Surface Modification of Biodegradable Magnesium Alloys

Abstract

The use of self-assembling monolayers (SAM) of long chain carboxylic acids has a potential for designing specific interface architectures in degradable implants technology. In this paper, the native and anodically formed oxide films on the Mg-alloy (AZ91D) surface were modified with the SAMs of palmitic acid (PA) and stearic acid (SA) to protect the alloy degradation in a physiological solution. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to evaluate chemical composition, bonding modes of acids to the substrate, and structural properties (ordering and close-packing) of the surface layers. The results obtained by electrochemical impedance spectroscopy (EIS) have shown a beneficial effect of anodization and especially of the oxide layer modification by forming SAMs of PA and SA on the corrosion properties of AZ91D alloy in physiological solutions.Magnesium and its alloys are ideal structural materials for the lightweight engineering applications due to their remarkable properties such as low density, high specific strength, high thermal conductivity, and electromagnetic interference resistance. But so far, their application has been limited mostly due to the poor corrosion and wear resistance. 1 As a potential biodegradable implant material, magnesium shows many advantages over current metallic materials as well as biodegradable polymeric and ceramic materials because it is an essential element and in the human body its concentration is high . 1-3 However, in a solution containing Cl − ions like body fluids magnesium and its alloys easily corrode, and their successful application as degradable orthopedic implants has been mainly inhibited due to the high degradation rates and a consequent loss of the mechanical integrity. 2-6 Various surface treatments have been applied to Mg and its alloys to improve their corrosion resistance and the lifetime of implants including anodization, 7, 8 deposition of chemical conversion layers, 9 treatments based on the sol-gel application, 10 and the surface modification by formation of SAMs of long-chain carboxylic acids. 11, 12 The chemisorption of saturated long-chain (C 16 , C 18 ) carboxylic acids has been also studied on many surfaces such as aluminum oxide, 13-15 iron, 16, 17 zinc, 17 and steel. 18 As endogenous compounds and the main components of fats, fatty acids are very biocompatible substances. According to the Food and Agriculture Organization of the United Nations (FAO) recommendation, the intake of saturated fatty acids by adults should not exceed 10% of the total energy consumption. 19 However; the results on the interaction of carboxylic acids with the magnesium/AZ91D alloy surface are not consistent. While the theoretical studies predict that the most stable adsorbed configuration is one in which the carboxyl group is attached by its each oxygen atom to two separate magnesium ions, 20 the experimental results have confirmed the existence of monodentate bonding, 11 and even the electrostatic attraction of magne...

Protection of copper against corrosion in simulated urban rain by the combined action of benzotriazole, 2-mercaptobenzimidazole and stearic acid

“…Carboxylic acids (CH 3 (CH 2 ) m COOH) have been used to generate hydrophobic surfaces on various metal substrates [20][21][22][23][24][25], including the copper-based substrates bronze and brass [26,27]. The length of the carbon chain affects the properties of the selfassembled layer on Al [20].…”

Functionalization of biodegradable magnesium alloy implants with alkylphosphonate self-assembled films

“…However, in a solution containing Cl − ions like body fluids magnesium easily corrodes, and its successful application as degradable orthopedic implants has been mainly inhibited due to the high degradation rates and a consequent loss of mechanical integrity [2][3][4][5][6][7]. Various surface treatments have been applied to magnesium to improve its corrosion resistance and the lifetime of implants including alloying [8], anodization [9,10], deposition of chemical conversion layers [11], treatments based on the sol-gel application [12], and the surface modification by formation of self-assembling monolayers (SAMs) of non-toxic organic molecules [13]. The potential molecules for this purpose must consist of headgroups that enable the molecules to anchor to the implant surface and hydrophobic tail-groups that prevent water molecules to contact the metallic surface and to determine the interfacial properties of the SAM [14].…”

“…The formation of SAMs using oxy-acid species can be either an acid-base reaction to give a carboxylate or phosphonate complex of a surface metal cation, or simple hydrogen bonded adducts [15]. The chemisorption of carboxylic acids on the native oxide surfaces of many metals including magnesium and its alloys yields stable SAMs in a simple immersion process [13,[16][17][18][19]. Phosphonic acid SAMs have also been reported on the native oxide surfaces on many metals and alloys such as aluminum [20][21][22][23], copper [24,25], nickel [26], iron [27], titanium [23,28,29], steel [30,31], nitinol [31,32], cobalt-chromium [33] and so on.…”

Effective and Environmentally Friendly Nickel Coating on the Magnesium Alloy

“…The electric equivalent circuit (EEC) used to fit the experimental data is shown in the inset of Figure 1. The EEC with two time constants is often used to describe the impedance data obtained on the surface of magnesium alloy covered with a film [29][30][31]. The meanings of the EEC elements are as follows: R el is the electrolyte resistance, R 1 -CPE 1 combination represents the outer part of the surface film (i.e., the interfacial film/solution boundary), R 1 is the charge transfer resistance within the pores of the interfacial oxide/electrolyte boundary, and CPE 1 is the constant phase element that represents a surface capacity of pores of the film.…”