JP3183512B2 - Two-step chemical / electrochemical method of magnesium coating - Google Patents

Abstract

A two-step process for coating magnesium and its alloys is disclosed. The first step comprises immersing the magnesium in an aqueous solution comprising about 0.2 to 5 molar ammonium fluoride at a temperature of about 40 to 100 DEG C. The second step is an electrochemical treatment in an aqueous electrolytic solution having a pH of at least about 12.5 and which solution comprises about 2 to 12 g/L of a soluble hydroxide, about 2 to 15 g/L of a fluoride-containing composition, and about 5 to 30 g/L of an alkali metal silicate.

Description

Description: FIELD OF THE INVENTION The present invention relates to a method of forming an inorganic coating on a magnesium alloy and to a product formed by this method. In particular, the present invention relates to a method comprising infiltrating an article comprising a magnesium alloy in a neutral pH chemical bath, followed by electrolytic coating of the article pretreated with an aqueous solution.

BACKGROUND OF THE INVENTION The use of magnesium for structural applications is growing rapidly. Magnesium is alloyed with any of aluminum, magnesium oxide, thorium, lithium, tin, zirconium, zinc, rare earth metals, or other alloys, and enhances its stability as a structural material. Such magnesium alloys are often used when high strength to weight ratio is required. Suitable magnesium alloys can also exhibit the highest strength at high temperatures relative to the weight ratio of ultralight metals. Further, alloys with rare earths or thorium can retain sufficient strength up to 315 ° C. or higher. The structural magnesium alloy is assembled in many conventional ways, including riveting, bolting, arc and resistance welding, brazing, soldering, and bonding. Magnesium-containing articles find use in the aircraft and aerospace industries, military equipment, electronics, automotive bodies and parts, hand tools, and material manipulation. Magnesium and its alloys show good stability in the presence of many chemicals, but do not require any additional metal protection, especially in acidic environments and saline conditions. Therefore, in marine applications, in particular, it is necessary to form a coating that protects metals from corrosion.

There are many types of coatings that have been developed and used. The most common coatings are chemical or conversion coatings, which are used as paint bases and provide some corrosion protection. Both chemical and electrochemical methods are used for the conversion of magnesium surfaces. Chromate films are the most widely used surface treatment for magnesium alloys. The coating of the hydrated gel-like polychromate structure forms a surface that is an excellent paint base, but with limited corrosion protection.

Anodizing of magnesium alloys is another electrochemical method for forming a protective film. At least two low-voltage anodizations, Dow 17 and HAE, have been used commercially. However, the corrosion protection provided by these treatments remains limited. The Dow 17 method uses potassium dichromate, a chromium (VI) compound, which is very toxic and is highly regulated. The basic component of the ehA anodic coating is potassium permanganate, but it is necessary to use a chromate sealant with this coating to obtain acceptable corrosion resistance. Thus, in each case, chromium (VI) is required in all steps to achieve the desired corrosion resistant coating. The use of chromium (VI) makes disposal of waste from these processes a serious problem.

More recently, metallic and ceramic-like coatings have been developed. These coatings are formed by an electroless or electrochemical method. Electroless deposition of nickel on magnesium and magnesium alloys using a chemical reducing agent for film formation is well known in the art. However, this method produces large amounts of hazardous heavy metal contaminated water that must be treated before being discarded. Electrochemical coating methods are used to form both metallic and non-metallic coatings. Metal coating methods also suffer from the production of heavy metal-contaminated wastewater. Non-metallic coating methods have been developed to partially overcome the problems associated with heavy metal contamination of wastewater. Kozak, U.S. Patent No.
No. 4,184,926 discloses a two-step process for forming a corrosion resistant coating on magnesium and its alloys. The first step is the acidic chemical pickling or treatment of the magnesium workpiece, using hydrofluoric acid at about room temperature to form a fluoromagnesium layer on the metal surface. The second step electrochemically coats the workpiece with a solution containing the alkali metal silicate and the alkali metal hydroxide. About 150
A potential of ~ 300 volts is applied between the electrodes, about 50-200 mA
A current density of / cm ² is maintained in the bath. The first stage of the process is a direct pickling stage and the second stage proceeds in an electrochemical cell that does not contain a source of fluoride. Tests of this method have shown that corrosion resistance and coating safety need to be improved.

