4.3 EC-characterisation of coatings
4.3.1 AZ31
As first corrosion test the plain material was tested to establish a baseline for further testing. The reference measurements on AZ31 only grinded with 1000er SiC paper is displayed in figure 4.22.
-200 0 200 400 600 800100012001400160018002000 -1,65
-1,40 -1,35 -1,30 -1,25 -1,20 -1,15 -1,10 -1,05 -1,00 -2 Figure 4.22 OCP and pitting results from grinded AZ31
After 1800s immersion in 0.05MNaCl solution the OCP stabilised at a value of 1.55V vs.SHE. The pitting test shows a distinct anodic behaviour with steadily increasing current density and a very small cathodic part. The corrosion current density is1.55·10−1cmµA2 at a potential ofEcorr =−1.263V vs. SHE In the following are the corrosion test results for the Zn and Al-Zn coating. The samples with a pure Al slurry were not tested, due to the lack of a diffusion layer.
Zn-coating
The zinc coating shows in general two different corrosion profiles, as it can be ex-pected from the coatings results. Figure 4.23(a) shows the OCPs from the zinc experiments. The two coatings at 450ï¿12C show an Ecorr about +100mV more an-odic than the reference. Similar results come from the corrosion test depict in figure 4.23(b). The shape of the corrosion current for the blank, 420ï¿12C and 380ï¿12C are indication for pitting followed by a stable corrosion.
-200 0 200 400 600 800 100012001400160018002000
(a) OCP measurements (b) Potentio dynamic polarisation curve Figure 4.23 Corrosion measurements of AZ31 Zn coatings
Al-Zn coating
Figure 4.24 shows the OCP and pitting measurements for the AZ31 coated with AlZn at various temperatures. The results are similar to the previous ones and only the coating at 450ï¿12C shows a significant increase in the OCP of about +160mV.
The potentiodynamic polarisation curve for 450ï¿12C shows a very long cathodic branch. Song et al. [4] described occluding of cracks and pits by hydrogen bubbles during corrosion and formation of local anodes and cathodes inside these isolated cracks. Furthermore, it is not possible to measure the corrosion inside the isolated cracks. Considering the experimentation setup, it seems likely that the anodic sides on the sample have been isolated by hydrogen bubbles, so that only the cathodic reaction could be observed.
-250 0 250 500 750 1000 1250 1500 1750 2000 -1,42
(a) OCP measurements (b) Potentio dynamic polarisation curve Figure 4.24 Corrosion measurements of AZ31 AlZn coatings
4.3. EC-characterisation of coatings 47
4.3.2 AZ91
Zn-coating
The corrosion measurements on AZ91 have similar results to those of AZ31. All samples have an OCP similar to that of the reference, except the 450ï¿12C sample as displayed in figure 4.25(a). The potentio dynamic polarisation results reflect the OCP measurements and only the 450ï¿12C sample shows an improved corrosion resistance.
-200 0 200 400 600 800 100012001400160018002000 -1,34
(a) OCP measurements (b) Potentio dynamic polarisation curve Figure 4.25 Corrosion measurements of AZ91 Zn coatings
Al-Zn coating
Similar to the Zn coating, only the sample heated at 450ï¿12C showed a changed OCP and Ecorr during potentio dynamic polarisation measurements.
-200 0 200 400 600 800 100012001400160018002000 -1,350
-1,325 -1,300 -1,275 -1,250 -1,225 -1,200 -1,175
UvsSHE[V]
t [s]
AZ91 blank
AZ91 Al-Zn 450°C
AZ91 Al-Zn 420°C
AZ91 Al-Zn 380°C
(a) OCP measurements (b) Potentio dynamic polarisation curve Figure 4.26 Corrosion measurements of AZ91 AlZn coatings
49
5. DISCUSSION
The experiments of this work gave a few interesting results and the most significant is the formation of a diffusion layer only at a temperature of 450ï¿12C regardless the slurry or basic material, except that no diffusion layer could be produced using a pure Al-slurry. Several effects might have hindered its diffusion and promoted Zn and Zn-Al slurries. Hirmke et al. [29] studied packed powder diffusion with Zn-Al-Zn on AZ91E and concluded that the addition of Zn to the Al powder significantly promotes the formation of intermetallic layers. In their paper they report of diffusion layers as thick as 600µm for T as low as 350ï¿12C. Nevertheless, their procedure requires treatment times of up to 18h. The results show that the addition of Zn in the slurry powder promotes the diffusion of Al and the formation of ternary Mg-Al-Zn phases.
All of the phases are very Mg rich and their composition is summarised in table 5.1 Phase Material T [ï¿12C] t [hr] Coating Composition
Fine structure AZ31 450 2 Zn Al1Mg35Zn5
Coarse structure AZ31 450 2 Zn Al2Mg91Zn10
Light grey AZ31 450 2 Al-Zn Al14Mg28Zn1
Dark grey AZ31 450 2 Al-Zn Al6Mg72Zn1
White precipitations AZ31 450 2 Al-Zn Al6Mg25Zn2 Table 5.1 Formed phases in diffusion layers on AZ31
In all the samples the diffusion coating contained areas of the crude basic material in the form of dendrites. EDX analysis showed that oxidic Mg particles segregated at the upper part of the dendrites. Additionally, the samples at 420ï¿12C, with no diffusion layer, contain a thin oxide layer on the surface. The corrosion measure-ments on these samples show pitting characteristics and the local breakdown of a protective film as common for magnesium [30]. The oxidic Mg particles seem to prevent diffusion effectively and should be the reason why no pure Al was observed.
