US3479293A - Process and composition for etching ferrous metal surfaces - Google Patents

Description

United States Patent ice 3,479,293 PROCESS AND COMPOSITION FOR ETCHING FERROUS METAL SURFACES Kenneth P. Bellinger, Rockville, and Le Verne Verzier, Cheshire, Conn., assignors to Conversion Chemical Corporation, Rockville, Conn., a corporation of Connecticut No Drawing. Filed Sept. 26, 1966, Ser. No. 581,736 Int. Cl. C09k 3/00; C23f 1/00 U.S. Cl. 25279.4 9 Claims ABSTRACT OF THE DISCLOSURE A dry powder composition upon addition to water provides an acid bath for etching ferrous surfaces and consists essentially of urea nitrate, a water-soluble auxiliary dry acidic component and optionally a finely divided inert filler. The acidic components may be organic or inorganic acids or the acid salts thereof but do not include reducing agents or film forming radicals.

The present invention relates to the chemical etching of metals, and more particularly, to a method for etching ferrous surfaces and to a powder composition for preparing the bath therefor.

Methods and compositions for etching steels and various other metals are, of course, old in the art. In many cases, however, the rates of attack of the available etchants are not readily controlled. Moreover, it is often difficult to obtain a desirable degree of uniformity in etching. Generally, the etchant employed for ferrous surfaces is nitric acid and the liquid acid employed creates dangerous-and otherwise undesirable Working conditions.

Accordingly, it is an object of the present invention to provide a process for etching ferrous metal surfaces at a controlled rate and in a manner that will give substantially uniformly etched surfaces.

It is also an object to provide a self-contained dry powder formulation which, upon addition to water, will produce a highly effective bath for the chemical etching of ferrous surfaces and which is convenient and safe to ship and handle, relatively economical in operation and precontrolled in the optimum proportion of essential ingredients so as to provide a balanced bath composition.

Another object is to provide such a self-contained dry powder formulation with which the rate of attack upon the metal workpieces can be readily varied by varying the amount of powder formulation added to water and/ or the temperature.

It has now been found that the foregoing and related objects can be readily attained by use of a self-contained dry power composition comprising urea nitrate and dry, auxiliary acidic component. The components and their concentrations are carefully selected so that upon solution of the prepared powder in a suitable quantity of water, a bath will resut having a pH of about 0.2 to 2.0 which will etch ferrous surfaces substantially uniformly. In addition to urea nitrate and the auxiliary acidic composition, a powdering or anti-caking agent, and possibly other additives, are beneficially included in the powder. The bath formed by the dissolved composition may be used at ambient and elevated temperatures, i.e., about 60 to 150 Fahrenheit, with the time of immersion necessary for etching the workpiece varying with the concentration and temperature of the bath.

More particularly, the powder compositions of the present invention comprise 80.0 to 95.0 percent, and preferably 85.0 to 90.0 percent, by weight of urea nitrate. Accordingly, the dry, auxiliary acidic component comprises 5.0 to

0 20.0 percent by weight, and preferably 10.0 to 15.0 percent 7 by weight, of the composition.

31,479,293 Patented Nov. 18, 1969 Although the theory of operation is not understood, it is believed that the urea nitrate provides a controlled source of nitric acid for the bath and that the urea exerts some surface activity enhancing the etching activity. At higher concentrations and elevated temperatures with certain alloys, the urea nitrate acts to substantially eliminate fuming. The auxiliary acidic component contributes to the controlled etching activity and appears to improve the surface appearance while facilitating shipment of the composition as a desirably dry composition which is readily soluble in water. It is possible that the auxiliary acidic compounds tie up certain deleterious ions which occur in the solution. It is thought that, for example, ferrous ions which are produced as a natural consequence of the etching process are tied up by the sequesting of chelating action of certain of the compounds such as the gluconic acids.

To be suitable, the auxiliary acidic component not only must be acidic, in the sense of the Lewis acid-base theory, but it must, of course, also be dry and water-soluble. Moreover, it must not be an active reducing agent and should not include a film-forming radical (i.e. phosphate and chromate radical-containing compounds are unsuitable as the second component). Accordingly, various compounds may be employed as the acidic compound such as the dry acids exemplified by boric, tartaric, maleic, citric, succinic and amalic acids. In addition to acids per se, numerous acidic salts can also be utilized exemplified by acidic tartrates and sulfates of the alkali metal, alkaline earth metals and ammonia. The bisulfates are particularly advantageous in that they aid in the control of pH in the solution. Other compounds which are particularly suitable are those which conform to the above criteria and are also chelating or sequestering agents. Exemplary of such compounds are gluconic acid and particularly the delta-lactone thereof.

