Galvanic Corrosion
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 temperature.
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.
Dealloying 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)
Graphite
Platinum
Ni-Cr-Mo Alloy C
Titanium
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
Silver
Nickel 200
Silver-bronze alloys
70-30 copper Nickel
lead
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
Copper
Tin
Naval brass, yellow brass, red brass
Aluminum bronze
Austenitic nickel cast iron
Low-alloy steel
Low carbon steel, cast iron
Aluminum alloys
zinc
Most Active or Anodic
|
