The Monel alloy 400, nickel and Inconel alloy 600 rapidly used with the solutions of 3gm per litre or less available chlorine in irregular operations like in cyclic textile bleaching with hypochlorite solutions where the bleaching cyclic is followed by rinsing

1. Influence of acidic media on Nickel based alloys
Hydrogen chloride and chlorine
The entire nickel alloys taken in this study offer good resistance to dry chlorine and hydrogen
chlorine, most of them even at very high temperatures. Monel alloy 400 is a standard alloy for trim
on chlorine cylinder and tank valves, for orifice plates in chlorine pipelines and is used popularly for
components of chloride dispensing device.
Wet chlorine at temperatures lower than dew point will often act about the same as concentrated
hydrochloric acid. Hastelloy alloy B is an excellent nickel based resistant alloy. Wet chlorine at
temperature lower than dew point of aqueous solutions comprising of significant contents of free
chlorine are extremely corrosive to these alloys excluding Hastelloy alloy C. The Monel alloy 400,
nickel and Inconel alloy 600 rapidly used with the solutions of 3gm per litre or less available chlorine
in irregular operations like in cyclic textile bleaching with hypochlorite solutions where the bleaching
cyclic is followed by rinsing and acid souring operations in the same vessel. In highly concentrated
attack the corrosion is more severe and accompanied by pitting.
The short time tests of Monel alloy 400 in sodium hypochlorite solution of 3 gm per litre chlorine
have provided average rates of corrosion of about 0.001 ipy in contact with the solutions for 8 hours
daily over an year period in cyclic textile bleaching. Small or larger rates are found to implement for
shorter or longer periods of daily contact with the solutions. Nickel acts similarly as of Monel alloy
400 in weak hypochlorite solutions but it is normally of lower-grade than Monel 400 in
concentrations where the presence of chlorine content is more than 3 gm per litre. The analysis with
Inconel alloy 600 have shown that it is more resistant than Monel 400 or nickel to corrosion due to
hypochlorites particularly where the content of chlorine is more than 3 gm per litre.
Monel alloy 400, nickel, Inconel alloy 600 offer resistance to highly dilute hypochlorite solutions,
normally containing less than 500 ppm, existing chlorine utilized for sterilizing operations. Prucha
provided the outcomes of tests in sodium hypchlorite sterilizing solutions as shown in the following
table:
Corrosion rate
35 ppm chlorine 100 ppm chlorine 500 ppm chlorine
Mdd ipy Mdd ipy Mdd ipy
Monel alloy
400
0 0 1.3 0.0002 5.1 0.0008
Nickel 0.61 0.0001 2.1 0.0003 5.1 0.0008
The inhibitors like sodium silicate or trisodium phosphate have provided remarkable effect in
decreasing the corrosion rate of Nicklel, Monel alloy 400 and Inconel alloy 600 in hypchlorite
solutions. The restricting effect retains for solutions having 6.5 gm per litre chlorine. The data
showing the resistive features of these salts in sodium hypchlorite solutions is mentioned as
following:
Solution composition gm per
litre
Monel alloy 400 Nickel Inconel alloy 600

2. Availabl
e
chlorine
Sodiu
m
silicate
Trisodium
phosphat
e
Corrosion
rate
Max.
depth
pittin
g
inch.
Corrosion
rate
Max.
depth
pittin
g
inch.
Corrosion
rate
Max.
depth
pittin
g
inch.
Md
d
Ipy Md
d
Ipy Md
d
Ipy
6.5 0 - 692 0.113 0.011 321 0.05
2
.022 69 .012 .027
6.5 0.5 O 107 0.018 0.005 64 0.01
0
.014 15 .003 .022
6.5 0.5 51 0.008 0.006 122 0.02
0
.024 15 .003 .017
6.5 2 O 13 0.002 0.007 9 0.00
1
None 6 .001 .008
6.5 - 2 21 0.003 0.005 57 0.00
9
None 7 .001 .012
3.3 0 - 243 0.040 .007 183 .030 .014 29 .005 .032
3.3 0.5 6 .001 None 22 .004 None 7 .001 .008
3.3 - 0.5 26 .004 .003 40 .006 None 6 .001 .007
0.1 O - 23 0.004 .003 26 .004 none 12 .002 .006
0.1 0.5 O 2 0.000
3
None 3 .000
5
none 4 .000
7
.003
0.1 - 0.5 8 0.001
3
None 4 .000
6
none 4 .000
7
none
Nickel-Iron alloys
The inclusion of Nickel to Iron increases its resistance to hydrochloric acid solution at atmospheric
temperature. The enhancement is roughly in proportion to the nickel concentration where inclusion
of nickel is more than 12%. So far a nickel steels are concerned, the ones most likely to have practical
applications in dilute hydrochloric acid applications are the series of Nickel-Iron alloys containing 25
to 60% nickel used for their specific expansion or magnetic features. For example Invar or Elinvar
alloys containing 36% nickel. The following table provides the results of various corrosion tests if
high nickel steels in hydrochloric acid solutions at room temperature:
Acid content %
25 % Nickel steel 32% nickel-2% chromium steel
HCl
Mdd Ipy Mdd Ipy
1.8 70 0.013 O O
5 O O 96 0.017
9 160 0.029 O O
10 O O 168 0.036
15 576 0.105 O O
17.2 - _ 360 0.064
The corrosion rates of Elinvar alloy in dilute solutions at the different temperatures are described in
the following table:
Acid concentration % HCl Corrosion rate
Room temp 52oC or 125oF 74oC or 165oF
1 25 .005 48 0.009 73 .013

3. 2 26 .005 50 0.009 131 .024
4 27 .005 54 0.010 291 .053
8 26 .005 206 0.038 1260 .023
The Nickel-iron alloys most commonly utilized with dilute hydrochloric acid at room temperature
are the nickel and nickel-copper cast irons of the NI-resist cast iron series. The corrosion rates of Ni-
Resist kind 1 cast iron (13.5 to 17.5 % nickel, 5.5 to 7.5 % copper and 1.75 to 2.5% chromium) in
unaerated hydrochloric acid solutions at room temperature are shown as following:
Acid content %
Corrosion rate
HCl
Ni- resist kind 1 cast iron Plain cast iron
1.8 25 0.005 4476 0.9
3.6 74 0.015
5 88 0.018 7440 1.5
10 81 0.016 6186 1.2
20 230 0.045 -
Ni-resist type 3 comprising 28 to 32% nickel, due to its high nickel content, often has superior
resistance to dilute HCl solutions than Nickel resist type 1 and Nickel resist type 2.
Both aeration and increase in temperature significantly increase the corrosion rates of Ni-resist
alloys. For example, tests in aerated 2% hydrochloric acid at 49oC provided corrosion rates of 520
mdd for Ni-resist type 1 and 15,000 mdd for plain cast iron.