In addition to the urea nitrate and the dry acidic component, as was mentioned above, other components may also be advantageously included in the powder compositions. One such component is a powdering, or anti-caking, agent which is a finely divided inert filler such as a finely divided calcium silicate. When employed, such a powdering agent is advantageously included in an amount within the range of 0.5 to 5.0 percent based on the weight of powder. The admixture of other components such as surfactants, etc., which will occur to those skilled in the art is also within the scope of the present invention.

Thus, one of the preferred compositions in accordance with the present invention is composed of about 84.6 percent by weight of urea nitrate, about 8.7 percent by weight of boric acid, about 4.75 percent by weight of ammonium acetate and about 1.95 percent by weight of powdering agent. A second preferred composition comprises a quantity of urea nitrate and Micro-Cell E similar to the above, but includes the delta-lacone of gluconic acid in lieu of the boric acid and ammonium acetate.

The etching solutions may be prepared in any suitable manner, one of the outstanding advantages of the present invention being that the dry composition need only be dissolved in water at the point of utilization to produce the etchant. Hence, the desired components are premixed in precise ratios at the point of manufacture where quality control is most effectively maintained, leaving only the most rundimentary measurements to be carried out by the ultimate operator who need not be especially skilled to achieve desirable results. The bath may be readily re plenished merely by addition of the powder formulation to again obtain a high degree of desirable etching activity.

Since the water is added by the ultimate user, the formulation is dry during transit. This not only decreases the expense of transportation by keeping the weight to a minimum, but it also facilitates packaging and handling at all stages. The working solution will be prepared in any suitable manner by dissolving the prepared powder in a suitable quantity of water. An amount of powder ranging from about 3.0 to 15.0 percent, and preferably from about 4.0 to 10.0 percent, by weight on the basis of the water has been found to yield etchants of the proper pH which provide high-quality surfaces. The quantity of powder composition which can be dissolved is dependent upon the temperature of the water so that the higher concentrations require elevated temperatures.

Although it is possible to employ baths of the present invention which have pH values near 2.0, it is preferred that the pH of the etchant be less than 1.0, and most desirably at about 0.6. Thus, while the operable range-of pH values is from about 0.2 to 2.0, the preferred range is'from 0.5 to 1.0. The pH of the solution is, of course, determined by the components of the dry powder and the concentration of powder in the solution.

As is indicated above, the present compositions and the process employing same are suitable for the etching of ferrous surfaces. The term ferrous surfaces as used herein refers to surfaces of iron and ferrous alloy articles and other metallic articles having a coating of iron or ferrous alloy produced by any means. Thus, the various steel alloys are included. It is to be pointed out, however, that the so-called nitric acid-passivated metals such as stainless steels are not well suited for etching in accordance with the present invention. Passivation is due, it is believed, to the generation of an oxide film on the surface of the metal which interferes with the etching reaction. It is hypothesized that the reaction by which such an oxide film is generated in the presence of nitrate ion is catalyzed by chromium or nickel so that passivatable metals should not be utilized in baths of the present invention.

The instant process may be effectively carried out by a relatively simple series of steps. Preliminary to the etching operation, it is usually desirable, and sometimes essential, that the surface of the article to be etched be freed of dirt, oil, grease and other contaminants which might tend to interfer with the etching process or otherwise reduce the desirability of the surface of the product obtained. Methods for such cleaning operations are well known in the art. Having cleaned and, normally, dried the workpiece, it is then immersed in a solution previously prepared from a powder formulation as discussed above, the solution having a proper pH value and being maintained at a suitable temperature. Although it is not essential, it is usually desirable to provide agitation in the etching bath or to the workpiece so that a uniformly etched product is obtained. After a short time, the article is removed from the etchant and subjected to subsequent treatments which may include rinsing and neutralization steps. Solutions of compounds such as sodium hydroxide and sodium cyanide are suitable for the neutralization step when it is employed, sodium cyanide being a particularly useful neutralizer since it also provides some corrosion protection to the etched surface. Other salts, such as those of potassium, can be similarly utilized. As an additional subsequent step, it is frequently desirable to desmut the surface of the article such as with an anodic alkaline technique. This is particularly true when plating of the surface is contemplated. The composition of the smut, which is in many cases believed to be a carbide, will depend upon the components of a particular alloy. Oxidizing washes tend to stain the etched surfaces and should, therefore, be avoided.

