Question

A fire-tube boiler made from carbon steel in which fuel gas flows within the tube while the boiler feed-water along with condensate from the plant flows on the shell-side. Visual inspection revealed heavily localized tubercles were seen on the outside surface of several tubes. After about 11 months of operation, the pipe was leaked and visual inspection found pin holes outside surface contain loose brownish rust. The rust contained chloride and was acidic in nature. Select appropriate corrosion prevention to apply and identify types of corrosion happen.

Answer 1

There are seven types of chemically induced
corrosion failures: general corrosion, pitting corrosion,
stress corrosion, dezincification, galvanic corrosion,
crevice corrosion, and condensate grooving .

1. General Corrosion -- This type of corrosion is
characterized by a relatively uniform attack over the tube,
tube sheet, or shell, and there may be no evidence that
corrosion is taking place.

Fairly stable aggressive conditions generate this type of
attack. Low pH (less than 7) combined with carbon dioxide
or oxygen produces the attack on copper. The blue or
bluish-green color on the tubes are the results
of carbon dioxide attack on the inside of a copper tube.
Various chemicals, such as acids. also produce this type of
metal loss.
Selecting a material with adequate corrosion resistance
for its environment, along with using proper treatment
chemicals, maximizes heat exchanger life .

2. Pitting Corrosion -- Localized pitting is frequently
encountered in ferrous and nonferrous metals. It results
from the electrochemical potential set up by differences in
oxygen concentration within and outside the pit, and is
frequently referred to as a concentration cell. The
oxygen-starved pit acts as an anode and the unattacked
metal surface as a cathode. A small number of pits may be
present; however, any one can cause a heat exchanger
failure.
Pitting corrosion is most likely to occur during
shutdown periods a when there is no flow and the
environment is most suitable for the buildup of
concentration cells. The susceptibility to pitting corrosion
is further enhanced by scratches, dirt or scale deposits,
surface defects, breaks in protective scale layers. breaks in
metal surface films, and grain boundarv conditions.

3. Stress Corrosion -- This form of corrosion attacks the
grain boundaries in stressed areas. Heat exchanger tubes
usually have both avoidable and unavoidable residual
stresses. These stresses are the result of drawing or
forming the tube during manufacture, forming U bends, or
expanding the tubes into tubesheets.
Failures from this corrosion take the form of fine cracks,
which follow lines of stress and material grain boundaries.

The corrodent that causes stress corrosion on stainless
steel tubes is the chloride ion, which is potentially present
in any compound formulated with chlorine. All naturally
occurring waters contain the chloride ion in varying
degrees. The chloride stress corrosion phenomenon is not
well understood, but it is known that the frequency of
occurrence rises with an increase in temperature and
chloride ion concentration. Keeping tube wall temperatures
below 115 F (calculated with maximum, not average. fluid
temperatures) prevents stress corrosion cracking problems
with chloride ion concentration up to 50 ppm.

The corrodent that causes stress corrosion cracking on
copper or copper alloy tubes is ammonia. Very small
concentrations (1 ppm or less) can create a problem.
The vacuum breaker used on steam-heated exchangers
draws in ammonia from any leak in the ammonia
refrigeration machine. The ammonia causes stress cracking
problems, particularly in the inner U bends of the heat
exchanger tubes. Copper-nickel alloys have good
resistance to stress corrosion cracking and should be used
in applications where low concentrations of ammonia are
expected

4. Dezincification : Here it is not possible since the given metal structure has no zinc added .

5. Galvanic corrosion : Not possible because happens only when dissimilar metals are joined .

6 . Condensate Grooving -- This problem occurs on the
outside of steam-to-water heat exchanger tubes.
particularly in the U-bend area. It is recognized by an
irregular groove, or channel. cut in the tube as the
condensate drains from the tubing in the form of rivulets.
A corrosion cell usually develops in the wetted area
because of the electrical potential difference between the
dry and wet areas. The condensate, which must be aggressive for this grooving to occur, wears away the protective oxide film as it drains from the tubing.
Controlling condensate pH and dissolved gases and
cleaning the tube bundle outside surface to remove oils
that prevent uniform wetting of the tube usually reduce the
problem potential.  

7 . Crevice Corrosion -- This type of corrosion originates in
and around hidden and secluded areas, such as between
baffles and tubes,