MXPA99011466A - Process for pickling stainless steel - Google Patents

Abstract

Chemical and/or electrochemical pickling of high grade steel is carried out in an acidic liquid that contains no nitric acid. Consumed electrolyte solution from electrochemical neutral salt pickling lines, preferably sodium sulphate pickling lines, is added.

Description

PROCESS FOR DECAPATING STAINLESS STEEL FIELD OF THE INVENTION The invention relates to a process for the chemical and / or electrochemical pickling of stainless steel in an acidic liquid that does not contain nitric acid.

BACKGROUND OF THE INVENTION When a strip or strip of hot rolled stainless steel is pickled, the entire scale or oxide scale must be removed from the strip after the hot rolling and annealing process. This pickling process is usually carried out in an acid mixture (hydrofluoric acid (= HF) and nitric acid (= HN? 3)). However, nitrous vapors (N0X) are generated in this process, which leads to the development of more and more "pickling processes without nitric acid" for a few years. The principle of all these pickling processes is that the nitric acid is replaced by a different mineral acid (sulfuric acid, hydrochloric acid) in which the redox potential is set exactly by the content of iron ions of different valence (Fe2 + / Fe3 +). In the pickling process, the iron in the form of divalent ions dissolves and the trivalent iron ion is produced either by partial oxidation of Fe2"1" or by the addition of Fe3 + salts.

F1732 / 99MX Processes are known in which trivalent iron ions are produced by the oxidation of Fe ^ + with hydrogen peroxide (H2O2) • In other processes, salts of Fe ^ + (sulfate salts, chloride) are added to the pickling bath. , nitrate or fluoride). When the hot rolled and pickled strip has been reduced by rolling to smaller strip thicknesses, it has to be re-annealed. In this process, a thin layer of oxide is formed, which has to be removed "in a smooth way", without damaging the surface quality (brightness). The electrochemical pickling process in sodium sulphate (neutral electrolyte) solutions has gained worldwide acceptance for cold rolled stainless steel strip. In this neutral pickling process (pH 7 to 4) very toxic chromate (Cr ^ +) is produced. Therefore, the pickling bath used is detoxified or purified, that is, the Cr ^ + is reduced to Cr3 +, in a separate treatment process, before it can be neutralized and the metal hydroxides can be removed by filtration. The detoxification of chromate is very expensive. The reducing agent normally used is a2S2Ü5 or similar sulfur compounds. The following formula describes the reduction process: P1732 / 99HX 4 H2Cr04 + 3 Na S2? 5 + 3 H2SO4 = > 2 Cr2 (S04) 3 + 3 a2SÜ4 + 7 H2O The reduction can also be achieved with divalent iron sulphate FeS04: 2 H2Cr? 4 + 6 FeS04 + 6 H2SO4 = ¡> 2 Cr (804) 3 + 3 Fe2 (SO4) 3 + 8 H20 SUMMARY OF THE INVENTION The objective of the invention is to avoid the disadvantages of the two processes, particularly the costly detoxification of chromate. In this way, the invention is characterized by the electrolyte solution used from electrochemical pickling lines with neutral salt, preferably, pickling lines of sodium sulfate, which is added. As a result, on the one hand the chromate can be reduced ("detoxified"), without requiring expensive additional reducing agents and, on the other hand, the iron is oxidized from Fe2 + to Fe3 +. A further favorable development of the invention is characterized by the redox potential of the pickling liquid which will be set at a value determined by the amount of electrolyte solution added, wherein the oxidation-reduction potential (redox potential) of the Pickling is measured and the amount of electrolyte solution used from the electrochemical pickling lines with sodium sulfate can be added in P1732 / 99HX the appropriate amount. This is an advantageous method for fixing the pickling effect for the stainless steel strip in the acid pickling liquid. An advantageous configuration of the invention is characterized by a divalent iron salt and a trivalent salt, preferably iron sulphate., which is added when the acid pickling liquid is freshly prepared. This is a favorable method to start the process. An advantageous further development of the invention is characterized by a different oxidizing agent, preferably hydrogen peroxide, which will be added to the pickling liquid in addition to the electrolyte used. If sufficient neutral electrolyte is not produced to set the desired redox potential or required for the pickling process, this redox potential can be easily obtained by means of the aforementioned development of the invention. A favorable configuration of the invention is characterized in that inhibitors are added. In this way, corrosion attack of the pickling material can be controlled in a more effective manner. A further favorable development of the invention is characterized by acid and neutral electrochemical etching steps that combine with each other, wherein the electrolyte solution used coming from the neutral electrolyte stage can P1732 / 99MX added to the acid stage. This combination allows the advantage of the process of being used in a pickling line. An advantageous configuration of the invention is characterized in that the acid etching liquid will be neutralized using caustic soda (NaOH) or sodium carbonate (a2C? 3) so that the generated metal hydroxides can then be recovered, preferably, filtered off, using, for example, microfiltration, which allows the sodium sulphate (Na2S04) produced to be recirculated as a neutral electrolyte. In this way, not only the acid etching liquid is neutralized and regenerated but also the neutral electrolyte can be regenerated, resulting in considerable cost savings. A further favorable development of the invention is characterized in that the electrolyte solution used will be taken from one or more of the pickling lines of cold rolled strips and added to the acid pickling liquid, free of nitric acid, coming from one or more lines of stripping of hot rolled strips. As a result, the most favorable variant, in terms of investment and operating costs, of a stainless steel plant can be selected.

