Note: Descriptions are shown in the official language in which they were submitted.
21 64403
We/wa 95403
NICKEL ALLOY AND CONSTRUCTIONAL M~MR~RS MADE THEREFROM
The invention relates to a nickel alloy having improved
resistance to corrosion, more particularly in relation to acid,
low-oxygen media, such as occur, for example, in smo~e gas
desulphurization plants, and constructional mem~ers produced
therefrom.
EP-A-0 334 410 discloses a number of corrosion-resistant nickel
alloys containing molybdenum and chromium as the main alloying
components, namely alloys containing (all details in % by
weight): 14 to 26% chromium; 3 to lg% molybdenum; 14.S to 23
chromium; 14 to 17~ molybdenum; 20 to 24% chromium~ 12 to 17%
~olybdenum; 2 to 4% tungsten; 2 to 8% iron; 22 to 24% chromium;
15 to 16.5% molybdenum. These alloys are more particularly used
in the chemical industry and environmental techniques where
conditions of corrosion are so aggressive that high-alloy steels
no longer provide adequate resistance to corrosion. In spite of
the wide scope of use described for the last-mentioned alloy in
EP-A-0 334 410, very recent experiences of smoke gas
desulphurization have shown that to an increasing extent it
creates conditions for which the resistance to corrosion of the
known nickel/chromium/moly~denum alloys are inade~uate. These
are the very acid and at the same time low-oxidant ranges of high
concentration of chloride ions which may occur in the wet
desulphurization of smoke gas, more particularly with deposits
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and encrustations of gypsum. Another problem is represented ~y
the heat displacement systems of smoke gas desulphurization
plants. The reprocessing of technical sulphuric acid is also
frequently formed in extremely low-oxidant conditions, in which
the known metallic material alloyed with chromium move with the
result of an impermissibly high corrosive detrition outside their
passive range. In such a case, therefore, materials such as
graphite must be used in the prior art for tanks and piping,
although such materials have the disadvantages of deficient
ductility and lack of weldability in comparison with metallic
materials. Even metallic materials on a nickel basis, as a rule
containing 26 to 30% molybdenum and up to 7% iron and up to 1.5%
chromium, which have hitherto been developed exclusively for the
handling of heavily reducing acids, have not proved satisfactory
for such applications.
Further to this, U. S. Patent 4,861,550 discloses nickel alloys
containing 25 to 35% molybdenum, 2 to 8% iron, 0.3 to 2%
aluminium, 0.1 to 4% of at least one of the elements vanadium,
tungsten, chromium and copper and also possibly 0.5 to 5% cobalt
and further admixtures of boron, calcium, titanium, zirconium,
niobium and yttrium. Their resistance to corrosion also proves
to be inadequate in some cases.
It is therefore an object of the invention to provide a metallic
material which has adequately adapted resistance to corrosion
even in the aforementioned more stringent conditions.
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In an attempt to solve the above-mentioned problems,
the present invention provides a nickel alloy which consists
of (in % by weight):
20.5 to 25% molybdenum
5 to 11.5% chromium
more than 5 to 8% iron
0.1 to 0.6% manganese
0.1 to 0.3% vanadium
0.1 to 0.5% aluminium
0 to 0.3% silicon
0 to 0.5% copper
0 to 0.1% carbGn
0 to 1% cobalt
0 to 0.04% magnesium
0 to 0.01% calcium
and the remainder of nickel and usual unavoidable impurity
elements accompanying due to manufacture, such as phosphorus
and sulphur.
Preferably nickel is contained 58 - 63%. Preferably
molybdenum is contained 23 - 25%. Preferably chromium is
contained 5 - 10%. Preferably iron is contained 6 - 8%.
The present invention also provides a construction
member of a smoke gas desulphurization plant or a reprocessing
plant for waste sulphuric acid or a mixture of inorganic acids.
The member is made of the nickel alloy mentioned above.
The invention will now be explained in detail with
reference to the following examples.
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- 3a -
A number of tests were carried out with nickel
alloys having the compositions set forth in Table 1. They
could readily be converted into sheets by hot, followed by
cold shaping. The sheets were finally solution annealed for
30 minutes at 1190~C, the result being a uniformly
recrystallized structure. Alloys No. 1 and No. 2 belonged
to the prior art, being the nickel/chromium/molybdenum Alloy
C-276 (German Material No. 2.4819) and the nickel/chromium/-
molybdenum Alloy 59 (German Material No. 2.4605). The alloys
No. 3 and No. 4 were two examples of the material according
to the invention.
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-- 4
To compare their resistance to corrosion, the alloys were tested
in media bel~nging to the range of reducing and weakly oxidizing
conditions of corrosion. In accordance with the preferred
conditions of use of the alloy according to the invention, a
simulate solution of smoke gas desulphurization was selected for
this purpose. The solution was an aqueous sulphuric acid
solution, adjusted to a pH value of 1, with the addition of 7
chloride ions and 0.01% fluoride ions and also 15% gypsum
deposited in smoke gas desulphurization. The alloys were tested
in this solution at 100 C for 30 days, the stirred solution being
covered with nitrogen. Experience showed that this gave a
satisfactory imitation of the very acid and simultaneously low-
oxygen conditions of high chloride ions concentration which are
formed in wet desulphurization of smo~e gas in constructionally
conditioned gaps and with deposits and encrustations of gypsum.
