Corrosion performance of a hot-dip galvanized steel treated by different kinds of conversion coatings

The anticorrosive performance of various kinds of conversion coatings including Cr(VI) and Cr(III) were studied on hot galvanized steel (HDG). The effect of Co(II) and Ni(II) ions was also examined on the Cr(III) coating performance. DC and electrochemical impedance spectroscopy (EIS) were conducted when the specimens were immersed in 3.5 wt.% sodium chloride solution. The coating morphology was observed using SEM. Micro-cracks were observed on both Cr(III) and Cr(VI) treated samples. The results indicated a decrease in the corrosion rate of HDG when the Cr(III) or Cr(VI) conversion coatings were found on the surface. The greater anticorrosion resistance of a Cr(VI) treated sample (compared with a Cr(III) one) was obtained at low immersion times. However, a significant decrease in self healing behavior and therefore corrosion resistance of the Cr(VI) treated sample was observed at longer immersion times. The anticorrosion resistance of Cr(III) treated samples was significantly improved using Co(II) or Ni(II) conversion coatings on Cr(III) treated samples. On the other hand, a lower decrease in corrosion performance of samples treated by Cr(III) and Co(II) or Ni(II) was obtained after a long immersion time in corrosive environment. It was found that, the passive and barrier layer which the Co(II) or Ni(II) conversion coating can produce on the Cr(III) coating can superiorly improve its anticorrosion resistance.     

Effect of molybdate post-sealing on the corrosion resistance of zinc phosphate coatings on hot-dip galvanized steel

The technique of post-sealing the phosphated hot-dip galvanized (HDG) steel with molybdate solution was addressed. The composition and corrosion resistance of the improved phosphate coatings were investigated by SEM, EDS, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements, and neutral salt spray (NSS) test. The results showed that molybdate films were formed in the pores of phosphate coatings, and the compact and complete composite coatings composed of phosphate coatings and molybdate films were formed on the zinc surface, resulting in that both the anodic and cathodic processes of zinc corrosion were inhibited remarkably; the corrosion protection efficiency values were increased; and the electrochemical impedance values were enhanced at least one order of magnitude. The low frequency impedance values for the composite coatings were increased at the initial stages of immersion in 5% sodium chloride solution, indicating the self-repairing activity of the composite coatings.     

Corrosion behavior of Cr electrodeposited from Cr(VI) and Cr(III)-baths using direct (DCD) and pulse electrodeposition (PED) techniques

A comparison was made between the electrochemical corrosion behaviors of chromium deposited from hexavalent [Cr(VI)] and trivalent [Cr(III)] chromium baths using direct current (DCD) and pulse electro deposited (PED) techniques. Chromium coatings were deposited on mild-steel (MS) substrate. The corrosion behavior of both DCD and PED chromium from Cr(VI) and Cr(III)-baths in 3.5%NaCl solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results indicated that PED chromium from Cr(VI) and Cr(III)-baths have higher charge-transfer resistance R_ct and very low I_corr than that of DCD chromium on mild-steel substrate.     

Corrosion behaviour of molybdate–phosphate–silicate coatings on galvanized steel

In this work, a Cr-free conversion layer based on molybdate–phosphate–silicate (MPS) was synthesised on a galvanized steel by simple immersion and its corrosion behaviour was compared to that of a typical chromate layer. Stationary electrochemical techniques and electrochemical impedance spectroscopy (EIS) were employed to highlight the corrosion mechanisms of both coatings in different NaCl concentrations, immersion times and pH. Contrary to the chromate layer, the MPS coating showed good electrochemical stability even in concentrated NaCl solutions and remarkable electrochemical efficiency. With increasing time, two corrosion stages were associated with the two likely sublayers of the MPS coating. Furthermore, the MPS coating behaved better than the chromate layer in acidic and alkaline pH, especially the latter as a compact corrosion product layer formed. Finally, each coating/electrolyte interface was characterised by an electrical equivalent circuit giving a satisfactory correlation between the experimental and the calculated impedance. It derived that the MPS could be an environmentally friendly alternative to chromating.     

