Oil Pan Drain Area Corrosion
 

I took the belly pan off for the first time ever. Considering I have a 10+ year old Q5, driven entirely in Wisconsin, most everything looked to be in decent shape except for some significant rust around the oil drain plug. The circular pattern of the rust suggests to me that this is primarily galvanic corrosion caused by the crush washer, which appears to be aluminum. Aluminum is not terribly reactive to carbon steel but I am not sure what else could cause the rust to expand in a near perfect round pattern. Here is a photo of the problem... it looks a bit wet because I decided to spray some WD40 on the area to help remove any moisture:
https://cimg0.ibsrv.net/gimg/www.aud...c0ea12c376.jpg
In order for the rust to have started, the black finish would have likely been damaged or scratched at some point. For the first 7 years, I only used the local Audi dealership for oil changes, until I started doing the oil changes myself using a vacuum extractor. So I think the problem started at the dealership or perhaps even on the production line.

My plan is to prep (wire brush) the area and then cover it with the following rubberized undercoating and rust reformer:
And then I think I may replace the crush washer with an over-sized rubber washer and stainless steel (non crush) washer, with the rubber washer up against the oil pan. This would stop any further galvanic corrosion but not too sure how the rubber seal would hold up.

Just wondering if others have encountered a similar problem on the oil pan ? And would love to hear any other suggestions to stop/prevent further corrosion ? I don't think the oil pan is very thick and I would not be surprised if this corrosion could begin to cause a leak in a few years. I did clean up a little bit of old oil on the bottom of the pan and it was obvious it came from the drain area but it may have been there for many years. I also looked at changing out the pan myself. It is only about $50 but I discovered that it is not a simple job (due to access) and not one I really want to tackle.

If the car has a drain plug, I would recommend removing and reinstalling it at every change. You can still use the vacuum but the presence of a plug typically indicates the oil circuit is designed to fully drain with the plug.

It isn't worth the time to repair the pan. That is more than surface rust. You can't remove all the rust with the plug installed. Get a new pan, seal, fasteners and have a shop change it all at the next oil change. Can't be more than two hours of labor and the oil change would be done at the same time. Regularly install a new washer in the future.

A rubber washer may seal for a little but it may become brittle and start to leak. Especially as the surface continues rusting. A rust converter won't stop the rust, only pauses it for a short while.

Not the answer you wanted to hear, but that's what I would recommend.

My recommendation is to removed the drain plug during your next oil/filter change, sand the corroded area, prime, and re-paint. Unless there is another reason, no need to replace the pan.

This is what mine looked like the last time I did a drain (use an extractor now) just for another data point. The corrosion really isn't hurting anything as long as your oil isn't coming into contact with it.

https://cimg3.ibsrv.net/gimg/www.aud...5c9b5bc052.jpg

Thanks guys! I will definitely do the next oil change from the bottom (no extractor) and see how feasible it is to clean and sand down the pan for a coat of paint/primer. Silver_tt's photo is very helpful... and appears to be another rust belt Q5. :D The drain area is obviously much thicker than the rest of the pan and perhaps the extra thickness extends even beyond my rusted area. If so, I don't think I have much to worry about for another 10 years... as long as the drain holds a seal.

As for using an extractor, I have no concerns. I always get very near a full 5 quarts out before refilling. And, don't know if it is true, but I've heard in the past that some Audi dealers use an extractor for oil changes. However, I think I may pick up a magnetic drain plug from ECS or 034 for this next oil change.

Yep, my car has lived around snow/salt in the winter its entire life :)

I went ahead and "repaired" my oil pan corrosion prior to doing an oil change and I expect I will have to do a little touch-up after removing the drain plug for the oil change. I used a Dremel with a small wire wheel first and then followed up with a little sanding drum... worked well and was able to clean up the visible exterior rust in about 10 minutes. And then I used 3 coats of the Rust-Oleum rubber undercoating that I linked above. It is easy to apply with a brush with good coverage and easily adheres to bare metal or slightly rusty metal, no primer needed. It dries quickly and a second coat can applied after just an hour or so. Not a pretty job but it is out of sight and I was only concerned about function. Time will tell how well it holds up and prevents rust from re-forming.
https://cimg8.ibsrv.net/gimg/www.aud...ae4f207e23.jpg

Since I discovered this problem, I have come across several videos where others have had similar corrosion on the oil pan of their B8 Audi. FCP Euro also has a how-to video and they state in the beginning that this is a common problem:

Thanks. Interesting, as you mention he says one of the reasons is the corrosion but it’s so bad you can almost “poke your finger through the pan” as he puts it. Wow…..

​​​​​​The oil pan seems to have the same poor/thin finish as the rear subframe. I know Audi changed the design of the rear subframe on the B9's, and I suspect the oil pan was changed too. An aluminium pan would be a nice upgrade.

B9 Q5 Oil Pan is plastic.