By Branko N. Popov, Project Director*and Prabhu Ganesan, Center for Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina
Hard chromium has been used in automotive, aerospace, mining and general engineering industries due to its excellent wear resistance and low coefficient of friction. However, hexavalent chromium-based coatings are under severe regulations owing to their toxicity and research and development of alternate coatings are in progress.1 Ni-P and Ni-B graded coatings have been proposed as possible replacement coatings for the existing hard chromium coatings.
Engineering components are subject to failure through surface degradation processes such as wear, oxidation, corrosion and fatigue under varied circumstances. Different techniques, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), etc., are widely used for engineering the surface to impart desirable mechanical properties. Electrodeposition and electroless plating processes have received widespread acceptance owing to their less complex nature and cost-effectiveness. Alloying of phosphorus or boron along with nickel has improved hardness, corrosion resistance and wear resistance.2-5
In addition to Ni-P and Ni-B graded composite coatings, Ni-SiC and Ni-Co/SiC coatings are under development to replace hard chromium in many applications.6-13 Bi-layer Ni-SiC composite coatings were prepared by electrodeposition using a mixture of nickel chloride and nickel sulfamate as the nickel source. SiC particles of sizes varying from 1.2 to 20 µm were dispersed in the electrolyte for the preparation of Ni-SiC coatings with improved wear resistance.6 Besides Ni-SiC, Ni-WC and Ni-W composite coatings have also received technological interest due to their improved wear and mechanical properties.14,15 Other coatings such as Ni-Al2O3 are also under development with high wear resistance and improved hardness.16,17
Zinc offers good sacrificial properties and also offers good barrier properties when nickel is included in smaller amounts (~10-15 wt%). The drawback to the use of pure zinc coatings is that it shows a higher potential difference when coupled with iron due to its electronegativity (-0.760 VSHE). The large potential difference exerts a driving force for rapid zinc dissolution and the underlying steel substrate is protected only for a short period of time. In the case of Zn-Ni alloys, the life of the coating is enhanced by the presence of nickel, which offers good barrier protection. However, due to the high zinc content in the deposit, these alloys exhibit a more negative potential than cadmium and hence dissolve rapidly in corrosive environments. However, Zn-Ni alloys do not offer corrosion protection for longer time without having chromium-based chemical conversion coatings as a post-treatment.18,19
Chromate conversion coatings are widely used to protect stainless steel or zinc-coated steel substrates.20,21 These coatings are deposited from hexavalent chromium-based solutions, which are highly toxic and carcinogenic. In order to replace the chromate conversion coatings, colloidal silicate coatings are under extensive research. Silicate conversion coatings can be deposited from environmentally-friendly silicate-containing solutions by a dip-coating method, followed by drying and heat-treatment at high temperature.20-22
Silica-silicate coatings have been obtained from alcoholic silica sol that are either derived from hydrolyzed alkoxysilanes,23,24 aqueous silica/silicate sol22,25-37 or aqueous silicate solution containing organosiloxane polymers.21 Most of these studies were performed on zinc or zinc-plated steel22,25-31,35-38 and the presence of the silica-silicate coatings increased the corrosion resistance. It was reported that the corrosion protection resulted from silicate adsorption on the pits, which caused suppression of anodic as well as cathodic processes at the zinc surface and additionally produced a diffusion barrier for the corrosive species.25,35 In one of our earlier studies, we found that silicate layers can be formed on zinc under anodic polarization conditions,26 which causes the pH to increase at the interfacial region resulting in polymerization of silicates on the surface. This polymerization process of the surface-bonded silicates leads to SiO2 formation.21
Silicate coatings were also deposited from solutions that contain silica,30,31,34 silicates26-30,34 and metasilicates.31 Some researchers have added either organic or inorganic compounds to the bath to improve the corrosion protection or the quality of the silicate film.30,31,34-38 Earlier, our research group developed high corrosion-resistant silica coatings for zinc and Zn-Ni substrates by different methods.26-29 In this report, we present the deposition of alkaline Zn-Ni coating using sulfate electrolyte and surface modification followed by the silica conversion coating process. The corrosion properties of the Zn-Ni-SiO2 coating were evaluated in 5% NaCl solution and the results are compared with Zn-Ni coated steel.