EIS study of the corrosion behaviour of zinc-based coatings on steel in quiescent 3% NaCl solution. Part 1: directly exposed coatings

The possibilities offered by the EIS technique for studying the progress of corrosion in several types of galvanised coatings were investigated. Corrosion tests were performed in a 3 wt.% NaCl aqueous solution, at approximately neutral pH, without stirring and in contact with the air. A tendency was seen towards an electrochemical overestimation of the corrosion values of essentially pure zinc and Zn-5%Al (Galfan) coatings compared with the gravimetric reference values. In contrast, the opposite tendence was revealed with the Zn-10%Fe (Galvanneal) coating. These deviations have made it necessary to use empirical values of the constant B of the Stern-Geary equation. The approximately linear progress of the attack with immersion time suggests a slight barrier effect of the corrosion products layer.     

Electrodeposited zinc composite coatings with embedded carbon nanotubes – advanced composite materials for better corrosion protection

The aim of the present investigation was to demonstrate a preparation method for obtaining an electrodeposited composite zinc coating with incorporated carbon nanotubes (CNT). The composite coatings were additionally treated in a newly developed conversion solution based on a Cr(III)-containing compound. The surface morphology of the Cr(III) and non-Cr(III) samples was investigated with scanning electron microscopy (SEM), the metallographic structure with X-ray diffraction analysis (XRD) and the composition with X-ray photoelectron spectroscopy (XPS). Composite zinc coatings without and with the surface conversion film (CF) have been investigated in a standard test medium of 5% NaCl solution by use of potentiodynamic polarisation (PDP) curves, electrochemical impedance spectroscopy (EIS) and polarisation resistance (R_p) measurements. The results obtained show the synergistic effect of the incorporated CNT and conversion film for achieving better corrosion resistance of the system composite Zn/CF in aggressive medium containing chloride ions as corrosion activators.     

Effect of Co and W on the corrosion behaviour of 1% Cr steel in flowing synthetic seawater

The aqueous corrosion characteristics of 1% Cr‐steel alloyed with small amounts of Co and W in synthetic seawater was studied by using immersion weight‐loss tests, electrochemical corrosion tests (potentiodynamic tests and electrochemical impedance spectroscopy (EIS) measurements) and analytical techniques. 1% Cr steels containing Co or W from 0.2 to 0.4 wt.% showed higher corrosion resistance than the 1% Cr steel in the immersion and potentiodynamic tests. EIS measurements showed that the Nyquist plot presented one time constant. Furthermore, the Co‐ and W‐bearing steels present higher R_p values than the 1% Cr steel through all the test period. The better corrosion resistance of the Co‐and W‐bearing steels is attributed to the protectiveness of the surface layer. The corrosion products were examined using electron probe microanalysis (EPMA) and X‐ray photoelectron spectroscopy (XPS). The results of EPMA indicated that Cr was concentrated in the inner region of the rust layer, while Co and W were distributed all over the rust layer. XPS results showed that Co existed as a trivalent oxide in the rust layer and W in the rust appeared in the form of a WO₄ compound. These compounds act as a factor for corrosion resistance in aqueous solutions.     

Electrochemical corrosion behavior of plasma sprayed Al2O3‐13%TiO2 coatings in aqueous hydrochloric acid solution

One kind of conventional and two kinds of nanostructured Al₂O₃‐13%TiO₂ coatings were prepared by plasma spray process. The phase composition and microstructure of coatings were examined by means of scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to investigate the corrosion behavior of coatings in aqueous hydrochloric acid solution. The results showed that nanostructured coatings had superior corrosion resistance compared to conventional Metco 130 coating. The corrosion resistance of coatings was mainly related to their microstructure and defects density. The EIS measurement for long time immersion in hydrochloric acid solution revealed that the corrosion resistance of coatings decreased with the increasing of immersion time. During the immersion period, electrochemical corrosion mainly occurred on the carbon steel substrate under NiCrAl coatings. In addition, the Al₂O₃‐13%TiO₂ coatings were also failed during corrosion in aqueous hydrochloric acid solution.     

