EP0171817A2

EP0171817A2 - Composition and process for electrodepositing a Zn or Zn/Si/P coating on metal substrates

Info

Publication number: EP0171817A2
Application number: EP85110271A
Authority: EP
Inventors: Yu-Ling Teng; Charles Mccoy; Francis Defalco; Richard Mayernick; Chong Tan Liu
Original assignee: Kollmorgen Technologies Corp; Kollmorgen Corp
Current assignee: Kollmorgen Corp
Priority date: 1984-08-16
Filing date: 1985-08-16
Publication date: 1986-02-19
Also published as: AU4576685A; CA1249790A; IL75974A0; EP0171817A3

Abstract

The present invention provides an aqueous composition and process for electrodepositing a layer of a ductile, strongly adhering, adsorptive and/or absorptive zinc coating or zinc coating comprising silicon and phosphorous on a metal substrate. The electrodeposition composition is prepared by reacting metallic silicon and zinc with phosphoric acid and an alkali metal hydroxide in the ratio of between 0,4 and 1,3 moles of alkali metal hydroxide per mole of phosphorus acid, and adjusting the solution to a pH of 1 or higher after completion of the reaction. The coating is deposited on the metal substrate by electrodeposition and comprises about 70 to 99,5% by weight of zinc, and about 0,1 to 10% by weight of silicon, and about 0,5 to 20% by weight of phosphorus.

The resulting Zn/Si/P coating improves the resistance of the metal substrate to corrosion, wear, galling, stress corrosion cracking and cracking during mechanical forming operations. While essentially all metals of industrial importance may be coated, this process is especially important for ferrous metals, steels, stainless steels, copper, aluminium, chromium and titanium.

Description

The present invention relates to an aqueous composition and process for electrodepositing a layer of a ductile, strongly adhering, adsorptive and/or absorptive zinc coating or zinc coating containing silicon and phosphorus on a metal substrate to improve wear resistance, protect against galling, and to improve resistance of the metal- against-, corros-ion and stress corrosion cracking.
The ductile Zn or Zn/Si/P coating of the present invention is resistant to cracking during subsequent mechanical forming operations and the metal articlestreated in accordance with the invention, including formed areas, are surprisingly highly resistant to corrosion, stress corrosion cracking, wear and galling.
The coating may also be subjected to further treatments, including the application of functional or decorative coatings or paintings.
While essentially all metals of industrial importance may be plated, this process is especially important for ferrous metals, steels, stainless steels, copper, nickel, chromium, aluminum and titanium and alloys of these metals. Many attempts have been made in the past to improve the surface properties of metals in order to widen their applications. The most commonly used methods include barrier coating and cathodic protection, i.e., providing a "sacrificial" metal coating which is anodic to the metal substrate. Zinc has been widely used for this purpose, and may be applied in the form of a zinc rich paint or by providing a layer of zinc metal, with the latter being the most commonly used method to improve corrosion resistance of ferrous metals and steels. Processes for forming such zinc metal layers include hot-dip, hot-spray and electrodeposition.
However, the zinc rich paints tend to contain non-conductive binders which coat the zinc particles and interfere with or prevent the "sacrificial" galvanic reaction from proceeding. Galvanization by a hot-dip or hot-spray process consumes large quantities of energy and is rather costly. Moreover, it results in brittle, macro-crystalline zinc coatings difficult to form and which will not accept paint except after their surfaces have been treated by chromate conversion or phosphating.
The degree of protection provided by immersing the substrate in a bath of molten zinc is highly dependent on the bath temperature, immersion time, rate of cooling or subsequent reheating. Moreover, the strength and impact toughness of the substrate is generally reduced, and the zinc coating tends to craze or crack if the hot-dip galvanized substrate is subsequently formed by sharp bending.
It is also known that zinc may be electroplated from an acid solution at pH of about 3 to 4,5. (Modern Electroplating, 3rd Edition, John Wiley & Sons, New York (1974), pp. 442-460). However, such zinc platings have not been found to produce satisfactory and commercially interesting results lacking high ductility and good adhesion to difficult to plate metals. Furthermore, such zinc platings have proven to be unsatisfactory for protection against the corrosive effects of severe industrial environment. Without being bound by theory, it is believed that this is because these previous deposits are interfered with by the presence of detrimental inclusions in the crystal lattice, thus decreasing their ductility and adhesion characteristics. Whereas, the zinc electrodeposited according to this invention does not have these detrimental inclusions in the crystal lattice. Based on experiments, none of the known acid zinc electroplating processes provide ductile deposits which can be bent or deformed and still provide sufficient corrosion resistance even when chromated.
It is also known that zinc coatings can be further treated. Chromate conversion coating significantly improves the corrosion resistance of metal substrates which are galvanized or electroplated with zinc. Phosphatizing is used to improve adhesion of paints to galvanized surfaces. However, the chromate conversion as well as phosphatizing processes also result in brittle coatings.
Besides providing corrosion resistance, metals have been coated with cadmium to provide lubricity, solderability, and compatible electrical conductivity. Cadmium coated steels are used in the aerospace and automotive industries, e.g., for aerospace fasteners, disc-brake components, radiator hose fittings, door latches and torsion- bar bolts. However, because of the toxicity of cadmium and the potential health hazards resulting therefrom, there are stringent regulations controlling the use of cadmium limiting its use and increasing cost.
Metals are also surface treated to provide galling and wear resistance and lubricity. For example, metals have been coated with cadmium, phosphatized, galvanized or provided with Cu or Zn coatings to provide the desired properties.
Other methods of providing galling and wear resistance and improved lubricity include oxalate conversion coating, coating with fluorocarbon polymers and coating with electrolessly formed copper, nickel or hard chromium deposits. Oxalate conversion does not provide corrosion resistance. The usable temperature range of fluorocarbon polymers is very limited and there tends to be excessive flow under stress. Therefore, it is not suitable for applications where the metal substrate is to be subjected to high temperatures and stress.
Copper coatings induce corrosion of ferrous metal substrates. Nickel and hard chromium coatings tend to breakdown under high stress loads; they provide wear resistance but poor galling resistance and lubricity.
It is important that the coating or plating on threaded parts be very thin so that it does not interfere with the thread make up. It is also important that the coating or plating adhere to the base metal, provide a low coefficient of friction and protect against corrosive attack.
Methods commonly used today suffer from many disadvantages. As an example, an ASTM B-7 bolt should have a maximum tensile strength of 80 000 lbs. and a usable temperature range of subzero to 600°C. Galvanizing can only provide a coating with a tensile strength of about 40 000 lbs. The deposited layer is thick and thus requires special nut designs. Further, the smallest breakdown in the coating provides sites for accelerated corrosion whereby the nuts and bolts fuse together.
Prior art zinc electroplating provides poor tensile strength and low resistance to corrosion.
Cadmium electroplating may provide tensile strengths of 70 000 lbs. and a low coefficient of friction. However, it only provides moderate protection against corrosion and any breakdown in the coating accelerates corrosive attack. Phosphating with zinc oxide in phosphoric acid has also been used to improve adhesion of paints to difficult to coat metals. (US-PS 2,743,205). However, phosphating makes the surface very brittle, so that the treated mezal article cannot be formed without losing corrosion resistance. Moreover, large quantities of sludge are produced in the process and must be properly disposed.
Furthermore, phosphating by itself does not provide satisfactory protection against corrosion.
Fluorocarbon polymer coatings do provide good corrosion resistance and a low coefficient of friction. However, the usable temperature range is very limited and fluorocarbon polymers tend to flowexcessively under stress.
Metal substrates may also be "siliconized" or implanted with phosphorus to improve their wear resistance. However. these processes are difficult to control, expensive and are impractical.
Therefore, there is a critical need for a coating that is resistant to corrosion and will prevent galling, particularly in the oil exploration field.
Galling is a problem encountered frequently in oil and gas exploration. High temperatures and pressures coupled with agressively corrosive environments such as hydrogen sulfide, hot chlorides, carbon dioxide gas have compounded the problems of oil exploration and expensive metal alloys have been developed to meet the challenge. Since it is not uncommon to drill wells of 15 000 feet or deeper, drilling pipes must be threaded together. Further, tool joints, pulsation dampers, blow out preventers, valves, electric measuring devices are used and all of these have threaded connections. The galling of threaded connections has been a severe problem in oil and gas exploration causing increased expenses in time and money. Attempts have been made to overcome this problem with specially designed pipe threads and coatings such as electroless nickel, hard chromium deposits. However, none of the coatings provide a satisfactory solution.
Another serious problem recently encountered is stress corrosion cracking of high strength alloys. These high strength alloys are used in satellites, space vehicles, airplanes, cars, bridges and nuclear reactors and are subjected to highly stressful environment. No viable solution was available to solve the stress cracking of high strength alloys.
It is an object of this invention to provide ductile, strongly adhering zinc and Zn/Si/P electroplated coatings on metallic surfaces which are highly ductile, dull and non-lustrous having high resistance against corrosion, stress cracking, wear and galling.
It is another object of the invention to provide a coating which is mechanically formable and provides an excellent base for barrier-coatings including paints, adhesives, lubricants and electro-overplates.
It is a further object to provide a method for providing electroplating solutions suitable for forming said coatings.
It is still another object of this invention to provide a simple electroplating process for depositing said coatings.
A still further object of the present invention is to provide metallic articles, including articles made of metals known to be hard to provide with coatings, with securely adhering coating layers which can successfully be provided with additional coatings by electroplating, chromate conversion treatment, phosphating and painting.
Applicant has found that, surprisingly, these and other objectives can be achieved by the process of the present invention for forming^astrongly adhering, electroplated coating on metallic articles, said coating comprising zinc, characterized in that said process comprises preparing an electroplating solution comprising 5 to 90 g/1 zinc ions and an effective amount of an agent suitable to maintain the solution at a pH selected in the range of 1 or greater, and further comprising a conductivity salt in an amount from 0 to 4 moles/I; and immersing a cleaned metallic article in said solution; and electroplating with the metallic articles as the cathode and at a current density of at least 0,5 A/dm² for a period of at least 1 second, thus forming a dull, highly ductile, adsorptive and/or absorptive coating which is resistant to corrosion, stress corrosion cracking, wear, galling and cracking during mechanical forming operations.
Applicant further found that improved wear and antigalling properties can be achieved with the use of electroplating solutions further comprising silicon.
Such solutions can be made by either preparing a silicon comprising solution and adding a zinc comprising solution, and mixing both solutions such that the ratio of zinc to silicon in the electroless plating solution is in the range between 8:1 and 30:1, and adjusting the pH of the solution; or by reacting zinc and silicon simultaneously. The thus prepared aqueous solution is viscous.
The coating formed by the process of this invention should be at least 0,01 µm, preferably between 3 and up to 5 µm thick.
Scanning electron microscope studies of a layer of about 15 µm show that the ductile and adhesive electroplated coating of the invention comprises hexagonal, platelet-like crystals ranging in size from about 4 microns to about 8 µm along their . longest axis. The platelet-like crystals are stacked face to face against each other. The thus produced coating is very adsorptive and absorptive and receptive to adherent paint, lacquer or chromate deposits; it allows the paint, lacquer or chromate deposits to penetrate deeply into the zinc coating, thereby promoting very strong adhesion to the metal substrate.

