Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

Analysis of the electrochemical reaction behavior of alloy AZ91 by EIS technique in H3PO4/KOH buffered K2SO4 solutions

The EIS technique was used to analyze the electrochemical reaction behavior of Alloy AZ91 in H₃PO₄/KOH buffered K₂SO₄ solution at pH 7. The corrosion resistance of Alloy AZ91 was directly related with the stability of Al₂O₃ · xH₂O rich part of the composite oxide/hydroxide layer on the alloy surface. The break down of the oxide layer was estimated to occur mainly on the matrix solid solution phase in Alloy AZ91. The mf capacitive loop arose from the relaxation of mass transport in the solid oxide phase in the presence of Al₂O₃ · xH₂O rich part and from Mg⁺ ion concentration within the broken area in the absence of Al₂O₃ · xH₂O rich part in the composite oxide structure on the alloy surface. The lf inductive loop had tendency of disappear when the dissolution rate of the alloy decreased as a result of the formation of the protective oxide layer.     

Corrosion behavior of AZ91D magnesium alloy in sodium sulfate solution

The corrosion behavior of as‐cast AZ91D magnesium alloy in 0.1M sodium sulfate solution at the corrosion potential (E_corr) was investigated by using electrochemical impedance spectroscopy (EIS), environmental scanning electron microscopy (ESEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffraction (XRD). The results showed that the corrosion of AZ91D started at both the primary α‐Mg and the eutectic α‐Mg. The surface first was covered by a film (MgO, Mg(OH)₂) which became thicker with time. Due to the dissolution of the eutectic α‐Mg, the concentration of aluminum increased, MgAl₂(SO₄)₄ · 2H₂O precipitated at the primary α‐Mg and progressively spread to the eutectic α‐Mg areas. The surface film changed from two‐layer to three‐layer structure with the increase of immersion time.     

Self-healing mechanism of a protective film prepared on a Ce(NO3)3-pretreated zinc electrode by modification with Zn(NO3)2 and Na3PO4

Self-healing mechanism of a protective film against corrosion of zinc at scratches in an aerated 0.5 M NaCl solution was investigated by polarization measurements, X-ray photoelectron spectroscopy (XPS) and electron-probe microanalysis (EPMA). The film was prepared on a zinc electrode by treatment in a Ce(NO₃)₃ solution and addition of aqueous solutions containing 9.98 or 19.9 μg/cm² of Zn(NO₃)₂ · 6H₂O and 55.2 μg/cm² of Na₃PO₄ · 12H₂O. After the coated electrode was scratched with a knife-edge crosswise and immersed in the NaCl solution for many hours, polarization measurements, observation of pit formation at the scratches, XPS and EPMA were carried out. This film was remarkably protective and self-healing against zinc corrosion on the scratched electrode. The cathodic and anodic processes of zinc corrosion were markedly suppressed by coverage of the surface except for scratches with a thin Ce₂O₃ layer containing a small amount of Ce⁴⁺ and the surface of scratches with a layer composed of Zn₃(PO₄)₂ · 4H₂O, Zn(OH)₂ and ZnO mostly.     

Corrosion of AZ91D magnesium alloy with a chemical conversion coating and electroless nickel layer

A chemical conversion treatment and an electroless nickel plating were applied to AZ91D alloy to improve its corrosion resistance. By conversion treatment in alkaline stannate solution, the corrosion resistance of the alloy was improved to some extent as verified by immersion test and potentiodynamic polarization test in 3.5 wt.% NaCl solution at pH 7.0. X-ray diffraction patterns of the stannate treated AZ91D alloy showed the presence of MgSnO₃ · H₂O, and SEM images indicated a porous structure, which provided advantage for the adsorption during sensitisation treatment prior to electroless nickel plating. A nickel coating with high phosphorus content was successfully deposited on the chemical conversion coating pre-applied to AZ91D alloy. The presence of the conversion coating between the nickel coating and the substrate reduced the potential difference between them and enhanced the corrosion resistance of the alloy. An obvious passivation occurred for the nickel coating during anodic polarization in 3.5 wt.% NaCl solution.     

