Effect of Al on the galvanic ability of Zn-Al coating under thin layer of electrolyte

The effect of Al on the galvanic ability of Zn-Al coating has been studied under thin electrolyte layers by measuring surface potential and surface pH. The changes of surface potential and surface pH over Zn-Al/steel galvanic couple corroding in artificial sea water (ASW) were measured at 60% and 90% RH at 298 K. In the initial stage of corrosion, Zn-55Al coating has shown better galvanic protection ability than Zn-5Al coating in both 60% and 90% RH. However, Zn-5Al coating was better in long term corrosion. The better galvanic ability of Zn-55Al coating in the initial stage of corrosion was related to the observation of pH as low as low as 2 on its surface. The low pH value was due to hydrolysis of Zn²⁺ and Al³⁺ ions. The low pH value was further confirmed by observing evolution of gas due to H⁺ reduction on the Zn-55Al coating. With the progress of corrosion, the low pH region of coating layer extended towards the base steel. This helped expand the deposition of zinc corrosion products on the steel surface. The enhanced dissolution of zinc in Zn-55Al coating led to the formation of a barrier layer which limited the galvanic protection of remaining steel. This was not the case in Zn and Zn-5Al coating. The X-ray analyses of the corroded samples have shown the deposition of zinc corrosion products on the steel surface, which greatly depended on the RH value. The part of the steel surface covered with zinc corrosion products has shown relatively less noble potential than other part indicating that zinc corrosion products took a role to protect the base steel against corrosion. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test.     

The spatial distribution of Zn during galvanic corrosion of a Zn/steel couple

The spatial distribution of Zn²⁺ during galvanic corrosion of a model Zn/steel couple in 0.01 M NaCl was investigated using a scanning zinc disk electrode. The couple had a coplanar arrangement of a steel substrate with an electroplated zinc layer at the center. During galvanic corrosion, the marked changes in the Zn²⁺ concentration were confined to a thin solution layer ca. 1.0 mm thick above the couple surface. In this thin solution layer above the zinc layer, a higher concentration region of Zn²⁺ in the range of 5-18 mM extended around the zinc layer in the solution during galvanic corrosion. Conversely, above the steel surface distant from the zinc layer, the surface concentration of Zn²⁺ was almost zero during galvanic corrosion. On this surface, the precipitation of zinc corrosion products due to the hydrolysis reaction of Zn²⁺ was observed. The distribution of the Zn²⁺ concentration supported that Zn²⁺ acted as a buffer that suppressed the increased pH due to the cathodic reaction on the steel surface near the zinc layer and almost no corrosion products formed there. The spatial distribution of Zn²⁺ is discussed in relation to the distributions of potential and pH and the surface morphology of the galvanic couple.     

Distribution of pH during galvanic corrosion of a Zn/steel couple

The spatial distribution of pH during galvanic corrosion of a model Zn/steel couple immersed in 0.01 M NaCl was investigated using a scanning tungsten pH electrode. The couple consisted of a steel substrate with a 3 μm-thick layer of zinc electroplated at its center. During galvanic corrosion, marked changes in pH occurred, which were confined to a thin layer of solution ca. 1.5 mm over the couple surface. As the surface was approached, the pH over the zinc layer decreased slightly from ca. 5.6 in the original bulk solution, while the pH over the steel surface distant from the zinc layer increased to ca. 11.5. The area of low pH extended not only over the zinc layer, but also over the steel surface adjacent to the zinc. Zinc corrosion products precipitated in the region of high pH. The pH distribution arising from the galvanic corrosion of the couple is discussed in terms of the potential distribution and the morphology of the precipitated zinc corrosion products.     

