Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Patent
1995-11-09
1998-07-28
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
205712, 205714, 204206, 204207, 204208, C25F 106, C25F 700
Patent
active
057865566
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 continuation of PCT/SE94/00406 filed May 4, 1994.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for removal of oxide layers, chromium depleted zones and the like in pickling of a metal, in the first place stainless steel, more particularly high alloy stainless steel in the form of plates or strips, continuously passing in an electrolytic bath. The invention also relates to a device for performing said method.
2. Decsription of the Prior Art
The development of new high alloy stainless steel, both austenitic and ferritic-austenitic steels, requires new or improved pickling methods. The methods used up to now are principally the same used for some decades for lower alloyed stainless steels.
A known pickling technique is pickling in different mineral acids or mixtures of acids. Further electrolytic pickling in neutral salt solutions is used, see the Swedish patent 205 105.
Electrolytic pickling in mineral acids or mixtures of acids is used to get a fast pickling in continuous annealing/pickling lines, where the process control is related to the strip speed, see report by S Owada et al, A new electrolytic descaling in HNO.sub.3 --HCl acid for development of functional stainless steels; in Proc. International Conference on Stainless Steels, 1991, Chiba, ISIJ, p 937. Electrolytic pickling with alternating current in mineral acids or mixtures of acids is also known according to the Swedish patent 132 298.
For high alloy stainless steel the mentioned methods have problems both in obtaining a clean surface without any annealing oxide and in removing the chromium depleted zone, about 2-20.mu.m deep, below the annealing oxide. For the new high alloy stainless steels and their demanding applications it is of outmost importance that the surface has the properties of the alloy, that the composition is perfect in the surface, thus that the chromium depleted zone has been removed. The lower chromium content in the surface zone means a considerable deterioration of the surface properties, e. g. the pitting resistance, compared to the properties of the bulk material below the surface. The critical pitting temperature in potentiodynamic test in 1M NaCl is for several high alloy steels over 90.degree. C. in the bulk material, but in presence of a chromium depleted zone in the surface the critical pitting temperature can be only 70.degree. C. in the surface. It is known that if pitting once has started in the deteriorated surface zone, the pitting attacks will continue down to material with the right composition. Grinding has been tested to remove the chromium depleted zone, but causes microcrevices in the surface and impurities from the grinding belt and thus deteriorated corrosion properties in the new surface.
The industry has even been forced to accept a certain chromium depletion in the surface of high alloy steels due to the pickling problems, see report by J F Grubb, in Proc. International Conf. Stainless Steels, 1991, Chiba, ISIJ, p 944.
The industry has also tried to and still tries to solve the process technical problems by combining several different pickling methods in a production line, e. g. electrolytic pickling in neutral salt solution, followed by mixed acid pickling. Further, mechanical stages as shot blasting, brushing and possibly grinding are often included.
For mixed acid pickling, in which a high speed (e. g. 30 m/min) is used in continuous processes for passing through the furnace, long pickling baths, high acid concentrations and high temperature are required to manage to achieve an acceptable pickling effect. This means a great strain on handling and environment. Big volumes of acids, HF and HNO.sub.3, and big volumes of air with reaction gases, nitrogen oxides , must be handled in refining and retardation stages. For the high alloy stainless steels a final pickling according to this method cannot manage the previously mentioned problem with chromium depletion in the surface.
Electrolytic pickling in neutral salt solutions g
REFERENCES:
patent: 5425862 (1995-06-01), Hartmann et al.
Gronlund Conny
Gronlund Kaj
Lezdey John
Phasge Arun S,.
Swedish Pickling AB
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