FIELD OF INVENTION
The present invention relates to a variety of an alloy metal and ,in particular, to Monel
alloy which is stress corrosion crack resistant in hydrofluoric acid or where the add
formation is a possibility through chemical reactions with fluoride salts. The Monel alloy
and more particularly, the Monel 400 of the invention which is stress corrosion crack
resistant is directed to wide variety of industrial applications even in environments where
the alloy comes in direct contact with hydrofluoric acid. The invention is also directed to
a simple and effective manner of manufacture of Monel-400 to make it resistant to stress
corrosion cracking in hydrofluoric acid and facilitate production of Monel-400 for more
reliable and wider applications in environments where the alloy is subject to exposure to
stress corrosion conditions.
BACKGROUND ART
Monel-400 is known to be naturally resistant to hydrofluoric acid (HF) but stress
corrosion cracking (SCC) occurs in it when stress is present. Such stress corrosion of
Monel 400 in l-lydrofluoric acid was reported by Frazer, O.B.3., ^Symposium on Stress
Corrosion of Metals' ASTM-AIME, Philadelphia (1944). Copson and Cheng, ^Corrosion 12,
647t (1956)' also disclosed and reported the failure of the alloy 400 in hydrofluoric add
vapour where the U-bend specimens were exposed over 48 % hydrofluoric acid in dosed
polyethylene containers partially immersed in a water bath 60 °C. It was further reported
that the mode of cracking was intergranuiar (IG) in the annealed material but
transgranuiar (TG) in cold drawn materials. The SCC of Monel-400 in other environment
/ media such as in caustic solutions , in primary water and in mercury salts have also
been reported.
Everhart and Price "Corrosion Testing and Evaluation: Silver Anniversary Volume, ASTM
STP 1000, R. Baboian and S.W. Dean, Eds, ASTM, Philadelphia (1990)" have also
reported use of slow strain testing (SSRT) to investigate SCC of Monel-400. They have
reported no cracking in 26% of HF solution but the development of extensive transverse
cracks of up to 3 grains deep in 0.063M CuFz + 0.38M HF solution. In this case the
cracking was inter granular (IG) at low strain levels in the specimens but trans granular
(TG) at higher strains.
It is also known as suggested by Boyd and Berry "Stress Corrosion Cracking-A state of
art' ASTM, Philadelphia (1972)' based on the reduction of susceptibility to TGSCC In Fe-
Ni-Cr alloys with higher nickel addition due to an increase in the stacking fault energy
that this increase in stacking fault energy of nickel would account for the predominantly
intercrystalfine cracking of nickel base alloys.
Also a transition in the mode of cracking in cold worked alpha brass in ammoniacal
solution is also reported (Althof, F.C, Z. Metallkunde, 36, 177 (1944); Chatterjee, U.K.,
PHD thesis, irr Kharagpur 1968). Presumably, beyond a critical value of cold
deformation, the increasing structural heterogeneity inside the grains reach a condition
when the grain interior becomes baser with respect to grain boundaries. The dissolution
is favoured more inside the grains creating sites for transgranular cracking.
It is well known Monel-400 has many industrial applications in direct contact hydrofluoric
acid or where the acid formation is a possibility through chemical reaction with fluoride
salt. In particular it is known to be useful in the fluorination of uranium oxide in the
production steps of Uranium but is found to be vulnerable to SCC.There is thus a
requirement in the art to provide for a Monel variety which could guard against such SCC
susceptibility and favour a much wider and reliable effective use of the alloy.
OBJECTS OF THE INVENTION
It is thus the basic of the present invention to provide for Monel alloy with stress
corrosion crack resistant characteristic so that it can be effectively and advantageously
used also in environments were it is in direct contact with hydrofluoric acid or where the
acid formation is a possibility through chemical reactions with fluoride salts.