The improvement of anticorrosion properties of zinc-rich organic coating by incorporating surface-modified zinc particle

The corrosion behaviors of zinc-rich coating with various zinc contents, ranging from 0 to 60 volume percent, in thin organic coatings (below 5 μm) were characterized by electrochemical impedance spectroscopy (EIS), free corrosion potential (E_corr) measurement and cycle corrosion test (CCT). It was verified that both coatings with 60 volume percent of zinc powder and without zinc powder showed good corrosion resistance mainly due to the cathodic protection and barrier effect, respectively. On the other hand, coatings with an intermediate concentration (10–40 vol.%) of zinc powder was not successful in protecting a steel substrate efficiently. To improve anticorrosion property of zinc-rich coating, the surface modification of zinc particle was carried out with derivatives of phosphoric and phosphonic acid in the aqueous solution. The effects of the surface modification of zinc particle on corrosion resistance of the coating were investigated with scanning vibrating electrode technique (SVET) and X-ray photoelectron spectroscopy (XPS). The best anti-corrosion performance was achieved when the incorporated zinc particle was treated with phosphoric acid 2-ethylhexyl ester and calcium ion simultaneously, which induced the formation of alkyl-phosphate-calcium complex layer of 190 nm in thickness on zinc particles. Corrosion resistance was improved by the decreased zinc activity and the increased compatibility between the formed complex layer on zinc surface and polymer binder matrix.     

汽车用钢板纳米晶镀锌层在氯化钠溶液中的腐蚀机理

将汽车用钢板置于碱性镀锌液中电镀,制备了纳米晶镀锌层.采用场发射扫描电子显微镜(FE-SEM)观察镀锌层在3.5%NaCl溶液中腐蚀前后的表面形貌,以能谱仪(EDS)研究腐蚀产物组成,运用塔菲尔(Tafel)曲线和电化学阻抗谱(EIS)考察纳米晶镀锌层的电化学腐蚀行为.结果表明,镀锌层由直径为80～100 nm的针状结...     

Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: Role of Zn content on corrosion protection mechanism

Effect of zinc content in the epoxy zinc rich coating on the mechanism of corrosion protection was investigated using electrochemical impedance spectroscopy (EIS). The zinc content in the coating (on dry film) was varied from 40% to 90% in steps of 10%. Open circuit potential (OCP) measurements and salt spray tests were also carried out to generate supporting evidences and to deepen the understanding in the area of zinc rich coatings. The healing or bridging ability of these coatings was studied by making a scribe on the coating and monitoring the evolution of OCP. EIS was also utilized to screen the amount of zinc particles required to provide efficient galvanic protection and to study the effect of purity of Zn on corrosion protection performance. Both EIS and OCP measurements indicate that coatings containing 40% Zn (on dry film) provides very good barrier protection coming mainly from polymer whereas the one containing >80% Zn provides excellent galvanic protection to the steel substrate. When the zinc loading is between 60% and 70%, coatings neither show barrier protection nor galvanic protection.     

Performance of zinc phosphate coatings obtained by cathodic electrochemical treatment in accelerated corrosion tests

The formation of zinc phosphate coating by cathodic electrochemical treatment and evaluation of its corrosion resistance is addressed. The corrosion behaviour of cathodically phosphated mild steel substrate in 3.5% sodium chloride solution exhibits the stability of these coatings, which lasts for a week's time with no red rust formation. Salt spray test convincingly proves the white rust formation in the scribed region on the painted substrates and in most part of the surface on unpainted surface. The protective ability of the zinc corrosion product formed on the surface of the coated steel is evidenced by the decrease in the loss in weight due to corrosion of the uncoated mild steel, when it is galvanically coupled with cathodically phosphated mild steel. Potentiodynamic polarization curves reveal that E_corr shifts towards higher cathodic values (in the range of −1000 to −1100 mV versus SCE) compared to that of uncoated mild steel and conventionally phosphated mild steel substrates. The i_corr value is also very high for these coatings. EIS studies reveal that zinc dissolution is the predominant reaction during the initial stages of immersion. Subsequently, the non-metallic nature of the coating is progressively increased due to the formation of zinc corrosion products, which in turn enables an increase in corrosion resistance with increase in immersion time. The zinc corrosion products formed may consist of zinc oxide and zinc hydroxychloride.     

