Electrochemical corrosion behaviour of microcrystalline aluminium in acidic solutions

The electrochemical corrosion behaviour of microcrystalline pure aluminium coating, fabricated by a magnetron sputtering technique, has been investigated in both 0.5 mol/l NaCl and 0.5 mol/l Na₂SO₄ acidic (pH = 2) aqueous solutions. The corrosion resistance of the microcrystalline Al coating has deteriorated more compared with that of the cast pure Al in Na₂SO₄ acidic solution. However, its oxide film has a higher pitting resistance in the NaCl acidic solution. Chloride ions play a big role in the formation of the oxide film on the microcrystalline Al coating. The higher pitting resistance was attributed to the more acidic isoelectric point which the oxide film achieved.     

Effects of dithionite ion on corrosion of type 304 stainless steel in sulphuric acid solution

Corrosion behaviour of type 304 stainless steel was investigated, with particular attention to additive effects of hydrosulphite (Na₂S₂O₄) on corrosion in 0.1 mol/l H₂SO₄ solution with various amounts of Na₂S₂O₄ up to 60 mmol/l.Corrosion of SUS304 occurred below pH 3.0 at 30 °C in a 0.1 mol/l H₂SO₄ solution in which Na₂S₂O₄ was added to 0.1-20 mmol/l. The maximum corrosion rate at 30 °C was measured as 7.2 g/m² h (7.9 mm/y) in 0.1 mol/l H₂SO₄-10 mmol/l Na₂S₂O₄ at pH 1.2. Microscopic surface observation revealed that active dissolution was accompanied by intergranular corrosion at the metal surface.The SUS304 was easily passivated in 0.1 mol/l H₂SO₄ solution with more than 30 mmol/l Na₂S₂O₄. NiS was detected in the passivated film.     

Corrosion behaviour of die-cast AZ91D magnesium alloy in aqueous sulphate solutions

The corrosion behaviour of die-cast AZ91D magnesium alloys in sulphate solutions was investigated by SEM, FTIR and polarization measurements. For immersion times less than 48 h, no pitting corrosion occurred and only generalized corrosion was apparent. According to the polarization curves, the corrosion rate order of the die-cast AZ91D Mg alloy in three aqueous solutions was: NaCl > MgSO₄ > Na₂SO₄. The main corrosion products were Mg(OH)₂ and MgAl₂(SO₄)₄·22H₂O in the sulphate solutions and the product film was compact. Precipitation of MgAl₂(SO₄)₄·22H₂O required a threshold immersion time.     

Corrosion behavior of alumina ceramics in aqueous HCl and H2SO4 solutions

The corrosion behavior of cold isostatically pressed (CIP) high purity alumina ceramics in aqueous HCl and H₂SO₄ solutions with various concentrations has been studied simultaneously at room temperature (25 °C). Corrosion tests were also performed with 0.65 mol/l HCl and 0.37 mol/l H₂SO₄ solutions at 40, 55 and 70 °C for 48 h. Chemical stability was monitored by determining the amount of Al³⁺, Mg²⁺, Ca²⁺, Na⁺ Si⁴⁺ and Fe³⁺ ions eluted in different concentrations of HCl and H₂SO₄ solutions by means of atomic absorption spectrometry (AAS). By increasing the concentration from 0.37 to 6.5 mol/l, it was notified that the corrosion susceptibility in HCl and H₂SO₄ solutions for the CIP alumina specimens at room temperature decreases.     

Inhibiting effects of butyl triphenyl phosphonium bromide on corrosion of mild steel in 0.5 M sulphuric acid solution and its adsorption characteristics

