Backplate Question

SeaJay · Aug 13, 2002

Has anyone used an aluminum backplate for salt water diving? Have there been corrosion problems in this setup?

I'm also interested in finding out what people's experiences have been with ABS plastic backplates, like Dive Rite sells.

See, my buddy and I have been doing about 50% freshwater dives and 50% saltwater dives. In an effort to create one BP/Wing setup that we could use in either case (salt or fresh), I'm a bit confused as to where to turn.

A stainless steel backplate would be terrific, and great in salt water. However, I think that I personally would dive with little to no weight at all in freshwater with this rig (six to eight pounds is what I'm diving with anyway) and that means no ditchable weight would be available to me... Fine in a cave or spring experience, but a really bad idea in open fresh water. While I could easily swim that kind of weight up, if I ever needed to float for a long period of time in fresh water (like if I lost the boat or whatever) it'd be hard to stay buoyant.

An aluminum backplate would be better in fresh water, as I could then weight it correctly with ditchable weight. In salt water, I'd have to carry 10-15 lbs of weight, which means that the situation wouldn't really be as ideal as with a stainless backplate. Not a big deal though, since I could easily overcome that problem with more weight. But I'm concerned about the reputation for aluminum in salt water... Do aluminum backplates have corrosion problems in salt water?

The last option... And an interesting one, is the ABS plastic backplate that Dive Rite is making. At over a pound negatively buoyant in freshwater, it's got pretty much the same buoyancy characteristics as an aluminum plate... Yet obviously no corrosion problems. However, I'm a bit concerned about the durability of an ABS plastic backplate. Anyone have any problems with these?

Thanks in advance to all the help I've gotten on this board...

Scubaroo · Aug 13, 2002

I've seen photos of cracked ABS backplates. I'll try and find a link to the images later. Basically the backplate had cracked from the top bolt hole to the top of the backplate.

Uncle Pug · Aug 13, 2002

The aluminum plate should be fine if you rinse it after diving salt.

But I'm curious... the concern is no ditchable weight diving fresh water with a SS bp if you should surface and not find the boat... where are you diving fresh? Is it really *big water*? Do you dive wet? Do you need the ditchable weight at the end of the dive?

SeaJay · Aug 13, 2002

I didn't give you enough information!

As you know, I live on the islands of the coastal Carolinas, not far from Hilton Head. I'll be diving salt water whenever I can, but sometimes I dive inland, too... I've been to the springs in North Florida several times, and there are springs in "upstate" South Carolina, near the tail end of the Appalachian Trail. So my diving's about 50/50 fresh and salt.

The "big water" fresh is Lake Murray, near Columbia, SC. I have relatives on the lake and plan on diving there quite a bit. The water's freshwater, but the lake's very large... In fact, at some points you can be in a boat and not see either shoreline.

Most of my diving, because of the warm nature of the area here, will be in a 3/2 mil suit. My offshore diving is very much in the Gulf Stream... It comes close to shore here. The springs in Florida are a constant 72 degrees, making a 3/2 mil quite comfortable. Only for about three or four months out of the year would I be diving something warmer offshore... Or if I were to do some deeper (more than 130') dives in the lakes locally (like Lake Murray) would I want something warmer. I probably will never see a dry suit.

Frankly, I am beginning to fear the concept of a broken ABS backplate... It just doesn't seem like a real good idea as compared to a metal one. And knowing how quickly salt can eat just about anything and everything (it even makes my stainless steel dive knife rust if I don't keep it oiled, even if I DO rinse it), I'm thinking that an aluminum backplate might not do so hot either.

So I'm beginning to think that stainless might be the way to go. Would your experience agree with that logic?

I saw at Ginnie last weekend a Halcyon polished stainless BP... Niiiiice.... Wow...

The only BP's that I'm familiar with were what the LDS at Ginnie had for sale... The complete lines of both Dive Rite and Halcyon's BP systems. Yet, I'm hearing that FredT's stuff is pretty hot... Got a favorite, you guys?

I'm particularly attracted to "donut style" wings instead of "horseshoe style" ones. I plan basically do dive with AL80's most of the time, so Dive Rite's "Travel Wing" and Halcyon's "Pioneer 27" are really looking nice. I like the pull dump (go ahead and shoot me now) on the Travel Wing... And I like the tinyness of it over the Pioneer 27... Yet the Pioneer's got that cool as heck STA-less system for diving singles. What's your take on this, you guys?

Yeah, I've read some of the stuff written here before... I'm just looking for y'all's latest perspective and opinions...

