Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.     

Morphology and phase composition of corrosion products formed at the zinc-iron interface of a galvanized steel

The corrosion products formed on the inner wall of pipes made of galvanized low carbon steel, exposed for ∼2 years to water flowing in a large household heating system, were analysed using X-ray diffraction, Mössbauer and Raman spectroscopic techniques, as well as metallographic techniques. Products grew in the form of large-sized tubercles that gradually developed causing base metal losses up to perforation of the steel pipe. Considerable differences in the phase composition were found between the products formed in contact with the steel and those constituting the outer part of tubercles. The former were mainly made of FeCO₃ (siderite), with small amounts of Zn₅(CO₃)₂(OH)₆ (hydrozincite), ZnCO₃ (smithsonite), (Fe,Zn)CO₃ mixed carbonate and CaCO₃ (calcite), the latter mainly by Fe(III) oxyhydroxide goethite. Both parts of the tubercles also contained small amounts of other ferric oxyhydroxides, γ-FeOOH (lepidocrocite) and β-FeOOH (akaganeite), and very small amounts of hematite. The procedures used proved effective for an adequate identification of both the iron-containing and iron-free compounds in the corrosion products as well as for suggesting a corrosion mechanism.     

Atmospheric corrosion of zinc induced by runoff

Atmospheric corrosion and runoff of zinc were investigated during two years in humid tropical climate on hot dip galvanized steel and zinc samples. The high zinc mass loss (14.70 g m⁻²) is induced by the intensive zinc release (12.40 g m⁻²). No corrosion phase containing chloride was detected on the zinc surface, while a variety of sulfates not dissolved by rains reveals the sensitivity of zinc to SO₂ pollutant. However, two chloride-containing corrosion products were detected on the galvanized steel. Exponential equation is proposed that fits well the experimental data for zinc mass loss induced by runoff process as a function of the time of wetness. The formula gives possibility to predict the mass loss even before a steady state in the corrosion process has been reached. This equation can converge to a Benarie lineal function (C = At_w), when the coefficient b = 1 for the corrosion which is accelerated with the partial removal of the corrosion layer during the runoff phenomena.     

Role of zinc compounds on the formation, morphology, and adsorption characteristics of β-FeOOH rusts

To simulate the corrosion of galvanized steel in marine zone, β-FeOOH was prepared by aging the FeCl₃ solutions containing ZnCl₂ and zinc rusts such as ZnO and zinc hydroxychloride (Zn₅(OH)₈Cl₂·H₂O:ZHC). Adding ZnCl₂, ZnO, and ZHC inhibited the crystallization and particle growth of β-FeOOH and the inhibitory effect was in order of ZHC ≈ ZnO > ZnCl₂. The adsorption of H₂O and CO₂ was suppressed by adding ZnCl₂, ZnO, and ZHC. These results imply that the rust formed on galvanized steel in marine environment is more compact, amorphous, and hydrophobic in nature which may lead to improve the corrosion resistance.     

Corrosion of X65 steel in CO2-saturated oilfield formation water in the absence and presence of acetic acid

Electrochemical corrosion behavior of X65 steel in CO₂-saturated formation water in the absence and presence of acetic acid was studied by electrochemical measurements, scanning vibrating micro-electrode (SVME), localized electrochemical impedance spectroscope (LEIS) and surface analysis techniques. It is found that, when steel is immersed in formation water, the dissolution of Fe dominates the anodic process and the steel is in active dissolution state. Adsorption of intermediate product on the electrode surface results in generation of an inductive loop in the low frequency range of EIS plot. As corrosion proceeds, the concentration of Fe²⁺ in the solution increases. When the product of [Fe²⁺] × [] exceeds solubility product of FeCO₃, FeCO₃ will deposit on the electrode surface, and protects the steel substrate from further corrosion. The steel is in a “passive” state. When the electrode surface is completely covered with FeCO₃ film, the inductive loop in the low frequency range disappears. In the presence of acetic acid in formation water, the cathodic reaction will be enhanced due to the direct reduction of undissociated acetic acid. Addition of acetic acid degrades the protectiveness of corrosion scale, and thus, enhances corrosion of steel by decreasing the FeCO₃ supersaturation in solution.     

