atw 2018-02

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

DECOMMISSIONING AND WASTE MANAGEMENT 108

a) mass loss

b) mass loss rate

| | Fig. 3.

Measured mass loss and mass loss rates of fine-grained steel (1.6210) in MgCl 2 -rich (red) and NaCl solution (blue) as function of time at 150 °C.

Hastelloy and for the two Cr-Ni steels.

The Hastelloy experiments covered a

temperature range between 90 °C and

170 °C, whereas the CR-NI steels were

investigated at 150 °C, only.

For Hastelloy, all mass loss rates

were found below 12 g m -2 yr. -1 showing

no distinct time dependence. For

the experiments with Cr-Ni steels, the

initial mass loss rates decreased and

remained for the long term below

15 g m -2 yr. -1 . The effect of the solution

type on the mass loss rates for Cr-Ni

steels was not significant. Also, the

differences of the mass loss and mass

loss rates between 1.4306 and 1.4833

steels were marginal. Concerning the

temperature effect of the general

corrosion of Hastelloy, relatively high

mass losses were found at 90°C after

676 days. At higher temperatures, the

exposure period remained below 500

days. The reason for the increased

mass losses could be explained by

crevice corrosion of the Hastelloy C22

in MgCl 2 rich solution showing pit

depths of about 200 µm. The scatter

of mass losses is correlated to local

corrosion processes.

For comparison, the mass loss and

mass loss rates of the fine-grained

steel 1.6210 is shown in Figure 3. In

this case, the mass loss rates were by a

factor of 50 higher in comparison to

the Cr-Ni steel in NaCl solutions and

by a factor about 100 higher in MgCl 2

solution after about 500 days (150 °C).

The uniform mass loss rates of

the Ni-Resist steels were found in

the range of the Cr-Ni steels at

20 ± 7 g m 2 yr. 1 for steel 0.7660

and 12 ± 9 g m -2 yr. -1 for 0.7680,

respectively. These values are also

significantly lower in comparison to

the fine-grained steel 1.6210.

| | Fig. 4.

Crevice corrosion in Hastelloy C22 after 676

days in MgCl 2 rich solution at 90 °C showing

depths of about 200 µm.

Local corrosion phenomena

The breakdown of passivity (the

breaching of the protective barrier

provided by the passive film) initiates

the most damaging kinds of corrosion,

the localized forms of corrosion,

pitting, crevice corrosion, intergranular

attack, and stress corrosion. The

induction period for pitting corrosion

starts with the initiation of the breakdown

process by the introduction of

breakdown conditions and ends when

the localized corrosion density begins

to rise. Unfortunately, electrochemical

corrosion studies were applied

only for carbon steel and the influence

of chemical species in brines have

been investigated [21]. For this

reason, corrosion potential for pitting

corrosion have not been determined

for the investigated alloyed steels.

In brine media, localized corrosion

has been investigated over the complete

range of chloride concentrations.

The Cl- concentration, however,

is not as critical as pH and temperature,

since the attack can occur at any

concentration over the minimum

value. Factors such as incubation

time, severity, and frequency of

occurrence can be influenced by the

concentration.

Localized corrosion was observed

for all alloyed steels. In the case of

Hastelloy C22, the first pits occurred

after 275 days in the MgCl 2 rich

solution at 90 °C. These pits had

depths of about 10 µm. After 552

days, the depths increased to 20 µm

and after 676 days, a pit’s depth of

200 µm was found in a crevice. In the

MgCl 2 solution 2, even deeper pits

were detected. In NaCl solution, after

552 days, the pit’s depth amounted to

16 µm.

The average pit depths as function

of time in the steels 1.4306 and 1.4833

are shown in Figure 5.

In contrast to the observations

for Hastelloy, the depths of the pits

were significantly deeper after about

3 months. The pits showed relative

a) Steel 1.4306 at 150°C

| | Fig. 5.

Average pit depths determined in untreated Cr-Ni steels as function of time.

b) Steel 1.4833 at 150°C

Decommissioning and Waste Management

Corrosion Processes of Alloyed Steels in Salt Solutions ı Bernhard Kienzler

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