Galvanic corrosion occurs when two dissimilar metals are connected electrically
and are in contact with an electrolyte solution. One of the two metals is corroded
preferentially; this metal is the anode and the unattacked metal is the cathode
in the galvanic couple.
One example found in the oilfield is when a new section of pipe is added to an
older section. The new pipe becomes anodic and corrodes preferentially.
The Galvanic Series is a list sorted by corrosion
potentials for various alloys and pure metals in sea water. It should not be confused
with the emf series
. The emf series is a list of half-cell potentials for standard state conditions
measured with respect to the standard hydrogen electrode, while the Galvanic Series
is based on corrosion potentials in sea water.
Each metal or alloy has a unique corrosion potential, Ecorr
, when immersed in a corrosive electrolyte. The most negative or active alloy is
always attacked preferentially by galvanic corrosion, whereas the more noble metal
becomes cathodic (where reduction of hydrogen ions or oxygen takes place) and is
protected from corrosion.
Often the relative areas of each metal exposed are more important than their
position in the galvanic series. If the anode (more active metal) has a large area
with respect to the cathode (more noble metal), the small area of the cathode will
not provide enough current to support uniform corrosion of the anode. However, if
the anode is small in comparison to the cathode, the rate of corrosion of the anode
will be greatly accelerated and corrosion will be localized adjacent to the more
noble metal. When using coatings to prevent galvanic corrosion, it is important
to coat the more noble metal rather than the active metal, so that when defects
are introduced to the coat, the effects are not catastrophic.
There are some well-known examples of bimetallic (galvanic) corrosion. For example,
N-80 couplings connected to J-55 tubing always corrode preferentially to the J-55
grade at fairly rapid rates in wet CO2 environment. Stainless steel valve in cast
steel body also create a galvanic couple. Corrosion occurs immediately adjacent
to the more noble metal.
Galvanic corrosion is also frequently observed in downhole pumps. Pump barrels,
balls and cages are usually made of different alloys that may form galvanic couples.
Pump barrels are also chromium plated for increased abrasion resistance. However,
chromium plate may be scored by sand grains or crack, which leads to severe galvanic
corrosion that is rapid and usually catastrophic. Electroless nickel plating also
suffers from galvanic effects
There are many subsets of galvanic corrosion. A piece of metal is not uniform
on the microscale, but contains grain boundaries and precipitates. These precipitates
are electrochemically different from the base metal, and may act as cathodes or
anodes with respect to the base metal.
Stainless steel, an alloy of chromium(Cr), nickel and iron, requires at least
12% Cr for passivity. If stainless steel is heated to a high temperature (such as
425 C), chromium carbide precipitates will start to form along grain boundaries,
leaving a zone depleted of chromium. The precipitates will dissolve back into the
grain structure when heated above 850 C and fast cooled (quenched) back to room
Stainless steel may become sensitized during welding. The area surrounding the
weld bead is known as a heat affected zone (HAZ), a zone depleted of chromium, which
will preferentially dissolve away. Therefore, post-welding heat treatment or the
use of low-carbon varieties is needed to prevent grain boundary corrosion.
The following picture shows a weld at the granular level:
Another well-known example of HAZ corrosion in wet CO2 service is the failure
of upset J-55 tubing that has not been full-length normalized (heat treated) after
upsetting. This form is known as “ringworm” corrosion and it usually
occurs 4-6 inches below the upset in the heat-affected zone that has a different
microstructure from the rest of the tubing.
occurs when one or more components of an alloy are more susceptible to corrosion
than the rest, and are preferentially dissolved. The most important example of dealloying
is the removal of zinc from brass, known as dezincification.
Another common example is graphitic corrosion, which occurs in gray cast iron. In
graphitic corrosion, the graphite acts as a cathode, anodically dissolving the iron
and leaving a graphite frame. This frame maintains its shape but loses mechanical
strength. Graphitic corrosion is observed in buried cast iron pipe after many years
exposure to soil; it can also be seen in cast iron cannons in ships that have been
sunk at sea.
Galvanic Series for Seawater
- Most Noble or Cathodic (resistant to corrosion)
- Ni-Cr-Mo Alloy C
- Ni-Cr-Mo-Cu-Si Alloy G
- Ni-Fe-Cr Alloy 825
- Alloy 20 stainless steels, cast and wrought
- Stainless steel Types 316, 317
- Nickel copper alloys 400 K-500
- Stainless steel Types 302, 304, 321, 347
- Nickel 200
- Silver-bronze alloys
- 70-30 copper Nickel
- Stainless steel Type 430
- 80-20 copper nickel
- 90-10 Copper nickel
- Nickel silver
- Stainless steel Types 410, 416
- Tin bronzes (G & M)
- Silicon bronze
- Manganese bronze
- Admiralty brass, aluminum brass
- 50 Pb-50 Sn solder
- Naval brass, yellow brass, red brass
- Aluminum bronze
- Austenitic nickel cast iron
- Low-alloy steel
- Low carbon steel, cast iron
- Aluminum alloys
- Most Active or Anodic