Protective properties of organic phosphate-pigmented coatings on phosphated steel substrates

Investigations have been carried out on properties of coatings, differing by their pigmentation and binder, and applied on different chemical pre-treatments of the steel surface. Paints based on alkyd and alkyd-melamine binders, pigmented with zinc phosphate and modified basic zinc phosphate were applied on amorphous and crystalline phosphated steel surface and, for the comparison purpose, on degreased steel surface. The effect of the binder, the pigment and the pre-treatment of the steel surface on the protective properties of the coatings were determined by measurements of adhesion, water absorption and water permeability and by results obtained in salt spray and Prohesion tests. Coatings based on alkyd binder show a lower damage degree and good retention of adhesion in corrosion conditions, in spite of a higher water absorption and water permeability and a lower initial adhesive strength. Protective properties of coatings have been found to be highly dependent upon the substrate pre-treatment. Chemical pre-treatment of the steel substrate increases the protective properties of the system, which is particularly evident in the case of crystalline phosphating and the coating pigmented with modified basic zinc phosphate. This phenomenon can be explained by the synergism between this phosphate pigment with crystalline phosphate layer.     

Composite multilayered coatings on mild steel

A composite multilayered coating which consisted of an electrodeposited Zn–Fe alloy layer, a zinc phosphate conversion layer, and one, two, or three organic layers was deposited on a mild steel substrate. The adhesion between these multilayered coating and the mild steel substrate was studied with the aid of a scratch testing technique. Observation of the worn surface of different multilayered coatings was performed with the aid of metallurgical microscopy. The same multilayered coatings were examined with FTIR spectroscopy and X-ray diffraction techniques. Finally, the corrosion behavior of bare and multilayered coated mild steel in 0.1 M NaCl solution (pH = 5.5, T = 25°C) was studied.     

A study of semiconducting oxide-glass coatings on mild steel

Preliminary attempts to develop inexpensive, protective conducting coatings on mild steel for use as insoluble anodes are described. From the consideration that a mixture of acid resistant semiconducting oxide and glass might form a satisfactory coating when heated on a steel substrate, Sb-SnO₂ and Ti-Fe₂O₃ were selected as conducting oxide materials for evaluation. With 3–30 wt.% glass added to the oxides, Sb-SnO₂ was found to be unsuitable as pellets of the glass-oxide mixture did not densify on heating up to 1400° C. 3% Ti-Fe₂O₃ plus 20% glass pellets fired at 1250° C showed low resistivity and porosity. An oxide-glass mixture of this composition fired on a mild steel substrate at 1250° C gave a coating with a porosity of 0.5%, on which chlorine could be evolved in HCl solution. The Ti-Fe₂O₃ oxides were very acid resistant under conditions of chlorine evolution. A reduction in the porosity of the coating, or the use of an acid resistant metal substrate, would be required to make the oxide-glass coating a practicable proposition.     

Protective properties of intact unpigmented epoxy coated mild steel under cathodic protection

Electrochemical Impedance Spectroscopy (EIS) has been used to characterise intact unpigmented epoxy coated mild steel with and without the application of cathodic protection (CP). Coated specimens were exposed to 0.6 M NaCL solution. Cathodic protection was applied at −0.78 V and −1.1 V (SCE). Coated specimens were also tested at Open Circuit Potential (OCP). The application of cathodic protection at −1.1 V was shown to affect the protective properties of the coating, causing the coating resistance to fall below the border line between fair and poor coating. The coating maintained a resistance in the order of 1 × 10⁶ Ω cm² when CP was applied at −0.78 V but a resistance of 1 × 10⁵ Ω cm² when CP was applied at −1.1 V. It was shown that water uptake by the coated specimens was considerably affected by the application of CP. The water uptake by the coated specimens was increased as a result of increasing the level of CP. The application of CP at −0.78 V, and −1.1 V was found to reduce the extent of corrosion on the coated specimens.     

