Cold galvanized / Hot-dipped = Difference?

[volyblmn]

I was curious if there were any pros or cons to either method of galvanized steel tanks. I see most tanks are now hot-dipped but a few (like the LP-77 in the deals forum) is cold galvanized. What makes one better than the other? Or is this another one of those "personal preference" type things?

 

[PhilEllis]

I was curious if there were any pros or cons to either method of galvanized steel tanks. I see most tanks are now hot-dipped but a few (like the LP-77 in the deals forum) is cold galvanized. What makes one better than the other? Or is this another one of those "personal preference" type things?

Hi Volyblmn. I am sure there will be plenty of opinions on which is better...hot dipped or galvanized zinc spray. I can tell you that the coating on the new Worthington LP-77 cylinders has been very well tested and stands up quite good to weather extremes. I will send Mike Pantoni at XS Scuba an email and ask him to address this thread. He will be able to give you much better information. Thanks.

Phil Ellis

 

[Hoosier]

As I know, PST and Worthington are only HDG tanks. It has the better rust protection compared to simple Zinc spray like an Asahi tanks.


Here is a good info from Diveriteexpress....

Tank Paints, Coatings, and Finishes

Uncoated steel tanks quickly develop a coating of rust in the presence of water, so all steel tanks are coated. The most durable coating for steel tanks is hot-dip galvanizing. When a steel tank is galvanized, a zinc metal coating is bonded to the surface of the steel. The hot-dipped galvanized coating comes from the factory as a shiny silver color that rapidly ages to a mottled gray once exposed to sea water. Some experts believe that environmental concerns about the zinc galvanizing operation itself outweigh the benefits of hot-dipped galvanization. Steel tanks are also available in a durable and environmentally friendly cold-galvanized coating that is further protected with an epoxy paint that seems to adhere very well.



We have observed debates over the corrosion resistance and durability of hot-dipped galvanized steel versus cold-galvanized painted steel tanks. In fact, we have seen steel tanks of every type and manufacture in excellent condition after many years of heavy service and we have also seen them fail the first annual inspection. The truth is both types of steel tanks will give many years of corrosion free service when they receive proper care. All tanks must be thoroughly rinsed in fresh water after use in salt water and during storage the boot should be removed. Both unpainted and painted steel tanks require some additional care in comparison to unpainted aluminum. Any deep gouges or scratches that cut through the galvanizing and expose bare metal need to be touched up with either a rust-inhibiting paint or a cold-galvanizing compound, both available in aerosol sprays and brush applicators at hardware stores.

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[captain]

Hot dip is the way to go. I have a 50 year old hot dip tank that has no rust even in the hydro date stamps. I don't use boots because I don't like the hassle of taking them off to rinse the tanks.

 

[XS Scuba]

Hey V:
Here's the deal on the exterior finishes:

Hot Dip Galvanization
1. All currently manufactured models of Worthington cylinders, except LP77.
2. The cylinder is submerged in 840 degree zinc alloy for about 10 minutes.
3. The cylinder is manufactured in Canada, shipped to the US for HDG, then shipped to XS Scuba or Sea Pearls.
4. Without doubt the best, longest lasting exterior finish to prevent rusting on a steel scuba cylinder (as example posted by captain, 50 years without exterior rusting).
5. Relatively expensive for the HDG process and double shipping costs.

Cold Spray Galvanization
1. Only the LP77 model.
2. The cylinder is grit blasted to bare metal, primed and sprayed with zinc rich exterior finish.
3. The cylinder is manufactured and cold galvanized in Canada, then shipped to XS Scuba or Sea Pearls.
4. An excellent, long lasting exterior finish.
5. Less expensive and no double shipping costs.

Several years ago, when we started the steel cylinder program with Worthington we evaluated many different exterior finishes. One of these is the current cold galvanizing process on the LP77. Nearly three years ago we cold galvanized a couple of LP95. After the galvanizing process, one of the cylinders was beat hard with 3/8 BBB chain to simulate the wear and tear of years of diving. Both cylinders were given to my old partner in the scuba charter boat business in Long Beach, California. Both of these cylinders continue to "live" on the back deck of his 65' charter boat. No love is given to these cylinders. They're on the back deck 24/7, exposed to the salt water/spray and direct sun. They are still there, used on an average of 20 dives per month. I've even asked them to avoid hosing them off when the back deck is washed. They remain rust free, even the cylinder we beat with a chain.

So, last year we started a project to offer a steel cylinder (LP77) that could compete with an aluminum 80 from all perspectives, including price. We choose the cold galvanizing process of the excellent results of the testing and the cost savings.

So, that's the whole story.

Both exterior finishes are covered by our Lifetime No Rust Guarantee (attached).

I was curious if there were any pros or cons to either method of galvanized steel tanks. I see most tanks are now hot-dipped but a few (like the LP-77 in the deals forum) is cold galvanized. What makes one better than the other? Or is this another one of those "personal preference" type things?

 

[Sadamune]

Here's a question for the steel cylinder experts.

Regarding the issue of corrosion resistance in steel cylinders, has any manufacturer considered or attempted to produce cylinders with a carbonitride surface treatment? The extreme hardness and corrosion resistance imparted by carbonitriding appears to be an ideal solution to the corrosion problem that afflicts steel in marine environments.

For those unfamiliar with carbonitriding, read Wikipedia's entry on it here: Carbonitriding

Sadamune

 

[Leadking]

Here's a question for the steel cylinder experts.

Regarding the issue of corrosion resistance in steel cylinders, has any manufacturer considered or attempted to produce cylinders with a carbonitride surface treatment? The extreme hardness and corrosion resistance imparted by carbonitriding appears to be an ideal solution to the corrosion problem that afflicts steel in marine environments.

For those unfamiliar with carbonitriding, read Wikipedia's entry on it here: Carbonitriding

Sadamune

We used that process in die cast dies. It wears very well but makes the steel very brittle, I am sure it would cause problems in steel cylinders as they expand and contract under pressure changes and would cause cracks in the steel.

 

[SparticleBrane]

Mike, that's a fantastic picture. Got any more? I'd love to see how they're made!!

 

[Jimmer]

I just read the Dive Rite part about using a spray cold galvanizing compound to touch up a tank. I'm curious if I was to shot blast or glass bead glast the paint and minor surface rust from a couple of old 72's, could I just spray 3 or 4 coats of this stuff on them and dive them like that?

 

[Luis H]

I just read the Dive Rite part about using a spray cold galvanizing compound to touch up a tank. I'm curious if I was to shot blast or glass bead glast the paint and minor surface rust from a couple of old 72's, could I just spray 3 or 4 coats of this stuff on them and dive them like that?

Galvanizing paint has been used by many divers on steel tanks (that were not galvanized at the factory) with good results. The paint is not like the hot dip galvanized, in that the paint can get easily scratched. The beauty of the galvanizing paint is that the zinc is sacrificial and will create a chemical bond with the steel which will prevent rust from migrating under the finish.

Any scratches that break through the finish will need to be touch up on a regular basis, but since no rust will form under the paint you only have to re-treat the affected area.