Chemical Engineering January 2017 - 32

Corrosion, mm/yr (mils/yr)
10
(400)
5.0
(200)
Alloy 904L
1.0
(40)
High-chromium alloy
0.5
(20)
Alloy C
Alloy G
Alloy 20Cb3
Alloy 825
0.05
(2)
H3PO4 = 70%
H2SO4 = 4%
F-
CI0.01
(0.4)
60
(140)
70
(160)
80
(175)
FIGURE 6. The influence of
temperature on the corrosion
rate of various alloys in
contaminated phosphoric acid
shows the effect of chromium
content
90
(195)
easily maintained.
Chromium also lowers the corrosion rate
in the passive state and increases the transpassivity
potential (Etr). The latter is especially
important, since the corrosion potential of
stainless steel in nitric acid at high concentrations
is often close to the transpassive state.
Therefore, a steel with too low a chromium
content, or a steel containing elements that
will reduce the transpassivity potential, is likely
to corrode at a high rate. An electrochemical
study of the influence of various alloying
elements on the transpassivity potential will
indicate how a steel should be composed in
order to obtain a good corrosion resistance.
Table 1 measures the influence of various elements
on the corrosion of stainless steels
in HNO3 both in quench-annealed (QA) and
sensitized plus quench-annealed form. For
QA, material is heat-treated followed by rapid
cooling (quenching). Sensitized means that
the material has been held at a temperature
at which carbides can precipitate.
Table 1 clearly demonstrates that chro=
0.5%
= 60 ppm
Fe3+ = 0.6%
100
(210)
Temperature, oC (oF)
sure the same quality is achieved in all steel
melts, with very narrow tolerances for the
chemical composition.
Chromium for nitric acid service
All stainless steels contain chromium, and the
element has a powerful effect on the corrosion
resistance of stainless steel in nitric acid
service, due to the influence it exerts on the
material's fundamental electrochemical characteristics.
This is illustrated in Figure 8, a schematic
anodic polarization curve. Here, chromium
lowers the maximum current density
(imax), which implies that the passivity is more
Energy
110
(230)
120
(250)
mium is the most effective element in promoting
the corrosion resistance of stainless
steels in nitric acid concentrations of up to
65%, whereas carbon (C), silicon (Si), phosphorus
(P), sulphur (S), molybdenum (Mo), titanium
(Ti) and niobium (Nb) all have a negative
effect. This shows that a high-chromium
grade is needed for the most severe conditions.
The reason is that good corrosion resistance
depends largely on the ability of the
stainless steel to form a protective oxide layer
consisting mostly of chromium. Therefore,
materials with higher chromium content have
an enhanced ability to develop the protecting
layer. The content needs to be about 10 wt.
% in order for the oxide film to form at normal
atmospheric conditions. This oxide layer
could last indefinitely were it not for the corrosive
effects of the environment and changing
service conditions. These factors become
more aggressive and cause the layer to lose
its protecting properties.
Ammonia NH3
900oC
Nitrogen
oxides
(NOx)
Air
Nitric acid HNO3
FIGURE 7. The typical process
for producing nitric acid
(HNO3) is shown
32
NOx production
Nitric acid production
Testing alloying elements
A simple means to evaluate the effect of
alloying elements on stainless steels is the
boiling nitric-acid test or Huey test, standardized
as ASTM A262-Practice C. The Huey
test is an intergranular corrosion test where
a sample is boiled in nitric acid for five periods
of 48 h each. Because it is performed in
nitric acid, the test offers a very good indication
of how well the material will perform in
the plant by easily revealing susceptibility to
intergranular corrosion.
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Chemical Engineering January 2017

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