A new approach to improve high temperature corrosion resistance of thermally sprayed coatings by using electrochemical corrosion tests

In order to increase reliability and lifetime of heat exchangers in waste‐to‐energy plants corrosion resistant thermally sprayed coatings are applied in a higher extent. From the selection of a plant specific spray material and process via laboratory and field experiments up to a successful coating a high amount of time and money is necessary. In particular, experiments in corrosive surroundings at high temperature which are needed to optimize the coating quality are time‐consuming. In order to decrease development times and to reduce costs, corrosion resistance of a thermally sprayed coating under high temperature conditions is compared with its behavior in an aqueous solution. If the high temperature corrosion resistance can be forecast by electrochemical tests, a fast, efficient and cheap possibility to improve the coating quality would be available and the efforts for tests in high temperature environments could be decreased. To accomplish this objective, Diamalloy 1005 was sprayed with the APS‐process on a ferritic steel. Since the performance of thermally sprayed coatings is not only influenced by the coating material but also by the spraying parameters, three different spray parameter sets were used. After analyzing the results of the corrosion tests under high temperature conditions and in aqueous solution an identical assessment of the spray parameter‐specific corrosion protection could be obtained. Ongoing field tests in a waste‐to‐energy plant are expected to prove the laboratory results. So, a first step in the development of a fast, efficient and cheap method to predict the high temperature corrosion resistance of a thermally sprayed coating might have been done.     

Development of microarc oxidation process to improve corrosion resistance on AZ91HP magnesium alloy

A new anodizing process, which does not contain chromate but can improve the corrosion resistance of magnesium alloys significantly, was developed using a microarc power supply. Surface morphology was observed and the coating was compact and ceramic-like. In addition, the corrosion resistance of samples before and aider anodization by the new process and a method in US Patent 5470664 was compared by potentiodymaic polarization curves, electrochemical impedance spectroscopy （EIS） and salt spray test. The results show that the anodization can improve the corrosion resistance of magnesium alloy. The samples obtained by the new process and the method mentioned in the US Patent 5470664 achieve 9 and 7 rates aider 336 h salt spray test, respectively.     

Carbonation of Mg powder to enhance the corrosion resistance of Mg-rich primers

This paper is a continuation of our investigation into the characteristic dichotomy of Mg-rich primers between accelerated salt-fog testing and natural weathering. Our earlier study suggested that magnesium powder reacted with atmospheric CO₂ to form a protective carbonate layer on its surface. In this study, magnesium powder was treated with aqueous carbonic acid to accelerate magnesium carbonate development. The treated magnesium powder was formulated into a Mg-rich primer and evaluated for its corrosion resistance. The Mg-rich primer formulated with the treated Mg powder performed better in the salt-fog test than the control primer based on untreated Mg powder.     

Non‐destructive detection of corrosion applied to steel and galvanized steel coated with organic paints by the pulsed phase thermography

Organic coatings in the automobile industry have to resist corrosion and mechanical damage from stone chipping. Currently, no tool is established in industrial non‐destructive applications for analyzing the damage of stone‐impacts and the following corrosion after accelerated corrosion tests. Measurement methods such as the scanning Kelvin probe can analyze the corrosion progress in a detailed manner, but with a long measurement time. The pulsed phase thermography (PPT) is a non‐destructive tool to analyze inhomogeneities and defects in materials, with a huge field of applications existing. The present work shows advances in using the PPT to detect propagation of corrosion under coatings. Physical principles of the mechanism of the corrosion detection under coatings are described. Results of measurements of organic coatings on carbon steel as well as of organic coated galvanized steel show the corrosion propagation. Influencing factors to the measurement such as the thickness of the coatings are investigated, but no significant effect on the quality of the analysis was found. The corrosion progress can be monitored by the PPT fast and reliably. The achieved results correlate with the theoretical basis and the test results after surface characterization and destructive analysis of samples.     