Butyl triphenyl phosphonium bromide (BuTPPB) has been evaluated as a corrosion inhibitor for mild steel in 0.5 M H₂SO₄ solutions using galvanostatic polarisation and potentiostatic polarisation measurements. The study was also complemented by infra red (IR) spectroscopy, scanning electron microscopy (SEM) and quantum chemical calculations. Galvanostatic polarisation measurements showed that the presence of BuTPPB in aerated 0.5 M H₂SO₄ solutions decreases corrosion currents to a great extent and the corrosion rate decreases with increasing inhibitor concentration at a constant temperature. At 298K, inhibition efficiency was found to be 94.5% for 10⁻⁷ M BuTPPB which increased to about 99% for the BuTPPB concentration of 10⁻² M. The effect of temperature on the corrosion behaviour of mild steel was studied at five different temperatures ranging from 298 to 338K. The polarisation curves clearly indicate that BuTPPB acts as a mixed type inhibitor. Adsorption of BuTPPB on the mild steel surface follows the Langmuir isotherm.Potentiostatic polarisation measurements showed that passivation was observed only for lower BuTPPB concentrations (10⁻⁵ and 10⁻⁷ mol l⁻¹) for the mild steel in 0.5 M H₂SO₄. IR and SEM investigations also confirmed the adsorption of BuTPPB on the mild steel surface in 0.5 M H₂SO₄ solutions. The molecular parameters obtained using PM3 semi-empirical method, were correlated with the experimentally measured inhibitor efficiencies.     

Effect of ZrO2 on corrosion behaviour of chromium coatings

To study the effect of ZrO₂ particles on corrosion behaviour of Cr coating, steel samples were plated in Cr(VI) baths without and with ZrO₂. The corrosion behaviour of plated samples was studied at different exposure times in a solution containing 0.01 mol l⁻¹ H₂SO₄ + 0.5 mol l⁻¹ Na₂SO₄ using cyclic voltammetry and impedance spectroscopy. The equivalent circuit model R_e(Q_cR_pore)(Q_s[OR_s]) was proposed to fit the corrosion process and the parameters Y₀(Q_c),Y₀(Q_s) and R_pore reflecting corrosion behaviour of samples were evaluated. From the results, it was found that samples plated in bath containing ZrO₂ exhibited improved protective properties as a result of the structural characteristics of the coatings obtained; namely, the size and shape of pores.     

Hot corrosion behaviour of low Al NiCoCrAlY cladded coatings reinforced by nano-particles on a Ni-base super alloy

Three NiCoCrAlY coatings with Al content lower than 5 wt.% reinforced by different kinds of nano-particles with the same addition and one without nano-particles were prepared on a Ni-base super alloy using laser cladding technique. Hot corrosion of the NiCoCrAlY coatings in Na₂SO₄/K₂SO₄ (75:25, wt./wt.) mixture was performed at 1050 °C in static air. Results indicate that the hot corrosion resistance of the coatings with nano-particles is better than that of the one without nano-particles, among which the one with nano-CeO₂ presented the best hot corrosion resistance. Effects of nano-particles on the hot corrosion behaviour were also discussed.     

Evaluation of hydrogen absorption behaviour during acid etching for surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys

The hydrogen absorption behaviour during acid etching for the surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys has been investigated on the basis of the surface morphology, electrochemical behaviour and hydrogen thermal desorption analysis. To simulate the conventional acid etching for the improvement of the biocompatibility of Ti alloys, the specimens are immersed in 1 M HCl, 1 M H₂SO₄ or 0.5 M HCl + 0.5 M H₂SO₄ aqueous solution at 60 °C. Upon immersion, commercial pure Ti absorbs substantial amounts of hydrogen irrespective of the type of solution. In H₂SO₄ or HCl + H₂SO₄ solutions, the hydrogen absorption occurs for a short time (10 min). For Ti-6Al-4V alloy, no hydrogen absorption is observed in HCl solution, whereas hydrogen absorption occurs in other solutions. For Ni-Ti superelastic alloy, the amount of absorbed hydrogen is large, resulting in the pronounced degradation of the mechanical properties of the alloy even for an immersion time of 10 min, irrespective of the type of solution. The hydrogen absorption behaviour is not necessarily consistent with the morphologies of the surface subjected to corrosion and the shift of the corrosion potential. The hydrogen thermal desorption behaviour of commercial pure Ti and Ni-Ti superelastic alloy are sensitively changed by acid etching conditions. The present results suggest that the evaluation of hydrogen absorption is needed for each condition of acid etching, and that the conventional acid etching often leads to hydrogen embrittlement.     

