Effects of KF, NaOH, and KOH Electrolytes on Properties of Microarc-Oxidized Coatings on AZ91D Magnesium Alloy

Abstract

Oxide coatings were formed on AZ91D Mg alloy by microarc oxidation ͑MAO͒ in a sodium aluminate ͑NaAlO 2 ͒-based electrolyte, and the effects of additive electrolytes, KF, NaOH, and KOH, on the physical and chemical properties of the MAO coatings were investigated, particularly focusing on their corrosion resistance determined by potentiodynamic and potentiostatic tests. The anodizing characteristics ͑breakdown voltage and ignition time͒ and the phase, surface morphology, and thickness of the MAO coatings were strongly dependent on the additive electrolytes. The intense and continuous sparking in the KF-NaAlO 2 electrolyte resulted in a thick MgAl 2 O 4 coating containing F − ions, whereas the MAO coating obtained in either NaOH or KOH-NaAlO 2 electrolyte was thin and poorly crystalline, resulting from the unstable and discontinuous sparking. The MAO coating obtained in the KF-NaAlO 2 electrolyte exhibited the highest corrosion potential, the lowest corrosion current density, and the highest polarization resistance, which were closely related to the crystalline MgAl 2 O 4 phase and the relatively thick dense inner barrier layer. The pitting corrosion occurred in AZ91D Mg alloy, and the pitting was significantly protected by the MAO coating obtained in the KF-NaAlO 2 electrolyte. The pitting potentials for the MAO coatings have been determined by potentiostatic tests.Magnesium ͑Mg͒ and its alloys have been used in a wide range of applications such as construction, automotive, aerospace, and computer parts due to their excellent physical and mechanical properties including low density, high specific strength, high dimensional stability, and good machining and recycling ability. 1,2 However, poor corrosion and wear resistance, particularly when exposed to aggressive electrolyte species such as Cl − ions, severely limits their widespread uses in many applications. 3 The most effective way to improve the corrosion performance is surface modification by protective coatings. 4 Several coating technologies have been explored for Mg and its alloys including anodizing, 5-8 conversion coatings, 9 electrochemical plating, 10 physical vapor deposition coatings, 11,12 and organic coatings. 13,14 Among them, microarc oxidation ͑MAO͒ based on the traditional anodic oxidation is highly attractive because it can produce a thick, hard, and well-adherent ceramic-like coating that has the potential to significantly enhance the corrosion and wear resistance. [15][16][17][18] The nature and microstructure of the MAO coatings depend on numerous parameters, 19-28 but the chemical composition of the electrolyte is known to play a decisive role to determine the properties of the MAO coatings. [19][20][21][22][23] The presence of forsterite ͑Mg 2 SiO 4 ͒, spinel ͑MgAl 2 O 4 ͒, Mg 3 ͑PO 4 ͒ 2 , and MgF 2 in addition to MgO and Mg͑OH͒ 2 is beneficial for the improvement of corrosion resistance of the MAO coatings. 8,17,[22][23][24][25][26][29][30][31][32][33][34][35] Thus, in many studies, alkaline phosphate, silicate, and aluminate-based elec...

Polarization Behavior of Magnesium Alloy AZ91D with Micro-Arc Oxidation Coating in NaCl Solution

“…The results indicated that the properties of MAO coating are related to its primary composition and structure which depend on several factors, such as the compositions of the substrate, 15,16 the electric parameters, [17][18][19] the concentration and chemical composition of electrolyte. 9,10,20 In these investigations above, polarization curve is also an important electrochemical method to evaluate corrosion behavior of magnesium alloy with MAO coating. The polarization curves obtained in above papers showed various patterns which were considered to only depend on the composition and structure of the MAO coating.…”

Anodizing of AZ91D Magnesium Alloy in Borate-Terephthalic Acid Electrolyte

“…1 However, the poor anti-causticity of magnesium alloy in harsh environment hinders its use in many applications. [2][3][4] Various technologies including alloying, surface modification, chemical conversion, electroplating, anodic oxidization, and plasma electrolytic oxidation (PEO) [5][6][7][8] have been developed for the protection of magnesium and its alloys. Among them, PEO technology is the most studied in recent years 9 because of the significant properties of the anodized film, such as good adhesion, high corrosion resistance, and hardness.…”

Microstructure Evolution During Plasma Electrolytic Oxidation and Its Effects on the Electrochemical Properties of AZ91D Mg Alloy

“…[13][14][15][16] The PEO coatings are commonly composed of outer porous layer and inner dense layer, [16][17][18][19][20] and the corrosion resistance strongly depends on the properties of inner dense barrier layer. 17,18,[21][22][23][24][25][26][27][28][29][30][31] The chemical composition of the electrolytes is known to play a decisive role that determines the physical and chemical properties such as the phase, composition, and structural characteristics of the barrier layer. 20,22,[25][26][27][32][33][34] In the previous study, 25 the effects of additives (KF, NaOH, and KOH) in a sodium aluminate (NaAlO 2 )-based electrolyte on the physicochemical properties and corrosion characteristics of the PEO coatings on AZ91D Mg alloy were investigated.…”

“…17,18,[21][22][23][24][25][26][27][28][29][30][31] The chemical composition of the electrolytes is known to play a decisive role that determines the physical and chemical properties such as the phase, composition, and structural characteristics of the barrier layer. 20,22,[25][26][27][32][33][34] In the previous study, 25 the effects of additives (KF, NaOH, and KOH) in a sodium aluminate (NaAlO 2 )-based electrolyte on the physicochemical properties and corrosion characteristics of the PEO coatings on AZ91D Mg alloy were investigated. The PEO coating obtained in KF-NaAlO 2 electrolyte exhibited the highest corrosion resistance, which has been attributed to the presence of considerably thick, fluoride (F)-containing inner dense barrier layer with crystalline MgAl 2 O 4 .…”

“…The influence of applied current density, electrolyte concentration, and treatment time of the PEO process in KF-NaAlO 2 electrolyte has been reported previously, 20,35,36 but the presence and development of inner dense barrier layer and its effects on the electrochemical corrosion properties have not been well addressed in this electrolyte system. In addition, fluoride is known to be incorporated into the oxides during PEO in the F-containing electrolytes such as KF, NaF, NH 4 F, K 2 ZrF 6 , and Na 3 AlF 6 , [18][19][20][24][25][26][27][28][32][33][34][37][38][39] and the fluoride compounds such as KMgF 3 , MgF 2 , and AlF 3 were also found near the coating=substrate interface. 19,20,37 The MgF 2 formation was considered to be beneficial for the improvement of corrosion resistance because of the lower solubility of MgF 2 than magnesium oxide or hydroxide.…”