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Question: Is there a way to calculate a suitable electrolyte path length vs. electrolyte resistivity to relegate the effects of galvanic corrosion between aluminum and stainless steel to insignificant?
Answer:
Galvanic Corrosion is a function of several factors like:
electrolyte resistivity, ratio of the surfaces exposed,
potential of the two materials.. Usually, in piping
design, when two different materials needs to be coupled
the solution adopted is to provide a spool of pipe that
isolate the two materials and avoid the possibility of
galvanic corrosion.
The spool can be a plastic pipe or a metallic pipe with and
internal lining in plastic materials.
Another possibility is to paint the more noble material.
Experiments have been performed to assess the lenght of pipe
spool considering different material couples:
http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=A369229&Location=U2&doc=GetTRDoc.pdf
NORSOK (Norvegian Petroleum Association) standard M-001
"Materials Selection",
state that:
At galvanic connections between dissimilar materials without
insulation or distance spool, it can be assumed that the
local corrosion rate near the interface is approximately 3 times higher than the average corrosion
rate, decreasing exponentially away from the interface
within a length of 5 pipe diameters
Due to the difficulty in calculating the real effect on
corrosion the normal approach is to use good engineering
pratice like the one proposed by NORSOK (Norvegian Petroleum
Association):
- Install a distance spool between the dissimilar metals so
that they will be separated by at least 10 pipe diameters
from each other. The distance spool may be either of a solid electrically non-conducting material (e.g. GRP) or of a metal that is coated internally with an electrically non-conducting material, e.g. rubber.The metal in the distance spool should be the most noble of the dissimilar metals.
- Apply a non-conducting coating on the most noble of the
dissimilar metals. The coating shall extend at least 10 pipe
diameters into the most noble pipe material.
Other Companies suggest for example a minimum of 50 cm to
avoid Galvanic Corrosion.
This are other good reference regarding galvanic corrosion
between Aluminium and Stainless Steel:
Bimetallic
(galvanic) corrosion risks from contact with galvanised
steel or aluminium
http://www.bssa.org.uk/topics.php?article=89
Another good reference is the
Mil Standard 889 "Dissimilar Metals":
http://www.everyspec.com/MIL-STD/MIL-STD+(0800+-+0899)/download.php?spec=MIL-STD-889B_NOTICE-3.011336.pdf
The final solution to avoid Galvanic Corrosion is also a
function of the criticality of the service, the possibility
to make maintenance, the consequence in case of failure... Sometimes galvanic corrosion can be tolerated in case of
low impact of a failure, and foreseen periodic inspection
and replacement of the corroded pipe.