WO1999032690A1

WO1999032690A1 - Pickling process with at least two steps

Info

Publication number: WO1999032690A1
Application number: PCT/US1998/026235
Authority: WO
Inventors: Dane G. Armendariz; Lawrence E. Faw
Original assignee: Henkel Corporation
Priority date: 1997-12-23
Filing date: 1998-12-23
Publication date: 1999-07-01
Also published as: ZA9811837B; AU2086299A

Abstract

A process for pickling a scale covered solid surface of a metallic article includes contacting the substrate with a first liquid to loosen the scale but not completely remove it, followed by electrolysis in which the substrate with the loosened scale is an electrode and external electromotive force is applied to the substrate and a counter-electrode. This electrolysis completes the pickling by removing the loosened scale from the substrate.

Description

PICKLING PROCESS WITH AT LEAST TWO STEPS

BACKGROUND OF THE INVENTION

During manufacture, or intermediate heat treatment such as annealing, of hot- rolled steel articles, the steel becomes covered with a layer of oxidation products of varying thickness. Because of the need to obtain a bright surface finish for the final product, most or all of these oxide layers have to be removed. This is achieved by the well known pickling processes, for which solutions in water of inorganic mineral acids such as hydrochloric, sulfuric, nitric and/or hydrofluoric acids are used, either alone or in mixtures of varying proportions.

For stainless steel articles, the pickling process most commonly or indeed almost exclusively used until a few years ago involves the use of a mixture of nitric acid and hydrofluoric acid, concentrations of which vary according to the type of equipment used for pickling, the type of stainless steel to be pickled, its surface characteristics and the geometry of the article to be pickled. The process is economical and enables excellent results to be obtained. It has, however, considerable problems of an ecological nature: Because of the use of nitric acid, nitrogen oxide vapors are emitted into the atmosphere around the pickling bath. Nitrogen oxide ("NO_x") vapors are strongly irritating and can be toxic to humans who breathe them, and these vapors are also highly polluting and aggressive towards most metals and many non-metals with which they come into contact. In addition, the use of nitric acid in pickling baths leads to high nitrate levels in the rinse liquids used in pickling and in the spent pickling baths, with the consequent problem of their lawful disposal. Abatement of the NO_x vapors in the air and of the nitrates in spent baths creates considerable plant problems, high operating costs, and no certainty that the results will satisfy future emission limits.

Various non-nitrogenous pickling compositions and processes for stainless steel substrates have been disclosed. With most of these, however, contrary to the situation with conventional pickling, it has been found that complete removal of all of the scale desired to be removed can not practically be accomplished in a single step by chemical treatment alone. Instead, at least the first chemical treatment primarily serves to loosen the scale present from its attachment to the underlying metal of the substrate. A second step is then required to complete removal of the scale loosened in the first step. This second step heretofore has been normally mechanical, the use of high pressure water spraying having been found to be particularly suitable. These and other mechanical methods of removing loosened scale, however, suffer from the disadvantage that they require special equipment that is not readily available at all locations where pickling is normally performed.

Accordingly, a major object of this invention is to provide a two step process for pickling scale covered metal, particularly stainless steel, substrates that will avoid both the use of nitrogenous materials that result in an environmental hazard or nuisance and the need for most if not all special mechanical equipment, particularly the need for an exceptionally high pressure water supply. Other alternative or concurrent objects are to reduce the cost of operations and/or materials required to achieve satisfactory pickling of metals, particularly stainless steel, without the use of nitric acid or other nitrogenous compounds that have the hazard of generating, and releasing into the atmosphere around the pickling bath, toxic oxides of nitrogen and the certainty of generating large amounts of nitrate in process waters. Other more detailed objects of the invention will be apparent from the description below. Although most of the description below is written in terms of application to stainless steel, a preferred embodiment of the invention, all of it is to be understood as applying also, mutatis mutandis, to pickling other types of scale covered metal articles.

Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, "parts of, and ratio values are by weight or mass; the term "polymer" includes "oligomer", "copolymer", "terpolymer" and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not necessarily preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word "mole" means "gram mole", and the word itself and all of its grammatical variations may by used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, unstable, hypothetical, or in fact a stable electrically neutral substance with well defined molecules; and the terms "solu- tion", "soluble", "homogeneous", and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18 - 25 °C.

BRIEF SUMMARY OF THE INVENTION

It has been discovered that electrolysis provides an efficient and satisfactory second step for pickling of stainless steel articles, following a chemical pickling step that loosens but does not fully remove the scale desired to be removed by the pickling 5 process.

DETAILED DESCRIPTION OF THE INVENTION

Any treatment that loosens but does not completely remove scale from stainless steel may be used as the first step in a process according to this invention. However, the invention is particularly advantageously applied when its first step is treatment by o contacting the article to be pickled with an aqueous liquid pickling composition that comprises, preferably consists essentially of, or more preferably consists of water and the following components:

(A) sulfuric acid;

(B) hydrofluoric acid; 5 (C) iron(lll) cations, alternatively called "ferric cations"; and

(D) elemental oxygen; and, optionally, one or more of the following components:

(E) a dissolved peroxy compound other than gaseous oxygen;

(F) surfactants, exclusive of any that may be part of one of the immediately previously recited components (A) through (E); and o (G) corrosion inhibitors, exclusive of any that may be part of one of the immediately previously recited components (A) through (F).

This preferred type of aqueous liquid pickling composition for use in the first step of a two step pickling process according to this invention preferably further satisfies the following conditions (1) and (2) at all times during use of the aqueous liquid pickling composition 5 in a process according to the invention: (1) the concentration of iron(lll) cations in the composition remains at least 10 grams per liter of composition (hereinafter usually abbre- viated as "g/l") and (2) (2.1) the redox potential of the composition, as measured by the potential of a platinum or similar inert metal electrode in electrical contact with the composition is at least 300 millivolts (hereinafter usually abbreviated as "mv") more oxidizing than a standard hydrogen electrode (hereinafter usually abbreviated as "SHE"), or (2.2) the ratio of iron(lll) cations to iron(ll) cations dissolved in the composition is at least

1.0:1.0, or (2.3) both of alternative conditions (2.2) and (2.1) are satisfied.

In this type of preferred first step pickling composition used in a process according to this invention, the concentration of H₂S0₄ preferably is at least, with increasing preference in the order given, 100, 110, 120, 130, 140, or 145 g/l, and independently the concentration of HF preferably is at least, with increasing preference in the order given,

5, 10, 13, or 15 g/l of HF and if very high pickling speed is more important than minimizing pickling bath material costs still more preferably is at least, with increasing preference in the order given, 25, 35, 40, 45, or 50 g/l of HF. Independently for each acid, the concentration of H₂S0₄ preferably, primarily for reasons of economy, is not greater than, with increasing preference in the order given, 600, 500, 400, 300, 250, 200,

180, 160, or 150 g/l and the concentration of HF preferably, primarily for reasons of economy, is not greater than, with increasing preference in the order given, 300, 200, 100, 80, 70, 60, 55, 50, 45, 40, 35, 30, 25, 20, or 17 g/l. These acids are believed to have various functions, of which the most important are to maintain the pH of the pickling composition as noted above; to dissolve the oxides originating from the heat treatment; and, for the hydrofluoric acid, to complex the Fe⁺³ and Cr⁺³ ions dissolved in the pickling composition during its use.

As the concentrations of the two acids, and particularly the hydrofluoric acid, tend to fall during the pickling process, these acids preferably are replenished continuously or periodically by additions of the acids to the composition on the basis of its analytically determined free acid and fluoride ion values.