Kozak, U.S. Pat. No. 4,620,904, teaches a one-step method of coating an article made of magnesium using an electrolytic cell containing an alkali metal silicate, an alkali metal hydroxide, and fluoride. This tank is about 5-70 ° C
And a pH of about 12-14. The electrochemical coating is applied under a potential of about 150-400 volts. Testing of this method also shows that the need to increase corrosion resistance remains.

Based on the teachings of the prior art, there is a need for a method of coating a magnesium-containing article that produces a uniform coating with improved corrosion resistance.

In addition, there is a need for a more economical coating method that has low equipment requirements and does not produce heavy metal contaminated wastewater.

SUMMARY OF THE INVENTION The present invention contemplates a method of coating a magnesium-containing article. The article has a pH of about 5-8 and a temperature of about
Infiltrated with an aqueous solution containing about 0.2-5 mol of ammonium fluoride at 40-100 ° C. This infiltration stage cleans the article and produces ammonium fluoride on the surface of the article to penetrate the article. Next, the infiltrated article should have a pH of about 12.5.
This solution is immersed in the above electrolytic solution, and the solution contains a soluble hydroxide solution of about 2 to 12 g / L, and a fluoride-containing composition selected from the group consisting of fluoride and silicate fluoride of about 2 to 2 g. Fifteen
g / L, about 5-30 g / L silicate. A voltage difference of at least about 100 volts is applied between the infiltrated article and the cathode, and is immersed in an electrolytic solution to provide about 2-90 mA / cm ²
Is formed. By this step, a silicon oxide-containing coating is formed on the magnesium-containing article.

The term "magnesium-containing article" as used in the description and in the claims means a metal article which is wholly or partly metallic magnesium itself or a magnesium alloy. Preferably, the article is formed of metallic magnesium or a magnesium alloy and contains a high amount of magnesium, specifically a high magnesium content alloy containing about 50 weight percent magnesium, and more particularly, the article. Contains about 80 weight percent magnesium.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a coated magnesium-containing article of the present invention.

 FIG. 2 is a configuration diagram of the present invention.

 FIG. 3 is an explanatory diagram of the electrochemical method of the present invention.

FIG. 4 is a scanning electron micrograph of a cross section of a magnesium-containing substrate and the coating of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a cross-sectional view of a magnesium-containing article coated by the method of the present invention. The magnesium-containing article 10 is shown having a primary ammonium fluoride layer 12 and a second ceramic-like layer 14 formed. Layers 12 and 14 combine to form a corrosion resistant coating on the surface of the magnesium-containing article.

The coating includes a coating containing ceramic silicon oxide. FIG. 2 shows the steps used to form this coated article. The untreated article 20 is first placed on a chemical layer 22 that cleans and forms an ammonium fluoride layer on the article.

Next, the article is treated with the electrochemical layer 24, so that
A coated article 26 is produced. Chemical layer 22 contains an ammonium fluoride aqueous solution. Preferably, the vessel contains from 0.2 to 5 moles of ammonium fluoride in an aqueous solution, more preferably from 0.3 to 2.0 moles of ammonium fluoride, more preferably from 0.5 to 1.2 moles of ammonium fluoride. Accommodating. The reaction conditions are shown in Table I below.
Is shown in

Table I Conditions Preferred More Preferred Most Preferred pH 4-8 5-7 6-7 Temperature (° C) 40-100 55-90 70-85 Hours (min) 15-60 30-45 30-40 The tank is too acidic or At high temperatures, a vigorous oxidation (etching) reaction occurs, and when the bath is too alkaline or low temperature, the reaction proceeds very slowly to the actual production of a coated article.

The magnesium-containing article is retained in the chemical layer for a time sufficient to clean impurities on the article surface and to form a base layer containing ammonium fluoride into the magnesium-containing article. As a result, magnesium-containing articles coated with a layer containing the predominant metal ammonium fluoride and / or metal ammonium oxofluoride are produced, the majority of the metal being magnesium depending on the nature of the alloy. If the residence time in the chemical tank is too short, cleaning of the fluoride-containing base layer and / or the magnesium-containing article will be insufficient. This ultimately causes the coated articles to have low corrosion resistance. Long residence times tend to be uneconomical as the processing time increases while the base layer is only slightly improved. The base layer is substantially uniform in composition and thickness over the surface of the article, forming an excellent base from which the second ceramic-like layer is deposited. Preferably, the thickness of the fluoride-containing layer is about 1-2 microns.