Degradation mechanism of galvanized steel in wet-dry cyclic environment containing chloride ions

The wet-dry cyclic test of a galvanized steel (GI) and pure zinc (ZN), which simulates marine atmospheric environment, has been conducted to clarify the degradation mechanism of galvanized steel. The samples were exposed to alternate conditions of 1 h-immersion in a 0.05 M NaCl solution and 7 h-drying at 25 °C and 60%RH, and the corrosion was monitored for 10 days (30 cycles) using a two-electrode type probe. Simultaneously, the corrosion potential was measured every three cycles only during the immersed conditions. The reciprocal of polarization resistance R_p⁻¹ was taken as an index of the corrosion rate. Several sample plates of GI and ZN were exposed, together with the monitoring probes. They were removed from the test chamber at the end of 1st, 3rd, 9th, 18th, and 30th cycles of exposure and were analyzed for the corrosion products with XRD and laser Raman spectroscopy. Further, their cross sections were analyzed with FESEM-EDS. The FESEM photographs and elemental analysis of cross sections confirmed that the R_p⁻¹ value commences to decrease when the corrosion front reaches Zn-Fe alloy layers (boundary layers of zinc coating and steel substrate) due to localized nature of attack. A schematic model of degradation mechanism and the role of galvanic protection have been discussed.     

Corrosion characteristics of copper microparticles and copper nanoparticles in distilled water

The corrosion characteristics of copper microparticles and copper nanoparticles in distilled water were investigated in this paper. The Cu²⁺ transformations of copper microparticles and copper nanoparticles in distilled water were tested by using absorbance measurement, the structures of their corrosion products were determined by using XRD and TEM techniques. The results of absorbance measurement show that the corrosion characteristics of copper nanoparticles in distilled water are quite different from that of copper microparticles. The Cu²⁺ transformations ratio of copper microparticles increases slowly with the increasing of immersion time and levels off eventually, but the Cu²⁺ transformations ratio of copper nanoparticles increases sharply with the increasing of immersion time and gets to peak rapidly, and then decreases as the immersion time increases and levels off finally. The results of XRD present that they have different corrosion products, the corrosion products of copper microparticles in distilled water are Cu and CuO, but the nanoparticles are Cu, CuO, Cu(OH,Cl)₂ · 2H₂O and Cu₂(CO₃)(OH)₂. All these differences owe to the size effect of copper particles.     

On the fundamentals of electrochemical corrosion of X65 steel in CO2-containing formation water in the presence of acetic acid in petroleum production

Electrochemical corrosion behavior of X65 steel in CO₂-containing oilfield formation water in the presence of acetic acid (HAc) was investigated by various electrochemical measurements and analyses as well as thermodynamic calculations of ionic concentrations, reaction rate constants and equilibrium electrode potentials. A conceptual model was developed to illustrate corrosion processes of steel in oilfield formation water system. The anodic reactions of the steel contain a direct dissolution of Fe, Fe → Fe²⁺ + 2e, and the formation of corrosion scale, FeCO₃, by Fe + → FeCO₃ + H⁺ + 2e. The cathodic processes contain the reduction of H⁺, , H₂O and HAc, where reduction of HAc has the least negative equilibrium potential and thus dominates the cathodic process. With addition of HAc in the solution, both cathodic and anodic reaction rates increase remarkably. It is attributed to the fact that HAc inhibits or degrades the formation of protective scales due to the decrease of solution pH. Upon electrode rotation, the measured impedance decreases with the increase in HAc concentration. The FeCO₃ scale will not form on electrode surface. When HAc concentration is less than 1000 ppm, the adsorbed intermediate product is not significant, resulting in generation of a low-frequency inductive loop in EIS plots. When HAc concentration is more than 3000 ppm, the adsorption of intermediate product is significant, generating overlapped capacitive semicircles in EIS measurements.     

