JProsser99 wrote on 4/25/97 rec.scuba.equipment:
>
> Seem to have missed the original metal tank coatings post which I did not
> see listed now. Looks like I missed a good post: can someone forward me a
> copy of it? Thanx! (jays address removed)
It's available on Deja news archive.
HTTP:\\
www.dejanews.com
but you _asked_ for it so here goes.
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Carl Heinzl wrote:
>
> >Now Carl - what's wrong with OMS tank? I was about to buy one tomorrow.
> >Seriously!
>
> I too was going to buy some OMS tanks before I found out all this. Ok
> this is from memory as some of this stuff is till put away in boxes.
>
> 1) Their buoyancy characteristics aren't as good as pressed steel.
>
> 2) I forget what type of galvanizing they do, I think it is sprayed on,
> but it is NOT hot dipped galvanized like pressed steel (and that is a
> big one).
>
> There was more but I don't remember it off the top of my head. Pick up
> tanks that are *hot dipped* galvanized though.
>
> Perhaps others can expound on this more fully than I.
>
> -Carl-
Carl,
I don't have an OMS tank and know of no incidents of coating failure (or success for that matter) with them, but I am relatively familiar with coating technology.
There are 3 commonly used methods for galvanizing in production operations. Normally flame spray is not one of them, but it may be used if there was a concern for the heat treatment of the tank. The normal methods and some of the differences are: (all dimensions in inches)
1. Hot Dip
This process involves mechanically or chemically cleaning the object to be coated to bare steel, sometimes preheating the object, and then dipping the object into a tank of molten zinc. The zinc will form a metallurgical bond with the steel similar to a brazed joint where a small percentage of each metal goes into solution in the other forming a bond at the atomic level. The amount of coating remaining on the object is dependent on zinc pot temperature, withdrawal rate and any mechanical removal done as the hot coated object is removed from the tank. Normal coating thickness is often in excess of 0.005". Air knives and mechanical wipers are used on some products to limit film thickness in critical areas. A multiple dip process can obtain heavier coatings in excess of 0.030.
++Hot Dip normally provides the best adhesion, heaviest film thickness, and least pinholes in the coating of any method.
++ Pinholes tend self-seal when in contact with water.
+Can be done in an automated process on a production line with minimum labor involvement.
+Process quality control is easy to monitor and has relatively wide acceptable levels for the critical variables.
-Energy intensive as the entire object must be heated to above the melting point of zinc and the molten zinc must normally be maintained at plating temp round the clock to provide production throughput.
-Expensive USA EPA and OSHA requirements for zinc fume scrubbing and worker contact can run up the costs significantly over previous years.
-Effectively prohibits any heat treatment step requiring soaking at or below the zinc pot temperature from being effective. Normal realities involved in heat treatment of steel will usually prohibit engineers from incorporating the galvanizing process into the middle of a heat treatment process following the hot forming of the tank itself since beginning and end of day in process work will have different time constants. This limits material choices.
-(For the manufacturer) thin films are often difficult to maintain with this process. This uses more zinc and thus adds cost to the product. This heavier minimum coating I consider a + for scuba tank owners. Makes it much harder to get a tank off spec on the thin side.
2. Electrogalvanized
This is simply an application of zinc to the surface of the steel by the electroplating process. The object to be coated is etched to remove any surface contamination and immersed in a chemical bath with an applied voltage. The coating drops out of solution on the surface to be coated. The bond is normally at the metal crystal and/or surface irregularity level. Normally used where the required coating thickness is measured in parts of a thousandth of an inch to 0.002". Heavier coatings can be obtained at longer bath dwell times but thick (.005 and up) single coat electroplate of _any_ metal has a tendency to bond failure and will often chip off during localized impact. Coating will preferentially be deposited on sharp outside corners and be thinner in valleys. This may be a problem at tank necks. The process is relatively cold and does not limit heat treatment choices. Coating variables include bath chemical concentrations, dwell time, current density per square inch, temperature, and driving voltage.
+Well understood and developed technology.
+(For manufacturer) Easy to maintain constant coating thickness with minimal material usage.
+Can be set up as part of a production line with limited impact to start and end of day work in progress.
+No constraints on heat treatment/wider material selection.
-More variables and relatively narrower acceptable bandwidth for quality control purposes. Monitoring equipment is more maintenance sensitive than that needed for HD.
-Chemical baths often require treatment as hazardous waste during disposal driving up costs.
-Coating can have unbonded areas not immediately visible on inspection and pinholes do not normally self heal as rapidly as HD, if at all.
-Chemical baths require a similar level of US OSHA and EPA safety hardware and pollution control as HD.