The concentration of Fe⁺³ ions in a pickling composition used in the first step of a process according to this invention preferably is at least, with increasing preference in the order given, 10, 13, or 15 g/l from the beginning of use of the composition; independ- ently, primarily for reasons of economy, at the beginning of use of a composition, the concentration of total iron in the composition preferably is not more than, with increasing preference in the order given, 100, 75, 50, 25, or 20 g/l. As a composition is used, the concentration of total iron in it normally substantially increases as a result of dissolution of the iron-containing oxides that the composition is operated to remove. So long as a sufficient concentration of the dissolved iron in the composition is maintained in its +3 oxidation state, this_concentration of dissolved. iron(l II) cations_preferably being main- tained at least as high as its initial concentration in the pickling composition before the composition began to be used, accumulations of at least 100 g/l of total iron are harmless to the efficacy of the composition, although its speed of pickling may be somewhat decreased. The primary function of the ferric cations in a pickling composition used in the first step of a process according to this invention is believed to be oxidizing iron(0) atoms, from the metal surface underlying the oxides to be removed from the pickled surface, to iron(ll) cations so that they will become soluble in the pickling composition. In this process, the ferric cations are themselves reduced to ferrous cations, so that in the absence of oxidizing agent added during operation of the pickling composition, the composition will cease to be effective for pickling because its redox potential will no longer be sufficient to promote an appropriate rate of metal dissolution to remove all the surface oxides desired to be removed.

The oxidation of Fe⁺² ions to Fe⁺³ ions during the first step of a preferred process according to the invention, to maintain the concentration of the Fe⁺³ ions, is believed to be accomplished by the action of the elemental oxygen introduced into the composition during its use. If peroxide compounds are also present initially and/or introduced during use, the oxidizing action of the peroxide compounds also is believed to contribute to the oxidation of Fe⁺² to Fe⁺³ cations.

In one embodiment, the preferred type of aqueous liquid pickling composition for the first step of a process according to the invention, from the beginning of its use, comprises hydrogen peroxide, in a concentration that preferably is at least, with increasing preference in the order given, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0 g/l and independently preferably is not more than, with increasing preference in the order given, 50, 35, 30, 20, 18, 16, 14, 12, 10, 8, 6, or 5.1 g/l. Whenever hydrogen peroxide is used in an aqueous liquid pickling composition in the first step of a process according to the invention, it is preferable to use along with the hydrogen peroxide a stabilizer effective in preventing or at least substantially retarding peroxide decomposition under the pickling conditions (temperature up to 70 °C, very acid composition pH, iron up to 100 g/l, presence of Ni and Cr ions). Stabilizers for H₂0₂ known to be effective in acid media are, for example: 8-hydroxyquinoline, sodium stannate, phosphoric acids, salicylic acid, pyridinecarboxylic acid, and, most preferably, one or both of p-hydroxy-benzoic acid and phenacetin (i.e., acetyl-p-phenetidine). Such stabilizers when used preferably have a concentration of from 5 - 20 parts of stabilizer per million parts of the total pickling composition, a concentration unit that may be used hereinafter with respect to any material as well as such stabilizer and is usually abbreviated hereinafter as "ppm". In an alternative embodiment of the preferred aqueous liquid pickling composition for a first step of a process according to this invention, no peroxide is used initially. This is often preferred for reasons of economy over the use of peroxide initially. However, either of these alternative embodiments can give equally good results. When hydrogen peroxide or any other peroxide compound is present initially in a preferred first step aqueous liquid pickling composition in a process according to the invention, the replenishment of constituents required for long continued use of a given volume of first step aqueous liquid pickling composition may or may not include replenishment of the peroxide. If the peroxide is replaced and is hydrogen peroxide as preferred, the replenishing peroxide is preferably mixed with a stabilizer as described above at the time of addition, and independently it is preferably added, at frequent intervals or most preferably continuously, at a rate from 0.3 to 1.0 g/l of total aqueous liquid pickling composition per hour of operation in pickling.

