March 24, 2011

Derusting the Fuel Tank

[Click the pictures for a better view]

To other pages

I'm getting ready to paint the tank and fenders.  To ramp up to this there is all the standard sheet metal prep work, but before that, I wanted to make sure the tank was in good enough shape to use.  A lot of bad things can happen to tanks that sit for 30 years unattended.  Even though this bike was always in a garage, temperature and humidity cycles can deposit condensation on the unprotected metal inside.  It's possible that a tank that looks sound from the outside may have places that are essentially only paint and rust.

My tank looked solid from the outside:

A few of the attachments would need some attention.  The petcock had leaked for months before I put the bike away (see petcock rebuild), and something had turned some of the rubber mounts to goo.

To do an internal inspection, I first had to get by this bad boy:

Back in my younger days some of my so called friends thought it was funny to put fluffed up cigarette filters in peoples' gas tanks, believing that the fibers would eventually clog the fuel filter, causing hilarious trouble, but no real damage.  They were right about the trouble.  The key lock solved that problem, but now 30 years later, I seemed to have misplaced the key.  I tend to keep keys, especially if I can't remember what they go to, so I had a pile of candidates.

None of them worked, of course, either because the right key was not among them, or because the lock was so corroded inside that it was siezed.  I even read up on picking locks and tried for over an hour to pick it.  While I could feel the pins move, I couldn't turn the plug enough to open it.  Finally, giving in to my baser instincts, I got out the big drill. Even faced with this, the lock didn't give up easily--it had some hardened parts inside, I think.  Anyway, half an hour later, success.

Peering into the tank through the filler neck with the aid of a little flashlight, I saw what I expected:  The sidewalls had some rust, but most of it appeared superficial.  The bottom had a rough black crust, which I assumed was rust covered by gasoline varnish.  

In thinking out a cleaning protocol, it seemed that a systematic scientific approach would be good, focusing on each type of deposit in order.  Also, I wanted to be able to asses the progress over an area larger than the little area I could see through the filler neck.  

This is when the video TankCam was conceived.  While certainly not an original idea, I needed to do it on the cheap.  I'll try to cover the details of how I ended up doing it elsewhere, but I'll show some results here.

Here's a snip of how the untouched tank looked inside (these maybe aren't for dialup).   Mostly what you see is dark brown or black varnish.  A note about the videos:  I know it will be impossible to tell what part of the tank is being shown.  When I did this, I had the advantage of looking at the screen while manipulating the camera on the end of a long flexible positioner, so I knew generally where the camera was and what direction it was pointing.  These video clips tend to fucus on the bottom of the tank.

When gasoline gets elderly, it can deposit a "varnisn".  If the gasoline evaporates, the varnish is left behind.  Varnish is probably the small fraction of the fuel that is not volatile, or possibly some oxidation, decomposition or polymerization product of gasoline components.  At any rate, to remove it, wimpy solvents won't do.  It takes something like acetone to remove it.  I used half a can of carb cleaner (this one was a mixture of acetone and toluene), added to the tank and agitated in all positions for half an hour.  

This clip shows what the inside of the tank looked like after it dried:


Now it is just rust to deal with.  There are a lot of ways to remove rust from steel.  One way is acids.  Strong ones like muriatic (hydrochloric), or weaker ones like phosphoric, acetic (vinegar), citric, or oxalic.  Hydrochloric isn't usually the best choice since it can attack the underlying steel, and it also leaves the steel so bare that it will often immediately flash rust after the treatment.  The others have varying degrees of speed and safety.  I like to use phosphoric.  It is easy to get, not too expensive, relatively strong, not particularly toxic, and has the advantage of leaving an insoluable coating of iron phosphate on the steel that affords some limited protection against subsequent rusting.  Some soft drinks have phosphoric acid as an ingredient, and this has led some to use it for derusting.  It probably does work, if you're not in a hurry.

There are also non-acidic rust removers that use complex organic molecules called  chelates that can grab and encapsulate certain metal ions, like the iron in rust.  These tend to be non toxic and environmentally benign.  Molasses is a very old rust remover.  It probably works by chelation.  Commercial tank de-rust products that brag about no acid or being biodegradable are probably chelates.

Another method is electrolytic rust removal.  This process is a little like reverse electroplating.  It works well, and there are some who have tried it for the interior of tanks.  The main difficulty with tanks, especially motorcycle tanks, is that an electrode(s) must be inserted deep into the tank, ideally so that it has line-of-sight visibility to every area needing derusting, all while guaranteeing that the electrodes(s) doesn't touch the tank.

There is also pure mechanical rust removal using abrasives.  For the inside of a tank that you can't crawl into, this would usually mean some kind of tumbling or vibratory action using a loose abrasive media inside.

So I made up maybe a gallon of phosphoric acid solution and put it in the tank.  I decided to add a little abrasive action to speed up the process.  I've used gravel (like on my
oil tank).  Small drywall screws or other metal hardware is also popluar.  On this tank, I used a couple of pounds of a media made for the purpose.  It is little abrasive-loaded plastic tetrahedrons.

After maybe 30 minutes with acid and agitation, the tank still had a little rust on the bottom in the rear of the tank, so I repeated the phosphoric process.  I emptied the acid and media from the tank, rinsed it several times with water, and then rinsed with alcohol to scavenge the leftover water and get it to dry quicker.