The difference in the Zn experiments can be explained by the liquefaction of Zn at a temperature of 420ï¿12C. In the 450ï¿12C experiments the Zn was liquid and caused a breakthrough through the protective surface layer. At lower temperatures the Zn did not melt totally and could not aid the oxide breaking process.
In the SEM micrographs it is visible that the slurry particles show a similar in-termetallic phase structure as the diffusion layer. Hirmke et al [29] presented a schematic description of the diffusion dynamics during the PPDC process. It seems viable to assume a similar mechanism here, namely that the particles also diffuse into each other, into the magnesium and the magnesium into them. Zn will diffuse mainly into the Al-particles, since the solubility of Zn in Al is much higher (≈64%
[29]) than Al in Zn (≈2% [29]). Thus when the Zn started to melt it got solved in the Al particles and the magnesium substrate. However, Czerwinski [31] investigated the oxidation behaviour of AZ91D at high temperatures and reported sublimation of magnesium. The sublimation was guided by the oxidation process and Al content.
High Al contents of 10% seem to increase the rate-constant at 673K by over two order of magnitude. In general it was assumed that an increased oxidation rate was the major driver for magnesium sublimation. Furthermore, Czerwinski pointed out a similar oxidation acceleration effect for zinc. The slurries which produced diffusion layers had all a high zinc content (50wt.%&25wt.%) and half of them additionally a high aluminium content (25wt.%). It seems likely that the zinc and aluminium together supported the formation of semi-solid magnesium, which then sublimated and interacted with the powder particles on top of the sample.
The EC measurements showed that the diffusion coatings increase the corrosion resistance of AZ31 and AZ91. The shape of the potentio dynamic polarisation curves is typical for magnesium and its alloys [3], [4], [21], [22], [29], [32]–[34]. From this it follows, that the specimen corroded locally plus figure 5.1 shows very localised black spots on the surface.
51
Figure 5.1 AZ31 after corrosion test
The potentio dynamic polarisation curves seem to follow the course of a typical tafel plot. King et al. [21] compared several techniques regarding the preciseness of the measured corrosion rate and reported that Tafel extrapolation gives too small values. Tafel extrapolation is based on the assumption that the surface corrodes uniformly and not locally as with magnesium [16]. The corrosion mechanism of all samples is the same that a thin partially protective layer (not the diffusion layer) exists on the surface and leads to the phenomena of pitting after local breakdown.
The higher Ecorr of the successfully coated samples results from the enrichment of zinc in the diffusion layer. Joensson [35] describes in his PhD thesis that zinc does not increase the corrosion protection of magnesium, but introduces an anodic shift of Ecorr.
6. CONCLUSION
1. The diffusion process can easily be hindered by the natural surface oxide layer of magnesium. At high temperatures (>450ï¿12C) the protection of the layer is not enough to stop the diffusion, but at lower temperatures (<= 420ï¿12C) all diffusion is blocked.
2. Zn aids the diffusion of Al by lowering the melting point and promotes the for-mation of ternary Mg-Al-Zn phases. Molten zinc on the surface can break the protection of the surface oxide layer and enable diffusion into the magnesium.
3. The diffusion process is guided by a mass exchange between the Zn particles, the Al particles and the Mg substrate. All of them diffuse into each other and change their physical properties respectively. Furthermore the addition of zinc causes the aluminium and magnesium to melt at lower temperatures and thus supported the evaporation of semi-solid magnesium.
4. Zinc as major part in the diffusion layer does not bring the hoped for corrosion protection. Ecorr is increased due to the higher zinc content, which would reduce galvanic corrosion potential, but icorr is not influenced by the zinc content.
53
7. OUTLOOK
The topic of corrosion protection through diffusion layer coating proved to be a challenging and interesting topic with a great potential to supply easy and cheap access to corrosion resistance improved magnesium. This work shall be seen as proof of principle and the basic layout for further research. Many variables play a role in the formation of good diffusion coatings and the most important, from my point of view, are:
Treatment time Influences the layer thickness, ability to break the surface oxide layer and economical efficiency of the process.
Surface oxide layer Critical parameter on the way to low temperature applica-tion and hinders diffusion of third metals effectively. In further research the introduction of an oxide breaker as known from packet powder diffusion might be helpful. Another option is the removal by pickling, e.g. with an aluminium pickle.
Gas atmosphere Oxygen seems to be a harmful element to the whole process and methods of reducing the residual oxygen level to an ǫ level (0%) seems worthwhile.
Slurry composition Different compositions yield different diffusion layers. Worth-while implementing should be the design of different slurries according to the other parameters, such as treatment time and temperature.
Temperature Similar to treatment time, the temperature plays a major role in the whole process and an optimum between layer thickness, composition, time and temperature should be found. Nevertheless, from an economic standpoint the aim should be for lower temperatures.
Process Integration into existing industrial processes would not only reduce the costs, but can also benefit the overall environmental conditions.
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