научная статья по теме MORPHOLOGICAL, MECHANICAL, CORROSION AND HYDROGEN PERMEATION CHARACTERISTICS OF NI NANO-TIO2 COMPOSITE COATING COMPARED TO NI ELECTRODEPOSITED ON LOW CARBON STEEL Физика

Текст научной статьи на тему «MORPHOLOGICAL, MECHANICAL, CORROSION AND HYDROGEN PERMEATION CHARACTERISTICS OF NI NANO-TIO2 COMPOSITE COATING COMPARED TO NI ELECTRODEPOSITED ON LOW CARBON STEEL»

ПОВЕРХНОСТЬ. РЕНТГЕНОВСКИЕ, СИНХРОТРОННЫЕ И НЕЙТРОННЫЕ ИССЛЕДОВАНИЯ, 2011, № 2, с. 90-96

УДК 539.548

MORPHOLOGICAL, MECHANICAL, CORROSION AND HYDROGEN

PERMEATION CHARACTERISTICS OF Ni-NANO-TiO2 COMPOSITE COATING COMPARED TO Ni ELECTRODEPOSITED

ON LOW CARBON STEEL © 2011 Amir Sadeghi1, R. A. Khosroshahi1, Z. Sadeghian2

department of Materials and Metallurgical Engineering, Sahand University of Technology, Tabriz, Iran 2Gas Institute, Research Institute of Petroleum Industry, Tehran, Iran Received August 12.2010

Ni and Ni—nano-TiO2 composite coatings with various amounts of TiO2 in electrolyte, on low carbon steel, have been prepared from Watts-bath using electrodeposition process. The morphological, mechanical, corrosion and hydrogen permeation characteristics of Ni and Ni-nano-TiO2 coatings were studied and compared with each other. The results revealed that, existence of nano-TiO2 particles in Ni matrix improved the microstructure as well as microhardness, whereas increasing particle incorporation from 4.33 to 7.62 vol. % concluded to microhardness enhancement. The corrosion behavior of Ni and Ni-nano-TiO2 composite coatings with various amount of particle content was studied by the anodic polarization curves in 5% H2SO4 solution at room temperature. It was seen Ni-nano-TiO2 composite coatings exhibited higher corrosion resistances comparing to pure Ni coating and corrosion protection improved with increasing nano-TiO2 in coatings. In addition to the corrosion and engineering properties, comparison of hydrogen permeation characteristics of the Ni coating was made with Ni-nano-TiO2 composite coating through Devanthan-Stachurski hydrogen permeation test. From the resulting data analysis, Ni-nano-TiO2 composite coating was seen not only to provide longer life under corroding media, but also reduces greatly the risk of the substrate being exposed to hydrogen permeation when compared to electrodeposited Ni coating.

INTRODUCTION

Steel substrates, when used in engineering applications, are prone to hydrogen embrittlement failure due to the hydrogen discharge reaction occurring during corrosion processes. During this process, the evolved hydrogen gets adsorbed on the metallic surface. Subsequently, some of this adsorbed hydrogen diffuses into the crystalline lattice of the substrate, following an adsorption-absorption equilibrium process. From the diffused hydrogen atoms, H2 molecules also may be formed in micro-voids ofthe material. Such an accumulation of molecular and atomic hydrogen in metals causes hydrogen-assisted cracking, bilisters and ultimate failure of the structure [1-3]. Several techniques have been used to decrease the problem of hydrogen embrittlement of steel. Some of these include post treatment annealing, laser surface modification and shot peening [1, 4, 5]. One of these methods is using catalytic effects [5].The rate at which molecular hydrogen dissociates and enters a metallic surface can be limited by controlling the surface kinetics through the use of surface catalysts [5]. Due to the investigations, inorganic oxides such as ZrO2, TiO2 and SnO2 are known as significant catalysts [2, 6]. S.K. Yen and his coworkers studied effects of electrolytic ZrO2 coating on hydrogen permeation of AISI 430 stainless steel and observed that ZrO2 film on the AISI 430 stainless steel demonstrated the retarding effect on hydrogen entry and the

occurrence of blistering [2]. On the other hand, electroplating of the substrate with a suitable coating holds promise in modifying the hydrogen discharge kinetics of the substrate and thereby reducing the hydrogen influx into the metal [3, 7]. However, the necessary prerequisite for such coatings is that they should have the capability of providing sacrificial protection should the substrate be exposed. The two main issues to be considered while choosing a coating are (1) damage to the coating and subsequent local uptake of hydrogen on the exposed steel surface and (2) show corrosion of the coating (with or without local damage) leading to some hydrogen generation by water reduction and subsequent permeation of hydrogen into the substrate. Based on this, the choice of coating should be one that is mechanically strong (dose not abrade or scratch easily) and one that has low corrosion rates, thereby preventing hydrogen evolution on the surface [5]. Previously, Popov et al. [3] developed a variety of electrodeposited procedures to develop coatings such as Zn-Ni-Cd and Zn-Ni-P with corrosion and hydrogen permeation properties.

The objective of this work was to determine the morphological, mechanical, corrosion and hydrogen permeation characteristics of the Ni-nano-TiO2 composite and to determine its effectiveness as compared to Ni elec-trodeposited coating.

RTChkl =

-w-100%.

