Steel tank and rust

[Gombessa]

And on a side note Steeltanks are not tumbled, but sandblasted.

Global, sure. But the OP is in Davis, California, and the statement as written above is irrefutably wrong

 

[reefugee]

Thanks everyone for your responses. The dive shop is a great place, so this post wasn't meant to bash them. I do dive in the water from 46 to 60F, so captndale explanation could be correct. I guess I will just budget potential annual tumbling into my equipment maintenance fund.

Minh

 

[captain]

I have 10 steel tanks, the newest is 30 years old, most have never been tumbled. All were hydroed in the last year, most have some flash rust, no problem.

 

[oxyhacker]

The math and logic in Capn Dale's post may be correct, but the fact is, most steel tanks that get filled with decent Grade E air don't rust, even if used in very cold water, at least enough to matter.

So I would still be suspicious that either your fills are a bit wetter than they should be, or that your shop is one of those shops that panics at even the slightest trace of rust. We dive here in water down to 40 F or so, and my steels get a casual whipping every few years, but that's it. Sure there is often a very light reddish rust bloom if you look carefully enough, but that is not enough to disqualify them for either VIP or hydro-time inspection. So I would suggest that you, rather than budgeting for annual tumblings, find a shop with either better air or better understanding of the inspection process, or try to educate your shop.
You say your dive shop is a great place. That doesn't guarantee either good air or educated inspections. A lot of very good shops have blind spots, which is why the wise customer tries to educate them rather than getting mad.

Thanks everyone for your responses. The dive shop is a great place, so this post wasn't meant to bash them. I do dive in the water from 46 to 60F, so captndale explanation could be correct. I guess I will just budget potential annual tumbling into my equipment maintenance fund.

Minh

 

[Randy43068]

Typically how much flash rust will a hydro facility tolerate in a steel tank before they test them?

 

[Nemrod]

Typically how much flash rust will a hydro facility tolerate in a steel tank before they test them?

Flash rust is a thin, light dusting of oxidation. It is fairly uniform over an area and is typically somewhat self limiting or at least of it's own accord does not progress to pitting rust.

Pitting rust, is dangerous and will destroy a tank quickly. Pitting rust shows as localized bubbles or pits with a dark ring of converted iron.

I have had steel tanks fail due to pitting rust, my suspicion is that dive shops often improperly fill, don't blow the valve and fill nozzle clear and have high moisture content air due to improper compressor set up and filtering.


"Air" and mositure is not entering your tank via the O ring at the valve and neck inteface unless you have a leak there and the tank is empty. If the "tech" told you that then perhaps you need to find a tech that knows what he/she is doing because this one does not.

N

 

[Randy43068]

Thanks N.

One thing I'm noticing about steel tanks and flash rust, some do, some don't. Maybe some have some sort of inhibitor against rust, others don't. Flash rust appears to be, at least anticdotally anyway, a random occurance.

 

[pescador775]

Capt Dale's opinion about dew point is a little dated. Yes, Grade E air is now supposed to have an equivalent dew point of -60F. At 3500 psi the dewpoint is more like 30F, not 55F as the capt suggested. So, a full tank of air exposed to sub freezing temps might develop a little internal frost which would quickly evaporate when the tank was warmed up and/or the press dropped. I fill my own tanks and they are stored in an unheated diving locker, exposed to temps down to 0F. Back in the day, they developed rust but not anymore, not since I updated the filtration system on the compressor. The added filtration is not $1500 exotic hyper stuff, just regular equipment seen on most new compressors. So, friends, if your tanks are pit ugly rusty you should smell a rat. If there is nothing beyond a thin brown patina it is probably normal for the industry and not cause for rejection or immediate remedy. Truth be told, if your tanks are being inspected in a humid environment that could be the source of a little flash rust right there.

 

[Ben_ca]

Minh,
I have had the same thing happen o me... (tanks from same shop a few years back) On the first vis a whole bunch of rust powder.. What we think happened was more the result of the tank sitting in the back waiting to be sold than anything I could have done...

Have them tumble and see if it happens again next year... It could have been there even before you bought it... not like you don't take the shop's word that the tank is good when you buy it....

Ben

 

[Luis H]

Capt Dale's opinion about dew point is a little dated. Yes, Grade E air is now supposed to have an equivalent dew point of -60F. At 3500 psi the dewpoint is more like 30F, not 55F as the capt suggested.

Below is the graph in captndale link. I have been searching for a better graph or table. The resolution of the picture doesn’t allow me to enlarge it in order to make it easier to read. It also only goes to 3000 psi.
Do you have a better source?

Fig. 6. Dew point conversion chart assists in determination of dew point of air at variety of pressures. Moisture-content scales chart quantity of moisture contained in atmospheric air at indicated dew points.

From:
Air dryers