JP3297695B2 - Electrode for cathodic electrolysis in nitric acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for cathodic electrolysis in nitric acid produced from an alloy having a small amount of corrosion in cathodic electrolysis in nitric acid.

[0002]

2. Description of the Related Art A Cr-containing Fe-based alloy represented by stainless steel is extremely stable in medium and low concentrations of nitric acid (for example, several percent to 40%), and its surface is passivated and hardly melts. It has been known. However, it is known that in concentrated nitric acid containing an oxidizing agent such as hexavalent Cr, the surface potential becomes a noble potential in a transpassive region and dissolves.

[0003] On the other hand, the dissolution behavior of stainless steel when it is kept at a low potential in nitric acid has not been known so far, but Japanese Unexamined Patent Publication (Kokai) No. 3-267399 discloses a dissolution behavior.
It is described that when ferrous and austenitic stainless steels such as SUS304, 316, 430, 420, 410, etc. are used as a cathode in electrolytic nitric acid in electrolytic nitric acid, they will not dissolve due to cathodic protection. I have.

However, the present inventors have actually tested and found that all of these stainless steels melt at a considerable rate and do not have satisfactory performance as a cathode material in nitric acid. did.

[0005] Today, not only steel materials but also stainless steels are often pickled by anodic and cathodic electrolysis in a nitric acid solution of about 10% in order to further improve the corrosion resistance. Although high-silicon cast iron is used, a special manufacturing method is required, so that cost increase is unavoidable, and the development of more inexpensive materials is required.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide an inexpensive material which has a small amount of corrosion when electrolyzed as a cathode in nitric acid.

[0007]

Means for Solving the Problems The present inventors have conducted various studies in order to solve such problems, and as a result, have obtained the following findings. As a result of examining the effect of alloying elements in stainless steel on corrosion resistance during cathodic electrolysis in nitric acid, it was found that the effect of Cr was large. On the other hand, since Ni has almost no effect of improving corrosion resistance, Fe-Cr-based stainless steel (ferritic stainless steel, etc.) is more advantageous than Fe-Cr-Ni-based stainless steel (austenitic stainless steel) in consideration of economy. It is.

[0008] The higher the Cr content, the better the corrosion resistance in cathodic electrolysis in nitric acid. However, if the Cr content is too high, it is difficult to produce a plate, and the raw material cost increases. ~ 40% is appropriate. As alloy elements for improving corrosion resistance in this way, Nb,
Al is present, and when added in an amount of about 0.2 to 2.0% and about 0.5 to 5.0%, the same effect as Cr is exhibited. Thus, the gist of the present invention is that, by weight%, C
r: Electrode for cathodic electrolysis in nitric acid made of stainless steel containing 20-40%.

[0009]

The reason why the amount of corrosion is reduced when the Cr content is high in the present invention is not clear at present, but is presumed as follows. In other words, stainless steel retains its corrosion resistance by a passive film mainly composed of Cr oxide, but when it is electrolyzed as a cathode in nitric acid, the oxide in the passive film is expressed by the following reaction formula. Is reduced and destroyed.

[0010] ^{(1) FeO + 2H + +} 2e → Fe + H 2 O (2) Fe 2 O 3 + 6H + + 6e → 2Fe + 3H 2 O (3) Cr 2 O 3 + 6H + + 6e → 2Cr + 3H 2 O (1) and (2) Comparing the reactions of Eqs. (3) and (3), Eqs. (1) and (2) are much more likely to occur. In addition, the higher the Cr content in stainless steel, the higher the concentration of Cr oxide in the passive film, so that the passive film is less likely to be destroyed by the reactions of equations (1) to (3). On the cathode surface where the passivation film was destroyed, reduction of nitrate ion by the following equation (4), and by the following equation (5)
H ₂ gas is generated, and at the same time Fe and Cr in stainless steel
Are eluted as ions by the reactions of the equations (6) and (7).

NO ₃ ⁻ + 3H ⁺ + 2e → HNO ₂ ⁻ + H ₂ O (4) 2H ⁺ + 2e → H ₂ (5) Fe → Fe ²⁺ + 2e (6) Cr → Cr ²⁺ + 2e (7) Figure 1 shows 4%, 12% and 20% SUS304 with platinum as anode
The relationship between the amount of dissolution and the potential when performing cathodic electrolysis in HNO ₃ (50 ° C.) is shown. It can be seen that the higher the nitric acid concentration, the faster the dissolution rate, and the dissolution rate varies depending on the potential. The potential at which the dissolution rate is the fastest is -0.7 to -1.4 V (vs. SCE), and this potential tends to become lower as the nitric acid concentration increases. Since this tendency is the same regardless of the type of stainless steel, 50 ° C, 12% HNO ₃ , potential −1.0V (vs. SCE) were selected as typical conditions. The dissolution rate (corrosion rate) in electrolysis was compared.

