The Preparation, Characterization and Formation Mechanism of a Calcium Phosphate Conversion Coating on Magnesium Alloy AZ91D

Abstract

The poor corrosion resistance of magnesium alloys is one of the main obstacles preventing their widespread usage. Due to the advantages of lower cost and simplicity in operation, chemical conversion coating has drawn considerable attention for its improvement of the corrosion resistance of magnesium alloys. In this study, a calcium phosphate coating was prepared on magnesium alloy AZ91D by chemical conversion. For the calcium phosphate coating, the effect of processing parameters on the microstructure and corrosion resistance was studied by scanning electron microscope (SEM) and electrochemical methods, and the coating composition was characterized by X-ray diffraction (XRD). The calcium phosphate coating was mainly composed of CaHPO4·2H2O (DCPD), with fewer cracks and pores. The coating with the leaf-like microstructure provided great corrosion resistance to the AZ91D substrate, and was obtained under the following conditions: 20 min, ambient temperature, and no stirring. At the same time, the role of NH4H2PO4 as the coating-forming agent and the acidifying agent in the conversion process was realized, and the formation mechanism of DCPD was discussed in detail in this work.

Potential bioactive coating system for high-performance absorbable magnesium bone implants

“…The crystallinity of the layer is an important factor that is affected by the Ca/P ratio, and higher amounts of Ca 2+ or PO 4 3− can lead to the formation of amorphous phases such as Ca 3 (PO 4 ) 2 , dicalcium phosphate dihydrate (DCPD; CaHPO 4 .2H 2 O), and CaHPO 4 .H 2 O [ 149 , 150 ]. The chemical conversion coating of CaP coating can increase the corrosion resistance of Mg alloys and enhance cell proliferation and bone growth on the interface of bone/implant [ 148 , [151] , [152] , [153] , [154] ]. Another bioactive chemical conversion coating is fluoride conversion coating which forms an insoluble Mg fluoride (MgF 2 ) layer.…”

Corrosion Protective Film Formation on Mg Alloy AZ31 by Exposure to Dilute Selenite Solutions

“…Compared with other conventional Mg coatings in a similar testing environment, the Se-based protective film has better performance than vanadate, stannate and Ce 3+ , has similar performance with PO 4 3− and Ca 2+ , and can be comparable to chromate [ 12 , 15 , 19 , 26 , 27 , 28 , 29 , 30 , 31 , 46 , 47 ]. Since the study of the selenite-based protective film was blank before.…”

“…Extensive research has been conducted in order to find a high-performance coating to substitute the highly toxic chromate [ 25 , 26 , 27 ]. Common corrosion protective coatings for Mg alloys include those based on fluoride, phosphate, calcium, and rare earth metals [ 27 , 28 , 29 , 30 , 31 ]. However, the study of corrosion protection by selenite is very limited.…”

The one-step preparation and corrosion resistance of a Ca–Mn–Zn ternary phosphate conversion coating on magnesium alloy AZ91D

“…In our previous researches, a method to prepare SPCCs on MA surface is obtained, and the mainly innovation of our method compared with others is as follows: metallic oxides (CaO, MnO or ZnO) rather than metallic salts [Ca(NO 3 ) 3 , Mn(NO 3 ) 3 or Zn(NO 3 ) 3 ] as film-forming agent decrease the cost of coating preparation; further, in phosphating bath, keeping the concentrations of other components and only changing the types of metallic oxides, different SPCCs, such as Ca@SPCC, Mn@SPCC and Zn@SPCC, can be prepared on MA surface (Zhou et al , 2014; Xiong et al , 2015; Zhou et al , 2015; Xiong et al , 2017; Zhou et al , 2017a; Liu et al , 2018; Yang et al , 2019). After that, based on above results, we find that some BPCCs can also be prepared when two metallic oxides are added into phosphating bath at the same time.…”