Ihkl

EXPERIMENTAL

Fabrication of coatings. The pure Ni coating and Ni—nano-TLO2 composite coatings were prepared by elec-trodeposition technique from a typical Wktts-bath. The bath solution composition and plating conditions: NiSO4 • • 6H2O - 300 g/l; NiCl2 • 6H2O - 40 g/l; H3BO3 - 40 g/l; Sodium Dodecyle Sulfate (SDS) - 0.5 g/l; TiO2 powder, Degussa (dm = 30-50 nm) - 0, 10, 50, 100 g/l.

Analyze and morphological surveys. Scanning electron microscope (SEM), OXFORD Model, was applied in order to study the microstructure, morphology and composition of the surface of different coatings. The amount of incorporated TiO2 particles was evaluated using an energy dispersive spectrometer (EDX) with accelerating voltage of 20 kV coupled with the SEM. Also map analyses of Ti with aid of EDS were performed on the surface of the each composite coating for demonstrating of particles distribution and density in coatings. The phase structure of the coatings was analyzed with X-ray diffraction, D8 ADVANCE-BRUCKERS AXS Model, with CuKa.

The texture and preferred orientation of the nickel films was estimated using the relative texture coefficient RTChki:

t IT0

1 hkll 1 h..

^ Ihkll Th

Where Im are the relative intensities ofthe hkl reflections, ~LIhkJ is the sum ofall intensities, in our case 111, 200, 220, 311 and 222.

Microhardness test. The microhardness was determined using a microhardness tester, Micromet Model, with load of 50 g for 10 s, corresponding with ASTM B578. The final value was evaluated by taking 10 identita-tions on each specimen and after omitting the maximum and minimum the average was calculated.

Corrosion test. Potentiodynamic polarization studies were made at 298 K using Potentiostate EG&G 273 A. A three-electrode corrosion cell was used. The specimen to be tested was used as a working electrode; the reference electrode was SCE and graphite was used as a counter electrode. The electrolyte used was 5% H2SO4 with pH = = 1. The coated specimen with 28.6 mm2 area was exposed to the electrolyte. Samples were rinsed with etha-nol and deionized water before corrosion testing. Poten-tiodynamic polarization studies were carried out by polarizing the working electrode at a scan rate of 1 mV/s from -400 mV to +400 mV For each analysis, Icorr, Ecorr, Rp (corrosion resistance) obtained due to the Linear extrapolation.

Hydrogen permeation test. For the hydrogen permeation studies, one side of the steel membranes was elec-trodeposited with Ni and Ni-nano-TiO2 (50 g/l). To ensure uniformity in testing, both membranes were elec-trodeposited for 5 minutes to reach the same thickness. The prepared membrane samples were then fitted into

DC-power

V vyv^y-1 I

(Gr)

0.5 M H3BO3

0.5 M Na2SO4

Potantiostate

(Gr)

(SCE)

0.2 M NaOH

Specimen (W)

Fig. 1. Scheme of Devanthan-Stachurski hydrogen permeation cell [3].

the Devanthan-Stachurski permeation cell (Fig. 1) [3]. This cell setup consists of two working compartments (cathodic and anodic chambers), and the membrane is placed between two chambers. The side with the deposit faces the cathodic side and the other side faces the anodic side. Permeation studies were performed using a power supply (2 V-15 A) and a potentiostat (EG &G 273 A) connected to membrane. Graphite was used as the counter electrode in each side and the membrane was used as a bipolar working electrod and the SCE electrode was used as a reference electrode. The anodic compartment was filled with 0.2 M of sodium hydroxide (NaOH) solution and the potential was kept at 0.5 V until the background current fell below 0.5 mA/cm2. This value corresponds to a practically zero concentration ofatomic absorbed hydrogen on the surface. The cathodic compartment was filled was filled with 0.5 M Na2SO4 + 0.5 M H3BO3 solution at pH 6.0. The membrane on the cathodic side of the cell was polarized potentiostatically at current density of 10 A/dm2, creating conditions for hydrogen evolution. Hydrogen generated on the cathodic side permeates through the membrane and gets oxidized on the anodic surface of the membrane. The steady-state currents associated with anodic (permeation current) and cathodic (charging current) reactions were monitored continuously by charging the overpotential for hydrogen evolution reaction at the cathodic side.

RESULT AND DISCUSSION

Particles content of Ni—nano-TiO2 composite coatings.. Figure 2 and 3 respectively show EDX and Ti Mapping analysis results for Ni-TiO2 composite coatings with different amount of nano-TiO2 particles suspended in electrolyte (10, 50, 100 g/l). As it is seen with increasing amount of particle content in electrolyte, the percentage of incorporated TiO2 and its density in composite coating enhances. According to the EDX analysis it was seen, the amount of TiO2 particle incorporation varied between 4.33 vol. % for Ni-TiO2 (10 g/l) and 7.62 vol. % for

(a)

Sum Spectrum 90 sec

(a)

Ni

Ni

Jit

Ti Ti 1 1

Ni

0 1 2 3 4 5 6 7

Full Scale 26869 cts Cursor: - 0.021 (978 cts) (b)

8 9

10

keV

90 sec

N i

N i

O

C Ni

Ji) Ti Ti 1 —1—-J— 1 1 J \ lA 1 1

0 1 2 3 4 5 6 Full Scale 26869 cts Cursor: 0.000

(c)

Ni

78

9 10 keV

Sum Spectru

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