Effect of hybrid aluminum dihydrogen phosphate on the corrosion resistance of PTFE composite ceramic coatings

In this study, the polytetrafluoroethylene (PTFE) composite ceramic coatings with hybrid aluminum dihydrogen phosphate (AP) are prepared on AISI 304L stainless steel by spraying and heat curing to improve the corrosion resistance of the coatings. AP is hybridized with methyltriethoxysilane (MTES), and the structure of the hybrid AP is characterized by Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Contact angle tests, scratch tests, and electrochemical experiments are used to investigate the corrosion behavior of composite ceramic coatings. In addition, scanning electron microscopy is used to examine the microscopic morphology of the coatings after corrosion to investigate the mechanism of the hybrid AP on the corrosion resistance of the composite ceramic coatings. The findings reveal that MTES successfully hybridizes with AP and implants the –CH₃ hydrophobic group in AP, which improves the hydrophobicity of composite coatings. The corrosion potential of hybrid AP coatings all move in a positive direction, and the corrosion current density is lower than that of unhybridized AP coatings. The corrosion current density of the coating is about 2.931e?008 A/cm² when the MTES content is 5 wt%, which is 20% less than that of the unhybridized AP coating. Results indicate hybrid AP can significantly improve the corrosion resistance of PTFE composite ceramic coatings with the best corrosion resistance occurring when the content of MTES is 5 wt%.     

Al2O3 eliminating defects in the Al coating by laser cladding to improve corrosion and wear resistance of Mg alloy

Although Al produces a solid metallurgical bonding with Mg alloy substrates, micropores or crevices in the Al coating can reduce the resistance of Mg alloy to corrosion. In this study, a composite coating with a defect-free microstructure was prepared on the AZ31 Mg alloy substrate by introducing Al₂O₃ into the Al matrix via the method of laser cladding. On the one hand, Al₂O₃ with thermal insulation had a low thermal expansion coefficient and was not very prone to voids during laser melting. On the other hand, Al₂O₃ particles with a small size acted as the filler in the micropores or crevices. The Al/Al₂O₃ coating exhibited a smaller current density (2.1 × 10⁻⁶ A/cm²) in comparison with those of bare substrate and Al coating (158.4 × 10⁻⁶ and 3.1 × 10⁻⁶ A/cm², respectively), which was mainly ascribed to the pore-free microstructure and high resistance to corrosion of Al₂O₃ phase. A favorable microhardness value of 95.3 HV was achieved for Al/Al₂O₃ coating, approximately 1.8 times higher than that of Al coating (52.8 V), which was mainly ascribed to the dispersion hardening of Al₂O₃ phase. Meanwhile, the Al/Al₂O₃ coating significantly reduced wear volume from 2.8 mm³/m of Al coating to 0.4 mm³/m, showing great potential for weight reduction applications.     

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.     

The use of microcapillary techniques to study the corrosion resistance of AZ91 magnesium alloy at the microscale

The AZ91 alloy is composed of Mg₁₇(Al, Zn)₁₂ precipitates, an eutectic phase around these precipitates, AlMn intermetallic particles and an α-Mg solid solution (matrix). The corrosion behaviour of AZ91 was investigated at the microscale by means of the electrochemical microcell technique, which uses extremely small capillaries (diameters between 5 and 10 μm). Experiments were conducted in 0.1 M NaClO₄ at 25 °C. The β-Mg₁₇(Al, Zn)₁₂ precipitates were found to have the highest corrosion resistance, whereas the eutectic phase was very active (pitting potential of approximately −1400 mV vs. Ag/AgCl). The α-Mg solid solution displayed better corrosion resistance than the eutectic phase.     

Stress distribution in thick-walled cylinder due to non-uniform radial pressure on the example of reinforcement corrosion in concrete

This paper presents an analytical solution to the non-uniform pressure on thick-walled cylinder. The formulation is based on the linear elasticity theory (plain strain) and stress function method. As an example, the proposed solution is used to model the stress distribution due to non-uniform steel reinforcement corrosion in concrete. The model is formulated considering different scenarios of corrosion pressure distribution. It is validated against the finite element model for different cases of non-uniform pressure distributions. The results show that the corrosion-induced cracks are likely to start just beyond the anodic zone. This is confirmed by the experimental tests on concrete cylinder exposed to non-uniform accelerated corrosion of steel reinforcement. The model can be effectively used to calculate the distribution of corrosion-induced stresses in concrete.     

Additional modification to two-dimensional polymer films of a hydroxymethylbenzene self-assembled monolayer with alkyltriethoxysilanes for enhancing the protective abilities against iron corrosion

Additional modification of the ultrathin two-dimensional polymer film, a p-hydroxymethylbenzene C₆H₄CH₂OH self-assembled monolayer modified with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ (BTESE) and alkyltriethoxysilane C_nH_2n+1Si(OC₂H₅)₃ (C_nTES, n = 8 or 18), was attempted to improve the protective ability of the film against iron corrosion. The ability of the film was examined by polarization measurement of an iron electrode coated with the film in an oxygenated 0.5 M NaCl solution after immersion in the solution for 1.5 to 72 h. Marked improvement of the protective efficiency, P was not obtained by additional modification to the polymer film with BTESE. The P values of the two-dimensional polymer films were markedly increased by additional modification with C₈TES. The increases in P were ascribable to improvement of alkyl tail arrangement and additional interconnection in the polymer films. The film on the iron surface was characterized by contact angle measurement, FTIR reflection spectroscopy and X-ray photoelectron spectroscopy. The protective abilities of the two-dimensional polymer films additionally modified with C₈TES were persistent during immersion for 72 h.     