The elemental oxygen required in a process according to this invention may be monatomic (a form which is unstable under most conditions), diatomic (the type predominantly stable under most terrestrial conditions), or triatomic, also known as "ozone" (a form which is also unstable under most terrestrial conditions but can be generated by many known methods and used before it spontaneously decomposes). All of these forms of elemental oxygen are gases at any combinations of temperature and pressure that are practical for use in pickling, and the elemental oxygen may be introduced into the pickling composition in pure form or in a mixture with any other gas(es) or vapor(s) that do not act adversely to an object of the invention. Usually, for reasons of economy, ordinary air from the ambient natural atmosphere is preferred and is fully suitable.

It is advantageous, from the viewpoint of the extent of oxidizing iron(ll) cations to iron(lll) cations that is obtained from a constant mass of elemental oxygen used, for the elemental oxygen supplied as part of a process according to this invention to be heated above ordinary ambient human comfort temperature (i.e., 18 - 25 °C). More particularly, the temperature of the elemental oxygen, immediately before it is introduced into the first step pickling composition during its use in a process according to this invention, preferably is at least, with increasing preference in the order given, 30, 35, 40,

45, 50, 55, 60, 65, 70, 75, or 80 °C. On the other hand, heating often is associated with increased cost, so that for this reason it may be more advantageous to minimize the amount of heating used. The adequacy of the supply of elemental oxygen can be readily measured by the redox potential and/or iron(lll) cations to iron(ll) cations ratio, as already noted, and independently of all other expressed preferences, the amounts of elemental oxygen and of any peroxide material introduced as replenishing ingredients into an aqueous liquid pickling composition used in a first step of a process according to this invention should be managed together so as to maintain the redox potential and/or iron(lll) cations to iron(ll) cations ratio within their preferred values as described elsewhere herein. Any volume of gaseous material as defined herein is to be understood to be measured at, or "corrected" to by calculation using well known mathematical relations, 1 bar of pressure and 25 °C. If elemental oxygen is supplied in a mixture with other gases, the effective volume of oxygen is to be understood as the mathematical product of the thus- measured or calculated total volume of the mixture multiplied by the volume fraction of elemental oxygen in the mixture. In order to maximize the speed of the pickling process, when the elemental oxygen is supplied in the form of diatomic oxygen gas as is usual and the aqueous liquid pickling composition does not contain any peroxide component other than this, the effective volume of the elemental oxygen introduced into the pickling composition during each hour of its operation preferably is at least, with increasing preference in the order given, 0.60, 0J0, 0.80, 0.90, 1.0, 1.2, 1.4, 1.6, 1.8, or 2.0 times as large as the volume of the pickling composition into which it is introduced. Independently, however, for reasons of economy it is preferable for the effective volume of diatomic oxygen gas introduced per hour of operation into a pickling composition being used in a process according to the invention to be not more than, with increasing preference in the order given, 20, 15, 10, 8, 6, 4, or 3 times the volume of the pickling composition, and even more preferably to be little or no more than is needed to maintain the redox potential and/or the ratio of iron(lll) cations to iron(ll) cations dissolved in the pickling composition within a preferred range. If optional peroxide component (E) is also present initially and is replenished during use of a preferred aqueous liquid pickling composition in the first step of a process according to this invention, smaller volumes of elemental oxygen than are otherwise required will usually be sufficient, but even if a preferred amount of peroxide as indicated above is used either initially or as replenishment, a total volume of gas, not all of which needs to be elemental oxygen, that is at least 3 times the volume of the total aqueous liquid pickling composition being used preferably is introduced into the aqueous liquid pickling composition during each hour while the composition is being used, to accomplish mechanical agitation of the laminar layer of aqueous liquid pickling composition near the article(s) being pickled, so that the chemical constitution of this layer will remain as close as possible to that of the bulk of the aqueous liquid pickling composition, even when rapid dissolution of the surface of the article being pickled is occurring. To ensure a preferred extent of mechanical agitation and homogenization of the pickling composition, it is advisable to blow the gas used into the bottom of the tank via perforated feed tubes, or by the use of other suitable gas distributing equipment as is generally known in the art.