Baking treatment effect on materials characteristics and electrochemical behavior of anodic film formed on AZ91D magnesium alloy

The composition and microstructure of the anodic films formed on AZ91D Mg alloy, with or without baking, were investigated. The associated corrosion behavior of the anodized alloy in 3.5 wt% NaCl solution was also examined using electrochemical impedance spectroscopy (EIS). The results show that MgO was the main component in the anodic film which also contained some Mg(OH)₂, Al₂O₃, Al(OH)₃, and MgAl₂O₄. Both the amorphous and crystalline forms of anodic film were identified. The degree of crystallinity depended on baking temperature, which increased with increasing temperature in the range of 50-250 °C. The amounts of MgO and Al₂O₃ increased as a result of a dehydration reaction. The polarization resistance of anodized Mg alloy was improved significantly by increasing the oxide content in the anodic film. An optimum value of polarization resistance of anodic film was obtained for the alloy baked at 150 °C for 2 h followed by air cooling.     

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.     

Corrosion behaviour of an alloy formed by inductive melting of a cobalt-based alloy and AISI 4140 steel

A bimetal of Co-based alloy/AISI 4140 steel was fabricated by induction melting. The microstructure of the Co-based alloy was examined and the influence of on the alloy of acid solutions, the temperature of these solutions, and the immersion time was investigated. The results show that the microstructure of Co-based alloy consisted of a Co-rich phase, a Cr-rich phase, and W₃CoB₃. In a solution of 20% HCl and 6% FeCl₃, the Co-rich phase was attacked to form porous channels but transformed to a Si-rich passivation film to prevent further attack in the solution of 72% H₂SO₄. Therefore, the relative corrosion resistance of Co-based alloy to acid solution is 72% H₂SO₄ > 20% HCl > 6% FeCl₃.     

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 protection of magnesium alloys by cerium, zirconium and niobium-based conversion coatings

A new Ce, Zr and Nb-based conversion coating was designed for AZ91 and AM50 magnesium alloys. The corrosion protection provided by this coating was evaluated by electrochemical measurements (polarization curves, electrochemical impedance spectroscopy) in Na₂SO₄ electrolyte, and accelerated atmospheric corrosion tests (humid, SO₂ polluted air, and salt spray). Its chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements showed that Mg alloys treated during 24 h in the Ce-Zr-Nb conversion bath exhibit: (i) increased corrosion potential, (ii) decreased corrosion and anodic dissolution current densities, and (iii) increased polarization and charge transfer resistances. The accelerated corrosion tests revealed excellent atmospheric corrosion resistance for all Ce-Zr-Nb-treated samples, with or without an additional layer of epoxy-polyamide resin lacquer or paint. XPS analysis showed that the coating includes CeO₂, Ce₂O₃, ZrO₂, Nb₂O₅, MgO, and MgF₂ as main components. No significant modification of the chemical composition was observed after cathodic and anodic polarization in Na₂SO₄. This new coating provides improved corrosion resistance, and excellent paint adhesion. It offers an alternative to the chromate conversion coating for magnesium alloys.     

Corrosion of copper and silver plates by volcanic gases

Corrosion products that had been formed on copper and silver plates exposed in Miyake Island, where suffered a volcanic eruption in 2000, were analyzed by X-ray techniques to get better understanding of copper and silver corrosion in harsh environment. The exposure experiment was carried out from September 2004 to April 2005. Many kinds of patina were found on copper such as cuprite (Cu₂O), posnjakite (Cu₄SO₄(OH)₆ · H₂O), brochantite (Cu₄SO₄(OH)₆), antlerite (Cu₃SO₄(OH)₄), and geerite (Cu₈S₅). For silver, silver chloride (AgCl) and silver sulfide (Ag₂S) were formed. Although the volcanic activity had greatly subsided, the atmospheric corrosion of copper and silver plates exposed on Miyake Island was mainly affected by volcanic gases, wet-dry cycles in the environment, and sea-salt aerosols.     

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).     

Application of some ferrocene derivatives in the field of corrosion inhibition

Three ferrocene derivatives, namely 1,1′-diacetylferrocene (Diacetyl Fc), 1,1′-diformylferrocene (Diformyl Fc) and 2-benzimidazolythioacetylferrocene (BIM Fc) were synthesized and their inhibitive effects against mild steel corrosion in aerated 0.5 M H₂SO₄ and 1 M HCl solutions were evaluated. Corrosion measurements based on polarization resistance (R_p), potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) indicate that Diacetyl Fc, in most cases, accelerates mild steel corrosion in HCl while Diformyl Fc and BIM Fc act as weak inhibitors. In H₂SO₄ solution, ferrocene derivatives show good inhibition performance. The efficiency of the inhibitors follows the order: BIM Fc > Diformyl Fc ? