Influence of Ce surface treatments on corrosion behaviour of A3xx.x/SiCp composites in 3.5 wt.% NaCl

The influence of silicon carbide particles (SiCp) proportion and matrix composition on aluminium metal matrix composites A3xx.x/SiCp modified by cerium-based conversion or electrolysis coating was evaluated in 3.5 wt.% NaCl at 22 °C using potentiodynamic polarization and gravimetric measurements. Ce-treated surfaces presented better corrosion behaviour in chloride media than original composite surfaces without treatment. Both treatments preferentially covered the intermetallic compounds and SiCp. The electrolysis afforded a higher degree of protection than conversion treatment because the coating was more extensive. Coating microstructure and nature of corrosion products were analysed by scanning electron and atomic force microscopy (SEM, AFM) and low angle X-ray diffraction (XRD).     

The effect of molybdate concentration and hydrodynamic effect on mild steel corrosion inhibition in simulated cooling water

The effects of two important factors namely concentration and hydrodynamics on the inhibition efficiency of molybdate were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy tests. Results showed that molybdate was capable of inhibiting the corrosion of mild steel in simulated cooling water. The inhibition efficiency of molybdate was increased with increasing both its concentration and water circulation velocity. These two factors seem to promote the adsorption of molybdate and oxygen ions on the metal surface, leading to the formation of a protective layer with a greater charge transfer resistance and lower permeability. A model is also proposed which facilitates the probable mechanism of inhibition.     

Corrosion behavior of a cerium-based conversion coating on alumina borate whisker-reinforced AA6061 composite pre-treated by hydrogen fluoride

Cerium-based conversion coatings have been made on alumina borate whisker-reinforced AA6061 composite pre-treated with hydrogen fluoride. The pre-treatment prior to coating significantly affects the microstructure and the corrosion behavior of the ultimate coated composites. The coating deposited on composite pre-treated for 20 s provides better corrosion protection than that pre-treated for 180 s. A self-healing effect is observed on the coated composite with pre-treatment for 20 s in the early stage of immersion, while no self-healing effect appears with pre-treatment for 180 s. It is largely because the morphologies of the coatings are remarkably different under the two pre-treatment conditions.     

Corrosion behaviour of an aluminized nickel coating in a carbon dioxide capture process using aqueous monoethanolamine

Carbon steel A106 with an aluminized Ni coating was tested in a 5 mol/l (M) monoethanolamine (MEA) with a carbon loading of 0.2 mol/mol MEA at 80 °C using polarization resistance, Tafel, and electrochemical impedance spectroscopy (EIS) techniques. Surface morphology and elemental line profile of the coating were examined before and after a 900 °C heat-treatment in atmosphere environment. X-ray diffraction (XRD) results showed that the coating surface constitutes transformed from Ni₂Al₃ to Al-rich NiAl after heat-treatment. The electrochemical results show that the heat-treated coating displays outstanding corrosion performance with the conditions tested.     

Corrosion behaviour of a magnesium matrix composite with a silicate plasma electrolytic oxidation coating

The corrosion behaviour of AZ92 magnesium alloy reinforced with various volume fractions of silicon carbide particles (SiCp) and treated by alternating current (AC) plasma electrolytic oxidation (PEO) was investigated in humid and saline environments. For untreated composites, corrosion attack started around the Al-Mn inclusions and gradually developed into general corrosion without significant galvanic coupling between the matrix and the SiCp. PEO coatings consisted mainly of MgO and Mg₂SiO₄, and revealed increased hardness, reduced thickness and slightly higher corrosion resistance with increasing proportion of reinforcement. Pit formation and hydration of the outer layer were the main mechanisms of corrosion of PEO-treated specimens.     

Electrochemical study of growth behaviour of plasma electrolytic oxidation coating on Ti6Al4V: Effects of the additive

The growth behaviour of plasma electrolytic oxidation (PEO) coating on Ti6Al4V was studied by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test, focusing on effects of the electrolyte additive - calcium hypophosphite. The EIS analysis of the outer layer of the PEO coating provided insight into the structure of the coating, which was confirmed by SEM results. The EIS analysis of the inner layer of the PEO coating provided information matching well with the results of the potentiodynamic polarization test.     

Improving the corrosion and tribocorrosion resistance of Ni–Co nanocrystalline coatings in NaOH solution

Ni–Co nanocystalline coatings were electrodeposited from a modified Watts bath. Increasing the deposition current density had no significant effect on structure, corrosion and tribocorrosion behavior of the coatings. Adding saccharin into the bath reduced the grain size, increased the hardness, changed the texture component from (2 0 0) to (1 1 1), smoothed the surface morphology, increased the corrosion resistance and improved the tribocorrosion behavior of coating. Presence of sodium lauryl sulfate in the bath increased the corrosion resistance of coating by producing a more compact surface morphology. However, the coating showed low tribocorrosion resistance, probably due to its lower hardness.     