Corrosion monitoring of galvanised coatings through electrochemical impedance spectroscopy

In this work, corrosion of industrial galvanised coatings is monitored through potentiodynamic and electrochemical impedance spectroscopy (EIS) methods and supported by real-time immersion tests. For such purpose, the corrosion behaviour is studied in different media (NaCl, NaOH and rain water), at different concentrations and varying immersion times. The results show that EIS allows to establish the interfacial reactions and the dissolution mechanisms occurring in three corrosive media, hence to foresee the protection conferred by these coatings. The impedance diagrams of the coated steel do not provide information on the slowest reactions, which only occur in natural rain water. Finally, each Zn/medium interface is characterised by a specific equivalent circuit giving a similar impedance response.     

烧结NdFeB永磁体的腐蚀行为研究

通过静态腐蚀实验,研究了烧结钕铁硼磁体在3.5 wt.% NaCl溶液中浸泡不同时间段的腐蚀特征。采用扫描电镜、拉曼光谱分析各个阶段的腐蚀形貌和腐蚀产物组成,通过动电位极化曲线、电化学阻抗谱研究不同腐蚀阶段磁体表面结构的变化及腐蚀机理,采用等效电路模型模拟各阶段的腐蚀行为。研究结果表明,烧结NdFeB磁体在3.5 wt.% NaCl溶液中依次发生点蚀、选择性晶间腐蚀、全面腐蚀行为,形成的腐蚀产物层结构疏松,只能产生短时减缓腐蚀速率的效果,而富钕相优先溶解会造成主相颗粒脱落,导致磁体粉化、破坏。腐蚀产物类型随腐蚀时间延长而有所变化,主要为铁和钕的氧化物或氢氧化物。     

Influence of drying temperature on the corrosion performance of chromate coatings on galvanized steel

The influence of drying temperature on the corrosion performance of chromate coatings on electro‐galvanized (EG) steel has been investigated using electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements in 3.5% NaCl solutions. The chromate coatings were applied to the EG steel in a solution (pH 1.2) containing sodium dichromate and sulfuric acid at room temperature. The coatings were dried in an oven at three different temperatures: 60, 110 and 210°C. The surface of the chromate coatings was analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM) combined with energy‐dispersive spectrometry (EDS). The results show that the drying temperature significantly affects the morphology of the chromate coatings and consequently affects their corrosion behavior. The chromate coatings dried at 110°C had few cracks and the lowest corrosion current. The chromate coatings dried at 60°C showed passivity. The EIS results show that the chromate coatings dried at 60°C has the largest impedance in a neutral 3.5% NaCl solution. Drying at higher temperature (210°C) degrades the chromate coatings by widening the cracks and reducing soluble Cr(VI) in the chromate layer. The favorable drying temperature for the chromate coatings on the EG steel is between 60 and 110°C.     

Corrosion study of Ni/Zn compositionally modulated multilayer coatings using electrochemical impedance spectroscopy

Ni/Zn compositionally modulated multilayer (CMM) coatings were deposited using dual bath technique. Coatings corrosion performance was evaluated using electrochemical impedance spectroscopy (EIS) during extended immersion times up to 48 h. The results of electrochemical impedance spectroscopy showed that Ni/Zn CMM coatings had better corrosion resistance compared to that of the zinc single layer coating. The modified corrosion product which is formed on the Ni/Zn CMM coatings during extended exposure times and also a good barrier effect of the nickel layer against aggressive species in these coatings can be two important reasons for high corrosion performance and so protection performance of the Ni/Zn CMM coatings.     

Acid violet 6B as a novel corrosion inhibitor for cold rolled steel in hydrochloric acid solution

The inhibition effect of acid violet 6B (AV6B) on the corrosion of cold rolled steel (CRS) in 1.0–5.0 M HCl solution was studied for the first time by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) methods. The results show that AV6B is a very good inhibitor in 1.0 M HCl, and the adsorption of AV6B on CRS surface obeys Langmuir adsorption isotherm. Polarization curves reveal that AV6B behaves as a mixed-type inhibitor. EIS spectra exhibit one capacitive loop and confirm the inhibitive ability. Effects of immersion time and acid concentration on inhibition performance were also discussed.     

