Corrosion study of Neodymium-Iron-Boron alloys:
Neutron reflectometry of the ultra-thin polymer film interfaces and effectiveness of polymer coatings
S. Benfattoum
1, A. Lounis
1*, M. Azzaz
1, D. Miroud
11
Laboratory of Sciences and Material Engineering.
University of Science and Technology Houari Boumediene, 16111 Algiers. Algeria
e-mailsad_djia@live.fr
B. Guedioura
22
Nuclear Research Center. Reactor Division. 16050 Draria. Algiers. Algeria
Abstract— Neodymium-iron-boron alloys are very sensitive, particularly to the air humidity. The aim of this work is to study the effectiveness of polymer coatings against corrosion in the cases cited above. This work is financially supported by the Research Thematic Agency in Science and Technology. In this context we introduced the neutron reflectometry to study the ultra-thin polymer film interfaces of polymethylmethacrylate (PMMA). Reflectometry experiments were conducted at the nuclear research center of Draria (Algiers). The preparation of the PMMA films is performed by means a technique based on depositing a thin film on a planar solid surface. This method known as the “spin-coating’’ provides dense, homogeneous materials very thin after a rapid evaporation of the total solvent. First, we realized calibration settings "zero" reflectometer:
one seeks the zero position of the detector, the optimal position of the sample and its angle of rotation. The measured reflectivity as a function of the scattering vector Q, gives information on the thickness of layers, roughness of multilayer system. From the spectrum of the reflectivity vs. the wave vector (scan -2 of the PMMA film), we find a thickness of the PMMA film.
Electrochemical measurements were performed in a three electrode cell using an VoltaLab Master4, Model 273A potentiostat. The samples of Nd-Fe-B magnets covered by the PMMA film are used as working electrode in the NaCl (0.5M) solution, a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The polarization parameter values, corrosion current density (Icorr) and the polarization resistance (Rp) were determined. The corrosion rate values obtained show interesting perspectives for our work. The corrosion resistance is improved in the presence of PMMA layers. We note that for variable thickness of PMMA film 485nm and 1,5µm the polarization resistance are 518cm² and 6550cm² respectively. The corrosion rate values obtained show interesting perspectives for our work. The corrosion resistance is improved in the presence of PMMA layers
Keywords— rare earths, magnet, corrosion, polymer coatings
Experimental
The preparation of the films is performed by means of a preparation technique based on deposing a thin film on a planar solid surface. The PMMA (0.1g) is dissolved in 10 ml of chloroform (CHCl3). The substrate is coated with the solution and initiates rotation (4000tours/ min). This method known as the
“spin-coating’’ provides dense, homogeneous materials very thin after a rapid evaporation of the total solvent
A neutron reflectometry experiment consists in measuring the reflection of a neutron beam on a thin solid at grazing incidence. The spectrometer used was a vertical neutron reflectometer which operates at fixed wavelength (4.7Å). NUR reflectometer operates in the angular dispersive mode by running a -2 scan. The measured reflectivity as a function of the scattering vector Q, gives information on the thickness of layers, roughness of multilayer system and on the variation of refractive index perpendicular to the sample surface n(z). [1][2]
First, we realized calibration settings "zero"
reflectometer: one seeks the zero position of the detector, the optimal position of the sample and its angle of rotation.
Results and discussion
Optimal position of the detector: Positioning the detector in front of the monitor of the incident beam, it is actually to look for the zero position of the detector relative to the incident beam (maximum reflection Fig.1).
Position (X0) of sample maximum illumination:
The spectrum of the sample translation allows to find the optimal position. The average value of the spectrum (Fig. 2) gives us gives us the coordinates (X0 , Y0 ) of the sample.
The Figure 3 shows the rotational position.
To test the performance of the reflectometer a 10 Ni-Ti bilayers monochromator obtained from HMI, Berlin, was analyzed. The nominal thickness of each bilayer is 15nm for Ni and 15nm for Ni, i.e. the total thickness of the monochromator is 300nm. The multilayers are deposited on float-glass substrates. The Fig. 4 reports the obtained neutron reflectivity profile. The monochromator shows two peaks at Q= 0.021 and 0.041Å. Their spectral position gives the expected multilayer period of 30nm
After determining the rotation and translation for PMMA (Fig.5 and Fig.6), we perform a scan -2 of the film
From OriginLab fit we find a thickness of the PMMA e=485nm, calculated by the relation d=2/Q. We note that the spectrum is noisy ; the background area is reached quickly (Q= 0.0051 Å-1) and therefore the Parrat32 fit did not converge (Fig.7).
Figure 8 shows micrographies of PMMA film: we are in the presence of cavities and split with a very specific way. We also observe a roughness on the thin film as well as bubbles of varying sizes, this phenomenon is probably due to the rapid evaporation of the solvent during annealing
Figure 8: Micrographies of PMMA film
Electrochemical measurements were Performed in a three electrode cell using an VoltaLab Master4, Model 273A potentiostat. The samples of Nd-Fe-B magnets are used as working electrode in the NaCl (0.5M) solution, a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode. Table 1 gives the polarization parameter values [3], corrosion current density (Icorr) and the polarization resistance (Rp).
Tabel 1:Electrochemical data obtained from polarization curve
Parameter Sample Icorr
(A/cm²) Rp (cm²)
Without layer 58.5 518
One layer of PMMA 19.5 1280
Five layers of PMMA 5.5 6550
Acknowledgment
We acknowledge financial support from the Research Thematic Agency in Science and Technology. The authors are
grateful for CRTI Algiers, Algeria.
References
[1] Penfoldt, J., K Thomas, R., 1990. The application of the specular reflection of neutrons to the study of surfaces and interfaces. J. Phys:
Condens. Matter 2: 1369-1412.
[2] Sugiura, S., Nakajima, M., Seki, M. 2002. Preparation of Monodispersed Polymeric Microspheres over 50 μm Employing Microchannel Emulsification. Ind. Eng. Chem. Res. 41: 4043-4047.
[3] Basu, S., Singh, S., Ghosh, S.K., 2006. Morphology of electrodeposited Ni/Cu
