電漿電解質氧化(PEO)用於鋁酸塩電解質中使用定電流雙極脈衝模式在 AZ91D 的鎂合金上鍍製具有保護效果之陶瓷膜層。本研究的目的是探討電性參數和操作時間對製備的 PEO 膜層的形成機制、微觀結構和化學反應的影響。為了分析 PEO 膜層，使用了掃描式電子顯微鏡 (SEM)、X 光繞射 (XRD)、膜厚偵測儀、恆電位移阻抗分析儀 (EIS) 技術。
首先，我們研究了IR(IR=I+/I-)和CR(CR=Q+/Q- =(I+×T+ on)/(I-×T- on) 關於PEO膜層的形成。這 2 個參數的組合決定了膜層上的電弧或軟火花。電弧火花會在膜層形成過程中促進破壞性放電的形成，而軟火花會軟化火花，這有助於形成孔隙較少的膜層。此外，電弧火花的發生是由於膜層的氧化和還原機制引起膜層上的軟火花。然而，從 XRD 數據中注意到，電弧火花能夠形成穩定的 MgAl2O4，而在軟火花期間，膜層中會產生不太穩定的 MgO。
已經發現，當 IR < 1 時，經過一定時間後，還原機制變得比氧化機制更佔優勢，並且會發生軟火花。 PEO 操作 30 分鐘後，雖然軟火花可以減少破壞性放電、孔隙率以及增加形成的膜層的耐腐蝕性 (4.816×107 Ω·cm2)，但開發的膜層的厚度 (< 20 µm) 及其均勻性較低。然而，如果（IR<1.0；CR<1.0），膜層的耐腐蝕性（7.32×104 Ω·cm2）和厚度（18.7 µm）下降得更多。另一方面，如果（IR>1.0；CR>1.0），膜層的厚度（>100 µm）和均勻性提高，但耐腐蝕性降低（4.419×105 Ω·cm2），孔隙率增加。這是因為在這種情況下，在膜層形成過程中只發生電弧火花。然而，當（IR>1.0；CR<1.0）時，PEO 膜層的質量顯著提高，因為電弧和軟火花在膜層形成過程中同時發生。這種條件提高了耐腐蝕性 (1.09×108 Ω·cm2) 和厚度 (81.55 µm)，同時降低了塗層中的孔隙率。
塗層在 (IR>1.0; CR>1.0) 中微觀結構的研究表明，隨著我們將 PEO 製程時間從 3 分鐘增加到 30分鐘，膜層厚度增加（22.67 µm 到 113.78 µm）但耐腐蝕性降低（3.274×109 Ω·cm2 至 4.419×105 Ω·cm2）。這是因為膜層的孔隙率增加了。或者，當（IR < 1; CR > 1.0）時，膜層厚度幾乎沒有變化，但耐腐蝕性增加（5.32×104 Ω·cm2 到 7.32×104 Ω·cm2），在 3 分鐘和 30 分鐘後分析 PEO 膜層。在這種情況下，弱火花有助於堵塞膜層中的孔隙。儘管如此，如果 (IR>1.0; CR<1.0) 用於 PEO 製程，操作 30 分鐘後，厚度（20.79 µm 至 81.55 µm）以及耐腐蝕性（7.389×106 Ω·cm2 至 1.09×108 Ω·cm2 ) 增加但孔隙率降低。
此外，由於（IR>1.0；CR<1.0）提供了相對較厚和最高的耐腐蝕性膜層，有系統地研究了不同電性參數對PEO膜層的影響。這些參數影響膜層的顯微結構、氧化量和還原量。已知，在 PEO 膜層形成過程中保持氧化和還原之間的平衡是最重要的。如果任何電性參數抵消了平衡，則膜層的微觀結構差異、膜層-基材界面和耐腐蝕性會發生變化。例如，如果正電流 (I+) 從 1.3 A 變為 1.0 A，而所有其他參數保持不變，則耐腐蝕性從 1.09×108 Ω·cm2 變為 1.62×108 Ω·cm2，膜層厚度從 81.55 µm 變為110.45 微米。

Plasma electrolytic oxidation (PEO) was used to create protective ceramic coatings on magnesium alloy called AZ91D using constant current bipolar pulsed mode in aluminate electrolyte. The aim of this study is to investigate the effect of electrical parameters, and Operation Time on coating formation mechanism, microstructure evolution, and chemical stability of the fabricated PEO coating. In order to analyze the PEO coating, Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), thickness measurement, and Electrochemical Impedance Spectroscopy (EIS) techniques have been used.
