Wencheng Su

Corrosion performance of polypyrrole coating applied to low carbon steel by an electrochemical process

The corrosion performance of polypyrrole and poly(N-methylpyrrole) coated steel was evaluated by DC polarization and Electrochemical impedance spectroscopy (EIS). Our results show that the presence of polypyrrole coatings significantly increases the corrosion potential and drastically reduces the corrosion current and corrosion rate of steel. The corrosion resistance of poly(N-methylpyrrole) coated steel was lower than that of polypyrrole coated steel.

Electrodeposition mechanism, adhesion and corrosion performance of polypyrrole and poly(N-methylpyrrole) coatings on steel substrates

Polypyrrole and poly(N-methylpyrrole) coatings have been successfully electrodeposited on steel substrates from aqueous oxalate solutions. In acidic solutions, the electrodeposition process was characterized by three distinct stages. In this work the electrodeposition process was first investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and FTIR techniques. The adhesion and corrosion performance of the electrodeposited coatings were then evaluated by single lap shear tests and tafel tests, respectively. Our results reveal that deposition of FeC2O4·2H2O crystal layer first established the passivation of steel at the end of the first two stages, but the formed FeC2O4·2H2O passive layer is subsequently decomposed followed by electropolymerization of pyrrole. Our results also show that electrochemical process parameters had significant effects on the adhesion and corrosion performance of the electroposited polypyrrole and poly(N-methylpyrrole) coatings. Polypyrrole coatings exhibited better adhesion and corrosion performance than poly(N-methylpyrrole) coatings.

Electropolymerization of pyrrole on steel substrate in the presence of oxalic acid and amines

Abstract Electropolymerization of pyrrole on steel substrate was carried out in aqueous oxalate solutions in the presence of amines. Triethylamine and allyamine were the amines used in this study. The electropolymerization process of pyrrole in acidic medium was found to be different from that in alkaline medium. In acidic medium, the reactions were characterized by an induction period while no such period was observed in alkaline medium. Our results show that the pH of the reaction medium and the applied current density had a great influence on the induction time. Effects of triethylamine and allyamine on the electropolymerization process of pyrrole were very similar. The composition of the coatings was studied by FTIR and elemental analysis. The coatings formed in different medium had a similar composition. Smooth, uniform, strongly adherent coatings could be formed on the steel substrate by proper choice of the reaction parameters.

Characterization of the passive inorganic interphase and polypyrrole coatings formed on steel by the aqueous electrochemical process

Polypyrrole coatings have been successfully formed on steel from aqueous oxalic acid-pyrrole solutions by electrochemical polymerization. Formation of the coatings was found to be dependent on the pH of the reaction solution and the applied current. In acidic medium, the formation of polypyrrole was characterized by an induction (passivation) period before electropolymerization of pyrrole. At the end of the induction period, a crystalline passive interphase was formed. The morphology and composition of the electrodeposited passive interphase and the resultant polypyrrole coatings were investigated by scanning electron microscopy, reflection-absorption infrared spectroscopy, and X-ray photoelectron spectroscopy. Our results reveal that the chemical composition of the passive interphase was similar to that of iron(II) oxalate dihydrate, FeC 2 O 4 . 2H 2 O, crystals. Size and orientation of the crystalline passive interphase varied with electrochemical process variables.

IR and XPS studies on the interphase and poly(N-methylpyrrole) coatings electrodeposited on steel substrates

Abstract Poly( N -methylpyrrole) coatings have been electrodeposited on steel substrates from aqueous oxalate solutions. The formation process of the coatings was found to be different between acidic and basic mediums. In an acidic medium, the reactions were characterized by two stages: formation of the passive interphase and electropolymerization of the N -methylpyrrole. The composition of the formed interphase and poly( N -methylpyrrole) coatings were examined by infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). Our results show that the interphase formed in an acidic medium is composed of iron(II) oxalate dihydrate. The formation of poly( N -methylpyrrole) coatings on steel occurred after the passivation of steel was completed. Interaction between the poly( N -methyl pyrrole) coatings and the interphase is shown by the broadening of the –CO absorption peak.