Another object of the present invention is directed to provide stress corrosion crack
resistant Monel-400 by way of simple and cost effective process such that Monel-400
which would be adapted for wide variety of applications including industrial applications
in direct contact in hydrofluoric acid or where such acid fomfiation is possibility due to
chemical reaction with fluoride salt.
Another object of the present invention is directed to effectively and advantageously
make use of Monel-400 in adverse conditions subject to exposure to hydrofluoric add
especially such as in the fluorination of Uranium Oxide in the production of Uranium and
resist the problem of component vulnerability due to stress corrosion cracking.
Yet another object of present invention is directed to advantageous use of articles /
components made of Monei 400 in conditions wherein the said articles would come in
direct contact with hydrofluoric acid.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present Invention there is provided Monei alloy
rendered resistant to stress corrosion cracking in hydrofluoric acid comprising Monei
alloy, basically a Nickel-Copper alloy having elemental constitution in the range of Ni 61-
67 % by wt., Cu 28-32 % by wt., Fe 0.7-2 % by wt., Mn. 0.7-1% by wt., SI 0.1-0.5% by
wt., Ti < 0,5 % by wt., S < 0.005% by wt., Al 0- 2.75 % by wt., Co. 0.1- 0.2% by wt.
and C<0.2 % by wt., and selecting at least one of (i) said Monei alloy being cold worked
to at least 60% or more (ii) said Monei alloy grain refined by selective grain size diameter of 4
micron or less and (iii) said Monei alloy galvanic coupled with a protective metal which is
anodic to said Monei alloy.
In accordance with yet another aspect the stress corrosion crack resistant Monel-400
comprise Monel-400 galvanic coupled with stainless steel preferably 316 stainless steel.
In the above disclosed stress corrosion crack resistant Monel-400 the Monei alloy
comprises elemental constitution of Ni in the range of 63 to 66% by weight. Cu in the
range of 29 tO 31 % of weight. Fe in the range of 0.9 to 2 % of weight, Si in the range of
3 to 4 % of weight. Al In the range of 2 to 3% by weight, C in the range of 0.10 to 0.15
% by weight, Mn less than 1% by weight, Ti and Co in the range of 0.05 toO.1% by
weight.
In accordance with the preferred aspect the Monel-400 stress corrosion crack resistant in
hydrofluoric add comprises Monel-400 aJJoy having and elemental constitution of Ni
64.89% by weight, Cu 31.80% by weight, Fe 1.95% by weight, Mn 0.73% by weight, Si
0.12% by weight, Ti 0.05% by weight, S 0.004% by weight, Al 0.07% by weight, Co
0.09% by weight and C 0.122% by weight.
Importantly the above disclosed Monel-400 of the invention provides for improvement in
ductility ratio (DR) of the alloy thereby confirming the desired decrees in the SCC
sususceptibility of the altoy. In aocordance with a preferred aspect the Ductility Ratio
(DR) of the Monel-400 is above 0.85.
In accordance with another aspect of present invention there is disclosed a proce^ for
rendering Monel alloy, basically a nictel-copper alloy, with elemental constitution in the
range of Nl 61-67 % by wt., Cu 28-32% by wt, Fe 0.7-2% by wt, Mn 0.7-1% by wt. Si
0.1-0.5% by wt, Ti <0-5% by wt, S <0.005% by wt, Al 0-2.75% by wt, Co 0.1-0.2% by
wt and C <0.2% by wt., resistant to stress corrosion cracking In hydrofluoric add,
which comprises subjecting the alloy to treatment selected from any one or more of the
following:
(a) subjecting the said alloy to cold working to at least 60% by cold rolling for sheets
and by cold drawing though dies for bars or rods,
(b) subjecting the said altoy to grain refinement to 4 micron diameter or less ; and
(c) subjecting the sakl alloy to gah^anic coupling with a metal which is anodic to the
said alloy.
