Abstract
The principal possibility to grow layered double hydroxide (LDH) at ambient pressure on plasma electrolytic oxidation (PEO) treated magnesium alloy AZ91 in the presence of chelating agents is demonstrated for the first time. It avoids hydrothermal autoclave conditions, which strongly limit wide industrial application of such coating systems, and the presence of carbonate ions in the electrolyte, which lead to the formation of “passive” non-functionalizable LDH. A combination of chelating agents (sodium diethylenetriamine-pentaacetate (DTPA) and salicylate) were introduced to the treatment solution. The role of each additive and the influence of treatment bath composition on the LDH formation processes are discussed. A synergistic effect of DTPA and salicylate during LDH formation is discovered and its possible explanation is proposed.
Highlights
Plasma electrolytic oxidation (PEO) treatment is an effective way to improve corrosion and wear resistance of Mg alloys due to the formation of ceramic oxide layers[1,2,3,4,5,6]
We demonstrate the possibility of using environmentally friendly chelating agents for direct hydrothermal synthesis of Mg,Al-layered double hydroxide (LDH) on the surface and in the pores of plasma electrolytic oxidation (PEO)-treated AZ91 alloy at 95 °C without employing autoclave conditions
The acceleration effect of different chelating agents on LDH formation on bare AZ91 alloy is related to the increase of soluble Mg2+ concentration in the pH range favorable for LDH growth[45]
Summary
Plasma electrolytic oxidation (PEO) treatment is an effective way to improve corrosion and wear resistance of Mg alloys due to the formation of ceramic oxide layers[1,2,3,4,5,6]. The process is significantly more environmentally friendly in comparison with conventional chromic acid anodizing, previously widely used for corrosion protection and forbidden due to the high toxicity for humans and for environment[9] In previous publications it was shown, that corrosion behavior and wear resistance of PEO-coated magnesium alloys depend strongly on the treatment conditions, such as voltage, current density, treatment time and electrolyte composition[2,10,11,12]. Sodium salts of salicylic, ethylenediaminetetraacetic and nitrilotriacetic acids were chosen with different complex stability constants with magnesium (log KMg-L) of 4.7, 8.64 and 10.2, respectively It was shown, that in the solutions containing chelating agents the concentration of soluble forms of magnesium (as Mg2+ ions and Mg2+-ligand complexes) was maintained relatively high in the pH range necessary for LDH formation NTA is widely used in cleaning products, because it is biodegradable and is almost completely removed during wastewater treatment[46], but it is considered as carcinogenic[47], which could cause problems for further industrialization of the process
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