Abstract:
The present invention describes a cost-saving and environmentally conserving process for the electrochemical polishing of steel, in particular of low-alloy steels. The workpieces are rinsed after electropolishing in an arid phosphoric acid-sulfuric acid-bath in a first rinsing step with a phosphoric acid-containing solution, whereby a chemical attack on the freshly polished surfaces is avoided without the use of environmentally harmful and unhealthy inhibitors such as chromates. By recycling of the acids and the rinsing solution, these acids and solutions can be regenerated, whereby the process can be designed effluent-free.

Description:
[0001]    The present invention describes a method for the electrochemical polishing of steel workpieces, in which corrosive attack of the surface in the rinsing operation can be avoided even without the use of expensive and environmentally harmful inhibitors. Said method is also suitable in particular for workpieces of low-alloy steels, which are especially susceptible to chemical attack. 
         [0002]    Electrochemical polishing is a process that is used for the deburring, smoothing and brightening of metal surfaces. Owing to the higher current density at fine scratches and other irregularities, the metal is ionized and dissolved at these points faster than on the smooth regions of a metal workpiece, so that its irregularities are leveled out. For this purpose, the objects that are to be electropolished, which are suspended on suitable carriers or are arranged in baskets or the like, are immersed in an electrolyte, the polishing bath, and are removed from the latter after a certain time. After the bath liquid has drained from the polished surfaces, the objects are immersed in rinsing baths, to remove the electrolyte completely. These electropolishing methods have found extensive industrial application, in particular for steels with a chromium content above 12%, which are generally called stainless steels, special steels or acid-resistant steels. The electrolytes used are mainly based on mixtures of phosphoric acid and sulfuric acid, and brighteners and inhibitors can be added to them for further improvement of the action. 
         [0003]    However, steels with a chromium content below 12%, i.e. low-alloy steels such as structural steels and tool steels, which form the majority of the steel grades used, cannot be electropolished with adequate quality with the methods used for the processing of special steels. The reason for this is the lower acid resistance of these steels, which leads to uncontrollable chemical and corrosive attack on the surfaces by the electrolyte during the electropolishing process. 
         [0004]    For successful electropolishing of low-alloy steels with electrolytes based on phosphoric acid and sulfuric acid, generally a notable concentration of chromic acid, i.e. an oxyacid of hexavalent chromium (chromate), is added as inhibitor to the electrolytes, and prevents chemical attack on the surfaces of the workpiece during electropolishing. 
         [0005]    Chromates are highly toxic, embryotoxic and carcinogenic, so their industrial use is increasingly being restricted and is subject to stringent safety requirements regarding labor protection and environmental protection. Patent JP-A 5 163 600 describes the addition of ascorbic acid or salts of ascorbic acid as a possible means of reducing hexavalent chromate to chromium-(III) ions, which are less toxic. However, the use of chromic acid also constitutes an appreciable cost factor, which further limits the economic effectiveness of the electropolishing process. 
         [0006]    Various steels, aluminum, nickel and alloys thereof are electropolished, according to U.S. Pat. No. 2,773,821, in solutions of sulfuric and phosphoric acid without chromic acid, although addition of hydroxyacetic acid, benzenesulfonic acid and toluenesulfonic acid is necessary. The organic additives account for up to 40% of the electropolishing solution. EP-A 0 249 650 uses, for the chromium-free electrochemical polishing of objects of steel, special steel, nickel alloys, aluminum and aluminum alloys, a chelating agent based on phosphonic acid. EP-A 1 443 129 describes the addition of up to 50% of alcohols and other surface-active substances to the electropolishing solution, before the electropolished objects are varnished. All these additives represent a not inconsiderable cost factor. 
         [0007]    The rinsing process that follows the electropolishing operation, and is intended to remove adhering electrolyte from the surfaces, is of decisive importance for the brightness and smoothness of the object that is processed. The resultant decrease in the concentration of the acid on the surface of the electropolished object increases the corrosive action of the electrolyte. This effect should be suppressed by the addition of inhibitors such as chromic acid. Without these additives, the freshly polished metal surfaces are attacked again, so that to a considerable extent the effect of smoother and brighter surfaces achieved by the electropolishing is lost. 
         [0008]    Therefore an electropolishing method for low-alloy steels, in which this chemical attack by the dilute acid can be avoided even without the addition of expensive substances that are also harmful to the environment and to health, and is comparable to the methods for the processing of special steels with respect to costs and potential risks, would be of considerable advantage to industry. 
         [0009]    DE 808 519 B describes a method for the polishing and deburring of high-carbon or low-carbon steels and low-alloy steels by electrolytic means. The electrolyte contains 5 to 60 wt. % sulfuric acid and 30 to 80 wt. % phosphoric acid. The electrolyte bath can additionally contain a trivalent dissolved metal, e.g. iron, among others. 
         [0010]    AT 190 769 B describes a method and an electrolyte for the electrolytic cleaning of metal objects. The electrolyte consists of hydrochloric acid and is rinsed following electrolysis, and phosphoric acid in the proportion of about 0.05% to 3% can be added to the rinsing water. The purpose of adding phosphoric acid is to prevent the formation of oxides on the metal of the treated object. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The invention presented here is based on an electropolishing method, which, like the electropolishing method for special steel, is based on mixtures of phosphoric acid and sulfuric acid, in which the first rinsing step following the actual electropolishing step is performed with a solution containing phosphoric acid, preferably with a solution that has a phosphoric acid content of at least 50 wt. %. In particular, the use of concentrated phosphoric acid, with a content of 85 wt. % H 3 PO 4 , is suitable here as the starting solution. This method manages without the addition of chromic acid or other inhibitors and therefore offers considerable, and not only economic, advantages. 
