corrosion hole in firewall

[rydfly]

I did an oil change on my bird yesterday w/ my A&P handy and we noticed that corrosion from the battery has eaten a small hole in the firewall near the corner of my battery box. He's sent me on a mission to determine what the C170 experts would recommend as an appropriate fix so that we'll be ready come annual time this Sept.

I've attached a picture. I am assuming that we'll be removing the battery box for thorough cleaning of both it and the firewall, but how to patch/seal the hole?

Also, how tough is it to remove the battery box... are we talking rivets or screws? And replacement batteries... any specific type stand out as the best (i.e., least maint. required)?

Thank you!

-Kennet

[Bruce Fenstermacher]

I believe the there is no clear cut repair and you will get as many varied schemes as posts to this thread. You already mentioned cleaning and neutralizing the area. I might be inclined to simply seal the hole with a fire retardant material such as used were the controls and wires exit the firewall. It is a bit close but you might also drill the corrosion out to a round hole the fill it with a wilkie plug and sealant. If the area flexes and you think it needs to be reinforced then a patch repair riveted in place would be the way to go.

Don't know what a wilkie plug (button) is? Look here: http://www.cessna170.org/forums/viewtop ... 44&start=0

[GAHorn]

The method of repair should be found in Section 19, Para 52B of the 100 Series Service Manual, under "Structural Repair, Firewall".

"Firewall damage may be repaired by removing the damaged material and splcing in a new section of the material. The new section should be lapped over the old material and sealed with EC-539 compound or equivalent, and secured with stainless steel rivets. Damaged or deformed angles or stiffeners should be replaced. A severely damaged firewall should be replaced as a unit."

All batteries produce some corrosive gases, however recombinant batteries probably reduce the risk. Batteries and battery boxes are expensive, and are owner-serviceable (preventive maintenance) so there's little excuse for anyone to allow corrosion to get away from them. Check your battery at least quarterly, when you do your oil changes, right? (Take the battery out, wash the box out with a baking-soda/water solution, dry the box, restore any acid-proof paint necessary, check the battery fluid level, top-charge the battery with a good charger (not one of those cheap trickle chargers from china), clean the terminals, and re-install it. You can save yourself a $1000 (approx) battery-box/firewall repair/replacement expenditure.) Remember, your battery is your in-flight emergency-source of electrical power! Take care of it.

[170C]

Some years ago I experienced a similiar problem with my plane. Every time I opened the battery box there was the old white build up, corrosion I believe, and I would clean it out, repaint the box and sure enough the next time I checked, same thing. Its possible I may have had an overcharging problem, but that was never determined. While at a flyin I inadvertenly left the master on (so I could charge my cell phone ) and ran the battery down. I removed the battery box top so I could take the battery up to the hangar & charge it. More corrosion & it had eaten a 3-4 inch hole in the battery box. Fortunately it had not eaten into the firewall. I removed the battery box, fiberglassed the hole and replaced that battery with a sealed unit and thus far have had no more problems. I think, & hope, the solution was the sealed battery.

[n2582d]

Kennet,
You might want to consider installing a sealed Odyessy battery. The battery box is STC'ed by F. Atlee Dodge but your mechanic would have to get a field approval to install the battery. My FAA inspector in Fresno said it would be no problem to get a field approval but it would pay to check with your inspector before spending any money.

Good advice on checking to make sure that your voltage regulator is not set too high. Might also be worth checking for mud dabbers blocking the battery box drain if you live in mud dabber territory.

[rydfly]

The method of repair should be found in Section 19, Para 52B of the 100 Series Service Manual, under "Structural Repair, Firewall".

"Firewall damage may be repaired by removing the damaged material and splcing in a new section of the material. The new section should be lapped over the old material and sealed with EC-539 compound or equivalent, and secured with stainless steel rivets. Damaged or deformed angles or stiffeners should be replaced. A severely damaged firewall should be replaced as a unit."

Where would I be able to get the identical material to make a patch? Isn't it some kind of aluminum-clad steel? Is there a suitable substitute?

