SK61296A3 - Solution for chemical decontamination of steels - Google Patents

Abstract

Solution for chemical decontamination of high-alloy low-alloy steels with reducing and complexing The properties include 0.01 to 0.9% by weight percent mineral acid, 0.008 to 0.5 weight percent percent hydrazine hydrate, 0.07 to 2 percent by weight salts of nitrilotriacetic acid and the remaining content water up to 100 percent by weight with its value The pH is 1.8 to 3.5.

Description

Chemical decontamination solution for steels

Technical field

The invention relates to a composition of a reducing solution for chemical decontamination of steel surfaces from nuclear power plant operation.

BACKGROUND OF THE INVENTION

The current method of reducing surface contamination of steels and dissolving their corrosion layers is by applying various decontamination solutions by circulating them in in situ equipment or by applying them in decontamination baths where the decontaminated objects are submerged. Depending on the type of decontamination steels and their corrosion layers, the decontamination solutions are decontaminated in two stages first in the oxidation solution and then in the reduction solution, or in one stage in the reduction solution. The two-stage decontamination in oxidizing and then reducing solution is used in the decontamination of high chromium stainless steels in a contaminated corrosion layer. Single-stage decontamination in a reducing solution can be used in the decontamination of both stainless steels and low-alloy steels with a low chromium content in the corrosion layer. The oxidizing solutions contain a strong oxidizing agent, e.g. potassium permanganate and nitric acid or sodium hydroxide. The reducing solutions contain components allowing for the possible reduction, removal of oxidizing solution residues and dissolution, in particular of iron oxides. Dissolution of iron oxides can take place by a reduction mechanism in which the reduction of Fe ³⁺ to Fe ²⁺ ions at the interface between the oxide and the solution is possible. Reductive solutions used in the past include a solution containing citric acid and oxalic acid. This solution, in addition to its very good reducing properties and the ability of oxalic acid to dissolve iron oxides, is also known to cause the formation of a precipitate of oxides of dissolved metals (Fe, Ni) and both recontamination of steel surfaces and local corrosion attack of steel surfaces and welds. D. Majerský (Patent No. 277909, SR;

respectively. AO č. 267173, ČSSR) by adding a suitable complexing agent and adjusting the pH of the solution of oxalic acid and citric acid so as to avoid adverse corrosion and deposition of oxalate. The disadvantage of this solution has been the need for careful analytical control of the process, limited solubility of the complexing agent at lower pH values of the solution, and the need for frequent dosing of chemicals into the solution at a higher iron ion content in the solution. Another possibility of eliminating the adverse properties of oxalic acid is discussed by H. Spychala (patent 9215722, France) using a reducing solution no longer containing oxalic acid and containing nitric acid and a reducing complexing agent ascorbic acid. The disadvantage of this solution may be the consumption, decomposition of the decontamination and complexing agent, in this case ascorbic acid, in the reduction of oxidant residues after previous oxidation of steels e.g. with potassium permanganate solution and the possibility of thermal decomposition of ascorbic acid to oxalic acid at higher application temperatures.

SUMMARY OF THE INVENTION

The above-mentioned drawbacks and problems are solved by a new reduction solution for chemical decontamination of steels, the nature of which results from a mineral acid content of 0.01 to 0.9 percent by weight, hydrazine hydrate of 0.008 to 0.5 percent by weight and sodium and / or potassium and / or ammonium salts. nitrilotriacetic acid having a content of 0.07 to 2% by weight and a residual water content of up to 100% by weight, the pH of the solution being equal to 1.8 to 3.5.

The composition of the solution according to the invention allows rapid and corrosion-safe dissolution and removal of contaminated corrosion layers from steel surfaces without subsequent deposition of precipitated metal salts from the solution.

The improved effect of the new reducing decontamination solution is achieved by finding suitable solution components that provide optimal decontamination and reducing conditions and action as well as complexing properties of the solution.

Embodiment of the invention

After oxidation treatment of the contaminated stainless steel tube samples of a pressurized water reactor of a nuclear power plant with an oxidation solution containing 2% by weight potassium hydroxide and 0.5% by weight potassium permanganate for 1.5 hours at a temperature of 90 ° C and subsequent reduction treatment with a solution according to the invention. weight percent sodium nitrilotriacetic acid and 0.4 weight percent nitric acid and 0.3 weight percent hydrazine hydrate pH 2.5 at 90 ° C for 3 hours resulted in a 6.2-fold reduction in sample contamination. The corrosion rate in these stainless steel treated corrosion coupon solutions was 0.04 gm ^-2 per decontamination cycle. Decontamination of parallel contaminated samples with the same oxidative treatment with sodium hydroxide and potassium permanganate solution resulted in a 4.5-fold reduction in contamination after a reduction of 2% by weight of citric acid and 2% by weight of oxalic acid at 90 ° C for 1.5 hours. samples. The corrosion rate of the corrosion coupons treated in these solutions was 0.05 gm &^lt; ^{2 >} per 1 decontamination cycle.

Industrial Applicability

The solution according to the invention can be used in nuclear power plants for chemical decontamination of steel surfaces coated with contaminated corrosion layers.

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Claims (1)

PATENT CLAIMS A chemical decontamination solution for steels, used alone or in combination with an oxidation treatment, characterized in that it consists of 0.01 to 0.9% by weight of nitric acid and / or phosphoric acid and / or sulfuric acid and 0.008 to 0.5 % by weight of hydrazine hydrate and 0.07 to 2% by weight of sodium and / or potassium and / or ammonium salts of nitrilotriacetic acid and the remaining water content up to 100% by weight, the pH of which is equal to 1.8 to 3.5.