Exemplary of the eflicacy of the present invention are the following specific examples wherein baths were prepared by addition to water of dry powder compositions in accordance with the present invention.

Example 1 A dry powder formulation is prepared containing by weight 84.6 percent urea nitrate, 8.7 percent boric acid, 4.75 percent ammonium acetate, and 1.95 percent of finely divided calcium silicate.

Into 1.5 liter of water at ambient temperature is dissolved 90.0 grams of the above powder formulation to produce an acid bath having a pH measured at 0.6. Hull cell panels (zinc-plated steel) 2% inches by 4 inches were previously prepared by stripping the zinc with hydrochloric acid, rinsing, treating with sodium nitrite to prevent rusting, desmutting, drying and weighing them. Ten such panels were immersed in the etchant which was at ambient temperature. After immersion in the agitated bath for about two minutes, the panels were withdrawn from the solution, rinsed with water and weighed. All panels were observed to be uniformly etched with highly desirable dull-matte finishes.

A second group of ten panels was treated with the same solution in the above-described manner. They were observed to have equally well-etched surfaces.

Third and fourth batches were also sequentially etched in the same bath. The quality of etch on these panels was observed to be somewhat lower than that of the panels in the first two replicates. Therefore, an additional charge of 90.0 grams of powder was added to the bath.

Three replicates of ten panels each were subjected to etching in the thus-rejuvenated bath under conditions similar to those which were employed with previous replicates. The first group of panels subjected to the rejuvenated bath had surfaces which were equally as well etched as the original two batches of panels. Thereafter, the quality of etch began to diminish in the two subsequent replicates.

An additional charge of 90.0 grams was accordingly added, followed by two replicates of ten panels each. Thereafter, the bath was again rejuvenated with 90.0 grams of powder and three replicates of panels were etched. The solution was rejuvenated twice more with 9.0 gram charges, the solubility thereof in the solution at ambient temperature being exceeded upon the addition of the final charge.

In all cases of the first replicate after rejuvenation of the bath, the quality of the etched surfaces obtained was good. The data obtained from these tests are summarized in the table which follows, which indicates the average weight loss per panel as a result of each etching, the points of addition of rejuvenating charge and panel appearance for each group of panels. In the table, a rating of A indicates good etching and appearance. Panels rated B exhibited somewhat less etching at the center than at the edges. Those rated C were poorly etched in the center and slightly etched near the edges. The D-rated panels were classified as poorly etched.

TABLE Panel 1.5 liters of bath appearance l Solubility of solution exceeded.

Example 2 A test similar to that of Example 1 was performed. In this test, however, the dry powder was composed of 89.0 percent of urea nitrate, 1.0 percent of the powdering agent and 10.0 percent of the delta-lactone of gluconic acid. The etched panels which were obtained with the thus-modified formulation exhibited surfaces more finely and even more uniformly etched than those obtained with the formulation of Example 1.

Example 3 An etching composition was prepared as in Example 1, the powder composition containing 89.0 percent urea nitrate, 1.0 percent of powdering agent and 10.0 percent of sodium bisulfate, all by weight. Panels obtained from an etching operation, carried out in substantial accordance with the procedure of Example 1 exhibited etched surfaces comparable in quality to those obtained in that example.

The temperatures and times employed in the etching process of the present invention are generally interdependent and dependent upon the concentrations of the active agents of the solution and also upon the desired degree of etching. It is highly advantageous to operate the present process with the solution maintained at ambient temperature; i.e., 60 to 100 Fahrenheit. This is, of course, the most economical and convenient temperature, and also one at which well-etched products are obtained. A more rapid reaction can generally be effected by raising the temperature of the solution. This in itself will serve not to increase the rate of reaction, but it will also increase the solubility of the powdered components in water and thereby make possible a more concentrated and, hence, an even more active, solution. Surprisingly, the activity of baths containing the delta-lactone of gluconic acid Was found not to increase appreciably with elevated temperatures. Although higher temperatures are feasible, temperatures of about 150 Fahrenheit are generally most advantageous; temperatures in the range of 60 to 100 Fahrenheit are preferred.

