“Pickling” is a process applied to metallic surfaces, particularly those of iron and steel but also to various alloys of nickel, copper, and other metals. Usually, pickling is the first chemical treatment applied to a metallic surface after the surface has been formed into the shape and size desired for at least intermediate use by some means that cause the metal surface to be coated with a visible oxide film that may be called “scale”, “smut”, “discoloration”, “oxide”, red, white, or black “rust”, or some similar term.
The first chemical treatment in a pickling process is generally a concentrated acid or alkaline aqueous solution, for example 10-25% sulfuric, nitric, or hydrochloric acid or 10-40% sodium hydroxide solution. Often such solutions contain an inhibitor that reduces the rate of dissolution of elemental metal without substantially reducing the rate of dissolution of metal oxides. For example, various aromatic amines are very useful inhibitors in acidic pickling solutions and silicate salts are useful inhibitors in alkaline solutions. These concentrated solutions used at the beginning of pickling, however, rarely leave the metal surface in a desirable condition for immediate application of protective coatings of any type. Therefore, the last step of a complete pickling process usually is a chemical treatment operation with at least one and often both of the following two major goals: (a) to obtain a thoroughly clean and bright metallic surface by removing any soil, smut, spots, and/or any other dullness or discoloration formed during or left behind by the primary pickling step and (b) to passivate the metal surface by promoting the formation of a thin oxide or other oxygen-atom-containing layer, with corrosion resistance superior to that of the bare metallic surface, over the metallic surface.
In most instances, both of these goals are achieved by contacting the metal surface after the primary pickling treatment with an oxidizing aqueous solution, such as nitric acid or chromic acid solutions, alone or in mixture with a minor amount of other acids (e.g. HF).
In the more specific case of stainless steel pickling, a major traditional pickling process using mixed HNO3/HF acids includes, after one or more primary pickling operation(s) in which the stainless steel surfaces are contacted with HNO3 (10 to 15%)—HF (2 to 5%) solutions, a final operation in which the metallic surface is contacted with an aqueous liquid, the chemical composition of which is dependent on the type of stainless steel. Generally, this last pickling treatment liquid, which is designated hereinafter as a “brightening and passivating liquid”, contains the following ingredients and is used at the following temperatures: HNO3 for mostferritic and/or martensitic grades of stainless steel that are cold and hot rolled, used at a low temperature (<30° C.); HNO3 combined with a very small amount of HF (<1%), used at a low temperature (<30° C.), for specific ferritic and/or martensitic grades that need a little etch to remove the smut from both cold and hot rolled surfaces; and HNO3+HF (1 to 2%) for austenitic grades of stainless steel that have been cold and hot rolled, used at a medium temperature (20-50° C.).
Recently, new pollution-reducing pickling processes utilizing aqueous solutions of sulfuric acid, HF, and Fe3+ have been developed to overcome the pollution problems generated by the presence of nitric acid. In such pickling processes, hydrogen peroxide is added to the solution, and/or air or another source of elemental oxygen is blown through the solution, to restore the desired concentration of trivalent iron cations as the latter are depleted by being reduced to divalent iron cations during a pickling process. In practice, hydrogen peroxide is usually used in preference to relying on elemental oxygen alone in such a process, because the amount of elemental oxygen needed and the fact of the existence of elemental oxygen under normal conditions of temperature and pressure only as a gas require that special equipment be provided in many existing plants to prevent losses of pickling solution by entrainment in the large volume of gas flow required through the pickling solution. These new processes give pickling performances fully as satisfactory as those obtained with the traditional high polluting HNO3/HF mixed acid system.
In principle, hydrogen peroxide could be used in such a process by constantly monitoring the oxidation-reduction (hereinafter usually abbreviated as “redox”) potential of the solution and adding hydrogen peroxide continuously and only to the extent needed to maintain the potential. In practice, however, many pickling plants are not equipped for such continuous monitoring and addition, so that hydrogen peroxide is added from the beginning of a pickling process and afterwards is replenished only at fairly long time intervals. When so used, hydrogen peroxide needs to be stabilized against spontaneous decomposition in order to achieve the highest possible oxidation reaction yield during the oxidation of bivalent iron to trivalent iron. Many stabilizers for this purpose, such as acetamide, phenacetine, tertiary alcohols, and the like, are known in the art and are available commercially, and many of these stabilizers work quite well when used in the primary stages of pickling.
The main problem in completely eliminating HNO3 from pickling processes using these new pollution-reducing pickling solutions is in the final brightening and passivating liquids. These liquids generally (with some exceptions) have much lower dissolution rates for metal and/or metal scale than do the primary pickling solution(s) used earlier in a pickling process. As a consequence, the brightening and pickling liquids can usually be used for one week up to several months before these liquids accumulate a sufficient concentration of metal ions, dissolved from the surfaces being pickled, to require replacing the brightening and passivating liquids.
Attempts have been made to substitute for HNO3 in traditional brightening and passivating liquids a combination of non-oxidizing acid such as H2SO4 or H3PO4 (sometimes with a small amount of HF) with sufficient hydrogen peroxide to achieve a redox potential of the solution of at least about 550 millivolts (this unit being hereinafter usually abbreviated as “mV”) more oxidizing than a silver-saturated silver chloride electrode. (The redox potential can be measured, as is known to those skilled in the art, by immersing a chemically inert electrode such as platinum in the solution and measuring, by some method that draws only minimal electric current, the potential of this electrode compared to a standard reference electrode also immersed in the same solution.) Stabilizers of the types listed above have been included in such brightening and passivating liquids, but their useful lives have nevertheless been observed to be very short. As a consequence, in many industrial applications using pollution-reducing primary pickling process as described above, the final brightening and passivating liquid is still an HNO3 or HNO3—HF solution. When a brightening and passivating liquid that does not contain any nitrogenous acid must be used, very high concentrations of hydrogen peroxide are needed to obtain technically satisfactory results, and the costs are very high compared to the traditional process because of rapid spontaneous decomposition of hydrogen peroxide in such high concentrations. This is particularly true in pickling ferritic/martensitic grades of stainless steel, for which the use of a brightening and passivating liquid with a high redox potential is necessary to remove black smut formed during the earlier stage(s) of pickling.
Accordingly, a major object of this invention is to provide technically and economically satisfactory brightening and passivating liquids for completing a pickling process without any need to use nitric acid or any other ingredient that has a hazard of releasing toxic oxides of nitrogen into the environment of the pickling process. Other alternative and/or more detailed objects will become apparent from the detailed description below.
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 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 be 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, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; the terms “solution”, “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.; the first definition of an acronym or other abbreviation applies to all subsequent uses of the same acronym or other abbreviation; and the term “paint” and its grammatical variations includes all similar types of coatings that may be described by more specialized names such as “lacquer”, “varnish”, “primer coat”, “top coat”, or the like.