Magnesium Alloys - Part 5 - Safety Precautions

Safe handling and Pre-Treatment Procedures / Continued from Galvanotechnik 6/2024: Magnesium is a reactive and highly combustible metal. In certain forms, like chips, granules, powder or thin ribbon, it ignites easily in the presence of water or cutting fluids containing fatty acids. Hydrogen produced in this process adds to an explosion hazard in addition to fire. However, magnesium is an excellent conductor of heat, in solid forms, like ingots, its combustion is difficult to get started until the entire piece is brought too close to melting point. The flammability of magnesium is greatly reduced by a small amount of calcium in the alloy. In this article safe handling, firefighting and pre-treatment procedures for magnesium alloys are discussed.

Safe handling and storage of magnesium

Airborne magnesium dust burns explosively if ignited in the critical air-dust ratio. Smoking and the use of open flame or electrical welding must be prohibited in areas where magnesium is machined, sawed, or grounded. All electrical connections and motors must be explosion proof. Non-sparking conductive tools must be used where magnesium dust is present [1-2].

Dust collector system

While dry grinding of magnesium alloy parts, the grinding dust should be captured in a dedicated wet dust collector vacuum system [3]. The entire system should be grounded and the power supply to the exhaust fan and the liquid level controller should be interlocked. The system should be designed in such a way that no dry dust is accumulated at any point before being converted to sludge. The collector must also be designed in such a way that hydrogen generated in the sludge is vented at all times, even in the event of a power failure. The magnesium slag in the vacuum cleaner should be cleaned up periodically to prevent excessive accumulation. Avoid dry dust on the high-speed moving parts. Sludge from the liquid separators should be removed at least daily and transported to the disposal site or staging area in covered and vented steel drums.

If the magnesium parts are to be wet ground on a belt sanding device or a disc grinder, a sufficient amount of cutting fluid should be used to collect all the dust. Do not ground the parts with a chromated surface or those containing steel inserts because both can spark. Sparks from any source can ignite the grinding dust and the resulting fire can travel through the entire system.

Machining

Machining of magnesium requires less power than any of the other commonly used metals, allowing for maximum speeds and feeds that produce large chips. Cutting tools should be kept sharp and never allowed to ride on the metal without cutting. Use only high flash tapping fluid, water-oil emulsion fluids that inhibit hydrogen gas formation. Do not use water soluble oils or oils with > 0.2 % fatty acids [3].

Molten magnesium

Those working around molten magnesium must wear adequate protective clothing and equipment: safety glasses, hard hat with safety shield, fire retardant clothing, safety shoes, insulated gauntlet gloves. Molten magnesium will ignite and burn when exposed to air, so it must be protect­ed during the melting operations. The traditional method was a cover of chloride salts that fluidized, excluding air contact at the surface. Today the common practice is to use a protective gas, such as sulfur hexafluoride (SF6), in very low concentrations with air – or air and carbon dioxide. SF6 forms a film on the melt surface, which prevents excessive oxidation [4]. Any material, such as ingots or tools, that are introduced into molten magnesium must be pre-heated well above 100 ºC to drive off all moisture or other volatiles.

Molten magnesium can react exothermically with iron oxide in a thermite reaction which generates temperatures in excess of 2200 °C and a large amount of heat. Since most magnesium melting pots are made of steel, it is extremely important to keep the inside of the pot clean and free of scale. The refractories used for the furnace should be high alumina or magnesia since molten magnesium can react violently with even small amounts of silica which may be present in ceramic materials.

Magnesium foundry structures should be built with non-combustible materials and the floors around melting operations should be hard burned or vitreous paving blocks. The heat from molten magnesium can release the water of hydration in concrete which will cause it to spall, sometimes explosively.

Storage and transportation

Magnesium products should be stored at ground level in a well-drained area where water will not puddle. Preferred storage for ingots and parts is a single level, non-combustible building. If stored with other combustible materials, the National Fire Protection Association recommends the followings [1]:

parts weighing 11 kg or more: limit to 36 cubic meters; and

parts weighing less than 11 kg: limit to 28 cubic meters.

Magnesium storage in quantities greater than 50 cubic feet (1.41 m³) shall be separated from storage of other materials that are either combustible or in combustible containers by aisles. Piles shall be separated by aisles with a minimum width of not less than the pile height. Aisles should be wide enough to permit effective use of equipment by firefighters.

Dry magnesium powder, chips, granules, and turnings should be stored in tightly sealed, non-combustible containers such as steel drums that are well separated from other combustible materials. The use of automatic sprinklers in those areas should be prohibited. Wet magnesium turn­ings, fines, or sludge should be kept under water in a covered and vented non-combustible container and stored outdoors. When damp magnesium is exposed to air, it will generate heat which accelerates the evaporation and will eventually result in ignition. Containers should never be stacked, and ignition sources should be kept away from the vent.

