Development of a Novel, Environmentally Compliant Magnesium Coating System for Corrosion Protection of Magnesium Alloys

Due to its low density and good mechanical properties, Mg and its alloys are very attractive materials for aerospace applications. However since Mg is the most electrochemically active structural metal, protecting it from environmental degradation presents a real challenge.  Corrosion of Mg alloys depends on their chemical composition and microstructure. The intermetallic phases formed during production processes are more noble than the Mg matrix and result in the establishment of microgalvanic cells within the Mg alloy; thus, accelerating the overall corrosion of the Mg.  To shield Mg and its alloys from corrosive environments, their surfaces are usually protected by chemical and conversion inorganic coatings, sol-gel coatings, anodization, or thermal spraying.  But one has to be aware that defects present in the coating or developed during operation often result in the formation of galvanic cells leading to accelerated corrosion of Mg component. 

An ideal, effective coating should be able to simultaneously provide corrosion protection through: sacrificial anode-based cathodic protection, active corrosion protection to minimize corrosion at defects (for example by releasing active inhibitors), and formation of a protective barrier layer on the substrate surface. Producing a protective coating which could fulfill the above requirements is extremely challenging. For example, creation of sacrificial coatings for Mg substrate is not an easy task, because there are not many engineering materials more active than Mg alloys. However physical vapor deposition technology can be employed to produce anodic coatings capable of sacrificially protecting Mg alloy components.

Our preliminary results show that physical vapor deposition process indeed can produce thin films of Mg alloys with sacrificial properties. These thin films were tested in artificial sea water in ambient conditions:

 

Alloy	Form	OCP (V vs. SCE)	Corrosion rate, mpy
Pure Mg	bulk	-1.6	131.3
	Thin film	-1.9	27.6
AZ31	bulk	-1.6	34.3
	Thin film	-1.8	3.6
AZ91	bulk	-1.6	9.5
	Thin film	-1.8	6.3

The electrochemical parameters show that thin films not only exhibit lower corrosion potential (on average lower by 200 mV) but also have lower corrosion rate. Obviously, the corrosion performance of the thin film is a function of the microstructure of the deposits and this in turn depends on deposition parameters. The key factors in deposition process which enable production of  films with tailored properties will be discussed.