Abstract  The inhibition of bis-diisobutylaminomethyl-urea (BDMU) for the atmospheric corrosion of mild steel was investigated by volatile inhibiting sieve test (VIS) and electrochemical measurements. Electrochemical impedance spectroscopy of a vapor phase corrosion inhibitor monitor cell (VpCIM) was applied to study the effect of BDMU on the corrosion inhibition of mild steel under thin electrolyte layer. The results show that BDMU has good protection effect for steel. It suppressed the anodic reaction of steel electrode and rendered the Ecorr to more positive direction. Reflected Fourier transform infrared (FT-IR) spectroscopy was used to characterize the adsorption of BDMU on steel surface.
Keywords  Steel; Bis-diisobutylaminomethyl-urea; Vapor phase corrosion inhibitor; Electrochemical impedance spectroscopy (EIS);

1. INTRODUCTION
The use of vapor-phase corrosion inhibitors (VpCIs) is one of the most effective and convenient methods for preventing metallic articles and equipment from corrosion during transport and storage. There are numerous investigations on corrosion inhibitions by aliphatic amines, alicyclic amines, and their salts as VpCIs for various industrial metals and alloys. Among them, cyclohexyl ammonium carbonate (CHAC) and dicyclohexyl ammonium nitrite (DICHAN) are the most commonly used VpCIs and have been used in industry for several decades ^{[1, 2]}. However, the disadvantage of CHAC and DICHAN is their toxicity. An attempt was made to develop new environment-friendly organic compounds as VpCIs. Saurbier suggested toluylalanine as an effective temporary inhibitor of steel in wet atmosphere ^[3]. Vuroinen reported a series of morpholine Mannich base derivatives as vapor-phase corrosion inhibitors ^[4]. Some 6-methoxy-aminobenzothiazole derivatives were also used by Quraishi et al. to inhibit the metal corrosion under vapor-phase conditions ^[5].
    A VpCI needs to be a volatile compound that can be capable of forming a relatively stable bond at the interface of the metal. Thus, the vapor pressure and its volatile corrosion-inhibiting ability are the two very important properties for VpCIs ^[6]. Urea is one of the main ingredients of corrosion inhibitor formulations for many volatile corrosion-inhibiting works. It is safe and cheap. But the disadvantage is its too high vapor pressure to give lasting protection ^[7]. For this reason, work on urea derivatives having greater inhibitory activity than that of the parent urea molecule is desirable. In this paper, bis-diisobutylaminomethyl-urea (BDMU), which has a urea moiety and two diisobutylamino moieties, was developed as a novel VpCI. The molecular structure is described in Fig. 1. It has bulry molecular size and more N atoms in its molecule, and therefore may have larger coverage on the metal surface and provide better inhibition effect. Inhibition of mild steel corrosion by a BDMU film forming on the metal surface was studied by a volatile inhibiting ability test and electrochemical measurement. Its adsorption on the mild steel surface was investigated by reflected FT-infrared spectroscopy.

Fig.1 Molecular structure of Bis-diisobutylaminomethyl-urea (BDMU)