Structure and properties of Ti(IV)-doped zinc hydroxychloride rusts formed at various temperatures

To simulate the atmospheric corrosion of steels galvanized with Ti–Zn alloys under different atmospheric temperatures, Ti(IV)-doped zinc hydroxychloride (Zn₅(OH)₈Cl₂·H₂O: ZHC) was prepared at various aging temperatures of 6–120 °C. Adding the Ti(IV) inhibited the crystallization and particle growth of ZHC, showing a minimum at 50 °C. Higher aging temperature promoted the formation of TiO₂ nano-particles. Elevating the aging temperature suppressed the adsorption of H₂O and CO₂ on Ti(IV)-doped ZHC. These results suggest that the alloying Ti in galvanized steel forms compact zinc rust layer at various atmospheric temperatures in marine environment, which would lead to the enhancement of corrosion resistance.     

Effects of NH4 +, Na+, and Mg2+ ions on the corrosion behavior of galvanized steel in wet–dry cyclic conditions

The effect of cations on the corrosion of galvanized steel (GS) is scarcely reported. In this study, a wet–dry cyclic test was conducted to study NH₄ ⁺, Na⁺, and Mg²⁺ cation effect on the corrosion behavior of GS available in Nepal. Fourteen wet–dry cycles (18 h wet and 6 h dry) were performed by exposing samples at 298 K with a relative humidity of 90% in a wet cycle and 50% in a dry cycle for 14 days. The cations strongly affect the corrosion rate of the GS sample estimated by weight loss and potentiodynamic polarization. The potentiodynamic polarization curves showed the inhibition of cathodic and anodic reactions by Mg²⁺ ion, while the NH₄ ⁺ ion only changed the cathodic reaction. Mg²⁺ ion was found to shift the corrosion potential to noble values compared with NH₄ ⁺ and Na⁺ ions. A compact and thin corrosion products layer was developed in Mg²⁺ salt solution in contrast to a thick and porous corrosion products layer in NH₄ ⁺ and Na⁺ salt solutions. Red rust due to corrosion of underlying steel appeared in the presence of NH₄ ⁺ and Na⁺ salt solutions. Finally, the weight loss data revealed that the corrosivity of cations for GS decreased in the order Na⁺ > NH₄ ⁺ > Mg²⁺.     

Understanding corrosion via corrosion product characterization: II. Role of alloying elements in improving the corrosion resistance of Zn–Al–Mg coatings on steel

Corrosion products are identified on Zn, ZnMg and ZnAlMg coatings in cyclic corrosion tests with NaCl or Na₂SO₄ containing atmospheres. For Mg-containing alloys the improved corrosion resistance is achieved by stabilization of protective simonkolleite and zinc hydroxysulfate. At later stages, the formation of layered double hydroxides (LDH) is observed for ZnAlMg. According to thermodynamic modeling, Mg²⁺ ions bind the excess of carbonate or sulfate anions preventing the formation of soluble or less-protective products. A preferential dissolution of Zn and Mg at initial stages of corrosion is confirmed by in situ dissolution measurement. The physicochemical properties of different corrosion products are compared.     

A3钢在富营养化东湖水中初期腐蚀研究

    以A3钢为研究对象,采用失重实验和电化学技术研究了A3钢在富营养化东湖水中的腐蚀行为.结果表明：A3钢在实验过程中腐蚀电流随时间增长逐渐减小;自腐蚀电位先降后升,而极化电阻逐渐增大,说明电极表面刚开始处于活化状态,然后电极表面“保护膜”不断生长,将电极表面覆盖,减缓了基体的腐蚀;电化学方法计算出富营养化水中A3钢的腐蚀电流与线性极化电阻的比例常数B值为23.6242;通过XPS分析得出A3钢腐蚀产物的成分为FeOOH,Fe³⁺与C、N、S结合的有机产物和FeSO₄,其中Fe²⁺/Fe³⁺=0.24.     