Mechanical properties of plasma sprayed nanostructured TiO2 coatings on mild steel

This paper discusses the effects of plasma spray parameters on the mechanical properties of nanostructured TiO₂ coatings deposited on mild steel substrates. The design of experiment method was applied to investigate the significant effects of each property and to optimize the operational spray parameters. Plasma power, powder feed rate, and stand-off distance were selected as independent variables. Agglomerated and sintered nano-TiO₂ powder was deposited on A-36 commercial mild steel. The microstructural and mechanical properties of the coatings such as porosity, microhardness, surface roughness, and wear rate were evaluated. Both plasma power and powder feed rate were found to be the main factors affecting all four responses. It was also noted that the stand-off distance was a significant factor mainly in influencing the surface roughness of the coatings. All in all, the optimized properties can be achieved by applying a plasma power of 30 KW (high level), a powder feed rate of 22 g/min (high level), and a stand-off distance of 80 mm (low level).     

功能型环氧树脂基防腐涂层的研究进展

环氧树脂涂层因其优异的耐蚀性能、对金属表面良好的附着力而在金属防腐领域得到广泛应用。但涂层固化过程中会形成缺陷和孔洞，腐蚀介质可以直接接触到金属表面。为了开发出具有优良耐久性的防腐涂层，研究人员使用不同的方法和材料制备了多种环氧树脂防腐涂层。该文综述了纳米粒子改性环氧防腐涂层、超疏水型环氧防腐涂层、自修复型环氧防腐涂层、导电聚合物改性环氧防腐涂层及生物基材料改性环氧防腐涂层的制备及性能。但这5种防腐涂层各有局限性，未来应该探究出新的环氧防腐涂层，在提升涂层防腐性能的同时，兼顾其他性能，使环氧树脂基防腐涂层朝着功能化、智能化的方向发展。     

Critical effect of pH on the formation of organic coatings on mild steel by the aqueous electropolymerization of 2-vinylpyridine

Coatings of poly(2-vinylpyridine) have been formed on mild steel substrates in aqueous medium by electrochemical polymerization of the 2-vinylpyridine monomer. The pH of the solution has been found to be critical for this electropolymerization coating process. At low pH (below 3.5), even with an efficient initiation reaction, the propagation process was impeded and no substantial polymer film was formed. At high pH (above 6.0), only a thin and irregular film formed due to the lack of an effective initiation reaction. Only when the solution pH is in the range of 4 to 5.5 can good quality coatings be formed on mild steel substrates. The detailed effects of the pH on the electropolymerization are discussed in terms of a proposed free radical polymerization mechanism. This research has also resolved the issue of some of the non-reproducible experimental results reported in the literature and confirmed the feasibility of forming poly(2-vinylpyridine) coatings on a mild steel substrate by electropolymerization of the monomer.     

Corrosion protection of mild steel with nanofibrous polyaniline-based coatings

Polyaniline (PANI) nanofibers were successfully synthesized by a modified rapid mixing method, that is, by the rapid mixing of solutions of aniline and ammonium peroxydisulfate in either hydrochloric acid or filtrates of oxidative polymerization of aniline. Scanning electron microscopy examination showed that nanofibrous PANI products were achieved in all cases; this indicated that the initially presented excessive anions and cations had no evident effect on the formation of nanosized PANI. The nanofibrous PANI exhibited excellent dispersability in both water and organic solvents, and an ultraviolet–visible spectrum was successfully recorded by dispersion in cyclohexanone. Composite coatings were fabricated with the dispersions of nanofibrous PANI and solutions of epoxy. Greatly enhanced corrosion protection performances were demonstrated by the coatings loaded with a small quantity of nanofibrous PANI. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Crosslinked main-chain-type polybenzoxazine coatings for corrosion protection of mild steel

Bisphenol A/diaminodiphenylmethane (BA-ddm)-based polybenzoxazine precursor was synthesized from 4,4′-diaminodiphenylmethane, bisphenol A, and paraformaldehyde. The curing behavior of BA-ddm was studied by using differential scanning calorimeter and Fourier transform infrared spectrometer techniques. To verify the anticorrosion property, cured polybenzoxazine (PBA-ddm) coatings were prepared on mild steel (MS) through dip-coating and thermal curing methods. The surface properties of cured PBA-ddm coatings were characterized by microscopy and contact angle measurement. The electrochemical measurements were carried out to investigate the corrosion properties of PBA-ddm-coated MS. The results showed that PBA-ddm-coated MS samples exhibited high anticorrosive performance with the corrosion current reduced by two orders of magnitude than that of pristine MS.     