Formation and breakdown of surface films on magnesium and its alloys in aqueous solutions

The influences of surface films formed by open-circuit exposure to neutral solutions on the corrosion and electrochemical behaviour of pure Mg and Mg alloys have been examined by in situ ellipsometric analysis and electrochemical measurements. Surface films mainly composed of Mg(OH)₂ grew rapidly during open-circuit exposure to 0.1 M NaCl and 0.1 M Na₂SO₄ solutions. These films had protective ability to passivate Mg in the solutions. However, they suffered local breakdown under anodic polarisation. The passive current density decreased and the breakdown potential increased with increasing immersion time and film thickness. Influences of purity and alloying elements on the passivity and its breakdown of Mg have been discussed.     

Hot corrosion of YSZ/Al2O3 dispersed NiCrAlY plasma-sprayed coatings in Na2SO4-10 wt.% NaCl melt

This study examines the effect of ytrria stabilized zirconia (YSZ) dispersion on hot corrosion behaviour of NiCrAlY bond coat. Hot corrosion studies were conducted on NiCrAlY and NiCrAlY containing 25, 50 and 75 wt.% YSZ coatings obtained through the air plasma spray technique, in Na₂SO₄ + 10 wt.% NaCl environment at 800 °C. The results show that YSZ dispersion lowers the overall hot corrosion tendency of the NiCrAlY, though it enhances the inherent hot corrosion tendency of its metallic constituent (NiCrAlY). Furthermore, there exists a threshold oxide level beyond which it adversely affects the hot corrosion of the coating.     

The corrosion behaviour of WC-VC-Co hardmetals in acidic media

The effect of increasing vanadium carbide (VC) content on the corrosion behaviour of tungsten carbide - 10 wt% cobalt hardmetals was investigated in 1 M hydrochloric (HCl), and sulphuric (H₂SO₄) acids solutions. Increasing VC content makes the open circuit potential (OCP) in the test solutions more negative than the base alloy. Specimens exhibited pseudo passivation in all the test solutions. Increasing VC led to decreasing corrosion current density. However, the corrosion current densities during chronoamperometric tests were lower for 0 wt% VC. XRD and Raman spectroscopy showed that hydrated WO₃ formed in the surface films of all specimens in hydrochloric acid (HCl), while hydrated vanadyl sulphate also formed for higher VC content specimens in sulphuric acid (H₂SO₄).     

The influence of CO2, AlCl3 · 6H2O, MgCl2 · 6H2O, Na2SO4 and NaCl on the atmospheric corrosion of aluminum

The influence of salt deposits on the atmospheric corrosion of high purity Al (99.999%) was studied in the laboratory. Four chloride and sulfate-containing salts, NaCl, Na₂SO₄, AlCl₃ · 6H₂O and MgCl₂ · 6H₂O were investigated. The samples were exposed to purified humid air with careful control of the relative humidity (95%), temperature (22.0 °C), and air flow. The concentration of CO₂ was 350 ppm or <1 ppm and the exposure time was four weeks. Under the experimental conditions all four salts formed aqueous solutions on the metal surface. Mass gain and metal loss results are reported. The corroded surfaces were studied by ESEM, OM, AES and FEG/SEM equipped with EDX. The corrosion products were analyzed by gravimetry, IC and grazing incidence XRD. In the absence of CO₂, the corrosivity of the chloride salts studied increases in the order MgCl₂ · 6H₂O < AlCl₃ · 6H₂O < NaCl. Sodium chloride is very corrosive in this environment because the sodium ion supports the development of high pH in the cathodic areas, resulting in alkaline dissolution of the alumina passive film and rapid general corrosion. The low corrosivity of MgCl₂ · 6H₂O is explained by the inability of Mg²⁺ to support high pH values in the cathodic areas. In the presence of carbon dioxide, the corrosion induced by the salts studied exhibit similar rates. Carbon dioxide strongly inhibits aluminum corrosion in the presence of AlCl₃ · 6H₂O and especially, NaCl, while it is slightly corrosive in the presence of MgCl₂ · 6H₂O. The corrosion effects of CO₂ are explained in terms of its acidic properties and by the precipitation of carbonates. In the absence of CO₂, Na₂SO₄ is less corrosive than NaCl. This is explained by the lower solubility of aluminum hydroxy sulfates in comparison to the chlorides. The average corrosion rate in the presence of CO₂ is the same for both salts. The main difference is that sulfate is less efficient than chloride in causing pitting of aluminum in neutral or acidic media.     

