Microstructural characteristics and corrosion property of non‐chromate surface treatments on AZ91D magnesium alloy

Chromate conversion coatings can be successfully used for corrosion protection of magnesium alloys. However, the environmental laws have imposed severe restrictions on chromate use in many countries. In this study, a novel protective environmental‐functionally gradient coating was formed on AZ91D magnesium alloy by non‐chromate surface treatments, which consisted of pre‐etching followed by cerium‐based chemical conversion before applying the sol–gel CeO₂ film. It was determined by the analysis of X‐ray diffraction that the gradient coating was mainly composed of CeO₂. The calculation, based on the Scherrer formula, further revealed the formation of nanocrystalline structure in the coating. Scanning electron microscopy (SEM) observations showed that the coating was homogeneous and compact, no obvious cracked structure occurred. According to the immersion tests, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests, the corrosion resistance of AZ91D magnesium alloy was found to be greatly improved by means of this novel environmental‐functionally gradient coating.     

Energy storage ability and anti‐corrosion protection properties of TiO2–SnO2 system

TiO₂/SnO₂ and TiO₂–SnO₂ coatings were prepared on type 304 stainless steel by sol–gel method, respectively. TiO₂/SnO₂ coating is compared with TiO₂–SnO₂ coating in terms of energy storage ability and anti‐corrosion property. The two coatings can be charged with reductive energy under UV irradiation in 3 wt% aqueous NaCl. The self‐discharging time of the TiO₂/SnO₂ coating is slower than that of the TiO₂–SnO₂ coating. The slow discharging may be suitable for an anti‐corrosion application for metal. In the case where TiO₂/SnO₂ coating electrode is electrochemically charged at ?0.38 V (vs. SCE) for 1 h, it can maintain a good cathodic protection for type 304 stainless steel for 7 h in the dark, while the TiO₂–SnO₂ coating electrode can only maintain a good cathode protection for 0.5 h in the dark.     

Erosion–corrosion characteristic of nano‐particulates reinforced Ni–Cr–Mo–Cu surface alloying layer in acidic flow and acidic slurry flow

In order to improve the corrosion and erosion–corrosion resistance of 316L stainless steel in engineering application, two kinds of composite alloying layers were prepared by a duplex treatment, consisting of Ni/nano‐SiC and Ni/nano‐SiO₂ predeposited by brush plating, respectively, and a subsequent surface alloying with Ni–Cr–Mo–Cu by double glow process. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were performed on the two kinds of composite alloying layer using 10 wt% HCl solution to assess the corrosion behavior. Erosion–corrosion tests were carried out by erosion–corrosion test rig in acidic flow and acidic slurry flow for test time of 20 h at four different rotational speeds. Results of electrochemical tests indicated that the corrosion resistance of composite alloying layer with brush plating Ni/nano‐SiO₂ particles interlayer approximated to that of single Ni‐based alloying layer, whereas the corrosion resistance of the composite alloying layer with brush plating Ni/nano‐SiC particles interlayer was apparently inferior to that of Ni‐based alloying layer in 10 wt% HCl solution at static state. Under the conditions of acidic flow and acidic slurry flow, the mass losses of tested samples increased with increase in the time of erosion–corrosion tests and the rotational speeds of samples. The mass losses of composite alloying layer with brush plating Ni/nano‐SiO₂ particles interlayer were lower than that of single Ni‐based alloying layer at all rotational speeds, except at 1.88 m/s in acidic flow. The mass losses of composite alloying layer with brush plating Ni/nano‐SiC particles interlayer were higher than that of single Ni‐based alloying layer at all rotational speeds, but were obviously lower than that of AISI 316L stainless steel. The influences of second phase on the corrosion and erosion–corrosion of the two kinds of composite alloying layer were discussed in this paper.     

Synthesis and properties of high corrosion resistant Ni–cerium oxide nano‐composite coating

The effect of cerium ion on the formation, morphology, composition, and corrosion behavior of Ni–cerium oxide coatings was investigated by SEM, FESEM, XRD, EDS, XPS, EIS, and potentiodynamic polarization. The extremely highest corrosion resistant coating was obtained when the cerium ion concentration in the plating bath was 16 mM. It has been observed that the presence of cerium ion in the plating bath led to changes in the morphology of the coating from pyramid nodular structure to coaxial structure. By adding cerium ion to the plating bath, a considerable grain refinement in the nanometer region was observed.     

SKPFM investigations of intermetallic compounds of innovative Er‐ and Zr‐containing Al–Si–Mg alloys and their influence on corrosion localization in saline solution

The paper aims at characterizing the influence of intermetallic compounds on the corrosion localization of innovative Al–Si–Mg Er‐ and Zr‐containing casting alloys. Samples of the investigated materials were studied by means of optical and scanning electron microscope micrographs, immersion tests, and scanning Kelvin probe force microscope (SKPFM) analyses in the T6 temper. Combination of immersion tests and SKPFM analyses allowed to identify those classes of intermetallic compounds promoting localization of the corrosion process. It was found that intermetallic compounds richer in Fe were the most critical for corrosion localization; furthermore, additions of Er caused a marked decrease of the potential difference of intermetallic compounds with respect to the Al matrix and a consequent less intense microgalvanic coupling, which translates into slower corrosion kinetics. Further, Zr additions slightly increased the potential difference of intermetallic compounds with the Al matrix, promoting a faster corrosion process.     

Preparation and corrosion protective properties of titania‐containing modified self‐assembled nanophase particle (TMSNAP) sol–gel on AA2024 aluminum alloy

A new method of preparing water‐based sol–gel containing titania nanoparticles for the protection of aluminum alloy AA2024 against corrosion was presented and performance of the coating in Harrison's solution was studied. The coating was prepared using alkoxysilanes, tetraethylorthosilicate (TEOS) and 3‐glycidoxypropyltrimethoxysilane (GPTMS), and in additional metal alkoxide, titanium(IV) tetrapropoxide (TPOT), as a source of titania particles. Poly(ethylene imine) (PEI) polymer was utilized to create cross‐linking and also to improve the coating quality. In addition, the molar ratios and amount of components and factors affecting performance were assessed to improve coating properties and its performance. Potentiodynamic scan (PDS) and electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the corrosion protection performance of coatings. Also, scanning electron microscopy (SEM) was employed to investigate surface morphology. The stability of the best prepared coating and its corrosion protective effects on the alloy were evaluated in Harrison's solution up to 15 days. The results revealed that this new sol–gel coating provides significant protection against corrosion of the AA2024 alloy in Harrison's solution.