Why do people use brass freeze plugs, thermostats,
and hardware in the cooling systems of engines with
aluminum heads and blocks?

These two materials are dissimilar enough that
galvanic corrosion is prone to occur. IIRC, brass is
the more cathodic or 'noble' while aluminum is the
more anodic or sacrificial. Put 'em together, add an
electrolyte (the coolant) and watch the aluminum rot
away.

I noticed today that my aluminum radiator has a
brass petcock. My old 525 received a brass
thermostat to replace a galvanized . I've seem
several photos here of aluminum blocks with brass
freeze plugs.

What gives here? Have the elements been rearranged
on the periodic table or are people unaware of the
consequences of such actions?

.

 

 

Well your choices are limited for the items you mention...An alumnium petcock won't be strong enough, and anything alumnium with male threads isn't going to last, the threads will strip off too easily. So, for those things its either brass or steel and steel & alumnium together is a very bad combination. As far as freeze plugs go...alumnium won't be strong enough either.
I work in the aerospace industry and we use alumnium and brass together on lots of things and the brass is usually the first of the 2 metals to corrode...Remember, the alumnium found in heads, radiators and such aren't pure alumnium, they are alloys and these alloys are much less prone to galvanic and electrolytic corrosion than pure alumnium

 

I think the big factor is the use of coolant to help prevent corrosion. BMW is using magnesium engine blocks in their new six cylinders and that stuff corrodes real well. I guess they figure with long life coolant it will make the warranty.

 

The other interesting possibility is using Magnesium somewhere in the coolant system as a sacrificial anode. Does anyone have a good source for these? I know that these sacrificial anodes are used in some aluminum block engines.

 

Certain conditions need to be present for corrosion to
occur.

• dissimilar metals
• conductive path
• an ion path from the anode to the cathode

The greater the voltage difference between the
materials and the large the cathode is compared to an
anode, the more pronounced corrosion will be.

With an anodic index of 0.85 V, mild steel appears to
be much closer to conventional wrought or cast
aluminum (0.90-0.95 V) than is brass or bronze
(0.40-0.45 V).

Aluminum, steel and iron are toward the anodic end of
the chart and are sacrificial (they rot away) relative
to brass which is toward the cathodic end of the chart.

Coolant when new is pretty benign, but in time it can
become much more conductive.

Reducing or eliminating the dissimilarites between
materials is ideal. Freeze plugs are, or used to be
available in steel as well as brass. As you mentioned,
petcocks are available in steel and suitable plastic,
too, I believe. In at least some cases, thermostats
are available in steel.

Coatings can interupt the paths and prevent or slow
corrosion. We see this in use on the U-joints for
the aluminum drive and half-shafts.

Keeping the coolant fresh reduces or eliminates its
conductive role in corrosion.

Here are a couple of short, informative discourses on
the matter. Searches will turn up many others.

Galvanic Corrosion by Dr. Stephen C. Dexter.

Galvanic Compatibility Unknown - (Engineers Edge).

I know there's a potential for problems. I don't have
the answers. I'd wondered about the things I've seen
done as folks make changes to the OEM configurations
and I thought I would raise a flag.

.

 

Yes, I took note of that when the new 3-Series was
reviewed and it was a key reason why the car was
not our list when we bought new this fall.

I have learned the hard way - three times, in fact,
that BMW and I have different expectations of the
cooling system.

.

 

Adding anodes came up in a thread I recall here last
year. In that case, Zinc was mentioned.

Someone mentioned that this is used for boat
protection amd can be sourced from marine supply
houses. It was indicated that placing the material
in the coolant passage would be insufficient for
conductivity. There was a reply that the material
could be tightened into an unused sensor hole.

If I locate the post, I will add a link.

.

 

I am afraid that its no solution. The current path will be very local and does not give a protection anywhere else. The zinc blocks in ships have to be mounted close the the propeller in order to protect the material for to local currents near the mass of brass / bronze.

 

How thick is a freeze plug and what is its mass compared to the block? I suspect the strategy for the brass plugs is to protect the plugs themselves. Automotive coolant has specific protection for aluminum (radiators, heater cores and blocks).

 

Aluminum dart blocks have threaded anadized aluminum freeze plugs with o-rings.

 

As long as the cooling system is properly maintained with fresh coolant there should be not problems. It contains specific coorrosion inhibitors that will prevent damage. I found an interesting article that is worth a read: "Proper coolant use is a chemical balancing act"

 

Enjoyed the article. Pat Bedard consulted with Dr.
Turcotte for two columns Bedard wrote in C&D in June
and July of 2002.

I agree about the role of coolant but they are talking
about stock configurations. I wrote out of concern
about cases where parts are changed for ones made
of different materials with the result of unwittingly
increasing the tendency toward corrosion (and
making the job of coolant more difficult.)

I thought the following remark was interesting.(I and most of my neighbors came to similar
conclusions in much less than ten years time.)

Thanks,
Ken R.

.

 

I don't have a full understanding of the chemistry at work here, but I have always connected a reasonably heavy copper cable between my alum heads and iron block in order to keep them at the same potential, and to create an easier path for any current, so the coolant doesn't carry current.. Is this worthwhile, or a waste of time?

 

Waste of time. Metals have an inherent electrical potential, that can't just be shorted out against eachother.

Lead acid batteries work based on this potential.

Your heads will conduct plenty well to the block through the head bolts and head gasket, which is partly metalllic.

 

Here's a product that along with proper coolant maintenance will counteract reactions from dissimilar metals and other materials in a system. I've used and have seen this product used for years and I can tell in an instant when I tear an engine down, whether or not it's been used in it. Not only will there be zero deposits anywhere, the inside of the block will look like brand new, not even any discoloration.

http://www.penray.com/hdcatalog/liquid3000.htm

 

The Pencool 3000 you suggested and Fleetguard DCA4
appear to be popular with the HD diesel crowd. I did
not find any negative remarks, nor anything counselling
against use in gasoline applications in this brief research.

Also, while looking into the Pencool, I came across a
reminder about how the introduction of new/virgin
aluminum such as a replacement radiator, heads,
intake and/or water pump increases the consumption
of the SCAs (corrosion inhibitors) in the coolant until
such time as the normal coating of oxidation forms
on the surfaces of the passageways that the coolant
moves through. This means to me that the coolant
should be replaced earlier or additional SCA should
be added to help replentish the inhibitors.

Thanks,
Ken R.

.

 

That's exactly what it does, maintains the SCAs. The 3000 is for all types of engines. The 2000 and 3500 is for diesels specifically.

 

From what I can understand, Pencool 3000 is strictly
an SCA (SCA - Supplemental Coolant Additive), as
opposed to an alternative coolant (as I initially mistook
it to be.)

Also, the intent isn't that it is included with new
coolant, rather it is added to existing coolant as part
of routine (annual) maintenace during the lifecycle of
the coolant. IIUC, the coolant is still replaced on a
periodic basis.

Others would want to verify this interpretation and
see whether they come to the same conclusion.

I wonder whether it has any perceptible negative
effect on cooling performance. You mention that
engines with it show no build-up inside so I'd suppose
cooling would be the same/better compared to an
engine without this maintenance.

.

 

Can you clairify whether you are commenting
specifically about use of a cable between Al heads and
an iron block - or about the concern over mixing
dissimilar metals when reassembling with new parts.

.