Corrosion behaviors of arc spraying single and double layer coatings in simulated Dagang soil solution

Three kinds of single layer coatings of Zn, Zn15Al, 316L stainless steel and two kinds of double layer coatings with inner layer of Zn or Zn15Al and outer layer of 316L stainless steel by arc spraying were developed to protect the metal ends of prestressed high-strength concrete (PHC) pipe piles against soil corrosion. The corrosion behaviors of the coated Q235 steel samples in the simulated Dagang soil solution were investigated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and natural immersion tests. The results show that the corrosion of the matrix Q235 steel is effectively inhibited by Zn, Zn15Al, Zn+316L and Zn15Al+316L coatings. The corrosion rate value of Zn15Al coated samples is negative. The corrosion products on Zn and Zn15Al coated samples are compact and firm. The corrosion resistance indexes of both Zn and Zn15Al coated samples are improved significantly with corrosion time, and the latter are more outstanding than the former. But the corrosion resistance of 316L coated samples is decreased quickly with the increase in immersion time. When the coatings are sealed with epoxy resin, the corrosion resistance of the coatings will be enhanced significantly.     

2-Mercaptobenzimidazole as a copper corrosion inhibitor: Part I. Long-term immersion, 3D-profilometry,and electrochemistry

The high corrosion inhibition effectiveness of 2-mercaptobenzimidazole (MBIH) in 3 wt.% aqueous NaCl solution is reported using long term immersion tests, 3D-profilometry, electrochemical impedance spectroscopy, and potentiodynamic curve measurements. The high corrosion inhibition performance was proven after 180 days of immersion. The impedance spectra were characterized by two time constants relating to charge transfer and finite layer thickness or semi-infinite diffusion of copper ions through the surface layer, therefore Cu corrosion in solution containing MBIH follows kinetic-controlled and diffusion-controlled processes. Moreover, it is shown that MBIH is a mixed-type inhibitor.     

Corrosion behaviour in physiological fluids of surface films formed on titanium alloys

The properties of the surface films formed on Ti and its Ti–5Al–4V and Ti–6Al–3.5Fe alloys by chemical (immersion in 10 M NaOH) and thermal treatment (500 °C) were studied in this paper. Their corrosion behaviour in Ringer solution was investigated. After treatments, the sample microhardness was measured. Also, the sample surfaces were observed by an optical microscope before and after immersion in Ringer solutions. The electrochemical behaviour of the surface films obtained by the chemical and thermal treatments was studied using the methods of the electrochemical impedance spectroscopy (EIS) and monitoring of open circuit potentials at different immersion periods (up to 60 days) in Ringer solution. Microhardness increased with the increasing loads for the studied materials, showing the existence of a compact protective layer. Microscopic observations exhibited more compact coatings after 60 days of exposure in Ringer solution, due to the formation of an apatite layer. EIS spectra revealed surface films with two layers: an inner, barrier protective layer and an outer, porous apatite layer. The impedance increases in time denoting that the films have grown by apatite nucleation. Also, EIS spectra showed that the complex treatment by chemical + heat method is the most efficiently. For the samples treated by the two processes (chemical + heat) the values of the open circuit potentials are nobler than of the chemical treated samples, denoting better protective, bioactive films.     

Effect of Al additions in WC-(Cr1−xAlx)N coatings on the corrosion resistance of coated AISI D2 steel in a deaerated 3.5 wt.% NaCl solution

Multilayered WC-(Cr_1−xAl_x)N coatings were deposited on AISI D2 steel using cathodic arc ion plating (CAIP) process. Five kinds of WC-(Cr_1−xAl_x)N coatings were prepared: WC-Cr_0.6Al_0.4N, WC-Cr_0.57Al_0.43N, WC-Cr_0.53Al_0.47N, WC-Cr_0.48Al_0.52N and WC-Cr_0.45Al_0.55N. The Al concentration could be controlled by using evaporation source for Al targets and fixing the evaporation rate of the other metals (WC alloy and Cr). In this study, the corrosion behavior in deaerated 3.5 wt.% NaCl solution was investigated by electrochemical corrosion tests (potentiodynamic polarization test, galvanic corrosion test, electrochemical impedance spectroscopy (EIS)) and surface analyses (glow discharge optical emission spectroscopy, X-ray diffractometry, scratch adhesion test, scanning electron microscopy, electron probe micro-analyzer).

