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.     

Numerical analysis of galvanic corrosion of Zn/Fe interface beneath a thin electrolyte

A numerical analysis of galvanic corrosion of a Zn/Fe interface beneath a thin layer electrolyte is presented. Specifically, a circular defect, where the zinc coating has been removed, is considered. It is assumed that both oxygen reduction and iron oxidation can occur on the Fe surface, while only zinc oxidation occurs on the Zn surface. The importance of electrolyte thickness and conductivity and defect radius is considered. It is assumed that the iron and zinc oxidation rates are described by a Tafel relationship. If the kinetic parameters of the oxidation reactions are known, the cathodic protection of Fe is a function of a Wagner number, the ratio of the electrolyte thickness to the defect radius, and the ratio of the radius of the defect to the outer radius of the zinc layer.     

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.     

SKP and FT-IR microscopy study of the paint corrosion de-adhesion from the surface of galvanized steel

Scanning Kelvin Probe (SKP) and FTIR microscopy were applied to study the atmospheric corrosion of galvanized steel coated by electrophoretic epoxy resin (ED) at a defect.     

Surface chemistry of galvanized steel sheets relevant to adhesion performance

A review is presented on the recent development of surface treatment technologies for hot-dip galvanized steels relevant to adhesion of organic coatings. Applications of surface analytical techniques have elucidated that the surface layers of the nanometer scale dramatically govern the adhesion performance of painting or adhesive bonding. Surface enrichment of aluminium in the zinc layer deteriorates paint adhesion due to the reduction in phosphatability on the galvanized steel sheets and decreases the adhesive strength of the epoxy/dicyandiamide-bonded sheets due to the loss of acid-base interaction at the adhesive-substrate interface. In addition, the co-segregation of Al and Pb into the surface layer is responsible for the intergranular corrosion of zinc and facilitates the formation of a weak boundary layer, resulting in poor bond durability in a wet atmosphere. Improved adhesion performance has been established by developing new technologies that reduce the surface enrichment of minor elements or impurities in the zinc layer on the galvanizing line or that adopt a surface conditioning process prior to pretreatment in subsequent coil coating lines.     

Zn／A3钢在海水中电偶腐蚀的研究

研究了金属Zn与A3钢在海水中偶接时的电偶腐蚀行为。考察了偶接时间、溶液中Cr浓度、电偶对中阴阳极面积比、实验温度等因素对阳极腐蚀速度的影响。结果表明：偶接时间为24h时,腐蚀速度趋于稳定。腐蚀速度随温度升高,溶液中Cr浓度的增大;阴阳面积比的增大,A3钢的腐蚀速度会相应减慢。     

Localized corrosion of carbon steel in a CO2-saturated oilfield formation water

In this work, corrosion and localized corrosion behavior of X65 pipeline steel were studied in a simulated, CO₂-saturated oilfield formation water by various electrochemical measurement techniques, including electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curves, galvanic current and localized EIS (LEIS). The morphology and composition of the formed corrosion scale were characterized by scanning electron microscopy and energy-dispersive X-ray analysis. A conceptual model was developed to illustrate the occurrence of localized corrosion of the steel under scale. Both galvanic current and LEIS measurements showed that a galvanic effect existed between the bare steel and the scale-covered region. The scale-covered region served as cathode and the bare steel site as the anode. The big cathode vs. small anode geometry accelerated the local corrosion reaction. At an elevated temperature, a compact, crystalline scale was formed on the steel surface, enhancing the galvanic effect. Moreover, the stability of the scale was increased with time, and localized corrosion of the steel under scale experienced mechanistic changes with time.     

Synthesis and characterisation of surface gradient thin conversion films on zinc coated steel

Surface gradient layers on hot-dip galvanised steel were synthesised in order to determine the barrier properties and corrosion resistance of thin amorphous conversion coatings as a function of layer thickness and processing time. For this purpose, a dip coating procedure was established that yields well-defined gradient layers. As a model system for conversion film formation on zinc coated steel, a zirconium based bath chemistry was used. The synthesised zirconium oxyhydroxide gradient films were characterised by localised electrochemical techniques, such as Scanning Kelvin Probe (SKP) and electrochemical impedance spectroscopy using an electrochemical capillary cell. Microscopic infrared reflection absorption spectroscopy (μ-FT-IRRAS) measurements and small-spot X-ray photo electron spectroscopy (XPS) were used as complementary surface analytical techniques. The applied analysing techniques provide a spatial resolution of 100-1600 μm. Thereby, a complete variation of thin film properties, such as thickness, barrier properties, corrosion resistance and chemical composition can be measured as function of the time of film growth on a sample with a length of a few centimetres. This approach allows a precise and accurate determination of structure-to-property relationships of thin conversion films. Moreover, it could be shown that a surface gradient film analysis significantly rationalises experimental time and increases the reliability of the experimental results.     

