IL93672A - Method for coating metals with metalloids - Google Patents

Description

REF 1486/90 93672/2 METHOD FOR COATING METALS WITH METALLOIDS 1486/90 -1- The present invention relates to metalloid coatings and to methods for obtaining them. More particularly, the invention refers to metalloid coatings containing arsene or arsene and sulfur, which are useful to impart a variety of properties to coated surfaces.

The coating, e.g. of metals, is a technique which is well known in the art. Coatings have a very large number of applications, including -but not limited to - protection against environmental attack, such as corrosion, erosion, corrosion-erosion, oxidation or cavitation, or decorative coating, or coating designed to impart specific electric or mechanical properties to the surface.

Coating can be achieved by a variety of methods, such as electrolytic coating, coating from the gas phase, such as CVD (chemical vapor deposition), electroless, hot dipping, PVD (Physical Vapor Deposition), sputtering, plasma and/or arc spraying.

The art of coating is constantly searching for better, easier and non-expensive ways to treat surfaces employed in different industrial applications. It is an object of the present invention to provide metalloid coatings, which can be employed to produce a variety of coatings for a variety of industrial applications. 1486/90 -2- It is another object of the present invention to provide a method which permits to obtain an effective protective coating against environmental degradation of metals and alloys.

The term "metalloid" as used herein, is meant to indicate a solid having the appearance of a metal but which does not conduct electricity. Other examples of metalloids are Si, Ge and Te. Some metalloids can behave as semiconductors.

The method for providing a metalloid coating according to the invention, comprises contacting the surface to be coated with a Zn,M/X2 brine, wherein M is Ca, Na or K and X is Br or CI, and mixtures of different M and X, containing As+3 and optionally NaCNS. In practice, this is done by immersing the metal to be coated into the brine, which brine is kept at the desired temperature.

According to a preferred embodiment of the invention, the density of the Zn,M/X2 brine is up to 21 ppg (pounds per gallon). As+3 may be provided, e.g., in the form of AS2O3, NaAs02, AsBr3 or ASCI3, or organic arsenates, although different suitable arsene compounds will be recognized by the skilled chemist.

A parameter which is of paramount importance in order to carry out the invention is the diluted pH of the brine. This parameter (indicated by pHio) is the pH of a solution resulting from a ten-fold dilution of the brine with water, and should not be higher than about 6.0, 1486/90 -3- regardless of the combinations of metal atoms M and of halogen atoms X which constitute a specific brine.

It should be noted that the coating method of the invention substantially differs from such methods as the electrolytic deposition. In the process of the invention a compound is employed as the starting material for the coating process, which compound undergoes a chemical change during the coating process, and may even result in an elemental coating. As will be apparent to a person skilled in the art, a coating process of this type is temperature-dependent, and employing the appropriate temperature is important to obtain acceptable coating rates. The inventors have found that appreciable coating with the brines of the invention begins when the temperature of the brine reaches about 80°C. Furthermore, for most metals, the preferable coating temperature will be comprised between 80°C and about 200°C.

As will be further understood by the skilled person, overcoating, or coating with a coating rate which is too high, may result in coatings which are too thick, or which do not adhere effectively to the coated surface. The skilled person will be able to ascertain which specific conditions are preferable for a specific surface to be coated. However, the abovementioned temperature, range of 80-200°C, as well as coating times of about 1-3 hours, embrace most of the convenient operating conditions for metals. In particular, a coating temperature 1486/90 -4- of about 80°C is preferred when coating brass, a temperature of about 90° when coating copper, and a temperature of about 100°C, when coating 'carbon steel. Metals such as carbon steel and copper, if treated at the appropriate temperature, do not overcoat. Brass, on the other hand, may be overcoated at a temperature of about 80°C , resulting in an extremely thick and nonadhesive coating, which peels off. Care, therefore, should be taken not to exceed the desired thickness of coating. A useful thickness of coating, which results in an effective coating which protects the coated metal against atmospheric corrosion attack for long periods of time, can be obtained when the coating is such that a weight addition of about 2 x 10"4 gr/cm^ of a metal is obtained. As the skilled engineer will appreciate, appropriate coating conditions and final coating weight or thickness can be determined by simple laboratory measurements.

The amount of AS2O3 and NaCNS (if present) in the brine shall preferably be about 1000 ppm and about 2000 ppm, respectively. The lowest permissible concentration of As+3, however, in the case that a metal is coated, should be about 100 ppm. Below this value, metal corrosion in the brine, which is a highly corrosive environment, will take place. It is therefore important to maintain at least this limiting concentration in the brine throughout the deposition process.

NaCNS accelerates the coating process and promotes the coating of the surface, and therefore may be conveniently employed in the 1486/90 -5- process of the invention, although its presence is normally not necessary. Presence of NaCNS - or, in general, of any other material which liberates CNS" ions in the brine - is necessary, however, when it is desired to obtain coatings which embody sulfur as one of the components. EDS (energy dispersion spectroscopy) analysis with a SEM will easily show the composition of the coating. On the other hand, care should be taken not to employ too high NaCNS concentrations, which may result in a homogeneous reaction in the brine, with resulting precipitation therefrom.

