GB2220005A - Process for removing oxide layer and scale from metals and metal alloys - Google Patents

Abstract

A process for removing oxide layer and scale from the surface of metals and metal alloys comprises treating the metal at 20-80 DEG C, for 5-40 minutes in an aqueous solution of iron (III) chloride sulphate iron (III) chloride, iron (III) sulphate or a mixture of iron (III) chloride or iron (III) sulphate containing 150-200 g/l of iron (III)-ion and said solution being free of acid or containing up to 30% by weight. The process can be used to treat individual metal articles or continuous metal material in a continuous length.

Description

Process for Removing Oxide Layer and Scale from Metals and Metal Alloys The invention relates to a process for remox- ing oxide layer and scale from metals and metal alloys, in particular alloyed steels, high-allofed steels and carbon steels by using the aqueous solution of iron chloride sulfate and,'or iron chloride and/or iron sulfate, containing iron(III)ion as active ingredient.

Several methods are known for chemical processing and refining of the surface cf metals and metal alloys.

According to known methods mostly sulfuric acid and hydro- choloric acid are used for pickling carbon steels and lcw- -allcfed steels within the temperature range of 20 to 80 OC.

It is known that in 10 %-ual sulfuric acid sixtimes more iron dissolves than iron oxide forming but a smaller part of scale. (Electrochem. Soc. V. 9 (1962), p 103.) According to NM. Zetvin, F. Sz. Rohovszkaja and V.I. Usakov ("Removal of oxide layer from a metal surface' p. 195, Moscow, 1964) by the use of hydrochloric acid a far higher pickling velocity can be obtained and about 40 % of the scales can be removed b direct chemical dissolution, in a consequence the metal loss is less.

According to the solution disclcsed in HU-PS 163 685 - simultaneouslf maintaining advantageous characteristics of the pickling methods, using hxdrochloric acid and sulfuric acid, respectively - the aforementioned two methods of pickling are combined so, that instead of the last one or two pickling baths using sulfuric acid a bath of hfdrochloric acid is used, whereby original pickling output can be increased by 10 to 15 %.

However, in course of using hbdrochlorid acid, sulfuric acid and the combination thereof one is confronted with the problem, in so far as without additives metal loss is considerable, while atomic hydrogen, released as a result of chemical reaction, diffuses into the texture of the basic metal and deteriorates mechanical properties of the metal. A further disadvantage lies in that the scale-layer covering the surface of high-alloyed steels contains in acids hardly soluble or insoluble metal oxide.

In addition to sulfuric acid and hydrochloric acid nitric acid and hydrogen fluoride, or the mixture thereof are used for the removing of oxide layer and scales from the aforementioned steels in acidic solutions.

When working with hydrochloric acid and hydrogen fluoride harmful vapours, in case of nitric acidic systems NOx gases and in course of pickling with sulfuric acid strong fogs of sulfuric acid arise, which - when getting into the atmosphere of the plant - influences negatively the health of the workers.

The mcst efficient method for the removing of the oxide layer and for descaling of high-allofed steels is the pickling in salt melt. One of these methods is the salt melt-pickling, working according to oxidation mechanism, which contains sodium nitrate as oxidizing reagent in addition to sodium hydroxide. The pickling temperature is generally in the temperature range between 500 and 530 OC. After the salt bath hfdrochloric acid or sulfur acid pickling solutions are used or the residual iron and nickel oxides are reduced in hydrogen flow.

From the presently applied processes the pickling method in salt bath based on the principle of reductIon is considered as the mOKL modern one, which salt bath con tains the mixture of sodium hydroxide and sodium hydride.

Sodium hxdride is produced in situ in the tank containing the descaling melt bf the continuous addition of metal -sodium to the anhydrous sodium hydroxide melt.

The HU-PS No. 158 872 discloses a process for the quick and reliable production of a melt with a high content of active reagent, in course of which in a separate autoclave provided with an agitator, metal-sodium is added to sodium hydroxide in the presence of hydrogen or gases with hydrogen content, at a temperature between 350 and 430 OC, at a pressure being less, than 10 bar.

The disadvantage of the processes with salt melt lies in that due to the materials applied and the temperature used the material- and energy costs are high and costs of investment of the equipments are consiHeraLle, too. In course of operation a further disadvantaSe as- pears - in particular, when the pickling is carried out with hydroxide compounds - in so far as under the effect of oxygen and moisture contained in air the active ingredient decomposes explosion-like and it can be used only with an extraordinary technological discipline and a pErsonnel of high professional skill. Further problems are involved in elimination cf the environment-contaminating effect of waste resulting from pickling.

According to page 54 of the book "Cleaning of metal surfaces in industry (Technical Publishers, Budapest, 1972) copper and copper alloys are pickled at 80 0C, in a 10 % sulfuric acid. If said requirements are met, copper oxide dissolves well in the pickling bath. zweLer, if the product is strongly oxidized, an oxidizing agent potassium bichromate - is to be added to the sulfuric acid.