Electrochemical studies on the alternating current corrosion of mild steel under cathodic protection condition in marine environments

Alternating current (AC) corrosion of mild steel in marine environments under cathodic protection (CP) condition was studied. Electrochemical studies at the two protection potentials namely −780 and −1100 mV versus SCE were examined by different techniques. DC polarization study was carried out for mild steel in natural seawater and 18.5 g/L NaCl solution to evolve corrosion current density. The corrosion rate determination, pH of the end experimental solution and surface morphology of the mild steel specimens under the influence of different AC current densities were studied. The amount of leaching of iron into the solution was estimated using inductively coupled plasma spectrometry. All these techniques revealed that AC influences the corrosion of mild steel in the presence of marine environments even though CP was given. Surface micrographs revealed that spreading of red rust products noticed on the mild steel surface. At −780 mV CP, red rusts are visually seen when the AC source was above 10 A/m² in both the media but red rusts are appeared after 20 A/m² in the case of −1100 mV CP. Weight loss measurements coupled with surface examination and solution analysis is a effective tool to characterize and quantify the AC corrosion of mild steel in marine environments.     

Corrosion protection of 1Cr18Ni9Ti stainless steel by polypyrrole coatings in HCl aqueous solution

Electrically conducting polypyrrole (Ppy) coatings doped with sodium dodecylsulfate (SDS) have been deposited on 1Cr18Ni9Ti stainless steel by anodic polymerization from aqueous solutions of pyrrole and sodium dodecylsulfate. The corrosion behavior of Ppy coated steel was investigated in 0.3 M HCl aqueous solution at room temperature by a combination of electrochemical measurement techniques and scanning electron microscopy. The steel is in active state at the open circuit potential and suffers from pitting corrosion when the polarization potential is higher than 210 mV versus SCE. The Ppy coating can increase the corrosion potential of the steel by more than 600 mV versus SCE, and the pitting corrosion potential by more than 500 mV versus SCE. Fifty-day exposure experiments indicated that the Ppy coating shows high stability, and can inhibit effectively the corrosion of the steel.     

SAE 1045 steel/WC-Co/Ni-Cu-Ni/SAE 1045 steel joints prepared by dynamic diffusion bonding: Microelectrochemical studies in 0.6 M NaCl solution

Corrosion of SAE 1045 steel/WC-Co/Ni-Cu-Ni/SAE 1045 steel interfaces was investigated in 0.6 M NaCl solution using an electrochemical microcell, which enables local electrochemical characterization at the micrometer scale. Two pieces of steel, one with a WC-Co coating covered with Ni (12 μm) and Cu (5 μm) layers, and the other with a Ni (15 μm) layer, were welded by dynamic diffusion bonding. A WC-Co coating was applied to the steel by the high velocity oxygen-fuel process, and Ni-Cu and Ni layers by electroplating. Polarization curves were recorded using an electrochemical microcell. Different regions of welded samples were investigated, including steel, cermet coating, and steel/cermet and steel/Ni-Cu-Ni/cermet interfaces. Optical and electronic microscopes were employed to study the corroded regions. Potentiodynamic polarization curves obtained using the microcell revealed that the base metal was more susceptible to corrosion than the cermet. In addition, cermet steel/cermet and steel/Ni-Cu-Ni/cermet joints exhibited different breakdown potentials. Steel was strongly corroded in the regions adjacent to the interfaces, while the cermet was less corroded. Iron oxides/hydroxides and chloride salts were the main corrosion products of steel. After removal of the superficial layer of corrosion products, iron oxides were mainly observed. Chloride ions were detected mainly on a copper-enriched layer placed between two Ni-enriched layers.     

EIS study of the effect of high levels of SO2 on the corrosion of polyester-coated galvanised steel at different relative humidities

The effect of SO₂ on the degradation of polyester-coated galvanised steel at different relative humidities was investigated using electrochemical impedance spectroscopy. Measurements were performed on specimens which had been tested in an accelerated gaseous corrosion test. For this purpose the samples were subjected to SO₂ gas for 16 days in atmospheric test cells with adjusted relative humidity (RH) from 60 to 100%. Subsequently, the impedance response of the coated material was measured and evaluated. The results indicated that the coating performance varies with RH. Thus, under condensing conditions, the organic coating and galvanised layer was totally removed, the impedance response being interpreted as the formation of an iron sulphide film on the surface. At lower RH, remarkably, the coating remained effectively intact with the coating resistance varying inversely with RH. This work is relevant to the application of such organic-coated products adjacent to combustion flues where high levels of SO₂ occur in association with high humidity.     

