EP0182932A1

EP0182932A1 - A method for protecting an apparatus from corrosion

Info

Publication number: EP0182932A1
Application number: EP84114388A
Authority: EP
Inventors: Tetsuya Ohrui; Masao Okubo; Masayoshi Miki
Original assignee: Nippon Stainless Steel Co Ltd; Sumitomo Chemical Co Ltd
Current assignee: Nippon Stainless Steel Co Ltd; Sumitomo Chemical Co Ltd
Priority date: 1984-11-28
Filing date: 1984-11-28
Publication date: 1986-06-04

Abstract

There is provided a method for protecting an apparatus from corrosion, which apparatus treats a nitric acid solution containing heavy metal(s) and/or compound(s) thereof, in which method stainless steel with a silicon content of 1% by weight or blow is used as the material composing the apparatus, and an NOx gas is compulsorily added into the nitric acid solution.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method for protecting a stainless steel-made apparatus treating a nitric acid solution containing heavy metal(s) and/or compound(s) thereof from corrosion.
It is well known that stainless steel, when placed in a moderately oxidative fluid such as nitric acid, forms a protective oxide film, or a so-called passive state film on its surface owing to its own oxidative action to exhibit an excellent corrosion resistance. However, in a nitric acid solution containing heavy metal(s) and/or compound(s) thereof, stainless steel is heavily corroded to such an extent as could hardly be imagined from its showing in nitric acid alone.
Such unexpectedly heavy corrosion of stainless steel in a nitric acid solution containing heavy metal(s) and/or compound(s) thereof can be accounted for by the acute turn of surface potential of stainless steel to noble potential due to sharp increase of oxidative activity of nitric acid by the ions of heavy metal(s) dissolved in nitric acid or direct deposition of solid heavy metal(s) and/or compound(s) thereof on the stainless steel surface to cause a so-called "over-passive state corrosion", as diagrammatically illustrated in Fig. 1 attached hereto. Such overpassive state corrosion may occur even in nitric acid alone under high-temperature or high-concentration conditions, but it tends to take place very readily and extensively when heavy metal(s) and/or compound(s) thereof are contained in nitric acid.
Nitric acid solutions containing heavy metals and/or compounds thereof are treated in various production plants and apparatuses, for example, nitric acid production plants using oxidation process of ammonia, apparatus for oxidation of organic compounds with nitric acid, apparatus for recovering metals from scraps of semiconductor materials, radioisotope reprocessing apparatus, and stainless steel surface cleaning apparatus, and therefore the development of the techniques for providing stainless steel with an excellent corrosion resistance against nitric acid solutions containing heavy metals and compounds thereof is an immensely great industrial contribution.
Generally, stainless steel with a low silicon content shows an excellent corrosion resistance in a system of nitric acid alone, while stainless steel with an increased silicon cntent shows an anticorrosive disposition in a nitric acid solution containing heavy metal(s) and/or compound(s) thereof.
However, in the latter case, the increase of silicon content to, for instance, about 2% by weight can not provide a satisfactory corrosion resistance improving effect while too much increase of silicon content can impair excellent workability and weldability of stainless steel, and no decisive solution to these problems has yet been found.
The present inventors have carried out extensive researches on the method for preventing corrosion of stainless steel which has good workability and weldability and is relatively easy to obtain economically and, as a result, found that stainless steel is amazingly prevented from corrosion in a nitric acid solution when an NOx gas is compulsorily added into the solution while defining the silicon content in stainless steel to at most 1% by weight so that the excellent workability and weldability of stainless steel are not impaired. The present invention was achieved on the basis of such finding.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method for preventing excess corrosion of an apparatus in which a nitric acid solution containing heavy metal(s) and/or compound(s) thereof is treated, characterized in that stainless steel with a silicon content of 1% by weight or below is used as the material composing said apparatus, and that an NOx gas is compulsorily added into said nitric acid solution.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph for illustrating the general relation between corrosion potential and corrosion rate of metal by nitric acid. In this graph, (a) indicates the region of active state, (b) the region of passive state, and (c) the region of overpassive state.
_Fig. 2 is a schematic illustration of the experimental apparatus used in Example 1 and Comparative Example ₁. In this drawing, (a) denotes NOx gas feed, (b) feed of 45% HNO₃ + 0.054% NH₄V0₃+ 0.109% CuO solution, (c) mantle heater for boiling, (d) multi-necked flask, (e) corrosion test piece, (f) nitric acid solution used for corrosion test, (g) discharge of nitric acid solution used for corrosion test, and (h) water-cooling condenser.