Corrosion behavior of steel under wet and dry cycles containing Cr ion

The corrosion behavior of mild steel has been investigated during the wet and dry cyclic transitions containing Cr³⁺ ion added as sulfate in order to gain a better understanding of the influence of Cr on the atmospheric corrosion of steels. The corrosion rate during drying is greatly suppressed by the existence of Cr³⁺ ion in the electrolyte covered with the surface. Lower corrosion rates are observed during drying even if the surface have been polarized to negative potentials below −200 mV_SHE during the wet corrosion conditions in which the surface-covered electrolyte contains Cr³⁺ ion. This corrosion behavior is identical to the case of Cr-containing steel for the wet and dry cyclic transitions without the addition of Cr³⁺ ion. The composition of rust layer after the wet and dry cyclic transitions is composed of α-FeOOH, γ-FeOOH and Fe_3−δO₄ for both cases of non-Cr³⁺ and Cr³⁺-containing condition, and no significant difference in the mass fraction of the above rust substances between two conditions is observed by means of Mössbauer spectroscopy. The only difference in the rust layer is that the rust formed under the wet and dry cyclic transitions containing Cr³⁺ ion contains a certain amount of Cr near the steel/rust interface. Those results suggest that the role of Cr during the wet and dry cyclic transitions is the inhibition of the rust reduction and the formation of Fe²⁺-state intermediate by the existence of Cr in the rust layer. This can lead to the inhibition of the oxygen reduction during drying.     

X-ray photoelectron spectroscopy study of the corrosion behaviour of galvanised steel implanted with rare earths

The present paper studies the effect of ion implantation of 2 × 10¹⁶ ions/cm² of Ce⁺ and 2 × 10¹⁶ ions/cm² of La⁺ at 150 keV on the corrosion behaviour of hot-dip galvanised steel. After implantation, galvanised steel was characterised by means of XPS previous to and following immersion in the medium. The results revealed incorporation of cerium and lanthanum on the surface as Ce₂O₃ and La₂O₃, respectively. Electrochemical impedance spectroscopy was carried out in order to evaluate its corrosion behaviour in 0.6 M NaCl during 1 month of immersion. The corrosion resistance was improved by an increase in the charge transfer resistance of the implanted specimens in the medium. This effect could be associated with changes in the morphology/microstructure of the corrosion products layer rather than in its composition variations.     

The influence of Mgon the formation of calcareous deposits on a freely corroding low carbon steel in seawater

The purpose of this study was to determine how magnesium in seawater influences the corrosion behaviour of freely corroding steel. This was done by studying if Mg(OH)₂ is formed and if calcite and aragonite differ in their protective properties. No Mg(OH)₂ was detected after immersion of steel in a Mg²⁺-containing artificial seawater. Magnesium seems to influence the corrosion behaviour of freely corroding steel by causing calcium carbonate to precipitate as aragonite. Aragonite is more effective in covering the surface than calcite and is therefore more functional in preventing oxygen from reaching the steel surface, thereby lowering the corrosion rate.     

Corrosion behaviour of carbon steel in different concentrations of HCl solutions containing H2S at 90 °C

The corrosion behavior of SAE-1020 carbon steel in H₂S-containing solutions with different concentration of HCl at 90 °C was investigated by weight loss, electrochemical measurements, SEM and XRD analysis. The results showed that the corrosion rate of carbon steel increased with increasing HCl concentration. Uniform corrosion was found on the carbon steel surface in H₂S + HCl solutions, while corrosion cavities were observed in the solution only containing H₂S. The ratio of Faradaic process of total corrosion process increased with the increase of HCl concentration. The corrosion products were solely composed of mackinawite in the H₂S-containing solutions with or without HCl.     

Understanding corrosion via corrosion product characterization: I. Case study of the role of Mg alloying in Zn-Mg coating on steel

