Process for continuous titanium sheet pickling and passivation without using nitric acid

Process for sheet titanium pickling followed by passivation. Pickling is carried out in an aqueous solution containing H.sub.2 SO.sub.4, HF and Fe.sup.3+ at .ltoreq.50.degree. C., continuously fed with an air flow, and with a stabilized H.sub.2 O.sub.2 quantity adjusted to the bath redox potential to be kept at -200 to 0 mV. Passivation is carried out in an aqueous bath containing 4 to 10 g/l HF, .ltoreq.1 g/l Fe.sup.3+, and not containing H.sub.2 SO.sub.4, with a redox potential kept at 500-600 mV by continuous feeding of stabilized H.sub.2 O.sub.2.

TECHNICAL FEATURE 
As is known when, during the manufacturing process, iron and steel industry 
products undergo hot-rolling of intermediates undergo heat treatment, such 
as for instance annealing, the material is coated with a thinner of 
thicker oxidation layer. In consideration of the final products having to 
exhibit a polished and glossy finish, the oxidation layer is to be removed 
entirely. This is done through the well-known pickling process generally 
using mineral inorganic acids, such as hydrochloric acid, sulphuric acid, 
nitric acid, and hydrofluoric acid, either individually of as mixtures. 
According to the industrial processes currently applied, titanium pickling 
is normally, of almost exclusively, based on the use of a 
nitric-hydrofluoric acid mixture, i.e. a bath similar to that used for 
stainless steel. It follows that the same plants and apparatus as used for 
stainless steel treatment can be employed. Although the process is 
undoubtedly economic and leads to excellent results, it involves extremely 
serious ecological problems hard to solve, brought about by the use of 
nitric acid. Actually, while on the one hand highly polluting nitrogen 
oxide vapours having general formula NO.sub.x, aggressive toward metallic 
and non-metallic materials with which they come into contact, are vented 
to the atmosphere, on the other hand high nitrate concentrations are 
reached in wash water and spent baths, both types of pollutants requiring 
treatment prior to disposal. The removal of NO.sub.x from air and of 
nitrates from baths involves huge plant operation problems and high 
operating costs, with no certainty about the obtainment of targets 
complying with the regulations in force. This means that the resulting 
industrial plant investment costs can be hardly borne in most cases. 
A pickling method not requiring the use of nitric acid is therefore 
demanded by industry and various proposals in this sense have been made in 
these last ten years. 
DESCRIPTION OF THE INVENTION 
The process consists in a pickling step followed by passivation and can be 
applied to commercially pure titanium semimanufactured products having 
undergone hot- or cold-rolling and heat treatment, e.g. annealing, if any. 
The process was particularly developed for application to strips. 
The process is based on the use of a pickling bath containing iron ions, 
H.sub.2 SO.sub.4, HF, H.sub.2 O.sub.2 and a maximum of 1 g/l of 
conventional additives--such as wetting agents, emulsifiers, polishing 
agents, inhibitors--continuously blown into with a strong air flow, equal 
to 3 m3/h per m3 bath min. The operating temperature must not exceed 
50.degree. C., and should preferably range from 35.degree. C. to 
40.degree. C.: bath cooling is required, the process being highly 
exothermic. The starting bath consists of an aqueous solution containing: 
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HF 10 to 30 g/l 
H.sub.2 SO.sub.4 
20 to 50 g/l 
Fe.sup.3+ .gtoreq.15 g/l. 
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Once the treatment has started, the bath redox potential tends to fall 
below 0 mV while the Fe.sup.3+ content in the bath decreases. Optimal 
operating conditions are restored by keeping the redox potential between 
-200 to 0 mV, which is secured by a combined action due to the air 
continuously blown into the bath--which produces bath agitation--as well 
as to hydrogen dioxide added either continuously or periodically to the 
bath. In the course of treatment, proper free acidity values are 
maintained by periodically feeding HF and H.sub.2 SO.sub.4 : in 
particular, pH must be kept at .ltoreq.1.5. 
Passivation is carried out at room temperature in a bath consisting of an 
aqueous solution containing a low amount of HF (10 g/l max.) and not 
containing H.sub.2 SO.sub.4 and iron ions. The redox potential is kept at 
high values (between 500 and 600 mV) by continously feeding low amounts of 
H.sub.2 O.sub.2. 
Bath agitation is secured by a continous air flow. 
Continuous addition or stabilized hydrogen peroxide during pickling and 
passivation phases. 
Needless to say that to secure process economics it is necessary to use as 
little hydrogen peroxide as possible. This is why it is very important to 
use hydrogen peroxide containing a known stabilizer capable of preventing, 
or at least of reducing significantly, the peroxide decomposition process 
under the following conditions: temperature up to 50.degree. C., strongly 
acid bath pH, presence of iron ions in the pickling bath, presence of free 
or complex Ti ions. Stabilizers for H.sub.2 O.sub.2 effective in acid 
medium are for instance: 8-hydroxyquinoline, sodium stannate, phosphoric 
acids, salycylic acid, pyridincarboxylic acid. As a particularly suitable 
stabilizer came out phenacetin (i.e. acetyl-p-phenetidine) used in amount 
corresponding to 5+20 ppm to the pickling bath. 
A suitable stabilizer is the one sold by Interox (Solvay) under the trade 
name Interox S 333 or Interox S 333C, removable from spent solutions by 
the traditional chemo-physical methods and not containing any pollutant. 
The use of duly stabilized H.sub.2 O.sub.2, combined with the use of air 
blown into the bath, has made it possible to develop a process based on 
the use of H.sub.2 O.sub.2, which has resulted to be economic, an 
advantage that no known process has ever been capable of offering. The 
pickling bath is prepared with a starting H.sub.2 O.sub.2 quantity (as 130 
vol. commercial product) ranging from 1 to 20 g/l, preferably from 2 to 5 
g/l. 
As already mentioned, the addition of H.sub.2 O.sub.2 during the process 
cycle is substantially adjusted to the pre-set oxidation potential of the 
bath, both of pickling and of passivation.

EXAMPLE 
Hot-rolled strip titanium was treated in a pickling bath having the 
following starting composition: 
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H.sub.2 SO.sub.4 20 to 50 g/l 
Fe.sup.3+ 40 to 60 g/l 
F.sup.- (from free HF) 10 to 20 g/l 
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During operation, the bath temperature was kept at 35.degree. to 50.degree. 
C. by cooling the solution. 
The redox potential was kept at -200 to 0 mV by feeding H.sub.2 O.sub.2 
stabilized with Interox S 333. 
Bath agitation was secured by an air flow equal to 3 m3/h per m3 bath. 
The treated material, after a 90-sec. residence in the bath, was fed to the 
passivation bath consisting of an aqueous solution containing 4 to 10 g/l 
free HF, 1 g/l Fe.sup.3+ max., and practically not containing free H.sub.2 
SO.sub.4. 
The bath temperature was room temperature, the redox potential was kept at 
500 to 600 mV by continous H.sub.2 O.sub.2 feeding. Bath agitation was 
secured by a strong air flow. 
The material surface was perfectly clean, polished, and free from corrosion 
phenomenon.