Mild steel was electroless coated with nickel–phosphorus alloy from a bath containing sodium hypophosphite and glycin–citrate complexing agents. The effect of coating time on phosphorus content, thickness, structure and hardness of the deposits were analyzed. Corrosion parameters such as current density, corrosion potential and corrosion rate were obtained from tafel polarization curves and immersion corrosion tests in aerated 3.5% NaCl solution. The X-ray diffraction (XRD) patterns, anodic polarization curves and scanning electron microscopy (SEM) of heat-treated specimens obtained in various coating times were also studied. The X-ray diffraction patterns showed that by changing the coating times, amorphous or crystalline structure could be obtained. The coatings containing 11.1–13.1% phosphorous were amorphous, and showed better corrosion resistance than microcrystalline structure. The heat-treated specimens obtained in different coating time had more hardness and corrosion resistance than unheated samples. According to the polarization studies and scanning electron microscopy images, the heat-treated samples with 10.8 and 10.1% phosphorous show the intergranular corrosion, while the coatings with 11.7 and 12.2% of phosphorus have the least corrosion rate. Immersion corrosion test data at the 4-month interval had good agreement with electrochemical polarization results.

Effect of coating time and heat treatment on structures and corrosion characteristics of electroless Ni-P alloy deposits

Adsorption of arsenic, phosphorus and chromium by bismuth impregnated biochar: Adsorption mechanism and depleted adsorbent utilization

The operating process, versatility and the increasing research interest in optimising the process and products technology in the electroless plating method of metal coating, particularly, the electroless nickel plating of metallic substrates such as mild steel, necessitates the writing of this review. It is also aimed at providing more literature information, both of the past and the present published research in this field. In this paper, electroless nickel plating is introduced. The various nickel plating solutions and baths’ operating parameters; main types of electroless nickel plating; the mechanism involved in the plating process; application of the nickel plating process to iron powders; advantages and disadvantages and the process’s other applications are reviewed. Electroless nickel plating produces an amorphous deposit in the as-plated condition. The deposit is not dependent on current distribution and hence it is almost uniform in thickness. Electroless nickel plating is far more difficult to remove chemically than conventional nickel deposits due to its superior corrosion resistance. The deposit has a good wettability and is generally hard. However, its bath control is more complex than with electroplating. The bath also has lower efficiency and higher operating costs, even without the use of electricity.

Electroless Nickel Plating – A Review

http://eprints.nmlindia.org/311/1/Ni-P_N-B_duplex_coating_MCP_paper.pdf

Electroless Ni–P/Ni–B duplex coatings: preparation and evaluation of microhardness, wear and corrosion resistance

Electroless Ni–P coatings provide high hardness and excellent resistance to wear and abrasion The present work aims to study the formation of electroless Ni–P graded coatings, with varying nickel and phosphorus contents of the individual layers and to evaluate their corrosion resistance by polarization and electrochemical impedance spectroscopic studies The possibility of preparing electroless Ni–P graded coatings by sequential immersion in three different plating baths is discussed The study reveals that electroless Ni–P graded coatings offer better corrosion resistance than non-graded Ni–P coatings

Deposition of electroless Ni–P graded coatings and evaluation of their corrosion resistance

Electroless Ni-P deposits with phosphorus content ranging from 4.8 to 12.8 weight percent (w/o) were examined using potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) techniques to characterize the effect of phosphorus on the corrosion behavior of electroless nickel-plated mild steel in deaerated 40 w/o NaOH solution. Anodic polarization of the electroless Ni-P alloys in caustic NaOH solution shows that the passive current density decreases with increasing phosphorus content in the deposits. At an applied potential of -1.2 V vs. saturated calomel electrode (V SCE ) (close to their E corr ), EIS data indicate that the R ct for Ni-P alloys in NaOH solution increases with increasing phosphorus content. X-ray photoelectron spectroscopy (XPS) results suggest that the primary constituent formed on the Ni-P surface after EIS measurement in 40 w/o NaOH solution at an applied potential of -0.4 V SCE (in the passive region) is Ni(OH) 2 , which is responsible for the passivity of the Ni-P alloys. The polarization resistance of Ni-P alloys in NaOH solution at -0.4 V SCE also increases with increasing phosphorus content

The Electrochemical Behavior of Electroless Plated Ni‐P Alloys in Concentrated NaOH Solution

Electroless Ni-P deposits with phosphorus contents ranging from 4.8 to 12.8 wt.% were examined using d.c. polarization curves and a.c. electrochemical impedance spectroscopy techniques to characterize the effect of phosphorus on the corrosion behavior of electroless nickel-plated mild steel in neutral and alkaline 3.5 wt.% NaCl solutions. The anodic polarization behavior of the Ni-P alloys in NaCl electrolyte was very similar for all deposits except Ni-4.8P. According to the electrochemical impedance data, the charge transfer resistance R ct and film resistance R f of these deposits have subsequently been studied as functions of the phosphorus content. It was shown that the R ct and R f values increased with increasing phosphorus content of the electroless nickel coating. The Ni-P deposits remained bright in appearance during the periods of electrochemical experiment, and no evidence of pitting was observed after testing. Based on X-ray photoelectron spectroscopy analysis, the passivation of Ni-P alloys was attributed to the formation and adsorption of a layer of hypophosphite.

Ming-Pan Hung

Papers

The Electrochemical Behavior of Electroless Plated Ni‐P Alloys in Concentrated NaOH Solution

Role of phosphorus in the electrochemical behavior of electroless Ni-P alloys in 3.5 wt.% NaCl solutions

Electrochemical Behavior of Electroless Plated Ni‐P Alloys in Concentrated NaOH Solution.