Hydrocarbon Processing - January 2021 - 25

Corrosion

A. ANWER AND N. QURESHI
Contributing Authors, Abu Dhabi, UAE

Address damage caused by galvanic corrosion
At petrochemical and refining facilities, it is common for U-clamps over piping runs to become corroded, sometimes
to the extent where they are completely
thinned out. Localized thinning of supports can also happen, which may remain
unnoticed. Fasteners lose their coating
and a thin layer of rust appears that can
potentially turn into something more significant that requires action.
These forms of galvanic corrosion can
be observed externally with visual aids
only. Slowly, these damages lead to the
implementation of refurbishment projects to eliminate this form of corrosion.
While notable cases exist where galvanic
corrosion happens within process equipment, the focus here is the monitoring
and elimination of those areas where galvanic corrosion happens externally and is
often noticeable with visual aids only.
Galvanic corrosion can start where
dissimilar metals are in contact without
any precautions taken. One of the metals must be a noble-type; the other must
be an active metal. While this contact can
sometimes be avoided during the design
stage, it is not always possible and designers may have many places onsite where a
noble metal is in contact with an active
metal. Another required element for galvanic corrosion is the presence of an electrolyte. Because this article is focused on
the external areas that are vulnerable to
galvanic corrosion, atmospheric conditions play an important role in defining
the electrolyte present in the galvanic triangle: electrolyte, noble metal and active
metal. Any missing element eliminates
the chances of galvanic corrosion.
Vulnerable areas. Galvanic corrosion

can become evident in numerous places,
and some are more critical to plant integrity than others. Supports over or under
austenitic stainless-steel piping are critical areas that should be inspected closely
for any signs of damage by galvanic corrosion. Vessel internals constructed from

dissimilar materials can cause serious
damages. In heat exchangers in which the
baffles or tube sheet are of different material than the tube, galvanic corrosion can
aggressively attack the active metal.
Areas that are less critical but still experience unnoticed and unreported galvanic corrosion include instrument tubing supported through U-bolts over the
length. Instrument tubing is normally of
austenitic stainless-steel metallurgy and
the installed U-bolts are of plain carbon
steel material. Also, non-pressure parts
can be attached together without considering galvanic corrosion damages.
This is sometimes done inadvertently
when a superior material is used just in
case the required is unavailable, and that
better material becomes the noble and
the other active. A classic case is when
austenitic stainless-steel nuts and bolts
are placed over carbon steel flanges,
leading to the inevitable and noticeable
galvanic corrosion.
Knowledge of active and noble materials is important, and information tables
are available where the significance of
galvanic corrosion is shown when any
two materials come in contact. Based
upon this significance, galvanic corrosion should be monitored across the site.
This galvanic series is useful during the
design stage as well, especially when selecting piping support materials for different piping networks.
Prevention. Prevention of galvanic corrosion is normally done by applying coating systems. Essentially, coating should
be applied over the noble metal if austenitic stainless steel is in contact with carbon steel-the austenitic stainless steel
must be coated, but what normally is
found onsite is that the coating has been
applied on carbon steel material, which is
the active metal. Installation of structural
steel supports like U-bolts over austenitic
stainless-steel piping with rubber pads or
other foam material in between is a com-

mon practice that is incorrect. Over time,
these rubber pads degrade, and close inspection of piping at all levels to ensure
that these insulating consumable pads
still exist is rare.
To preventively address damages by
galvanic corrosion, a 5-yr survey is recommended across the site to identify those
areas where this damage already exists or
can potentially exist and to take remedial
action. This survey should essentially remove all existing and potential threats
due to galvanic corrosion found externally. At the same time, there must be a
record of equipment in which dissimilar
materials have been installed as internals.
That record can be kept as a list and for all
such equipment, a check for galvanic corrosion should be made part of the written
scheme of inspection.
For those areas where galvanic corrosion is seen or can potentially exist, corrective action must be taken.
To be identified, galvanic corrosion
does not demand state-of-the-art inspection techniques. Visual aids can easily
identify problematic areas and corrective
actions can be taken accordingly based
upon the situation. Once galvanic corrosion is eliminated at sites, major replacement or refurbishment costs can be significantly lowered, especially for those
locations where operating assets are installed in a marine environment.
ASHFAQ ANWER is an Inspection
Professional with more than 15 yr of
experience in ammonia-urea
complexes, petrochemical units, and
the oil and gas industry. He has
extensive expertise in material
selection, corrosion mapping and
control, fitness-for-service studies, defining inspection
framework and implementing inspection plans for old
and new units.
NABEEL QURESHI is an Inspection
and Corrosion Specialist with more
than 13 yr of experience in
deploying advanced inspection
methodologies, corrosion
monitoring and static equipment
repair and integrity assurance.
Hydrocarbon Processing | JANUARY 2021 25

Hydrocarbon Processing - January 2021

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