In the note you sent the Prestons, you mentioned that the steel inserts
within the head gasket itself were acting as a sacrificial anode when in the
presence of coolant. You are correct in assuming that a galvanic couple
exists, but I think its really the opposite effect. I don’t have a
“galvanic series” chart handy … so I’m going by memory (a dangerous
thing).
Within an iron (steel insert) and aluminum (block and head) couple, the iron
is considered the cathode and aluminum is the anode. Aluminum will be
displaced in this type of situation. In the case of the Jaguar, this type
of corrosion is considered relatively benign as the area ratio between the
aluminum material and the iron based material is very high. The loss of
aluminum is spread over a larger area with less impact than if the situation
was reversed. From an engineering perspective, its a very acceptable
compromise.
I’m not saying there will not be damage, but its typically spread over a
larger area of the aluminum and it takes considerably longer time frame to
develop a problem. But if given enough time, you will develop a notable
loss of aluminum material. Another nasty characteristic of galvanic
corrosion (or most any corrosion type for that matter) is that it is
temperature sensitive. Increase the operating temperature and the corrosion
rate increases significantly. There are limits to this rule of thumb, but
its a player in the equation for this situation.
I’ve seen the effects of these “area based galvanic couples” on piping
systems (stainless steel interfacing with carbon steel) for years and its
very striking to see. A large anode (carbon steel) to cathode (stainless
steel) area ratio couple can last 20-25 years in a fresh water environment
before a problem (leak) will surface. Reverse the situation with a large
cathode-to-anode couple, and you may see major damage within one year or
less.
Sorry for boring you with the details, but its interesting stuff to see in
person. The moral of this story … all metals seek an oxidized state or
“Jaguar engines don’t last forever”. But they will last a very long time
before extensive repair on the aluminum is necessary.
I think your are right in your electrolysis theory and the resulting
corrosion of the aluminum. It is supported by the corrosion on the surface
of my cylinder head. Unfortunately the corrosion is concentrated around the
coolant passages in the cylinder head and every one has some evidence of
galvanic action. The worst area is the size of a quarter (5 pence) and is
about 10 thou deep. The other are all smaller and about 2 thou deep. The
worst area is being ground down and then welded back up. The entire head
will be resurfaced by removing 5 thou.
My machine shop guy says that this fairly typical and recommends using
distilled water only in the coolant and perhaps a small zinc anode suspended
in the coolant.
Unfortunately, I think that some time everyone on the list will be faced by
this problem.
In the note you sent the Prestons, you mentioned that the steel inserts
within the head gasket itself were acting as a sacrificial anode when in
the
presence of coolant. You are correct in assuming that a galvanic couple
exists, but I think its really the opposite effect. I don’t have a
“galvanic series” chart handy … so I’m going by memory (a dangerous
thing).
Within an iron (steel insert) and aluminum (block and head) couple, the
iron
is considered the cathode and aluminum is the anode. Aluminum will be
displaced in this type of situation. In the case of the Jaguar, this type
of corrosion is considered relatively benign as the area ratio between the
aluminum material and the iron based material is very high. The loss of
aluminum is spread over a larger area with less impact than if the
situation
was reversed. From an engineering perspective, its a very acceptable
compromise.
I’m not saying there will not be damage, but its typically spread over a
larger area of the aluminum and it takes considerably longer time frame to
develop a problem. But if given enough time, you will develop a notable
loss of aluminum material. Another nasty characteristic of galvanic
corrosion (or most any corrosion type for that matter) is that it is
temperature sensitive. Increase the operating temperature and the
corrosion
rate increases significantly. There are limits to this rule of thumb, but
its a player in the equation for this situation.
I’ve seen the effects of these “area based galvanic couples” on piping
systems (stainless steel interfacing with carbon steel) for years and its
very striking to see. A large anode (carbon steel) to cathode (stainless
steel) area ratio couple can last 20-25 years in a fresh water environment
before a problem (leak) will surface. Reverse the situation with a large
cathode-to-anode couple, and you may see major damage within one year or
less.
Sorry for boring you with the details, but its interesting stuff to see in
person. The moral of this story … all metals seek an oxidized state or
“Jaguar engines don’t last forever”. But they will last a very long time
before extensive repair on the aluminum is necessary.