The results of potentiodynamic polarization test showed that the WC-Cr_0.48Al_0.52N coating with lower porosity exhibited the lower corrosion current density. The galvanic corrosion current between the coating and the substrate showed low values. In EIS measurements, the charge transfer resistance (R_ct) value of WC-Cr_0.48Al_0.52N coating only increased with the immersion time, when compared to the other coatings. It can be due to the corrosion products plugging the pores and increasing the pathway resistance.     

环氧铝粉涂层和氟碳涂层耐蚀性能的比较

采用电化学阻抗谱技术(EIS)研究了环氧铝粉涂层和FEVE氟碳涂层/碳钢体系在天然海水介质中的电化学腐蚀行为,通过对两涂层的涂层电容分析及腐蚀后表面形貌的观察,评价了两种有机涂层的防腐蚀性能。结果表明,随着浸泡时间的延长,两种有机涂层体系的保护作用都有所降低。环氧铝粉涂层在浸泡初期呈现单容抗弧特征,浸泡57天时出现了双容抗弧。氟碳涂层在浸泡周期内EIS曲线均呈现单容抗弧特征,浸泡110天时低频阻抗模值仍高于108Ω.cm2。在整个浸泡周期内,氟碳涂层的涂层电容基本维持在1.6×10-10～1.8×10-10 F.cm-2,约为环氧铝粉涂层电容的1/20,表现出低渗水性。     

Temperature effect in the corrosion resistance of Ni-Fe-Cr alloy in chloride medium

The corrosion susceptibility of alloy 33 in 0.5 mol/L sodium sulphate solutions containing or not 0.1 mol/L sodium chloride was tested at three different temperatures: 22 °C, 40 °C and 60 °C. Electrochemical studies were performed using corrosion potential measurements (E_corr) as well as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion potential measurements showed that alloy 33 was passivated by a previously air formed film which was not destroyed during immersion in both solutions. No corrosion was observed during these tests although the temperature affected the film. Potentiodynamic polarization experiments showed that at high anodic potentials the previous film was broken up, and localized corrosion occurred in both solutions and at the three temperatures tested. Electrochemical impedance spectroscopy tests confirmed the presence of a stable passive film on the alloy surface at open circuit potential. Mott-Schottky analysis indicated that the passive film is an n-type semiconductor due to the presence of point defects of donor species, such as oxygen vacancies and interstitial metallic cations. As the potential increases the Cr(III) present in the barrier layer oxidizes producing Cr(VI) soluble species. The dissolution creates metallic cation vacancies that are acceptor species and the film changes from n-type to p-type semiconductor. The passive film rupture and the following localized attack are related to the drastic oxidative dissolution of the film at high anodic potentials, independent of its p-nature, chloride presence or increased temperature.     

Corrosion evaluation of microarc oxidation coatings formed on 2024 aluminium alloy

Dense alumina ceramic coatings of 7 μm thickness were fabricated on 2024 aluminium alloy by microarc oxidation (MAO). The corrosion behaviour of the MAO coated alloys was evaluated using potentiodynamic polarisation and EIS measurements. The results show that the corrosion process of the coated alloy can be divided into three stages: (1) the initial stage (the first 2-6 h of immersion): penetration of corrosion medium into the aluminium alloy was inhibited by coating; (2) the second stage (after 24 h of immersion), corrosion medium penetrated to attack the interface between the substrate and the coating; (3) the final stage (after about 96 h): corrosion process was controlled by the diffusion of corrosion products.