Anyone using a Halcyon polished stainless BP with a Dive Rite Travel Wing and an STA? Are they all compatible? How badly does that STA affect trim in the water? I hear it keeps the tank too far from the diver's center of gravity...

Scubaroo · Aug 13, 2002

People dived for years with STAs no problems before Halcyon brought out the integrated wing - but the integrated STA would bring the tank closer to your body. For personal preference and trim I opted to go with a seperate 2 piece FredT STA.

I haven't dived a Dive Rite wing, but I own and dive a Halcyon 36 lb wing, and have not found a reason to change brands.

Scuba446 · Aug 13, 2002

I have the Halcyon Pioneer wings and a Koplin 6# SS BP and the Halcyon STA. I liked the idea of the solid 'lock up' of an STA vs.the STA-less new design. Whether real or perceived, I like the STA, plus, it added 2 additional pounds to my rig - and that means -2# off my waist. Somehow, I can't think that the new STA-less method can be that rock solid - but heck, maybe it is. Might just be in MY mind...

If I had it to do over again, I'd go the Koplin weighted SS STA as well...add +4-5 more pounds whith that rig.

FredT · Aug 13, 2002

Aluminum is not all the same. Most aluminum plates are made from 6061-T6. The reason for that is that it is the most generally available, thus least expensive, alloy for sheet and structural uses. It is very resistant to dry land/rain weathering, takes anodizing well, and pits like crazy if contaminated with chlorine ions. The pitting problem with this alloy in sea water is well documented and inherent in the microstructure of the 6000 series alloys.

A better alloy of aluminum for direct seawater exposure is a member of the 5000 series. The alloy series simply indicates the primary alloying elemet in the series. The "smaller" weather buoys off US and Canadian coasts manufactured of 5000 series aluminum show little or no corrosion on the bare submerged hulls after over 15 years of service if not painted or otherwise covered. Painting can cause localized corrosion on any aluminum alloy in the presence of seawater and a dissimilar metal. This boils down to 6000 series AL working well dry or in fresh water, and 5000 series working in fresh or salt water. Reduce dissimilar metal contacts and 5000 series Al plates will work fine.

Note to the aluminum tank suporters: ALL aluminum tanks are/were manufactured of 6000 series aluminum. After 30+ years of throwing things in seawater I'm convinced the mix 6000 series and seawater is not a good idea. The result has always been a bad thing for the aluminum.

My aluminum plates are manufactured of 5000 series aluminum.

Now to SST. Again not all are created equal, and surface finish counts!

Several manufactures use 304 Stainless. This is well know to pit in seawater, especially in areas deprived in seawater. Not only is the chrome and nickel content lower, but the lack of Moly and high carbon content allowable in 304 allows corrodable carbides to precipitate out at the grain boundaries as the metal hardens from the melt. To provide a bit better corrosion protection it's necessary to increase the chrome and nickel at the surface. This is easily done by electropolishing the plate. This process involves selective removal of iron atoms from the surface layer with an acid bath and electric current. It rounds edges, removes burrs, gives the plate a nice mirror finish on the shelf, doesn't cost much as it is not hand labor, and creates a very noble surface that is resistant to corrosion. The drawback is that the noble surface created will eventually get scratched. The noble surface then becomes a cathode driving the corrosion of the less noble anode inner core of the plate, greatly accelerating the corrosion of the base metal. Corrosion rate is determined by surface ratios. The andoe "goes away" at a rate proportional to the the surface area of the cathode and the anode A large cathode surrounding a small anode will create a corrosion rate that is almost possilbe to see with the naked eye. Many knives are also electorpolished. If you have ever found a knife that's been down in the ocean for a while the pitted and cratered surface, especially if the knife has ever been resharpened, is what to expect from this "dissimilar metal" between the surface and the core. I've had even 316 SST pressure vessels fail from this "electropolish failure" mechanism after less than a month in seawater!

The "better" alloy for seawater exposure is 316L. This has an already higher chrome and nickel content, as well as a significant loading of Moly and almost no carbon. This provides a significant reduction to the corrosion rate of 304, and is still a fairly readily available alloy. Cost is up to 2x the 304, but it WILL survive seawater immersion much better. Again the electropolish is not a good way to go for immersion, although it works well above water. Finish by mechanical polishing will work either place.