Development of a galvanic sensor system for detecting the corrosion damage of the steel embedded in concrete structure: Part 2. Laboratory electrochemical testing of sensors in concrete

The correlation between sensor output and the corrosion rate of steel bar was confirmed in concrete environment. Open-circuit potential, linear polarization resistance (LPR) measurement and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion behavior of steel bar embedded in concrete. Also, galvanic current measurements of designed sensors were conducted to obtain the charge of sensor embedded in concrete.In this study, the results of corrosion behavior of reinforcing steel showed a consistence among the data obtained by open-circuit potential monitoring, LPR and EIS measurements. Steel/copper sensor showed a good correlation in concrete environment between sensor output and corrosion rate of steel bar. However, there was no relationship between steel/stainless steel sensor output and corrosion rate of steel bar due to the low galvanic current output. Through the relationship between the steel/copper sensor output and the corrosion rate of reinforcing steel, the real corrosion damage of the reinforcing steel can be detected. Consequently, this confirms that the galvanic sensor system is a good method for detection of corrosion in reinforced concrete.     

Effect of variable amounts of rust at the steel/paint interface on the behaviour of anticorrosive paint systems

This paper analyses the effect of the presence of rust at the metal/paint interface on the behaviour of different paint systems used for protecting the structural steel exposed to the atmosphere. The paint systems were applied as films of variable thickness over rusted steel surfaces cleaned to different grades of surface preparation (Sa3, Sa2(1/2), Sa2 and St2). Pre-rusting of steel was carried out in a clean (uncontaminated) rural atmosphere. Atmospheric exposure tests were conducted for 14 years at three Spanish test sites of different atmospheric corrosivity. Exposed specimens were evaluated for rusting and blistering, as well as for delamination of the paint system on both sides of a scribe made in the paint film. The results reveal that in some cases the presence of rust has a negligible influence on the durability of the paint system applied. Those systems including a zinc-rich (ethyl silicate) primer were found to provide the most effective protection against corrosion under all types of conditions tested.     

Surface and electrochemical characterization of pitting corrosion behaviour of 304 stainless steel in ground water media

The effectiveness of aminotrimethylidene phosphonic acid (ATMP) as a corrosion inhibitor in association with a bivalent cation like Zn²⁺ and non-ionic surfactant like polyoxyethylene sorbitan monooleate (Tween 80) were investigated by measuring corrosion losses using electrochemical techniques. The corrosion of 304 stainless steel in the ground water medium was inhibited by complexation of the inhibitor. A combined inhibition effect was achieved by adding both ATMP and Zn²⁺ along with Tween 80. The formulation functioned as a mixed type inhibitor. The synergistic effect of the inhibitor compound is calculated. Luminescence spectra, FTIR spectra, XRD, XPS and scanning electron microscopic studies were carried out to understand the mode of corrosion inhibition and also the morphological changes on the metal surface.     

Recession behavior of Lu2SiO5 under a high speed steam jet at high temperatures

Study of recession behavior of Lu₂SiO₅ bulk was performed in high speed steam jet with a velocity of ∼50 m/s temperature range between 1300 and 1500 °C for 100 h. X-ray results showed that no phase change was observed for all samples after steam exposure. Detailed scanning electron microscopy examinations showed some cracks formation was observed on the bulk surface for the samples of 1400 and 1500 °C. Also, porous structure was formed on the bulk surface for the samples of 1300 and 1400 °C. As for 1500 °C sample, the porous structure disappeared after exposure test. The high magnification images of 1300 °C sample showed the depletion of grain boundary glassy phase. However, for 1400 °C sample, boundary phase was formed again, and the grain growth can be identified for the sample of 1500 °C. The recession mechanism can be explained by a mass transfer of evaporated species from the bulk surface and the weight loss rate measured can be expressed by Arrhenius plot.     

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.     

Influence of substrate treatment temperatures and bias potential on capacitive manganese–cobalt–zinc oxide thin films deposited by radio frequency sputtering

Manganese–cobalt–zinc oxide films are deposited on graphite foils by a dry process, simpler one-step radio frequency sputtering with different substrate treatment temperatures and bias potential. The best long-term operational stability (only reduce about 7% specific capacitance at the 8000th cycle of potential cycling) and good specific capacitance are obtained at a substrate treatment temperature of 200 °C and without substrate bias potential. Furthermore, the lower the substrate treatment temperature, the better the stability. Moreover, the specific capacitance of the manganese–cobalt–zinc oxide electrode decreases with increasing substrate bias potential.     