In accordance with a preferred aspect the pfx>cess for manufacture of stress corrosion
crack resistant Monel comprising subjecting Monel 400 to treatment selected from any
one or more of the following:
(a) subjecting the Monel 400 to step of cokf working to at least 60%
(b) subjecting the Monel 400 to grain refinement to less ttian 6micron and more
preferably less than Amicron; and
(c) subjecting the Monel 400 to galvanic coupling with a protecting metal such as 316
SS which is anodic to Monet altoy.
It is found that 60% cold working provides the much required immunity against stress
corrosion cracking in hydrofluoric add. Cold working higher than 60% would also enable
maintaining the intended immunity from stress corrosion cracking.
As regards the other aspect of preventton of stress corrosion cracking by controlling tiie
grain size it is found that while a grain size of less than 6 microns and more preferably
less than 4 microns is more advantageous to resist ^ress corrosion cracking.
The galvanic coupling of Monel to arrest stress corrosion cracking can be achieved by
carrying out galvanic coupling involving a protective metal which should be anodic to the
Monel alloy t>etng protected. For such galvanic coupling of Monel stainless steal can be
advantageously used.
In accordance with a preferred aspect of the invention the method of manufacture of
stress corrosion crack resistant Monel in hydrofluoric add can be obtained by subjecting
the Monel alloy to a combination of 60% of cold working and galvanic coupling or small
grain size and galvanic coupling for better protection.
In accordance with yet another aspect of the present invention there is provided use of
Monel 400 stress corrosion crack resistant in hydrofluoric acid for use as components
immune to SCC in hydrofluoric acid such as in fluorination of uranium oxide in production
of Uranium.
The details of the invention, its objects and advantages are explained hereunder in
greater detail in relation to non limiting exemplary illustrations of the Monel alloy of the
invention and its manner of manufacture as per the following example:
EXAMPLES:
A selective composition of Monel-400 was used to carry out the following exemplary
illustration .The elemental constitution of the Monel-400 used is given in Table 1
hereunder.
Table 1
Under the following Examples I to III the stress corrosion crack characteristic of Monel-
400 after hot rolling, after 50% cold working and after 60% work condition respectively
were studied.
For the purpose of stress corrosion crack study in hydrofluoric acid tests were conducted
in a CORTEST slow strain rate testing machine. Cylindrical specimens of 60 mm length, 4
mm diameter and 25mm gauge length were use. The corrodent 40% HF was contained
in a temperature vessel made of Hastelioy. The level of acid in the vessel was
maintained at just below the gauge length of the specimen. The solution was heated to a
temperature of 70 degree centigrade by external electrical heating of the container.
The result of stress corrosion crack test were noted as discussed hereunder:
Example I : SCC result in Monel-400 obtained after hot rolling.
Under this example the above Monel-400 composition as obtained after hot rolling was
studied to determined its stress corrosion crack characteristics. The results obtained and
in particular the SSRT data noted were as shown in Table 2 hereunder
Table 2 : SSRT data ffbr Monel-400 (hot rolled condition)
As would be apparent from the result in Table 2 above, a considerable low value of
ductility ratio (DR) shows that the material is susceptible to SCC under the test
condition.The cracking mode is intergranuiar. Metailographic and Factrographic
evidences of such intergranuiar cracking are further shown in Figures 1 and 2 respective.
Example II : SCC result of 50% cold work material.
The SSRT data noted under the above condition are reproduce in Table 3 hereunder :
Table 3: SSRT data for Monel-400 {SO^ cold worked).
As would be evident from the results in Table 3 above, even 50% cold working can not
arrest of problem of stress corrosion cracking of Monel-400 in hydrofluoric acid . The
factrographic shown in Figure 3 gives evidence of transgranular cracking.
Example III : SCC result of 60% cold work material.
SSRT data obtain of 60% cold worked material In accordance with the present invention
are presented in Table 4 hereunder.
