Process for Electrolytically Pickling Chromium-Containing Stainless Steel

The present invention relates to a process for the eloctrolytic pickling of chromium-containing stainless steel, in which initially pickling takes place in an aqueous Na.sub.2 SO.sub.4 - solution and thereafter in acid, preferably mixed acid, optionally without electric current. Such a process as described in AT-PS No. 252.685, wherein hydro-fluoric acid plus nitric acid serve as the mixed acid. In the first step the millscale is removed; whilst in the second step the chromium-depleted layer underneath the millscale, formed during annealing, is dissolved off. That process has gained worldwide recognition, e.g. for the pickling of stainless steel strip, but suffers from the disadvantage, that the Cr in the millscale is oxidised by the current to CrO.sub.4.sup.2-, whilst Fe, when dissolved, is precipitated immediately as Fe(OH).sub.3. The CrO.sub.4.sup.2- which is formed, remains in solution and is only removed during de-sludging, respectively together with the stainless steel strip and is only then detoxified by the reducing agent. For that purpose, primarily the reduction with an aqueous FeSo.sub.4 -solution in the pH-range of 0-2 respectively 7-8 have found acceptance. In both cases it is necessary subsequently to neutralise again, in order to precipitate all metal ions in the solution in the form of hydroxide. Moreover, in the event of a sudden availability of very large quantities of CrO.sub.4.sup.2- -containing solutions, there always exists a risk of a breakthrough of CrO.sub.4.sup.2-, which may then enter into the effluents. A further drawback of the process is that only a fraction of the CrO.sub.4.sup.2- formed is removed from the aqueous solution by the Na.sub.2 SO.sub.4, whilst the balance increases the concentration in the solution and results in increased attack on plastics pipelines and pumps.

In the publication "Chemical Abstracts", Vol. 87, No. 14, Oct. 3rd, 1977, on page 396, Abstract No. 108322s, the formation of CrO.sub.4 "respectively Cr.sub.2 O.sub.7 " is indeed described, but no solution to this problem is offered.

It is an object of the invention to provide a process of the type defined in the introduction which avoids the aforesaid drawbacks.

This object is attained according to the invention in that acid and a reducing agent are added to the aqueous Na.sub.2 So.sub.4 - solution in accordance with the pH-value and the redox potential, such that a pickling solution is obtained which is free of CrO.sub.4.sup.2-.

A further development of the inventive concept provides that the pH-value of the solution is adjusted to less than 3, preferably to 1.5 to 2.5, advantageously 2, by the addition of H.sub.2 SO.sub.4. In a further development of the inventive concept, provision is made that the redox potential of the solution as measured in relation to a calomel electrode is reduced by the addition of acid and reducing agent by 50 to 100 mV, a reducing agent being used in the form of a substance selected from the group Na.sub.x H.sub.y S.sub.z O.sub.v, wherein x=0 to 2, y=2 to 2, z=1 to 6 and v=2 to 6, and that in the reaction Na.sub.2 SO.sub.4 is formed as well.

The following reaction mechanisms may illustrate the manner in which the reduction proceeds:

In addition the dissolved Fe.sub.2 (SO.sub.4) .sub.3 will also be reduced by the reducing agent, as exemplified in the following:

and the Fe.sub.2 SO.sub.4 thus formed will react further with H.sub.2 CrO.sub.4 according to the following reaction equation:

which once again adds up to the overall reaction according to reaction 1.

Due to the oxidation of these substances Na.sub.2 SO.sub.4 is formed as well which in turn serves as a conductor salt in the Na.sub.2 SO.sub.4 solution, and by an appropriate selection of the pH value of this solution it is possible to cause the dissolved Fe.sup.3+ to precipitate after the solubility has been exceeded in the form of Fe(OH).sub.3, the result of which is that the solution, after a certain Fe concentration has been reached, need not be discarded completely, but need only be freed of Fe(OH).sub.3 sludge. The concentration of Na.sub.2 SO.sub.4 in the solution amounts to 10 to 250 g/l, preferably 170 to 200 g/l.

The pH value for these reactions is selected according to the invention lower than 3, preferably 1.5 to 2.5, advantageously 2. At that pH value the reaction rate in the solution is adequately high and the redox potential measured against a calomel electrode in the CrO.sub.4.sup.2- -containing solution exceeds that of the solution free of CrO.sub.4.sup.2- by 50 to 100 mV. It stands to reason that in addition to the redox potential other analytical methods may be utilised as well for the determination of the CrO.sub.4.sup.2- -content of the solution, although that particular method was found to be the simplest and most cost effective.

The following illustrative examples should be read against the background of and together with the preceding general description to enable the skilled person to practise the invention with the claimed scope thereof.

A stainless steel strip 1000.times.6.0 mm, having a strip velocity of 8.4 m/min was pickled in an electrolytical pickling plant with an aqueous solution of Na.sub.2 SO.sub.4 whereafter the chromium depleted layer positioned below the millscale layer was removed in a mixed acid trough using nitric acid-hydrofluoric acid. In a freshly made up aqueous Na.sub.2 SO.sub.4 solution the rise of the Cr.sup.6+ concentration amounted to 0.2 g Cr.sup.6+ /1 over a period of eight hours.

After adjustment of the pH value to 2.0 by addition of H.sub.2 SO.sub.4, 96%, the total Cr.sup.6+ was reduced by the further addition of 8.8 ml 10% Na.sub.2 SO.sub.3 solution and 3.7 ml 96% H.sub.2 SO.sub.4 per liter of the aqueous solution whilst the redox potential of the solution changed from a previous 620 mV to 530 mV, measured against a calomel electrode.

During the subsequent eight hours this redox potential was kept constant by further constant addition of Na.sub.2 SO.sub.3 solution as well as sulphuric acid. Towards the end of the eight hour period no further Cr.sup.6+ could be detected analytically in the aqueous solution.

Subsequent to Example 1 the addition of reducing agent was stopped until the redox potential had again risen to 620 mV. After approximately a further 4 hours the analytically determined Cr.sup.6+ concentration amounted to 0.11 g Cr.sup.6+ /1. By the addition of solid Na.sub.2 S.sub.2 O.sub.5 in an amount of 0.9 g Na.sub.2 S.sub.2 O.sub.5 (62%) per liter, it was possible to reset the redox potential once again to 520 mV, as measured against a calomel electrode and analytically no Cr.sup.6+ could be detected any more. During the addition of the Na.sub.2 S.sub.2 O.sub.5 the pH value of the solution dropped from 2.0 to 1.9.

After the addition according to Example 2 the addition of a reducing agent to the aqueous solution was interrupted once again until again a Cr.sup.6+ concentration of 0.16 g Cr.sup.6+ /1 had been attained. By the addition of 3.9 ml 10% Na.sub.2 S.sub.2 O.sub.4 solution as well as 1.3 ml 96% H.sub.2 SO.sub.4 per liter of solution the redox potential was reset to 515 mV and no Cr.sup.6+ could be detected any more analytically.

In all examples the stainless steel strip was free of millscale and had a silvery lustre after the treatment with acid or mixed acid respectively.

The claims which follow are part of the present disclosure.