Firefighting

With enough heat, magnesium can catch fire and burn fiercely giving off a bright light and billowing clouds of white smoke. Flame temperatures can reach up to 3,100 °C. Once ignited, such fires are difficult to extinguish because combustion continues in nitrogen (forming magnesium nitride), carbon dioxide (forming magnesium oxide and carbon), and with oxygen molecule of water it reacts violently producing hydrogen, creating an explosion hazard. Many other conventional fire extinguishing agents, like foam, halogenated agents, dry chemicals containing mono or diammonium phosphate, will also accelerate a magnesium fire.

Magnesium burns by direct oxidation, therefore the only way to control it is to „smother“ with dry, air-excluding agents. A magnesium fire is a Class D fire and it should be handled carefully or let it burn to extinction. Class D fire extinguishers for Mg contain completely unreactive sodium chloride powder, and sand or silica. Examples include: Met-L-X powder; Dry G-1 powder; magnesium foundry flux; FEM-12 SC [1]. Pull the pin of fire extinguisher, hold the nozzle above the fire (max. reach of the nozzle is 6 feet), squeeze the discharge lever, and sweep the nozzle back and forth so that it covers the base of the fire. In confined areas, such as a storage tank, it can be suppressed with argon.

The use of magnesium is rapidly growing worldwide and millions of tons of magnesium have been melted and processed without incident by following well-developed safety practices. With proper safeguards, magnesium alloys can easily be welded, rolled, extruded, forged, and heat-treated.Fig.1: Magnesium-water Pourbaix (E-pH) diagram at 25 °C

Corrosion resistance and surface modification

The highly reactive nature of magnesium is clearly indi-cative by its position in the electrochemical series (E°= - 2.37 V). The electrochemical behaviour of magnesium in aqueous solutions can be understood from Pourbaix diagram (Figure 1) [5], which shows that magnesium dissolves readily as Mg²⁺ in all solutions with a pH < 10.

Surface modification by wet chemical processes, laser treatment, physical vapour deposition, polymer/powder coatings etc. enable improvement in corrosion resistance of magnesium without affecting bulk properties. The wet chemical processes are most effective and widely used surface modification techniques due to their inherent low cost and ease of operations:

• Chemical conversion coatings: provide temporary protection or paint base
• Anodic oxide coatings: impart superior wear and corrosion resistance as well as a good paint base
• Metallic plating: offers surface conductivity, solderability and corrosion protection
• The oils and waxes can be applied for temporary storage of parts, until a suitable surface treatment is implemented.

Pre-Treatment procedures

As all other metallic objects being prepared for finishing, a clean surface of substrate material is necessary for subsequent surface treatment. Therefore, all surface contaminants such as oil, grease, dirt, die forming components, surface oxides, etc. must be removed. This requires establishment of a suitable cleaning cycle depending on the condition of the object and nature of the contaminants. In some cases, it may be prudent to remove the previous surface treatment that was applied for temporary protection. The general cleaning and pre-treatment methods described herein are derived from the standard literature [6-11].

Mechanical cleaning

The methods of mechanical cleaning of magnesium alloys are similar to those used on zinc and aluminium alloys. These include polishing, buffing, blasting, sanding and brushing. However, dry blasting is usually avoided because of the cathodic-particle contamination that arises when employing typical blasting media. Stainless steel or aluminium wire brushes should be used. Aluminium wool and aluminium oxide papers also give good results. Regular steel wool and brushes as well as silicon carbide cloth should not be used. When blasting method of any type employed there is danger of embedding surface contaminants which may increase the basic surface corrosion rate. In such cases subsequent acid pickling must be used to etch up to 50 µm from the surface prior to the application of any coating.

Solvent cleaning

Solvent cleaners are employed to remove abnormal amounts of grease oil from the job. These are also used for cleaning of finished products which have been soiled in handling. Solvent cleaning prevents a rapid build-up of oil and grease in the subsequent alkaline cleaner. Vapor decreasing, ultrasonic cleaning or immersion methods may be employed using chlorinated solvents, hydrocarbons or alcohols. Emulsion cleaners can also be used. These cleaners are essentially a mixture of emulsifying agents and hydrocarbon solvents in which it is possible to water rinse the solvent and contaminants from the surface after cleaning, by dip or spray method.