Fig. 1 is a scanning electron microscope picture of a layer of the ductile and adhesive electroplated zinc coating according to the invention at 4000X.
Fig. 2 is a scanning electron microscope picture of a bent layer of the ductile and adhesive electroplated zinc coating according to the invention at 50X.
Figs. 3 to 6 are scanning electron microscope pictures at 50X of zinc coatings on steel substrates formed or bent after plating and electroplated from a commercial acid chloride process (Fig. 3);
- from a commercial cyanide process (Fig. 4);
- from a commercial alkaline process (Fig. 5); and
- from a commercial hot-dip galvanization process (Fig. 6).
Fig. 7 and 8 are scanning electron microscope pictures of zinc coatings electroplated from a commercial acid chloride process (Fig. 7) ; and
from a commercial cyanide process (Fig. 8).
Fig. 9 is a typical EDX spectrum of the surface of the steel substrate after electrodeposition using the process of claim 2. The spectrum shows the presence of zinc, silicon and phosphorus in the surface layer of the steel.
Fig. 10 is a scanning electron microscope picture of the surface of the Zn/Si/P coating.
Fig. 11 is a graph plotting torque against loss of weight of the block in milligrams. This shows the degree of wear of the objects tested. The straight line shows the rate of wear of a coated block against an uncoated ring. The curve shows the rate of wear of an uncoated block against an uncoated ring.
Fig. 12 is a graph plotting degree of stress versus time- to-failure in hours for coated and uncoated casing materials after these have been subjected to various stress levels. The upper curve is the result obtained for a coated casing, the lower curve is the result obtained for an uncoated casing.
Fig. 13 is a graph comparing the corrosion rate (mpy = mils per year) of coated and uncoated metal specimens made of AISI 41.0, 9Cr-1MO and AISI 4130, steels.

