atw 2018-02

atw Vol. 63 (2018) | Issue 2 ı February
104
DECOMMISSIONING AND WASTE MANAGEMENT
Corrosion Processes of Alloyed Steels
in Salt Solutions
Bernhard Kienzler
Introduction For many years, in Germany POLLUX canisters were considered as reference concept for spent
nuclear fuel disposal casks. The cask consists of the shielding cask with a screwed-in lid and the inner cask with bolted
primary and welded secondary lid. The spent fuel should be inserted in the final disposal cask in bins. The cylindrical
wall and bottom of the inner cask consist of fine-grained steel 15 MnNi 6.3. The thickness of the cylindrical wall was
designed according to the mechanical and shielding requirements and was 160 mm thick. The primary lid of the inner
cask was also made of fine-grained steel. This lid was designed to keep the sealing function prior to and during the
welding of the secondary lid. A plate made of neutron-moderating and absorbing materials (carbon/boron mixture)
was attached to the primary lid. The secondary lid is designed as a welded lid. The base body of the shielding cask
consisted of ductile cast iron (GGG 40). The wall thickness was designed according to the requirements for the shielding
and was 265 mm thick. The weight of the POLLUX cask was 65 Mg [1]. The whole POLLUX cask consisted of actively
corroding steels.
The corrosion behavior of the POLLUX
materials in salt solution for temperatures
up to 200°C were investigated
[2]. Both materials showed high corrosion
rates especially at elevated
temperatures and frequently the question
was asked why not using alloyed
steels. In fact, alloyed steels are developed
to be corrosion resistant, and the
steels are widely used especially for
corrosion-resistant applications.
Alloyed steels such as stainless
steels do not readily corrode, rust or
stain in contact with water as finegrained
or cast iron steels. However,
the alloyed steels are not fully stainproof
in low-oxygen or high-salinity
environments. There are various
grades and surface finishes of stainless
steel to suit the environment the
alloy must endure. Stainless steel is
used where both the properties of
steel and corrosion resistance are
required.
Stainless steels differ from carbon
steel by the amount of chromium
present. Unprotected carbon steel
rusts when exposed to air and
moisture. The iron oxide film has
lower density than steel, the film
expands and tends to flake and fall
away. In comparison, stainless steels
contain sufficient chromium to
undergo passivation, forming an inert
film of chromium oxide on the surface.
This layer prevents further corrosion
by blocking oxygen diffusion to the
steel surface and stops corrosion from
spreading into the bulk of the metal.
Passivation occurs only if the proportion
of chromium is high enough
and oxygen is present.
In the scope of corrosion studies
of high-level waste canister materials,
the corrosion behavior of several
alloyed materials was investigated.
The materials comprised nickel based
alloys (Hastelloy C22 and C4), and
chromium-nickel steels. Furthermore,
titanium alloys and copper-nickel
alloys were taken into the investigations.
These alloys are not covered in
this contribution.
The recommendations of the
German High-Level Waste Commission
[3] are reflected in the German law for
amendment of the site selection law
(passed by the German Parliament,
March 23, 2017 [4]). Especially the
maximum temperature condition has
been changed. The maximum temperature
at the canister surfaces is now
limited to 100 °C, and the retrievability
of the wastes during the operational
phase of the disposal and the recoverability
of the wastes for a period of 500
years is need to be taken into account.
Corrosion mechanisms
of alloyed steels
The corrosion resistance of stainless
steel (Cr-Ni steel) known under
atmospheric conditions depends on
the chromium content of the alloy.
Chromium leads to the formation of a
passive layer, the so-called chromium
oxide skin, which spontaneously
forms in air and protects the material
underneath from corrosion. By
alloying different chromium and
molybdenum fractions, the corrosion
resistance can be adjusted to the
environmental conditions. The low
corrosion rates of Cr-Ni steels are due
to the build-up of passive layers (oxide
layers) on the surface, which are
re-established under the conditions of
low-concentrated solutions.
The stability of container materials
in a deep underground disposal is
influenced by various uniform and
local corrosion processes. These
processes are controlled by the local
geochemical conditions, in particular
pH, redox potential and chloride
concentration. Iron and steels are not
thermodynamically stable in contact
with water or saline solution. A
number of different corrosion processes
are described depending on a
variety of factors [5]. For metals, two
types of corrosion occur: general and
localized corrosion.
• General or uniform corrosion
results in a relatively uniform mass
loss over the entire area of the
sample. General corrosion effects
are predictable. Cast irons and
steels corrode uniformly when
exposed to open atmospheres, soils
and natural waters as well as in salt
solutions.
• Localized corrosion occurs at discrete
sites on the metal surface.
The areas immediately adjacent to
the localized corrosion normally
corrode to a much lesser extent.
These types of corrosion are less
common in atmospheric exposure
than in immersion exposures.
Corrosion activity at localized
corrosion sites may vary with
changes of the water composition,
defects in passivation layers,
changes in contaminants or
pollutants, changes in the electrolyte
and by formation of
galvanic cells. The predominant
forms of localized corrosion are
pitting and crevice corrosion.
• Pitting corrosion is especially
prevalent in metals that form a
protective oxide layer. Pitting
can be initiated on an open,
freely-exposed surface or at
imperfections in the passivation
layer. Deep, even fully penetrating
pits can develop with
Decommissioning and Waste Management
Corrosion Processes of Alloyed Steels in Salt Solutions ı Bernhard Kienzler