2.2.2 Electrochemical measurements
Electrochemical measurements were conducted both in stimulated atmospheric corrosion water and under thin electrolyte layer. A three-electrode cell, consisting of a mild steel rod working electrode (WE), a platinum foil counter electrode (CE), and a saturated calomel electrode (SCE) reference electrode was used for solution electrochemical measurements. The WE was mechanically polished on wet silicon carbide (SiC) paper (grades 120, 600, and 1,200), rinsed with redistilled water, degreased with acetone and ethanol, and dried at room temperature. The WE was introduced into an epoxy resin holder exposing a 1cm² surface to the solution. The solution was prepared using redistilled water and containing Cl^- 0.1kg/m³, HCO₃^-0.1kg/m³, SO₄^2- 0.1kg/m³, respectively. These anions were used as their sodium salts. As for Potentiodynamic polarization curves test, the potential changed up to +250mV around open circuit potential at speed of 1mV/s. The EIS experiments were performed at open circuit potential over a frequency range of 0.05 Hz to 100 kHz. The sinusoidal potential perturbation was 5 mV in amplitude. The potential values reported here were versus SCE. All measurements were conducted in a non-stirred electrolyte. The cell was opened to the laboratory air and the measurement was conducted at room temperature.
    A vapor phase corrosion inhibitor monitor cell (VpCIM) was used to obtain electrochemical impedance data with thin electrolyte. VpCIM consisted of 20 foils of mild steel (80.0mm×8.0mm×0.5mm) and each steel plates was separated by a 170mm thick layer of an insulating sheet. Alternate steel foils were connected to establish a 2-electrode configuration technique ^[10]. A block of 10 steel foils acted as the working electrode, and the other block of 10 steel foils served as the reference electrode and the counter electrode. The above ensemble (steel foils and insulating sheets) was encapsulated in an epoxy resin. One side of the ensemble was polished with emery paper of different grades (#1, #4 and #6), degreased with alcohol, and dried. The VpCIM was placed upon a 300 mL-beaker, which contained a lid of 5g inhibitors. The edges of the polished steel foils faced down to the lid. After a specific period of film-forming time of the inhibitor, the VpCIM was then covered with a filter paper saturated by stimulated atmospheric water. The VpCIM was placed in a vessel at a relative humidity of 90%. The EIS experiments were done according to a process described as above.
2.3 FT-Infrared Reflection Test
The steel specimens of 5mm×5mm×1mm were abraded with silicon carbide (SiC) paper and polished with diamond paste to 1mm. The samples were washed with acetone and isopropanol of analytical grade and then immediately transferred to a desiccator. The samples were moved to a vessel for BDMU treatment. After 8 hours inhibitor film-forming period, the BDMU-treated steel samples were transported for FT-Infrared Reflection Absorption Spectra analysis. A background spectrum was recorded for a sample without BDMU treatment. A Nexus 470 FT-IR spectrometer (Niclet) was connected for FT-IR surface analysis. The infrared light from FTIR spectrometer bench was directed on the sample via plane and spherical windows and strikes the sample surface 80⁰ from the surface normal. The reflect spectra were obtained by using p-polarized light at a resolution of 4 cm^-1. All subsequent spectrums were recorded in absorbance units (R_t/R₀×100%), where R_t is the reflected intensity of the BDMU-treated sample surface and R₀ is the reflectivity of the background spectrum. The FT-Infrared adsorption spectrum of BDMU with a potassium bromide (KBr) disc was also recorded for comparison.

3 RESULTS AND DISCUSSION
3.1 Volatile corrosion inhibition test
Visual inspection is the criterion for the vapor phase corrosion inhibition test. After VIS test, the corrosion rate and inhibition effectiveness for the BDMU film-forming specimens were 23.8 mg·m^-2·h^-1 and 84.6%, respectively. This shows that BDMU can volatilize and adsorb on the mild steel surface to protect the steel.
3.2 Electrochemical measurements in stimulated atmospheric corrosion water
Potentiodynamic polarization curves are shown in Fig. 2 for freshly polished mild steel electrode after 2 hours immersion in stimulated atmospheric corrosion water.

Fig. 2 Potentiodynamic polarization curves for steel electrode after 2 hours immersion in simulated atmospheric corrosion water（1-blank，2- 0.2 % BDMU）

    The anodic curve for the steel electrode in stimulated atmospheric corrosion water exhibits an active dissolution property. The mechanisms of anodic process of steel in electrolyte solutions are ^[11]:

    The cathodic portion of the polarization curve is a composite and represents oxygen reduction. The cathodic corrosion reaction of steel is:

    The total corrosion reaction of steel in aqueous solutions is as follows ^[12]:


    It is clear that BDMU renders E_corr more possitive to –417.0mV for the steel electrode in stimulated atmospheric corrosion water. In the presence of BDMU, both the cathodic and anodic current densities were greatly decreased near E_corr region. BDMU decreases the anodic reaction rate more strongly than the cathodic reaction rate. This indicates the effective corrosion inhibition by BDMU in stimulated atmospheric corrosion water.
    Fig. 3 shows Nyquist plots of a steel electrode in stimulated atmospheric corrosion water with and without BDMU.

