Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Treating process fluid by means other than agitation or...
Patent
1993-12-20
1997-06-10
Niebling, John
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Treating process fluid by means other than agitation or...
205 99, 205100, 205101, 205130, 205138, 205141, 205210, 205215, 205216, 205245, 205305, C25D 2106, C25D 2116, C25D 706, C25D 356
Patent
active
056372051
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the electrolytical coating on one or both sides of an object of steel, connected as a cathode, on one or both sides, preferably a steel strip, or made of hot dip galvanised steel, preferably a hot dip galvanised steel strip, with zinc or a zinc-iron alloy in a galvanic cell comprising as the electrolyte liquid an aqueous solution of zinc chloride and iron chloride having a pH of 0.1-3.0, preferably 1.0-2.0 and using insoluble anodes.
In recent years a multitude of processes for zinc coating on one or both sides, in particular of steel strip has been described. In this context, on the one hand zinc electrolytes of the chloride type in combination with soluble anodes as well as zinc electrolytes of the sulphate type in combination with soluble or even insoluble anodes are employed. Examples of such processes are described, e.g. in EP-OS 151 235 or DE-OS 3 428 277. The advantages of zinc chloride electrolytes in combination with soluble anodes are described extensively in the literature and relate essentially to the improved conductivity as well as the somewhat improved electrical efficiency as compared with sulphate electrolytes. Iron contained in the electrolyte liquid does not enter into the coating or only slightly so. In conjunction with insoluble anodes extensive safety measures are often necessary due to the liberation of chlorine. On the other hand sulphate electrolytes are very sensitive to iron ions. As from a concentration of iron ions of about 4 g/l not only the appearance but also the protective properties of the galvanisation is impaired very considerably because of the substantially increased deposition of iron. In addition, the electrical efficiency is lowered from about 98% to 94% and less.
In WO91/04359 a process for the electrolytical coating of a metal object with zinc or zinc-iron alloy is described, involving the use of insoluble electrodes and an electrolyte of the chloride type, wherein the replenishment of the content of zinc and/or iron in the electrolyte takes place by way of regenerating tanks. The electrolyte is passed through these vessels charged with iron and/or zinc, metal entering into solution, whereafter the regenerated electrolyte liquid is again passed to the coating unit. However, this process, because of the additionally required regeneration tanks and the need for zinc and iron as raw materials, is complex and expensive.
The object of the present invention is a process of the type referred in the introduction in which, in a simple and economical manner, a continuous coating of the object is possible.
In order to attain this object the invention provides, when coating an object of steel, that the object prior to the galvanic coating is rinsed with the electrolyte liquid in the absence of current, iron entering into solution and the electrolyte liquid after the rinsing of the object, being passed to the galvanic cell.
When coating a hot dip galvanised object on the other hand, the invention provides that the object prior to the galvanic coating is rinsed with the electrolyte in the absence of current, zinc entering into solution and the electrolyte liquid after the rinsing of the object being passed to the galvanic cell.
Due to the rinsing of the object in the absence of current prior to coating with the electrolyte liquid provided for such coating, the adherence of the coating deposited is improved on the one hand and furthermore the object itself serves as the source for replenishing the metal content in the electrolyte liquid, so that additional regenerating tanks or dissolution columns, charged with the metal to be replenished, can be avoided.
In order to avoid the deposition of metallic iron onto the object to be coated almost completely and thereby to create a pure zinc coating, it is provided that for the deposition of metallic zinc a concentration of the zinc chloride solution of 50 to 1000 g/l, preferably 300 to 600 g/l zinc chloride and a concentration of iron-2-ions of 0.5 to 60 g/l, preferably 10 to 40 g/l is set up, a m
REFERENCES:
patent: 4444629 (1984-04-01), Martin
patent: 4541903 (1985-09-01), Kyono
patent: 5246563 (1993-09-01), Maresch
Krupicka Ulrich
Maresch Gerald
Andritz-Patentverwaltungs-Gesellschaft m.b.H.
Niebling John
Wong Edna
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