The aqueous electroplating solution of this invention may be prepared by dissolving zinc in the form of zinc metal or zinc salts in concentrated phosphoric acid. The zinc salts may be selected from the group comprising zinc acetate, zinc carbonate, zinc oxide, zinc chloride, zinc sulfate, zinc sulfamate and zinc phosphate. The solution may be produced in concentrated form and diluted with water to provide a solution containing about 5 g/1 to about 90 g/l of zinc ions and about 40 g/l to about 300 g/1 of phosphate ions, preferably about 10 g/1 to about 60 g/l of zinc ions and about 100 g/l to about 250 g/l of phosphate ions.
The pH of the solution should be in the range of about 1 to about 3,5, preferably below 2,5, and most preferably below 2,0. The pH may be adjusted by using concentrated acids such as hydrochloric acid, phosphoric acid, or sulfuric acid and strong base such as sodium, potassium, lithium hydroxide or ammonium hydroxide. It is to be noted that when the zinc ionconcen.tration is low, i.e., in the range of about 5 g/l to about 25 g/l, the pH should be in the range of about 2,5 to 3,5; when the zinc ion concentration is high, in the range of about 30 to about 90 g/l, the pH should be about 1,5 to 2,5.
It is believed that the presence of an agent suitable for maintaining the pH at the selected value within the range of 1 to 3,5 so that the pH does not change significantly during the electrodeposition process, is important for achieving a uniform and even layer of dull, ductile coating-having the desired properties. Suitable agents include phosphoric acid, orthophosphoric acid, pyrophosphoric acid, chloroacetic acid, dichloroacetic acid, bromoacetic acid, other strong acids and their salts. The preferred agent is orthophosphoric acid and dihydrogen orthophosphoric salts.
In the electrodeposition process insoluble anodes, lead or precious metal coated titanium (_DS_A ^RTM anode) as well as soluble anodes, e.g., zinc metal, may be used.
It has been found that the addition of conductive salts containing anions such as chloride, sulfate and fluoroborate ions is beneficial to the plating operation.and decreases the voltage required for the electroplating process. However, when an amount greater than 50 g/1 of chloride ion is added to the electroplating solution, only soluble anodes, e.g., zinc metal, may be used to avoid the evolution of significant amounts of chloride gas.When sulfate or fluoroborate anions are used to increase solution conductivity, insoluble anodes may also be used. The ratio of the area of the anode to the cathode is preferably about 1:1 or higher. The anode and cathode are preferably placed about 2,5 cm to 20 cm apart, most preferably 5 cm apart. The current density is about 0,5 A/dm² to about 60 A/dm², preferably about 5 A/dm² to about 40 A/dm2.
Electrodeposition from a solution according to the present invention shows a cathodic efficiency of about 75% to 90%. At an optimum current density of 30 A/dm², a layer of about 6 nm is deposited on a metal substrate in about 1 minute.
Depleted zinc can be replenished by using zinc oxide or a concentrated solution of zinc ions in phosphoric acid. Metal articles electroplated in accordance with the above described process are provided with a zinc coating which is ductile and securely adhering. The zinc coating is further characterized as comprising hexagonal platelet-like crystals ranging in size from about 4 microns to 8 microns along their longest axis. Coated articles may be mechanically formed into desirable structures and when further provided with a second protective coating, such as chromate conversion coating or paint, are surprisingly highly corrosion resistant. Even if the article is cut through to the base metal layer or bent at sharp angles, the combined coating is extremely corrosion resistant. Moreover, the zinc coating is highly adherent on difficult to plate metals such as stainless steels, aluminum, nickel, copper and the like and forms a base receptive to additional coatings.
Surprisingly, applicants found that the zinc-containing solutions made as described herein, when added to silicon-containing solutions made as described hereinafter, silicon was routinely and reliably co-deposited with zinc on the article to be plated.
It was further found by applicants that when solutions are prepared according to this invention, a co-deposit of zinc, silicon and phosphorus was formed on the surface. Because a deposit containing these three species was unknown prior to the present invention, the properties of such a deposit were also completely unknown. It was surprising to find superior galling-resistant, wear-resistant, corrosion resistant and stress corrosion-resistant properties.
The solution according to this invention preferably comprises 5 to 50 g/1 of zinc, 0,1 to 50 g/l of silicon, and 10 to 250 g/l of phosphorus. Preferably, a solution for electrodepositing a zinc/silicon/phosphorus coating comprises 5 to 20 g/1 of zinc, 0,1 to 10 g/1 of silicon, and 40 to 200 g/1 of phosphorus. More preferably, the solution comprises 10 to 20 g/1 of zinc, up to 2 g/l of silicon and 50 to 120 g/1 of phosphorus.
The aqueous solution is prepared either by contacting zinc and silicon metals with a phosphorus containing acid and/an alkali metal hydroxide or ammonium hydroxide in the presence of each other, or by preparing zinc and silicon comprising solutions in separate vessels and mixing these solutions subsequently.
In one method, the aqueous solution is prepared by contacting silicon metal in the presence of zinc in an aqueous solution of a phosphorus containing acid and adding an alkali metal hydroxide or ammonium hydroxide in increments until the pH is in the range of 1,5 to 4. Preferably, the solution is allowed to react for from about 16 hours to a few days without stirring. Alternatively, the solution may be prepared by contacting silicon metal in the presence of zinc with concentrated alkali metal hydroxide solution and then adding a solution of a phosphorus containing acid in increments until one mole of said acid has been added per 0,4 to 1,3 moles of the alkaline hydroxide. The solution is then allowed to react for from 16 hours to a few days without stirring. In both cases, the reaction continues either until all the metal has dissolved, or, as is more common, the solution is decanted from the excess metal when the desired metal ion concentration in the product solution is reached. In another method, zinc and silicon concentrated solutions are prepared separately, and the solutions are mixed after preparation. The separate concentrated solutions are first prepared by contacting zinc metal and silicon metal in separate vessels with a phosphorus containing acid and adding increments of an alkali metal hydroxide or ammonium hydroxide. Alternatively, the separate solutions can be prepared by contacting zinc metal or silicon metal with concentrated alkali metal hydroxide or ammonium hydroxide and then a phosphorus containing acid in increments.
Instead of the zinc comprising solution prepared as here described, any other zinc comprising solution of this invention may be used.
The zinc containing solution is then mixed with the-silicon containing solution such that the ratio of zinc to silicon is in the range of from 8:1 to 30:1.
The addition of alkali metal hydroxide to a phosphorus containing acid or a phosphorus contairtng acid.to alkali metal hydroxide generates heat and raises the solution temperature. During the contacting of the metal, metals or metal compounds with such solutions, the solution temperature should not exceed its boiling point and, preferably, should not exceed 100°C, and more preferably, 75°C. The temperature can be controlled by controlling the rate of addition of the phosphorus containing acid to the alkali metal hydroxide comprising solution, or the rate of addition of alkaline hydroxide to the acid comprising solution, or by conventional cooling means.
The alkaline hydroxide is selected from sodium, potassium, and lithium, and ammonium hydroxide, preferably sodium or potassium hydroxide. The phosphorus-containing acid may be phosphorus acid, phosphoric acid or orthophosphoric acid, preferably orthophosphoric acid.
Alternatively, silicon metal is reacted with concentrated aqueous alkali metal hydroxide or ammonium hydroxide. The resulting product is then combined with a solution of zinc in phosphoric acid and allowed to react without stirring for several days.
Electrodeposition is carried out at a pH in the range of 1 to 3,5, preferably at a pH of 1,5 to 3. The pH of the solution prepared according to the methods described above is adjusted by using a concentrated alkaline hydroxide solution or concentrated acid solution or-solid alkali metal hydroxide or, preferably, phosphoric acid solution, as the case may be.
Electrodeposition from a solution according to the invention shows a cathodic efficiency of about 75%. At a current density of 3,3 A/dm² a layer of about 10 µm is deposited on a metal substrate in about 15 minutes.
The depleted zinc and silicon in solution is replenished by addition of concentrated solution of zinc and silicon, or when a zinc anode is used, by addition of a concentrated solution of silicon.
An electrodeposition bath according to this invention has very good macro-throwing power. However, for metal parts with intricate or special shapes, conforming anodes or auxiliary anodes may be required to procide sufficient micro-throwing power.
The Zn/Si/P coating of this invention formed by electrodeposition is dull, matte gray in color. If desired, the appearance of coated parts may be improved by dipping into a solution of about 0,5 to 1% nitric acid, rinsed with water and dried. It has been found that the coated surface treated with nitric acid is whiter and smoother. The coated parts may also be subjected to chromate conversion coating to provide a clear blue or gold finish. The chromate conversion coating process further improves the corrosion resistance of the parts with a Zn/Si/P coaling. Parts which have been subjected to the electrodeposition process according to this invention were analyzed by electron dispersive x-ray analysis (EDX) to determine the presence of zinc, silicon and phosphorus on the surface of the metal part.
It is extremenly difficult to measure the composition of surface coatings. EDX is a good compromise combining good sensitivity with reasonable cost and can be used for routine analysis.
It is believed that the Zn/Si/P coating of this embodiment of the invention comprises at least 70% by weight of zinc, at least 0,1% by weight of silicon, and at least 9,5% by weight of phosphorus. The coating is believed to contain oxygen in the form of metal oxides and of oxygenated-phosphorus moieties; however, oxygen is not detected by EDX.
The process for electroplating the zinc coating in accordance with this invention is illustrated in the following examples.

