Mechanical properties, corrosion behaviors and microstructures of 7075 aluminium alloy with various aging treatments

Industrial application of electroless Ni P coatings are limited by their low toughness and cracking under various loading conditions that in turn requires improvement of toughness using ternary coating systems. In this research work, graphene suspension was added to electroless Ni P plating bath in various concentrations to produce four different compositions of Ni-P-G (graphene) coatings. Micro-structural and surface attributes were studied using SEM and white light profilometer respectively. Scratch and indentation behaviors of substrate and coatings were investigated. Graphene incorporation in Ni P coating matrix improved the hardness as well as wear resistance. Also graphene supported Ni P coating exhibited toughness through crack shielding and crack deflection phenomena.

Preparation and tribological characterization of graphene incorporated electroless Ni-P composite coatings

Erosion–corrosion is one of the major damage mechanisms incurring higher maintenance cost for hydrocarbon industries. Various damages resulting from service conditions have necessitated protective coatings such as Ni–P. Although Ni–P electroless coatings are known for their corrosion resistance and anti-wear behaviors, still there is a dire need for increasing the anti-wear and anti-corrosion behaviors to expand potential application of Ni–P coatings in oil and gas and petrochemical sectors. In this work, graphene enrichment in Ni–P coating matrix was performed achieving various compositions of ternary Ni–P-graphene coatings through the addition of various concentrations of graphene (30 mg/L, 60 mg/L and 100 mg/L) into electroless plating bath. Surface topography and micro-structural examinations of coatings were conducted. Slurry pot erosion–corrosion testing (using AFS 50–70 silica sand and 3.5 wt% NaCl solution) was employed to characterize the erosion–corrosion behavior of the coatings. SEM analysis was performed on the wear scars to identify the dominant wear mechanisms. Addition of graphene improved the pure erosion and erosion–corrosion resistance. Ni–P-100 mg G coating displayed best slurry pot erosion–corrosion resistance. It is believed that higher hardness, impermeability, and hydrophobic nature of graphene are responsible for improved performance of ternary coatings.

Effect of Graphene Nanoplatelets (GNPs) Addition on Erosion–Corrosion Resistance of Electroless Ni–P Coatings

This study is an effort to investigate the synergic effect of nano-diamond (ND) addition and plasma nitriding (PN) treatment on tribological and structural properties of nickel‑boron (NiB) coatings. NiB/ND nanocomposites containing different amounts of NDs (0.0, 0.1, 0.5, and 1.0 g/L) were produced in an electroless bath. Then, plasma nitriding was conducted on the samples for 1 h at 400 °C in an N2-H2 environment. To study phase analysis and examine the surface morphology of the samples, X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) were utilized. The wear behavior and friction were determined under the normal load of 10 N using the pin-on-disk method. The surface roughness, microstructure, and microhardness were also assessed. The results showed that, as the ND concentration increased, the amorphous structure of as-plated (AP) Ni-B transformed to a semi-crystalline one. Moreover, the amorphous structure of the as-deposited NiB/ND coatings changed to a crystalline one after plasma nitriding, which resulted in a higher hardness value. NiB/0.5ND coating showed the highest hardness value (1523 HV0.1) and the highest wear resistance (0.8×10−10 Kg/N.m). The smoothest wear trace with no visible cracks is related to the worn surface of PN NiB/0.5ND sample, suggesting that the NiB/ND nanocomposite can be a promising candidate for surface engineering processes. 

Effect of plasma nitriding on tribological properties of nickel‑boron-nanodiamond electroless coatings

The corrosion of AA 7075-T6 aluminum alloy is a critical issue for many industries. In this study, aluminum coatings were deposited onto AA 7075-T6 by cold gas spray and the effect of the porous outer layer on different properties of the coating, including corrosion–resistance was investigated. As-prepared and polished samples were used to study the microstructure, morphology, mechanical properties and corrosion resistance of the coating in 3.5 wt.% NaCl solution. Cross-sectional analysis showed a dense structure, low porosity (0.8%) and thickness up to 300 µm (~ 100 µm for the porous outer layer and ~ 200 µm for the compact inner layer). The sliding wear test resulted in a volume loss of 3.2 × 10−4 mm3/Nm with an adhesive wear mechanism. The abrasive wear test showed a wear rate of 1.1 × 10−4 mm3/Nm for the as-prepared coating and 0.8 × 10−4 mm3/Nm for the polished coating. The as-prepared coating pores and interparticle spacing in the outer layer were mostly responsible for the increase in wear rate. For the polished coating immersed in 3.5 wt.% NaCl solution during 900 h, the electrolyte reached some specific points of the substrate as revealed by the cross-sectional analysis. Inspection of the as-prepared coating demonstrated that the coating/substrate interface was completely damaged after long immersion times (~ 900 h).