Applicants do not want to be limited to this theory, but the first chemistry tank binds firmly to the substrate, forms a green layer that protects it, forms a second layer, forms a second layer, The first chemical layer appears to be advantageous because it is compatible with the composition joining the layer to the substrate. It is believed that the base layer contains ammonium fluoride and oxofluoride which strongly bond to the metal substrate. Due to the compatibility of the compounds of the second layer with these compounds,
It is believed that there is no significant etching of the metal substrate and that silicon oxide is deposited uniformly among other compounds.

This green phase protects the metal substrate but does not have the abrasion resistance and hardness that a full two-layer coating provides.
On the other hand, if the silicon oxide-containing layer is deposited on the metal substrate without first depositing on the base layer, the corrosion resistance and abrasion resistance of the coating are reduced because the silicon oxide-containing layer does not bond well to the substrate.

Between the chemical layer 22 and the electrochemical layer 24, the infiltrated article is washed with water to remove any unreacted ammonium fluoride. This cleaning process prevents contamination of the electrochemical layer 24.

Next, the cleaned and infiltrated article is subjected to the electrochemical coating process shown in FIG. Electrochemical layer 24
Is about 2 to 12 g / L of a soluble hydroxide compound, about 2 to 15 g / L of a soluble fluoride-containing compound selected from the group consisting of fluoride and fluorosilicate, and about 5
~ 30g / L is accommodated. Suitable hydroxides include alkali metal hydroxides. More preferably, the alkali metal is lithium, sodium, or potassium, and most preferably, the hydroxide is potassium hydroxide.

The fluoride-containing compound may be an alkali metal fluoride, for example, a fluoride such as lithium, sodium, potassium, or a fluoride such as hydroxide fluoride or ammonium bifluoride. Fluorosilicates such as potassium or sodium fluorosilicates can also be used. Preferably, the fluoride-containing material comprises an alkali metal fluoride, an alkali metal fluorosilicate, a hydroxyfluoride, or a mixture thereof. Most preferably, the fluoride containing compound comprises potassium fluoride.

The electrochemical layer also contains a silicate. Useful silicates include alkali metal silicates and / or alkali metal fluorosilicates. More preferably, the silicate comprises a lithium, sodium, or potassium silicate, and most preferably, the silicate is a potassium silicate.

The composition ranges of the electrolytes are shown in Table II below. Table II Compound virtuous applied more preferred and most preferred pH 2~12g / L 4~8g / L 5~7g / L Temperature (℃) 2~15g / L 3~10g / L 8~10g / L time (min) 5 30 g / L 10-25 g / L 15-20 g / L The impregnated article 30 is immersed in the electrochemical cell 24 as an anode. The container 32 containing the electrochemical cell 24 can be used as a cathode. The anode may be connected to a rectifier 36 via a switch 34, and the container 32 may be connected directly to the rectifier 36. Rectifier 36 rectifies the voltage from voltage source 38 and sends a direct current to the electrochemical cell. Rectifier 36 and switch 34
Is connected to a microprocessor controller 40 for controlling the electrochemical composition. Preferably, the rectifier generates a pulsed DC signal that drives the deposition process.

The conditions for the electrochemical deposition step are preferably as shown in Table III below. From Table III Conditions favorable suitable preferred and most preferred pH 12 to 14 12 to 13 12.5-13 Temperature (° C.) 5 to 30 10 to 20 10 to 20 hours (minutes) 5-80 15-60 20-30 Current Density 2 90 5 to 70 10 to 50 (mA / cm ² ) Depending on these reaction conditions, about 80 minutes or less
A ceramic coating up to 40 microns is formed. If this voltage difference is maintained for a longer time, a thicker film is deposited. However, for most practical purposes, from about 10
A 30 micron thick coating is preferred and is obtained with a coating time of about 10 to 30.

The coating formed by the above-described method is ceramic and has excellent corrosion resistance, wear resistance, and hardness characteristics.
While not wishing to be bound by this theory, it is believed that these properties are the result of the structure and bonding of the coating on the metal substrate. Preferred coatings include a mixture of molten silicon oxide and fluoride along with an alkali metal oxide. The joints of the coatings of the present invention appear to perform significantly better than all known commercial coatings. This is the result of a tight interface between the metal substrate and the coating. The coherent interface indicates that the interface includes a continuum of magnesium, magnesium oxide, magnesium oxofluoride, and silicon oxide.

