US3544390A - Phosphatizing process for iron products and products obtained thereby - Google Patents

Description

United States Patent Office 3,544,390 Patented Dec. 1, 1970 3,544,390 PHOSPHATIZING PROCESS FOR IRON PRODUCTS AND PRODUCTS OBTAINED THEREBY Edward Labib Ghali and Jacques Voeltzel, Saint-Germainen-Laye, Yvelines, France, assig'nors to Institut de Recherches de la Siderurgie Francaise, Saint-Germain-en- Laye, France No Drawing Filed Dec. 3, 1968, Ser. No. 780,893 Claims priority, application France, Dec. 4, 1967, 130,842 Int. Cl. C23f 7/10 US. Cl. 1486.15 8 Claims ABSTRACT OF THE DISCLOSURE An anti-corrosion iron phosphate coating with a superposed zinc phosphate coating is formed on iron products by applying a potential difference between the product and a counter-electrode while the product is immersed in a zinc-containing phosphatizing bath, which potential difference is discontinued after a surface solution of the iron has been effected and an iron phosphate coating has been formed followed by formation of a superposed zinc phosphate coating.

BACKGROUND OF THE INVENTION The invention relates to a phosphatizing treatment for iron or iron alloy products wherein electrochemical phenomena are used as occur through the passage of an electric current through the product.

The idea to employ electrochemical phenomena caused by the passage of an electric current in order to improve the quality of protective coatings obtained by a phosphatizing treatment is not novel in itself. It has for instance been proposed to apply the phosphatizing treatment to parts to which an alernative voltage is applied or to which a current is applied of which the passage is reversed in a sustained manner so that the parts alternately constitute during the phosphatizing treatment the cathodes and anodes of an electrolytic cell. In another proposal, the parts to be protected are first subjected to phosphatizing treatment of conventional nature by immersion only. The quality of the phosphate coating is then improved by electrolytic deposition of zinc thereon. It has also been contemplated to apply during the entire period of the phosphatizing treatment a sustained positive voltage to the products in order to increase the regularity of the crystal formation of the phosphate zinc.

SUMMARY OF THE INVENTION It is an object of the present invention to increase the corrosion resistance of the protective coatings constituted by metal phosphates which are deposited at the surface of iron or iron alloy products.

This object is accomplished by a process of forming a protective coating of a metal phosphate on iron or iron alloy products comprising the steps of immersing the product in a phosphatizing bath containing zinc in solution; applying a sustained potential difference between the product as anode and a counter-electrode as cathode and thereby exerting a superficiary dissolving action on the iron in said product so as to increase the number of iron ions at the metal product-solution interface; then, terminating the said potential difference and thereby causing a first phosphate coating to form at the surface of the product by virtue of the iron ions at first being present at said interface, the said first phosphate coating being high in iron content; a second phosphate coating forming on top of said first phosphate coating after substantial reduction of the number of iron ions through formation of said iron phosphate coating, the said second coating being high in zinc content by virtue of the zinc initially introduced in said phosphatizing bath.

The invention also embraces iron or iron alloy products formed by the process described which are characterized by a protective phosphate coating that is high in iron adhering to its metal surface and a superposed zinc phosphate coating adhering to said first coating.

DESCRIPTION OF PREFERRED EMBODIMENTS From the above summary of the invention, it will be understood that the process of the invention comprises providing, at the metal-solution interface for conditions that favor the formation of a subcoating of iron phosphate on top of which a second coating of zinc phosphate is being deposited as soon as the iron contents of the solution is substantially reduced.

It is preferred in the embodiment of the invention to adjust the potential difference in a manner that the electrochemical reaction which arises at the surface of the product embraces only the reaction involving the dissolving of iron at the surface of the product. Preferably for this purpose the potential difference is maintained during a time suflicient to permit the passage of an amount of current through the electrolytic cell formed which is between 2 and 5 coulombs per cm. of surface to be treated.

