AU693609B2 - Nickel alloy and constructional members made therefrom - Google Patents

Abstract

A Ni alloy displaying improved corrosion resistance, esp. in acidic and low O2 media, comprises (wt.%): 20.5-25 Mo, 5-11.5 Cr, over 5 to 8 Fe, 0.1-0.6 Mn, 0.1-0.3 V, 0.1-0.5 Al, 0-0.3 Si, 0-0.5 Cu, 0-0.1 C, 0-1 Co, 0-0.04 Mg, 0-0.01 Ca, balance Ni and impurities.

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): Krupp VDM GmbH ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

.of performing it known to me/us:a INVENTION TITLE: Nickel alloy and constructional members made therefrom The following statement is a full description of this invention, including the best method of performing it known to me/us:- *~oo U 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 smoke gas desulphurization plants, and constructional members 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 18% molybdenum; 14.5 to to 23% chromium, 14 to 17% molybdenum; 20 to 24% chromium, 12 to 17% o molybdenum, 2 to 4% tungsten, 2 to 8% iron; 22 to 24% chromium, 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/molybdenum alloys are inadequate. Thee 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 P1 _I II__ 2 and encrustations of gypsum. Another problem is represented by 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 chromium, which have hitherto been developed exclusively for the handling of heavily reducing acids, have not proved satisfactory for such applications.

Further to kis, US-PS 4 861 550 also 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.

a.

Advantageously, the invention provides a metallic material which has adequately adapted resistance to corrosion even in the aforementioned more stringent conditions.

ap- 3 According to the invention there is provided a nickel alloy which consists of (in% by weight): 20.5 to 25 molybdenum to 11.5 chromium 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 carbon 0 to 1 cobalt usual admixtures of accompanying elements due to such as phosphorus and sulphur, residue nickel.

and also the manufacture, The invention will now be explained in detail with reference to the following examples.

A number of tests were carried out with nickel alloys having the S:o' 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 :4 terial according to the invention.

4 To compare their resistance to corrosion, the alloys were tested in media belonging 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 lowoxygen conditions of high chloride ions concentration which are formed in wet desulphurization of smoke 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 described in detail in "Werkstoffe und Korrosion", Vol. 37, 1986, pp. 183-190, 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 0.029 and 0.006 mm/year, in the case of the prior art alloys No. 1 and

I-

5 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 lowmaintenance 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 sulphuric acid at 125°C were selected as further test solutions.

These sulphuric acid concentration ranges and temperatures o.oe :....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.

,elee S The results of the comparative test are reproduced in Table 3.

It can be 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 much 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 mixtures of low-oxidant inorganic acids, such as are typically used in metal pickling departments, is illustrated by way of example in Table 4. Due to the enormous aggressiveness of these pickling acids at relatively high -I I 6 temperatures, hitherto metallic components have been 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 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 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.

a If satisfactory 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.

0 *0 o a *0 a 0* 000 **a Table 1: Chemical composition of' the test alloys 0*a a a a .a a. a. *e a a..

a a a a 0 a -a a a a a a Alloy No. Composition in by weight 'i Mol C1. Fe [VIII S i Al NI C Others 56,25 16.15 15,9 6,19 0,39 0,01i 0,19 0,005 3,46\'W 0,9 1 Co- 0, 12 Cui- 2 60,25 15,50 22,55 0,99 0,17 0,03 0,24 0,13 0,006 0,03 Co 3 62,23 '24, 80 15,95 6,05 0,28 0,03 0,24 0,18 0,003 19,15 Co 0,09 Cu 24,86 9,93 J6 60 7 0,9 0, 04 0,'25 02 ,0 ,4C ,8C Table 2: Resistance to corrosion in smoke gas desulphurization simulate solution with pH 1 SO~) chloride ions. 0.01%/ fluoride ions, 15%/ smoke gas desuiphurization gypsum, stirre unde~r nitrogen cover at 100 0 C after 30 days Alloy No.

Surface corrosion mmn/vear Crevice corrosion depth

MM

11 0,029 2 10,006 0, 17 3 0,103 0 4 10,061 t0 0a a a..

a a a a a Table 3: Resistance to corrosion in hot medium concentrated sulphuric acid Alloy No. Surface corrosion, mrm/year

%H

2 S0 4 0 g% H 2 S0 4 1200 C 1250 C 5,85 1,36 2 7,61 1,9] 3 0,09 0,07 4 0,25 0,12 V.

V.

V

V 0 S V V V

V

V Table 4i: Linear detritional rat,- after 21 days immersion test in 10% H~l with and without N 2cover at boiling temperature Alloy No. 10% HUl with N 2cover 10% 110 200 ppm, Fe 3 without N 2cover 1 6,56 7,55 2 8,49 6,31 3 0,31 0,42 4 1,31 .1

Claims (2)

1. A nickel alloy having improved resistance to corrosion, more particularly in relation to acid and simultaneously low-oxygen media and which consists of (in by weight): 0** 0 eo *04. 0000

20.5 to 25 molybdenum to 11.5 chromium 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 carbon 0 to 1 cobalt 0 to 0.04% magnesium 0 to 0.01% calcium residue nickel, including unavoidable impurities. 2. Constructional members of smoke gas desulphurization plants or reprocessing plants for waste sulphuric acid or mixtures of inorganic acids, such as are deposited, for example, in metal pickling processes, made from a nickel alloy according to claim 1. 1':0'UMAXDU7722--11 PS-559 3. A nickel alloy and/or elements made therefrom substantially as hereinbefore described with reference to the Examples. DATED this FIFTEENTH day of MAY, 1998 Krupp VDM by DAVIES COLLISON CAVE Patent Attorney's for the Applicants I o 0 SO S S S 55 S S S S S. S S 555 S tL~ C' Tf r ABSTRACT The invention relates to a nickel alloy which contains 20.5 to molybdenum, 5 to 11.5% chromium, more than 5 to 8% iron as the main alloying elements and which has adequate resistance to corrosion, mcre particularly in relation to acid, low-oxygen media such as occur, for example, in smoke gas desulphurization plants, and constructional members produced therefrom. 0 9* 0 e s 0* 0* 0 a