Hydrocarbon Processing - July 2021 - 81

Maintenance
and Reliability
R. KADIKAR, ITT Corp., Vadodara, India
Is coating required on stainless-steel components?
Stainless steel is a generic term used for
Generally, stainless steel suffers from
a large group of corrosion-resistant alloys
containing at least 10.5% chromium (Cr),
and possibly containing other
elements like nickel (Ni), molybdenum
(Mo), manganese (Mn) and nitrogen (N).
Stainless steels are divided into five
categories based on their microstructure
and properties:
1. Austenitic stainless steel
2. Ferritic stainless steel
3. Martensitic stainless steel
4. Duplex stainless steel
5. Precipitation-hardened
stainless steels.
These grades are further subdivided
into super austenitic, super ferritic and
super duplex stainless steel. Super (austenitic,
ferritic and duplex) stainless steel
grades may contain higher Cr, Ni, Mo
and N, depending on their material type
and grade.
Corrosion in stainless steels. In stainless
steels, Cr content above 10.5% is
necessary to form a stable passive chromium
oxide (Cr2
O3) layer. This passive
layer protects underlying material from
corrosion damage from the surrounding
environment. The passive Cr2
O3 film is
extremely thin, approximately 10-100
atoms thick (approximately 2 nm); however,
it prevents further oxygen diffusion
into the base metal. A Cr2
O3 passive layer
in stainless steel acts as a defender of the
material. Any mechanical activity, like
grinding or cutting, damages the passive
Cr2
O3 layer. However, Cr has very high
affinity with oxygen, and so it immediately
reacts with surrounding oxygen and
forms a passive Cr2
O3 film, thereby protecting
stainless steels. In a corrosive environment,
damage to the passive layer cannot
be restored, which leads to corrosion
of the underlying material.
alloying
corrosion damage in the presence of
halides, such as chlorides, bromides,
etc. Damage due to chloride presence is
common in stainless steels around industrial
areas, buried vessels under soil and/
or water, and vicinity to a marine environment.
These locations contain high
chlorides and lead to chloride-related
damage mechanisms.
Chloride ions from wet and humid environments
can combine with Cr of the
passive layer, forming soluble chromium
chloride. As Cr dissolves, free iron (Fe) is
exposed to chemically reactive surroundings
containing chlorides, and the surface
reacts with the corrosive environment to
initiate corrosion. In this way, the chloride
ion acts as a nemesis of the material.
Damage mechanisms of corrosion
types. Depending on chloride concentration,
temperature and operating conditions,
chloride ions can cause three different
corrosion damages in stainless steels:
1. Crevice corrosion
2. Pitting corrosion
3. Stress corrosion cracking.
Crevice corrosion is highly localized
corrosion that occurs within the crevices
and shielded areas on metal surfaces exposed
to corrosives. A typical example of
crevice corrosion is under-deposit corrosion,
which forms below sand, dirt and
corrosion products.
Pitting corrosion is an extremely localized
attack that results in holes on
metal surfaces. Pitting is one of the most
destructive and insidious form of corrosion.
It causes equipment to fail because
of perforations with only a small percent
of weight loss of the entire structure.
The damage mechanism for crevice
and pitting corrosion is the same result:
the passive Cr2
O3 layer is damaged, and
the chloride ion forms hydrochloric acid
within the pits/crevices, thereby enlarging
the pits and crevices over time.
FIG. 1A shows a typical crevice and pitting
corrosion mechanism. FIG. 1B and
FIG. 1C show how crevice and pitting corrosion
looks in stainless steel.
Stress corrosion cracking involves the
cracking and sudden rupture of equipment
without any warning, which makes
stress corrosion cracking in stainless steel
more damaging to equipment, to personnel
working around the equipment
and to the surrounding environment.
Austenitic stainless steel is susceptible
to chloride stress corrosion cracking if
the temperature is above 60°C (140°F).
FIG. 2 shows chloride stress corrosion
cracking in austenitic stainless steel.
Critical factors that increase susceptibility
for chloride damage in stainless
steel involve chloride concentration, temperature,
pH, oxygen and residual stresses
for stress corrosion cracking. IncreasFIG.
1. Crevice and pitting corrosion
mechanism (A), crevice corrosion appearance
in stainless steel (B), pitting corrosion
appearance in stainless steel (C).
Hydrocarbon Processing | JULY 2021 81
Hydrocarbon Processing - July 2021

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