Environmental Engineering Reference
In-Depth Information
power year) for Alloy 600 LTMA (low temperature mill annealing), but
they were detected only after 10 EFPY in Alloy 600 HTMA (high tem-
perature mill annealing). However, some Korean power plants that have
installed Alloy 600 HTMA in an explosive expansion method experienced
circumferential SCC in three to seven years. PWSCC of the U-bend zone
usually causes axial cracks. If there is mechanical expansion, axial cracks are
primarily found, but in a French power plant, a circumferential crack was
found in kiss roll expanded tubes in the sludge accumulation area.
Denting of Alloy 600 was fi rst reported in the tube support plate (TSP)
region in 1975 immediately after the secondary system water treatment
changed from phosphate to all volatile treatment (AVT). Denting refers to
the state where the cross-section of a tube does not maintain its original form
due to the growth of oxides around the pipe. This damage mechanism was a
major cause of tube plugging from 1976 to 1980, although more recently it
has not been particularly serious. If chloride intrudes into the secondary sys-
tem by steam condenser leakage and is concentrated between the tube and
the support structure, it creates an acidic environment. When the oxygen
content is high, corrosion of carbon steel tube support plates increases and a
porous magnetite is formed which has double the volume of the base metal.
The degree of superheat, presence of chloride and concentration of oxy-
gen in the secondary system infl uences the corrosion rate of carbon steel.
Copper oxides or copper ions can also be factors that supply oxygen to the
water. Sulphate, like chlorides, can accelerate corrosion of carbon steel. The
most serious cases occurred when seawater was used as the coolant of the
steam condenser, such as in the RSG steam generators of Westinghouse
and CE (Combustion Engineering in the United States). For a power plant
that uses phosphate treatment, the environment remains alkaline so denting
hardly occurs. Denting occurs rapidly, not occurring or spreading gradually
like PWSCC or outer diameter stress corrosion cracking (ODSCC). When
exposed to seawater containing chloride, denting occurs in a very short time
(20 ppb acid chloride: 2.5 years), though when exposed to water containing
neutral salt, it takes a more considerable length of time (20 ppb neutral
chloride: 50 years).
The secondary IGSCC and IGA of Alloy 600 have been considered a
serious corrosion problem since they were fi rst reported in the early 1970s.
These problems have been found in many freshwater cooling power plants,
whereas fewer problems have been found in the power plants that use sea-
water coolant. The causes of IGSCC corrosion are impurities concentrated
by surface boiling due to coolant fl ow not being smooth in a certain area, as
well as stress, materials, temperature, etc. IGA is different because it occurs
when there is no stress; however, sometimes stress creates IGA.
Erosion-corrosion describes pieces of corroded metal peeling off the
metal surface by the action of solid particles repeatedly colliding with the
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