Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Reexamination Certificate
2000-07-19
2002-12-31
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
C205S771000, C134S041000
Reexamination Certificate
active
06500328
ABSTRACT:
The present invention relates to a method for pickling steels and, more specifically , carbon steels, magnetic steels (containing Si) and stainless steels with a chrome content lesser or equal to 15% (i.e. AISI 409), wherein the Fe
3+
ion is used as an additive in the bath to increase the reaction kinetics.
In order to realise an efficient pickling process of the carbon steels, class whereto also magnetic steels containing Si belong, hydrochloric (HCl) or sulphuric (H
2
SO
4
) acid is normally used, or mixtures thereof, at a temperature that generally varies between 60 and 75° C. For stainless steels with a low chrome level, e.g. AISI 409 of the ferric class, analogous baths can be adopted as well.
The main reaction of pickling, to allow the removal of the scale of thermal oxide, is the dissolution (through oxidation) of the iron base according to the anodic half-reaction:
Fe→Fe
2+
+2
e.
(1)
The corresponding cathodic half-reaction in acid environment is the ion H+reduction that develops hydrogen:
2H
+
+2
e
→H
2
(2)
Therefore, the resulting total reaction is:
Fe+2H
+
→Fe
2+
+H
2
(3)
As is known from the study of electro-chemical reactions, the kinetics thereof is strongly influenced by the values of the electrode potential.
An additive of oxidising species is added to the pickling bath, enabling to obtain a more noble electrode potential of the steel to be pickled, thereby allowing higher reaction kinetics.
The species preferred as oxidant is the Fe
3+
ion.
The possibility of obtaining a more noble potential of the steel to be pickled derives from the fact that the added ferric ions Fe
3+
behave as oxidant (being reduced to Fe
2+
ions) with respect of the steel (Fe) that is to be pickled according to reaction (1) (that derives from the reversible electrochemical potential scale: E
rev
=−447 mV SHE for the Fe
2+
/Fe couple; E
rev
=+771 mV SHE for the Fe
3+
/ Fe
2+
couple).
Therefore, the Fe
3+
ion is capable of being reduced to Fe
2+
ion during the pickling process, according to the cathodic half-reaction below:
Fe
3+
→Fe
2+
+e
(4)
Also the cathodic half-reaction (4) occurs at the same time of the anodic half-reaction (1). The resulting reaction (1)+(4) therefore is:
Fe+2Fe
3+
→3Fe
2+
(5)
When the Fe
3+
ion is present as an additive the reactions (3) and (5) occur at the same time, with an increase of the global iron (Fe) dissolution kinetics.
The working electrode potential Ew results, in this case by effect of the addition of the oxidising species (Fe
3+
) in the bath, more noble than the potential in absence of additions.
The amount of Fe
3+
ion that has to be added must balance the amount of Fe
3+
ion consumed in the cathodic reaction(4).
A good stirring of the bath further improves the pickling kinetics, allowing the depolarisation of the total cathodic reaction resulting from the sum of the reactions (2) and (4) that would tend, without stirring, towards diffusion control conditions.
In EP 0792949A1 a pickling process for steel products is disclosed in which the concentration of Fe
+3
ions is comprised in the range 1-300 g/l by oxidation of the Fe
+2
ions obtained during the pickling.
According to the present invention, the Fe
3+
ion can after all be added directly, e.g. as ferric chloride or ferric sulphate. However, it can be obtained in the bath by the addition of other oxidants, particularly H
2
O
2
(hydrogen peroxide), ozone or permanganates. In fact, once added to the pickling bath those oxidants come to contact with a solution already rich in Fe
2+
(due to the primary pickling reaction (1) the bath is always rich in Fe
2+
ions) and induce the oxidation of Fe
2+
to Fe
3+
. Even oxygen alone, added in conditions of stirring by air bubbling, or admixed in an external reactor with the pickling solution, works as an oxidising agent capable of producing Fe
3+
.
A preferred embodiment of the method described here according to the invention is that of obtaining Fe
3+
electro-chemically, sending the pickling solution as anolyte in an electrolytic cell, and carrying out an anodic oxidation of the Fe
2+
ion that, as aforestated, is always present in the bath, according to the reaction:
Fe
2+
→Fe
3+
+e.
(6)
Therefore, it is an object of the present invention to provide a method for pickling steel products, wherein in the pickling bath Fe
3+
is present in a concentration comprised in the range 6-60 g/l, directly added or produced in the pickling bath itself by the addition of an oxidising agent selected from the group comprising: hydrogen peroxide, ozone, permanganates, persulphates, and oxygen.
Advantageously, the obtained increase of the pickling kinetics is a function of the added Fe
3+
concentration that is maintained in the pickling bath itself, thereby improving also the productivity of the industrial lines.
A further advantage, according to the invention, lies in the fact that the maintenance and/or the control of the Fe
3+
ions concentration in the pickling bath allows a strict control of the most critical parameter of the process (that is the potential redox value of the system) with further advantages on the final quality of the product as well.
According to the present invention, continuous pickling lines of carbon and/or magnetic steels can be employed advantageously also for the pickling of stainless steels with a <15% Cr content. This result was made possible by the fact that, according to the present invention, the Fe
3+
/Fe
2+
ratio is employed as a control parameter of the reaction kinetics, jointly with the acid concentration control. In fact, when it is desired to use the same carbon and magnetic steels production line for the stainless steels, e.g. of the AISI 409 type, the presence and the maintenance of appropriate Fe
3+
concentration values allows in any case to widely control the pickling kinetics, for the carbon steels as well as for the stainless ones, thereby making possible the combined utilisation of these production lines.
Advantageously, the method according to the invention proves to be compatible with the possible presence of corrosion inhibitors, normally employed to avoid drawbacks of carbon steels over-pickling.
Furthermore, always according to the present invention, it is possible to pickle these abovementioned steel types without resorting to mechanical processes of descaling such for instance as peening.
A further object of the present invention is to provide different embodiments, as hereinafter specified, of the method for the direct production of the oxidant Fe
3+
within the pickling bath:
(i) direct addition of Fe
3+
as reactant (for instance: ferric chloride or sulphate);
(ii) addition of oxidants for the production Fe
3+
by oxidation of the Fe
2+
ion present in the bath to Fe
3+
;
(iii) oxidation of the Fe
2+
ion in an electrolytic cell present in the bath itself to Fe
3+
.
According to the third embodiment, the same pickling solution itself (composed of aqueous solution of hydrochloric acid and/or sulphuric and, optionally of phosphoric acid) constitutes the cell anolyte, the oxidising agent to be added as additive being the ferric ion produced at the anode by oxidation of the ferrous ion present within the bath.
The catholyte is preferably composed of an aqueous solution of hydrochloric and/or sulphuric acid. Also the catholyte is preferably sent out continuously into the pickling solution, to reintegrate the HCl or the H
2
SO
4
that is consumed during the pickling reaction.
According to the method of the invention, an electro-chemical cell of the membrane type is preferably employed as shown in FIG.
1
.
The use of a cell to generate the oxidising species Fe
3+
, according to the third embodiment, i
Fortunati Sandro
Mancia Franco
Musso Augusto
Acciai Speciali Terni S.p.A.
Browdy and Neimark PLLC
Phasge Arun S,.
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