An ultrathin polymer coating of carboxylate self-assembled monolayer adsorbed on passivated iron to prevent iron corrosion in 0.1 M Na2SO4

For preparing an ultrathin two-dimensional polymer coating adsorbed on passivated iron, a 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer (SAM) was modified with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. Protection of passivated iron against passive film breakdown and corrosion of iron was investigated by monitoring of the open-circuit potential and repeated polarization measurements in an aerated 0.1 M Na₂SO₄ solution during immersion for many hours. The time required for passive film breakdown of the polymer-coated electrode was markedly higher in this solution than that of the passivated one, indicating protection of the passive film from breakdown by coverage with the polymer coating. The protective efficiencies of the passive film covered with the coating were extremely high, more than 99.9% in 0.1 M Na₂SO₄ before the passive film was broken down, showing prominent cooperative suppression of iron corrosion in the solution by coverage with the passive film and polymer coating. The polymer-coated surface was characterized by contact angle measurement and electron-probe microanalysis (EPMA). Prevention of passive film breakdown and iron corrosion for the polymer-coated electrode healed in 0.1 M NaNO₃ was also examined in 0.1 M Na₂SO₄.     

Corrosion study of HK-40m alloy exposed to molten sulfate/vanadate mixtures using the electrochemical noise technique

Corrosion performance of HK-40m alloy obtained from electrochemical noise technique and polarization curves during 24 h of exposure in high sulfate (80 mol% Na₂SO₄-20 mol% V₂O₅) and high vanadate (80 mol% V₂O₅-20 mol% Na₂SO₄) molten salts at 700 °C are reported. Electrochemical noise signals were analyzed in the time and frequency domain. A statistical analysis obtaining the resistance noise, the current standard deviation and the localization index are presented as well as the determination of corrosion rates. Corrosion rates were supported by X-ray diffraction analysis of corrosion products and scanning electron microscopy analysis of corroded samples. Results from optical microscope examination of the corroded samples showed that HK-40m alloy suffered inter-granular corrosion when was exposed to the high vanadate salt, whereas exposed to the high sulfate salt, HK-40m corroded through a mixed corrosion process. A corrosion mechanism of HK-40m alloy was obtained together with the corrosion rate, showing the different behavior when exposing the alloy to a high vanadate and high sulfate molten salts.     

A comparative study on the corrosion behaviour of 304 austenitic stainless steel in sulfamic and sulfuric acid solutions

Potentiodynamic polarization and EIS have been employed to compare the corrosion behaviour of 304 stainless steel in NH₂SO₃H and H₂SO₄ solutions. Corrosion tests were carried out as a function of the acid’s concentration (0.1-0.5 M) and solution temperature (20-60 ^oC). The corrosion rate is higher in H₂SO₄ than in H₂NSO₃H in all concentrations and temperatures. Values of the activation energy (E_a) revealed that the corrosion process is faster in H₂SO₄ than in NH₂SO₃H solution. EIS data showed that the display of Nyquist plots, and hence the mechanism of corrosion, depends not only on the acid concentration but also on the solution temperature. In 0.1 M concentration, the equivalent circuits R_e(R_ctQ_dl) and R_e(R_ctQ_dl)(RQ)_ads describe the corrosion systems in H₂NSO₃H and H₂SO₄ solutions respectively. At concentrations ?0.2 M, the equivalent circuit R_e(R_ctQ_dl)Q_diff is applicable. Adsorption of the counter ion of the acid on the steel surface and the stability of the surface complex may explain the observed corrosion rates.     

