WO2010025567A1

WO2010025567A1 - Corrosion inhibitor for mg and mg-alloys

Info

Publication number: WO2010025567A1
Application number: PCT/CA2009/001245
Authority: WO
Inventors: Zhong Xie; Debabrata Ghosh
Original assignee: National Research Council Of Canada
Priority date: 2008-09-05
Filing date: 2009-09-04
Publication date: 2010-03-11

Abstract

The invention disclosed is a composition for inhibiting the corrosion of a magnesium or magnesium alloy object in an aqueous media e g an aqueous solution containing halide ions e g chloride ions, up to their solubility limit, and in seawater The composition can also be used for inhibiting the corrosion of magnesium or magnesium alloys in engine coolants The composition comprising a hydrolysed silane e g bis- silane, an aqueous solvent, and an emulsifing agent.

Description

Corrosion Inhibitor for Mg and Mg-alloys

BACKGROUND OF THE INVENTION

This invention relates to inhibiting the corrosion of magnesium or magnesium- alloys in aqueous media, and in particular to a composition for inhibiting the corrosion of a magnesium or magnesium-alloy object, in an aqueous solution containing halide ions, specifically a high concentration of chloride ion (Cl-) up to its solubility limit, and in seawater. The invention can also be used for inhibiting corrosion of magnesium or magnesium-alloys in aqueous engine coolants.

In aqueous solutions containing halide ion, specifically chloride ion, Mg and its alloys undergo an intensive corrosion due to a negative difference effect (NDE). Various corrosion inhibitors and/or corrosion protection films/coatings have been developed. However, the majorities were targeted on low chloride ion concentration (e.g. NaCI normally less than 0.5 M (3%wt)). Under high saline concentrations (e.g. NaCI up to 2M (12%wt), the self-corrosion of Mg and Mg alloys significantly reduces the usefulness of Mg and Mg alloys as electrode materials in electrochemical cells. The significant corrosion of Mg and Mg alloys in such an environment has raised severe challenges and limited its widespread applications.

A prior art approach to corrosion inhibition involves the use of silanes, a group of silicon-based organic-inorganic chemicals with the general formula of R'(CH2)nSi(OR)3 , where R'= an organic functionality; OR = a hydrolysable alkoxy group, e.g., methoxy (OCH3) or ethoxy (OC2H5) and n= 2 to 6. Such silanes have been extensively used in surface treatments for corrosion protection of metals or as adhesive promoters. Studies associated with the corrosion protection of metals by silanes have been carried out extensively since early 1990s. It is believed that the stronger interfacial adhesion and denser films of silanes is one of the key factors that contribute to their corrosion inhibition performance on metals.

Some examples of such prior art include: US published patent application no. 20030026912 and US Patent no. 7,011,719, both of Ostrovsky.

Both references disclose a composition comprising a hydrolysed silane (specifically Bis silane) in an aqueous solvent including ethanol or acetone, and the use of the composition to "treat" a metallic object, particularly a magnesium or magnesium alloy object to form a protective coating.

Firstly, both references clearly convey the idea that while the hydrolysed silane composition is contacted with the object in an aqueous solution, this is only a first step in creating a layer with Si-O-Me bonds on the object, the other steps involving drying and usually a curing heat treatment, plus an application of a paint or another medium.

Secondly, neither reference mentions utility in a saline aqueous solution as the ultimate environment in which the corrosion protection is to be attained.

Thirdly, neither reference mentions use of an emulsifier as a component of the inhibitor composition or emulsifying process to prepare silane-based solution for corrosion protection.

Specifically, at paragraphs [46] and [50] of the US patent no. 7,011 ,719, the mechanism is described as follows: "The ability of hydrolysable silanes (for example, those having one or more alkoxy or acyloxy substituents) to bind to metal surfaces is well known to one skilled in the art. The binding of silanes with a metal surface can generally be described as a three-step process. First, a hydrolysable moiety is hydrolyzed. Second, the hydrolysed silane migrates to the surface of the metal where it binds to a hydroxyl group on a metal surface. Third and last, water is liberated and a covalent Si-O-Xx bond is formed, Xx being a metal atom." However, it is emphasized that the references are silent regarding the corrosion inhibiting effect of the hydrolysed silane composition in aqueous corrosive media, without drying and the formation of a solid coating on the Mg surface.

Moreover, the existing type of corrosion inhibitors work quite well in low saline environment, but are unable to function properly when employed in high concentration chloride ion solutions e.g. aqueous NaCI electrolytes in electrochemical cells including Mg or Mg-alloy electrodes, due to strong chloride ion attacking the magnesium oxide/hydroxide electrode film.

It is also noted that in the prior art use of silane as corrosion inhibitor, the silane has to be condensed onto the metal surface by a curing process to form a crosslinked siloxane film (Si-O-Si network). However, it is generally known that SiOH groups cannot be condensed on metal surfaces in aqueous media. Therefore, silanes have not heretofore been considered as a solution phase corrosion inhibitor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a silane-containing corrosion inhibitor, workable in an aqueous solution to form a cross-linked Si-O-Si network without a curing or condensation step.

According to the invention, we have developed a corrosion inhibitor composition, and a preparation process therefore.

According to one aspect of the invention we provide a composition for inhibiting the corrosion of a magnesium or magnesium-alloy object in the presence of an aqueous medium, comprising a) a hydrolyzed silane; b) an aqueous solvent including an organic co-solvent, and c) an emulsifying agent. In embodiments of the invention, the silane is a Bis-3 silane: (RO)3Si(CH2) n R'(CH2) nSi(OR) 3 where R¹ = an organic functionality group such as a lower- alkyl group; OR= a hydrolyzable lower-alkoxy group, e.g., methoxy (OCH3) or ethoxy (OC2H5) and n= 2 to 6. Examples include:

Bis[3-(triethoxysilyl)propyl]tetrasulfide or Bis-sulfur silane (OC2H5)3Si(CH2)3S4(CH2)3Si (OC2H5)3), Bis[3-(triethoxysilyl)propyl] amine or bis-amino silane, (OCH3)3Si(CH2)3NH(CH2) 3Si (OCH3)3 , and

Bis[3-(triethoxysilyl) propyl] ethane or BTSE, (OC2H5)3- Si(CH2)2Si(OC2H5)3) .

Further, the co-solvent is typically selected from but not limited to ethanol, methanol, acetone and ethyl acetate.

Yet, further, the emulsifing agent is typically selected from but not limited to ethylene glycol (EG), polyethylene glycol (PEG), and a glycerol, e.g. tetraglycerol.

In some embodiments, the co-solvent is ethanol, and the weight ratio of aqueous ethanol to Bis-3 silane is from 2:1 to 1 :5, preferably 1 :1.

In some embodiments, the emulsifying agent is ethylene glycol or polyethylene glycol in an amount in the range from 2 to 10%, preferably 5.5 % v/v.

In some embodiments, the silane is Bis[3-(triethoxysilyl)propyl]tetrasulfide in an amount of 0.1 to 10 mM, preferably in an amount of 2.0 to 5.0 mM.

According to one embodiment of the invention, the proposed corrosion inhibitor composition can be used in aqueous halide solution up to its solubility limit, to inhibit the magnesium or magnesium-alloy self-corrosion reaction. According to another embodiment of the invention, the proposed corrosion inhibitor composition can be used in aqueous concentrated chloride solution (up to its solubility limit) to effectively mitigate the magnesium or magnesium- alloy self-corrosion reaction.

According to yet another embodiment of the invention, the proposed corrosion inhibitor composition can be used in the aqueous medium, which comprises sodium chloride in a concentration range from 0.1 to 5M.

According to another embodiment of the invention, the proposed corrosion inhibitor composition can also be used in seawater for corrosion protection of magnesium or a magnesium-alloy.

According to yet another embodiment of the invention, the proposed corrosion inhibitor composition can be used in aqueous engine coolants for inhibiting magnesium or magnesium-alloy self-corrosion and increasing the lifetime of an engine chamber.

According to another aspect of the invention, we provide a process for the preparation of a composition for inhibiting the corrosion of a magnesium or magnesium-alloy object in the presence of an aqueous medium, comprising

(a) dissolving/dispersing a Bis-3 silane in an aqueous solvent including an organic co-solvent, to form a mixture,

(b) adding water to hydrolyze and cross-link the silane to form cross-linked Si- O-Si network, and

(c) adding an emulsifying agent.

In an embodiment of the invention all of the process steps are conducted ex- situ and the resulting treatment composition is subsequently added to the aqueous medium. According to the process aspect of our invention, the mechanism involves the formation of a protective network or film on the surface of a magnesium or magnesium-alloy object, which significantly reduces attack of the surface by corrosive media, and suppresses the self-corrosion and/or pitting reaction.

According to another aspect of the invention, a method is provided for inhibiting the corrosion of a magnesium or a magnesium-alloy object in the presence of an aqueous solution, comprising adding to the solution the corrosion inhibiting composition as described above.

According to another aspect of the invention, a method is provided for inhibiting the corrosion of a magnesium or a magnesium-alloy object in the presence of an aqueous solution including halide ions, comprising adding to the solution the corrosion inhibiting composition as described above.

According to yet another aspect of the invention, a method is provided for inhibiting the corrosion of a magnesium or a magnesium-alloy object in the presence of an aqueous solution including a high concentration of chloride ions up to its solubility limit, comprising adding to the solution the corrosion inhibiting composition as described above.

According to another aspect of the invention, a method is provided for inhibiting the corrosion of a magnesium or a magnesium-alloy object in the presence of an aqueous solution of sodium chloride in a concentration range from 0.1 to 5M₁ preferably from 0.5 to 3M.

According to yet another aspect of the invention, a method is provided for inhibiting the corrosion of a magnesium or a magnesium-alloy object in the presence of seawater, comprising adding to the seawater the corrosion inhibiting composition as described above.

According to another aspect of the invention, a method is provided for inhibiting the corrosion of a magnesium or a magnesium-alloy object in the presence of an aqueous engine coolant, comprising adding to the coolant the corrosion inhibiting composition as described above.

In a further aspect of the invention, an electrochemical device e.g an electrochemical cell, is provided, wherein an electrode e.g. the anode is made of magnesium or a magnesium-alloy and the electrolyte is an aqueous solution containing halide ions, and wherein the corrosion inhibiting composition as described above is included in the electrolyte.

In a further aspect of the invention, an electrochemical device e.g an electrochemical cell, is provided, wherein an electrode e.g. the anode is made of magnesium or a magnesium-alloy and the electrolyte is an aqueous solution containing chloride ions in a wide concentration range up to their solubility limit, and wherein the corrosion inhibiting composition as described above is included in the electrolyte.

In a further aspect of the invention, an electrochemical device e.g an electrochemical cell, is provided, wherein an electrode e.g. the anode is made of magnesium or a magnesium-alloy and the electrolyte is an aqueous solution containing sodium chloride in a concentration range of 0.1 to 5M, and wherein the corrosion inhibiting composition as described above is included in the electrolyte.

In a further embodiment of the invention, the corrosion inhibiting composition as described above is used in an electrochemical cell, wherein an electrode e.g. the anode is made of magnesium or a magnesium-alloy and the electrolyte is seawater, and wherein the corrosion inhibiting composition is included in the electrolyte.

It is noted that in electrochemical cells for generation of hydrogen, corrosion inhibition is provided when the cell is in open circuit i.e. when the electrolysis process is stopped. When the cell is operating, the presence of the inhibition composition does not interfere with the process. In other words, the corrosion inhibition composition works when the cell is turned off and process of hydrogen generation is stopped, to protect the magnesium or magnesium- alloy electrode exposed to the aqueous electrolyte containing halide ions.

In yet a further embodiment of the invention, the magnesium-alloy is selected from but not limited to ASTM designations, AZ31, AZ 61, AZ91, AM 60 etc. See e.g. catalogue from All Metals & Forge, Prototype Casting Inc. http://www.protcast.com/specs.htm etc. AZ91D is an exemplary magnesium alloy used in the working Example below.

AZ91D of a composition 8.3-9.7% Al, 0.15% Mn min., 0.35-1.0% Zn, 0.10% Si max., 0.005% Fe max., 0.030% Cu max., 0.002% Ni max., 0.02% max. other (each) impurity <0.01%, and balance Mg.

In another embodiment of the invention, the magnesium alloy is AM60B of a composition Al 5.6-6.4%, Mn 0.26-0.5%, Zn<0.20%, Si <0.05%, Cu<0.008%, NiO.001%, Fe<0.004%, other (each) impurity <0.01%, and balance with Mg.

In yet a another embodiment of the invention, the magnesium alloy is AZ31 of a composition Al 2.5-3.5%, Mn 0.2-1.0%, Zn 0.7-1.3%, Si <0.05%, Cu<0.01%, NiO.001%, FeO.002%, other (each) impurity <0.01%, and balance with Mg.

Other useful magnesium-alloys are described in our US patent No. 7,393,440 the Disclosure of which is incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1a and 1b are graphs illustrating the results of electrochemical testing to determine the minimum workable concentration of the Bis-3 silane: Bis[3-(triethoxysilyl)propyl]tetrasulfide. 2M NaCI electrolyte. 3-electrode cell (150cc electrolyte). Working Electrode AZ91 D Mg alloy with a surface area of 0.4 cm2. Platinum wire mesh counter electrode, Saturated calomel reference electrode (SCE) Figures 2a and 2b are graphs illustrating the effect of the inclusion of an emulsifier i.e. ethylene glycol in the in electrochemical testing. Electrolyte composition: 2M NaCI electrolyte. 5.5 % v/v ethylene glycol and 2.5 mM bis. 3-electrode cell (150cc electrolyte). Working Electrode AZ91D Mg alloy with a surface area of 0.4 cm2. Platinum wire mesh counter electrode, Saturated calomel reference electrode (SCE).

Figure 3 is a graph illustrating the positive effect on corrosion inhibition provided by compositions according to the invention. Electrolyte composition: 2M NaCI electrolyte in 3.5 L vessel. 8 plates stack with a surface area of 170 cm2/plate.

Figures 4a and 4b are pictures illustrating the visual difference in a sample exposed to a corrosion inhibitor composition according to the invention, compared to a sample not exposed to the corrosion inhibitor.

Figure 5 is a graph illustrating the positive effect on corrosion inhibition provided by inhibitor compositions according to the invention including various types of emulsifiers, particularly ethylene glycol (EG) or polyethylene glycol (PEG). Electrolyte composition: 2M NaCI electrolyte with 2.5 mM corrosion inhibitor in 3.5 L vessel. 4 plates stack with a surface area of 170 cm2/plate.

Figure 6 is a graph illustrating the positive effect on corrosion inhibition provided by a corrosion inhibitor composition according to the invention in 0.5 M NaCI or seawater according to the invention. Electrolyte composition: 0.5 M NaCI or seawater with 2.5 mM corrosion inhibitor in 3.5 L vessel. 4 plates stack of area of 170 cm2/plate.

Figure 7 is a graph illustrating the positive effect on corrosion inhibition effect of Mg-alloy (AZ91 D) exposed to engine coolant in the electrochemical testing. Electrolyte composition: 55 % v/v ethylene glycol balanced with water with or without 2.5 mM bis- 3 corrosion inhibitor. 3-electrode cell (150cc electrolyte). Working Electrode AZ91 D Mg alloy with a surface area of 0.4 cm2. Platinum wire mesh counter electrode, Saturated calomel reference electrode (SCE). DETAILED DESCRIPTION OF THE INVENTION

A water insoluble Bis-3 silane was first dispersed with an aqueous solvent including an organic co-solvent to form a mixture. Then, water was added to the mixture to initiate hydrolysis and cross-linking of Bis-3 silane to produce an 'activated' form of Si-O-Si. After this, an emulsifying agent was mixed with the hydrolysed Bis-3 solution so formed and added to an aqueous corrosive medium and stirred for a certain period of time before exposure of a magnesium- alloy to the solution for corrosion testing.

Example

A 1:1 weight ratio of aqueous ethanol to Bis-3 silane (Bis[3- (triethoxysilyl)propyl]tetrasulfide) in a 20 mL scintillation vial is mixed with a magnetic stirring bar for a minimum of 60 hours at 300 rpm and room temperature (25°C). Then 2.5 ml water was added to the mixture for hydrolysis within 24 hours. The hydrolyzed Bis-3 solution was mixed with 5.5% v/v ethylene glycol and added into a beaker with 4I_ of 2M NaCI solution and was aged for few minutes to three days with a magnetic stirrer at 300 rpm. The aqueous solution was transferred to a stainless vessel. A stack of 4 or 8 plates was put into the solution to run corrosion testing.

Electrochemical Corrosion Testing Minimum workable concentration has been determined by electrochemical screening of a Bis-3 Silane (Bis[3-(triethoxysilyl)propyl]tetrasulfide). Dynamic potential (DP) scanning and electrochemical impedance spectroscopy (EIS) were applied to determine the minimum additive level of Bis-3 silane. Electrochemical testing is performed using a Solartron 1480 multistat and 1255 B FRA, connected with an electrochemical 3-electrode cell containing 150 mL of the solution. An electrochemical cell was set up so that the working electrode, consisted of a 6.30 x 6.30 mm square face of AZ91 D (a magnesium alloy) in the centre of the cell and its surface was polished to a roughness of ~8 RMS. A platinum wire mesh and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode respectively. The aqueous solution is used as the electrolyte. The test was conducted under ambient environment.

Exemplary results were presented in Figures 1a and 1b. When Bis-3 silane (Bis[3-(triethoxysilyl)propyl]tetrasulfide) concentration increases from 0.1 mM to 2.5 mM. a noticeable shifting in Ecor and pitting potential from negative toward positive, for example, from -1.6 to -1.55 V vs SCE, and reducing of anodic Mg corrosion current under identical potential were observed. EIS also indicates an increased corrosion resistance (large arc diameter) for 2.5 mM Bis-3 silane. When 5.5% v/v ethylene glycol (EG) was added, a further increment of corrosion resistance from EIS and positive shifting of Ecor and pitting potential from DP, as shown in Figures 2a and 2b, were found. A wide passivation plateau observed in the potentiodynamic curves of magnesium- alloy electrode immersed into the electrolyte with Bis-3 silane, and into the electrolyte with Bis-3 silane and emulsifier EG₁ demonstrates an improved corrosion resistance.

Further exemplary results were presented in Figure 7. When Bis[3- (triethoxysilyl)propyl]tetrasulfide of a concentration of 2.5 mM was added into a mimic engine coolant aqueous electrolyte of a composition of 55% solution of ethyl eneglycol in water, a reduction of anodic Mg corrosion current under identical potential was observed demonstrating an improved corrosion resistance of AZ91 D magnesium alloy in engine coolant.

Mg and Mg alloy Corrosion testing by measurement of hydrogen generation

The test system (not shown) includes a large stainless steel pot equipped with gas vent lines, pressure gauge and transducer, safety relief valves, mounting rod, thermocouple, mass flow meters (MFM), filters, and a lab view data acquisition system (DAQ). Eight or four pieces of die-casting AZ91 D magnesium alloy, each with a size of 14.5 x 5.7 x 0.16 cm, were used as test samples and were assembled to form a stack. The Mg alloy stack was immersed into various electrolyte compositions prepared as outlined above in testing vessels. The Mg alloy self-corrosion rate is determined by collecting the amount of H2 generation from the closed vessel for a certain time period. Figure 3 shows the data collected during more than 3 days. As can be seen, the total accumulated H2 flow reduced 5-6 times in the presence of the corrosion inhibitor Bis-3 silane (Bis[3-(triethoxysilyl)propyl]tetrasulfide) or Bis-3 silane+EG in the electrolyte, which indicates that the self-corrosion of AZ91 D alloy in 2 M NaCI electrolyte was suppressed effectively. Without EG, it takes a significant time period to form an emulsion from immiscible Bis-3. However, with emulsifying agent included, the emulsion forms faster and the required concentration of inhibitor in solution is achieved while providing better inhibition effects. A minimum workable concentration of the inhibitor should be achieved in the solution, and the emulsifier helps transfer the inhibitor in soluble form. As noted above, based on Fig 2a and Fig 2b, the role of emulsifier is significant.

Figures 4a and 4b reveal the visual difference in a sample exposed to the corrosion inhibitor composition, compared to a sample not exposed to the corrosion inhibitor. It is seen that the sample not exposed to the additives undergoes a progressive pitting corrosion.

Fig 5 shows the comparison of corrosion inhibitor effect of Bis-3 dispersed in ethanol solvent (Bis-3: Ethanol=1 :1wt% ratio) and emulsified by ethylene glycol (EG) and/or polyethylene glycol (PEG) of 5.5% v/v on stacks formed by four Mg plates. The addition of EG and/or PEG demonstrates enhancement of the corrosion inhibiting effect, while significantly reducing the ageing period.

Fig 6 shows the corrosion inhibition effect of prepared Bis-3 (Bis[3- (triethoxysilyl)propyl]tetrasulfide) composition (Bis-3: Ethanol=1:1wt% ratio and 5.5% v/v EG as emulsifier) applied to seawater. A significant decrease in Mg corrosion rate was observed due to the corrosion-inhibiting additive. Figure 7 illustrates the positive effect on corrosion inhibition of a Mg-alloy (AZ91D) exposed to engine coolant in the electrochemical testing. Electrolyte composition: 55 % v/v ethylene glycol balanced with water with or without 2.5 mM bis- 3 corrosion inhibitor. 3-electrode cell (150cc electrolyte). Working Electrode AZ91D Mg alloy with a surface area of 0.4 cm2. Platinum wire mesh counter electrode. Saturated calomel reference electrode (SCE).

Claims

1. A composition for inhibiting the corrosion of a magnesium or magnesium-alloy object in the presence of an aqueous medium, comprising a) a hydrolysed silane; b) an aqueous solvent including an organic co-solvent, and c) an emulsifing agent.

2. A composition according to Claim 1, wherein the silane is a Bis-3 silane of formula (RO)3Si(CH2) n R'(CH2) nSi(OR) 3 where R' = an organic functionality group such as a lower-alkyl group, OR= a hydrolyzable lower- alkoxy group, e.g., methoxy (OCH3) or ethoxy (OC2H5) and n= 2 to 6.

3. A composition according to Claim 1 or 2, wherein the co-solvent is selected from but not limited to the group consisting of ethanol, methanol, acetone and ethyl acetate.

4. A composition according to Claim 1, 2 or 3, wherein the emulsifing agent is selected from but not limited to ethylene glycol (EG), polyethylene glycol (PEG), and a glycerol.

5. A composition according to any one of the previous Claims, wherein the Bis-3 silane is selected from the group consisting of Bis[3- (triethoxysilyl)propyl]tetrasulfide or Bis-sulfur silane (OC2H5)3Si(CH2)3S4(CH2)3Si (OC2H5)3), Bis[3-(triethoxysilyl)propyl] amine or bis-amino silane, (OCH3)3Si(CH2)3NH(CH2) 3Si (OCH3)3 , and Bis[3-(triethoxysilyl) propyl] ethane or BTSE₁ (OC2H5)3-

Si(CH2)2Si(OC2H5)3).

6. A composition according to any one of the previous Claims, wherein the co-solvent is ethanol, and the weight ratio of aqueous ethanol to Bis-3 silane is from 2:1 to 1 :5, preferably 1 :1.

7. A composition according to any one of the previous Claims, wherein the emulsifying agent is ethylene glycol or polyethylene glycol in an amount in the range from 2 to 10%, preferably 5.5 % v/v.

8. A composition according to any one of the previous Claims, wherein the Bis-3 silane is Bis[3-(triethoxysilyl)propyl]tetrasulfide in an amount of 0.1 to 10 mM. preferably in an amount of 2.0 to 5.0 M.

9. A composition according to any one of the previous Claims, wherein the magnesium alloy is selected from AZ31 , AZ 61, AZ91 , AM 60 etc.

10. A composition according to any one of the previous Claims, wherein the aqueous medium is an aqueous solution including halide ions, up to their solubility limit.

11. A composition according to any one of the previous Claims, wherein the aqueous medium is an aqueous solution including chloride ion (Cl-) of concentration up to its solubility limit.

12. A composition according to any one of the previous Claims, wherein the aqueous medium comprises sodium chloride in a concentration range from 0.1 to 5M.

13. A composition according to any one of the previous Claims, wherein the aqueous medium comprises seawater.

14. A composition according to any one of the previous Claims, wherein the aqueous medium comprises an aqueous engine coolant.

15. A process for the preparation of a composition for inhibiting the corrosion of a magnesium or magnesium-alloy object in the presence of an aqueous medium, comprising (a) dissolving/dispersing a Bis-3 silane in an aqueous solvent including an organic co-solvent to form a mixture,

(c) adding an emulsifying agent

16. A process according to Claim 15, wherein the aqueous medium is an aqueous solution including halide ions, up to their solubility limit

17. A process according to Claim 15, wherein the aqueous medium is an aqueous solution including chloride ion (Cl-) of concentration up to its solubility limit.

18. A process according to Claim 15, wherein the aqueous medium comprises sodium chloride in a concentration range of 0.1 to 5M.

19. A process according to Claim 15, wherein the aqueous medium comprises seawater.

20. A process according to Claim 15, wherein the aqueous medium comprises an aqueous engine coolant.

21. A method for inhibiting the corrosion of a magnesium or a magnesium- alloy object in the presence of an aqueous medium, comprising adding to the medium the corrosion inhibiting composition as claimed in any one of Claims 1 to 14.

22. An electrochemical device, comprising an electrode, made of magnesium or a magnesium-alloy and an electrolyte containing halide ions and a corrosion inhibiting composition as claimed in any one of Claims 1 to 9 included in the electrolyte.

23. An electrochemical device according to Claim 22, wherein the electrolyte is an aqueous solution including halide ions up to their solubility limit.

24. An electrochemical device according to Claim 22, wherein the electrolyte is an aqueous solution containing chloride ions in a wide concentration range up to their solubility limit.

25. An electrochemical device according to Claim 22, wherein the electrolyte comprises sodium chloride in a concentration range of 0.1 to 5M.

26. An electrochemical device according to Claim 22, wherein the electrolyte comprises seawater.

27. An electrochemical cell according to any one of Claims 22 to 25, wherein the electrochemical device is an electrochemical cell.