The redox potential of a pickling composition of the above described preferred type used in the first step of a process according to this invention preferably is at least, with increasing preference in the order given, 310, 320, 325, 330, 335, 340, 345, or 350 mv more oxidizing than a SHE and independently, primarily for reasons of economy, preferably is not more than 1200, 1000, 900, 800, 700, 600, 500, or 450 mv more oxidizing than a SHE. The ratio of the concentration of iron(lll) cations to iron(ll) cations in a pickling composition of the above described preferred type used in the first step of a pro- cess according to this invention preferably is at least, with increasing preference in the order given, 1.10:1.0, 1.20:1.0, 1.30:1.0, 1.40:1.0, or 1.50:1.0. No upper limit preference is believed to exist for this ratio, because a freshly made pickling composition of the above described preferred type usually contains no deliberately added iron(ll) cations at all and functions satisfactorily. The pH value of a pickling composition of the above described preferred type used in the first step of a process according to this invention preferably is at least, with increasing preference in the order given, -2.0, -1.0, -0.50, -0.40, -0.30, -0.20, -0.10, or 0.00 and independently preferably is not more than, with increasing preference in the order given, 3.0, 2.0, 1.5, 1.2, 1.0, 0.90, 0.80, 0J0, 0.60, or 0.50. In formulating a pickling composition of the above described preferred type for use in the first step of a process according to the present invention, usual additives for pickling may advantageously be used, for example non-ionic surfactants and fluorinated surfactants acting as wetting agents, emulsifiers, brighteners, and acid attack inhibitors (also known as corrosion inhibitors). These additives may improve and favor the pickling action. When used, they preferably are present in a total concentration not greater than about 1 g/l of total composition.

Particularly advantageous additives are perfluorinated anionic surfactants and non-ionic surfactants that consist of polyethoxylated alkanol molecules, the alkanol molecules having at least, with increasing preference in the order given, 10, 12, or 14 carbon atoms per molecule and independently preferably having not more than, with increasing preference in the order given, 22, 20, 18, or 16 carbon atoms per molecule.

As acid attack inhibitor for the pickling composition, there can advantageously be used dicyclohexylthiourea, alkylbenzyldialkylsulfonium salts, dialkylsulfides, dialkylsulfox- ide, and monoethanolamine; many other suitable inhibitors are disclosed in the publica- tion Corrosion Inhibitors — Manufacture and Technology by M. William Ranney, published by Noyes Data Corp. (1976) at pages 45 - 64. During operation, as the concentration of bivalent ferrous ions in the composition increases, the redox potential of the composition would tend to decrease without the introduction of elemental oxygen and/or peroxide molecules. By constantly controlling the potential, it is possible not only to ensure effective pickling but also to ensure that the metal surface after pickling is largely passive and therefore not susceptible to undesirable irregular corrosive attack such as pitting or intergranular corrosion.

The time of contact between the pickling composition and an article being pickled in a first necessary step of a process according to the invention preferably is at least, with increasing preference in the order given, 1.0, 2.0, 3.0, 4.0, 4.5, 5.0, 5.5, or 6.0 minutes and independently preferably is not more than, with increasing preference in the order given, 30, 25, 20, 18, 16, 14, 12, or 10 minutes. As noted further below, this step of the process may often advantageously be repeated at least once after exposing the article being pickled to the second necessary step in the overall process. It has been found that any given total exposure time to a process of the type required for a first step in a pro- cess according to the invention is more effective when divided into at least two separate instances of use of the step. When this is done, a process of the type required for the second necessary step of a process according to the invention preferably is used between each two successive instances of a process step of the first required type.