The products of the invention may be unfinished, halffinished or finished and consist of iron or iron-base alloys. They are characterized by a protective corrosion-resistant coating formed by the phosphatizing process of the invention and comprising a protective first coating constituted by a phosphate which is high in iron and adheres directly to the metal and a second superposed coating which adheres to the first coating and is constituted by a zinc phosphate.

In the course of tests, the applicants have found that phosphate coatings offer a resistance against corrosion which is clearly higher when the zinc phosphate coating is formed on top of a subcoating of iron phosphate.

In order to clearly explain the invention, it will be first briefly recalled that the general principle of the phosphatizing of metals comprises a simple immersion in a phosphate bath. The mechanism of the phosphatizing treatment on one hand is an electrochemical process which arises between the metal and the bath and in the course of which the pH of the solution develops in the neighborhood of the metal surface. This stage is then followed by a second purely chemical stage which results in the precipitation of the zinc phosphate on the treated surface after the pH of the solution has reached the pH necessary for the phosphate precipitation. Formation of the zinc phosphate coating thus depends narrowly on the uncontrolled electrochemical reaction which takes place between certain preferred points of the metal. This electrochemical reaction is behind the imbalance of the pH in the solution and consequently the precipitation reaction of the phosphate.

The thus-obtained protection is due to the presence of a coating which is rich in Zinc phosphate at the surface of the metal.

In order to form a subcoating which is rich in iron phosphate and which in the present invention results in an improvement of the protection, it is obviously not sufficient to act only on the composition of the phosphatizing bath. Actually, if one were content to increase the contents in iron of the bath one would cause not only the formation of an iron phosphate coating but also the formation of a mixed iron and zinc phosphate coating which would not accomplish the desired protection.

It will be understood, on the other hand, that with the application of a positive voltage by means of a counterelectrode to the product to be treated, a superficiary anodic solution of the metal is induced which permits a localized enrichment of the solution in iron ions at the metal-solution interface. It will also be understood that the applied voltage must be adjusted in order that no release of gases occurs at the surface of the metal and in order that the only chemical reaction which is induced is the iron dissolving reaction.

While the product is maintained under current, there is produced through the electrochemical reactions a rise in hydrogen ions at a localized place towards a hydrogen ion concentration which would be favorable to the precipitation of phosphates. If the voltage is then cut off, the hydrogen ion conditions and the conditions for the concentration of iron are favorable to the precipitation of an ion phosphate at the surface of the metal. Once the concentration of iron in the solution and in particular at the metal product-solution interface is sufficiently reduced through the formation of iron phosphate, the zinc phosphate precipitation will set in and will continue until a complete coating of the surface with a layer of zinc phosphate is obtained.

It is a matter of course that the products must be maintained without movement in the bath during the entire duration of the treatment. It should also be noted that the pH necessary for the precipitation of the zinc phosphate is more rapidly reached if the concentration of the metal ions increases at the metal product-solution interface, and also the phasphate coatings which are formed in the course of this treatment will be more uniform and more regular than in case of a phosphatizing treatment by immersion only.

The amount of electricity which must circulate in the electrolytic circuit of which the products form the anodes is determined as a function of the initial surface condition of the metal of the products. Thus, it has been found that when the surface is irregular, oxidized or covered with scale, it may be necessary to effect a more strenuous anodic dissolution in order to obtain a surface condition which is favorable to the formation of regular uniform phosphate coatings i The invention and the advantages arising therefrom will be easier understood in the light of the following example and test results comparing products made by the inventions with products made by conventional processes.

EXAMPLE Phosphatizing tests were carried out with specimens formed in a steel, that had a surface condition as rolled with only a light finishing pass. The criterion of the coarseness, termed C.L.A. corresponded to 1.2. The phosphatizing treatment had been effected in these tests in the conventional phosphatizing bath with a zinc basis which had been accelerated by the addition of nitrates (7 g./ liter (NO Zn) and which bath contained 1.1% of P and 0.7% of ZnO. The ratio of free acidity and total acidity in this bath was of the order of 1:4.