Investigation of the mechanical properties of electrodeposited nickel and magnetron sputtered chromium nitride coatings deposited on mild steel substrate

Electrodeposition and magnetron sputtering techniques have been employed for the deposition of Ni and bilayer NiCrN coatings, respectively, on mild steel substrate. Ni electrodeposition was performed using sulfate Watt’s bath, while magnetron sputtering was performed on electrodeposited Ni using DC power 350 W and base pressure of 3 × 10^?5 Torr in order to prepare bilayer NiCrN coatings. Structural and mechanical properties of Ni and bilayer NiCrN coatings have been investigated using various characterization techniques such as SEM-EDX, XRD, hardness, adhesion testing, etc. SEM analysis reflects the formation of spherical/nodular particles of varying sizes in NiCrN coating whereas Ni coating shows irregular, agglomerated, and non-uniform distribution of particles. Formation of hard CrN phase in NiCrN coating has been confirmed by XRD and EDX. NiCrN coating exhibits better hardness in comparison with Ni coating due to the formation of nitride phase. Micro scratch testing of bilayer NiCrN coating shows better interlayer adhesion and adhesion with mild steel substrate. The combination of electrodeposition and magnetron sputtering can produce inexpensive NiCrN coating containing hard CrN phase with better mechanical properties for automotive applications.     

Electron conducting organic coating of mild steel by electropolymerization

The aim was to coat mild steel with an electron conducting polymer in order to replace any possible electrochemical corrosion of the metal by another electrochemical reaction occurring on top of the film. In view of potential industrialization, electropolymerization of a water soluble monomer was studied. In most cases, this was achieved by oxidation, and the substrate had to be passivated. Polyaniline, was obtained in nitric acid solution. Films had a good structure but were brittle and powdery on the surface. Except on tin free steel, they were insulating. Polypyrrole was a better candidate. Prepared from a Na₂SO₄ solution, films were conductive, ensured good corrosion resistance of the substrate but they were brittle and adhesion to the substrate was not high. Furance, thiophene and iron substituted vinylpyridine complex were tried without success. Further, research should consider copolymerization of pyrrole with other monomers.     

Corrosion of mild steel with composite polyaniline coatings using different formulations

The protective abilities of composite coatings based on electrochemically and chemically formed polyaniline powder against the corrosion of mild steel were investigated. A polyaniline powder has been prepared in the form of an emeraldine base and benzoate salt through chemical dedoping and doping. The composite coatings using polyaniline powders, which were obtained through different routes, and base coatings, which were not corrosion-resistant, with different formulations were prepared and applied on mild steel samples. The corrosion was investigated using an electrochemical impedance technique in 3% NaCl, and the atmospheric corrosion was assessed in a humidity chamber. Emeraldine–benzoate salts, which are a chemically synthesized polyaniline, offer the best protection with an optimal polyaniline concentration of approximately 5 wt%. The different corrosion behaviors were assessed relative to the presence of aniline oligomers in the samples after characterization using UV–vis spectrophotometry. Upon comparison between the corrosion behavior in 3% NaCl with commercial primer paint for iron and that with a paint containing 5 wt% PANI, the composite coating has superior anticorrosion characteristics. The mechanism for the protection of mild steel from corrosion through composite polyaniline coatings was also considered.     

