Metal treatment – Stock – Magnesium base
Reexamination Certificate
1997-09-22
2001-07-24
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
Magnesium base
C148S666000, C148S667000, C420S411000, C420S412000, C420S413000
Reexamination Certificate
active
06264762
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This application claims priority of German Patent Application 196 38 764.7, filed Sep. 21, 1996, the entire contents of which are incorporated herein by reference and can be relied on to practice the claimed invention.
Materials containing magnesium are important for lightweight construction in a number of fields. For example, lightweight magnesium compositions are employed in a variety of parts in the automobile industry, in engine construction technology, in aerospace technology, and in other structural, lightweight objects in the computer industry and the domestic appliance industry. The low specific weight of magnesium and its good strength characteristics allow a considerable weight reduction in components in comparison to parts made of aluminum or steel. The better pourability of magnesium alloys, in comparison to materials made of aluminum, also results in a reduction in processing steps and an increase in productivity. And, in contrast to materials made of aluminum, complicated, thin-walled parts can be made in large numbers by casting. Using materials made of magnesium in transport means also opens up a potential for lowering costs, saving fuel, and increasing payload.
The energy required for the primary production of magnesium competes with the price of primary production of aluminum. When magnesium is recycled, only 5% of the recovered energy is required to recycle it. This represents an improved overall energy balance for recycling materials made of magnesium by comparison to those made of aluminum. However, even if no recycling is performed, materials made of magnesium can readily be returned to the resource cycle in nature.
Today only a few percent of lightweight construction applications are made from magnesium alloys. Therefore, the use of magnesium compositions with the highest specific strength and the highest specific modulus of elasticity offers a high potential for increasing production economy and reducing environmental impacts.
However, an obstacle to using alloys made of magnesium is their corrosion behavior. Corrosion media that contains water can considerably affect the function of magnesium parts. In order to improve the corrosion resistance of magnesium parts, it is known to give them so-called conversion layers, especially conversion layers in which chromate (VI) ions are embedded in the surface of the part. Magnesium parts are also anodized. Both conversion layers and the anodization of parts, however, merely result in the passivity of the surface. This means that if the passivated surface layer is damaged, the corrosion protection fails at the damaged point.
The addition of cathodic impurities into materials made of magnesium can never be avoided completely. The quantity of cathodic precipitates has been reduced to a minimum since the development of highly pure magnesium alloys, but because of the manufacturing process, these precipitates are frequently present on the surface.
A goal of the invention is to provide a magnesium composition and/or material that has a high level of corrosion resistance in aqueous electrolytes. This is accomplished according to the invention with a magnesium composition or material that contains at least one of the elements from the group composed of sp-metals and manganese. The term “sp metals” refers to those metals whose outer s- or p-states of electron configuration are not filled.
In particular, the sp-metals consist of Zn, Cd, Hg, Ga, In, Tl, Ge, Sn, Pb, As, Sb, and Bi. The sp metals and/or manganese together account for a maximum of 5 wt. of the magnesium composition or material. Preferably, however, the content of these metal elements is only 0.1 to 2 wt., and especially 0.2 to 1 wt., since at higher concentrations intermetallic bonds can develop, which, as will be explained below, do not possess any corrosion-resisting properties, or in any event have reduced corrosion resisting properties.
The invention comprises, more specifically, a magnesium material characterized by a content of up to 5% by weight of at least one element from the group of sp-metals and manganese. Preferably, the content of the at least one element amounts to from 0.1 to 1% by weight. As noted, the sp-metal can be zinc, cadmium, mercury, gallium, indium, thallium, germanium, tin, lead, arsenic, antimony or bismuth, or a combination of metals.
Generally, the magnesium material is characterized in that the at least one element has an exchange current density for hydrogen reduction of no more than 10
−7
A/m
2
. Also, the magnesium material generally contains the sp-metal and/or manganese element(s) without the formation of intermetallic bonds or compounds, or substantially without intermetallic bonds or compounds. The invention also comprises the use of a magnesium material in a component that is exposed to aqueous electrolytes and methods for making corrosion-resistant parts.
DETAILED DESCRIPTION OF THE INVENTION
In general, magnesium materials can be pure magnesium metal or a magnesium alloy, especially a commercially available magnesium alloy.
The corrosion of magnesium metal in an aqueous medium proceeds according to the following reaction equations:
Mg
→
Mg
2
+
2
e
-
(
1
)
H
+
+
e
-
→
H
at
(2a)
H
at
→
H
ad
(2b)
Reaction (1) represents anodic oxidation while the reaction equations (2a) and (2b) are the reduction reaction or adsorption reaction according to Volmer. This means that according to (H
at
) and these are then adsorbed (H
ad
). The recombination of two H
ad
to molecular hydrogen proceeds according to the so-called Tafel or Heyrowsky reaction.
During the corrosion of magnesium parts, the reduction reaction proceeds especially at those points where, for example, a less negative potential is present as a result, for example, of cathodic impurities in a magnesium alloy. These cathodic impurities can be present as more noble metals, copper, iron, or nickel, for example, or a higher aluminum content. At these cathodic points on the part, partial reactions (2a) and (2b), hydrogen development, takes place.
The elements used in the compositions and materials according to this invention are characterized in that they result in a high hydrogen overpotential, in other words they inhibit both hydrogen reduction (2a) and hydrogen adsorption (2b). By virtue of their homogeneous distribution in the mixed crystal, these elements influence the electron configuration of the matrix and poison, so to speak, the development of hydrogen so that the anodic oxidation of the magnesium according to equation (1) does not take place.
The high hydrogen overpotential of these elements expresses itself in a correspondingly low exchange flow density for hydrogen reduction and this exchange flow density for these metals is no more than 10
−7
A/m
2
.
The elements should be distributed as homogeneously as possible in the magnesium material of this invention. In no event may intermetallic bonds form from these elements since they have different properties, especially a different exchange current density for hydrogen reduction, and thus will not lead to the desired high hydrogen overpotential.
The formation of intermetallic bonds can be prevented by adding as little as possible of the alloy elements or by appropriate manufacturing methods for the magnesium material. For example, formation of intermetallic bonds can be prevented by using powder metallurgy methods, using magnesium or magnesium alloy powder, and an alloy element powder, or heat treatment with rapid cooling. Other methods are also known the art.
In addition to their intervention in partial reactions (2a) and (2b), the sp-metal or manganese elements act as anodes when the magnesium part, as a result of impurities, for example, contains locations with a less negative potential acting as cathodes.
When the sp-metals and manganese are oxidized as anodes, they form compounds that are difficult to dis
Bommer Heike
Lang Jurgen
Nitschke Felix
Daimler-Chrysler AG
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Ip Sikyin
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