Table 4;: SSRT data for Monel-400 (60<%b cold worked)
As would be apparent from the results In Table 4, the 60% of cold working of Monel-400
in accordance with the invention achieved a ductility ratio (OR) around 1 indicating the
material has been rendered immune to the SCC.
The above therefore sufficiently confirm the advantages following 60% cold working in
Monel-400 which avoids the problems of stress corrosion cracking in hydrofluoric acid.
Example IV : Effect of grain size
,/'
Under the example the possible arresting of stress corrosion cracking by way of refined
grain size of the Monel-400 in accordance with a further aspect of invention was studied.
For the purpose 50% cold worked material which showed under example II above to be
susceptible to SCC was heat treated at different temperature for varied lengths of time in
a tube furnace flushed with hydrogen gas. Grain size obtained was varied from 4 to 13
microns. The SCC susceptibility of such varied grain sized Monel-400 was noted. As
represented in Figure 4, it was found that decrease in the grain size led to increase in DR
and therefore decreased SCC susceptibility. Importantly at the lower grain size 4 micron
there was a surprising and synergetic drastic reduction in susceptibility to SCC.
Example V : Effect of galvanic coupling
Under this example the effect of galvanic coupling of Monel 400 with 360 stainless steel
was studied.For the purpose 60% cold worked Monel was used and the SSRT data was
noted as shown in Table 5 hereunder.
Table 5 : SSRT data for Monel-400 coupled with SS316
As shown in Table 5 above galvanic coupling was found to the effective in arresting SCC
susceptibility (DR=0.86-0.98) of Monel-400 in HF vapour. It is found that use of
stainless steel- Monel-400 coupling safeguards the Monel-400 from corrosive attack in
HF environment.
The above therefore clearly and sufficiently confirms the advantages in arresting and
controlling the stress corrosion cracking in Monel-400 involving any one and or more of
(i) 60% of more cold working to render immune to SCC of Monel-400 (ii) decreasing the
grain size to less than 6 micron preferably less than 4 micron to surprisingly and
drastically arrest susceptibility to SCC of Monel (iii) galvanic coupling of Monel-400 to
316 stainless steel to appreciably reduce the susceptibility to SCC.
The invention therefore favours advantageous and wide scale application of Monel-400
specially in industrial application in direct contact with hydrofluoric acid or where acid
formation is a possibility through chemical reaction with fluoride salt. Importantly and
more particulariy the present invention would enable use of Monel-400 based
components in environments subject to generation /presence of hydrofluoric acid such as
in the fluorination of Uranium oxide in the production steps of Uranium without the
problems of SCC susceptibility.
WE CLAIM:
1. Monel alloy rendered resistant to stress corrosion cracking in hydrofluoric add
comprising Monel alloy, basically a Nickel-Copper alloy having elemental constitution
in the range of Ni 61-67 % by wt., Cu 28-32 % by wt., Fe 0.7-2 % by wt., Mn. 0.7-
1% by wt.. Si 0.1-0.5% by wt., Ti < 0.5 % by wt., S < 0.005% by wt., Al 0- 2.75 %
by wt., Co. 0.1- 0.2% by wt. and C<0.2 % by wt., and selecting at least one of (i)
said Monel alloy being cold worked to at least 60% or more (ii) said Monet alloy grain
refined by selective grain size diameter of 4 micron or less and (iii) said Monel alloy galvanic
coupled with a protective metal which is anodic to said Monel alloy.
2. Monel alloy rendered resistant to stress corrosion cracking in hydrofluoric acid as
claimed In daim 1 comprising said Monel alloy galvanic coupled with stainless steel,
preferably 316 stainless steel.
3. Monel alk>y rendered resistant to stress corrosion cracking in hydrofluoric add as
daimed in anyone of daims 1 to 2 wherein said Monel alloy comprises elemental
constitution of Ni 64.89 % by weight, Cu 31.8 % of weight. Fe 1.95 % of weight. Si
0.12 % of weight. Al 0.07 % by weight, C 0.122 % by weight, Mn 0.73 % by weight,
Ti 0.05 % by wt., S 0.004% by wt. and Co 0.09 % by weight.
