A chrome-free conversion coating on magnesium–lithium alloy was obtained from a phosphate–permanganate solution. The morphology, the composition and the corrosion resistance of this coating were examined. The thin and non-penetrating cracked morphology with some deposits existed on the phosphate–permanganate conversion coating. The main elements of the conversion coating were Mg, O, K, P and Mn. The results of the electrochemical measurements and the immersion tests demonstrated that the corrosion resistance of the magnesium–lithium alloy has been improved by the phosphate–permanganate conversion treatment.

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

A highly anticorrosion and self-cleaning superhydrophobic surface with hierarchical flowerlike structures fabricated on AZ91D magnesium alloy is reported in this work. The hierarchical micro/nanostructures were created through electrodeposition of Ni–Co alloy coating. Followed by modification of stearic acid, a superhydrophobic surface with a water contact angle as high as 167.3 ± 1.3° and an ultra-low sliding angle of about 1° was achieved. The anticorrosion performance of the superhydrophobic surface was studied by using a combination of potentiodynamic polarization and EIS in neutral 3.5 wt.% NaCl solutions. The superhydrophobic surface can present an excellent anticorrosion effect with the inhibition efficiency of 99.99% to the bare magnesium alloy and its corrosion rate being 0.06% of that of bare magnesium alloy. The resulting superhydrophobic surface showed excellent superhydrophobicity in a wide range of pH values from 1 to 14. Besides, the as-prepared superhydrophobic surface possessed good chemical stability and self-cleaning effect. It's believed that the highly anticorrosion and self-cleaning superhydrophobic surface can efficiently protect magnesium alloy from corrosion and expand its application scope.

Highly anticorrosion, self-cleaning superhydrophobic Ni–Co surface fabricated on AZ91D magnesium alloy

Electroless nickel (EN) coatings with phosphorus are preferred in many industries such as the oil, chemical, plastic, mechanical, and electronic industries because of their excellent corrosion and wear resistance. This work evaluates the corrosion and wear-corrosion resistance of electroless nickel–phosphorus (ENP) coatings on glass fiber-reinforced plastic (GFRP) composites that are frequently used in wind turbine blades. The results demonstrated that the micro-porosity, phosphorus content, thickness and corrosion and wear-corrosion properties of ENP coatings on the GFRP substrate were all strongly related to the grinding pre-treatment condition, meaning the corresponding surface roughness state of the substrate. A higher P content (P > 7 wt.%), lower micro-porosity, greater thickness and greater hardness of the ENP coatings on GFRP substrate were obtained as the surface roughness of the substrate increased over 0.3 µm (as it did upon grinding with emery paper of lower than 800-grade), improving corrosion and wear-corrosion resistance properties.

Corrosion and wear-corrosion resistance properties of electroless Ni–P coatings on GFRP composite in wind turbine blades

http://www.ysxbcn.com/down/upfile/soft/2010729/66-p630.pdf

Progress of electroplating and electroless plating on magnesium alloy

Nickel–phosphorus (Ni–P) coatings with controllable P content and desirable characteristics can be produced by tuning bath chemistry and processing conditions during the electroless process. Although pure medium- and high-P coatings offer a good combination of mechanical and corrosion properties, their performance attributes can be further enhanced through incorporation of silica (SiO2) nanoparticles. Using 15 and 30 g/L of sodium hypophosphite as reducing agent in the plating bath, Ni–P coatings with respective P content of 10.8 and 14.3 wt.% were produced. While the surface roughness and grain size increased with an increase in the P content, the average hardness value and corrosion resistance of medium-P coatings were superior to their high-P counterparts. The average hardness of the Ni–P– SiO2 coatings based on medium-P Ni–P matrix was determined to be ~ 11.0 GPa. Electrochemical impedance spectroscopy (EIS) studies of these coatings indicated area-impedance values on the order of 108 Ω·cm2, whereas the double-layer capacitance per unit area (Cdl) and charge transfer area-resistance (Rct) values, computed from theoretical fit of the EIS data, were in the range of 1.18 × 10− 10 F·cm− 2 and 3.14 × 108 Ω·cm2, respectively. The findings confirmed that addition of SiO2 nanoparticles modifies deposit morphology through grain refinement, reduction in the surface roughness and minimization of surface porosity in the nanocomposite coatings.

Influence of SiO2 nanoparticles on hardness and corrosion resistance of electroless Ni–P coatings

Studies of the porosity in electroless nickel deposits on magnesium alloy

In this paper, the electroless nickel-plating on magnesium alloy was studied, using NiSO4·6H2O as the main salt in the electroless plating alkaline solutions. The effects of the buffer agent and plating parameters on the properties and structures of the plating coatings on magnesium alloy were investigated by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). In addition, the weight loss/gain of the specimens immersed in the test solution and plating bath was measured by using the electro-balance, to evaluate the erosion of the alloy in the plating solutions. The adhesion between the electroless plating coatings and the substrates was also evaluated. The compositions of the non-fluoride and environmentally friendly plating bath were optimized through Latin orthogonal experiment. The buffer agent (Na2CO3) added to the plating bath was found to be useful in increasing the growth rate of the plating coating, adjusting the adhesion between the electroless plating coatings and the substrates, and maintaining the pH value within the range of 8.5–11.5, which is required for the successful electroless nickel-plating on magnesium alloy with NiSO4·6H2O as the main salt. Trisodium citrate dihydrate was found to be an essential component of the plating bath to plate magnesium alloy, with an optimum concentration of 30 g L−1. The obtained plating coatings are crystalline with preferential orientation of (111), having advantages such as low-phosphorus content, high density, low-porosity, good corrosion resistance and strengthened adhesion.

Jianzhong Li

Papers

Studies of the porosity in electroless nickel deposits on magnesium alloy

The electroless nickel-plating on magnesium alloy using NiSO4·6H2O as the main salt