         [0012]    First the objects that are to be electropolished are degreased in an optional step, to avoid contaminating the electrolyte and to make the surfaces of the workpieces completely accessible for the electrolyte. Any commercial degreasing solution can be used for this. Next, the workpieces are usually rinsed with water and then immersed in the electropolishing bath and connected as the anode. Undesirably severe chemical attack on the surfaces of the object being electropolished can be prevented during the electropolishing step by keeping the water content of the electrolytes low. Therefore highly concentrated acids, such as sulfuric acid, phosphoric acid and mixtures of sulfuric acid and phosphoric acid are used almost exclusively for the electropolishing of steels and steel alloys. Electrolytes with a water content of max. 20 wt. % give particularly good effects. 
         [0013]    Moreover, it proves advantageous if, right at the beginning, the electrolyte already contains iron-(III) ions at a concentration of at least 1 wt. %, preferably at a concentration above 2.0 wt. %. In order to achieve chemical activity that is sufficient for an economical process, the temperature of the electrolyte should be above 50° C., preferably 60° C. to 90° C. 
         [0014]    The problem of chemical attack during decrease of the concentration of acid in the course of the rinsing process, without the use of inhibitors, was solved in the method according to the invention by rinsing at room temperature with concentrated phosphoric acid with low water content, rather than with water, in the first stage of the rinsing process. Surprisingly, it is found that, following this first rinsing step, the surfaces can then be finish-rinsed with water without any problem, without chemical attack by the dilute acid being observed. It is advantageous to add to the rinsing water, in the last rinsing step, a certain proportion of a commercial corrosion inhibitor such as KORANTIN BH (2-butyne-1,4-diol), to prevent subsequent corrosion during drying. 
         [0015]    It is found that enrichment of the phosphoric acid with electrolyte in the first rinsing stage up to a sulfuric acid content of approx. 20 wt. % does not adversely affect the results. This offers the possibility of using the phosphoric acid, enriched with sulfuric acid electrolyte, in its turn as the basis for the production of fresh electrolyte. Recovery of the phosphoric acid entrained in the additional rinsing process from the rinsing water is possible without loss of quality. This makes recovery of the inorganic acids, in conjunction with circulating the rinsing water through an evaporator, extremely economical. In this way the electropolishing process can be designed to be almost effluent-free. 
         [0016]    The iron ions abstracted from the workpiece surface during electropolishing go into solution in the electrolyte and accumulate there. Beyond a critical concentration of approx. 8 wt. %, equivalent to approx. 140 grams of iron per liter in the electrolyte, there is a marked decrease in efficacy of the electrolyte. This necessitates a decrease in the iron content by partial exchange with fresh electrolyte. The used electrolyte can either be removed directly, or by drag-out to the rinsing process. 
         [0017]    After removal, the spent electrolyte should either be delivered to a licensed site for destruction, or should be made usable again by regeneration. Electrolytic precipitation of the iron in the form of Fe(II) sulfate from the concentrated electrolyte is eminently suitable for regeneration of the spent electrolyte. Thus, finally the iron removed in the form of iron(II) sulfate is the only waste product from the electropolishing process, and for its part it may find further industrial use, perhaps as a reducing agent. 
         [0018]    Using the method according to the invention it is therefore also possible for low-alloy steels to be electropolished just as efficiently and inexpensively as special steel. Moreover, this process also represents a method of electropolishing that is far less harmful to the environment and presents less risk to health. 
         [0019]    The invention is explained in more detail in the following examples. The examples only represent possible embodiments of the electropolishing method described here, and in no way imply a restriction to the conditions presented here. 
       EXAMPLES 
     Example 1 
       [0020]    A set of cutting tools made of hardened tool steel (material No. 1.3343) was electropolished in an electrolyte consisting of 50 wt. % phosphoric acid and 50 wt. % sulfuric acid with a specific gravity of 1.75 kg/l and an iron content of 4.5 wt. % at an electrolyte temperature of 80° C., current density of 40 A/dm 2  and voltage of 12 V for 6 min, and then prerinsed in concentrated phosphoric acid (85 wt. %) at room temperature, rinsed finally in water, then immersed in water at a temperature of 60° C., to which a commercial corrosion inhibitor was added at a concentration of 2 wt. %, and dried in air. 
         [0021]    A second set was electropolished in an electrolyte with 70 wt. % phosphoric acid, 2.5 wt. % sulfuric acid and 9 wt. % chromic acid with a specific gravity of 1.740 kg/l and an iron content of 2.5 wt. % at an electrolyte temperature of 50° C., current density of 40 A/dm 2  and voltage of 11 V for 6 min. The parts were then rinsed with water and dried. 
         [0022]    The result of electropolishing was the same in both methods with respect to leveling of the surfaces and smoothing of the cut edges. 
       Example 2 
       [0023]    Plates of heat-treatable steel, in the hardened and unhardened state, were electropolished in electrolytes according to Example 1. The current density was 25 A/dm 2  at 14 V and electropolishing time of 60 min. The rinsing process was carried out as described in Example 1, as well as drying in air. The results achieved for the hardened and the unhardened plates were the same in both methods with respect to material removal, brightness and leveling.