Verfahren zum beizen von edelstahl

The invention concerns a procedure for chemical and/or electro-chemical pickling of high-grade steel in a salpetersäurefreJen and sour liquid. When pickling high-grade steel hot-rolled strip glow process the volume must be released from the scale after Warmwalzund. This pickling procedure usually becomes in mixture acid (hydrofluoric acid (= HF) and nitric acid (= HNO3)) accomplished. Here however nitrose gases (NOx) develop, -; 0 that in the last years increasingly “nitric acid-free pickling procedures” were developed. The principle of all of these pickling procedures consists of it that the nitric acid by another mineral acid (sulfuric acid, hydrochloric acid) in which the redox potential by the portion of the iron ions of different priority (Fe2÷/Fe +) is exactly adjusted, is substituted. When pickling iron goes to salts as bivalent ion into solution and the trivalent Eisenion either through partial oxidation of Fe2÷ or through addition from Fe3÷ - is manufactured. There is procedures well-known, with which the production of the trivalent iron ions by oxidation of the Fe2÷ with hydrogen peroxide (H2 02) are produced. With other procedures Fe3+-Salze (sulfate, chloride, Nitratoder of fluoride salts) are added in the pickling bath. After the pickled hot-rolled strip by rolling to thinner Blechstärken was reduced must it be again glowed. Here develops a thin oxide layer, which must be removed in a “careful kind”, without destroying thereby the surface quality (gloss). World-wide for the Ede} stealing cold rolled strip became generally accepted the electro-chemical pickling procedure in sodium sulfate solutions (neutral electrolyte). With this neutral (pH = for 7 to 4) pickling procedures very toxic chromate (Cr6+ develops). The used up pickling baths are detoxicated from there in a separate treatment, i.e. CR is reduced to Cr3, before ste be neutralized and the metal hydroxides filtered off can. The chromate decontamination is much costs intensively. As reducing agents “neistens Na2 S2 OS or similar sulfur connections are used. The reduction takes place according to the following formula: 4 H2 CrO4 + 3 well S2 OS + 3 H2 SO4 => 2 Cr2 (SO4) 3 + 3 well SO4 + 7H2 0 the reduction can be accomplished also with bivalent Eisensulfat FeS04: 2 H2 CrO4 + 6 FeSO4 + 6 H2 SO4 => 2 Cr2 (SO4) 3 + 3 Fe2 (SO4) 3 + 8 H2 0 a goal of the invention is it. the disadvantages of both procedures, in particular the cost-intensive, chromate decontamination ", to avoid. The invention is characterized by from there that used up electrolytic solution from electro-chemical neutral salt pickling lines prefers, is metered, sodium sulfate pickling lines. Thus on the one hand chromate without additional cost-intensive reducing agents can become reduced (, detoxicates ") and reach on the other hand an oxidation of the iron from Fe2÷ to Fe3÷. A favorable further training of the invention is characterized by it, “lets the redox potential in the pickling liquid by the quantity of admitted electrolytic solution to a given value is adjusted, whereby the reduction oxidation potential (redox potential) of the pickling liquid be measured and the quantity of the used up electrolytic solution from electro-chemical sodium sulfate pickling lines can be metered accordingly. Thus the pickling effect for the high-grade steel volume can be stopped in the sour pickling liquid in favourable way. A favourable arrangement of the invention is characterized by that with a new beginning of the sour Beizffüssigkeit a bivalent and a trivalent Eisensafz, preferably iron sulfates, are added. Thus a start for the procedure can be achieved in favorable way. A favourable further training of the invention is characterized by that additionally to used up electrolytes another Oxidaticns means, hydrogen peroxide, is preferably supplied the pickling liquid. The quantity of resulting used up neutral electrolyte quantity should not be sufficient, over the desired and/or. necessary redox potential for the pickling procedure to adjust, can be obtained so this redox potential in simple way. A favorable arrangement of the invention is characterized by that inhibitors are added. In this way the Korrosionsangdff can be better checked for the pickling property. A favorable further training of the invention is characterized by that sour and neutral electro-chemical pickling stages are combined, whereby the used electrolytic solution from the neutral electrolyte stage can be metered into the sour stage. By this combination the advantage of the procedure in a pickling line lets itself obtain. 2 RKs 406,486 B a favourable arrangement of the invention it is characterized by the fact that the sour is neutralized pickling liquid by means of caustic soda solution (NaOH) or Natriumcarbonat (Na2 CO3) that afterwards the developed metal hydroxides are separated, filtered off preferably, for example by means of a micro filtration, and that the developed sodium sulfate (Na2 SO “) is reconducted again as neutral electrolyte. Thus also the neutral electrolyte can be regenerated apart from the neutralization and regeneration of the sour pickling liquid, which leads to substantial cost savings. A favorable further training of the invention is characterized by the fact that the used up electrolytic solution or several cold rolled strip pickling line (n) inferred and the nitric acid-free and sour pickling liquid of one or several hot-rolled strip pickling line (n) is metered. Thus for a high-grade steel work the gf3nstigste variant can be selected regarding