3. Impact coating.
This process involves placing zinc (or other metal powder) in a tumbler along with the parts to be coated and an appropriate transfer medium (ceramic chips, steel balls, etc.) if required and running the tumbler for several minutes up to several hours or days. The coating is applied in a _very_ thin coat by being cold welded to the object by repeated impacts as the parts and transfer media roll around inside the tumbler drum. This process can also provide a limited deburring action. Often used with nuts, bolts and other small solid parts.
+Minimal material usage as unapplied powder and transfer media can be reused batch to batch.
+Minimal energy consumption.
+Time and transfer media wear are the only variables not totally controlled up front. Effectively no problem for quality control.
-Only relatively thin coatings <~0.0005" can be applied.
-Applicable to batch processing only.
-Difficult with scuba tank sized objects.
4. Flame spray or metal spray process:
The fourth type of coating is relatively rare in production but has gained widespread use in the maintenance industry for building up worn shafts at seal and bearing points and applying expensive protective coatings or hard surfacing at known wear points on otherwise lower cost items. Shafts and electrical contacts are two applications that come immediately to mind. I see no reason why a scuba cylinder _couldn't_ be coated this way, and a couple of reasons why it would appeal to the manufacturer. As far as the process path goes normally the material to be coated is mechanically cleaned to near bare metal at the start of the process. The process itself involves an oxy-acetylene, oxy-hydrogen, or other high temp torch impacting the surface to be plated and adjusted to provide a slightly reducing (oxygen starved) flame. The metal to be plated onto the surface is powdered and introduced into the base of this flame. Any surface oxidation on the part is removed by reducing any metal oxides on the surface back into the parent metal as the excess fuel in the torch consumes the oxygen from the metal oxides. It's possible to melt or soften the very surface of most metals without transferring much heat to the core of the product by controlling the torch temperature, flame velocity, and time the flame is in contact with any spot on the object to be coated. The coating metal powder is melted by the torch between the introduction point and when it contacts the softened or melted surface of the object and forms a good atomic level bond with it. Coating thickness is controlled by the speed of the torch, amount of powdered metal introduced, and torch path. Thickness can be as low as tenths of a mil (0.0001") up to several inches depending on the material deposited and number of passes involved. The variables in a production operation would normally be tracked by automated machinery and include powder particle size, fuel/O2 mix ratios, flame velocity, distance to surface being coated, powder feed rate, object temperature and cooling, etc. An example of an uncontrolled variable would be any transient jamming in the powder feed tubing. Provided the quality control is tight enough, and the coating is thick enough, a metal-sprayed zinc coating should be about the same as a hot dipped coating galvanized one. The greater number of critical variables in the process and the natural stinginess of production foremen (must minimize costs you know) combine to create a greater potential for coating failure in the field than in HD.
++ Good control of coating thickness and thus of costs.
++ Minimal USA OSHA and EPA requirements since molten metals and hazardous chemicals are not obviously involved. Fume collection on a relatively small flame spray booth would be about it. No, or minimal, hazardous waste since unconsumed powder may be reused or sold as a by-product to impact or electro-platers and die casters.
+ Frees up heat treat and material choices since sidewall core temperature can be controlled to _any_ reasonable point. It should be possible to limit wall core temp to less than 400F during the process. This will allow use of almost any High Strength Low Alloy or alloy steel for tank construction from a heat treatment point of view.
-Complicated quality and process controls are required to provide an acceptable product. Process transients can produce thin spots in an otherwise good coating without being immediately obvious.
The last item is the big kicker! Top management commitment to quality control is MANDATORY for this type of coating to be consistent and suitable for repeated immersion in seawater. Thin or spotty coatings on a few tanks will kill their reputation with the tech crowd. From what you said earlier it sounds like this has already happened. The tech crowd also tend to be a vocal lot with well-formed and entrenched opinions and tend to pass these on to others not of the tech persuasion. Considering that the techies are often opinion leaders in this game it sounds like OMS may have stepped in it big time!
IMNSHO If OMS has developed this reputation the only saving grace it has open to it would be to match Luxfer's neck crack tank replacement policy with their coatings. As it was explained to me if any Luxfer tank fails a VIP due to neck cracks they will replace it, period. No limitation on number of owners or age of the tank. Ship it to them with the PSI inspectors paperwork and they'll ship back a new tank! If OMS
unconditionally stood by their coatings to the same extent no diver would think twice about buying one. If he got a bad one it would be replaced with a new one. No (hard) questions asked. No proof of purchase or other legal nastiness required. For the price of the postage to get the tank back to them with a VIP inspection report a new one would appear at your door in less than a month!
FT
BTW Not having ever personally dealt with them I have no idea what their warranty currently is.
