Incoloy alloy 800 tubing is used in coolers dealing with concentrated 98% sulfuric acid production however it is not popular as a sulfuric resistant alloy

1. Nickel-Chrome alloys resistance to sulfuric acid (H2SO4)
Iron based Ni-Cr alloy
Incoloy alloy 800 tubing is used in coolers dealing with concentrated 98% sulfuric acid production however
it is not popular as a sulfuric resistant alloy and its significance is limited to dilute or concentrated acid
solutions. In the specific plant conditions of acid concentration, temperature, impurity content and flow
velocity, coolers, Incoloy alloy 800 is attacked at smaller corrosion rates as compare to Stainless steel 304
and 316 grades. In conditions of heat transfer where Incoloy 800 surface is cooler as compare to bulk
sulfuric acid temperature and in a plant acid stream comprising of inhibiting contaminants, corrosion rates
would be significantly low. The higher content of nickel present in Incoloy 800 tends to increase resistance
to stress corrosion cracking from water side of heat exchanger pipes. The corrosion analyses in the plant
conditions are referred to find suitable alloys. The plant acids should be used if lab tests are employed, so
that contaminants that may increase or decrease corrosion rates are available.
Very small corrosion was noticed for Incoloy alloy 800 in sulfuric acid, lab tests in 5% H2So4 at 50oC or
122oF describe rates of 0.51mm/y or 20 mpy in unaerated acid and an increase to 1.27 mm/y or 50 mpy
when solution is aerated. In air saturated 75% acid at 60oC or 140oF the rate is more about 4.57 mm/y or
180 mpy. The lab experiment results in reagent grade 95% sulfuric acid are described in the following
table:
Alloy Corrosion rate
50oC or 122oF 70oC or 158oF
Mm/y Mpy Mm/y Mpy
Carpenter alloy 20 Cb3 0.26 10.1 0.42 16.4
Stainless steel 316 0.41 16.1 2.42 95.2
Stainless steel 304 0.74 29 0.92 36.4
Incoloy alloy 800 1.12 44.1 0.99 38.9
Carbon steel 1020 1.59 62.7 3.25 128
The apparent irregularity in which the corrosion rate for Incoloy alloy 800 is lower at 70oC or 158oF than
50oC or 122oF refers an unstable condition in which the corrosion potential is oscillating among the active
and passive regions.
Nickel base chromium alloys
Chromium inclusion to nickel tends to decrease critical current density, passivity over the broad strength
range and normally decreasing passive current density in sulfuric acid solutions in comparison to pure
nickel.
Inconel alloy 600
Inconel 600 is a wrought Ni-Cr alloy that is commonly used in the chemical and process plants. It can be
employed in air-free sulfuric acid solutions up to about 60% concentration and then above 90% at room
temperature. Air saturation, excluding in concentrated acid, or an increase in temperature at all acid
contents enhances the corrosion rates significantly as described by corrosion test outcomes for Inconel
alloy 600 (shown in following tables):
Corrosion rate of Inconel alloy 600 in sulfuric acid solutions

2. Corrosion rate at Room
temperature
Corrosion rate at Boiling
temperature
Acid content % Mm/y mpy Mm/y mpy
10 0.08 3.2 3.43 135
20 0.05 2 4.72 186
30 0.06 2.5 5.49 216
40 0.05 1.8 17.8 700
50 0.04 1.6 - -
60 0.05 1.9 - -
70 0.06 2.3 - -
80 0.57 22.3 - -
90 0.01 0.5 - -
98 0.19 7.4 - -
Lab corrosion analyses of Inconel 600 in sulfuric acid solutions
Acid
wt %
Corrosion rate
Temperature Test
time
Velocity Air-free Aerated
oC oF Hours m/sec Ft/sec m/sec Ft/sec m/sec Ft/sec
0.16 100 212 - - - 0.09 3.7 - -
1 30 86 120 0.08 0.26 - - 1.24 49
1 78 172 22 0.08 0.26 - - 2.79 110
5 19 65 100 Nil Nil 0.06 2.4 - -
5 30 86 20 0.08 0.26 0.23 9 - -
5 30 86 23 0.08 0.26 - - 2 78
5 60 140 100 nil Nil 0.25 10 - -
5 80 176 20 0.08 0.26 0.76 30 3.81 150
10 RT RT 24 Nil Nil 0.11 4.2 - -
70 30 86 20 0.08 0.26 1.2 46 - -
93 30 86 20 0.08 0.26 6.86 270 0.25 10
Strong oxidizing agents like nitric acid, ferric or cupric ions prevent corrosion of Inconel alloy 600 at low or
medium temperatures. Although the oxidizing effect of dissolved air solely is not adequate to retain
passivity excluding in concentrated acid.
The corrosion rates of Inconel alloy 600 in boiling sulfuric acid are large except in dilute concentrations or
where boiling temperatures are lowered by using vacuum.
There are several Ni-Cr alloys containing 60% to 80% nickel and 13% to 20% chromium, with the remainder
iron. Two common alloys belonging to this group are Nichrome 80 and Nichrome 60. These alloys are
commonly employed for heating purposes as these offer high electric and heating resistance. These also
offer corrosion resistance similar to alloy 600. The outcomes of lab corrosion analyses of these Ni-Cr alloys
in pure sulfuric acid solutions are shown in the following table:
Lab analysis of Ni-Cr alloys in H2SO4 solutions at ambient temperature:
Alloy composition Acid wt% Test time Corrosion rate
Ni Cr Fe Mn Hrs Mm/y mpy

3. 88 11 - 1 10 24 0.11 4.2
84 15 - 1 10 24 0.17 6.7
79 20 - 1 10 24 0.04 1.7
69 20 10 1 10 24 0.33 13
65 15 20 - 10 24 0.02 0.7
65 15 20 - 10 24 2.1 82
80 20 - - 31 - 2.5 82
80 20 - - 31 - 2.5 100
80 20 - - 60 45 2.2 87
61 16 23 - 60 45 32.3 1270