An environment‐friendly calcium phosphate conversion coating on AZ91D alloy and its corrosion resistance

Abstract

An environment‐friendly phosphate conversion coating free of chromate, fluorides, and nitrite was developed on AZ91D alloy through a facile chemical deposition method. The surface morphology and phase composition of the coating were characterized by field‐emission scanning electron microscopy (FE‐SEM) and X‐ray diffraction (XRD). The corrosion resistance of the coating was evaluated by potentiodynamic polarization test in 3.5 wt% NaCl solution. Results indicate that the calcium phosphate coating was composed of a large number of leaf‐like and flower‐like particles. The chemical composition of the conversion coating was CaHPO4·2H2O crystals. The coating can remarkably reduce the corrosion current density of the substrate. Introduction of triethanolamine (TEA) additive into the phosphating bath leads to denser coating and high anticorrosive property of the phosphate coating.

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

“…Based on the above discussions, the leaf-like coating on the AZ91D alloy provides the best corrosion resistance when the AZ91D is immersed in a Ca-P bath composed of 25 g/L Ca(NO 3 ) 2 ·4H 2 O and 15 g/LNH 4 H 2 PO 4 for 20 min at a static condition of ambient temperature. From an electrochemical viewpoint, the corrosion resistance of calcium phosphate coating in this work is higher than that in some reported references [ 26 , 27 , 28 , 29 , 30 , 31 ]. Meanwhile, compared with those reported references [ 26 , 27 , 28 , 29 , 30 , 31 ], the preparation process of calcium phosphate coating in this work is also the simplest.…”

“…The correspondent EDS analysis shows that the main elements of the coating are O, Ca and P. The atomic ratio of Ca:P:O listed in Figure 8 is approximately 1:1:4. Given the main composition of leaf-like Ca-P conversion coating in the reported literature [ 27 , 28 , 31 , 42 ], it can be inferred that CaHPO 4 ·2H 2 O should be the main composition of the coating. Figure 9 shows the XRD patterns of the bare AZ91D alloy and the coating for further affirmation of the composition of the conversion coating.…”

“…Although the phosphate conversion coating showed excellent corrosion resistance for Mg-8.8Li alloy, the operation conditions needed the specific temperature and pH, as well as complicated pretreatments including grinding, ultrasonic cleaning, polishing with 0.5 μm diamond paste, and etching. On this basis, Sun et al [ 31 ] optimized this process and studied the effect of the addition of triethanolamine on the morphology and corrosion resistance of calcium phosphate coating. However, the pretreatment was still complex, and the calcium phosphate bath needed the HNO 3 /NaOH solution to adjust the appropriate pH value.…”

Corrosion Behavior of AZ91D Magnesium Alloy with a Calcium–Phosphate–Vanadium Composite Conversion Coating

“…Some divalent cations such as Ca 2+ , Zn 2+ and Mn 2+ are usually added to the phosphating bath to further enhance the anticorrosion property of the phosphate conversion coating. In our previous study, we developed an environment-friendly calcium phosphate conversion coating on AZ91D alloy, and demonstrated an enhanced anticorrosion ability of the conversion coating [19]. Zeng et al prepared Zn phosphate coating and Zn-Ca composite coating on AZ31 alloy [20].…”

Preparation and Anti-Corrosive Properties of Cerium Oxide Conversion Coatings on Steel X52

“…Among these surface treatments, chemical conversion treatment is a simple and cost effective method and has been increasingly used in a wide range of applications. Conversion coatings are used in metal finishing to afford corrosion protection and to improve the adhesion of paint systems to the underlying metal 13,14 . The conversion coating acts as a physical barrier to protect the metal from corrosive environment.…”