The trick to 316 is to have a passive surface. Embedded steel and other impurities can cause localized staining and accelerated corrosion. I attempt to minimize that contamination, but occasionally a plate will show surface rust stains after the first few dives. The " industry standard" treatment is to etch the plate in concentrated nitric acid for 10 to 60 minutes after completing all machining and finishing to "passivate" the plate by removing all contamination. The problem here is that an individual (me for instance) gets on all sorts of radar screens he doesn't want to get on if he purchases concentrated nitric acid! It is _the_ precursor chemical necessary for manufacture almost all high explosives. ATF knows this, and given the current political climate I'd rather just avoid the discussion with them all together. Luckily there is a good work-around that is easily available and cheap. Simply marinate the plate over night in Coca-Cola Classic (the diet doesn't seem to work as well) to dissolve the contaminates. The phosphoric and carbonic acid in Coke is not as fast as hot concentrated nitric but seems to get the job done quite effectively. If you don't have a glass pan big enough a shovel and garbage bag works to create a temporary pan in a garden.

FT

SeaJay · Aug 13, 2002

FredT, in the flesh ladies and gentlemen.

I knew you were here on this board, but how cool it is to get a reply from the metalmaster himself.

I studied Physics at Rutgers University, and spent a few years racing mountain bikes years ago, so I am left with some residual knowledge of metallurgy... And some familiararity specifically with aluminum products. Your reference to the grade (is it called, "grade?") really hits home with me.

I'm more comfortable with the idea of an aluminum backplate than a stainless one for weighting purposes... For me, a bladder failure in 300 feet of freshwater, 10 miles from the shore would be catastrophic, I believe, with a stainless steel plate. I don't think I'd have much problem getting to the surface (since I wear 6-8 lbs anyway), but I would not be able to maintain buoyancy indefinitely like I could if I wore an aluminum plate and dropped ditchable weight. If I were wearing a thicker suit or a drysuit, it wouldn't be an issue. If I were always diving salt water, it wouldn't be an issue either.

But I was worried about the whole aluminum thing in salt water. You've addressed that issue.

I know that several people here have your backplates... Do you have a website where I can look at your products? I'd like to get prices, options, etc...

Your backplates are pretty much compatible with Dive Rite and Halcyon wings, right? I mean, I don't have to necessarily choose one or the other if I go with your backplate, right?

Do you do any customizing of the backplate? Something cool in a design, like etching, engraving, anodizing (if you recommend that), etc? I know it seems silly, but something unique would be seriously cool.

Fred, any other suggestions that you can think of?

Thanks in advance!

SeaJay · Aug 13, 2002

FredT, you know backplates probably better than anyone else here. What do you suggest?

I'm always diving an AL80 and almost always doing it in a 3/2 mil. I might rent a heavier suit when it gets cold, but I'd just as soon head to the Keys instead and dive those waters when it gets too cold up here. Bottom line is that I'm always diving a 3/2 mil.

Half of my diving is in salt water, half of it in fresh. I need the corrosion resistance in a BP that your materials can offer. I also will almost always be diving a single tank.

Proper weight placement is key for me... I have this huge pet peeve of "opinionated" BC's that try to float me in a particular position. I want to decide what my position in the water is, and that might be horizontal, might be vertical (like when I'm feeding fish) and it might be upside-down, inverted, and with my head down, inspecting the roof of a cavern or scraping the bottom of a boat.

I normally dive with about eight pounds with the above configuration in fresh water. I generally go with 10-15 lbs (12 if I can get that denomination) in salt... However, I also like to "fossil dive," where I dive totally overweighted (25 or 30 lbs) so that I "really stick" to the bottom of a no-vis river even if there's a current.

I am looking for one BP that can do all of that... What's your suggestion?

Before you answer, I want to add that some of my freshwater dives can be in BIG freshwater, so I think that in case of wing or bladder failure, I'd want to have a rig that's positively buoyant if I dropped weight. So I'm inclined to believe that I want aluminum, and then add weight in a variety of ditchable and nonditchable ways through a set of Halcyon or Dive Rite pockets and maybe some "poured" weights. But stainless seems better suited for salt diving, and if I got your FredT SS "light," I would be basically neutral with an empty tank...

I dunno... What's your advice? And what's your take on "Line X"-ing everything?

scottk · Aug 13, 2002

FredT once bubbled...

Fred, I have never approached or taken to task another manufacturer, but this post leaves me with no option.

You are pushing snake oil.

>Aluminum is not all the same. Most aluminum plates are >made from 6061-T6.

Pure baloney. 5000 series plate is used because it is more malleable without additional heat treatment. End of discussion.