EXAMPLE 1

48,4 g of 85% phosphoric acid was introduced intoacon- tainer. A slurry of 3,1 g of zinc oxide (_AZO 55^RTM from ASARCO) in 35,8 ml of deionized water was slowly added with stirring to the phosphoric acid. The mixture was cooled and stirred to maintain a temperature of 65°C to 70°C until all of the zinc oxide had dissolved.
12,7 g of sodium hydroxide pellets were added with stirring and cooling. The mixture was allowed to cool to room temperature and then filtered. 50 ml of the filtered solution were diluted with deionized water to 150 ml. The pH was adjusted to 2,8 with 50% sodium hydroxide. The solution contained about 14 g/1 of zinc ions and 196 g/1 of phosphate ions.
Four panels of 1010 cold rolled steel, 76 x 127 mm, (hereinafter referred to as Q-panels), were cleaned and immersed lengthwise up to 76 mm in the diluted solution. Both sides of the Q-panel were electroplated at room temperature, using a DSA anode from Daimond Shamrock, at a current density of 3 A/dm² for 23 minutes. The thickness of the coating was 12 to 13 microns. A dull, matte gray, non-lustrous coating was obtained.
The plated panel was rinsed with deionized water and dipped into an olive-drab chromate solution (M&T Unichrome 1072^RTM) for 60 seconds for chromate conversion coating treatment and then rinsed with deionized water and dried overnight. X-ray mapping examination of the cross-section of the panel showed the presence of chromium in the top 8 µm layer of the zinc coating.
rhe electroplated and chromated panel was then formed by bending in a brake to an angle of 135° at a curvature of about 0,198 cm in diameter.
rhe panel was then tested in a salt-spray chamber for 260 hours. No signs of corrosion of the zinc coating or the underlying steel panel were observed.

EXAMPLE 2

An electroplating solution was prepared using 11,9 g of zinc oxide (a mixture of 4 g of _AZO ^RTM55 and ₇,₉ g of ^AZORTM66), ^{44,8 g} of ^85% H₃PO₄, 3,7 g of potassium hydroxide and-39,6 ml of deionized water following the procedure of Example 1.
The solution was diluted 1:2,4 with deionized water, 9,5 of sodium chloride was added and the pH was adjusted to 1,9 with sodium hydroxide pellets with stirring. The zinc ion concentration in the plating bath was 42 g/l. Electrodeposition of zinc on Q-panels was carried out at a current density of 3 A/dm² and 1,6 V for 20 minutes using a zinc anode. The cathodic efficiency was found to be 84%. The zinc plated Q-panels were rinsed in deionized water, treated with M&T Unichrome 1072^RTM, rinsed in deionized water and air dried overnight. The samples were bent 135° as described in Example 1 and tested in a salt-spray chamber. No corrosion was observed after 200 hours of testing, either on the flat surface or at the bent line.

EXAMPLE 3

2,5 g of zinc dust (Grade 330^RTM) was added to a mixture of 48,4 g of 85% phosphoric acid and 23,7 g of water with slow stirring and heating to maintain a temperature of 80° to 90°C. After all of the zinc dust had dissolved, the solution was allowed to cool to room temperature.
12,6 g of sodium hydroxide pellets were dissolved in 12,7 ml of deionized water and slowly added to the zinc comprising phosphoric acid solution with cooling.
The resulting solution was diluted 1:2 with deionized water. A Q-panel was electroplated as in Example 1. The resulting zinc coating was observed to be similar to the coating of Example 1.

EXAMPLE 4

An electroplating solution was prepared using 25 g of zinc dusty 18 g of 85% phosphoric acid., 76 g of sodium dihydrogen phosphate, NaH₂PO₄, and 781 ml of-deionized water following the procedure of Example 1.
235 ml of the mixture were diluted with 259 ml of deionized water containing 2,8 ml of sodium silicate solution. The pH was adjusted to 2,5.
Electrodeposition was carried out at a current density of 3 A/dm² and 6,7 V. The cathodic efficiency was 88%.

