[DML] Re: Galvanic corrosion

["Toby Peterson" <tobyp@xxxxxxxxxxxx>]

Charles - This is a good site for basic information like this.  I 
haven't seen this kind of data presented this way.  The military 
specification manuals express it differently, but this site will work 
fine.  They propose an "anodic" index, which is just a measure of the 
galvanic potential for each material.  If the materials are close 
together in terms of this value, then no corrosion will occur (all 
things being equal).  The farther apart they are in the anodic index, 
the worse the corrosive potential will be.  The stainless steels that 
we have in the car are listed as the "18% chromium type", with a value 
of .50.  The engine block and other aluminum bits are probably in the 
category "Aluminum, cast alloys other than silicon", with a value 
of .95.  The distance between these values is .45.  The "normal" 
carbon-based steels used for most bolts are at .85, while the cadmium 
plating used on many bolts is also .95.  So, the difference in anodic 
values with the aluminum is only 10 at the most.  The paragraph above 
the table on the site says that in a harsh environment, the difference 
should not be more than .15 to avoid galvanic corrosion.  Therefore, 
all things being equal, there is a concern with using stainless steel.

However, the additive packages in most good aluminum-compatible 
antifreeze products are designed to prevent this kind of corrosion, or 
at least slow it way down.  When the antifreeze is old, the additives 
dissipate, leaving the system open to this type of damage.  As has 
been said on this List many times, change your antifreeze every two 
years and you should be fine.

Toby Peterson  VIN 2248 "Winged1"
DeLorean Parts Northwest, LLC
www.delorean-parts.com


--- In dmcnews@xxxxxxxxxxxxxxx, "Charlesdos Mavor" <charlesmajor@xxxx> 
wrote:
> Here is one more site, it gives the numbers and limits. I know that
> there are some really smart people on the list that can break it all
> down for us. So, my question is should we be using plastics? Small
> changes like the SS water return pipe and the SS coolant bottle may 
be
> problems in 15 or 20 years for the block. I am asking not slaming
> products (I have the SS pipe and want the bottle).
> 
> http://www.engineersedge.com/galvanic_capatability.htm
> 
> Charles







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