Time to Hot-Dip Galvanize?

[rhjameson]

Hello everyone,

I've got a major question to ask. I have six USD Steel 72, manufactured dates ranging from 1962 to 1972, and all are due for hydro-ed next month. All have passed their previous tests five years ago, and visual inspections in May 2011. Two of the tanks are showing signs of slight corrosion, but no pitting

I'm debating whether, or not, to have all the tanks Hot-Dip Galvanized prior to hydro-static testing. This is mostly to remove any existing, and prevent any further corrosion.

Any help, or suggestions, would be greatly appreciated.

[YankDownUnder]

I have limited experience with hot galvanizing. The company we use needs an old valve with a hole in it installed. The hole is to put a stainless steel hook into. The hook supports the tank and has a stamped stainless label on it. The old valve plugs the neck to prevent the cylinder from filling up with zinc. All nylon and other heat sensitive items have to be removed. Anything on the outside will be burned off in the process I am told. We have had other items galvanized, but not tanks so the information came from the sales staff.

You can see tank production at: http://www.youtube.com/watch?v=Z8R-I5I1Dgo

Galvanizing can be seen at: http://www.youtube.com/watch?v=c2J07n5hSbs

If you just want better looking tanks, home hardware companies sell Naval Jelly. It takes off paint and other items like stickers, and makes the galvanizing look new. However, it is the grey oxidation on the tank which actually protects the steel, so using Naval Jelly too often thins the galvanizing.

[Robohips77]

Hello everyone,

I've got a major question to ask. I have six USD Steel 72, manufactured dates ranging from 1962 to 1972, and all are due for hydro-ed next month. All have passed their previous tests five years ago, and visual inspections in May 2011. Two of the tanks are showing signs of slight corrosion, but no pitting

I'm debating whether, or not, to have all the tanks Hot-Dip Galvanized prior to hydro-static testing. This is mostly to remove any existing, and prevent any further corrosion.

Any help, or suggestions, would be greatly appreciated.

Personal experience with Galvanized tanks is to just prep them and put on a couple of coats of ZRC Galvalite. JMO for whats it worth. Good Luck. Personally I would not try to hot dip galvanize them.

[rhjameson]

I've been doing more research on this topic, and found some surprising results. I was reading a post on SCUBABOARD.com from 2007 on the differences between Hot-Dip and Cold-Dip Galvanizing.

From what I gather, Hot-Dip Galvanizing is the most durable coating, and offers the best results for exterior corrosion protection, versus an applied coating. At this point, I'm leaning towards HDG. I do have a few concerns about having my steel tanks heated to 830 degrees for a few minutes; but from a reply I read, from one of our fellow vintage dive enthusiasts, he had a 50 year-old tank Hot-Dipped Galvanized and was very pleased with the results.

At this point, I'm now looking into a way I can have the tanks immersed and suspended into the zink bath, without risking contamination of the tank's interior. Any suggestions, or feedback, would be appreciated.

[YankDownUnder]

Cold galvanizing is just paint with zinc supported in it. It insures that rust will not travel underneath when it is scratched. Normal spray zinc is not very hard. The Galvanite idea might be a better approach. I have no experience with flame spraying, but that is something you could also look into.

The steel was hot dipped to begin with and the age means just about nothing. If you hot dip, test the tanks after the treatment and you might be more comfortable. The tanks glow cherry red when they are formed, so a bit of hot zinc isn't much of a problem. Watch the links to tank making above and you will see torches being applied while they are made. The trouble is locating a place that does it. I live in a city of four million, so just about everything is available. They often do small jobs too. However, talking to the people who will do it is the best approach.