As it is also mentioned in the above reference, a mixture of sulfuric acid and nitric acid in a ratio of 2:1 is applied as well for pickling copper objects in a temperature range between 50 and 80 OC.

The disadvantageous feature of the aforementioned process lies in that in course of the pickling procedure strong fogs of sulfuric acid and vapours of hydrochloric acid arise, which - when getting into the atmosphere of the plant - endanger the health of the workers and elim na- tion thereof requires expense exhausting and/or a1hi.lat ing equipments. A further d sadvantace lies in that without using an additive the metal loss is considerable.

According to page 34 of the aforementioned reference aluminium products are pickled in acidic or alkaline solutions, as amphoteric oxide of aluminium (A1203) can be dissolved well both in alkali and acid. In practice first of all alkaline pickling is used, as in this case the dissolution of the oxide la)er cowering the metal surface can be carried out more efficientl. The pickling is carried out in a 10 - 20 % sodium hydroxide solution at a temperature between 50 and 80 OC for 2-3 minutes. According to known methods a better surfacial quality can be obtained by using the following pickling bath: 100 g sodium hydroxide, 20 g sodium chloride, 1 liter water.

To obtain a bright surface, the following pickling bath is applied: 50 to 100 g sodium hydroxide, 20 to 50 g sodium fluoride and 1 litre water. For pickling aluminium products with a high copper content nitric acid is added to the pickling bath. In case if silicium, nickel or other impurities are contained, hydrogen fluoride is added to the solution.

The disadvantage of the aforementioned pickling method lies in that the spent pickling bath contains components being harmful for the environment and neutralization, or destroy thereof involves a consIderable extra expenses.

The aim of the inventIon is to provide a pickling agent, which is well suitable for the remcv- ing of oxide layer and scale from metals and metal alloys, in particular alloyed steels, high-alloyed steels and carbon steels, enabling the reduction of material- and energy costs, as well as the costs of investment, and all these with advantageous operative parameters and under favourable conditions.

The invention is based on the recognition, that in course of the oxidation-eduction (redcx) system formed b the aqueous solution of iron chloride sulfate and/or iron chloride and/or iron sulfate and the surface to be cleaned, iron (III) reacts with the components having lcwer oxidation number contained in the basic metal and loosens and thereafter dissolves them together with the adherent oxide laser (scale).

After having cleaned the surface with water, we obtain the oxide-free surface of the basic metal.

AccordIngly, the invention relates to a process for the removing oxide layer and scale from metals and metal alloys, in particular alloyed steels, high -alloyed steels and carbon steels in pieces or in pulling-over system, in course of which the product is treated in one or more stages at a temperature between 20 and 8C 0C, preferably '10 and 60 OC, for 3 - 40 minutes, preferably 10 - 30 minutes in the aqueous solution of iron chloride sulfate and/or iron (III)- chloride and/or iron(III) sulfate containing iron(III)ions in a quantity of 15 - 200 g/l, free of acid or containing mineral acid of 2 - 30 % by weight.

According to the invention, when pickling individual pieces, we proceed so, that in the tanks staying at disposal pickling bath containing iron chloride sulfate or iron chloride or iron sulfate or the mixture thereof and mineral acid of 2 - 30 % by weight is used so that in the tanks following one another the active ingredient of 150 g/l iron(III)-.ion is1 contained in an ever increasing concentration. Tenperature of the pickling bath is kept at 40 to 60 OC, metal to be pickled is treated with the increasing concentration of active ingredient in dependence of surfacial quality for 10 to 30 minutes, thereafter the surface having been pickled is cleaned with water.

When carrying out the pickling according to pulling-over system one proceeds correctly so, that the products to be pickled are led through a plurality of pickling baths. In the pickling baths of the temperature of 40 to 60 C containing iron chloride sulfate or iron chloride or iron sulfate or the mixture thereof and mineral acid of 2 - 30 % by weight)iron (III) ion as active ingredient is present in an increasing concentration, in the quantity of 15 - 200 g/l.

The velocity of pulling is chosen depending on the quality of the pickled surface. Pickling process is finished with flushing with water.

With both methods of pickling spent pickling bath is regenerated in situ or in a separate equipment by chlorination.

The main advantages of the process according to the invention are the following: - the-spent pickling bath may be regenerated with chlorination several times, - it makes possible to replace or omit pickling methods using acids and salt melts at low costs, utilizing the existing equipments or only few complementary equipments, - in case of pickling alloyed, high-alloyed steels and carbon steels from the spent pickling baths having been used in course of earlier acidic pickl ing processes, the pickling bath according to the invention can be produced in situ or in a separate equipment with chlorination, - the pickling can be carried out in a reliable and-reproducible way and is useable in case of hot-produced, cast, formed and rolled pre-products and final products, - it reduces problems of already existing systems in respect of safety and ecology.

The process according to the invention will be illustrated in detail by the aid of the following examples.

Example 1 As a pickling system a panelboard pickling device with a basket - rocker-movement is used, with cold water and warm water flushing.

As a medium of pickling an austenitic steel sheet of quality of K037, acid-resisting, of the size of 1000 X 2060 X 4 mm is used, with a scale-layer formed in course of hot forming.