Fig. 3 is a graph showing the results of Example 2 and Comparative Example 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A detailed description of the present invention will be given below.
It has been found in this invention that in a nitric acid solution containing heavy metal(s) and/or compound(s) thereof, stainless steel with a low silicon content is markedly improved in its resistance to nitric acid when an NOx gas is compulsorily added into the nitric acid solution. This is quite disparate from the case of a system of nitric acid alone where no NOx gas is blown. However, quite contrary to the above phenomenon, stainless steel with a high silicon content is excessively deteriorated in its resistance to nitric acid when NOx is blown into the nitric acid solution.
The maximum amount of silicon allowed to be contained in stainless steel for affording thereto excellent corrosion resistance by forced addition of NOx into nitric acid containing heavy metal(s) and/or compound(s) thereof is 1% by weight, and if the silicon content exceeds 1% by weight, the desired corrosion resistance is not provided.
In this invention, therefore, it is essential that stainless steel with a silicon content of 1% by weight or below be used as the material constituting the apparatus used in the present invention.
The heavy metals and heavy metal compounds that may be contained in nitric acid include platinum, vanadium, cerium, gallium, ruthenium, uranium, plutonium and many other heavy metals and their compounds as well as mixtures thereof. These heavy metals and their compounds are usually used after decomposed and melted or by mixing them with nitric acid as an assistant for the ensuring oxidation reaction, but they may be inevitably contained in nitric acid in slight quantities.
The NOx gas that is added into nitric acid in this invention includes NO gas, NO2 gas and their polymers N₂0_4' N₂0_3' etc., as well as mixtures thereof. Also, N₂, 0₂, H₂0 and other like substances can be used while mixed in NOx gas as inert gas components. Blowing of NOx gas into nitric acid can be accomplished in various ways such as: directly blowing NOx gas into a nitric acid solution before the treatment thereof; blowing NOx gas directly into a nitric acid which has become strongly corrosive with the increase of the amount of heavy metal(s) and/or compound(s) thereof in the solution; blowing NOx gas into a gaseous phase above the nitric acid solution, etc. Such blowing can be accomplished more effectively by incorporating a proper mixing operation or pressure application. As the NOx gas source, besides those obtainable in the ordinary ways, it is possible to use the NOx gas produced when adding metal(s) or compound(s) thereof to nitric acid. Also, a separately produced NOx gas such as produced by reacting an ordinary acid with a nitrous acid compound can be used.
The amount of NOx gas which needs to be blown for achieving the desired anticorrosive effect can not be definitely specified as it is variable depending on the various factors such as concentration and temperature of nitric acid, kind, valence and concentration of heavy metal(s) and compound(s) thereof, type and composition of NOx gas used, etc., but basically, said amount of NOx gas to be blown may be defined as that which is necessary for shifting the corrosion potential of stainless steel in the region of over-passive state into the region of passive stage, and the present invention provides a quite specific and effective method for achieving such shift of corrosion potential. An excess amount of NOx gas does not hamper the anticorrosive effect of this invention if the silicon content in the corrosion preventive surface of stainless steel is not greater than 1% by weight.
Usually, an NOx gas removing device is attahced to and operated in correspondence to the run of the equipment which treats nitric acid, so the effective utilization of superfluous NOx gas will enable a reduction of required working rate of the NOx gas removing device.
The present invention will be further described below by way of its typical embodiments, but it is to be understood that the scope of the present invention is not limited by these embodiments. The chemical compositions of the test pieces of stainless steel used in the embodiments described below are shown in Table 1. Of these test pieces, SUS 304, SUS 304L, 310 ELC, NAR-310Nb and SN-5 are commercially available steel stocks. The 16 steel samples from A-0.3 to D-1.8 in Table 1 are the test pieces prepared by melting the respective steel compositions, and they were tested by subjecting them to a solution heat treatment at 1050°C for 15 minutes and air cooling as final heat treatment. Also, a part of these stainless steel test pieces were tested by subjecting them to a sensitization heat treatment at 650°C for 2 hours. and air cooling (hereinafter abbreviated as Sl) by presuming preferential intergranular corrosion at the weld.