At first, we studied the effect of IR (I_R=I^+/I^-) and CR (C_R=Q^+/Q^- =(I^+×T^+ on)/(I^-×T^- on)) on the formation of the PEO coating. The combination of these 2 parameters determines the Arc or Soft-Sparking on the coating. Arc sparking encourages the formation of destructive discharges during the coating formation whereas Soft-sparking softens the sparks, which helps to form less porous coating. Also, Arc Sparking happens because of oxidation of the coating and reduction mechanism induces the Soft-sparking on the coating. Nevertheless, it has been noticed from the XRD data that arc sparking enables the formation of stable MgAl2O4, whereas during soft-sparking less stable MgO is produced in the coating.
It has been found that when IR < 1, after a certain period, reduction mechanism becomes more dominant than oxidation, and soft-sparking happens. After 30 min of PEO operation, although Soft-sparking can reduce destructive discharges, porosity as well as increases corrosion resistance (4.816×107 Ω.cm2) on the formed coating, the thickness (< 20 µm) and uniformity of the developed coating is pretty low. However, if (I_R<1.0; C_R<1.0), corrosion resistance (7.32×104 Ω.cm2) and thickness (18.7 µm) of the coating decrease even more. On the other hand, if (I_R>1.0; C_R>1.0), the thickness (> 100 µm) and uniformity of the coating improves but the corrosion resistance decreases (4.419×105 Ω.cm2) and porosity increases. This is because only arc sparking happens during the coating formation in this condition. However, when (I_R>1.0; C_R<1.0), the quality of the PEO coating improves substantially because both arc and soft-sparking happen simultaneously during the coating formation. This condition enhances the corrosion resistance (1.09×108 Ω.cm2) and thickness (81.55 µm) as well as decreases the porosity in the coating.
Study of the microstructural evolution of the coating in (I_R>1.0; C_R>1.0) shows that as we increase the PEO operation time from 3 min to 30 min, thickness of the coating increases (22.67 µm to 113.78 µm) but corrosion resistance decreases (3.274×109 Ω.cm2 to 4.419×105 Ω.cm2). This is because porosity of the coating increases. Alternately, when (IR < 1; C_R>1.0), thickness of the coating barely changes but corrosion resistance increases (5.32×104 Ω.cm2 to 7.32×104 Ω.cm2) when PEO coating is analyzed after 3 min and 30 min of PEO operation. Soft sparking helps to block the pores in the coating in this case. Nonetheless, if (I_R>1.0; C_R<1.0) is used for the PEO operation, after 30 min of operation, thickness (20.79 µm to 81.55 µm) as well as corrosion resistance (7.389×106 Ωcm2 to 1.09×108 Ω.cm2) increases but porosity decreases.
Additionally, as (I_R>1.0; C_R<1.0) provides comparatively thick and highest corrosion-resistant coating, a systematic study has been done on the effect of different electrical parameters on the PEO coating. These parameters affect the microstructural evolution, amount of oxidation and reduction in the coating. It has been found that it is paramount to maintain an equilibrium between oxidation and reduction during the PEO coating formation. If any electrical parameter offsets the equilibrium, the microstructural properties, interface of coating-substrate, and corrosion resistance of the coating changes. For instance, if positive current (I+) changes from 1.3 A to 1.0 A while all the other parameters remain constant, corrosion resistance changes from 1.09×108 Ω.cm2 to 1.62×108 Ω.cm2, and coating thickness changes from 81.55 µm to 110.45 µm.

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