Corrosion resistance of colored films grown on stainless steel by the alternating potential pulse method

Colored oxide films were grown on AISI-304 stainless-steel samples using the alternating potential pulse method with different amplitudes (0.41, 0.42, 0.43, and 0.44 V) and electrolysis times (10, 15, 20, 25, 30, 35, 40 and 50 min). The resistance of the colored samples to uniform and pit corrosion was also evaluated. All the different colored samples were less susceptible to uniform corrosion than the non-colored ones. Furthermore, the colored samples whose oxide films were grown using potential amplitudes of 0.42 and 0.44 V showed lower uniform corrosion rates than those grown with other potential amplitudes. The susceptibility to pitting corrosion was practically the same for colored and non-colored steel samples.     

Effects of cathodic potential on the local electrochemical environment under a disbonded coating

A rectangular crevice assembly was used to investigate the effects of cathodic protection (CP) potential, bubbling CO₂ and surface condition on the crevice corrosion of X70 steel under a disbonded coating. The solution within the crevice becomes more alkaline due to the reduction of dissolved O₂. As a result, the potential of the steel reaches the protected potential range and thus the protection distance becomes longer when the applied CP potential is more negative. Potential drop (IR) mainly occurs in the vicinity of the opening. However, the introduction of CO₂ into the solution prevents the formation of an alkaline environment but gives rise to an environment with a nearly neutral pH and a uniform potential distribution in the crevice. In addition, it is found that the pre-corrosion product layer significantly decreases the polarization rate in the crevice.     

纺织剪刀用W9Mo3Cr4V钢镀TiN薄膜的表面性能

为改善纺织剪刀用W9Mo3Cr4V高速钢的表面性能,采用多弧离子镀技术对其进行镀TiN表面处理。以X射线衍射和扫描电镜对镀膜进行微观分析。在载荷500kg、加载时间1min的条件下,用摩擦磨损试验机测试了镀TiN薄膜前后W9Mo3Cr4V钢的耐磨性。在3%NaCl溶液中测试了W9Mo3Cr4V钢镀TiN薄膜前后的极化曲线。结果表明,镀TiN能明显提高W9Mo3Cr4V钢的抗磨损性能以及抗腐蚀性能。     

Corrosion of similar and dissimilar metal crevices in the engineered barrier system of a potential nuclear waste repository

Crevice corrosion is considered possible if the corrosion potential (E_corr) exceeds the repassivation potential for crevice corrosion (E_rcrev). In this study, potentiodynamic polarization and potentiostatic hold were used to determine the E_rcrev of similar and dissimilar metal crevices in the engineered barrier system of the potential Yucca Mountain repository in 0.5 M NaCl, 4 M NaCl, and 4 M MgCl₂ solutions at 95 °C. The results were compared with data previously obtained using crevices formed between Alloy 22 and polytetrafluoroethylene. It was observed that, except for Type 316L stainless steel, all other metal-to-metal crevices were less susceptible to crevice corrosion than the corresponding metal-to-polytetrafluoroethylene crevices. Measurements of galvanic coupling were used to evaluate the crevice corrosion propagation behavior in 5 M NaCl solution at 95 °C. The crevice specimens were coupled to either an Alloy 22 or a Titanium Grade 7 plate using metal or polytetrafluoroethylene crevice washers. Crevice corrosion of Type 316L stainless steel propagated without repassivation. For all the tests using a polytetrafluoroethylene crevice washer, crevice corrosion of Alloy 22 was initiated at open circuit potential by the addition of CuCl₂ as an oxidant, whereas no crevice corrosion of Alloy 22 was initiated for all the tests using Alloy 22 or Titanium Grade 7 metals as crevice washer. However, crevice corrosion propagation was found to be very limited under such test conditions.     

Electropolishing of 304 stainless steel: Surface roughness control using experimental design strategies and a summarized electropolishing model

The electropolishing (EP) settings for obtaining 304 stainless steel (304SS) with variable surface roughness factors (Ra) in a mixture containing phosphoric acid, sulfuric acid, and glycerol were achieved using experimental design strategies, including the fractional factorial design (FFD) coupled with the response surface methodology (RSM) and the path of the steepest ascent. The bath temperature and polishing time were found to be the strongest factors affecting Ra of 304SS in the FFD study. The glycerol content and polishing current density involved strong interactions with the bath temperature although both factors only showed marginal significant effects. The effects of bath temperature and polishing time on Ra of 304SS were examined in the study of the steepest ascent path for controlling the surface roughness. The results showed that Ra of 304SS is decreased with decreasing the bath temperature and polishing time but increased when the temperature was lower than 20 °C. A summarized model with a new idea on the molecular interactions in the polishing bath is proposed to elucidate the phenomena found in this work. The morphologies and Ra of 304SS electropolished under various conditions were examined by means of the SEM photographs and AFM analyses.