capital outlays and operating cost. The invention is more near described now on the basis of remark examples. Attempt 1: At the end of a long test series over electrolytic pickling of Edelstahlkaltbändem one had a used up ElektrolyUösung with metal contents, as her on large-scale installations usually • • 6÷ • 3+ •,2+ - • is. The CR concentration 4,8 g/1. (the iron (Fe) and nickel (Ni) amounted to m form of suspended hydroxide sludge preserves and the chrome is present in the form of chromate (CR), i.e. chemically solved.) Now a second solution was manufactured hydrofluoric acid and FeSO4 consisting of a mixture of sulfuric acid, so that the following chemical analysis resulted in the following values: free H2 SO4 =400g/! free HF = 50 gll Fe2÷ = 40 g/I now was made mixtures of these two solutions, so that the redox potential of the again manufactured mixture is with 400 -500 mVs. The pickling tests with scaled high-grade steel hot-rolled strip, following on it, resulted in a somewhat longer pickling time than in conventional mixture acid, but nevertheless satisfying results. AISI 304-Probebleche for example radiated was scale-free pickled in approx. 70 seconds. After several sample pickling the pickling liquid was chemically analyzed and it could be proven no hexavalent chrome (Cr6÷). As expected the available bivalent iron (Fez÷) reduced it to trivalent chrome (Cr3÷). No influence could be observed by sodium ions on the pickling result and it was also confirmed in the accomplished pickling series that an additive of inhibitors of advantage is, in order to suppress the corrosion attack of the sulfuric acid on the base material. The advantage of this “nitrate-free pickling procedure” with that nitrosen gases it do not develop is that the trivalent Eisenion (Fe3÷) is manufactured by a used up electrolytic solution - i.e. a double cost advantage results. As with the past procedures hydrogen peroxide (H2 02) as oxidizing agents for the Fe3÷ production and no reducing agent (Na2S2Os) for the chromate decontamination does not have to be bought to go with. The used up electrolytic solution contains the trivalent iron in form of the suspended hydroxide sludge (Fe (OH) 3) and it hexavalent chrome (Cr6÷) contains, which directly with the Cr3÷ resulting from the pickling procedure for bivalent iron (Fe2) reacted and Fe3÷ develop and. In the process of the pickling tests leaving the pickling effect with the simultaneous dropping of the redox potential observed • it is meaningful to after-meter the used up electrolytic solution as a function of the measured redox potential sequentially into the pickling bath. Attempt 2: A further attempt was accomplished with high-grade steel cold rolled strip. One bivalent iron salt (FeSO4) and 96% were admitted - ige sulfuric acid to the used up electrolytic solution, so that the concentration of the free sulfuric acid 100g/I and the redox potential amounted to 440 mm. $o the won liquid was used as electrolytic solution for electro-chemical pickling tests. With these pickling tests it could be observed that, compared to electrolytic pickling in pure sulfuric acid, which is smaller corrosion attack on the base material and which remains original gloss of the sheet metal samples after pickling. In relation to an electrolytic treatment within the neutral range, with which a quality rise (=Glanz) with sequential treatment is given, however a substantially higher pickling loss could be obtained. RK 406,486 B I0 for example with a AISI 304 high-grade steel sample and a treatment (=Ladungsdi (- hte) of 600 A'sec/dm2 approx. 1.2g/m2 could] ust in neutral and approx. 2,0 glm2 with the again manufactured sour electrolytic solution to be determined. Attempt 3: A further attempt was accomplished as follows: Afterwards the electrolytic treatment with a charge density of 200 A'sec/dm2 was accomplished in the acidified electrolytic solution, in which the redox potential was adjusted to 440 mVs, and the sample sheet metal in neutral electrolytes was electro-chemically further-treated. This combined pickling procedure has the advantage in relation to a conventional neutral electrolyte treatment that with the same charge density of e.g. 600 A'sec/dm2 approx. 20% more pickling loss with slightly smaller gloss to be reached could. For example a large-scale installation, which consists of 6 electro-chemical pickling cells, can then be optimized that the first two cells with sourer electrolyte with exactly stopped redox potential and the following four with neutral electrolyte can be operated. The advantages resulting from it are: higher pickling losses in the electro-chemical pickle-hurry, i.e. shorter subsequent treatment in on it the following chemical mixture acid (HF + HNO3) pickle-hurry. The final product has the same gloss degree. The most substantial advantage however is that the entire chromathaltige neutral electrolytic solution cannot to be used and thus the costs of the Cre+ - Redukt.ion any longer result. A further advantage of this optimization is that for pickling in the neutral pH. Range necessary sodium sulfate solution (Na2 SO4) by neutralization of of the sour electrolytic solution 2+ 3+ 3+ 2+ (H2SO4, Na2 SO4, Fe, Fe, CR, Ni with Natfiumhydroxid (NaOH) or Natfiumcarbonat (Na2CO3) and following separation of the metal hydroxides (Fe (OH) 2, Fe (OH) 3, CR (OH) 3, Ni (OH) 2) by filtration, to be recovered can. The filtration here favourable-proves accomplished as Mikroflltration the attempts to place only examples of the procedure guidance, how they are defined in the requirements.