black water tank issue

[txjeff]

Thanks for the replies. The tank had a split in one of the weld seams, which I tig welded to repair. The material is very thin so Im guessing there is another spot somewhere in the back weld. I was hoping for some kind of epoxy to put in the tank the seal all the welds. Since this is in the back 1/2 bay on my MC-7C not sure that plastic will give me much more than about 1/2 the size I have now(guessing about 75 gals now)
I will pull the tank again tonight (3rd times the charm I hope ) and try and pressure test it............

[JohnEd]

Give up on that internal sealing stuff.  It has to be applied to a clean surface to work.

Van,

If you keep your tanks separate you can drain the grey into a ditch or ? thru a garden hose.  I can get withing 50 feet of just about any hole.  You can also do that with black but you need a macerate or really tiny poopies.

If you combine then the chems you use in dishwashing, etc, will "kill" the bugs that digest the black and it will emit odors outside that those down wind will find offensive.  My grey water really reeked but I let food scraps and coffee grounds into the grey.

You can only stack plastic if you put framing between them to support the load.  I wanted fresh on the bottom and grey and black on top.  Heating the fresh would also protect the other two from freezing.  I thought building mine from ABS or PVC black sheeting 3/8 would be the best ever.  I could install the baffles to be internal support to allow stacking without framing.  Fit to close tolerance and that watery solvent welds the pieces into a single piece.  You can fit joint reinforcement with the water solvent or use heavy body like the stuff they use in plumbing.  You could make any shape you want to fill any space.  The best I have seen use multiple tanks for fresh and grey all stacked and plumbed with nary a cubic inch to spare.  Lots of options. The black sheeting is reasonable but the white food grade is spendy.  I would plan to cook and drink bottled water if you made your fresh tank out of anything other than food grade.  Common flange fittings take care of connections. 

[boogiethecat]

Kyle is right, if you weld up a stainless steel tank, you HAVE to passivate it.  How many SS tanks have I seen that have not been passivated? MANY.  quite a few welding shops including some major SS tank manufacturers don't do it... and the result is always leaky welds and rusted out seams!

Fortunately there is a very simple (fairly new on the block) way to passivate.  First use a wire wheel to get the colors off that you've created during the welding, then use a "wand" and a tig welding supply, set at about 20 amps, and do it with this stuff: http://www.citrisurf.com/9002data.htm

Basically you create a "wand" by wrapping some scotchbrite along with a layer of fiberglass tape around the end of a piece of 3/8 stainless rod, hook the rod to the same terminal on your tig welder as your torch, ground the tank, set the welder at 10amps, dip the scotchbrite into a solution of citrisurf 9002, and wipe it slowly down the weld.
It'll create a giant sulphur stink cloud, but the job is almost instant and the electricity does the job. It eats the scotchbrite and fiberglass pretty fast.

Browse around the website http://www.citrisurf.com/  These guys have many passivation products based on citric acid that is very safe to use, environmentally friendly and most imporant, you can do it yourself.  I use the stuff daily and it's all like magic...

[gus]

There are different grades of SS.

Much to my surprise many years ago I discovered that a small magnet would stick to a supposedly SS kitchen vent??

If a magnet will stick it has iron in it, iron will rust. Simple but true.

[boogiethecat]

Actually most of it has some iron in it.  But it won't rust... that's what passivation is all about.  The main ingredients in common SS (304, 316 etc) are iron, nickel and chromium.  When the stuff is manufactured, any surface iron is removed by passivation and the resulting chrome on the surface oxidizes, creating a very robust non corrodable surface.  But when you weld, you re-alloy everything near the weld and some iron comes back to the surface...that's what rusts.  Removing it with citric acid or other means (passivation) re-establishes an iron-free surface and once again it won't rust or corrode.

[kyle4501]

Well said Boogiethecat.

gus, all stainless steels, with the exception of the austenitic group (300 series), are strongly attracted to a magnet.
304SS can become magnetic if work hardened. It has to do with the atomic lattice straining and formation of martensite.

All stainless will corrode, it just corrodes slower than mild steel.

[bevans6]

The things you learn on a bus chatsite!  good info on welding stainless, I appreciate it.  I do it as little as possible, because I know that while I can run the bead just fine, I don't know how or have the equipment to do the second half of the job, the passivation.  Now I know a little more about what I don't know, and that's always good!

Brian

[gus]

Good things to know about SS. I assumed the purest SS had no Fe at all?

I do know that the SS screws I use on aircraft fairings can't be picked up with a magnet when I drop them into black holes!!

I've never seen corroded SS but have seen corrosion on other metals touching them and rust stains on the SS screws/bolts from plain steel.

[JohnEd]

Gus,

I am with you on this.  The test, for me, and for those that advised me, was that SS was only slightly magnet.  Not enuf to pick it up.  And when I bought SS fabrications I specified "Marine Grade" SS along with the # of the stuff called out by the ME.  The stuff I had installed was on the coast within a couple hundred yards of the Pac.  It did not rust but did get a yellowish cast to it.  I, or rather the Merican Tax Payer, paid a princely sum for some of that stuff.  The aluminum stuff that I was always replacing and was specified and procured by Sperry became a white p[owder as did the framing of the semi we had out there.  Did you know that you can spec a semi trailer for operation in a high salt and corrosive environment?  That would have been thinking ahead, using same and this conversation has not taken a dive into sexual references or innuendo.

Really torn on this as The Boogie Meister is held in the highest regards in my household and I might seem to have taken up arms.  Be assured I haven't, Mein Heir.

I think you can make SS without using any nickel but the good stuff has a lot of copper content.  The next step up on the metal ladder is MONEL and they have two grades of that and the difference is nickel content.  I don't think it sticks to a magnet either.

No numbers or mil standards or even a USDA reference in my musings of 20+ years ago.

John

[kyle4501]

I was involved with a textile machine that we used every grade of stainless available & they all corroded to failure within months of the install.
We had to use Inconel - seemed expensive at first, but it got cheaper the longer it lasted. Installed the Inconel over 15 years ago & it is still working great with no corrosion issues.

BTW, while Inconel has iron in it, Inconel 600 is usually slightly magnetic at room temperature, it will become non-magnetic when warmed mildly. Cold work or thermal treatment have little if any affect on its magnetic properties.


It is better to use 316L stainless steels when parts are to be welded. This is especially true if the weld may come in contact with chlorinated water. During welding, chrome-carbides are formed along the weld. This formation of chrome-carbides depletes the metal of chromium. Without the chromium, the protective passive film cannot form and corrosion can occur. This is called intergranular corrosion.

Damn, ain't nothing simple if you dig a little deeper. . . . 

[Len Silva]

Plastic is just looking better and better.  Of course, I am really old school and built plywood/fiberglass tanks.  As far as I know, they are still in the bus and working fine after twenty years.

[kyle4501]

If done right, stainless tanks are fine. Just as plastic tanks.

Also, same as plastic, things will go bad if you don't do it properly. 

FWIW, I've seen plenty of plastic tanks that leak too. 


A friend has a MCI-5 that was converted by custom coach from new (never a seated bus). His stainless tanks are around 50 years old & still no leaks.

And, yes, his bus is original, down to the 8V71 & 740 Allison that are still getting the job done. 


My point is, before deciding on what to use - do your own homework to determine what is best suited for your needs.