Anyone can pick up a copy of the Machinery's Handbook and see exactly what alloy elements there are, in what concentrations, and for what purposes these alloys are recommended.

Much more important is the heat treat used.

http://www.ae.msstate.edu/vlsm/materials/alloys/aluminum.htm

<snip>

>Note to the aluminum tank suporters: ALL aluminum tanks >are/were manufactured of 6000 series aluminum. After 30+ >years of throwing things in seawater I'm convinced the mix >6000 series and seawater is not a good idea. The result has >always been a bad thing for the aluminum.

I guess we better all throw our AL tanks away. I have several bare AL80's that are going on ten yers old, not pits, nothing but a dull surface oxidation. The insides are shiney as a new dime.

> My aluminum plates are manufactured of 5000 series aluminum.

As are all others.

>Now to SST. Again not all are created equal, and surface finish >counts!

>Several manufactures use 304 Stainless. This is well know to pit >in seawater, especially in areas deprived in seawater.

We'll put that aside as an spelling/grammer error. "pit in seawater, especially in area deprived in seawater"???

90% of boat fittings are 304L. 90% of railings and tubular structures such as bimini supports, boat rails, davits, etc. are fabricated of 304 DOM tubing. These parts spend a lot more time in salt spray and immersion than any backplate ever will. Prop shafts, that are run through brass, fiberglass and other supports and hulls are made of 'Aquamet":

http://www.aquamet.com/index/grade.html

http://www.escosteel.com/alloys.asp?s=81871&p=903348

316 is indicated for applications such as hot acids and strong alkalai concentrations, for the diver and a backplate, the issue is absolutely mute.

I will pay anyone here $50 to show or post a picture of a DiveRite, Halcyon, OMS or Koplin plate made of 304 that has pitted. Ever.

<snip>

304L is the low carbon designator for 304 series alloys that will be used in weldments. The idea being to reduce embrittlement from carbon precipitation at the welded joint. Same with 316L.

Additionally, the source of the alloy used is far more important than the alloy designation. Chinese 316 that we have seen in our shop in the past is mildly magnetic! And it displays rust after only one short immersion in seawater. We dont use it. We use American made, qualified 304L, or 316L and there isnt a thing wrong with using either alloy for any backplate. The price difference is negligeable, unless you compare good stainless to imported crap. I can buy imported 316L for *less* than 1/2 the price of domestic 304L.

Also the absolute alloy fractions vary widely from manufacturer to manufacturer, all calling them 316(L).

>The "better" alloy for seawater exposure is 316L. This has an >already higher chrome and nickel content, as well as a >significant loading of Moly and almost no carbon. This provides a >significant reduction to the corrosion rate of 304, and is still a >fairly readily available alloy. Cost is up to 2x the 304, but it WILL >survive seawater immersion much better. Again the >electropolish is not a good way to go for immersion, although it >works well above water. Finish by mechanical polishing will work >either place.

Passivation:

http://www.pfonline.com/articles/119806.html

Corrosion:
http://bosunsupplies.com/StainlessInfo2.cfm
http://www.diveweb.com/maritech/features/uw-su99.01.htm

Glass beading addresses all these issues in a much more environmentally friendly manner. Compressed air and glass beads, not noxious chemicals that need to be disposed of.

>The trick to 316 is to have a passive surface. Embedded steel >and other impurities can cause localized staining and >accelerated corrosion. I attempt to minimize that contamination, >but occasionally a plate will show surface rust stains after the >first few dives. The " industry standard" treatment is to etch the >plate in concentrated nitric acid for 10 to 60 minutes after >completing all machining and finishing to "passivate" the plate >by removing all contamination. The problem here is that an >individual (me for instance) gets on all sorts of radar screens he >doesn't want to get on if he purchases concentrated nitric acid!

Try glass beading.

>It is _the_ precursor chemical necessary for manufacture almost >all high explosives. ATF knows this, and given the current >political climate I'd rather just avoid the discussion with them all >together.

Pure baloney. If you arent using for explosive or other illicit production, you havent a worry.

>Simply marinate the plate over night in Coca-Cola Classic (the >diet doesn't seem to work as well) to dissolve the >contaminates.

Unbelievable. So, in stead of using acid to passivate, every manufacturer on earth has been blowing it by not using Coco-Cola.

You can buy phosphoric or weak hydrochloric/nitric acid (called muriatic acid) at any harware store or pool supply.

Fred, if your plates are better than the rest, sell them as such.

Putting out crap like this does nothing to serve anyone, least of all yourself.

Scott

FT

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