The surfaces of the test pieces had previously been ground using
grain size 120. Tests were made both for uniform surface
corrosion and also crevice corrosion. For this purpose, as
descri~ed in detail in "Werkstoffe und Korrosion", Vol. 37, 1986,
pp. 183-lgO, PTFE crack blocks had been prepared and attached to
the surfaces with a torque of 3 Ncm. The results are reproduced
in Table 2. It can be seen that the surface corrosion of the
alloys No. 3 and No. 4 according to the invention was slightly
increased in comparison with the prior art alloys No. 1 and
No. 2, but still low enough for the alloys to be regarded as
stable. The decisive factor was crevice corrosion. In spite of
the comparatively small surface corrosion removal of only~0.02g
and 0.006 ~m/year, in the case of the prior art alloys No. 1 and
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,-- .
No. 2 crevice corrosion reached depths of up to 0.17 mm even
after only 30 days. This soon led to local perforation of the
walls, resulting in an adverse effect on the operation of the
whole plant. In contrast, the alloys No. 3 and No. 4 according
to the invention were free from any signs of such crevice
corrosions. This makes it possible to construct metallic low-
maintenance components which at present are still regarded by
operators as wearing parts with a few months' service life.
In accordance with the preferred conditions of use of the alloy
according to the invention, 70% sulphuric acid at 120 C and 80%
sulphuric acid at 125 C were selected as further test solutions.
These sulphuric acid concentration ranges and temperatures
correspond to actual conditions of use in heat displacement
systems of smoke gas desulphurization plants. The indications
obtained from such test conditions also provide bases for
presumed satisfactory behaviour in the reprocessing of waste
sulphuric acid (30 to 90~ by weight) under reducing conditions.
The results of the comparative test are reproduced in Table 3.
It can ~e seen that the prior art alloys No. 1 and No. 2 show
such high rates of removal that their use cannot be considered.
In contrast, due to their ~uch lower detritional losses the
alloys No. 3 and No. 4 according to the invention are useful.
The use of the alloy according to the invention in the handling,
transportation and storage of ~ixtures of low-oxidant inorganic
acids, such as are typically used in ~etal pickling departments,
is illustrated by way of example in Ta~le 4. Due to the enormous
aggressiveness of these pic~ling acids at relatively high
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temperatures, hitherto metallic components have ~een regarded as
wearing parts which were regularly interchanged at corrosion
speeds of > 5 mm/a (cf. Alloys 1 and 2).
As can be gathered from Table 4, the alloys according to the
invention, more particularly No. 3, here have the advantage of
substantially prolonged durability.
The iron content of the alloy according to the invention (more
than 5 to 8%) on the one hand makes possible cheap manufacture on
the basis of works scrap from the production of the prior art
alloy No. 1 (Table l), which accumulates in relatively large -
quantities for environmental techniques; on the other hand it
makes it easier to control iron dilution in the welding
processing of rolling clad composites, comprising a thin deposit
of a nickel-based alloy on a plain carbon steel supporting sheet,
which are becoming increasingly preferred for reasons of cost.
If satisfactory - i.e., crack-free hot workability is to be
ensured, the nickel alloy according to the invention can also
contain (in % by weight) 0.001 to 0.04% magnesium and up to 0.01%
calcium.
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Table 1: Chemical composition of the test alloys
Alloy No. Composition in % by wei~ht
Ni M~ C~ Mll ~;i Al ~' COthers
56,25 16,1515,9 G,1(3 0,390,0~1 - 0,19 0,005 3,~16W (),91 Co 0~I2CIJ
2 60,25 15,5022 55 0,9~) 0,170,03 0,2' 0,13 0,006 0,03 Co
3 62,23 24,B05,95 6,0~ 0,28 0 030,2' 0,18 0,003 0,]5 Co 0,09 Cu
9 5~,14 24,8~9,93 6,07 0,29 0,0~0,2, 0,20 0,001 0,14 Co 0,08 C
~, ~
1--
Table 2: Resistance to corrosion in smoke gas desulphurization sirr.ulate solution with pH 1 (H SO4),
7% chloride ions, 0.01% fluoride ions, 15% smoke gas desulphurization gypsum, stirre~ under
nitrogen cover at 100~C after 30 days
Alloy No. Surface corrosion Crevice corrosion depth
~ nun/vear . mm .
0 02~ 0 17
2 0,00G 0,1G
3 0,103 0
0,061 ()
o
Table 3 Resistance to corrosion ln hot medlum concentrated sulphur acid
Alloy No. Surface corroslon, mm/year
2 480%H2SO4
120~C 125~C
1 5,85 1,36
tv
2 7,61 1,91
3 0,09 0l07 O
4 0,25 0,12
.P
o
Teble 4: Llnear detrltlonal rate efter 21 deys lmmer~lon test ln lO~ HCl wlth and wlthout N2
cover at bolllng temperature
Alloy No. 10% HCl wlth N2 cover 10% HCl + 200 ppm Fe3 wlthout N2 cover
1 6,56 7,55
2 8,49 6,31
3 0,34 0,42
4 1,31 1,55
o
