atw 2018-02

inforum

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

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