r temporarily because of startups, for
example. Different systems during shutdown may be
filled with air, and this may cause air pockets during
startup. The oxygen from air will then dissolve into
the primary water and local oxidizing conditions
temporarily emerge until the oxygen is consumed by
the oxidation of metal surfaces. The risk of pitting
corrosion (and TGSCC) is, however, highest in auxiliary systems, for example, at outer surfaces, where the
temperature is low enough for condensation to occur.
Thus, pitting corrosion can occur at nominally dry
locations. Accumulation of aggressive local conditions
is enhanced by crevices.
The sources of chlorides were listed earlier.
Sulfate sources have been introduced earlier, for
example, in molybdenum disulfide greases, but
since the harmful influence of this material was identified, it is not an allowed expendable material. Again,
copper can enter the system from copper-containing
structural components.
Pitting corrosion is seldom considered to pose a
safety problem, as the wall thicknesses of pressure
boundary components are usually large enough to
sustain pitting corrosion for long times without leakage. However, pitting corrosion is always an indication of a harmful environment existing at the location
and is often associated with the risk of TGSCC,
which can cause wall cracking in short time periods.
Pitting corrosion enhances the risk of SCC as the pits
increase the local stress concentration and thus act as
crack initiators. Observation of pitting corrosion shall
therefore not be omitted as insignificant.

5.05.3 Pitting Corrosion
Pitting corrosion occurrence has several similarities to

TGSCC, that is, it requires oxidizing conditions and
presence of water with harmful ions, such as chlorides,
fluorides, sulfates, and/or copper, but no stress is needed.
The Type 304 stainless steel is more prone to pitting
corrosion than Type 316 stainless steel. Pitting corrosion

5.05.4 Microbiologically Induced
Corrosion
A rather rare corrosion mode is microbiologically
induced corrosion, or nowadays, microbiologically influenced corrosion (MIC). MIC is normal


Corrosion and Stress Corrosion Cracking of Austenitic Stainless Steels

electrochemical corrosion where the microorganisms
either chemically or physically change the conditions
on the metal surface to be favorable to corrosion.59
MIC appears as localized corrosion rather than as
uniform corrosion, and in welds rather than in base
materials. Pitting corrosion in the weld metal can
cause preferential attack of either the austenite or
the ferrite phase of the weld metal. The microorganisms of interest in MIC are mostly bacteria and fungi.
The highest risk of MIC is at temperatures from 15
to 45  C and near neutral pH, that is, in the range
from 6 to 8. MIC has been observed in fire-fighting
systems, for example.
MIC is stopped with great difficulty once it is
established due to the high sustainability of the microorganisms involved. The quality of the water in all
phases of the lifetime of the equipment is, thus, very
important at locations with risk of MIC. Water of high

quality must be used, not only during normal operation,
but also during hydrotesting of the system, for example.

13.
14.

15.

16.

17.
18.
19.
20.
21.
22.

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