their cylinders with a one-second blast of HP air from a full cylinder, which, I'm told, gets rid of the water quickly enough that rust doesn't form. In general any kind of low-temperature drying will produce minimal rust.

It is also possible to dry the cylinders under high vacuum using a two-stage vacuum pump. These are commonly used for servicing refrigeration equipment, and I have one for that reason. It's simple enough to make a hose with a DIN fitting on one end that will fit the pump. Once the cylinder is evacuated, there is no oxygen to speak of, and rust cannot form.

It would also be possible to dry cylinders in nitrogen. While this has been discussed on SB, I'm not aware of anyone actually doing it.

 

[OkByMe]

I was trying to intimate that flash rust is NOT a problem. Oils are and other contaminates like water that can make it into your cylinder by way of gas fills. My 20cu.ft. 100%ppo cylinder has flash rust, I knew it when I screwed the valve in the day I inspected, tumbled and hot cleaned it. It is sitting in my garage @2500psi, doing just fine.

 

[fmerkel]

I've been successful with (final) rinsing tanks with HOT water. Steel being a good conductor, the whole tank gets quite warm. Dump out the water, then immediately stick a reg hose attached to a first stage, but without the 2nd stage reg, all the way to the back of the tank, Stand to the side and turn on the air. You can see the water and steam get blasted out. Pull the hose out slowly trying to move the end around a bit. Make sure you get the shoulder area and threads well. I haven't had rust issues doing it this way....so far.