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Effect of the Outer Layer of Al Coatings Deposited by Cold Gas Spray on the Microstructure, Mechanical Properties and Corrosion Resistance of the AA 7075-T6 Aluminum Alloy

The purpose of the present study is to evaluate the effect of the electroless Ni-P/diamond/graphene composite coating on the structure and surface hardness of 2024-T6 aluminum alloy as well as their effect on the corrosion and wear resistance of the alloy in 3.5 % NaCl solution. The electroless Ni-P plating solution was prepared by adding different size diamond (6-12 μm and 0.2 μm) and nanographene into the electroless Ni-P plating solution to obtain Ni-P/diamond, Ni-P/graphene and Ni-P/daimond/graphene composite coatings for comparison. Experimental results indicated that the Ni-P/diamond, Ni-P/graphene and Ni-P/daimond/graphene composite coatings can be successfully electroless deposited on anodized 2024-T6 aluminum alloy. The anodically oxidized films, that formed on the aluminum alloy using phosphoric acid as the electrolyte, was porous with high density of pores, and thus could enhance the adhesion of the composite coatings. The Ni-P/daimond/graphene hybrid coating had a higher hardness as well as better corrosion and wear resistance of 2024-T6 alloy in 3.5 wt.% NaCl solution as compared with other composite coatings. When the combination of nanographene and smaller diamond particles added this beneficial effect was significantly raised, especially the composite coating was further vacuum annealed at 400 °C for 24 h to obtain a more smooth and defect-free coating structure.

Electroless Ni-P/Diamond/Graphene Composite Coatings and Characterization of their Wear and Corrosion Resistance in Sodium Chloride Solution

In this study nanoTiO2 particles were incorporated into the electroless plating solution to prepare Ni-P-TiO2 composite coating on anodized AA7075 aluminum alloy to improve the wear and stress corrosion cracking resistance of the coated alloy in 3.5%NaCl solution. The anodized AA7075 aluminum alloy was also performed by a boiling water sealing treatment for comparison. The wear and stress corrosion cracking (SCC) characteristics were investigated using a self-designed block-on-ring machine and slow strain rate test. The effect of corrosion was evaluated by electrochemical polarization measurements. The surface morphology, element composition and surface hardness of the coating were analyzed by scanning electron microscopy (SEM), X-ray energy dispersive spectrometry (EDS) and Vicker′s hardness tester. Experimental results indicated that after boiling water sealing treatment the resistance properties of the anodized AA7075 aluminum alloy were further improved. The anodizing treatment of AA7075 aluminum alloy gave a thick film with high porosity. The porous film efficiently improved the cohesion, adhesion and hardness of the electroless Ni-P composite coating. Therefore, the electroless Ni-P composite coating deposited on the anodized AA7075 aluminum alloy offered a superior wear, pitting corrosion and stress corrosion cracking resistance properties than both anodizing and sealing treatment. By comparison with Ni-P and Ni-P-TiO2 coatings the incorporation of TiO2 resulted in a more uniform and crack-free surface structure of the composite coating. This is responsible for the higher hardness, better wear, pitting corrosion and stress corrosion cracking resistance of the electroless Ni-P-TiO2 coating.

Preparation of Electroless Nickel-Phosphorous-TiO2 Composite Coating for Improvement of Wear and Stress Corrosion Cracking Resistance of AA7075 in 3.5% NaCl

Ching Yi Yang

Papers

Electroless Ni-P/Diamond/Graphene Composite Coatings and Characterization of their Wear and Corrosion Resistance in Sodium Chloride Solution

Preparation of Electroless Nickel-Phosphorous-TiO2 Composite Coating for Improvement of Wear and Stress Corrosion Cracking Resistance of AA7075 in 3.5% NaCl