Electrochemical behaviour enhancement of stainless steels by a SiO2 PACVD coating

Amorphous silica coatings have been deposited by plasma assisted chemical vapor deposition (PACVD) process on M2 steel and 304 stainless steel substrates. The chemical inertia of the coating has been established by resonant quartz crystal microbalance (RQCM), and no porosity has been observed at the AFM scale. The barrier effect has been evaluated on M2 steel by applying the concept of dipolar relaxation. The protection is very efficient, and no deleterious galvanic coupling with the uncoated metallic surface can occur.

A coated stainless steel was also investigated in NaCl solution in order to understand how the passive properties are influenced by such a barrier coating. When deposited on stainless steel, coated part keeps the beneficial passive behaviour inherent to the substrate. They evidence almost no corrosion. The silica based layer behaves as a quasi perfect dielectric. The corrosion rate is then greatly reduced, and the pitting resistance is improved. The excellent localized corrosion protection has been explained by a tiny porosity rate, and a limited access of chloride ions through open pores.     

Effect of surface condition on the initial corrosion of galvanized reinforcing steel embedded in concrete

The present work was aimed at determining the effect of coating surface condition on the initial corrosion of hot-dip galvanized reinforcing steel bar (HDG rebar) in ordinary Portland cement (OPC) concrete. During zinc corrosion in OPC concrete, calcium hydroxyzincate (CHZ) formed on untreated HDG steel provided sufficient protection against corrosion. Therefore, it is concluded that treating HDG rebar with dilute chromic acid is unnecessary as a method of passivating zinc. A layer of zinc oxide and zinc carbonate formed, through weathering, on HDG bars increased the initial corrosion rate and passivation time compared with the non-weathered rebar exposed to concrete. HDG steel with an alloyed coating, i.e. containing only of Fe-Zn intermetallic phases, required a longer time to passivate than those with a pure zinc surface layer. The lower zinc content of the surface limited the rate of CHZ formation; hence, delayed passivation. Regardless of the surface condition, the coating depth loss after two days of embedment in ordinary Portland cement concrete was insignificant.     

Influence of reinforcement proportion and matrix composition on pitting corrosion behaviour of cast aluminium matrix composites (A3xx.x/SiCp)

The influence of silicon carbide (SiCp) proportion and matrix composition on four aluminium metal matrix composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) immersed in 1-3.5 wt% NaCl at 22 °C was investigated by potentiodynamic polarization. The kinetics of the corrosion process was studied on the basis of gravimetric measurements. The nature of corrosion products was analysed by scanning electron microscopy (SEM) and low angle X-ray diffraction (XRD). The corrosion damage in Al/SiCp composites was caused by pitting attack and by nucleation and growth of Al₂O₃ · 3H₂O on the material surface. The main attack nucleation sites were the interface region between the matrix and the reinforcement particles. The corrosion process was influenced more by the concentration of alloy elements in the matrix than by the proportion of SiCp reinforcement and saline concentration.     

Microbial corrosion resistance of galvanized coatings with 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one as a biocidal ingredient in electrolytes

Electrodeposition of galvanized coatings from electrolyte containing 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) can increase microbial corrosion resistance. Coatings were found to inhibit the growth and metabolism of sulphate-reducing bacteria (SRB). Open circuit potentials and corrosion rates of coupons revealed DCOIT effectively influences the coating property. Energy diffraction spectrum and infrared absorption spectra were used to detect DCOIT on the coating surface. Scanning electron microscopy and X-ray diffraction revealed morphological and structural modifications. Electrochemical impedance spectroscopy and polarization techniques determined the corrosion behaviour of coatings in SRB. Results showed coatings formed from electrolytes with DCOIT have improved microbial corrosion resistance and bactericidal action.     

Synergistic corrosion behavior of coated Ti60 alloys with NaCl deposit in moist air at elevated temperature

In the present paper, the corrosion behavior of Ti60 alloys with an aluminide, TiAlCr, and enamel coatings in moist air containing NaCl vapor at 700-800 °C were studied. The results showed that the TiAlCr and aluminide coatings failed to protect the substrate from corrosion due to the cyclic formation of volatile products during corrosion at 800 °C. However, an uneven continuous protective Al₂O₃ scale could form on the aluminide coating during corrosion at 700 °C. And the enamel coating could protect Ti60 from corrosion due to its high thermochemical stability and matched thermal expansion coefficient with substrates of Ti-base alloys during corrosion.     

Evaluation of the anti-corrosive effect of acid pickling and sol-gel coating on magnesium AZ31 alloy

The effect of different acid pre-treatment procedures on the corrosion of magnesium AZ31 alloy was compared by measuring the amount of hydrogen gas formed when the surface was in contact with aqueous 5% sodium chloride solution. A 4-7 μm thick sol-gel coating prepared by phosphoric acid catalyzed sol-gel processing of a methyltriethoxysilane/tetraethoxysilane mixture was applied to the differently pre-treated magnesium surfaces. The corrosion rate of the alloy decreased by a factor of up to 60 by combination of acid pickling and sol-gel coating. The addition of triethylphosphate or 1,2,4-triazole as corrosion inhibitors led to further improvements. Composition and texture of the films was investigated by scanning electron microscopy and energy dispersive X-ray analysis.     