The specimens were immersed-in the bath while the bath was maintained at a temperature of 95 C. and were anodically polarized by means of an external source of current with a voltage of about plus 0.2 volt, measured by relation to a standard satured calomel electrode. The current density under the conditions of the test was about 30 to 40 ma./cm. The current application was continued for about one minute, which corresponded to the passage of an amount of current between 1.8 and 2.4 coulombs per cm.

It should be understood that the conditions thus created in this test amounted to an input of current which would start solely the iron-dissolving reaction. The current was then interrupted and the phosphates were permitted to precipitate. There was thus obtained a protective coating constituted by a underlying layer of a phosphate which was high in iron and which latter coating was covered by a closely adhering coating of zinc phosphate. A micro- 4 graphic examination of a section through this product revealed that this was indeed the structure produced.

In another test, the potential of a specimen during the deposition of the phosphate was measured, and at the same time the potential of a specimen was measured which was immersed in an identical bath but without the polarization of the present invention. It was found that the potential of the specimen which had received a preceding polarization was stabilized in 15 to 20 minutes, while the potential of the control specimen was not stabilized until about 40 minutes later. This difference between the periods of stabilization of the potential shows that the precipitation of the phosphates and accordingly the speed of formation of the protective coatings is substantially accelerated when the specimens are subjected to a preceding polarization step.

The thickness of the protective coatings thus obtained with the process of the invention was about 15 while in the conventional method the coatings had a thickness between 20 and 25p.

In order to determine the quality of the protective coatings thus obtained, specimens which had been treated by the process of the invention and control specimens which had been phosphatized by simple immersion were subjected to exhaustive corrosion tests. It should be emphasized that these tests were carried out without subjecting the specimens to any supplemental protective coatings such as a rinsing and passivation with chromic acid, oil impregnation or application of protective paints.

ELECTROCHEMICAL CORROSION TESTS This test comprised a measurement of the potential at which cracking of the phosphate coating would occur using a specimen which had been immersed in an electrolyte of sodium phosphate and chlorine ions. The cracking potential measurement permitted to determine the resistance to the corrosion of the phosphate coating. The electrolyte used for this test contained 0.1 M of Na P-O H, l2 H 0 and 0.1 M of NaCl.

It was assumed that the cracking potential E, is obtained when a current of 300 ,ua./cm. passes through the specimen. As a result of the tests, it was found that a phosphatized steel which was treated by simple immersion had a cracking potential around +600 mv. measured with reference to a standard saturated colonel electrode (E.C.S.-saturated colomel electrode). On the other hand, a specimen phosphatized by the process of the invention had a cracking potential E which was above 1,200 mv./ECS. This clearly indicates a much higher corrosion resistance.

If, under the same conditions, the potential of a specimen was maintained at a constant potential of +600 mv./ECS, the amount of current passing through the specimen in a test period of 30 minutes was found to be 3 coulombs for a conventionally phosphated steel and 0.6 coulomb, that is five times less, for specimens which were phosphatized with a preceding polarization. This confirms the results earlier given.

TESTS REGARDING ATMOSPHERIC CORROSION TESTS WITH CORROSION IN OCEAN WATER This is a very'severe corrosion test which comprises immersing the specimens in ocean water and which permits a quick appraisal of the degree of protection.

With such immersion, it was found in the first few hours that the control specimens showed deep pitting while, on the other hand, the specimens which were phosphatized with a preceding polarization remained unchanged during twenty hours.

A definite value can be obtained for the corrosion by reference to the European Standard (Echelle Europeenne). By this standard, specimens are compared with photographic standards of corroded specimens. This is called the European scale of corrosion for anti-corrosion paints, (Echelle Europeenne du degre denrouillement pour peintures antirouilles; Le comite de corrosion de lAcademie des Sciences Techniques, Stockholm 1961).

Referring to this scale, it was found that the degree of corrosion of the specimens which had been phosphatized by simple immersion was equal to the reference mark or symbol R while the standard to which the specimens phosphatized with a preceding polarization corresponded was R The total of these tests thus shows in unquestionable manner that the corrosion resistance of portective coatings obtained by the process of the invention is clearly higher and that these coatings impart to the treated products an increased protection.