After the scale on an article to be pickled has been loosened by a first step as de- scribed above, the article is ready for the second step in a process according to the invention. In this step, the article to be pickled is connected to an external source of electromotive force and is also in contact with a liquid electrolyte solution. In order to avoid undesired changes in the chemical constitution of the second step electrolyte solution during its use, the surface of articles being pickled is preferably rinsed between completion of the first necessary step of a process according to the invention and the beginning of the second necessary step thereof. Ordinarily, tap water is fully satisfactory for such rinsing and is therefore preferred for economy. However, if tap water at a particular location is observed to cause any deleterious change(s) in the chemical constitution of the second step electrolyte solution, this rinse would then preferably be performed with deionized, distilled, or similarly pure water.

The most preferred second step electrolyte solution is a solution of a strong acid in water. Primarily for reasons of economy¹, an inorganic² acid is normally preferred as

^xAs is generally known, the aquated protons formed by the ionization of strong acid in water have the highest specific conductivities of any ions known in water solutions. Therefore, any given level of ionic conductivity in water is generally most eco- the electrolyte, with sulfuric acid most preferred. However, almost any other strongly ionized electrolyte in sufficient amount to give adequate conductivity may be used, and even non-electrolytes that do not harm the process according to the invention may of course be included, although their absence is normally preferred at least for reasons of economy. When a strongly ionized inorganic acid is used, its concentration preferably is such as to give the total electrolyte solution used in the second necessary step of a process according to this invention a "Free Acid" content, measured in "points" which are defined as the number of milliliters (hereinafter usually abbreviated as "ml") of 0.10 N strong base solution required to titrate a 10 ml sample of the electrolyte solution to a pH value³ of about 5, that is at least, with increasing preference in the order given, 10, 15,

20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or 75 points and independently, primarily for reasons of economy, preferably is not more than, with increasing preference in the order given, 200, 175, 150, 125, 100, or 80 points.

The article to be pickled may be connected to the external electromotive force so as to be either the anode or the cathode if direct current electrolysis, either with or without superimposed alternating current, is used in the second necessary step of a process according to the invention, or alternating current electrolysis may be used exclusively, without any superimposed direct current. More complex electromotive force variations, e.g., those in which only parts of a normal alternating current are supplied to the work to be pickled, may also be used. Ordinarily, direct current is preferred because it can be obtained with relatively inexpensive equipment and works more rapidly in removing loosened scale than simple 60 or 50 cycles per second alternating current, the other inexpensively available type in most instances and locations. If hydrogen embrittlement is a danger to the intended use of the article being pickled and the article,

nomically achieved by using acid to provide most or all of the cations of the electrolyte. A high ionic conductivity value minimizes the consumption of electrical energy, because a given current density can be obtained at a lower voltage when the ionic conductivity is increased. Furthermore, a high ionic conductivity minimizes variations in current density, from one part to another of the surface to be treated by any electrolytic process, that might be caused by anything less than a perfect conformance in surface shape between the surface being treated and the counterelectrode(s).

²Strongly ionized inorganic acids are generally less expensive than strongly ionized organic acids.

Conveniently measured with methyl orange indicator . because of its passivating tendency or inherent electrochemical "nobility", is not susceptible to extensive dissolution under anodization in the electrolyte solution used, direct current with the article to be pickled as anode is preferred.