P1732 / 99MX DETAILED DESCRIPTION OF THE PREFERRED MODALITY The invention will now be described in more detail by using examples of possible designs.

Test 1: The result obtained at the end of a long series of tests on electrolytic pickling of a strip of cold-rolled stainless steel was an electrolyte solution used with the normal metallic content that occurs in large-scale or industrial plants. The concentration of Cr ^ + was 4.8 g / 1. (The iron (Fe3 +) and the nickel (Ni2 +) were present in the form of a slurry of suspended hydroxide and the chromium in the form of chromate (Cr ^ +)), that is, chemically dissolved). Next, a second solution comprising a mixture of sulfuric acid, hydrofluoric acid and FeS? 4 is prepared, which results in the following data after chemical analysis: free H2SO4 = 400 g / 1 free HF = 50 g / 1 Fe2 + = 40 g / 1 The mixtures of these two solutions were then prepared, so that the redox potential of the new mixture was between 400 and 500 mV.

The subsequent tests of pickling with a strip of stainless steel hot rolled and oxidized, P1732 / 99MX resulted in a slightly longer pickling time than with the conventional acid mixture, however, the results were satisfactory. Here, for example, blades of AISI 304 metal cleaned with abrasive blasting were pickled to a glossy finish in about 70 seconds. After several test pickling, the pickling liquid was subjected to chemical analysis and no traces of hexavalent chromium were found (Cr ^ + J). As expected, the divalent iron (Fe2 +) present had reduced it to trivalent chromium (Cr3 +) None of the stripping tests in the series performed showed that the sodium ions had any influence on the pickling result and these tests also confirmed that it is an advantage to add inhibitors to suppress corrosion attack with sulfuric acid on the base material. The advantage of this "nitrate-free pickling process" in which no nitrous gases are formed, is that the trivalent iron ion (Fe3 +) is produced by means of a used electrolyte solution, that is, the advantage of the Unlike the processes used to date, there is no need to buy or purchase hydrogen peroxide (H2O2) as an oxidizing agent to produce Fe3 +, and neither is it necessary no reducing agent (Na2S2? 5 > for the detoxification of chromate. The electrolyte solution used contains P1732 / 99MX to trivalent iron in the form of suspended hydroxide sludge (Fe (OH) 3) and contains hexavalent chromium (Cr6 +) t e? which reacts directly with the divalent iron (Fe +) produced by the pickling process, resulting in Fe3 + and Cr3 +. In the course of the pickling tests, it was observed that the pickling effect fell as the redox potential fell. Therefore, it is a good policy to continue adding the electrolyte solution used to the pickling bath, depending on the measured redox potential.

