Internal rust & flash rust in steel tanks

[joens]

there is a brief explanation of the different steel tanks here http://www.northeastscubasupply.com/tanks/steel_tanks.html
no tanks steel or aluminium are meant to have moisture in them .I would have tumbled them to clean them.
joens

 

[DA Aquamaster]

What you have is probably what is called flash rust. A very thin dusting of rust is normal in a steel tank and is not an issue to be concerned about.

If it progresses beyond this or begins to develop spots of heavier rust that may lead to pitting, then it will need to be tumbled.

I always insure the tank valve is dry when the tank is filled or will open the valve and release enough air to dry the valve orifice before attaching the fill whip.

 

[Walter]

I always blast air out of my tanks prior to turning them over to get filled. It keeps water from rinsing (or spray on the boat) out of the tanks.

 

[mcrae]

I have a two year old PST 104 and saw the same thing this year. It is minor flash rust as already mentioned it does not hurt anything. In fact when I cleaned my tank with simple green andhot water to re-do it for Nitrox almost all of it came out.

All tanks will see some moisture, even "dry" compressed air say -40 deg F will cause this.
Not a concern.

Have fun.
Gary

 

[Denver]

I have an old steel 72 that I'm contemplating putting back into service. The tank was painted, not galvanized, when new and the inside appears to be bare metal. I plan on O2 cleaning the tank for pp blending.

Is there anything that can be applied to the inside of the tank to protect it from rusting?

What about using corrostop from OMS?

 

[DA Aquamaster]

In the distant past some steel 72's were made with a plastic coating inside to prevent rust. This was a good idea...until the coating develops pin holes whihc then trap moistrue and lead to rust pits. Consequently, the coating in these tanks needed to be removed by tumbling, a process that took about a week.

I would not coat theinside of the tank with anything to prevent rust as I'd prefer not to have leftover traces of various chemicals and coumponds in the air I breathe.

Also it isn't required as it takes both moisture an 02 to cause rust. If you use dry air from a quality source (which is pretty much a given with nitrox) you will not encounter problems with rust in the tank. You may get a just a bit of flash rust from the tank being dried after the hydro test process, but should be able to go for 10 yrs before you need to tumble the tank to remove any substantial accumlation of rust from regular use as long as you take reasonable care not to get water in the tank.

The outside of the tank is what will prove to be a challenge. Paint is hard to keep on a scuba tank given how they are used and abused. I have tried both Imron and MTK paints with no success long term. Both required periodic touch ups to take care of chips that exposed the metal of the tank.

Lately I have stripped the Imron and MTK on the two tansk i own that are not galvanized and just went to using plain old Rustoleum spray paint from Ace Hardware. It's cheap, works well and if it chips I can just add another coat once a year or so. If you get too much paint build up, it is also easy to strip.

 

[pescador775]

An old, painted steel tank is practically worthless except to paintballers. However, if you insist on using this tank, the interior must be cleaned of all rust/particles and oil residue. No coatings of any kind. The interior can be cleaned with Dawn detergent with a pound of marbles added. Roll tank across the yard, drain, rinse, drain and blow dry with hair dryer with long extension or similar. This is the budget, tedious, flash rust solution. Otherwise, purchase some Ensolv oxygen approved solvent and rinse out the tank, drain. No rust, no marbles, no blow dry. This stuff may cost more than your tank is worth, however.

Sand blast or strip the exterior. Coat the exterior with Zinc Rich paint- CRC, Rustoleum or similar type. Compared to hot dip this is a relatively weak, cold galvanizing but actually works although must be done yearly for SW use, every two years for FW.

 

[pdoege]

I popped open some used steels that I bought a while back and, good lord, they are rusty as hell!

I have no idea how they passed their last vis. WTF? Anyway, the bottoms of these tanks have quite a bit, well, more like tons of rust.

They definitely need some TLC.

I took them to the local shop and put them on the tumbler. The young lady there doesn't seem to be all the familiar with the removal of a lot of rust from steel tanks.

She showed me the results after a 30 minute "wet" tumble and said that it was good enough. The rust wasn't even touched!

No way am I going to button the tanks up unless the bottoms are at least a bit clean.

I'm thinking that these bad boys are going to need several hours (days?) of dry tumbling.

Does anyone have any experience in this area?

I am looking for advice on length of time, cutting agents, etc. I'm open to any tips and experience you can provide

Thanks!

Peter

 

[DA Aquamaster]

If you are looking at heavy rust, you are probably also looking at pits in the bottom of the tank - a tank that probably won't pass a VIP after tumbling. What you are talking about is way beyond flash rust.

A lot of it will depend on whether the rust was due to standing water in the bottom of the tank which will mean pits (which are nasty little stress risers and more than enough to condemn a tank) , or just due to sitting around for years with the tank valve open or off. This could mean a lot o surface rust in a relatively dry climate or pits everywhere in the tank in a relatively humid climate.

Tumbling may be required to tell, but maybe not - a lot of vertical development in the rust normally means a pit and I would not bother tumbling.

Tumbling a tank in this condition could take a while with one or more 6-8 hr sessions required to remove the rust and allow a thorough inspection.

My guess is you just may have the raw material for the biggest set of wind chimes in the neighborhood.

 

[pdoege]

The shop in question is willing to pass them with the rust, I doubt that pits will stop them.

That being said, I think 8 hours is a good starting point.

Peter