Protective mechanisms occurring on zinc coated steel cut-edges in immersion conditions

Electrochemical processes occurring on the cut-edge of a galvanized steel immersed in NaCl solutions were studied using numerical simulations, and in situ current and pH profiles measured over the cut-edge. These results clearly demonstrate that only the steel surface remote from the zinc coating is cathodically active, oxygen reduction being strongly inhibited in the vicinity of zinc. This trend was confirmed by local polarization curves recorded on these distinct areas. Ex-situ AES and SEM analysis and cathodic polarization curves in solutions containing Zn²⁺ ions led to conclude that this cathodic inhibition was related to the fast nucleation of a dense Zn(OH)₂ film on the steel surface. After a long term exposure, a new galvanic coupling takes place between the Zn(OH)₂ covered area, showing an anodic activity, and the remaining steel surface covered by bulky white zinc corrosion products.     

Scanning Kelvin probe force microscopy for the in situ observation of the direct interaction between active head and intermetallic particles in filiform corrosion on aluminium alloy

This article presents for the first time an in situ high-resolution study of the interaction between the active head in filiform corrosion (FFC) and intermetallic particles within an aluminium alloy. For the first time direct evidence will be provided that the intermetallic particles directly determine the so far seemingly random course of the filaments. Both the segments of active filaments and the intermetallic particles (IMPs) were successfully imaged in a humid air (ca. 85% RH) environment by scanning Kelvin probe force microscopy (SKPFM) through a plasma polymer coating of about 340 nm thickness. In order to be able to do that, the experimental parameters need to be adjusted in such a way, that the width of the filaments is small enough to be well within the scan window of SKPFM (100 μm × 100 μm). Also it is important that the small IMPs can still be mapped by SKPFM through the coating. This was successfully achieved by use of a HDMSO plasma polymer film. Surface potential values in the head region of the propagating filaments were found to be 200 mV lower than the interface between intact plasma polymer and the aluminium alloy, indicating the active region. On the other hand, the surface potential values in the trailing filament tail are found to be about 250 mV higher than background, pointing out the cathodic site and superpassivation due to the accumulated corrosion products in this region. It was found that the direction of the filament is determined by the location of the IMPs nearest to the active head.     

New hypotheses for hydrogenase implication in the corrosion of mild steel

The influence of [Fe]-hydrogenase from Clostridium acetobutylicum was studied on the anaerobic corrosion of mild steel. Two short-circuited mild steel electrodes were exposed to the same solution and hydrogenase was retained on the surface of only one electrode thanks to a dialysis membrane. The galvanic current and the electrode potential were measured as a function of time in order to monitor the difference in electrochemical behaviour induced by the presence of hydrogenase. A sharp potential decrease of around 500 mV was controlled by the deoxygenating phase. When hydrogenase was introduced after complete deoxygenation, significant heterogeneous corrosion was observed under the vivianite deposit on the electrode in contact with hydrogenase, while the other electrode only showed the vivianite deposit, which was analysed by MEB and EDX. The effect of hydrogenase was then confirmed by monitoring the free potential of single coupons exposed or not to the enzyme in a classical cell after complete deoxygenating. In both phosphate and Tris-HCl buffers, the presence of hydrogenase increased the free potential around 60 mV and induced marked general corrosion. It was concluded that [Fe]-hydrogenase acts in the absence of any final electron acceptor by catalysing direct proton reduction on the mild steel surface.     

锌铝基重防腐涂料的开发与应用

根据钢结构特殊部位重防腐需要,以片状锌铝粉为基本填料,改性树脂为成膜物,开发出锌铝基重防腐涂料。对该涂料的防护性能和成本进行分析研究,结果表明:锌铝基涂料成膜后,其自腐蚀电位为-980 mV,可以对基体提供良好的牺牲阳极保护,其耐盐雾寿命为1 964 h。该锌铝基涂料与现有的锌铝基涂料相比具有明显的成本优势。     

Galvanic studies related to the use in desalination plant of corrosion-resistant materials