Test 2; An additional test was carried out with cold-rolled stainless steel strip. To the electrolyte solution used were added a salt of divalent iron (FeS? 4) and 96% sulfuric acid, raising the concentration of free sulfuric acid to 100 g / 1 and the redox potential to 440 mV. The liquid thus obtained was used as an electrolyte for the electrochemical pickling tests. In these pickling tests, it was observed that, compared to electrolytic pickling in pure sulfuric acid, there was less corrosion attack on the base material and the original brightness of the metal foil samples was retained after pickling. However, in comparison with the electrolytic treatment in the neutral interval, where the quality (brightness) P1732 / 99MX increases as the treatment progresses, a significantly higher loss of pickling was obtained. For example, with a sample of stainless steel AISI 304 and a treatment (= load density) of 600 A * sec / dm2, a pickling loss of approximately 1.2 g / m2 in the neutral electrolyte and approximately 2.0 g / m2 was measured. m2 with the recently prepared acid electrolyte solution.

Test 3; An additional test was conducted as follows: An electrolytic treatment was carried out with a loading density of 200 A * sec / dm2 in the acidified electrolyte solution, where the redox potential was set at 440 mV and the test sheet was then treated electrochemically in additional form in the neutral electrolyte. This combined pickling process has the advantage over conventional neutral electrolyte treatment that a pickling loss of about 20% more can be achieved with a slightly lower brightness at the same loading density of, for example, 600 A * sec / dm2. For example, an industrial-scale plant containing six electrochemical pickling cells can be optimized to such an extent that the first two cells can be operated with acid electrolyte with precisely set redox potential and the following four P1732 / 99MX cells can be operated with a neutral electrolyte. The resulting advantages are: higher pickling losses in the electrochemical pickling section, ie a shorter post-treatment in the subsequent section of chemical pickling with acid mix (HF + HNO3). The final product has the same brilliance. The absolutely essential advantage is, however, that all the neutral electrolyte solution containing chromate can be used, thus eliminating costs by reducing Cr6 +. An additional advantage of this optimization is that the solution of sodium sulphate (Na2S? 4) needed for pickling in the neutral pH range can be recovered by neutralizing the acid electrolyte solution (H2SO4, Na2S04, Fe2 +, Fe3 +, Cr3 +, Ni +) with sodium hydroxide (NaOH) or sodium carbonate (Na2C? 3) and metal hydroxides (Fe (OH) 2, Fe (OH) 3, Cr (0H) 3, Ni (0H) 2)) can be recovered then by filtration. It is an advantage to use microfiltration here for this purpose. The tests only provide examples of how the process as defined in the claims could be implemented.

P1732 / 99MX

Claims (10)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property: 1. Process for the chemical and / or electrochemical pickling of stainless steel in an acidic liquid. but which does not contain nitric acid, characterized in that a used electrolyte solution is added coming from electrochemical pickling lines with neutral salt, preferably from pickling lines with sodium sulphate.
2. 2. The process according to claim 1, characterized in that the redox potential in the pickling liquid will be set at a determined value by means of the amount of electrolyte solution added.
3. The process according to claim 2, characterized in that the oxidation-reduction potential (redox potential) of the pickling liquid will be measured and the amount of electrolyte solution used coming from the electrochemical pickling lines with sodium sulfate will be added in the appropriate amount.
4. The process according to one of claims 1 to 3, characterized by a divalent iron salt and a trivalent salt, preferably iron sulphate, which will be added when preparing P1732 / 99MX new acid pickling liquid.
5. The process according to one of claims 1 to 4, characterized by a different oxidizing agent, preferably hydrogen peroxide, which will be added to the pickling liquid in addition to the electrolyte used.
6. 6. The process according to one of claims 1 to 5, characterized in that inhibitors are added.
7. The process according to one of claims 1 to 6, characterized by acid and neutral electrochemical pickling steps that are combined with each other.
8. The process according to claim 7, characterized in that the electrolyte solution used comes from the neutral electrolyte stage that will be added to the acid stage.
9. The process according to one of claims 7 or 8, characterized in that the acid etching liquid that will be neutralized using caustic soda (NaOH) or sodium carbonate (Na2CO), so that the metal hydroxides generated can then be recovered, preferably by filtration, using, for example, microfiltration, which allows the sodium sulfate (NaS04) produced to be recirculated as a neutral electrolyte.
10. 10. The process according to one of the P1732 / 99MX claims 1 to 6, characterized by the electrolyte solution used that will be taken from one or several pickling lines of a cold rolled strip and added to the acid pickling liquid, free of nitric acid, coming from one or more lines of hot strip strip pickling. P1732 / 99MX