Here is the inside after the rust removal.  There is  a brownish/yellowish mottled appearance to the inside surfaces due to the deposit of iron phosphate.  (Sorry about that cursor in the middle of the frame.  I'm new to video capture.)

Now that I knew I could clean up the inside of the tank, at least visually, I turned to the outside.  I have a small blast cabinet, but the tank just about fills it up, leaving little room to maneuver the gun.  For most of the tank, I used a chemical stripper to remove most of the paint, then sanded it.

For some of the hard to reach places, I made the blast cabinet work:

At this point, many people would automatically apply some sort of tank sealer to the inside of the tank.  While I understand their motivation, I believe that unless a tank is leaking, sealers are unnecessary, and maybe even unwise.  Sealing systems do sometimes fail, possibly causing new and worse problems.  Chemical sealing systems have their place, but are better used as a last measure to save an otherwise unusable tank.  Even if a tank is leaking, there may be better ways to fix it.

So I needed to determine if my tank was fuel tight.  I was fairly certain that there had not been significant rust damage to the inside of the tank, but leaks can come from other causes.

I decided to do a leak-down test on the tank.  This is a test where the tank is pressurized and left to see how long it will hold the pressure.  I found a pipe plug of the right size to seal the filler neck:


The only other oriface in the tank is for the fuel petcock.  To eliminate as many external sources of leaks as possible, I decided not to use the petcock, but use a simple hose barb adaptor instead.  This turned out to be a little more complicated than it appeared.  What appears to be a 1/4" NPT pipe thread in the bung hole is not.  The thread pitch is 19 instead of the NPT 18.  There wasn't even a 19 TPI leaf in my pretty complete thread gage set (determined 19 TPI by using the 38 TPI gage).  Turns out (seems obvious once you know) that it is British Standard Pipe (BSP) thead.  Possibly if I lived in Europe, this would not have been a mystery at all.

I found a 1/4" BSP-to-1/4" NPT adaptor.  Since it is a BSP straight (non-tatered) theread, there is a sealing washer included:

The pressurization setup consisted of a "MityVac" hand pump (normally used for vacuum, but there is a port on the pressure side), a pressure gage, and assorted tubing and fittings:

It doesn't take much pressure.  In fact, it could be destructive or dangerous to use more than a few PSI in a tank not designed for it.  I pumped up the system to 1 PSI.  At this low reading, my gage is not very accurate, but I'm really only interested in whether the tank can hold pressure over time or not.

The pressure in the system decayed to virtually nothing over an hour or so, indicating a leak somewhere.  

To find a leak, one common method is to dunk the pressureized assembly in a water tank and look for bubbles.  I've done it this way before, and find it messy and cumbersome, especially if the gage and pump are still attached.   I opted for what I think is an easier method.

Using a solution made for the purpose (probably just soapy water), I swabbed all the joints in the tubing watching for bubbles, and found nothing.  The bung hole and filler neck fittings appeared tight.  Next I swabbed the detector solution along all the welds on the tank.  The all appeared tight until...

Right where the rear mounting bracket was joined to the tank on its left side, there was an obvious leak.

The leak was where the rear mounting tab was welded to the tank.  It looked like an incompete weld, or a weld may have pulled loose or cracked.  This would likely be a high stress area, and it's possible that the years of vibration eventually fatigued the joint.  Looking back at the picture of the area around the petcock (fourth picture at top), it appears that the leaking was not all from the petcock.  The crack was small--in fact when I started probing it with a sharp tool to clean it out, it stopped leaking.  I probably shoved some metal or crud into the crack and sealed it.  Nonetheless, I would have to fix it.  Ignoring it would put any new paint at risk.

I decided to braze the area where the mounting tab meets the tank.  I did both sides.  

One more pressure leakdown test, and it looked like I still had a slow leak somewhere.  Using the leak detector solution, I found it on the weld seam near the previous leak.  I'm pretty sure this leak wasn't there before.  Maybe the heat stress from the previous repair caused it

After I fixed that, the tank held pressure for over eight hours with no detectable loss.
  Time for final prep for paint!

July 4, 2011--

After I posted the above, I got a couple of messages that I had overlooked one potential source of leaks.  The way I plugged the filler neck with an expanding stopper put the filler neck joint outside the pressurized area, so a leak at the neck would not have been detected.  Of course, this is correct. Here is the construction of the filler neck joint:

In thinking of ways to repeat the pressure test, I decided that sealing this joint would be a good idea whether it was leaking right now or not. First, I cleaned up the accessible parts of the joint.

Then soldered it:

I had all the stuff out, so I soldered around the base of the neck, too:

To pressure test, I saw problems in just using a gas cap on the neck.  Even plugging the cap vent, the spring loaded cork washer didn't seem positive enough, and besides, it would be hard to check for a small leak around a gas cap.  I ended up bonding a metal disk over the top of the filler neck with RTV.  The five-pound lead weight balances the internal pressure to reduce the stress on the RTV joint.

The tank went over night with no detectable loss of pressure.  Now I think I'm ready for prep and paint,

To other pages

Send comments to: