Author: Sophie Carter

Just as steel will rust and copper will tarnish, so magnesium will acquire a surface coating of oxide.

The need for surface protection of magnesium alloys depends entirely on the conditions of service, and where these are known to be non-corrosive no protection may be necessary. The alloys are all very stable in normal atmospheres and can be used quite as freely as, for example, mild steel. Just as steel will rust and copper will tarnish, so magnesium will acquire a surface coating of oxide, but no corrosion will take place unless a definite corrosive factor is present.

Designing magnesium components to last

When magnesium alloys are used in contact with other metals greater care must be taken to see that wet or very humid conditions do not arise; or, if they cannot be avoided, that adequate steps are taken to ensure that they do not penetrate to the metal surface. Magnesium is anodic to other metals and so will be ‘sacrificed’ to the other metal to which it is attached when both are wetted with an electrolyte. When this happens, the other metal secures a degree of protection by sacrificing its partner. This effect, however, cannot occur at all in the absence of an electrolyte. Merely damp conditions alone are not sufficient to cause any appreciable degree of galvanic corrosion. This is an important point which is often overlooked, and the danger is in consequence greatly exaggerated. The real danger lies not so much in the bi-metal union as in the persistent wetting of such a union, and persistent wetting is undesirable even in the absence of a bi-metal union.

Untreated magnesium has a lower corrosion rate in an industrial atmosphere than mild steel

The practical aspect of the protection of magnesium alloys, in severe conditions especially, is very closely bound up on one hand with design and on the other with assembly. Schemes that are otherwise good may miscarry for lack of thought at the design stage or because of faulty or careless assembly. In general, the corrosive substances to be avoided are aqueous solutions of salts and acids. They may be derived from salt spray or from industrial gases such as sulphur dioxide. Most of the impurities in the atmosphere which lead to the corrosion and rotting of structural iron and steel will also attack magnesium, though sulphur gases are less damaging than chlorides. In a similar way corrosive conditions to be avoided are the wetting, and especially the persistent wetting, of the metal surface by such solutions as those described above. They may take the form of rain, seawater splashes, condensate from the cooling of highly humid atmospheres and the like. If these can be kept in check, little trouble will result.

Minimising damage to magnesium alloy parts

Corrosive influences are those factors which affect the behaviour of the metal in its environment. For example, lack of ventilation, by preventing evaporation of condensed water, may make all the difference in the situation. With free evaporation little or no corrosion will occur, whereas in the continued presence of moisture corrosion may be severe. In the same way the accumulation of water in an undrained recess may bring about significant corrosion, whereas little or none would have occurred if the water had been free to drain away. Hairline fissures between mating surfaces, which by capillary action attract and retain films of moisture also come into this category.

Want to learn more about corrosion protection of Mg alloys?

Request a full copy of our magnesium design guide here.

Luxfer MEL Technologies offers a wide range of magnesium alloys. We’d love to hear about your applications and discuss how our materials can offer you a solution.

Magnesium is the Lightest Commercially Available Structural Metal

Magnesium is the lightest commercially available structural metal and is an ideal solution for weight critical applications in aerospace.  It provides a lightweight alternative to Aluminium, Titanium, Steel and Carbon Fibre.

Elektron® 21 and Elektron® 43 magnesium alloys are specifically developed for higher temperature applications in the range of 150 C to 350 C.  The alloys can be cast, extruded, rolled, machined, or forged.

Magnesium as an Alternative to Aluminum

For applications operating in the temperature range of 150 C to 200 C, cast Elektron® 21 and Elektron® WE43B offer a lighter alternative to the aluminum alloys such as A356 and C355.  The density of magnesium is 66% of aluminum enabling significant weight savings to be achieved.

Machined Magnesium versus Aluminum

Tool ranges used during the machining of aluminum can also be used for magnesium. These give satisfactory results. However, due to the free-machining characteristics, relatively low cutting pressures and slightly lower heat capacity of magnesium, best machining practice should take tool material, tool life and tool geometry into consideration. For more information on machining magnesium, read this article.

Surface Treatment

Magnesium alloys are very stable in normal atmospheres and can be used quite freely without any particular surface protection.  Elektron® 21 and Elektron® 43 both exhibit corrosion resistant behaviour similar to that of aluminum alloys.  In service, magnesium alloys often have a surface treatment applied to enhance their protection.  A range of surface treatments are available to suit all conditions of differing harshness.

Designing with Magnesium

It is frequently possible to take full advantage of the lower density of magnesium due to deliberate oversize by design to include features such as lugs and flanges.  These considerations negate the need to proportionally stiffen magnesium components as compared with those made from alloys with a higher modulus.

A magnesium section is about 22% as stiff as steel of the same thickness but if the depth of the magnesium section is increased to twice that of the steel then the magnesium section will be 70% more rigid and yet only weigh half as much.

Similarly, on the basis of equal rigidity, a magnesium section need weigh only 75% of the aluminium one, or when compared on the basis of equal weight the magnesium will be twice as stiff.

Luxfer MEL Technologies offers a wide range of magnesium alloys for aerospace. We’d love to hear about your applications and discuss how our materials can offer you a solution.

SoluMag is a seamless extruded tube capable of higher-strength and greater resistance to deformation.

Let’s examine the extruded magnesium tube, which comes in two forms: porthole and seamless.  Porthole and seamless tubes are both extruded meaning a billet of magnesium is forced through a hole in a die shaped to form the two-dimensional cross-section.  Often in O&G applications, that extruded magnesium needs to have a hole down the center, which for example, is a circular tube. Porthole and seamless extrusions are different ways of forming that hollow internal section (the ID of the tube).

Porthole extruded magnesium

In porthole, metal is forced around a shape that matches the hollow section to be extruded. Physics being what it is, the tooling needs ribs to hold that shape in place, and the metal must flow around them. That separates the metal as it extrudes, so a second die forces those sections back together. That forms a longitudinal seam running the length of the extrusion.

Seamless extruded magnesium tubes

As the name suggests, seamless extruded tubes avoid this. In this process, a forming mandrel is inserted into the billet of magnesium from the rear and pushed through until it’s very close to the opening in the die. The material flows through the gap between the mandrel and die, emerging with both internal and external dimensions fixed and without any seams.

The benefits of seamless tubes

Looking at the seam through a microscope often reveals a different crystal structure to the magnesium elsewhere in the extrusion. This is a result of the grain distortion caused by the deformation and subsequent pressure that welds the separate pieces together. The extent of the difference depends on how much deformation and pressure the metal undergoes. The net effect of this distortion is to create a weaker region along the seam.  In porthole tubes, welded seams create a region of weakness where the tube could rupture if subjected to high internal pressure or significant bending, as in the forces created during setting and frac pressures associated with the downhole components of a frac plug.

The importance for oil and gas

SoluMag is a seamless extruded tube capable of higher-strength and greater resistance to deformation and internal pressure during the downhole environments during a typical frac operation.

So for tubes requiring strength and consistency for downhole applications, SoluMag is the best choice for components used for frac plugs and other oil field completion equipment.

Luxfer MEL Technologies offers a wide range of extruded magnesium products for the oil and gas industries. We’d love to hear about your applications and discuss how our materials can offer you a solution.