The overall objective of our work is to develop electrolytic processes for plating dense, high wear-resistant Ni-(P, B, WC or SiC)-based nano-composite coatings for metal finishing industries in the USA. Considering the need for non-toxic alloys with superior coefficient of friction, hardness, ductility, strength and solderability, this work has enormous significance, since it will develop environmentally-benign processes for producing coatings that can be potential replacements for conventional hard chromium coatings.
The specific objectives are to develop novel plating processes for deposition of: (1) nanostructured coatings of Ni-P, Ni-B binary coatings and (2) Ni-Zn-X, (X=SiO2) ternary composite coatings by using DC and pulse deposition techniques.
The objectives of the research are:
- To develop a galvanostatic and potentiostatic DC and pulse plating method for the deposition of environmentally-benign Ni-P and Ni-B composite coatings.
- To study the deposition process as a function of parameters such as electrolyte composition, deposition current density/applied potential, deposition temperature and electrolyte agitation conditions.
- To determine the effect of the concentration of the particulates such as organic additives in the solution on the deposition process and mechanical properties.
- To study the mechanical and corrosion properties of Ni-P and Ni-B coatings.
- To prepare Ni-P-X and Ni-Zn-X (X=SiO2) composite coatings and to study their corrosion and mechanical properties including wear resistance, hardness, coefficient of friction etc.
Standard specifications for electrodeposited coatings of hard chromium were used to compare the properties of the new coatings.
This report covers the preparation, physical and electrochemical characterization of Ni-P coatings prepared by electrodeposition process.
Studies on Ni-P coatings
Preparation of Ni-P coatings. Ni-P coatings with varying phosphorus contents (between 7 and 11 wt%) were prepared using an acidic electrolyte containing nickel sulfate, phosphoric acid, phosphorous acid and sodium citrate. The bath composition is given in Table 1. Prior to plating, the carbon steel surface (low carbon, Q-Panel, 25 cm2) was polished using #600 and #1800 sandpapers until a mirror finish was obtained. The surface was then cleaned with soap solution followed by rinsing in tap water. Finally, the steel samples were pickled in 10% HCl solution for 1 min, followed by rinsing in running tap water and DI water. The specimen was then carefully introduced into the plating solution. The deposition was carried out galvanostatically using a potentiostat. The deposition current density was varied from 10 to 20 mA/cm2 and the deposition temperature was varied between room temperature and 60°C to study the respective parameters on the nickel and phosphorus composition in the final deposit.
Table 1 - Electroplating bath composition for preparing Ni-P deposits.
Current density (mA/cm2)
100 - 195 g/L
2 - 13 g/L
3.5 - 24.8 g/L
1.8 - 23.6 ml/L
45 - 125 g/L
Physical and electrochemical characterization
The composition of the Ni-P and Ni-Co-P deposits was determined by using an x-ray fluorescence spectrometer (Fischer XDAL) and the surface morphology was studied using a scanning electron microscope (SEM, ESEM Quanta FEI 200).
A variety of electrochemical techniques such as linear polarization and Tafel polarization were used to evaluate the barrier resistance properties of the coating. The coating thickness was kept constant at 3 µm for all the electrochemical characterization studies. The electrochemical characterization was done using an EG&G PAR model 273A potentiostat/galvanostat interfaced with a computer and a three-electrode setup in 0.5M sodium sulfate and 0.5M boric acid (pH=7) solution. The steel substrate with the coating was used as the working electrode and a platinum mesh was used as the counter electrode. A saturated calomel electrode (SCE) was used as the reference electrode. All potentials in this study are referenced to the SCE.
Results and discussion
Figure 1 shows the effect of the deposition current density on the nickel and phosphorus contents of the Ni-P coatings. The phosphorus content in the deposit increased from 7% to 11% as the deposition current density was increased from 10 to 20 mA/cm2. Deposition currents below 10 mA/cm2 produced non-uniform coatings with a very low deposition rate. Based on the results, a deposition current density of 15 mA/cm2 was selected to prepare Ni-P coatings with 11% P in the deposit with a deposition rate of 10 µm/hr.
The effect of deposition temperature was also studied by varying the solution temperature from 20 to 60°C. The deposition temperature had no influence on the phosphorus content up to 40°C and the phosphorus content changed from ~6% to 9% and 11% at 50 and 60°C respectively. Based on these studies, 60°C was selected since the desired Ni-P composition and deposition rate were achieved at this temperature. In addition, deposition at the higher temperature will reduce the stress developed in the deposit.