The Effect of Tightening on the Corrosion Properties of the PVD Layers on Magnesium AZ91D Alloy

Abstract

The corrosive properties of PVD TiN-Ti-Al type composite titanium nitride layer with titanium and aluminium sub-layers system on the AZ91D magnesium alloy and its sub-layers were investigated by using polarization method and impedance spectroscopy, specifically, in terms of the hydrothermal tightening effect. The aim was to explain the mechanisms which contribute to high anticorrosive properties obtained by hydrothermal tightening. It was stated that the hydrothermal treatment changes the surface topography of the examined layers, most probably due to the formation of a continuous titanium oxides film. Tightening significantly modifies the corrosion resistance of the particular component sub-layers of the TiN-Ti-Al layer which exhibits optimum electrochemical parameters. It was shown that the increase of the electrochemical parameters is controlled by tightening of the outside titanium nitride layer, whereas the aluminium sub-layer plays a critical role in creating proper conditions for the hydrothermal tightening of titanium nitride.

Improving the Properties of Composite Titanium Nitride Layers on the AZ91D Magnesium Alloy Using Hydrothermal Treatment

“…This leads to the conclusion that the effective sealing deep inside of the outside titanium nitride layer, and in particular of the titanium sub-layer, such as was obtained in the overheated steam hydrothermal treatment variant, is essential to effectively isolate the aluminum sub-layer on magnesium alloy from the environment, and therefore to eliminate the risk of galvanic cells formation to prevent the accelerated galvanic corrosion of the AZ91D alloy covered with the composite titanium nitride layer. Even though, contrary to the original assumptions [ 21 ], the aluminum sub-layer as shown earlier [ 27 ] does not act as an anti-corrosion barrier in hydrothermally processed composite TiN-Ti-Al titanium nitride layer, its presence in the structure of the layer is a key factor in making the tightening process effective. It is supposed [ 22 ] that role is to prevent any galvanic corrosion processes between the titanium nitride layer and the AZ91D magnesium alloy substrate during hydrothermal treatment in the boiling water bath, which could precede and in consequence effectively prevent the successful tightening of the composite layer with oxides.…”

“…In fact, in this case, whenever the oxides film, which tightens the layer’s surface, breaks down due to corrosive processes or mechanical damage, the layer is no longer sealed inside and is automatically exposed directly to chloride environment penetration through the titanium and aluminum sub-layers defects towards the magnesium alloy substrate, so it behaves like an as-deposited layer ( Figure 6 ), which is sensitive to intensive pitting corrosion. Even though the aluminum layer on magnesium alloy on its own is relatively corrosion resistant [ 22 ], as a sub-layer of the composite TiN-Ti-Al titanium nitride layer, neighboring directly with the titanium sub-layer as shown previously [ 27 ], it does not constitute a durable anti-corrosion barrier that would be able to effectively isolate the outer, relatively noble, titanium nitride coating from the highly active magnesium base substrate. This is most probably due to the formation of corrosion cells between the aluminum sub-layer and the titanium sub-layer, resulting in fast galvanic corrosion [ 27 ].…”

“…Even though the aluminum layer on magnesium alloy on its own is relatively corrosion resistant [ 22 ], as a sub-layer of the composite TiN-Ti-Al titanium nitride layer, neighboring directly with the titanium sub-layer as shown previously [ 27 ], it does not constitute a durable anti-corrosion barrier that would be able to effectively isolate the outer, relatively noble, titanium nitride coating from the highly active magnesium base substrate. This is most probably due to the formation of corrosion cells between the aluminum sub-layer and the titanium sub-layer, resulting in fast galvanic corrosion [ 27 ]. In consequence, it leads to the perforation of the aluminum sub-layer, exposing the magnesium AZ91D alloy substrate to the corrosive environment, and the formation of aluminum–magnesium cells, which accelerate galvanic corrosion processes, as magnesium is even more active than aluminum.…”

Deposition time effects on structure and corrosion resistance of duplex MAO/Al coatings on AZ31B Mg alloy

“…This can easily produce a galvanic cell at the defects near the interface, thus accelerating corrosion (Daroonparvar et al, 2014;Altun and Sen, 2006). Therefore, there is ongoing research to minimize the density of these defects formed during or after the coating process (Tacikowski et al, 2014;Kaminski et al, 2014;Hoche et al, 2014). In the future, we aim to improve the compactness of the coating to ensure corrosion protection of coated Mg alloys.…”