atw 2018-02

inforum

atw Vol. 63 (2018) | Issue 2 ı February

only a relatively small amount

of metal loss. Pitting can occur

isolated or as group of pits

which may coalesce to form a

large area of damage.

• Crevice corrosion occurs in

crevices where the environment

differs from the surrounding

bulk environment. The different

environments result in

corrosion because of differences

in concentration (e.g.,

oxygen, pH, and ferric ions). If

there is an oxygen concentration

difference, corrosion will

proceed at crevices where less

oxygen is available than in the

environment surrounding the

crevice. Crevices are formed

when two surfaces are in

proximity to one another, such

as when two metal surfaces are

in close contact.

• Contact (galvanic) corrosion

occur when different metals are

in contact in a common electrolyte.

At current flows between

the two metals, the less noble

metal (the anode) corrodes at a

faster rate than would have

occurred if the metals were not

in contact. In this case, the rate

of corrosion depends on the

relative areas of the metals in

contact and the composition

(conductivity) of the electrolyte.

• Stress corrosion cracking

(SCC) requires the simultaneous

presence of tensile

stresses (effect of external loads

or welding / bending) and

specific environmental factors.

• Intergranular attack is caused

by carbon diffusion to the grain

boundaries and precipitation as

chromium carbide. This effect

removes chromium from the

metal phase (solid solution)

leaving a lower chromium

content adjacent to the grain

boundaries.

Especially in environments with high

chloride concentrations, chloride

promotes the breakdown of the oxide

layer. In the presence of chloride ions,

oxygen can be displaced by chloride

ions in the oxide layer of the passivated

metal. The addition of further

chloride ions results in a region which

is no longer protected by the oxide

layer. This site now offers an attack

point for further corrosion. Under

favorable circumstances, a so-called

re-passivation may occur: the chloride

ion is displaced again by oxygen, and

the protective oxide layer is “repaired”

again. Otherwise, the pitting corrosion

continues. The rate of displacement

of oxygen by chloride in the

passivation layer is the measure of the

incubation period for the occurrence

of local corrosion processes. The

following mechanisms effect pitting

corrosion [6]:

• The dissolved oxygen concentration

outside of the pit is considerably

higher than in the hole. The

low oxygen concentration in the

pit hinders re-passivation of the

metal.

• The small pit forms an anode, the

remaining surface represents the

cathode. The corrosion rate is

determined by the ratio of the

cathode to anode area.

• The metal dissolves according

Me n+ + H 2 O + k MeOH (n-1)+ +

H + , reducing the pH.

• Critical potential must exceed a

certain critical potential value. In

salt solution, the critical potential

is defined by E pit = A + B log [Cl − ]

with Cl − is the bulk chloride

concentration. B is generally in

the range 60-90 mV [7]. Critical

pitting potentials (E pit ) of 1.4301

Cr-Ni steel (type 304, UNS S30400)

are reported by Yashiro et al [8] as

a function of temperature (373 K

to 523 K) and chloride (Cl − )

concentration (0.01 to 2 mol/kg-

H 2 O). Steady polarization tests

were performed at discrete intervals

around Epit. Results were

expressed by E pit = A − B log [Cl − ].

In regard to temperature dependency,

the constant A decreased

with temperature, while B was

almost constant up to 448 K.

• In the presence of Cl − , the dissolved

metal in the pit reacts with

chloride forming iron chlorides

which hydrolyses (FeCl 2 +H 2 O vk

FeClOH + Cl − + H + ) and reduce

the pH.

The actual water consumption for

pitting corrosion is substantially lower

than in the case of uniform surface

corrosion of unalloyed steels. Carbon

steels also shows a passivation in the

alkaline environment, e.g. at pH > 12

in concrete constructions [9].

In contrast to alloyed steels,

unalloyed carbon steels do not build

up a protective layer under low or

slightly basic pH conditions, since

the alloying element chromium is

missing. Under acidic to basic pH

conditions voluminous iron oxides /

iron hydroxides are formed, which

generally do not adhere to the underlying

material. Therefore, the steel is

not protected but the oxidation is

maintained under the influence of

moisture and oxygen. This reaction

observed in the unalloyed steels is

referred to as an active corrosion

process in which iron reacts to iron

oxide/hydroxide. Numerous experiments

have shown that the active

corrosion of the unalloyed steels is

uniform and at a largely constant rate

[10–16]. This behavior allows predicting

the mass loss or thickness

reduction of the disposal cask to a

certain degree.

The corrosion experiments reported

here were performed in salt

solutions. Under reducing conditions

as they prevail in a deep geological

disposal, the corrosion process of

carbon steel consumes water and

generates hydrogen. During the corrosion

process, dissolved iron reacts

with the aqueous medium forming

ferrous hydroxides with divalent iron

(Fe II ). At 7 < pH < 9, the observed

solid corrosion products are magnetite

(Fe 3 O 4 ) and amorphous iron

hydroxides. At sufficiently low redox

potentials (absence of oxygen) in

chloride solutions, Cl − ions react with

amorphous iron hydroxides forming

the reaction product “green rusts”.

This compound has the formula

[Fe II 3Fe III (OH) 8 ]Cl×H 2 O and can be

formed at [Cl − ]/[OH − ] > 1 [17]. It

consists of both Fe II and trivalent iron

(Fe III ). In contact with oxygen, green

rust transforms quickly to magnetite.

In the presence of Mg-rich brines,

(Fe,Mg)(OH) 2 and Fe(OH) 2 Cl compounds

were found and characterized

[18].

Materials and methods

When the corrosion experiments were

started, the boundary conditions for

the research on container materials

for highly radioactive waste resulted

from the requirements defined by

pouring the molten highly radioactive

glass directly into the canister, apply

the necessary welding and decontamination

of the containers and by

the requirement for transport, interim

storage and final disposal. For the

POLLUX canister, the influence of the

production and sealing of a final

storage canister was considered, and

U-shaped samples, welded samples

using different welding procedures, as

well as contact samples were prepared

for the experiments. In particular, to

assess the influence of the welding on

the corrosion processes, different

treatments of the samples were

applied, including the delivery state,

heat-treated samples, welded and

subsequently heat-treated samples.

DECOMMISSIONING AND WASTE MANAGEMENT 105

Decommissioning and Waste Management

Corrosion Processes of Alloyed Steels in Salt Solutions ı Bernhard Kienzler

More magazines by this user