Corrosion & treatment
Corrosion is an important issue when magnesium is applied and protection is in most cases needed. Atmospheric corrosion occurs when magnesium surfaces without protection and  exposed to the atmosphere. With the high purity alloys the loss of material to oxidation is lessend.

Tests show that on the high purity magnesium alloys, there is less corrosion than on the aluminium alloys with factor three and factor seven with carbon steel. In general use the magnesium parts does not need to be coated unless used in cosmetic applications.

Galvanic corrosion occurs when magnesium is connected to other metallic materials in the presence of an electrolyte. The anode and cathode reactions must balance each other, which means that by reducing the consumption of electrons in the cathode reaction the produced electrons in the anode reaction are reduced accordingly. Tests have shown that there are minor differences  between the various magnesium alloys concerning the extent of galvanic corrosion. The trend is that the most corrosion resistant alloys also show the lowest degree of galvanic corrosion. The differences are small, meaning that the relative resistance to galvanic corrosion is not a decisive factor in alloy selection for a specific application. The properties of the cathode (the other metal than magnesium) are far more important.

With proper design it is possible to influence the electrolyte resistance and it is important to avoid water accumulation. Efficient drainage will reduce the electrolyte thickness and thus increase the electrolyte resistance. Another method is to insert spacers or shims.
In case of an aluminium spacer or shim, a compatible material is inserted between magnesium and the metal that induces galvanic corrosion. The use of aluminium washers is very efficient. With an aluminium washer between the magnesium and the coated steel bolt, the galvanic corrosion is reduced to the same level as using aluminium bolts alone, i.e. the galvanic corrosion is fully controlled by the washer. When mud accumulates at the boltwasher area, the effect of the washer is reduced but still gives a significant reduction compared to a plated steel bolt alone. The mud bridges the gap between the bolt head and magnesium, and the cathode reaction on the metal-plated bolt contributes more to the galvanic corrosion than with no mud present. It is expected that a polymer coating on the steel fasteners is better than metal plating in the presence of mud, because an intact and resistant polymer coating insulates the steel from the mud environment.

The bolt head design is also important for coated steel bolts. As described previously, it is important that the coating is intact. During assembly, defects may easily have been made in the coating and it is important that these defects are located as soon as possible before contact point to magnesium. Therefore inner drive bolts are preferred to over drive bolts. Coatings over areas of the bolt head with sharp edges, as in hexagonal head bolts, have a tendency to become very thin or even absent and therefore it is recommended to select a bolt design that is round and free from sharp edges.

Coating the magnesium for protection against galvanic corrosion is not recommended. The reason is that coating defects are easily introduced during assembly and also during service. This defect in the coating on magnesium creates a small anodic area compared to the large cathodic area. When the coating is essential with for example automotive parts like inner door frames, robust coating systems are necessary so as to withstand the assembly forces as well as the dynamic forces during service.

Chromate-type conversion coatings are losing favour because of environmental issues but may provide additional corrosion resistance when applied before top-coatings or paints.
Some (commercial) chrome-free conversion coatings for magnesium are:
  • Magpass ™ mixed solution of vandate, molybdate and magnadate
  • Gardobond ™ X4707 titanium/zirconium hexafluoride
  • Gardobond ™ X4729
  • Gardobond ™ X4740 zirconium/ hexafluoride
Painting of magnesium is similar to that of other metals. The paint is often applied over a conversion coating, as described in the previous section.

Anodizing (Anomag ®) is an alternative treatment that can be used as final finish or as a base for subsequent painting. Anodizing gives excellent corrosion resistance to wear.

Metal plating is possible with magnesium and in a manner similar to aluminium. The cleaning and activation steps are firstly to remove the die lubricant and to provide a uniform and reactive surface for initial plating, whether it be zincating or electroless nickel.

Zincating is an electrolytic zinc deposition process giving 0.1-0.2 µm layer of zinc on the surface. In traditional processes, the zincating is followed by electrolytic deposition of copper. Aside from copper, any metal can be plated (chromium, silver, nickel, gold).

Electrolysed nickel can replace both the zincating. The copper strike steps to provide a surface for plating of other metals or it can serve as the final finish.

Keronite ® is a ceramic  treatment for magnesium which provides excellent protection against corrosion and also effective hardness and wear resistance. The porous structure of the outer layer provide an ideal base for subsequent finishing with decorative scratch-resistant topcoats for adhesive bonding or for composite layers of PTFE or even other metals. Keronite ® has high hardness (400 to 600HV depending on alloy and thickness layer) and the thickness of the coating can be controlled from 5 -50 microns.

Keronite ® withstands over 1000 hours in salt fog (ASTM B117) without sealing and contains no chrome or other heavy metal and processes at room temperature.