Surface Functionalisation through Nanoscale Science

Corrosion resistance evaluation of Ni‐P\nano‐ZrO2 composite coatings by electrochemical impedance spectroscopy and machine vision method

Corrosion resistance evaluation of Ni‐P\nano‐ZrO2 composite coatings by electrochemical impedance spectroscopy and machine vision method

A. Stankiewicz*, J. Winiarski, M. Stankiewicz, I. Szczygieł and
B. Szczygieł

Ni‐P\nano‐ZrO2 composite coatings were obtained on the AISI 304 steel substrate by the electroless method from a bath containing dodecyltrimethylammonium bromide (DTAB). This cationic surfactant prevents ZrO2 agglomeration in the bath and affects the ZrO2 content in the coating, hence it alters functional properties of the coatings. It has been found in this study that corrosion resistance of the composite coatings depends on the surfactant concentration in the bath. The estimation of corrosion resistance was carried out by electrochemical impedance spectroscopy. The degree of the sample surface coverage with corrosion products was determined by the machine vision method. The coating obtained from the 0.88 g/dm3 DTAB solution showed the best protective properties. The machine vision method was shown to be an effective complementary tool to evaluate protective properties of the coatings.

1. Introduction

Composite coatings, as promising materials, are being investi- gated in plenty of research institutions in the world. Metal composite coatings are produced by the electroplating method or the electroless deposition from suspension baths. The currentless procedure is mainly
employed for nickel‐based coatings. Nickel, co‐deposited with hard particles (ZrO2, TiO2, SiC), exhibits a good abrasion resistance and high hardness; and it is applied as a replacement for chromium coatings in some cases. Such composite coatings have been applied in machine and automotive industry, for instance to low power engines. 

Surface‐active compounds are typical components of suspension baths, they are added to enhance stability and prevent agglomeration of the second‐phase particles. Applications of such compounds have been described in the literature; sodium dodecylsulfate (SDS) [1–5], cethyltrimethylammonium bromide

A. Stankiewicz, I. Szczygieł
Department of Inorganic Chemistry, Wrocław University of
Econom- ics, Komandorska 118/120, PL-53345 Wrocław,

J. Winiarski, B. Szczygieł
Faculty of Chemistry, Wrocław University of Technology,
Wybrze˙ze Wyspian
skiego 27, PL-50370 Wrocław, (Poland)
M. Stankiewicz
Faculty of Mechanical Engineering, Wrocław University of Technology, Łukasiewicza 5, PL-50371 Wrocław, (Poland)

(CTAB) [1,4–6], dodecyltrimethylammonium bromide (DTAB) [7], dodecyltrimethylammonium chloride [7], 1,3‐toloyltriethano- lammonium chloride [8], fluorosurfactants [9], or other various nonionic surfactants [7]. Surfactants are responsible not only for uniformity, stability, and adsorption capacity in such colloidal systems, but also for the influence on the rate of deposition, structure, degree of second‐phase particle incorporation into the nickel matrix, phosphorus content, microhardness and corrosion resistance of coatings [7]. The presence of surface‐active compounds in a bath also affects the z‐potential of the particles that is the electrical potential at the interface of a solid particle and an electrolyte. The content of the solid phase in a coating is determined by the sign and value of the potential [10–13]. A general dependence describing this relation has not been proposed so far, which is due to a large variety of both the solid particles and the bath’s compositions as well as the method of coating formation. There are literature data confirming the advantage of adding both the anionic (SDS) [12], and the cationic (CTAB) [13] surface‐active compounds to enhance the deposition process and increase of the content of the phase dispersed in the
composite coating. For a specific system, controlling the second‐ phase content of a coating is seemingly possible via z‐potential modification, which means affecting the properties of the coating. The effect of various surfactants on the coating deposition process of Ni‐P\nano‐ZrO2 composites has already been described [14]. DTAB was selected as an additional component favorably influencing the deposition process. The obtained Ni‐P\ nano‐ZrO2 composite coatings had a nodular structure. A detaileddescription of the morphology and composition of the composite coatings is given in our paper [14]. Zirconium(IV) oxide content in the coatings increases with increasing DTAB
concentration in the bath as follows: 9.7, 12.4, 22.1, 21.9 wt% for the 0.11, 0.22, 0.44, 0.88 g/dm3 DTAB, respectively. This dependence is related to the change in z‐potential as a function of surfactant concentration. The concentrations of 0.44 and 0.88 g/dm3 correspond to the
critical micelle concentration (cmc) and 2 × cmc, respectively. The results indicate that the presence of DTAB in the form of micelles in the nickel‐plating bath leads to an enhancement of solid phase transport [14]. The aim of the present research was to analyze protective
properties of Ni‐P\nano‐ZrO2 coatings obtained from the baths containing DTAB. The effect of nano‐ZrO2 content in the coating
(depending on DTAB concentration in the bath) on the corrosion resistance of the coatings was investigated. A machine vision
technique was used as a complementary method in the analysis and interpretation of the corrosion investigation by the electrochemical impedance spectroscopy (EIS) method. The above technique is a useful analyzing tool that provides quantitative and objective information on the corrosion state of the Ni‐P\nano‐ZrO2 surface.

2. Materials and methods

Coatings were produced using the method described in [14]. The bath composition was: 0.10 mol/dm3 NiSO4 6H2O; 0.15mol/ dm3 NH2CH2COOH; 0.20 mol/dm3 NaH2PO2 H2O; 0.50 mol/dm3 HCOONa; 3.0 g/dm3 ZrO2; 0.11–0.88 g/dm3 DTAB. The bath temperature was maintained at 75 8C, and pH at 6. Deposition process lasted 30 min. Coatings were deposited on the specimens of d ¼ 14 mm made of AISI 304 steel. The samples were prepared using 1200 grade abrasive paper and next they were ultrasonically cleaned in acetone. All the specimens were then etched for 10 min in a 0.1 mol/dm3 SnCl2 solution and washed with distilled water. Subsequently the samples were
activated for 10 s in a PdCl2 solution (1.4 10—3 mol/dm3), washed with distilled water and immersed in the bath. Sigma–Aldrich reagents pure p. a. were used to prepare the solutions. Zirconium(IV) oxide with a grain size of up to 100 nm was selected for the composite coating deposition. The thickness of the deposited coatings was about 3 µm. 

In order to determine the corrosion resistance of the obtained Ni‐P\nano‐ZrO2 coatings, the EIS method was employed. The measurements were performed in a non‐deaerated 0.5 mol/dm3 NaCl solution at pH 6.5 by using a three‐electrode system with a Reference600 (Gamry) potentiostat. An electrochemical vessel CEC/TH (Radiometer Analytical SAS) with a Pt counter‐electrode was used. A saturated calomel electrode (SCE) was applied as the reference electrode. An area of 0.363 cm2 of the working electrode was exposed to the NaCl solution. The impedance spectra were recorded at the corrosion potential (Ecorr) and by a sinusoidal voltage excitation with the amplitude 10 mV in the frequency range from 10 kHz to 10 mHz. The impedance
data with fitting lines were displayed as Bode plots. The acquired data were curve fitted and analyzed using the ZView 3.2c (Scribner Associates) software.

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