Effect of surface treatments based on self‐assembling molecules and cerium coatings on the AA3003 alloy corrosion resistance

This work presents the effects of a cerium conversion coating, self‐assembling molecules (SAM) treatment, and a combination of these two treatments on the corrosion resistance of the AA3003 alloy. The results were compared to that of a conversion coating treatment with hexavalent chromium (Cr VI). The corrosion resistance of the surface‐treated AA3003 alloy samples was investigated by electrochemical impedance spectroscopy and anodic polarization curves in 0.5 mol/L sodium sulfate solution with pH adjusted to 4.0. The results showed that the SAM treatment offered better corrosion protection for the AA3003 alloy than that provided by cerium conversion coating. The combination of cerium conversion and SAM treatments improved the corrosion resistance of the AA3003 alloy due to SAM adsorption on the alloy substrate exposed at the defects in the cerium conversion layer.     

Effects of rare earth cerium addition on the synthesis and corrosion resistance of electroless Ni–PTFE–P coating

Electroless Ni–PTFE–P coatings have been successfully deposited on the surface of mild steel shaft from plating baths containing various concentrations of rare earth metal cerium (RE Ce). Surface morphology, Ce fraction, and thickness of the coatings were characterized by scanning electron microscope, inductively coupled plasma optical emission spectrometry, and reflection optical microscope, respectively. Salt spray test was used to determine the corrosion resistance of the coating. Results revealed that structure, compactness, and deposition rate of the Ni–PTFE–P coatings were increased significantly by addition of a small amount of RE Ce (10–20 ppm) to the plating bath. Electroless Ni–PTFE–P coating deposited from plating baths with 20 ppm Ce shows the highest corrosion resistance, owing to its high compactness and thickness. Deposition rate and corrosion resistance of the Ni–PTFE–P coating were deteriorated greatly as concentration of RE Ce in the plating baths exceeds 100 ppm.     

Effect of processing parameters on the microstructures and corrosion behaviour of high‐velocity oxy‐fuel (HVOF) sprayed Fe‐based amorphous metallic coatings

A FeCrMoMnWBCSi amorphous metallic coating was prepared by using high‐velocity oxy‐fuel spray. The influence of processing parameters on microstructure, porosity level, amorphous phase fraction and corrosion behaviour of the coatings was characterised by scanning electron microscopy, X‐ray diffraction, differential scanning calorimeter and electrochemical methods. The results indicated that the microstructures of the coatings were sensitive to the spray parameters considerably. Porosity and unmelted particle proportion decreased with the oxygen/fuel (O/F) ratio and increased with the powder feed rate. The trend of oxides content was opposite to the porosity and unmelted particle proportion. The coatings obtained with higher O/F ratio and lower powder feed rate exhibited higher hardness. The low coating hardness was mainly due to the high porosity especially when the porosity was higher than 1.21%. The spraying parameters strongly affected the amorphous phase fraction. There was a critical passive current density for balancing the porosity and the amorphous phase fraction. Corrosion resistance is dominant by the amorphous phase fraction when the porosity is less than 1.21%, while by porosity when it is higher than that. Open‐circuit potential, potentiodynamic polarisation and electrochemical impedance spectroscopy results showed that the coatings obtained with the O/F ratio of 4.2 and the powder feed rate of 40 g/min exhibiting the best corrosion resistance in 1 wt% sodium chloride solution.     

Influence of the iron content in Cu–Ni based inert anodes on their corrosion resistance for aluminium electrolysis

Mechanically alloyed (Cu_3.25Ni)_100−xFe_x materials (x = 0, 15 and 30 wt.%) were evaluated as inert anodes for aluminium electrolysis in KF–AlF₃ (700 °C) electrolyte. For x = 0, the cell voltage was unstable and high (5–6 V) due to the formation of an insulting NiF_x layer at the metal–oxide interface. For x = 15 and 30, the formation of a Cu₂O-rich external scale with a protective NiFe₂O₄-rich intermediate layer was favoured, resulting in a lower (∼4 V) and more stable cell voltage. The purity of the produced Al was 98.96, 99.31 and 99.20 wt.% for x = 0, 15 and 30, respectively.