Steel Tank Maintenance

[Denver]

I just broke down my doubles (PST). I made about a dozen dives in salt water two months ago as well as fresh water dives during the year using the doubles. The exterior of the tanks looked fine until I took the bands off. A white powdery substance marks the location where the bands touched the tanks. This substance is caked on the bands as well.

1. Do I need to do a better rinse job in the future or is this normal?

2. Should I remove the white powder off the tanks and bands before I reassemble them?

 

[C_C]

Rinse the heck out of em' and dive, dive, dive.

C_C

 

[Hallmac]

Rinse the heck out of them and reassemble. I see that salt build up in a lot of equipment I service.

I recommend a good soak in fresh water then rinse.

Fresh water Good, Salt water fun!

Hallmac

 

[AzAtty]

Here's sort of how it works. Since your tanks are Pressed Steel, that means they're galvanized using a hot dip process. So you have a nice even coating of zinc on top of the steel. The zinc corrodes preferentially and protects the steel.

Now here comes the catch: normally, when zinc corrodes it forms zinc hydroxycarbonate, which adheres to the metal, fills in pores in the metal and forms a protective layer on the steel. That's why your steel tanks have a mottled grey appearance now (or shortly will). However, when zinc is exposed to waters with a low alkaline content or high carbon dioxide concentrations, zinc carbonate forms, which does not adhere to the metal and is simply washed away.

So if the stuff is flaking off, it's probably zinc carbonate. Although it isn't hurting the tank, it isn't helping either. Dust it off, rinse it and go back to your diving.

One of the inherent problems with bands, boots and anything else that directly contacts the tank is that water can get trapped between the tank and the contact item. I'd give the tank a good soak in fresh water after every dive and then try to dry it as well as possible in the band area with a hair dryer (if available). The goal is to evaporate as much water from the contact points as possible, since corrosion will continue even after the tanks are out of the water if they stay wet.