Galvanic currents and potentials have been measured for bimetallic couples involving carbon steel, three grades of stainless steel, 90 Cu/10 Ni and titanium when exposed to seawater in the laboratory at temperatures of 18C and 60C and varying oxygen content. The galvanic currents flowing between couples consisting of two of the corrosion-resistant materials were very small. The galvanic corrosion rate of carbon steel, when connected to one of the other materials, was dependent upon the cathodic-polarisation characteristics of the noble-material surface with indications that, at a given area ratio, the galvanic corrosion of carbon steel was greater when coupled to the 90 Cu/10 Ni alloy than when connected to stainless steel or titanium. The usual effects of increasing the anode/cathode area ratio were observed and data were obtained to demonstrate quantitatively the significant beneficial effect of coating the cathode in a bimetallic couple. Intermediate air exposures were not found to promote any large, long-term, effects on galvanic corrosion. There was evidence that galvanic corrosion rates were very small when the dissolved oxygen content was controlled at low levels and some interesting aspects of galvanic corrosion at the higher temperature were noted.     

Polyaniline for corrosion prevention of mild steel coupled with copper

Polyaniline emeraldine base/epoxy resin (EB/ER) coating was investigated for corrosion protection of mild steel coupled with copper in 3.5% NaCl solution. EB/ER coating with 5-10 wt% EB had long-term corrosion resistance on both uncoupled steel and copper due to the passivation effect of EB on the metal surfaces. During the 150 immersion days, the impedance at 0.1 Hz for the coating increased in the first 1-40 days and subsequently remained constant above 10⁹ Ω cm², whereas that for pure ER coating fell below 10⁶ Ω cm² after only 30 or 40 days. Immersion tests on coated steel-copper galvanic couple showed that EB/ER coating offered 100 times more protection than ER coating against steel dissolution and coating delamination on copper, which was mainly attributed to the passive metal oxide films formed by EB blocking both the anodic and cathodic reactions. Salt spray tests showed that 100 μm EB/ER coating protected steel-copper couple for at least 2000 h.     

脉冲参数对镍镀层在NaCl溶液中耐蚀性的影响

采用扫描电镜观察镀层腐蚀前后的微观表面形貌,其腐蚀为小孔腐蚀,并分析镍镀层在NaCl溶液中的腐蚀机理.利用浸泡腐蚀试验及阳极极化曲线测试镍镀层的耐蚀性.利用阳极极化曲线测试结果作为正交试验指标,着重分析脉冲参数及pH值对镀层耐蚀性的影响规律.脉冲频率越大,脉冲平均电流密度在6 A/cm2左右,pH值为3.8时,镀层耐蚀性能最好.     

Influence of microstructure and composition on the corrosion behaviour of Mg/Al alloys in chloride media

The influence of the microstructure and aluminium content of commercial AZ31, AZ80 and AZ91D magnesium alloys was evaluated in terms of their corrosion behaviour in an aerated 3.5 wt.% NaCl solution at 25 °C. The corrosion process was monitored by electrochemical impedance spectroscopy (EIS). The surface was characterized by scanning electron microscopy (SEM), scanning Kelvin probe force microscopy (SKPFM) and low-angle X-ray diffraction (XRD). The extent of corrosion damage was strongly dependent on the aluminium content and alloy microstructure. Two key factors were observed for the lowest corrosion rates, which occurred for the AZ80 and AZ91D two-phase alloys: the aluminium enrichment on the corroded surface for the AZ80 alloy, and the β-phase (Mg₁₇Al₁₂), which acted as a barrier for the corrosion progress for the AZ80 and AZ91D alloys. Surface potential maps suggested that, between the β-phase and the α-matrix, the galvanic coupling was not significant.     

钢铁表面导电聚苯胺膜的制备

对Q195冷轧钢进行氟铁酸盐转化膜预处理,然后通过原位聚合反应在其表面成功制备墨绿色导电聚苯胺膜.采用扫描电镜和红外光谱分别对聚苯胺膜层的表面形貌的结构进行了表征,并通过极化曲线、中性盐雾试验对其耐腐蚀性进行了研究.结果表明,聚苯胺膜处于中间氧化态,导电聚苯胺膜使钢铁的腐蚀电位正移了49 mV,使钢铁的耐蚀性能明显提高...