Alkaline cleaning

Unlike aluminium, magnesium is not appreciably attacked by caustic solutions. Therefore, heavy duty alkaline cleaners high in caustic can be used. These solutions are particularly beneficial when old chromate type of coating as well as heavy oil and grease are to be removed. The follow­ing chemical formulation provides satisfactory results for most of the magnesium alloys:

Higher caustic soda concentrations are used to remove burned-on die lubrication and the lower concentration for special alloys high in zinc and zirconium that are less resistant to strongly alkaline solutions. If the solution is to be used as a soak cleaner, a wetting agent is added and the job is agitated to obtain uniform cleaning.

Electro cleaning shortens the cleaning time considerably, the same solution can be used except that the presence of a wetting agent is undesirable. The work is made cathode and a current density of 2-4 A/dm² is employed for 1-5 min. No agitation of work is required.

A mild alkaline cleaner (de-oxidizer) of following composition can be used to remove low scale oil and grease, where the jobs are only slightly contaminated:

Acid pickling

Acid pickling is required to remove the oxide layer and flux from the castings; previously applied coatings; burned-on drawings and forming lubricants or other water soluble and un-emulsifiable substances. The dimensional changes associated with acid pickling are high since these solutions dissolve the base material as well. Numerous pickling solutions have been used in the past for magnesium alloys but only those which are most commonly used are described here.

Chromic acid pickling is useful for close tolerance parts to remove superficial oxide. The parts can be cleaned by immersion in a solution of following composition:

Chromic acid-ferric nitrate bright pickle is effective on all common magnesium alloys and forms. It produces a chemical polishing effect providing a smut free bright surface. The parts can be dipped or sprayed with the solution of following composition:

In this solution approximately 4 µm of surface are removed per min. Increasing the fluoride contents tends to increase the reactivity. Low fluoride concentration is preferred for wrought products and high fluoride content for castings.

Chromic acid-sodium nitrate bright pickle is commonly used for wrought magnesium alloys that are low in alumin­ium contents. The burn-on graphite lubricants from hot formed sheet parts can be effectively cleaned prior to arc or gas welding.

Increasing the chromic acid and decreasing the sodium nitrate provides best results for basket pickling of small parts. A pH range of 0.5 to 0.7 is most effective. When the pH is increased above 1.7, it should be lowered by addition of chromium trioxide.

Acetic acid-sodium nitrate pickle is primarily used for the removal of mill scale oxides from wrought alloys and non-aluminium magnesium castings. A metal removal of approximately 25 µm per min from the job surface is expected by this pickling.

REFERENCES

[1]Safe Handling of Magnesium, International Magnesium Association, https://cdn.ymaws.com/www.intlmag.org/resource/resmgr/safety/Safe-Handlling-of-Mg.pdf, Accessed on 2023.01.31
[2]Safe Handling of Magnesium, http://www.magmachineparts.com/about-magnesium.php, Accessed on 2023.01.31
[3]Safety Analysis of Grinding and Machining of Magnesium Alloy Parts, https://metalpartss.com/Safety_Analysis_of_Grinding_and_Machining_of_Magnesium_Alloy_Parts/, Accessed on 2023.01.31
[4]S. Housh; J. Waltrip: Safe handling of magnesium alloys, SAE Technical Paper 900786, (1990) 1-10. doi: 10.4271/900786.
[5]Pourbaix, M.: Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, (1974) p.139
[6]J.E. Hillis: ASM Hand Book on Surface Engineering, Surface Engineering of Magnesium Alloys, C.M. Cotell, J.A. Sprague, F.A. Smidt Jr. (Editors): ASM International, 5 (1994) 819-834
[7]W. Canning: Canning Handbook of Electroplating, The Surface Finishing of Magnesium, W. Canning & Co. Ltd., 21st Edition, (1989) 722-727
[8]H.K. DeLong: Anodizing and Surface Conversion Treatments for Magnesium, Electroplating Engineering Handbook, L.J. Durney (Editor), Van Nostrand Reinhold, New York, (1984, reprinted 2000), pp.410-419
[9]A.D. Forno; M. Bestetti: Electroless and Electrochemical Deposition of Metallic Coatings on Magnesium Alloys Critical Literature Review (Chapter 8), Magnesium Alloys: Corrosion and Surface Treatments, F. Czerwinski (Editor), IntechOpen, London, (2011) 153-184. doi: 10.5772/13975
[10]80th Universal Metal Finishing Guide Book, American Electroplating Society, 110 (2012) 283-287
[11]D. Hoche; A. Nowak; T. John-Schillings: Surface Cleaning and Pre-conditioning Surface Treatments to Improve the Corrosion Resistance of Magnesium (Mg) Alloys, Corrosion Prevention of Magnesium Alloys, G.L. Song (Editor), Woodhead Publishing Ltd., (2013) 87-109