4. A process for rendering Monel alloy, basically a nickel-copper alloy, with elemental
constitution in the range of Ni 61-67 % by wt., Cu 28-32% by wt, Fe 0.7-2% by wt,
Mn 0,7-1% by wt. Si 0.1-0.5% by wt, Ti <0.5% by wt, S <0.005% by wt, Al 0-2.75%
by wt, Co 0.1-0.2% by wt and C <0.2% by wt., resistant to stress corrosion cracking
in hydrofluoric add,
which comprises subjecting the alloy to treatment selected fi'om any one or more of
the following:
(a) subjecting the said alloy to cold working to at least 60% by cold rolling for
sheets and by cold drawing though dies for t)ars or rods,
(b) subjecting the said alloy to grain refinement to 4 micron diameter or less; and
(c) subjecting the said alloy to galvanic coupling with a metal which is anodic to
the said alloy.
5. A process as daimed in daim 4, wherein the said metal which is anodic to the said
alloy used is Type 316 stainless steel,
6. A process as claimed in anyone of daims 4 to 5, wherein the said alloy used
comprises elemental constitution of Ni 64.89%, Cu 31.8%, Fe 1.95%, Mn 0.73%, Si
0.12%, Ti 0.05%, S 0.004%, Al 0.07%, Co 0.09% and C 0.122% (all wt%).
7. A process as claimed in anyone of claims 4 to 6, carried out such that the ductility
ratio (DR) of the alloy is above 0.85 in slow strain rate test (SSRT),
8. A process for rendering Monei resistant to stress corrosion cracking, as claimed in
anyone of daims 4 to 7, comprising the lowering down of grain size to 4 micron or
below, as a singular measure,
9. A process for rendering Monel resistant to stress corrosion cracking, as claimed in
anyone of daims 4 to 7, comprising cold working Monel to 60% or higher, as a
singular measure.
10. A process for rendering Monei resistant to stress corrosion cracking, as daimed in
anyone of daims 4 to 7, galvanic coupling with Type 316 SS, as a singular measure.
11. A process for rendering Monel resistant to stress corrosion cracking, as daimed in
anyone of daims 4 to 7, comprising subjecting the said alloy to a combination of cold
working 60% or more and galvanic coupling with Type 316 SS or small grain size of 4
micron and lower and galvanic coupling with Type 316 SS.
12. A process for rendering Monel resistant to stress corrosion cracking, as daimed in
anyone of daims 6 to 9, comprising subjecting the said alloy to a combination of cold
working of 60% or more and galvanic coupling with Type 316 SS, with or without a
grain size of 4 micron or below,
13. A process for rendering Monel resistant to stress corrosion cracking, as daimed in
anyone of claims 4 to 7, comprising subjecting the said alk>y to a combination of
gah^ank: coupling with Type 316 SS and controlling of grain size to 4 micron or below,
with or without a of cokj working of 60% or more,
14. A process for rendering Monel resistant to stress corrosion cracking in hydrofluoric
add substantially as herein described and illustrated with reference to the
accompanying examples.

Monel alloy, in particular Monel 400,which is stress corrosion crack resistant in
hydrofluoric acid or where the acid formation is a possibility through chemical
reactions with fluoride salts and its process for manufacture. The stress corrosion
crack protected Monel-400 is selected from atleast one of (I) Monel-400 cold worked
atleast 60% (ii) grain refined Monel-400 and (iii) Monel-400 galvanic coupled with a
protective metal which is anodic to Monel-400,The Monel alloy Is directed to wide
variety of industrial applications even in environments where the alloy comes in
direct contact with hydrofluoric acid. It would thus be possible to use of articles /
components made of Monei 400 in conditions wherein the said articles would come in
direct contact with hydrofluoric acid.