Influence of SiC particles addition on the corrosion behavior of 2014 Al-Cu alloy in 3.5% NaCl solution

The influence of silicon carbide particles (SiC_p) addition on the corrosion behavior of Al-Cu alloy (2014) was evaluated in 3.5% NaCl solution at 30 °C using microstrucural and electrochemical measurements. Addition of 10 wt.% SiC_p to the base alloy is found to increase its corrosion resistance considerably. Incorporation of SiC_p beyond this proportion leads to an increase in corrosion rate of the synthesized composites. Addition of 25 wt.% SiC_p to base alloy decreases corrosion resistances considerably. Microstructural studies reveal the agglomeration of SiC particles in the composites. This results an increase of corrosion reaction with the increase of SiC particles in the composites. EIS measurement indicates the occurrence of adsorption/diffusion phenomena at the interfaces of the composites that ultimately initiate the localized or pitting corrosion.     

A facile approach to fabricate superhydrophobic Zn surface and its effect on corrosion resistance

A superhydrophobic Zn surface was prepared by the following facile approach. First, a simple method, water-only hydrothermal oxidation, was employed to fabricate loose-stacked ZnO nanorods with hexagonal cross-sections, resulting in a superhydrophilic Zn surface. Then the superhydrophilic Zn was modified with a low-surface-energy compound, changing the surface character from superhydrophilicity to superhydrophobicity. The superhydrophobic surface reduced the Zn corrosion rate to about one-eighth of that with an untreated surface. Such effective impact on corrosion resistance could be attributed to the obstructive effect and capillary effect caused by the surface superhydrophobicity.     

Comparison between the effect of nanoglass flake and montmorillonite organoclay on corrosion performance of epoxy coating

Montmorillonite organoclay (OMMT) and nanoglass flake (GF) were incorporated into epoxy resin by mechanical agitation and sonication process. Optical microscopy was used to analyse the optical homogeneity of dispersions. To investigate anticorrosive properties of nanocomposite organic coatings, salt fog test, electrochemical impedance spectroscopy (EIS), polarization measurement, pull-off adhesion and water absorption tests have been employed. The time course of impedance parameters and polarization studies show that coating corrosion resistance is improved as the amounts of OMMT and GF are increased to 3 wt%. The results indicated that nano-GF filled specimens display better corrosion performance than the OMMT filled ones.     

Effect of Mn/Mo incorporated oxide layer on the corrosion behavior of AA 2024 alloy

The oxide layer formed over AA 2024 using 10 wt.% H₂SO₄ (plain oxide, PO) was modified by Mn/Mo oxyanions (permanganate/molybdate modified oxide, PMMO) as an alternative to Cr(VI) ions to enhance the corrosion resistance. The corrosion current density values obtained for PMMO was found to be 2.8% and 1.4% of hydrothermal treated oxide (HTO) and PO respectively after 168 h immersion in 3.5% NaCl solution. The electrochemical studies showed the higher barrier layer resistance for PMMO. The improved corrosion behavior of PMMO was observed based on the damage function calculated. Similar observations were confirmed by continuous salt spray test.     

Hot corrosion behaviour of plasma sprayed YSZ/LaMgAl11O19 composite coatings in molten sulfate–vanadate salt

Hot corrosion studies of thermal barrier coatings (TBCs) with different YSZ/LaMgAl₁₁O₁₉ (LaMA) composite coating top coats were conducted in 50 wt.% Na₂SO₄ + 50 wt.% V₂O₅ molten salt at 950 °C for 60 h. Results indicate that TBCs with composite coating top coats exhibit superior oxidation and hot corrosion resistances to the TBC with the traditional YSZ top coat, especially for which has a LaMA overlay. The presence of LaMA can effectively restrain the destabilization of YSZ at the expense of its own partial degradation. The hot corrosion mechanism of LaMA coating and the composite coatings have been explored.     

Inhibitive effect of sodium eperuate on zinc corrosion in alkaline solutions

The effect of sodium eperuate prepared from Wallaba (Eperua falcata Aubl) extract on zinc corrosion was investigated in alkaline solutions with chloride ions (i.e., simulated concrete pore solutions) by using electrochemical techniques. Sodium eperuate inhibits the corrosion of zinc in 0.1 M NaCl solutions with pH 9.6. As its concentration increases to 1 g/L, the inhibition efficiency reaches approximately 92%. In alkaline solutions with pH 12.6, sodium eperuate has no adverse effect on passivity of zinc, and retards the chloride attack. These suggest that sodium eperuate is an effective inhibitor for the protection of zinc in alkaline environments.     

High temperature corrosion behaviour of a gradient NiCoCrAlYSi coating II: Oxidation and hot corrosion

The high temperature oxidation and hot corrosion behaviour of a NiCoCrAlYSi coating and a gradient coating were investigated. The gradient coating showed better performance of re-healing alumina scale due to its possession of more β phase as Al reservoir. The degradation process of the gradient coating was favorably retarded by the formation of Cr(W)-rich σ precipitates in the interdiffusion zone. The corrosion results also confirmed an improved corrosion resistance of the gradient coating. The improved high temperature performance of the gradient coating owes to the Al enrichment of Al in the outer layer.