EXAMPLE 5

Alternative formulations similar to that described in Example 1 are made using zinc salts other than ZnO. Thus, to replace 3,1 g of ZnO and 35,8 ml of water given in Example 1, the following alternative raw materials are used: 4,8 g of zinc carbonate and 34,1 ml of water;
5,2 g of zinc chloride and 33,7 ml of water; 3,8 g of zinc hydroxide and 35,1 ml of water; 6,1 g of zinc sulfate and 32,8 ml of water; or 7,0 g of zinc acetate and 31,9 ml of water.
The procedure for preparing the concentrate formulations and the plating solutions as well as the plating conditions are all similar to those described in Example 1.

EXAMPLE 6

For comparison testing Q-panels were electroplated using

(a) _M&_{T 261} ^RTM Bright acid chloride plating solution;
(b) Harshaw Alka-Star 83^RTM alkaline zinc plating process;
(c) cyanide zinc plating process;
(d) a sulfuric acid plating solution (zinc oxide in sulfuric acid) operated and adjusted to pH 2,8 per this invention; and
(e) hot-dip galvanizing.

After plating, the panels were treated by chromate conversion process and bent to an angle of 135° with a curvature of about 0,198 cm in diameter. The bent samples were examined by scanning electron microscopy. The coatings in the bent areas of the Q-panels using procedures (a), (b), (c) and (e) were severely cracked.
The bent and chromated samples together with^aQ-panel electroplated and bent according to Example 1 were placed in a salt-spray chamber for 260 hours.
The results are as follows:
The numbers represent visual estimates of percent of corrosion of the indicated areas.
X-ray mapping examination of the cross-section of the Commercially galvanized, chromated samples prepared according to Examples 6 (a), 6 (c), 6(d) and 6 (e) together with the Q-panel electroplated and chromated according to Example 1 were made.
The results are depicted in the following Table.
These results indicated that chromium had penetrated about 8 µm into the zinc coating of the present invention and about 5 µm into the zinc coating using an acid sulfuric acid process and only about 0,5 µm into zinc coatings of commercial galvanizing processes.

EXAMPLE 7

Twenty Q-panels were electroplated in a solution prepared as in Example 1, using a current density of 3 A/dm². Twelve of the panels were plated for 12,5 minutes to obtain a layer of 6,4 µm of zinc coating, and eight panels were plated for 23,0 minutes to obtain a layer of 12,8 µm of zinc coating. Eight of the panels with the 6,4 µm zinc layer were chromated, four with yellow chromate solution (_I ridite 80 RTM and four with olive chromate solution (_M&_{T U}nichrome 1072RTM). The eight panels with the 12,8 µm zinc layer were also chromated, four with yellow and two with olive chromate. There are thus two groups of ten panels, each group consisting of pairs of similarly treated panels. One of each pair of panels was bent 45°. All of the panels were then spray painted with a layer of epoxy primer, about 33 µm, and heat cured at 163°C for 20 minutes. On each painted panel two crossing lines were scribed with a stainless steel stylus over the flat surfaces and the bent lines to expose the underlying steel substrate.
One group of ten panels, with five flat panels and five bent panels, was placed in a humidity chamber and the second group of ten panels, five flat panels and five bent panels, was placed in a salt-spray chamber for 480 hours.
The results indicated that, in the humidity test, only one panel, the non-chromated, bent sample with 6,4 µm zinc coating showed undercutting of paint along the scribed lines near the bent line. All others showed little or no undercutting or blistering of paint.
In the salt spray chamber test, all samples showed slight or no undercutting.

EXAMPLE 8

An electroplating solution was prepared using 317 g of zinc oxide (1:3 mixture of _{AZO 55} ^RTM and _AZO 66^RTM), 1191 g of 85% phosphoric acid, 1069 ml of deionized water and 82,5 g of potassium hydroxide using the procedure described in Example 1.
The.mixture was diluted to 5,5 liters with deionized water and the pH adjusted to 2,2. This gave a solution containing 46 g/1 of zinc ions and 178 g/1 of phosphate ions.
Cleaned Q-panels were immersed into the diluted solution and electroplated using a current density of 30 A/dm² for 3 minutes to deposit a layer of zinc 12,5 µm thick.

EXAMPLE. 9

Two copper sheets were cleaned and one was electroplated at 3 A/dm² for 5 seconds with a zinc solution prepared as described in Example 1, rinsed with deionized water and air-dried. A commercial, inorganic-based coating, _Aremco 348^RTM was applied on both copper sheets to a thickness of 76 mm, using a brush. The sheets were then air-dried overnight and baked at 82°C for 30 min. After cooling, both sheets were bent 90°. The Aremco 348^RTM coating adhered to the copper sheet with the electroplated zinc coating, whereas it peeled from the other copper sheet. The zinc coated copper sheet was subjected to 500°C for 30 min. and then cooled to room temperature. There was only minor flaking of the inorganic-based coating.

EXAMPLE 10

Two sheets of Nitronic 40 ^RTM stainless steel were cleaned with detergent.
One of the two steel sheets was electroplated at 3 A/dm² for 5 seconds with a zinc solution prepared as described in Example 1, rinsed with deionized water and air-dried. The second steel sheet was electroplated at 3 A/dm² for 5 seconds with a copper plating solution consisting of CuSO₄ ·5H₂O, 90 g/l, and H₂S0₄ (98%), 100 ml/l. Both sheets were subjected to the pick test. This test involves etching away a portion of the electroplated metal to form a well-defined interface between the electroplated metal and stainless steel, and picking at the interface to dislodge mechanically the electroplated metal from the steel.
Copper was readily removed from the surface. The electroplated zinc layer could not be removed.
The process for electroplating Zn/Si/P coatings in accordance with this invention is illustrated in the following examples.

EXAMPLE 11

An electroplating solution was prepared as follows: 50 g of silicon granules (20 mesh, 99,999%) was mixed with 252 ml H₃P0₄ (85%) and 520 ml deionized water in a 1 liter beaker. The temperature of the solution was maintained at 30° to 35°C in an ice bath. 135 g of sodium hydroxide pellets were added in increments of 10 g every 15 min. with gentle stirring. 8 g of zinc granules were added to the solution and the solution allowed to react without stirring for 5 days. The pH of the solution was found to be 2,97. A current density of 3,2 A/dm² was used to electrolytically co-deposit a 10 µm layer of Zn/Si/P on the surface of the steel substrate in 20 minutes. An electron dispersive X-ray analysis (EDX) indicated the characteristic X-ray lines of zinc, silicon and phosphorus in the coating.