Corrosion evaluation and surface characterization of the corrosion product layer formed on Cu-6Sn bronze in aqueous Na2SO4 solution

The binary bronze alloy Cu-6Sn corrosion, and formation and properties of corrosion product layer (patinas) during 12 days of exposure to 15 mM Na₂SO₄ aqueous solution were investigated by a range of diverse experimental techniques. For the reasons of comparison, some techniques were applied, in parallel, to copper. Gravimetric measurements revealed lower corrosion rates of bronze than those of copper, probably caused by the presence of tin compounds in the corrosion product layer. Cyclic voltammetry results showed that the oxidation processes on bronze are affected by the formation of tin oxide species. Electrochemical impedance spectroscopy showed that, as opposed to copper which produced only two time constants, bronze corrosion resistance was dominated by the additional high-frequency time constant representing redox processes occurring at the corrosion product surface. SEM, ATR FTIR and PIXE results suggest that Cu-6Sn bronze corrosion in 15 mM Na₂SO₄ solution was impeded by the formation of two-layered structure of corrosion products that formed due to selective dissolution of copper at the layer/solution interface, leaving the outer layer enriched in highly corrosion resistant Sn oxi/hydrohide species.     

Influence of fluoride ions on the amorphous phosphating of aluminium alloys

The influence of the F⁻ ions containing compounds: NaF, HBF₄, H₂SiF₆, NH₄F·HF and their concentrations on formation of phosphate coatings on Al in МоО₄²⁻ ions containing solutions, as well as protective properties of the phosphate coatings in a 0.5 M Na₂SO₄ solution have been studied. The studies of film composition by the XPS method have shown, that the phosphate coating consists of the metal phosphates from aluminium alloy and the Mo (IV, V) phosphates, which are formed during reduction of МоО₄²⁻ ions. The mass of phosphate layer and that of etched metal depend on the nature of fluoride ions, which can be arranged in the following order according to the decrease in strength of their influence: H₂SiF₆ > NH₄F·HF > HBF₄ > NaF. Polarization measurements in a 0.5 M Na₂SO₄ solution and the calculated electrochemical parameters testify that phosphated Al samples exhibit a lower corrosion rate and a higher corrosion resistance as compared to non-phosphated Al substrate.     

The effect of H2S concentration on the corrosion behavior of carbon steel at 90 °C

The electrochemical behavior of SAE-1020 carbon steel in 0.25 M Na₂SO₄ solution containing different concentrations of H₂S at 90 °C was investigated using the methods of weight loss, electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the corrosion rate of carbon steel increased significantly with the increase of H₂S concentration. H₂S accelerated the corrosion rate of SAE-1020 carbon steel by a promoted hydrogen evolution reaction. Severe corrosion cavities were observed on the carbon steel surface in the solutions containing H₂S due to cementites stripped off from the grain boundary. The loose corrosion products formed on the steel surfaces were composed of mackinawite.     

Electrochemical behaviour of chromium-implanted magnesium in hydroxide, chloride and sulphate solutions

Despite the development and subsequent improvement of new Mg-based alloys, their vulnerability to oxidation and corrosion continues to pose a major obstacle to their more generalized use. The possibility of blocking high diffusivity paths, such as grain boundaries, by ion implantation may help to improve their oxidation and corrosion resistance, because mass transport through these short circuiting paths is reduced.Electrochemical techniques were used to investigate the effect of Cr ion implantation in the electrochemical behaviour of Mg in aqueous solutions. In particular ion implanted fluences of 5 × 10¹⁶ and 5 × 10¹⁷ at./cm² have been evaluated in solutions of NaCl, NaOH and Na₂SO₄.The corroded surfaces and products were analysed by ion beam analysis, scanning electron microscopy and X-ray diffraction. A model of the corrosion mechanism is proposed to explain the obtained results.     

Effect of surface finishing of a Zr-based bulk metallic glass on its corrosion behaviour

Zr-based metallic glasses passivate spontaneously, but exhibit also a certain pitting susceptibility. On the example of the Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy studied in 0.01 M Na₂SO₄ + x M NaCl (x = 0-0.1) electrolytes it is demonstrated that the surface finishing state and the pre-exposure conditions can significantly influence the free corrosion and anodic polarisation behaviour. Mechanical fine-polishing procedures can lead to extremely smooth topographies but also to Cu enrichment at the surface. This yields a pronounced Cu dissolution at low anodic polarisation prior to stable passivity and increases the pitting initiation susceptibility as compared to mechanically ground surface states.