[track73]

I was reading the post about the plug being caught in the block and it brought back a conversation I had with my machinist. When putting the parts together for my build we were discussing brass plugs vs. steel. He made the comment about the electrolysis with dissimilar metals. I know that happens with plumbing so why doesn't it happen in auto engines?

[steve25]

Allow me to ask this, why would you not put in steel plugs when even if you drive the car thru 25 years of salty road northeast Winters they don't rust out ?

The only reason I find for brass plugs is that they do pop out sooner if the motor freezes up and may save the block from cracking!

[74Grandville]

I’ve had the center steel plugs rust through, but it could be related to the block being filled. The outers did not seem to have this issue at the time.


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[Tom Vaught]

Aluminum parts, for the most part try to protect themselves from corrosion by forming Aluminum Oxide on the surface of the aluminum.
That is why Holley aluminum carbs look crappy but are not corroded for the most part. The aluminum is trying to protect itself.

Most metals resist corrosion pretty well except when you mix copper with steels or aluminum materials. Think screwing a copper pipe into a aluminum thread. Brass is different vs copper.

It has to do with the pH range of the materials.

Galvanic corrosion of aluminum, for example, only occurs under specific conditions.

Galvanic corrosion does not occur in dry, indoor air. It does not occur in rural areas air. It does occur at times by the ocean.

So you need the material type and the location of where the part will be.

Carbs used to be made out of zinc material.
The carbs were coated by the manufacturer with a Dichromate coating to protect the carb material.

When the dichromate material is gone you get metal corrosion of the parts. The white powder inside Rochester 2 bbl carbs is not from the gas putting the powder in the fuel bowls. The fuel bowls have "Tooth Decay" and the materials are rotten (like rotten teeth).

So if you can clean the corrosion off of the zinc and have the carbs recoated with a fresh coating of dichromate the they look like brand new again.

So to answer your question. You can have corrosion in many materials. You can protect some of the materials like zinc with dichromate coatings or steel with Galvanized Coatings.

In engineering of materials they call it Major vs minor materials and the major material always wins when they come together.

Tom V.

[dataway]

Does happen on cars. Just look at aluminum components in a 60's - 70's era car that hasn't had the coolant maintained well. The most reactive metal goes first, generally "sacrificing" itself before the adjacent less reactive metals corrode. Apply a small amount of electrical potential and the process increases rapidly. Add in a substance that enhances the flow of electrons ... like an acid and it increases even more, add heat and again the process accelerates.

Also known as galvanic corrosion, or bi-metallic corrosion. A battery basically uses the same process in reverse to create voltage. Metals like iron, aluminum, magnesium, zinc are very reactive, while metals like gold, silver, bronze, nickel, SS (due to the nickel content) are less reactive.

Several years ago I used some bare 6061 1/4" thick aluminum angle to reinforce the goose neck hitch on my truck. They were mounted underneath the bed with the through bolts to the hitch going through the bed and into the hitch. They were the only pieces of aluminum within several feet of where they were mounted, and they probably received a bit of voltage with the trailer probably trying to ground out through the hitch when pulling a lot of current from the truck (trailer had living quarters) rather than grounding just through the trailer connector.
Within three years, living up north, those aluminum pieces turned to dust. Never saw anything like it, was the perfect storm of dissimilar metal corrosion. These were substantial pieces of metal, four of them, you could break them in half with your hands. Luckily I caught them before race season that year and replaced with stainless steel.

As a side note ... that truck had an incredibly small amount of rust on it for it's age and mileage in a northern climate .... makes me wonder if those aluminum pieces saved it from several years of corrosion by sacrificing themselves. Back in the day you used to see sacrificial zinc anodes for sale for cars, just like seen on ocean going ships.

[Tom Vaught]

Nice Post Dataway. I agree with you high current passing thru the metal (lots of electrons) will for sure destroy the anodes in short order.
In your case the anodes being those aluminum reinforcement plates.

Tom V.

[dataway]

I should try bolting on some aluminum pieces to the bottom of my new truck with ground straps that drag the road

[Tom Vaught]

Problem is you could not drive the trailer anywhere with the ground strap attached to the 10 foot long grounding rod required. LOL!

Tom V.

[track73]

When I was real young I remember seeing gasoline tanker trucks with chains dangling from them. I guess it was to rid them of static electricity.

[GRX]

Brass and iron are not far enough apart on the galvanic index to be a problem unless your coolant has electrolytes. It is formulated against that.

Always found customer's insistence of brass to be interesting, and totally unnecessary unless a marine application. There are the inevitable stories of corroded plugs from poorly maintained coolant systems of course. Fact is these engines are very unlikely to see that kind of neglect today.

On the other hand, people do like shiny stuff. Perfectly detailed engines with green corrosion on bare brass plugs because they looked good while on the engine stand.

[77 TRASHCAN]

Years ago I asked my engine builder about steel vs. brass. He said he only used brass on race cars that had straight water in them, and that proper coolant mix would give long life to steel plugs.
He also showed me how to properly install a plug. They are driven in with a driver like a cam bearing. Not using a socket....