EXAMPLE 12

Premix A was prepared by mixing 250 g of zinc with 126 ml of H₃P0₄ (85%)- and 360 ml of deionized water in a 1 liter beaker. The temperature of the solution was maintained at 30° to 35°C in an ice bath. The mixture was allowed to react for 1 hour. 85 g of potassium hydroxide was added with gentle stirring in increments of 3 g every 15 min. The solution was allowed to react for 5 days.
Premix B was prepared by mixing 50 g of silicon granules with 126 ml of H₃PO₄ (85%) and 360 ml of deionized water in a 1 liter beaker. The temperature was maintained at 30° to 35°C in an ice bath. 115 g of KOH pellets in increments of 5 g every 15 min. was added to the solution with gentle stirring. The solution was allowed to react for 5 days.
Premix A and Premix B were then mixed in a ratio of 1:1 by volume.
This mixture with a pH of 2,92 was used to coat a steel substrate by electrodeposition, and the plated surface subjected to analysis by EDX showed the presence of 82,2% by weight of zinc, 3,8% by weight of silicon and 14,0% by weight of phosphorus.

EXAMPLE 13

For comparison purposes, an electroplating solution in accordance with US-PS 4,117,088 was prepared as follows: 85 g of silicon lumps were washed with hydrochloric acid solution (HCl diluted 1:1 with water). The silicon was then filtered from the solution and added to a mixture of 50 ml of H₃PO₄ (85%) and 200 ml of deionized water in a 1 liter beaker. The reaction was allowed to proceed for 2 days at 60°C. Thereafter, the silicon solution was filtered; the concentration of silicon-containing species remaining in the solution was 44 g/l and the pH was 11,2. The pH was adjusted to 2,9 and the silicon concentration was adjusted to 1,3 g/l by adding H₃P0₄ (85%) solution. A current density of 5 A/dm² was then used to pass a current through the solution using a copper cathode and a pyrolytic graphite anode. An EDX analysis indicated no characteristic X-ray line of silicon, and it-was concluded that silicon-containing species were not.electrodeposited from the solution.

EXAMPLE 14

Premix A was prepared by mixing 150 g of silicon powder (20 mesh, 99,999%) with 150 ml of concentrated ammonium hydroxide. Ammonia gas was bubbled slowly through the solution. 125 g NaOH pellets were added to the solution over a period of 3 hours in increments of 3,5 g every minute. The reaction temperature was controlled at 30° to 35°C for 48 hours.
Premix B was prepared by reacting 30 g of zinc powder in 250 ml of H₃PO₄ (85%) and 750 ml of deionized water. The solution was gently stirred for about 5 hours until all of the zinc had dissolved.
The electroplating solution was prepared by mixing Premix A and Premix B in a ratio of 1:3 with stirring.
EDX analysis of the surface of a steel substrate electroplated in the above solution at pH 2,5 indicates the presence of 9% by weight of silicon, 3% by weight of phosphorus and the balance as zinc.

EXAMPLE 15

Solutions were prepared in accordance with the methods described. The results are indicated in the following Table.

EXAMPLE 16

10 1 of H₃PO₄ (85%) and 10 1 of water were added to a 5 gallon reactor. Cooling water (10°C) was run in the cooling water bath until the acid mixture had cooled to less than 25°C. 9 Kg of zinc metal granules were added to the acid mixture and allowed to react for 15 minutes. 168 g of NaOH pellets were added every 15 min. until 4,2 Kg total had been added. The solution mixture was controlled at 35°C (30°-40°C) for four days after which a clear solution containing the solubilized zinc was poured off.
A silicon premix solution was prepared as follows:

400 g granular silicon, 300 ml deionized water and 300 ml phosphoric acid (85%) were added to a 1 liter beaker. 30 g of NaOH pellets were added initially. A total of 480 g NaOH added in increments of 30 g every 15 min. The reaction temperature was controlled to 50°C and the reaction carried out for 24 hours. The solution was diluted with water back to 1 liter. The clear solution , silicon premix, was poured off.
600 ml of silicon premix solution was slowly mixed into 20 1 of the zinc premix solution.

A steel substrate was electrocoated in the above solution. The surface analysis of the coating by EDX revealed zinc, silicon and phosphorus.
Parts made of other metals can be electroplated using any one of the solutions described hereinbefore.
The electroplating process is carried out at ambient room temperature, i.e. in the range of about 15° to 35°C.

EXAMPLE 17

770 g of zinc metal nuggets and 19 g of granular silicon metal were added to a reactor containing 1 liter of water. 400 ml of H₃PO₄ (85%) were added slowly to the mixture with constant stirring. After 30 minutes, 38 g of sodium hydroxide were added every 30 minutes and the additions were made with constant stirring until the pH reached about 3. The reaction was allowed to proceed for 3 to 4 days. The solution was removed by decantation. This solution showed 11 g/1 of zinc, 28 mg/l of silicon and 90 g/1 of phosphorus.
A steel substrate was electroplated in the above solution. The surface analysis of the coating by EDX revealed 0,1% by weight of silicon, 0,5% by weight of phosphorus and 99,4% by weight of zinc.

EXAMPLE 18

2,3 kg of zinc metal nuggets were added to a reactor with 3 liters of water, and 57 g of granular silicon metal was added to the mixture. 1,2 1 of H₃P0₄ (85%) were slowly added with constant stirring over a 1/2 hour period. The reaction of phosphoric acid and zinc was allowed to proceed for another 1/2 hour. 114 g of KOH were added every 30 min. and reaction allowed to proceed with constant stirring and controlling the temperature of the reactor to between 21°and 32°C. When the pH reached about 2, feed of KOH was stopped. The reaction was allowed to proceed for 3 to 4 days with the temperature at 90°C or lower. As zinc granules reacted and went into solution, the pH gradually climbed to about 3,5. After 3 to 4 days, the solution was removed by decantation and used to electroplate a steel substrate. EDX analysis of the electrodeposited coating detected the presence of zinc, silicon and phosphorus.

EXAMPLE 19

25 ml of NaOH (50%) was added to 800 ml of deionized water. 20 ml of silicon concentrate as prepared in Example 16 was added to the alkali water. Then, 40 ml of zinc concentrate as prepared in Example 16 were slowly added with stirring. A white precipitate slowly settled. The volume was adjusted to a total of 1 1, the pH being 13,5.and the content of soluble zinc measured at.1 g/l. A current density of 3,2 A/dm² was applied for 15 min. to the cathode immersed in this clear solution. A smooth dark gray deposit resulted.

EXAMPLE 20

Samples of 1-1/8" diameter x 8" ASTM A-193B7 stud bolts with ASTM A-194 grade 2H nuts were electrolytically coated to provide a zinc-silicon-phosphorus coating layer of this invention of 8 nm of thickness. The bolts and nuts were torqued to 100% of minimum yield strength in a simulated flange fixture incorporating a strain gauged load cell for load monitoring. After this, they were placed in an ASTM B-117 salt fog test chamber for corrosion testing. Two bolts with nuts were removed after 300 hours; one bolt with nut was removed after 700 hours; another bolt with nut after 1000 hours and the final bolt after 1350 hours. Results of the salt fog tests were as follows:

After 300.and 700 hours, no visible corrosion product was observed. After 1000 hours, slight corrosion of the surface, but no pitting of the steel substrate was observed. After 1350 hours, the bolt threads were filled with salt residue and/or corrosion products. The residue was easily removed, and no gross deterioration of the fastener was observed. There was some minor corrosion pitting. However, on testing, it was found that the strength of the fastener had not been reduced by the minor corrosion found.