The metalloid coatings obtained by the method of the invention, as well as the metals coated with it, are novel, and as such also form part of the present invention. Also encompassed by the invention are the coating media comprising Zn,M/X2 brines and As+3, optionally containing NaCNS.

The above advantages and characteristics of the invention will be further illustrated by the following examples.

Example 1 A Zn,Ca/Br2 brine having a density of 19.2 ppg was employed as a deposition medium, and it contained 1000 ppm AS2O3 and 2000 ppm NaCNS. Six identical samples (CS 1020, having dimensions 2.5 x 2.5 x 1 mm) were tested with this brine, as follows. Two tests were run at 1486/90 -6- 80°C, the first one during 1 hour and the second during 3 hours. Three tests were run at 90°C, for 1 , 3 and 5 hours, and one test at 100°C, for 1 hour. At 80°C both samples resulted in a homogeneous coating with good adhesion properties and having a gray metallic appearance. The weight addition by the coating was not measurable. This coating was effective as a protection for carbon steel for at least 8 • months at ambient conditions. At 90°C a homogeneous and adhesive gray coating of metallic appearance was obtained, with a weight addition of 2.2 x 10"4 gr/cm^, which was almost identical for all samples at this temperature. After 1 month at ambient conditions the coating started peeling off. The results at 100°C were similar.

EDS analysis showed that the coating contained arsene and sulfur.

Example 2 Operating as in Example 1, brass specimens of the same dimensions were coated. At 80°C a gray adhesive homogeneous coating was obtained, with no measurable weight addition. This coating was stable for several months at ambient conditions. At 90°C the weight addition was 5.98 x 10"4 gr/cm^ after 1 hour, 7.75 x 10"4 gr/cm2 after 3 hours, and 9.43 x 10"^ gr/cm^ after 5 hours. The coating obtained after 3 and after 5 hours started to peel off after 1 month at ambient conditions, while the coating obtained after 1 hour was stable for several months. 1486/90 -7- At 100°C the coating obtained was nonadhesive, and it started to peel off after a short exposition to ambient conditions.

Example 3 A specimen of 1020 carbon steel (2.5 x 1.5 x 1 mm) was coated under the same conditions as in the preceding examples, but at 100°C. The coating obtained was adhesive and effectively protected carbon steel from corrosion attack for at least 8 months.

Example 4 Example 1 was repeated, but using AsBr3 (prepared by reacting stoichiometric amounts of AS2O3 and HBr) as the source of As+3, instead of AS2O3. Results comparable to those of Example 1 were obtained.

Example 5 Example 1 was repeated, but using NaAs02 (obtained by preparing a 30% aqueous solution of equivalent amounts of AS2O3 and NaOH) as the source of As+3, instead of AS2O3. The results obtained were as in Example 1. 1486/90 -8- The above examples have been given for the purpose of illustration and should not be considered to be limitative. Many variations can be effected and the conditions, coating media, and temperatures of the method of the invention, as well as different methods can be coated thereby, without exceeding its scope. 1 3690 93672/2 -9-

Claims (14)

1. . A method for coating' a surface with a metalloid coating, comprising immersing the surface to be coated in a Ζη,Μ/¾ brine containing As 3 and, optionally, NaC S , wherein -. M is selected from among Ca, Na, K and their mixtures; and X is Br or CI or their mixtures; and the Zn,M/X2 brine has a density between about 14.2 to 21.0 ppg.

2. The method of claim 1, wherein the temperature of the brine is kept at about 80°C to about 200°C, for a period of time of at least about one hour.

3. · The method of claim 2, wherein the brine contains at least about 1000 ppm As+3.

4. The method of any one of claims 1 and 2, wherein As+3 is provided by adding to the brine a compound selected from AS2O 3 , NaAs02 > AsB 3, ASCI3 or organic arsenates or their mixtures.

5. The method of claim 4, wherein the brine is essentially a Zn,Ca/Br2 brine. 148^90 93 672 / 2 -10-

6. The method of any one of claims 1 to 5, wherein the surface to be coated is a metallic surface.

7. The method of claim 6, wherein the metal to be coated is brass and the temperature is about 80°C.

8. The method of claim 6, wherein the metal to be coated is copper and the temperature is about 90°C.

9. The method of claim 6, wherein the metal to be coated is carbon steel and the temperature is about 100°-200°C.

10. A surface coated with a metalloid coating, whenever obtained by the method of any one of claims 1 to 9.

11. A surface coated with a metalloid coating containing arsene and sulfur, essentially as described.

12. A coating medium comprising a Zn,M/X2 brine and As+3 as defined according to any one of claims 1 to 9.

13. The medium of claim 12, having a density of about 14.2 to about 21.0 ppg. -11-

14. The medium of claim 13, comprising about 1000-2000 ppm of each of As+3 and CNS". L UZZATTO & LUZZATTO