If direct current electrolysis is used in the second necessary step of a process ac- cording to this invention, the current density over the surface of the substrate being pickled preferably is at least sufficient to cause evolution of gas, in an amount readily visible to normal unaided human vision, from the substrate being pickled. More particularly, the current density over the surface of the substrate being pickled preferably is at least, with increasing preference in the order given, 0.005, 0.010, 0.020, 0.040, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.24, 0.26, 0.28, 0.30, or 0.32 amp per square centimeter of surface being pickled, a unit hereinafter usually abbreviated as "amp/cm²", and independently, primarily for reasons of economy, preferably is not more than, with increasing preference in the order given, 7, 5, 3, 2.0, 1.5, 1.0, 0.90, 0.80, 0J0, 0.60, 0.50, 0.45, 0.40, or 0.35 amp/cm². It has been found that most of the useful effect of the second necessary step in a process according to this invention is accomplished fairly quickly when using preferred current densities, and additional time increases expense without any compensating benefit. Therefore, in a single continuous instance of a second necessary step in a process according to the invention, the time of application of externally imposed electric current through the surface of an article being pickled preferably is not more than, with increasing preference in the order given, 10, 8, 6, 4, or 2 minutes. If cleaning is not complete within one of these time intervals, repetition of both the first and second necessary steps is preferred over extending the time of the second step only.

As already noted, it is often advantageous and therefore preferred to repeat, at 5 least once each, process steps of the type required for the necessary steps one and two of a process according to the invention as described above. When this is done, it is often advantageous and therefore preferred to use direct current electrolysis in the operations of the second necessary type and to connect the article being pickled as cathode in the first instance of the second necessary type of process step and as the o anode in the second instance of use of this second necessary type of process step; or if the article being pickled is the anode in the first instance, to connect it as the cathode in the second instance. If third or still further instances of repetition of these steps are used, reversing polarity of the connection of the article being pickled from the electrical connection made to the same article in the preceding instance is generally preferred. 5 Practice of and benefits of this invention may be further appreciated from consideration of the examples described below. In these examples, the first necessary step of the process was performed with a peroxide containing example of the preferred type of pickling composition described above. Particulars of the pickling compositions used for this part of the process are shown in Table 1. In addition to the ingredients shown explicitly in Table 1 , air was blown vigorously through the composition while it was in use, and hydrogen peroxide was added whenever needed to obtain the redox potential values shown in the Table; the balance of the composition not otherwise accounted for was water.

Table 1

Abbreviations for Table 1 ^■'kg/I" means "kilograms of substance per liter of total composition; "n.m." means "not measured".

The electrolyte solutions used for the second necessary step of the process examples, and for any repetition(s) of this step, were aqueous sulfuric acid with no other deliberately added ingredients. Three concentrations, with 26, 79, and 41 ml of total acid were used and were designated Compositions 2.1 , 2.2, and 2.3 respectively.

The substrates pickled were lengths of cylindrical stainless steel wire of various conventional types as specifically indicated by their Type Numbers below. At the beginning, all of the surface area of each length of wire was completely covered with a black scale. Each wire had a diameter of 5.6 mm and a length in contact with the liquid treating compositions of about 10 centimeters, corresponding to a total area of about 18 square centimeters. Two counterelectrodes of aluminum, each in the form of a plate about 2.5 centimeters wide with a wider bottom part, were used, with the substrate(s) being pickled approximately centered between them. Substrates were rinsed with tap water between each step explicitly shown. The voltage supplied was full wave rectified from a conventional power supply that was itself powered from conventional U. S. electric utility 60 cycle alternating current, and the value reported below was the total DC voltage between the substrate(s) being pickled and the counterelectrodes, which were electrically coupled to each other, with no attempt to correct for resistance losses in the electrolyte solution. The current density values given below are for the substrate(s) being pickled only, with the entire immersed part of the substrate(s) being assumed to conduct the current; the current densities at the larger counterelectrodes were correspondingly smaller. A single substrate was used unless otherwise indicated. 0 Further process details and results are given in Table 2 below. If a repetition of the first and second steps is noted in the Table by a superscript "^R", conditions during the repetition were the same as in the first instance of the same step, except in one instance that is specifically footnoted in Table 2.

Table 2

This table is continued on the next page ...

Footnote. Abbreviations, and Other Notes for Table 2

*In this instance only, the "repetition" was for 2 minutes of contact time, twice as long as the original step.