EXAMPLE 21

A Timken block was coated electrolytically in a solution described in Example 18. The lubricity measurement was performed using the coated block and an uncoated ring, which were immersed in 15W-40 grade motor oil during the test. The ring and block did not seize at the maximum torque of the Timken Tester, i-.e., 410 in.-lb., after which the test was terminated. Results are shown in Fig. 11, a graph plotting torque vs. weight loss.

EXAMPLE 22

Three sets of Timken blocks and rings were tested on the Timken Tester and were identified as follows:

No. 1 Uncoated ring and block
No. 2 Coated ring - uncoated block
No. 3 Coated ring and block

The pieces were electrophoretically coated in a solution described in Example 11.
The loss of weight of the block in milligrams was plotted against the torque meter reading, giving a visual indication of the rate of wear and the torque value at which the scoring of parts was observed. Scoring of the uncoated ring and block started to appear at 350 in.-lbs. The coated ring and uncoated block showed a steady rate of wear but did not score even at the maximum torque of the Timken instrument. The coated ring and block also showed a steady but higher wear rate. There was some evidence of scoring at the maximum torque of 410 in.-lbs.

EXAMPLE 23

Four Timken blocks were coated by electroplating from a solution described in Example 18 and tested at the same time with one untreated block for comparison,on the Timken Test Machine using the "oil-off" procedure. All tests were performed using a standard untreated T48651 test cup.
The cup and block were mounted in the Test Machine and flooded with lubricant. The test machine was started and the speed adjusted to 1200 rpm.
Loads were added at a rate of one pound per minute until the total weight was ten pounds, A baseline running torque was established.
After a ten minute "run-in", the oil flow was stopped and residual oil at the cup-block interface was purged using an air nozzle.
The machine was then run until the torque had increased to 10 in.-lbs. above baseline or reached a total running time of 50 minutes.
The "oil-off" test blocks (four treated and one untreated) were started with an initial no-load, running torque of 12 lb-ft/in. with a range of 19 to 22 lb-ft/in. The torque remained fairly constant during the 10 min. "run-in" portion of the test.
Almost immediately upon removal of the oil flow, there was a drop in running torque of 1 to 2 lb-ft/in. in all of the runs. The untreated block exceeded the 10 lb-ft/in. criteria within 1 1/2 min. and was terminated. The treated blocks ran an average of 14,5 min. under the no-load condition. None of the wear patterns on the treated blocks reached the depth or width observed on the untreated block.

EXAMPLE 24

Four standard A.P.I. L-80 ^RTM couplings were electrolytically coated in the solution as described in Example 11. They were doped with regular A.P.L pipe on a regular "buck-on" machine to 800 1bs. torque and standard A.P.I. stand off. The couplings were then removed and inspected. No galling was apparent on the pin or the coupling. This operation was repeated eight times without any observed galling.

EXAMPLE 25

A set of threaded components 1-5/16 through 18 UNEF-2 were plated from the solution described in Example 16. The coated set was subjected to torsional loading to about 120 ft. to the pound. There was no thread galling when the components were unscrewed.
An uncoated set was loaded to about 40 ft. to the pound. When unscrewed, the threads galled.

EXAMPLE 26

Eight tensile tests were conducted on P-110 casing steel (yield strength, 128 psi) in accordance with NACE Standard TM-01-77. Four of these specimens were coated by electrodeposition in a solution described in Example 17. The other four specimens remained uncoated. Samples were prepared and tested using the NACE Standard. The samples were inserted in the NACE solution (5% NaCl, 0,5% acetic acid in distilled water saturated with H₂S at 75°F and 15 psi).
Four strese levels were tested, The results are shown in the following Table:

EXAMPLE 27

Five 4"x 5" 304 stainless steel sheets were alkaline degreased, immersed in phosphoric acid solution, and electroplated with the Zn/Si/P coating from a solution described in Example 17. The adhesion of the deposited coating on the stainless steel substrate was tape tested. All tests showed good adhesion.

EXAMPLE 28

Five 304 stainless steel sheets (4"x 5") were treated with nitric acid to passivate the surface (a common treatment to prevent adhesion of electroplated metal to the substrate) These passivated stainless steel sheets were electroplated with a Zn/Si/P coating from the solution described in Example 17. A thick coating of more than 25 µm was deposited. Edges of the coating were cut by a sharp razor. It was not possible to peel off the coating from the substrate.
The surfaces of another five sample sheets were-further passivated by anodic current and electrolytically coated with a Zn/Si/P coating. Again, no coating could be peeled from the substrate. The coated stainless steel samples were forced to bend and stretch many times. No peeling or breaking in the coating was observed.

EXAMPLE 29

Five 5052 aluminum coupons (2" x 3") were alkali degreased and immersed in acid without any special treatment. They were electroplated with a Zn/Si/P coating from the solution described in Example 16. The coupons were tape tested and bend tested. No peeling of the coating from the aluminum substrate could be observed.

EXAMPLE 30

A plating solution was prepared using the concentrates of Example 17. 10 1 of the zinc concentrate were diluted with 28 1 of deionized water, and then 0,79 1 of the silicon concentrate were slowly stirred into the solution.
A plating barrel was loaded with type 430 stainless steel stampings. The stampings were anodically cleaned at 6 V and 70°C by immersing the barrel in an alkaline cleaning solution (Dynadet^RTM). After the barrel was immersed in first a hot water rinse at 60°C and then a cold water rinse , it was immersed in a solution of 1 part phosphoric acid (85%) and 9 parts water. After rinsing in cold water, the barrel was immersed in the plating bath solution and the stainless steel stampings were provided with an electroplated Zn/Si/P coating of approximately 75 µm at a current density of approximately 2,5 A/d_M ². The zinc/silicone/phosphorus coating, after rinsing and drying, had surprisingly excellent adhesion to the stainless steel. The plated stainless steel stampings were painted with a coating and the adhesion of said coating over the coated stainless steel was excellent. There was no blistering or degradation of the adhesion of the coating or the zinc plating during humidity testing.

