Surface Functionalisation through Nanoscale Science

Surface functionalisation by the introduction of self-healing properties into electroless Ni-P coatings

Alicja Stankiewicz a, *, Zoi Kefallinou b, Grzegorz Mordarski c, Zofia Jagoda d, Ben Spencer b

a Edinburgh Napier University, School of Engineering and the Built Environment, 10 Colinton Road, Edinburgh EH10 5DT, UK
b University of Manchester, School of Materials, Oxford Road, Manchester M13 9PL, UK
c Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, PL-30239 Krakow, Poland
d Wroclaw University of Economics, Faculty of Engineering and Economics, Komandorska 118-120, PL-53345 Wroclaw, Poland

Ni-P\alginate microgels coatings, as potential metallic protective coatings with self-healing properties, were deposited by the electroless method. The alginate microgels contained nickel chloride and sodium hypophosphite. It was proven that the reduction of nickel ions released from the microgels is possible on the steel and Ni-P coating surface. The self-healing effect of this system was studied by X-ray fluores- cence (XRF), chronoamperometry and scanning vibrating electrode technique (SVET). An improved corrosion protection observed here is attributed to the reduction of nickel ions to metallic nickel on the tested surfaces. Differences in the surface concentration of nickel and phosphorous species in the corrosion tested coatings with and without microgels, as evaluated using X-ray Photoelectron Spec- troscopy (XPS), provided substantial evidence for the formation of a Ni-P coating from the compounds included in the microgels.

1. Introduction

The provision of robust, durable, low-cost, protective coatings is of benefit to local and global economic growth across many in- dustrial sectors. A beneficial feature of a relatively new category of advanced coating material is the ability to self-heal. Self-healing protective coatings investigated so far are mainly based on carrier systems that store self-healing substances [1e7], and most often capsules or fibres are included in the polymeric matrices [6,8e12]. Many polymer-based materials have been used in this role, how- ever, compared with materials based on metals or ceramics, poly- meric coatings have poor mechanical properties, which limits their applications.
The carcinogenic hexavalent chromium, was, until recently, a key compound employed in research into self-healing metallic coatings. In the context of wear resistance, hardness and corrosion resistance, nickel-phosphorous (Ni-P) coatings are comparable to chromium coatings. Although Ni-P coatings do not currently have the ability to regenerate, a viable solution to this problem is the encapsulation of corrosion inhibitors and the introduction of the capsules into the matrix of the coating. This approach has been described for electrochemically-generated nickel [13] and zinc [14] coatings. In the case of the nickel coating prepared nanocontainers did not contain any active substances. The self-healing protective zinc coating contained nanoaggregates of polyethylene oxide-b- polystyrene (PEO113-b-PS218). The self-healing properties of these systems resulted from the ability of amphiphilic polymers to shrink and swell reversibly.

A new nickel-phosphorous self-healing coating process, which promises enhanced material properties and a low toxicity pro- duction process compared to the chromium-based equivalent is proposed [15]. The introduction of microgels comprised of nickel salts and sodium hypophosphite into Ni-P matrices results in the surface functionalisation by the introduction of self-healing prop- erties. The reconstruction of a metallic coating at the time of damage is possible due to the autocatalytic reaction of nickel and phosphorous deposition [16]:

Ni2þ þ 2H2PO—2 þ 2H2O / Ni þ 2H2PO—3 þ 2Hþ þ H2 (I)

H2PO—2 þ H / P þ H2O þ OH— (II)

XRF, chronoamperometry and SVET were used to study the self- healing effect. The morphology of the Ni-P and Ni-P\alginate microgels coatings was examined by scanning electron microscope (SEM) and fluorescence microscope. The elemental and chemical composition of the coatings was performed by energy dispersive X- ray spectroscopy (EDX) and XPS.

2. Materials and methods

For microgels preparation, alginic acid sodium salt from brown algae, paraffin oil, Span 80, sodium hypophosphite, nickel chloride, fluorescein and acetone were used. For the electroless deposition of nickel-based coatings nickel chloride, sodium hypophosphite, so- dium citrate and sodium hydroxide were used. Watt’s bath con- taining nickel sulphate, nickel chloride and boric acid was used to produce thin nickel coatings for a proof of concept trial. NaCl so- lution was used for corrosion resistance analysis, Scanning Vibrating Electrode Technique (SVET) tests and X-ray Photoelectron Spectroscopy (XPS) analysis. All materials were purchased from Sigma Aldrich and were used without further purification.

The specimens of A1008 steel of different sizes dictated by the requirements of the testing method were used. The steel surface was ground from 600 to 1200 grit silicon carbide abrasive paper, then washed with detergent and then rinsed with deionised water and acetone.

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