How fast can rust form in a steel tank?

[ChrisA]

How long does it take for rust to form inside a steel tank? I have a PST LP80 with a May 2004 VIP and had to take the valve off today in june 04. (The tube under valve fell off.) There is a light bit of rust inside. Could it really have been clean in May and now in June be in need of cleaning already?

Since I bought it six weeks ago, The tank has been well cared for and always had at least 500 psi. Only about 10 fills that I know of.

I'm thinking the guy who did the May 04 VIP didn't really do anything other then place the sticker on the tank. I bought the tank "new" and it had been sitting in the store for a year.

I've asked this question also on my "Sounds like a marble" thread but this really is a new subject.

 

[Otter]

Anecdotally, I have heard it can happen very fast. Given the nature of the (likely) corrosive moisture and bare steel, I wouldn't be surprised to see some rust in a month. There are lots of advantages to steel, unfortuantely rust is one of the few disadvantages.

 

[captain]

I is just about impossible to keep flash rust from forming on bare steel in the presents of oxygen in the air. With high 02 percentage Nitrox the situation is worse.
Flash rust does not present a problem, as long as liquid water does not enter the tank the flash rust will act as a protective film to prevent further rust. If it is removed it will only rust again in a short period of time.

Captain

 

[ChrisA]

I is just about impossible to keep flash rust from forming on bare steel in the presents of oxygen in the air. With high 02 percentage Nitrox the situation is worse.

Captain

Ok, but I thought those PST tanks were hot dip galvanized.
You mean that they only galvanize the _outside_ of the tank?

 

[Aerodog]

I'm thinking the guy who did the May 04 VIP didn't really do anything other then place the sticker on the tank. I bought the tank "new" and it had been sitting in the store for a year.

Lets just say that when I bought my PST E7-80 in May 04, it was MFG and factory viz'ed in Feb 04 and got a brand new May 04 VIP sticker. I brought the tank from the display area to the counter, asked if the cost included a fill, tank was hooked up to compressor (i viewed entire sequence of events), asked if I get a new VIP since the one on the tank was 02-2004 and they slapped a new one on right then and there! Oh my! I didn't see anyone in a red cape and tights so I don't think anyone looked at the inside of the tank.

 

[Ontario Diver]

...
I'm thinking the guy who did the May 04 VIP didn't really do anything other then place the sticker on the tank. I bought the tank "new" and it had been sitting in the store for a year.
...

ChrisA

Rust itself is not a reason to fail a tank during a Visual Inspection. Metal pits, dings, damage, and neck cracks are the reasons for failing a tank.

A light film of rust (flash rust) is quite common and can happen in a very short time. It is not considered much of an issue. Serious rusting (the kind worth worrying about) happens with pitting and flaking. With salt water, this can happen in days.

You are required to check the inside of the tank during a Vis tho'.....



Hope that this helps.

 

[bcsean]

pretty sure PST tanks are hot dip galvanized in and out.

 

[Drew Sailbum]

There was an experiment cited by my PSI Cylinder Inspector instructor. In it a steel cylinder had some salt water poured in and then left to sit on its side. IIRC, about half the thickness of the cylinder had rusted away in one spot in only two weeks!

A little flash rust is not too bad. External rust can get out of control if you do not take care to rinse under the tank boot and let the cylinder dry before putting the boot back in place. Particularly important if you are diving salt water with steel cylinders.

 

[FredT]

Ok, but I thought those PST tanks were hot dip galvanized.
You mean that they only galvanize the _outside_ of the tank?

You got that correct. You do NOT want to breathe from a tank that has an interior galvanized coating. AFAIK stock tanks have NEVER been offered with an internal hot dipped galvanizing. Discounting the very real hazard of inhaling zinc oxide the steam explosion hazard involved in trying to do a an internal hot dip would discourage the manufacturer.

Visible flash rust can form in under 5 minutes unless the drying process is done exactly right. It is also VERY easy to remove. OTOH flash rust is not a problem. Flake or popcorn rust can be.

FT

 

[Mike Newman]

In the wrong conditions - open to the air - for example during a VIP process if left unattended and untreated - a tank can begin to form rust in a matter of hours.

Normally, the issue is not, 'how long does it take?' - but, has water (in any form) entered the tank. If water is present, no matter how little, it will form rust very quickly in a steel tank that does not have any form of protective lining.

The imperative is to ensure that you have air fills from a reputable, well-serviced compressor and that you keep your first stage dry and clean. It's also important to be sure that whoever VIPs your tank uses a rust preventive (safe!) inhibitor during the set-up and doesn't leave the tank open and untreated after completing the basic internal inspection.