"Ex. No." means "Example Number"; "#" means "Number"; "Min." means "Minutes"; "Temp." means "Temperature".

In the column headed "Results and Other Remarks", percentages refer to percents of area of the substrate surface exposed to the treatment liquids.

Comparison of Examples 3 and 5, for instance, shows that a given total time of exposure of the substrate to the necessary first and second steps of a process according to the invention is more effective in removing scale initially present when divided into two separate exposures than when concentrated in only one.

Claims

1. A process for pickling a scale covered solid surface of a metallic article substrate, said process comprising operations of:

(I) contacting said scale covered solid surface, without imposing electromotive force on said surface from any external source, with a first liquid composition for a sufficient time to loosen but not completely remove said scale from said scale covered surface; and

(II) after step (I), contacting the surface bearing the scale loosened but not removed in step (I) with a second liquid composition that is an electrolyte solution in water by immersion of the surface in a container for the second liquid composition, said container also containing at least one counterelectrode having a surface that is distinct from the surface bearing the scale loosened but not removed in step (I) and is not in direct physical contact with the surface bearing the scale loosened but not removed in step (I), while an external electromotive force is imposed be- tween said at least one counterelectrode and the metal article having the surface bearing the scale loosened but not removed in step (I) to cause electric current to flow between the surface of said at least one counterelectrode and said surface bearing the scale loosened but not removed in step (I) for a sufficient time that at least some of said scale loosened but not removed in step (I) is separated from the surface formerly bearing it.

2. The process of claim 1 wherein the first liquid composition comprises water, sulfuric acid, hydrofluoric acid, iron(lll) cations, and elemental oxygen.

3. The process of claim 2 wherein, during operation (I) of the process, the concentration of iron(lll) cations in the first liquid composition is always at least 10 grams per liter of composition, and the redox potential of the composition, as measured by the potential of a platinum or similar inert metal electrode in electrical contact with the composition, is always at least 300 millivolts more oxidizing than a standard hydrogen electrode.

4. The process of claim 2 wherein, during operation (I) of the process, the concentration of iron(lll) cations in the first liquid composition is always at least 10 grams per liter of composition and there is a ratio of iron(lll) cations to iron(ll) cations dissolved in the composition that always is at least 1.0:1.0.

5. The process of claim 2 wherein the first liquid composition further comprises at least one component selected from the group consisting of dissolved peroxy compounds other than gaseous oxygen, surfactants, and corrosion inhibitors.

6. The process of claim 5 wherein the first liquid composition comprises hydrogen peroxide.

7. The process of claim 6 wherein the first liquid composition additionally comprises a stabilizer of hydrogen peroxide.

8. The process of claim 2 wherein the elemental oxygen has a temperature from about 30 ┬░C to about 80 ┬░C when it is introduced into the first liquid composition.

9. The process of claim 1 wherein the redox potential of the first liquid composition is maintained between about 310 mV and about 1200 mV more oxidizing than a standard hydrogen electrode during the first step of the process.

10. The process of claim 1 wherein the second liquid composition comprises an inor- ganic acid in water.

11. The process of claim 10 wherein the second liquid composition comprises sulfuric acid.

12. The process of claim 10 wherein the free acid content of the second liquid composition is from about 10 to about 200 points.

13. The process of claim 1 wherein the surface bearing the scale loosened but not removed in step (1) is rinsed with water after step (1) is completed and before step (2) is begun.

14. The process of claim 1 wherein the current density over the surface of the substrate during operation (II) is from about 0.005 to about 7 amp/cm².

15. The process of claim 1 wherein the time of application of externally imposed electric current through the surface of the article being pickled is not more than 10 minutes.

16. The process of claim 1 wherein operation (II) is performed in at least two stages each using direct electric current and in the second and any subsequent stages the polarity of the connection of the externally imposed electric current to the substrate is reversed from the polarity of the connection made to the substrate in the immediately preceding stage.