General and Localized corrosion of Austenitic and Borated Stainless Steels in Simulated Concentrated Ground Waters Page: 3 of 10
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Table 2. Chemical composition of the studied stainless steel alloys (wt%)
Element 304 SS 316 SS Neutrosorb Plus Neutronit A978 Netronit A978
(S30400) (S31600) (S30467) Type 316B7 Type 316B7
Type 304B7 (Heat E084295) (Heat N156129)
Name 304 316 SSC SSE SSN
Fe -70.0 -68 -64 -64 -63 .
Cr 18.98 16.55 19.97 18.18 19.16
Ni 8.02 10.70 12.49 12.07 12.74
Mo 0.14 2.13 <0.005 2.11 2.22
B - - 1.57 (A) 1.00 (B) 1.17 (C)
C 0.070 0.034 0.034 0.056 0.039
Mn 1.96 1.84 1.70 1.43 0.97
Cu 0.35 0.21 0.02 0.11 0.10
Si 0.48 0.57 0.60 0.72 0.38
The nominal values are: (A) 1.75%, (B) 1.38% and (C) 1.62%.
The exposure time for the stainless steel specimens was
over 5 years. The actual testing time for each vessel is shown in
Table 3 along with each specimen label, vessel number and
weight loss (or gain) during the testing period. Each specimen
is designated with 3 letters or numbers (Table 2) followed by
two characteristic sequential numbers. The results in Table 3
and further analysis are for dry specimens as removed from the
vessels, that is, the specimens were not cleaned to remove
corrosion products or salt deposits.
After more than five-year exposure to each solution at
90 C, the specimens were removed from their respective test
vessels, rinsed in DI water and dried in air at ambient
temperature. In all of the tested conditions, the coupons were
covered with deposits, which formed by precipitation of salts
from the environment. In some environments, the coupons also
had corrosion products. In the analyses given in this report, the
coupons were not cleaned. That is, actual corrosion rates are
not calculated. Relative corrosion rates may be calculated using
CR(nm yr) = 8.76x104AW (1)
where 8.76 x 10 is the proportionality constant,
A W is the mass loss in grams after 5+ years,
p is the density of each of the stainless alloys in g/cm3
A is the exposed surface area of each coupon (cm ) and
t is the exposure time (hours).
RESULTS AND DISCUSSION
Table 3 shows the weight loss (gain) of the tested stainless
coupons. A total of 135 coupons were studied. Most of the
coupons exposed to the vapor phase in each of the three vessels
experienced weight gain due to the formation of deposits or
corrosion products that cannot be fully washed out due to the
restricted amount of condensed water in the vapor phase. The
liquid phase would provide a constant supply of corroding
solution and at the same time offer a vehicle to transport away
the corrosion products. Both of these mechanisms are limited in
the vapor phase. The coupons exposed to the water line had a
distinctive behavior depending on the testing vessel. In the
SAW vessel, most of the coupons suffered weight loss. The
largest weight loss corresponded to the SSC coupons followed
by the SSE/SSN coupons, then 304 SS and the lowest weight
loss was for the 316 SS coupons (Table 3). In the SCW vessel,
the coupons exposed to the water line also suffered weight loss,
although a small amount compared to the SAW vessel. All the
coupons exposed to the water line in the SCMW vessel
experienced weight gain, mainly due to the heavy precipitation
of a white/brown salt at the splash line (wet/dry) (Table 4). This
salt is probably CaSO4, but this needs to be confirmed.
Most of the coupons exposed to the liquid phase suffered
weight loss due to corrosion. SAW was the most aggressive
solution for all the materials and the borated stainless suffered
the largest amount of weight loss. The second most corrosive
solution was SCMW, which especially corroded the SSE/SSN
borated stainless. The least corrosive solution was SCW, which
caused little corrosion in the witness materials (SS 304 and
316). The least resistant material in SCW seemed to be the
powder metallurgy alloy SSC.
Figures 1 and 2 show the weight loss (gain) for the
coupons tested in SAW and SCMW liquid, respectively.
Assuming that the same amount of deposits forms on both the
witness alloys and the borated stainless, Table 3 and Figure 1
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Fix, D.; Estill, J.; Wong, L. & Rebak, R. General and Localized corrosion of Austenitic and Borated Stainless Steels in Simulated Concentrated Ground Waters, report, May 28, 2004; Las Vegas, Nevada. (https://digital.library.unt.edu/ark:/67531/metadc780565/m1/3/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.