The period of immersion can vary within wide limits, times of from about ten seconds to about ten minutes being feasible and of one to five minutes being preferred. While an increased degree of control is obtained when the reaction is slow, by virtue of a dilute solution and/ or a low temperature, it is often more economical and efficient to minimize the time necessary to carry out an etching cycle. In the present process, balancing of such considerations is a convenient and facile matter.

Since the baths are somewhat corrosive, the tanks and handling equipment should be fabricated from acidresistant material, such as rigid polyvinyl chloride or a glass fiber-reinforced resin, to avoid contamination. The dry powder formulations are not in themselves corrosive, but should be transported in suitable containers that will minimize the likelihood of inadvertent contact with water or excessive amounts of moisture.

As is seen from Example 1, replenishment of the baths of the present invention is a simple matter which can be effected so as to avoid the need for discarding of the used baths at frequent intervals. The limit of replenishment occurs only after multiple replenishment and buildup of ions in the bath makes further replenishment no longer feasible.

Thus, it can be seen that the present invention provides a process of etching ferrous surfaces at a controlled rate and in a manner which produces substantially uniformly etched surfaces. The self-contained dry powders provided by the invention produce, upon addition to water, baths which chemically etch ferrous surfaces in a highly effective manner. The powders are convenient and safe to ship and handle and are economical in operation. Their nature is such that the optimum proportions of essential ingredients of the baths are precontrolled and the rate of attack of metal workpieces is readily varied by varying the amount of the powder formulations added to the water.

Having thus described the invention, we claim:

1. A dry powder composition for addition to water to provide an acid bath for etching ferrous surfaces consisting essentially of 80.0 to 95.0 percent by weight of urea nitrate, 20.0 to 5.0 percent by weight of an auxiliary watersoluble dry acidic component selected from the group consisting of organic and inorganic acids and the acidic salts thereof and up to 5.0 percent of a finely divided inert filler, said dry powder composition providing a pH of about 0.2 to 2.0 when added to water in a quantity ranging from about 3.0 to 15.0 percent based on the weight of water, said acidic component excluding active reducing agents and film forming radicals.

2. The dry powder composition in accordance with claim 1 wherein said auxiliary acidic component consisting essentially of a mixture of boric acid and ammonium acetate.

3. The dry powder composition in accordance with claim 1 wherein said auxiliary acidic component additionally includes a sequestering agent of acidic nature.

4. The dry powder composition in accordance with claim 3 wherein said sequestering agent is the delta-lactone of gluconic acid.

5. In an etching process, the steps comprising dissolving in water from about 3.0 to 15.0 percent by weight, based upon the water, of a self-contained dry powder composition consisting essentially of about 80.0 to 95.0 percent by weight of urea nitrate, about 20.0 to 5.0 percent by weight of an auxiliary water-soluble dry acidic component selected from the group consisting of organic and inorganic acids and the acidic salts thereof and up to 5.0 percent of a finely divided inert filler to prepare a bath having a pH ranging from about 0.2 to 2.0, said acidic component excluding active reducing agents and film forming radicals; and immersing a workpiece having a ferrous surface in said bath maintained at a temperature from about 60 to 150 Fahrenheit for a period of time sufiicient to etch said ferrous surface substantially uniformly.

6. The process in accordance with claim 5 wherein said period of time ranges from about ten seconds to ten minutes.

7. The process in accordance with claim 5 wherein 4 to 10 parts by weight of said self-contained dry powder composition is dissolved in 100 parts by weight of water to prepare a solution having a pH of about 0.5 to 1.0, said self-contained dry powder composition consisting essentially of about to percent by weight of urea nitrate, 5 .0 to 14.5 percent by weight of an auxiliary acid component consisting essentially of boric acid and ammonium acetate, and 0.5 to 5.0 percent by weight of finely divided inert filler, said solution being maintained at ambient temperature and said period of time of immersion of said workpiece being about one to five minutes.

8. The process in accordance with claim 5 wherein 4 to 10 parts by weight of said self-contained dry powder composition is dissolved in parts by weight of water, said self-contained dry powder composition consisting essentially of 85 to 90 percent by weight of urea nitrate, 0.5 to 5.0 percent by weight of a finely divided inert filler, and 5.0 to 14.5 percent by weight of the delta-lactone of gluconic acid.

9. The process in accordance with claim 5 wherein said workpiece is removed from said bath and rinsed in an aqueous cyanide bath.

References Cited UNITED STATES PATENTS 3,171,767 3/1965 Bellinger et al. 252-79.4 XR

MAYER WEINBLATI, Primary Examiner US. Cl. X.R.