KR100611288B1 - A Process for Preparing a Corrosion Resistant, Chromate-Free Conversion Coating for Magnesium Alloys - Google Patents

Abstract

The chromate-free phosphate-fluoride chemical coating with or without vanadate, formed on a magnesium or magnesium alloy substrate, includes an active corrosion inhibitor selected from the group consisting of organic phosphonic acids. The phosphonic acid group reacts with the magnesium metal of the substrate to form an insoluble salt.

Magnesium, magnesium alloy, vanadate, chromate, phosphate-fluoride chemical coating, organic phosphonic acid

Description

Process for Preparing a Corrosion Resistant, Chromate-Free Conversion Coating for Magnesium Alloys

FIELD OF THE INVENTION The present invention relates to corrosion resistant chromate-free phosphate-fluoride chemically formulated coatings and coating solutions for use in products with or without vanadate for products formed from magnesium or magnesium alloys.

Magnesium alloys are light resistant, but are very susceptible to corrosion due to the reactive properties of magnesium. Magnesium alloys are protected from corrosion in all practical applications. A low cost anticorrosion treatment commonly used for magnesium alloys is dichromate based chemical coatings. Dichromate based chemical coatings provide good corrosion protection, but they are based on compounds (hexavalent chromium) that have a number of occupational exposure risks. Non-chromate corrosion resistant magnesium chemical coatings are needed to meet industrial needs.

Another treatment for protecting magnesium or magnesium alloy products is disclosed in US Pat. No. 5,683,522 to Joesten, which is incorporated herein by reference. In this treatment, paint adhesion and corrosion resistant coatings of magnesium phosphate and magnesium fluoride are applied to products formed from magnesium alloys. The method of applying the coating includes impregnating a magnesium alloy product in a solution containing phosphate and fluoride ions. This treatment provides a barrier film and very good paint adhesion but does not include an electrochemically active ingredient for inhibiting corrosion. Co-pending patent application Ser. No. 10 / 073,688, filed February 11, 2002 and incorporated herein by reference, discloses an improved phosphate-fluoride corrosion resistant coating on magnesium and a method of applying the same.

It is an object of the present invention to provide an improved chromate-free corrosion resistant chemically coating for magnesium and magnesium alloy products.

It is a further object of the present invention to provide a coating solution for forming a chromate-free corrosion resistant coating.

The above object is achieved by the present invention.

According to the invention, the chromate-free phosphate-fluoride chemical conversion coating, with or without vanadate, formed on a magnesium or magnesium alloy substrate comprises an active corrosion inhibitor selected from the group consisting of organic phosphonic acids. The phosphonic acid group reacts with the magnesium metal of the substrate to form an insoluble salt. Preferred organic phosphonic acids to be used as corrosion inhibitors according to the invention are straight or branched chain amino alkyl phosphonic acids, straight or branched chain alkyl phosphonic acids, and triphosphonic acids, in particular nitrilotris (methylene) triphosphonic acid (NTMP). Selected from the group consisting of: If the corrosion inhibitor comprises amino alkyl phosphonic acid, the amine groups can react with vanadate ions in the coating solution to increase vanadate incorporation into the chemical coating.

Details of the magnesium-magnesium alloy chemical coating of the present invention, and the accompanying objects and advantages, are described in the detailed description below.

According to the invention, a solution for forming a chromate-free corrosion resistant coating on a magnesium or magnesium alloy substrate comprises a solution containing an active corrosion inhibitor selected from the group consisting of phosphate and fluoride ions, and organic phosphonic acids. do.

The solution of the present invention may optionally comprise vanadate anions. The active corrosion inhibitor is selected from the group consisting of straight or branched chain amino-alkyl phosphonic acids, straight or branched chain alkyl phosphonic acids, triphosphonic acids and mixtures thereof. Particularly useful triphosphonic acids include nitrilotris (methylene) triphosphonic acid (NTMP).

As noted above, the chromate-free solution includes phosphate and fluoride ions. The phosphate and fluoride ions are about 1 g / l to 50 g / l and 1 g / l to 10 g / l, respectively, preferably 10 g / l to 25 g / l and 3 g / l to 5 g /, respectively. present in an amount of l. It is important to adjust the pH of the solution in the present invention, which is achieved by the amount of phosphate ions and fluoride ions in the solution. The pH of the solution is preferably in the range of 5-7. Certain phosphate and fluoride compounds used to form solutions with appropriate pH are disclosed in detail in co-pending patent application No. 10 / 073,688, filed Feb. 11, 2002, and The documents mentioned therein are incorporated herein by reference.