[GAHorn]

It is galvanized or aluminized steel.
You might be amazed to know that there is a company who sells all sorts of aircraft stuff (including galvanized sheet steel) named "Aircraft Spruce"

http://www.aircraftspruce.com/catalog/m ... _sheet.php

You can even buy it in small square quantities, plain, and galvanize it yourself with an aerosol can.

http://www.aircraftspruce.com/catalog/m ... 0sheet.php

877-477-7823

One of the FINEST resources in aviation for any aircraft owner is a FREE CATALOG from AIRCRAFT SPRUCE and Specialty. Ask them to send you one. Read it from COVER TO COVER...just like it was a BOOK.... and you will be so much better informed about airplanes and how to construct, maintain, repair them than you could learn from an entire semester at junior college. PLUS....it's a FREE WISH BOOK!

(I'd bet that the answers to 95% of all inquiries I get about "where can I" .... is solved by my most common, simple answer: Aircraft Spruce.)

[n2582d]

Where would I be able to get the identical material to make a patch? Isn't it some kind of aluminum-clad steel? Is there a suitable substitute?

I'd be glad to send you a square off of my old firewall (when I return from vacation) but if I were you I would use a sheet of stainless rather than the galvanized steel. I would also use Monel rivets which are more corrosion resistant than stainless rivets. Look at any older C-172 and you will see that the stainless rivets on the firewall are rusting.

[rydfly]

It is galvanized or aluminized steel.
You might be amazed to know that there is a company who sells all sorts of aircraft stuff (including galvanized sheet steel) named "Aircraft Spruce"

I know them well, and I'm sure they'll be happy to take my money once more... I'm putting together an order for stuff we'll need for this year's annual anyhow. My concern with the material was that I thought I had read in an earlier post that the firewall was some specific Cessna alloy... I must have misinterpreted.

Which side should the patch be on... engine-side, cockpit-side, or both (a firewall sandwich)?

[GAHorn]

[...Which side should the patch be on... engine-side, cockpit-side, or both (a firewall sandwich)?

I think I'd put it on the most-visible side. (I'd rather look at patch than corrosion-hole.) (Of course, if you were really perfection-minded, I suppose a dollar-and-a-dime* patch might be a good fix.) Just be certain to pick up sufficient bordering area to allay any questions of structural integrity.

* dollar-and-a-dime is a phrase describing a complete cut-out of the damaged area, filled with all-new material and backed by a doubler, ...similar to the appearance of a coin sitting on a piece of paper. The result would be a flush-patch. Lots of work however, to get to the backside of the firewall. A simpler fix might be to put the patch on the engine side and use self-plugging (Cherry Max) rivets.

[Roesbery]

I have found that you can almost eliminate battery corrosion by preventing any contact of the battery itself from any nearby metal. Wood, plastic, rubber, fiberglass all seem to work. I first noticed this on my commercial fishing boat, the batteries were in a fiberglassed wood box and never had any terminal corrosion. If someone could come up with a open top plastic box that would fit inside the metal battery box and a battery could be set into, it would solve most corrosion problems. Even a shallow tray a inch or so high would work as long as no other part of the battery could touch the surrounding metal.
A little dab of Corrosion Block on the terminals doesn't hurt either. My idea is that a electrical circuit forms through the acid that developes on the outside of the battery and any conductor it contacts.

[rydfly]

How is the battery box fastened to the firewall... is it screws/bolts or rivets?

[GAHorn]

How is the battery box fastened to the firewall... is it screws/bolts or rivets?

Rivetted. (In your '53 model, you can likely see most of them thru the heater crossover ductwork ventilators and fwd of the rudder pedals.)

[rydfly]

Oh boy, sounds like fun times are ahead...

Does the EC-539 sealing compound go by another name? Google and A/C Spruce searches haven't been very helpful.

[Brad Brady]

Poky,
Were you able to call someone with a fuly charged cell phone to fix your fully discharged battery?