Inhibition of nickel coated mild steel corrosion by electrosynthesized polyaniline coatings

Polyaniline (PANI) coatings were electrochemically synthesized on nickel (Ni) coated mild steel (MS) and their corrosion protection properties were investigated. In this work, the Ni layer (∼1 μm thick) was electrodeposited on MS under galvanostatic condition. Thereafter, the PANI coating was deposited over the Ni layer from aqueous salicylate medium by using cyclic voltammetry. These bi-layered composite coatings were characterized by cyclic voltammetry, UV–vis absorption spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The corrosion protection properties of Ni coated MS (Ni/MS) and PANI coated Ni/MS (PANI/Ni/MS) were investigated in aqueous 3% NaCl by using open circuit potential (OCP) measurements, potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). It was shown that the top layer of PANI exhibits a lower porosity behavior with respect to Ni coating and reduces the corrosion rate of Ni/MS almost by a factor of 3500 and increases the lifetime of Ni coating.     

Effect of surface modified WO3 nanoparticle on the epoxy coatings for the adhesive and anticorrosion properties of mild steel

The effect of introducing WO₃ (tungsten oxide) nanoparticle in the epoxy coating was analyzed by electrochemical impedance spectroscopy and scanning electrochemical microscopy (SECM) methods in 3.5% NaCl. The (3-glycidyloxypropyl)trimethoxysilane was treated with the nanoparticle for the proper dispersion and chemical interaction of nanoparticle with the epoxy resin. The introduction of WO₃ nanoparticle in the epoxy coating enhances the charge transfer resistance (R_ct) as well as the film resistance (R_f). The observation of iron dissolution and oxygen consumption was done by applying the appropriate SECM tip potential in the WO₃-modified nanocomposite coated steel. The epoxy and epoxy–WO₃ nanocomposite-coated samples were used to study the adhesion and anticorrosion properties. The analysis by SEM/EDX displayed that the enriched W was detected in the nanocomposite coating of steel. The presence of the nano level corrosion product containing W was confirmed by focused ion beam-transmission electron microscope analysis. The high corrosion protection properties of the epoxy-based nanocomposite coating was due to the complex nanoscale layer formed and chemical interactions of epoxy resin with surface-modified nanoparticle in nanocomposites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48323.     

不同表面处理条件下复合涂层体系失效过程的EIS特征

研究了不同表面处理条件下环氧富锌/环氧云母氧化铁/氯化橡胶涂层体系的电化学阻抗谱特征。利用Bode图、涂层吸水率、涂层电阻及特征频率的变化评价了表面处理对涂层防护性能的影响。结果表明,基材表面状态不同的复合涂层体系吸水率相对稳定阶段所持续的时间长短顺序为：手工打磨>表面锈蚀>表面未处理,与涂层的防护寿命长短、涂层/基材间的黏附力大小顺序一致。此外,不论基材表面处理程度如何,当涂层体系的特征频率增加到1400 Hz左右时,涂层电阻均发生较快降低,吸水率发生较大增长,涂层失去防护作用。     

In situ electrochemical studies of forming-induced defects of organic coatings on galvanised steel

A new electrochemical setup is presented for in situ measurements during uniaxial forming of thin film coated metal substrates. This approach allows the formability analysis of a zinc pigmented organic coating on a galvanised steel substrate. The aim is to monitor the formation of defects during the forming process. The setup comprises an electrochemical microcapillary cell in a three-electrode arrangement and a miniaturised linear stretching device. The development of forming-induced defects is monitored in situ by applying electrochemical impedance spectroscopy (EIS) and also microscopically analysed by means of field emission scanning electron microscopy (FE-SEM). The studies were supported by GOM^® grid measurements and finite element simulations of model sample forming degrees. The established technique enables the evaluation of the correlation between forming degree and degradation of the barrier properties of organic coatings.Finally a phosphating process on the unformed and formed specimen is electrochemically and microscopically analysed to correlate the respective defect size with its local reactivity. The results show that stretching-induced defects occur at the interface between spherical Zn particles and the epoxy binder matrix. The defect size increases with increasing strain values. The phosphating process leads to the nucleation of phosphate crystals especially in the forming-induced defects and thereby reduces the free zinc in the defect area. The kinetic of the phosphating is accelerated with increasing size of the defect.