For now, i am dealing with specimen Mg alloy AZ31...immersed it in 3.5% NaCl...for time being, the corrosion rate of polished condition faster than the grinding condition..how does it comes? Previous researchers strongly said that grinding conditions tend to corrode faster than polished surface condition...
In principle, the reverse is true, but the following explanation may apply for polished magnesium alloys:
As soon as the oxide layer is damaged by a magnesium alloy, the base alloy is subjected to extremely rapid corrosion. The protective oxide layer is thicker on a rough grinding surface than on a polished surface, which is why the oxide layer on the grinding surface is more resistant.
After polishing, the protective layer obtained from previous processing is blown. This layer may have been obtained by chemical patching or in the form of weathering as a surface oxide. After the polishing, the layer is peeled but the oxidation then activates.
I've thinked i already remove the oxide layer by grinding and cleaning it for certain time..does the filiform corrosion development or other types of corrosion slower because of the surface conditions? But, do we has a way to know either the material still has remaining oxide layer or not?
On such active metals as aluminum, magnesium, titanium, yttrium, rare earths and many others there is always an oxide layer! If you damage this layer, for example during mechanical processing, a new oxide layer will grow immediately. Only the layer thickness can be different: this layer is the thinnest on a polished surface.
The thickness of the oxide layer is also strongly dependent on temperature: the higher the temperature, the thicker the oxide layer. To prove that the thickness of the oxide layer has an effect on corrosion of the magnesium alloy, you can perform a simple experiment:
Take two equally polished samples, heat one of them to 200-300°C and then do the corrosion test. I believe that the sample without heat treatment will corrode more quickly.
thanks a lot Sir. Your explanations really help me. Really appreciate it.
While grinding, heat is generated. This increases the thickness of the oxide layer on the magnesium alloy. Polishing gives a smooth and thin oxide layer on the magnesium alloy.
The thicker the oxide layer, the higher the barrier to corrosion. The barrier will have to be removed either initially at flaws or imperfections in the oxide layer before corrosion activities can commence.
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