Fig. 3 Nyquist plots for steel electrode after 2 hours immersion in simulated atmospheric corrosion water（1-blank，2- 0.2 % BDMU）

The corrosion resistance of each of the samples was determined by R_p. R_p is given by ^[13]:

    where R_e represents the real part of the complex faradaic impedance, and ω corresponds to the angular velocity of the AC signal (w=2pf，where f is frequency, Hz). R_p values were obtained by fitting the experimental Nyqusit data to a simple semicircle and extrapolating to Z_im=0. Comparing the two Nyquist diagrams in Fig.3, it is observed that the size of the semicircle without BDMU is far smaller than that with 0.2 % BDMU. This suggests that BDMU has excellent protection effect for corrosion of mild steel in simulated atmospheric corrosion water.
3.3 Electrochemical impedance measurements under thin electrolyte layer
The atmospheric corrosion of mild steel is an electrochemical process that conducted under thin electrolyte layer. The thin electrolyte layer was formed by adsorption, condensation or precipitation of humidity in the air. A VpCIM was used to investigate the protection of BDMU for mild steel under thin electrolyte layer. Fig. 4 shows Nyquist plots for a VpCIM obtained with and without a period of BDMU film-forming.

Fig. 4 Nyquist plots for a VpCIM under simulated atmospheric corrosion water layer (1-blank, 2-after 4 hours BDMU film-forming)

    The electrode impedance with BDMU film-forming was higher than that without BDMU film-forming. The inhibition effect of BDMU for mild steel was also proved under thin electrolyte layer.
3.5 FT-Infrared Reflection Test
As described above, infrared reflection absorption spectrum of steel specimen was obtained after 8 hours BDMU film-forming periods. The infrared absorption spectrum of BDMU with a potassium bromide (KBr) disc was also measured for comparison. Table 1 enumerated the fundamental frequencies of the bands of BDMU and those of the corresponding reflection spectra recorded after 8 hours exposure up to BDMU for film-forming periods.

    The adsorption of BDMU on steel surface is detected by the reflectance spectrum of steel surface. The bands around 1605cm^-1 and 1294cm^-1 were shifted about 58cm^-1 to 70cm^-1 compared to the infrared absorption spectrum for BDMU. This effect can be explained by assuming the interaction of Fe with N, O heteroatoms ^[14]. There are four N atoms and one oxygen atom in BDMU molecule. BDMU is capable of entering into interaction with the metal, for it has one pair electron on the nitrogen atom and two pair electrons on the oxygen atom. The metal surface usually becomes an electron acceptor. The formation of a donor-acceptor bond between BDMU and metal hinders the transfer of a metal ion from the metal lattice into solution. The mode of BDMU bonding on the steel surface is suggested in Fig. 5. BDMU can attach itself to steel surface through weak chemical bonding and forms an adsorbed monolayer to shield this interface from penetration by corrosive agents such as water, erosive ions ( Cl^- or SO₄^2- )，etc. The adsorbed monolayer may change the rate of electrochemical reactions like the dissolution of metal or the reduction of oxygen.

Fig. 5 Schematic of BDMU absorbed on the steel surface

4. CONCLUSIONS
Bis-diisobutylaminomethyl-urea (BDMU) has been proved to be a good vapor phase corrosion inhibitor for the mild steel. BDMU suppressed the anodic reaction of steel electrode and renders the corrosion potential to more positive direction. Infrared reflection absorption spectrum suggests that interaction exists between Fe and N, O heteroatoms of BDMU molecule.

Acknowledgments   Supports from the National Natural Science Foundation of China (20576069) and from Shanghai leading academic discipline project (P1304) are gratefully acknowledged.

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