In the present work the corrosion inhibitive role of Mg in Zn-Mg coatings is considered for different stages of corrosion. Corrosion product characterization was carried out using XRD, IRRAS, MEB-FEG-EDS on technical Zn-Mg coatings after various exposure times in a standardized cyclic corrosion test. The results are compared with artificial corrosion products obtained by chemical and electrochemical synthesis. The importance of the ageing and the role of the atmospheric CO₂ on the nature and morphology of the corrosion products are discussed. The corrosion resistance of Zn-Mg alloy is correlated with the stabilization of simonkolleite against its transformation into smithsonite, hydrozincite, and zincite during ageing cycles in presence of CO₂. The stabilization appears to be due to the preferential formation of magnesium carbonates. Thermodynamic modeling and titrometric analysis demonstrate that Mg²⁺ enhances simonkolleite during dry-wet cycling by (1) removing carbonate from the environment and thereby limiting of the transformation of simonkolleite into zincite, smithsonite, and hydrozincite and by (2) buffering the pH of the electrolyte around 10.2 due to the precipitation of Mg(OH)₂ preventing the dissolution of zinc based corrosion products into soluble hydroxide complexes.     

Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D

Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with a deposition of 70 μg/cm⁻² NaCl. The corrosion products formed and the surface electrolyte were analysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffraction and Ion Chromatography. The results show that magnesium carbonates are the main solid corrosion products formed under these conditions. The corrosion products identified were the magnesium carbonates hydromagnesite (Mg₅ (CO₃)₄ (OH)₂4H₂O) and nesquehonite (MgCO₃ 3H₂O). The corrosion attack starts with the formation of magnesite at locations with higher NaCl contents. At 95% RH, a sequence of reactions was observed with the initial formation of magnesite, which transformed into nesquehonite after 2-3 days. Long exposures result in the formation of pits containing brucite (Mg(OH₂)) covered with hydromagnesite crusts. The hydromagnesite crusts restrict the transport of CO₂ and O₂ to the magnesium surface and thereby favour the formation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on the surface at the beginning was essentially preserved during the initial corrosion process. Since the applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolyte was present on the surface during the whole exposure. Thus, Na⁺ and Cl⁻ ions can participate in the corrosion process during the whole time and the availability of these species will not restrict the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosion behaviour of AZ91D is rather controlled by factors related to the microstructure of the alloy and formation of solid carbonate containing corrosion products blocking active corrosion sites on the surface.     

Chemistry of corrosion products on Zn-Al-Mg alloy coated steel

Zn-Al-Mg alloy (ZM) coating provides a decisively enhanced corrosion resistance in a salt spray test according to DIN EN ISO 9227 (NSS) compared to conventional hot-dip galvanised zinc (Z) coating because of its ability to form a very stable, well adherent protecting layer of zinc aluminium carbonate hydroxide, Zn₆Al₂(CO₃)(OH)₁₆·4H₂O on the steel substrate. This protecting layer is the main reason for the enhanced corrosion resistance of the ZM coating. Surface corrosion products on ZM coated steel consist mainly of Zn₅(OH)₆(CO₃)₂, ZnCO₃ and Zn(OH)₂ with additions of Zn₅(OH)₈Cl₂ · H₂O and a carbonate-containing magnesium species.     

Corrosion resistance of zinc-magnesium coated steel

A significant body of work exists in the literature concerning the corrosion behaviour of zinc-magnesium coated steel (ZMG), describing its enhanced corrosion resistance when compared to conventional zinc-coated steel. This paper begins with a review of the literature and identifies key themes in the reported mechanisms for the attractive properties of this material. This is followed by an experimental programme where ZMG was subjected to an automotive laboratory corrosion test using acidified NaCl solution. A 3-fold increase in time to red rust compared to conventional zinc coatings was measured. X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the corrosion products formed. The corrosion products detected on ZMG included simonkolleite (Zn₅Cl₂(OH)₈ · H₂O), possibly modified by magnesium uptake, magnesium hydroxide (Mg(OH)₂) and a hydroxy carbonate species. It is proposed that the oxygen reduction activity at the (zinc) cathodes is reduced by precipitation of alkali-resistant Mg(OH)₂, which is gradually converted to more soluble hydroxy carbonates by uptake of atmospheric carbon dioxide. This lowers the surface pH sufficiently to allow thermodynamically for general precipitation of insoluble simonkolleite over the corroding surface thereby retarding the overall corrosion reactions, leaving only small traces of magnesium corrosion products behind. Such a mechanism is consistent with the experimental findings reported in the literature.