Effect of Surfactants on the Corrosion Protectability of Calcium Phosphate Conversion Coatings on Duplex Structured Mg-8Li (in Wt.%) Alloy

Abstract

Calcium phosphate chemical conversion coatings with the additions of sodium lauryl sulfate (SLS) and dodecanesulfonic acid sodium (DSS), respectively, were prepared on the surface of the Mg-8Li alloy. The surface and cross-sectional corrosion morphologies, compositions, and corrosion behavior of the coated surfaces in 3.5 wt.% NaCl solution were respectively investigated by using a scanning electron microscope (SEM), energy dispersive spectrometer (EDS), electrochemical workstation, hydrogen evolution apparatus, and optical microscope (OM). The results demonstrated that Ca-P coatings had a petal-like structure being composed of leaf-like particles. After being respectively performed for 30 min in conversion solutions containing SLS and DSS, the corresponding average film thicknesses of surface coatings were 27 μm and 7 μm. In addition, the corrosion current densities of coated surfaces by using the conversion solutions containing SLS and DSS were 1.438 × 10−5 A·cm−2 and 4.719 × 10−5 A·cm−2, respectively. The effect of surfactants on phosphate chemical conversion coating was discussed in detail.

Influence of Surfactant Types on the Anti-Corrosion Performance of Phosphate Chemical Conversion Coated Mg-8wt.%Li Alloy

“…When the Li amount ranges from 5.7 to 10.3 wt.%, two-phase structured Mg-Li alloys are formed [7]. For Mg-Li based alloys with a two-phase structure characteristic, the micro-galvanic corrosion is very prone to occur in the corrosion medium due to the different potential values of α-Mg and β-Li [8]. Thus, it is vital to explore effective approaches for resolving the poor corrosion resistance issue of Mg-Li alloys [9][10][11][12].…”

“…During the early research on chemical conversion surface films, the excellent corrosion protectability of the chromate conversion technologies was widely used [22,23]. However, the chromate treatment solution contains highly toxic Cr(IV) ions and is carcinogenic and harmful to the environment [8]. Due to this, the preparation of chromium-free conversion coatings has been widely researched.…”

“…Due to this, the preparation of chromium-free conversion coatings has been widely researched. Among them, phosphate conversion coatings possess the potential for replacing the chromate conversion coatings because they have the advantages of good corrosion protectability and simplicity in operation [8]. Generally, for the preparation of chemical conversion surface films, the category of additive in surfactants plays an important role, and their inhibiting effect on corrosion development has been widely studied [8].…”

“…Among them, phosphate conversion coatings possess the potential for replacing the chromate conversion coatings because they have the advantages of good corrosion protectability and simplicity in operation [8]. Generally, for the preparation of chemical conversion surface films, the category of additive in surfactants plays an important role, and their inhibiting effect on corrosion development has been widely studied [8]. Generally, the main functions of additives consist of the following: (1) promoting the formation of a coating film; (2) reducing particle crystallinity; (3) controlling the conversion rate; (4) increasing the hydrophobicity of the metal surface; and (5) enhancing the solubility of the main components of the conversion fluid in water [24][25][26][27][28][29][30].…”

Effect of the Surface Film Formed by Hydrogen Charging on the Corrosion Behavior of an As-Cast Mg–8%Li (in wt. %) Alloy

“…As one of the lightest metallic structural materials, magnesium-lithium (Mg-Li) alloys have the high potential for reducing the weight of equipment and saving energy in industrial fields [1][2][3][4][5]. Generally, the mass fraction of Li determines the crystallographic structures of Mg-Li alloys [6].…”