Quantity of the scale-layer: 180 - 200 g/m2 Quantity of the pickling bath: 5400 1 Irqn+ chloride sulfate is used for pickling.

Fe content: 63 g/l Pickling temperature: 45 C Hydrochloric acid content: 2 % Pickling time: 17 minutes.

The pickled surface is free of oxide, silver -white, phenomenon of overpickling could not be observed.

Example 2 The equipment as described in Example 1 is used, the pickling medium is low-alloyed electrotechnical sheet of the quality Dinamo, size 1000 X 2000 X 5 mm, coated with scales formed in course of final heat treatment.

Quantity of scales: 98 g,m Iron chloride sulfate is used for pickling.

Fe content: 47 g/l.

Temperature of pickling: 55 oC Pickling time: 30 minutes Pickled surface is silver-white, free of oxide, mat.

Example 3 The equipment as described in Example 1 is used, the pickling medium is steel sheet of the quality A 38, size 1000 X 2000 X 2 mm, covered with scales resulting from heat treatment.

Iron(III)-chloride solution is used for pickling.

Foe32 content: 45 g/l Temperature of pickling: 40 OC Time of pickling: 25 minutes Surface of the steel sheet after pickling: white, mat Example 4 The equipment as described in Example 1 is used, the pickling medium is acid-resistent steel sheet of the quality K0 36, size 1000 X 2000 X 4 mm, having a scale-layer formed in course of heat treatment.

Quantity of the scale-layer: 120 - 150 g/m2 Composition of the pickling bath: FeC13 100 g/l Fe2(S04)3 100 g/l FeCl SO4 100 9/1 Total Foe3+ content 92 g/l Temperature of pickling 45 OC Time of pickling: 25 minutes The pickled surface is silver-white, free of oxide, mat.

Example 5 In the equipment as used earlier as pickling medium an Al sheet with oxidized surface, size 1000 X 2000 X 4 mm is used.

Iron chloride sulfate is used for pickling.

Fe3+ content: 50 g/l Temperature of pickling: 70 OC Time of pickling: 10 minutes After pickling the aluminium sheet has a white mat surface.

Example 6 The equipment as described in Example 1 is used, the pickling medium is a copper sheet with oxidized surface, size 100 X 200 X 1 mm.

Iron chloride sulfate is used for pickling.

Fe3+ content: 50 g/l Temperature of pickling: 70 OC Time of pickling: 10 minutes.

As a result of pickling a sheet free of oxide with a definitely mat surface is obtained.

Example 7 As pickling medium an acid-resistent austenitic steel sheet of the quality KO 37, size 100 X 2050 X X 4 mm is used with a layer of scales formed in the course of hot-forming. Pickling is carried out in several stages, in 3 panelboard pickling tanks with basket -rocket-movement, in a solution of iron chloride sulfate.

I. Pre-pickling bath: Fe content: 15 g/l Temperature of pickling: 40 0C Time of pickling: 5 minutes II. Pickling bath: Fe3+ content: 69 g/l Temperature of pickling: 50 0C Time of pickling: 10 minutes Hydrochloric acid content: 3 % III. Pickling bath: Fe3+ content: 98 g/l Temperature of pickling: 60 0C Duration of pickling: 10 minutes The pickled surface is silver-white, free of oxide, phenomenon of overpickling could not be observed.

Example 8 The equipment according to Example 1 is used, the pickling medium is low-allofed electrotechnical sheet of the quality Dinamo, size of 1000 x 2000 x 5 mm, covered with scales formed in course of the final heat treatment.

Quantity of the scales: 98 g/m2 Iron chloride sulfate is used for the pickling.

Foe3 content: 47 g/l Temperature of pickling: 40 OC.

Sulfuric acid content: 12 % by weight.' Time of pickling: 8 min.

The pickled surface is silver white, free of oxide, mat.

Example 9 The equipment as described above is used and steel sheet of the quality of A 38, of size 1000 x 2000 x 2 mm, covered with a scale layer formed in course of the heat treatment, is pickled.

Iron chloride sulfate is used for the pickling.

Fe3+ content: 79 g/l.

Sulfuric acid content: 22 8 bp weight.

Temperature of the pickling bath: 40 OC Time of pickling: 5 min.

The pickled surface is free of cxide, white, mat.

Claims (2)

1. i. Process for removing of oxide layer and scale from the surface of metals and metal alloys, in particular alloyed steels, high-allcfed steels and carbon steels in pieces or in pulling-over system in a pickling bath, c h a r a c t e r i z e d in that the metal product is treated at 20 to 80 C, preferably 40 to 600C, for 5-40 minutes, preferably 10-30 minutes in one or more stages in the aqueous solution of 15-200 g/l, e.g.

   150-200 g/l, iron(III)-ion, containing iron (III) chloride sulfate anc'or iron(III) chloride and/or iron (III) sulfate, being free of acid or containing mineral acid of 2 to 30 X, preferably 8 to 12 bi by weight.
2. 2. Metal or metal alloys vhen treated by a process as claimed in claim 1.