Example 1 and Comparative Example 1
A 45% nitric acid solution containing NH₄VO₃ and _CuO was supplied continuously at a flow rate of 100 ml/hr into a 1-litre-capacity flask shown in Fig. 2 (the solution being discharged out from the outlet so that the liquid level in the flask was kept constant), and each test piece was subjected to a corrosion test in said solution under the boiling temperature, condition for a period of 100 hours. From the results shown in Table 2, it can be seen that stainless steel is extensively corroded in the 45% nitric acid solution containing NH₄VO₃ and CuO, but if the silicon content in stailess steel is below 1%, the corrosion is drastically suppressed by the addition of NOx gas.

Example 2 and Comparative Example 2

The test pieces A, B, C and D shown in Table 1 were subjected to a corrosion test in a 20% NHO₃ solution containing 2,000 mg/l of Ce⁴⁺ at boiling temperature under normal pressure to obtain the results shown in Fig. 3. As the NOx gas, N0₂ gas was continuously blown into 250 cc of the testing solution at a rate of 5 cc/min for a period of 10 hours. The results dictate that stainless steel with an _Si content below 1% by weight is markedly improved in its resistance to nitric acid by the blowing of NOx gas.

Example 3 and Comparative Example 3

In a nitric acid production plant using oxidation process of ammonia, there was formed an accumulation of residue containing heavy metals of the analysis (% by weight) shown in Table 3 and their compounds at a section where the flow rate was low, and the occurrence of extensive corrosion was seen in the stainless steel below said accumulation of residue. By using this residue, a corrosion test was conducted (amount of testing solution: 600 cc; amount of residue added: 10 g; test time: 168 hours). The test results are shown in Table 4.
From the above test results, it is noted that excess corrosion of stainless steel by a corrosive solution can be prevented by using a stainless steel with a silicon content below 1% by weight and by intermittently blowing N0₂ gas into the solution (blowing N0₂ gas for a period of one hour at a flow rate of 10 cc/min in every 24 hours).

Claims

1. A method for protecting an apparatus from corrosion, which apparatus treats a nitric acid solution containing heavy metal(s) and/or compound(s) thereof, characterized in that stainless steel with a silicon content of 1% by weight or below is used as the material composing said apparatus, and an NOx gas is compulsorily added into said nitric acid solution.

2. The method of Claim 1, wherein said apparatus is a nitric acid production plant using oxidation process of ammonia, an apparatus for oxidation of organic compounds with nitric acid, an apparatus for recovering metals from scraps of semiconductor materials, a radioisotope reprocessing apparatus or a stainless steel surface cleaning apparatus.

3. The method of Claim 1, wherein said NOx gas is NO, ^N0 ₂, N204, N₂0₃ or a mixture thereof.

4. The method of Claim 1, wherein said NOx gas is ^N ₂₁ 0₂, H₂0 or a mixture thereof.

5. The method of Claim 1, wherein said heavy metal is, for example, platinum, vanadium, cerium, gallium, ruthenium, uranium, plutonium or a mixture thereof.

6. The method of Claim 1, wherein said compulsory addition of NOx gas is effected by directly blowing said gas into the nitric acid solution before treatment thereof.

7. The method of Claim 1, wherein said compulsory addition of NOx gas is effected by directly blowing said gas into the nitric acid solution which has become highly corrosive with the increase of content of heavy metal(s) and/or compound(s) thereof.

8. The method of Claim 1, wherein said compulsory addition of NOx gas is effected by blowing said gas into the gaseous phase above the nitric acid solution.

9. The method of Claim 1, wherein the NOx gas produced when adding a metal(s) or a compound(s) thereof to nitric acid is used as NOx gas source.

10. The method of Claim 1, wherein the NOx gas produced from a reaction of a nitrite and an acid is used as NOx gas source.

11. The method of Claim 1, wherein an NOx gas is blown in an amount necessary for shifting the corrosion potential of stainless steel in the region of over-passive state into the region of passive state.