There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

American Society for Testing and Materials

https://pureadmin.qub.ac.uk/ws/files/126683921/Edited_sudagar_R1.pdf

Electroless nickel, alloy, composite and nano coatings – A critical review

Tribology of electroless nickel coatings – A review

Advances in coatings on biodegradable magnesium alloys

Interest in electroless plating of nickel-based ternary alloys has increased because of their excellent corrosion, wear, thermal and electrical resistance. They also possess good magnetic properties. In the present investigation, the effect of copper and tungsten in alkaline electroless nickel baths has been studied in depositing Ni–Cu–P and Ni–W–P alloys and also the synergistic effect of ions in depositing Ni–W–Cu–P alloys. Deposits were characterized using XRD, scanning electron microscopy (SEM), energy-dispersive analysis of X-ray (EDX) and atomic force microscopy (AFM) techniques. XRD results revealed that not much variation in structure and grain size has been found in Ni–Cu–P deposit. A decrease in phosphorus content and a marginal increase in grain size have been observed due to the tungsten addition in the Ni–P deposit. Addition of copper in Ni–W–P baths has resulted in a quaternary deposit, Ni–W–Cu–P, with increased crystallinity. SEM studies showed that presence of coarse nodules in ternary Ni–Cu–P and Ni–W–P deposits. Addition of copper in Ni–W–P baths has resulted in a very smooth deposit. Studies by AFM on deposits have proved that the copper has suppressed coarse nodules by inhibiting their growth in quaternary deposit. No considerable change in hardness has been noticed in both as-plated and heat-treated deposits due to the inclusion of copper in Ni–W–P deposit. A marginal improvement in corrosion resistance has been observed in quaternary alloy compared to ternary (Ni–Cu–P or Ni–W–P) alloys.

Electroless deposition of Ni–Cu–P, Ni–W–P and Ni–W–Cu–P alloys

High phosphorus electroless nickel bath has been used to prepare composite coatings containing alumina powders (50 nm, 0.3 μm and 1.0 μm). Deposits were characterized for its structure, morphology and hardness. Incorporation of particle has a marginal influence on the composition. More amount of particle incorporation and uniform distribution was found in composite coatings obtained with 1.0-μm (C3) alumina particles compared to 50-nm (C1) and 0.3-μm (C2) alumina particles. XRD results showed a broad peak of nickel and low intensity alumina peaks present in C3 composite coating in as-plated condition. A marginal improvement in hardness was noticed in as-plated composite coatings. A 15% increase in microhardness was observed in the heat-treated (400 °C for 1 h) composite coatings. Potentiodynamic polarization measurements made on these deposits in 3.5% sodium chloride solution showed that uniform corrosion occurred in C1 and C2 composite coatings whereas localized corrosion was observed in C3 composite coating.

Influence of particle size on the microstructure, hardness and corrosion resistance of electroless Ni–P–Al2O3 composite coatings

In the present investigation, a zirconium–cerium conversion coating (ZrCeCC) was developed on aerospace aluminum alloy (AA2024) by simple immersion technique. Zirconium conversion coating (ZrCC) was also prepared for comparison. Surface morphology of ZrCC and ZrCeCC specimens exhibited smooth and cracked patterns respectively. Elemental analysis showed the presence of Zr and Ce in ZrCeCC specimen. XPS results indicated the presence of multiple oxidation states of Zr and Ce in the developed coatings. Potentiodynamic polarization results revealed the corrosion current density (icorr) values obtained for both coatings (between 0.6 and 0.85 μA/cm2) were lower than bare specimen (1.2 μA/cm2). However, ZrCeCC specimen exhibited the lowest icorr value of about 0.46 μA/cm2 after 168 h of immersion in 0.6 M NaCl solution. EIS studies showed that higher coating resistance (Rcoat) value was obtained for ZrCeCC even after 168 h of immersion. The results obtained from simulated scratch cell test revealed a continuous increase in charge transfer resistance (Rct) value for bare specimen which was exposed along with ZrCeCC in 0.1 M NaCl solution. The self-healing behavior of ZrCeCC was confirmed from all electrochemical tests. After 168 h of neutral salt spray exposure, the surface of ZrCeCC was almost comparable with the conventional chromate conversion coating.

Evaluation of corrosion resistance and self-healing behavior of zirconium–cerium conversion coating developed on AA2024 alloy

Micro-arc oxidation (MAO) coatings were prepared on AZ31B magnesium alloy using alkaline silicate electrolyte at different current densities (0.026, 0.046 and 0.067 A/cm2). Field Emission Scanning Electron Microscopy (FESEM) analysis of the coating revealed an irregular porous structure with cracked morphology. Compositional analysis carried out for MAO coating showed the presence of almost an equal amount of Mg and O (34 wt.%) apart from other elements such as F, Si and Al. The cross-sectional FESEM images clearly portrayed that the MAO coating was dense along with the presence of very few fine pores. The surface roughness (Ra) of the coatings increased with an increase in the current density. Potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) studies were carried out for both the bare and MAO coated AZ31B Mg alloy in 3.5% NaCl solution. The corrosion potential (Ecorr) and corrosion current density (icorr) values obtained for the bare substrate were − 1.49 V and 46 μA/cm2, respectively. The coating prepared at 0.046 A/cm2 exhibited the lowest icorr value of 7.79 × 10− 10 A/cm2 and highest polarization resistance (41.6 MΩ cm2) attesting to the better corrosion resistance of the coating compared to other samples. EIS results also indicated almost similar corrosion behavior for the MAO coatings. Mott–Schottky analysis showed n-type and p-type semiconductor behavior for the oxide layer present on the bare magnesium alloy and MAO coatings respectively.

J.N. Balaraju

Papers

Electroless deposition of Ni–Cu–P, Ni–W–P and Ni–W–Cu–P alloys

Influence of particle size on the microstructure, hardness and corrosion resistance of electroless Ni–P–Al2O3 composite coatings

Evaluation of corrosion resistance and self-healing behavior of zirconium–cerium conversion coating developed on AA2024 alloy

The influence of current density on the morphology and corrosion properties of MAO coatings on AZ31B magnesium alloy

Structure and phase transformation behavior of electroless Ni–P alloys containing tin and tungsten