Claims

1. A process for forming a strongly adhering, electroplated coating on metallic articles, said coating comprising zinc, characterized in that said process comprises preparing an electroplating solution comprising 5 to 90 g/1 zinc ions and an effective amount of an agent suitable to maintain the solution at a pH selected in the range of 1 or greater, and further comprising a conductivity salt in an amount from 0 to 4 moles/l; and immersing a cleaned metallic article in said solution; and electroplating with the metallic article as the cathode and at a current density of at least 0,5 A/dm² for a period of at least 1 second, thus forming a dull, highly ductile, adsorptive and/or absorptive coating which is resistant to corrosion, stress corrosion cracking, wear, galling and cracking during mechanical forming operations.

2. The process of claim 1, characterized in that the pH is selected at a value in the range of 1 to 3,5.

3. The process of claim 1, characterized in that the electroplating solution further comprises silicon and the selected pH is 2,5 or greater.

4. The process of claims 1 to 3, characterized in that the plating bath is maintained at a temperature from 15°C to 35°C.

5. The process of claim 1 or 2, characterized in that the plating current density does not exceed 60 A/dm².

6. The process of claims 1 to 4,characterized in that the pH is maintained by means of an agent selected from phosphoric acid, orthophosphoric acid, pyrophosphoric acid, chloroacetic acid, dichloroacetic acid, bromoacetic acid, sulfuric acid, hydrochloric acid and sodium dihydrogen phosphate.

7. The process of claim 1, characterized in that the conductivity salt is selected from chlorides, sulfates and fluoroborates.

8. The process of claim 1, characterized in that the electroplating solution is prepared by dissolving zinc- metal or a zinc-compound in phosphoric acid while maintaining the solution at a temperature between room temperature and 100°C, with the resulting solution comprising 40 to 300 g/1 phosphate ions; and adjusting the pH to between 1 and 3,5 with alkaline hydroxide.

9. The process of claim 8, characterized in that the zinc compound is selected from zinc oxide, zinc acetate, zinc carbonate, zinc chloride, zinc sulfate and zinc sulfamate.

10. The process of claim 1, characterized in that the electroplating is prepared by contacting metallic zinc with phosphoric acid; adding an alkaline hydroxide in increments such that the temperature of the solution caused by the reaction does not exceed the boiling point, and ntil 0,4 to 1,3 moles of alkaline hydroxide per mole of phosphoric acid have been added; and allowing the reaction to proceed until the pH is between 1,5 and 3,5; and removing residual metallic zinc from the solution.

11. The process of claim 1, characterized in that the electroplating solution is prepared by contacting metallic zinc with phosphoric acid and an alkaline hydroxide until 0,4 to 1,3 moles of alkaline hydroxide per mole of phosphoric acid have been added; and allowing the reaction to proceed until the pH is between 1,5 and 3,5 and gas evolution has substantially ceased; and removing residual metallic zinc.

12. The process of claim 3, characterized in that a silicon comprising solution is prepared by eontacting metallic silicon with an alkaline hydroxide solution; adding phosphoric acid in increments such that the temperature of the solution caused by the reaction does not exceed the boiling point, and until 0,2 to 0,4 moles of phosphoric acid per mole of alkaline hydroxide are present; and allowing the reaction to proceed until the pH is between 10 and 12; and removing the residual silicon from the solution; and adding said solution to the zinc comprising solution of claims 7 to 10 such that the ratio of zinc to silicon is in the range of between 8:1 and 30:1; and adjusting the pH of the solution to 2,5 or greater.

13. The process of claim 3, characterized in that a silicon comprising solution is prepared by contacting metallic silicon with phosphoric acid and an alkaline hydroxide until 0,2 to 0,4 moles of phosphoric acid per mole of alkaline hydroxide have been added; and allowing the reaction to proceed until the pH is between 10 and 12; and removing residual metallic silicon from the solution; and adding said solution to the zinc comprising solution of claims 7 to 10 such that the ratio of zinc to silicon is in the range of between 8:1 and 30:1; and adjusting the pH of the solution to 2,5 or greater.

14. The process of claim 3, characterized in that the zinc and silicon comprising electroplating solution is prepared by contacting metallic zinc and silicon with phosphoric acid and an alkaline hydroxide until 0,4 to 1,3 moles of alkaline hydroxide per mole of phosphoric acid are added; and allowing the reaction to proceed until gas evolution ceases; and removing residual metallic zinc and silicon from the solution; and adjusting the pH to 2,5 or greater.

15. The process of claim 3, characterized in that the zinc and silicon comprising electroplating solution is prepared by contacting metallic zinc and silicon with phosphoric acid; and adding alkaline hydroxide in increments such that the temperature of the solution caused by the reaction does not exceed the boiling point, and until between 0, 4 and 1,2 moles of alkaline hydroxide per mole of phosphoric acid have been added; and allowing the reaction to proceed until gas evolution ceases; and removing residual metallic zinc and silicon from the solution; and adjusting the pH to 2,5 or greater.

16. The process of claim 3, characterized in that the zinc and silicon comprising electroplating solution is prepared by contacting metallic zinc and silicon with a concentrated alkaline hydroxide solution and adding a solution of phosphorus containing acid in increments until one mole of the acid has been added per 0,4 to 1,3 moles of the alkaline hydroxide; allowing the reaction to proceed until gas evolution ceases; removing residual metallic zinc and silicon from the solution; and adjusting the pH to 2,5 or greater.

17. The process of claims 12 to 15, characterized in that the pH is adjusted to 3.

18. The process of claims 1 or 2, characterized ion in that the zinc/concentration in the electroplating solution is from 5 to 25 g/1 and the pH is adjusted to between 2,5 and 3,5.

19. The process of claims 1 or 2, characterized in that the zinc ion concentration in the electroplating solution is from 30 to 90 g/1 and the pH is adjusted to between 1 and 2,5.

20. The process of claims 8 to 15, characterized in that the reaction temperature of the electroplating solution during preparation is controlled not to exceed 75°C.

21. The process of one or more of claims 9, 10, 11, 14, 15 and 16, characterized in that between 0,6 and 0,9 moles of alkaline hydroxide per mole of phosphoric acid are added.

22. The process of one or more of claims 1 to 21, characterized in that the alkaline hydroxide is added in solid form.

23. A metallic article provided with a dull, highly ductile, adsorptive and/or absorptive coating layer formed by electroplating employing the process of one or more of claims 1 to 22.

24. A metallic article provided with a dull, highly ductile, adsorptive or absorptive coating layer formed by electroplating employing the process of claim 3, characterized in that said coating comprises at least 70% by weight of zinc, at least 0,1% by weight of silicon, and at least 0,5% by weight of phosphorus.

25. The article of claim 23, characterized in that it is made from a metal selected from ferrous metals including steels, copper, aluminum, chromium, titanium and their alloys.

26. The article of claims 23 to 25, characterized in that it is further treated with a second coating selected from the group of phosphating coatings, chromate conversion coatings and paint coatings.