The process of the invention has numerous advantages. For instance, among others, there is the advantage that the preliminary polarization treatment 'by its superficiary dissolving of the metal of the treated product causes a pickling which extends to the entire surface and thus permits to treat the product irrespective of scale or previous rusting directly by the method of the invention. Such immediate treatment would not be possible with the conventional methods which would require a preparation of the surfaces intended to be phosphatized because zones which are not sutficiently clean from contamination points of lower resistance and thus of lowered protection against corrosion. In addition, it is well known that a preceding chemical pickling treatment results in the formation of phosphate coatings having a coarse grain and that the protective value of these coatings is rather mediocre.

Another advantage of the anodic polarization is the activation of the surface to be protected. As a result the active centers where the precipitation reaction is started oil? are increased and, likewise, a much finer crystallization and more regular crystallization is obtained in the phosphate coatings. The corrosion resistance is thus appreciably increased. In addition, the coating which has a finely crystalled structure has a much better adherence for outer coatings like pains and has a rather weak absorptive power for coloring agents.

It should also be noted that the presence of metallic ions at the metal-solution interface permits attaining rapidly the pH necessary for the precipitation of the phosphates and thus increases the speed of formation of the coatings.

In addition, the improved behavior against corrosion in the coatings of the invention is found to be obtained irrespective what the composition may be of the phosphatizing bath. The process of the invention also permits to phosphatize metals which have undergone a surface passivation treatment, particularly alloy steels and light alloys.

It will also be understood that with the treatment of light alloys by the process of the invention the nature of the phosphatizing coatings may be different since the iron may be replaced by another metal. This, however, is

6 also embraced by the general concept of the invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A process of forming a protective coating of a metal phosphate on iron or iron alloy products comprising the steps of immersing the product in a phosphatizing bath containing zinc in solution; applying a sustained potential ditference between the product as anode and a counterelectrode as cathode and thereby exerting a superficiary dissolving action on the iron in said product so as to increase the number of iron ions at the metal productsolution interface; then terminating the said potential difference and thereby causing a first phosphate coating to form at the surface of the product by virtue of the iron ions initially being present at said interface, the said first phosphate coating being high in iron content; a second phosphate coating forming on top of said first phosphate coating after substantial reduction of the number of iron ions due toformation of said iron phosphate coating, the said second coating being high in zinc content by virtue of the zinc initially introduced in said phosphatizing bath.

2. The process of claim 1, wherein the potential difference is adjusted to permit solely the said superficiary solution of the iron to take place.

3. The process of claim 2, wherein the potential difference is applied for a time sufficient to cause the passage of between about 1.8 and 5 coulomb per cm. of surface to be treated.

4. The process of claim 1, wherein the application of the potential difference is adjusted to inhibit the formation of gaseous phenomena around the product.

5. The process of claim 1, wherein the phosphatizing bath contains 1.1% of P 0 and 0.7% of ZnO and 7 g./liter (NO Zn.

6. The process of claim 1, wherein the potential difference applied is sufficient to cause the passage of a current of between 1.8 and 2.4 coulomb per om.

7. A finished, semi-finished or unfinished article consisting essentially of iron and an iron-base alloy and having a protective phosphate coating that is high in iron adhering to its metal surface and having a superposed zinc phosphate coating adhering to said first coating, the said article being produced by the process of claim -1.

8. The article of claim 7, wherein the said coatings have a thickness of 15,.

References Cited UNITED STATES PATENTS 870,937 11/1907 Coslett 148-615 X 1,007,069 10/1911 Coslett 148-6.15 X 1,867,527 7/1932. Dunn 20434 2,097,211 10/ 1937 Davies.

2,132,438 10/1938 Romig 204-1 2,366,477 I/ 1945 Bayley. 2,590,927 4/ 1952 Brandt et al. 204150 RALPH S. KENDALL, Primary Examiner US. Cl. X.R.