This continuous interface is shown in the scanning electron micrograph of FIG. The metal substrate 50 has an irregular surface, and the interface including the ammonium fluoride-containing base layer 52 is formed on the surface of the substrate 50. The silicon oxide-containing 54 formed in the base layer 52 exhibits excellent integrity, and thus both coating layers 52 and 54 have excellent corrosion and wear resistant surfaces.

Abrasion resistance is determined by Federal Test Method Standard No.
Can be measured. Preferably, a coating formed according to the present invention having a thickness of 0.5-1.0 mil is SC-17.
With a load of 1.0 Kg on a wear disk, it withstands about 1000 wear cycles before an exposed metal substrate appears. More preferably, the coating withstands at least about 2000 wear cycles by the time the metal substrate is exposed, and most preferably, the coating has a CS-
With a load of 1.0 kg on 17 wear disks, it withstands more than about 4000 wear cycles.

The corrosion resistance can be measured according to the ASTM standard. These tests include the salt fog test ASTM B117, as assessed by ASTM D1654, Procedures A and B. Suitably, as measured by Procedure B, coatings formed according to the present invention achieve a rating of 9 or more after a 24 hour salt fog test. More preferably, the coating achieves a rating of 9 or more after 100 hours, and most preferably, a rating of 9 or more after a 200 hour salt fog test.

After being coated by the present method, the magnesium-containing article exhibits excellent finish and high corrosion resistance properties and is used as is or further coated with an optional finish coating such as paint or sealant. The structure and morphology of the silicon oxide containing coating allows the use of many additional finish coatings that further impart corrosion resistance or decorative features to the magnesium coated article. In fact, silicon oxide containing coatings have excellent corrosion resistance and under dry and wet conditions,
For example, in a water immersion test, STMD3359 forms an excellent paint base giving very good bonding. Optional finish coatings include organic and inorganic composites, paints and other decorative and protective organic coatings. Typical general purpose inorganic compounds used as outer coatings include alkali metal silicates, phosphates, borates, molybdates, vanadates. Representative general purpose organic coatings include polymers such as polyfluoroethylene, polyurethane, polyglycol, and the like. Other finish coatings are known to those skilled in the art. Again, these optional finish coatings are not required for good corrosion resistance,
Their use provides decoration and further improves the protective quality of the coating.

Excellent corrosion resistance is produced after further optional finish coating. Preferably, coatings formed according to the present invention having an optional finish coating as measured by Procedure B achieve an evaluation performance of at least about 8 after 700 hours of salt fog testing. More preferably, the coating achieves a rating of at least about 9 after 700 hours, and most preferably, at least about 10 after 700 hours of the salt fog test.

EXAMPLES The following specific examples, which include the best mode, may be used to further illustrate the invention. These examples are merely illustrative of the invention and do not limit the scope of the invention.

Example I Cleaning of a magnesium test panel (AZ91D) was performed.
Washing was performed by immersing the test panel in an aqueous solution of sodium pyrophosphate, sodium borate and sodium fluoride at 70 ° C. and a pH of about 10.5 for about 5 minutes. The panel was then immersed in a 0.5 M ammonium fluoride bath at 70 ° C. for 30 minutes. Thereafter, the panels were washed in and held in a bath containing silicate. This silicate bath was prepared in the following manner. A solution was prepared by dissolving 50 g of potassium hydroxide in 101 of water. 200 ml of commercially available potassium silicate concentrate (20 g
w / w SiO ₂ ) was added to the above solution. Finally 50 g in this solution
Of potassium fluoride was added. As a result, the pH value of the bath became about 12.5. The concentration of potassium hydroxide is about 5 g / L, the concentration of potassium silicate is about 16 g / L, and the concentration of potassium fluoride is about 5 g / L.
It became. These panels were raised in the bath and connected to the positive lead of the rectifier. The stainless steel used as the cathode was connected to the negative lead of the rectifier. This rectifier was capable of outputting a DC pulse signal. Voltage 30
The current was adjusted to 150 V over a period of seconds so that the current density became 30 mA / cm ² . After 30 minutes, the coating containing silicon oxide
It grew to a thickness of 20 microns.

Examples II-VIII Examples II-VIII were prepared in a manner similar to Example I using the compositions shown in Tables IV and V below.

