Copper waashers to the turbo banjo fittings

[Fetnas]

In my efforts to eliminate all the rust from the cooling system whilst completing a head gasket job, I've had all the removable steel coolant pipes zinc electroplated and yellow passivated. So now I don't have to worry about rust that much. But it occurred to me today that I might have made a mistake having the plating done where banjo fittings are required to be sealed with copper washers. Afterall copper and zinc aren't exactly the best metals to have touching each other in a wet environment! I think I just inadvertently created a rudimentary galvanic battery, which will strip of the nice shiny new zinc pretty quickly.

So what to do next? Clean the zinc off where the copper washers sit? Apply a paint coating over the zinc to electrically separate the metals, although the electrical path will still go thorough the bolt? My preferred option is to use dowty seals in place of the copper washers. I know I can get these seals with a 200 degree C rating and they're already used on the oil cooler, so they can easily handle the pressure. My gut feeling is that they should be able to handle the job with little issue, my main concern is whether the heat soak at the turbo exceeds the thermal rating of the dowty seal. In operation it shouldn't get hotter than the temperature of the coolant flowing through the pipe.

Does anyone have any other ideas or options, or am I worring too much about the copper to zinc galvanic issue? It's probably not an issue on the oil lines as oil is a non conductive fluid, but if I'm getting rid of the copper washers, I'll swap them all out.

Thanks in advance to the LEC brains trust on any in-coming advice.

[lotusflasherman]

Yes, you're correct about the potential for the zinc to disappear(couldn't resist a pun ) as on the Galvanic Series it's much more anodic than copper so will behave as a sacrificial anode. Zinc is a favourite sacrificial anode for cathodic protection anywhere that it won't get hot - like on ship's hulls. At high temperature zinc becomes passivated (less negative) and stops working as a sacrificial anode. All those passivated coatings are supposed to do that as well. Ford Motor Co developed the classic yellow passivation (using zinc chromate) nearly 100 years ago but it's now being phased out here due to European legislation because it's toxic and considered to be a carcinogen.

Actually the steel coolant pipe is more anodic than the copper washer so in theory that will corrode away before the copper washer too, just to cheer us up...

Dowty washers are synthetic rubber bonded to a zinc plated steel washer so will certainly sort out the galvanic problem as you'll have zinc to zinc..

[Fetnas]

Thanks for the reply Lotusflasherman. I must admit, it took me longer than I'd like to admit to get your pun, feeling a bit slow this morning!

You've basically confirmed my lack of foresight on this galvanic issue. I've done a bit more research into the use of dowty seal use on turbos since posting. Other forums have anecdotal evidence that they are fine and some people prefer them over copper. I think they were also use on late model SAABs so probably on many other modern turbo cars. Probably cheaper than copper on a production line scale!

There are a couple of Hydraulic supply places near where I work. I should be able to pick up some self centring dowty seals in the correct size and temp rating locally.

[Fetnas]

So sourcing dowty seals proved to be more annoying than anticipated.

I tried Pirtek, the didn't have any M12. I tried Enzed (similar to Pirtek) they didn't have any M12 either! Tired a big bearing and seals place, they pulled out an old multi box of dowty seals that was almost empty, no M12!

I gave up on the dowty option at that point and mail ordered some aluminium crush washer seals, the place I ordered them from sent out 16 & 18mm washers which wasn't what I ordered and the invoice they sent me didn't match my order, or what was sent!!!

Thankfully today I had success. Found an industrial oil seal specific supplier literally around the corner from where I work that had a whole range of dowty options. So I now have self centering M10 and M12 seals and 2 that are Viton to use on the oil line on top of the turbo, I expect that area will be the hottest and Viton can handle over 200 degrees C with short bursts over 300! That should do the trick.

[Simon_P]

Didn't see this before but may be helpful. The zinc and yellow passivated steel will be fine with a copper washer, as it is on things like brake fittings. That's how it would have been originally and how it would be today. Commercial plating will be passivated with trivalent Chromium. Hexavalent Chromium was banned years ago for most uses.

The coolant will have a corrosion inhibitor so it won't corrode from the inside either. That's why you should change the coolant regularly.

Personally I wouldn't use a Dowty washer on the Turbo.

[Fetnas]

Hi Simon,

Thanks for your additional advice on this one. In the end I've gone with the Dowty washers. The banjo fitting are all pretty accessible (unlike many other bolts on this car) and I'll keep an eye on them to make sure they're coping in this application. If I notice any weeping of these seals, it's not a big job to change them later.

One thing that give me confidence that this will be OK is that I painted the oil drain tube with POR15 when I swapped the turbo core whilst living in London. This paint has shown no signs of heat degradation when other brackets I painted at the same time, bolted to the exhaust manifold, have had the POR15 fail. The oil coming out of the turbo will be the hottest of the turbo fluids, so I think it will work.

If I notice any problems in the early days of this set up, I'll report back.