Formation of polypyrrole coatings onto low carbon steel by electrochemical process

Thin polypyrrole coatings (∼ 10 μm thick) were formed on low carbon steel by an aqueous constant current electrochemical polymerization using oxalic acid as the electrolyte. The amount of polypyrrole coatings formed on steel increased with the applied current and monomer concentration. No significant change in the electropolymerization of pyrrole occurred as a result of increased electrolyte concentration. The induction time for electropolymerization decreased significantly with current density but was unaffected by the initial monomer and electrolyte concentration. The electropolymerization potential of pyrrole increased with increased current density (Cd), i.e., Ep = 0.62 + 0.41 [Cd], and decreased exponentially with increased monomer and electrolyte concentration, Ep = E0 exp-[M]. Scanning electron microscopy (SEM) showed that the microstructure of the polypyrrole coatings formed on steel was dependent on the current density to the extent that smoother and more uniform coatings are formed at low current density.

Formation of polypyrrole coatings on stainless steel in aqueous benzene sulfonate solution

Abstract Polypyrrole coatings with varying surface morphology have been formed on a stainless steel working electrode using benzene sulfonic acid sodium salt as the electrolyte. The morphology of the coatings varied with the current density. The amount of polypyrrole formed during electropolymerization increased with current density and monomer concentration, but was unaffected by increased electrolyte concentration. The electropolymerization potential of pyrrole increased with increased current density but the current efficiency remained relatively unchanged at 99–105%. The rate of electropolymerization of pyrrole onto stainless steel increased slightly with increased monomer concentration. The electropolymerization potential decreased with increased monomer and electrolyte concentration.

Morphology and structure of the passive interphase formed during aqueous electrodeposition of polypyrrole coatings on steel

An iron(II) oxalate dihydrate passive interphase was electrodeposited on steel substrate from aqueous oxalate solutions prior to the electrochemical polymerization of pyrrole. The effect of electrochemical process parameters on the morphology and structure of the passive interphase was systematically investigated by scanning electron microscopy(SEM) and X-ray diffractometry (XRD). Our results reveal that the passive interphase is composed of a monolayer of closely packed crystals. The size of the crystals is dependent on the experimental conditions. The iron(II) oxalate dihydrate interphase formed on steel by electrochemical deposition has an orthorhombic structure irrespective of the reaction condition. The unit cell dimensions of the passive interphase formed by electrodeposition are slightly different from those formed by conventional chemical process.

Kinetics and efficiency of aqueous electropolymerization of pyrrole onto low-carbon steel

The effect of process parameters on the conversion, P, and current efficiency, η, for the aqueous electropolymerization of pyrrole on low-carbon steel has been investigated. The amount of polypyrrole coatings formed on steel, W p , increased with the charge passed, Q, and the initial pyrrole concentration [M], but was unaffected by the electrolyte concentration. The conversion of pyrrole into polypyrrole, P = W p / W M , increased with electropolymerization time, and the applied current, and decreased with the initial monomer concentration. The oxalic acid concentration had no significant effect on conversion. The current efficiency for the electropolymerization of pyrrole performed by using high applied current, I(I ≥ 40 mA), and high pyrrole concentration, [M] ≥ 0.5M, rose to its highest value at short polymerization times, t < 300 sec. It then decreased and leveled off at longer times, t ≥ 1,000 sec. At low applied current, I ≤ 20 mA, and low pyrrole concentration, [M] < 0.25M, the current efficiency increased gradually with increased reaction parameters ([M], I, and t) and reached a maximum value at t = 1,000 sec. A retrogression of the current efficiency occurred at t ≥ 1,000 sec, for the reaction performed by using applied current of 10 mA. Overall, the current efficiency varied between 39 and 130%, with the higher values occurring at high pyrrole concentration and high applied current. The current efficiency was determined from the ratio of the experimental and theoretical electrochemical equivalents for polypyrrole.

Electrodeposition of poly(N-methylpyrrole) coatings on steel from aqueous medium

Poly(N-methylpyrrole) coatings were formed on low carbon steel by an electrochemical method from aqueous oxalate solutions. The electrochemical reactions were performed in a wide range of pH of the reaction medium and applied current density. The formation of poly(N-methylpyrrole) on steel occurred in three stages: (i) dissolution of the steel, followed by (ii) passivation of the steel, and, finally, (iii) electropolymerization of N-methylpyrrole on the passivated steel. The time taken to form the passive interphase (induction time) is decreased by an increased applied current. Passivation occurred instantaneously at pH 8.4. Below pH 7, the shortest passivation time occurred at pH 2.6. The quantity of the charge consumed during passivation (passivation charge) remained independent of the applied current at pH 2.6 and decreased with the applied current at pH 4.1 and 5.7. The polymerization potential increased with the pH and the applied current. Polymerization potentials greater than 2.0 V resulted in film degradation. By controlling the electrochemical process parameters, good quality poly(N-methylpyrrole) was formed at a controlled induction time.