A wear resistance test (141C) was performed on these test panels. When a grinding wheel (CS-17) was tested under a load of 1.0 kg, it took at least about 2,000 for the metal substrate to appear.
It was found that it could withstand cycle wear.

Example IX A magnesium test panel was coated in the same manner as in Example I. After drying, additional coating was performed in the following manner. That is, the panel is exposed to 60 ° C for 12 minutes.
This additional coating was done by dipping in a solution of potassium borohydride at pH = 7.2. Then, after washing and drying the panel
The salt fog test specified in ASTM B117 was performed. 700 in salt fog
After standing for a long time, the proof stress level was found to be 10.

Example X Panels coated according to Examples I and IX were primed with an acidic catalyst primer and subsequently hot-enamelled. The panels were then immersed in 100 ° F. water for 4 days and tested according to Method B as specified in ASTM D3359. As a result, the proof stress level was 5/5. This level is the highest level possible since no spalling of the coating was observed.

The above description and the examples and data described so far are merely examples and are not intended to limit the scope of the invention. Various changes and modifications may be made without departing from the spirit and scope of the invention, the scope of which is as set forth in the appended claims.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Remu, Brian E. United States 56721 Minnesota, East Grand Forks, Fifth Avenue Northwest 2144 (72) Inventor Woolsey, Earl R. United States 58201 North Dakota, Grand Forks, South Elevens Street 901 (56) Reference JP-A-55-18540 (JP, A) JP-A-58-1093 (JP, A) JP-A-62-33783 (JP, A) 1-301888 (JP, A) JP-A 63-501802 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25D 11/30 C25D 9/06 C23C 22/57

Claims (4)

(57) [Claims] 1. A method for applying an improved corrosion resistant coating to a magnesium-containing article, comprising: (a) treating the article in a first aqueous solution, wherein the aqueous solution has a pH value of 5-8. Yes, the temperature is 40 ~
100 ° C., wherein the aqueous solution contains 0.2 to 5 mol of ammonium fluoride, so that a layer comprising metal ammonium fluoride is formed on the surface of the article, thereby obtaining a pretreated article (B) immersing the pretreated article in a second aqueous electrolytic solution having a pH value of at least 12.5 and containing the following components: (i) 2 to 12 g / L of water-soluble water Oxide (ii) a water-soluble fluoride-containing composition of 2 to 15 g / L,
A water-soluble fluoride-containing composition selected from the group consisting of fluoride, fluorsilicate and mixtures thereof; (iii) an alkali metal silicate of 5 to 30 g / L; (c) an anode comprising the pretreated article. And placing the cathode in an electrolytic solution and applying a potential difference of at least about 100 V between the two electrodes to generate a current having a current density of ^{2 to} 90 mA / cm ² , comprising the steps of: Is formed on the article. 2. A method for applying an improved corrosion resistant coating to a magnesium-containing article, comprising: (a) treating the article in a first aqueous solution, wherein the aqueous solution has a pH value of 5-8. Yes, the temperature is 40 ~
100 ° C., wherein the aqueous solution contains 0.2 to 5 mol of ammonium fluoride, so that a layer comprising metal ammonium fluoride is formed on the surface of the article, thereby obtaining a pretreated article (B) immersing the pretreated article in a second aqueous electrolytic solution having a pH value of at least 12.5 and containing the following components: (i) 2 to 12 g / L of water-soluble water Oxide (ii) 2 to 30 g / L alkali metal fluorosilicate (c) placing an anode and a cathode containing the pretreated article in an electrolytic solution and applying a potential difference of at least about 100 V between the two electrodes Producing a current having a current density of ^{2 to} 90 mA / cm ² by applying a coating comprising silicon oxide on said article. 3. A magnesium-containing article that provides improved corrosion and abrasion resistance in a magnesium-containing substrate and a first layer as a base layer comprising metal ammonium fluoride.
And a second layer as an outer layer containing silicon oxide. 4. A magnesium-containing article, comprising: a magnesium-containing substrate; a first layer as a base layer containing ammonium metal fluoride; and a second layer as an outer layer containing silicon oxide. The magnesium-containing article having a coating having a thickness of about 0.5 mils is
1.For grinding wheel CS-17 according to method 6192.1.
A magnesium-containing article capable of withstanding a wear cycle of at least 1,000 before the substrate emerges using a load of 0 Kg.