**6. Methods of reducing the degradation rate of magnesium alloys by modifying their surface (application of protective layers)**

High corrosion rate of magnesium-based alloys in tissue environment may be limited, in addition to modifying the chemical composition, by surface treatment technologies. The degradation process can be controlled by way of coating the surface or changing its structure [53, 60, 61]. There are two methods of coating: conversion and deposition processes. Conversion coatings are the product of complex interaction of metal dissolution and precipitation, usually during treatment in aqueous solutions, while deposition treatments consist of metallic, inorganic and organic coatings [53, 62, 63]. Modifications in the surface of magnesium alloys by mechanical treatment are also used [62]. The classification of the coating technology for magnesium alloys is shown in **Figure 6**.

Homogeneity of the corrosion process is an important aspect that determines the degradation rate and the physical condition of the implant at a specific treatment

**Figure 6.**

*General classification of surface treatment technologies applied on magnesium alloys [62, 63].*

stage [63]. Magnesium alloy coatings often have pores and cracks. Corrosion, which begins in these areas, leads to uneven rate of corrosion, accelerates destruction of the coating and premature degradation of the implant [63, 64]. Therefore, it is important to minimize the porosity of the coating by adjusting the parameters of the application process or the appropriate preparation of the substrate's surface [62, 65]. In addition, protective coatings on biodegradable magnesium alloys should be adapted to specific applications – e.g. vascular stents have different surface requirements than orthopedic implants, where osseointegration with newly formed bone is important [62, 63]. Selected technologies of forming coatings on magnesium alloys are discussed below with regard to their advantages and disadvantages in terms of use, with an aim to reduce the corrosion rate:


*Amorphous and Crystalline Magnesium Alloys for Biomedical Applications DOI: http://dx.doi.org/10.5772/intechopen.94914*


In choice of the coating technology, one should always take into account preservation of the alloy's biocompatibility, due to the potential toxicity of the elements introduced in coating treatments. In addition to coating, an alternative solution is mechanical treatment (shot peening, machining, burnishing, deep rolling), which solves the toxicity problem. The literature also describes hybrid techniques, which combine mechanical treatment with coating, as a promising solution for controlling the corrosion rate and mechanical properties at individual stages of treatment [63]. Biomimetic coatings are also noteworthy, as they are of biological origin and ensure excellent biocompatibility, but further work is required to improve their low adhesion to the substrate alloy [63, 67].

The types of protection coatings used to delay/reduce the degradation rate of magnesium alloys are shown in **Figure 7**. Besides phosphate and fluoride, most of the proposed ceramic coatings are non-resorbable. In the case of resorbable materials, considered polymeric coatings include PLA, PLGA and copolymers. Composite coatings increasing the corrosion resistance of magnesium alloys, tested by several researchers, include the following types of coatings: ceramic-metallic and ceramic-polymer.

As part of the authors' own research, tests of phosphate coatings on magnesium alloys were carried out. In this work [78], the chemical method was used for Ca-P coatings preparation. NaOH and ZnSO4 as accelerators were added to phosphatizing baths, with an aim to form a dense and uniform protective phosphate coating. It

*Types of protective coatings used to delay/reduce the degradation rate of magnesium alloys [68–77].*

should be noted, that NaOH and ZnSO4 are used to improve corrosion resistance of Mg alloys. The results of microscopic observations and phase identification of the obtained phosphate coatings (with the use of chemical composition of the phosphating bath) are shown in **Figure 8**.

XRD results indicate that obtained protection layers included dicalcium phosphate dihydrate (CaHPO4·2H2O). Both NaOH and ZnSO4 formed the morphology of the produced layers. The coating obtained by immersion in a phosphatizing bath with ZnSO4 addition (ZnAM50 sample) consisted of petals. The coating obtained by immersion in a bath with NaOH addition (NaAM50 sample) showed plate-like morphology.

The degradation tests of magnesium alloys with Ca-P layers were also performed (**Figure 9a** and **b**) in Ringer's solution at 37°C. The results of electrochemical tests indicated that coated samples have more positive value of Ecorr than non-coated AM50 sample (**Figure 9a**). In addition, the cathodic part of potentiodynamic curve

#### **Figure 8.**

*X-ray diffraction patterns and SEM images of Ca-P coatings on Mg alloy [78].*

#### **Figure 9.**

*Results of degradation tests of Mg alloys with calcium phosphate coatings in Ringer's solution at 37°C: (a) polarization curves, (b) hydrogen evolution [78].*

*Amorphous and Crystalline Magnesium Alloys for Biomedical Applications DOI: http://dx.doi.org/10.5772/intechopen.94914*

determined for the coated samples is located in a low current range, which indicated a low cathodic activity. It corresponds with the immersion tests results (**Figure 9b**). The volume of evolved hydrogen (hydrogen is a result of cathodic reactions) in an uncoated sample was higher than its level in coated samples.

The degradation rate of ceramic material determines the occurrence of defects, cracks and flaws in technology. Defected ceramics can be destroyed in contact with water. Inclusions of other phases are equally disadvantageous to ceramic materials, that lead to their degradation. In contact with water, these inclusions accelerate aging and increase volume. These processes also have a direct impact on deterioration of the mechanical properties of ceramic materials [79].