According to the invention, the corrosion inhibitor is present in the solution in an amount of about 1 ppm to 1% by weight, preferably 10 ppm to 0.5% by weight. Corrosion inhibitors are in the form of organic phosphonic acids. Preferred organic phosphonic acids include straight and branched chain amino-alkyl phosphonic acids, straight chain or branched chain alkyl phosphonic acids and triphosphonic acids. Particularly useful triphosphonic acid is nitrilotris (methylene) triphosphonic acid (NTMP).

According to the invention, the solution may optionally comprise vanadate anions. Sodium vanadate is a suitable source of vanadate ions for the solution of the present invention. According to the invention, the vanadate ions are present in an amount of about 1.0 g / l to 5.0 g / l. It has now been found that vanadate anions interact with the amine groups of amino alkyl phosphonic acids, increasing the incorporation of vanadate into the chemical coating, thereby improving coating performance. In addition, the phosphonic acid groups of corrosion inhibitors react with magnesium metal substrates to form insoluble salts that improve corrosion performance.

Preferred chromate-free solutions are phosphate ions in an amount of 10 to 25 g / l, fluoride ions in an amount of 3 g / l to 5 g / l, and vanadate anion 1 g / l to 5 g / l And the corrosion inhibitor in an amount of 10 ppm to 0.5% by weight.

Magnesium or magnesium alloy substrates with chemical conversion coatings according to the invention mainly comprise magnesium phosphate and magnesium fluoride. Amino alkyl phosphonic acids are considered to form insoluble magnesium salts as part of the chemical coating. This component, when vanadium is also used as a corrosion inhibitor, will increase the interaction of vanadium with the chemical coating, thus increasing the vanadium content in the coating.

The invention is not limited to the examples disclosed and presented herein, which are merely illustrative of the most preferred embodiments of practicing the invention and are considered to permit modifications of the form, size and arrangement of the elements and details of the embodiments. do. The invention is rather intended to embrace all such modifications as fall within the scope and range of the claims as defined by the claims.

According to the present invention, an improved chromate-free corrosion resistant coating is provided on a magnesium or magnesium alloy substrate, using a solution containing phosphate and fluoride ions, and an active corrosion inhibitor selected from the group consisting of organic phosphonic acids. can do.

Claims (16)

delete delete delete delete delete delete Magnesium or magnesium alloy substrates may contain vanadate anions, phosphate ions and fluoride ions, and straight chain amino-alkyl phosphonic acids, branched chain amino-alkyl phosphonic acids, straight chain alkyl phosphonic acids, branched alkyl phosphonic acids, triphosphonic acids, and these Treating with a solution consisting essentially of an active corrosion inhibitor selected from the group consisting of: wherein the phosphonic acid groups react with the magnesium metal to form insoluble salts. Method of Making a Coating Free An article comprising a magnesium or magnesium alloy substrate having a corrosion resistant coating prepared according to the method of claim 7. 8. The method of claim 7, wherein the active corrosion inhibitor is nitrilotris (methylene) triphosphonic acid (NTMP). 8. The method of claim 7, wherein said solution comprises 1 ppm to 1 weight percent corrosion inhibitor. The method of claim 10 wherein the phosphate ions are present in an amount of 1 g / L to 50 g / L and the fluoride ions are present in an amount of 1 g / L to 10 g / L. The method of claim 10 wherein the phosphate ions are present in an amount of 10 g / L to 25 g / L and the fluoride ions are present in an amount of 3 g / L to 5 g / L. 8. The method of claim 7, wherein said solution comprises 10 ppm to 0.5 weight percent corrosion inhibitor. The method of claim 13, wherein the phosphate ions are present in an amount of 1 g / L to 50 g / L and the fluoride ions are present in an amount of 1 g / L to 10 g / L. The method of claim 13, wherein the phosphate ions are present in an amount of 10 g / L to 25 g / L and the fluoride ions are present in an amount of 3 g / L to 5 g / L. 8. The method of claim 7, wherein the pH of the solution is between 5 and 7.