As a result, it was found that elements having a great effect of reducing the dissolution rate were Cr, Nb and Al. The reason why the corrosion resistance in cathodic electrolysis in nitric acid is improved when the Cr content is high is presumed to be that the passivation film is less likely to be destroyed during cathodic electrolysis as described above.
It is considered that the film is concentrated in the passivation film in the same manner as that described above to prevent the film from being destroyed during cathodic electrolysis in nitric acid.

As described above, the proper concentration of Cr is 20 to 40% in consideration of the corrosion resistance improving effect and the manufacturability. At a concentration higher than this, the corrosion resistance improving effect tends to be saturated. Nb is 0.2
%, The effect of improving corrosion resistance appears. The greater the amount added, the greater the effect. However, the upper limit is 2% from the viewpoints of productivity and economy.
And

The effect is exhibited when Al is added in an amount of 0.5% or more, but the upper limit is set to 5% in terms of productivity. As alloying elements other than Fe, Ni, Cr, Nb, and Al, Mo, Cu, Ti, W, Co, V, Zr, S
Although i and Mn may be contained at 5% or less, the effect of improving corrosion resistance is small. Ag, Au, Ru, Rh, Pd, Os, Ir, Pt are 1%
Addition of the above is effective, but is not preferable in terms of economy.

Since Ta usually coexists with Nb and its effect is almost the same as Nb, it is sufficient that Nb + Ta is 0.2 to 2.0 %. In addition, a small amount or a small amount of Si, Mn, Y, a rare earth element, Ca, Mg, U, etc. may be added for the purpose of deoxidation or the like, and C, N, P, S, As, Sn as inevitable impurities. , Pb and the like may be contained in a trace amount or a small amount. Next, the operation and effect of the present invention will be described more specifically with reference to examples.

[0016]

EXAMPLE Stainless steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace to produce a 10 kg ingot, and a 10 mm thick plate was formed by hot forging. After the solution heat treatment, a 2 mm thick test piece (30 x 50 mm) was cut out by cutting, the surface was finished by No. 600 wet polishing, and the lead wire was spot-welded to remove the silicone sealant except for the 25 x 40 mm test surface. Was applied. Potentiometer / galvanostat (Hokuto Denko, HA310 type), 50 ° C, 12% HN with platinum as counter electrode
After maintaining the potential at −1.0 V (vs. SCE) in O ₃ for 1 hour, the dissolution loss (corrosion loss) was measured with an analytical balance, and the dissolution rate was calculated.

[0017]

[Table 1]

As can be seen from Table 1, the dissolution rates of the alloys of the present invention are comparable to those of the comparative alloys No. 13 SUS304 and No. 14 SUS430.
1 order or more smaller than that of the. These alloys can be used as a cathode material in electrolytic treatment in nitric acid.

[0019]

According to the present invention, a material exhibiting excellent corrosion resistance can be obtained even when a cathodic electrolysis is performed in a nitric acid solution of 40% or less, so that an electrode can be manufactured by using the conventional high silicon cast iron. Thus, an electrode that is less expensive and superior to that obtained by the method described above can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the effects of nitric acid concentration and potential on the dissolution rate in cathodic electrolysis of stainless steel according to the embodiment of the present invention.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-56-58982 (JP, A) JP-A-57-145991 (JP, A) JP-A-57-145992 (JP, A) Ryohei Tanaka (4) Name) Edited “Encyclopedia of Metallic Materials” (January 25, 1990) Asakura Shoten Co., Ltd. 199− (58) Field surveyed (Int.Cl. ⁷ , DB name) C25B 1/00-15/08 C25F 1/06 C25F 7/00

Claims (2)

(57) [Claims] 1. An electrode for cathodic electrolysis in nitric acid made of stainless steel containing 20% to 40% Cr by weight. 2. Cr content: 20 to 40%, Al: 0.5 to 5% by weight.
% And / or Nb: stainless steel containing 0.2 to 2%
The electrode for cathodic electrolysis in nitric acid.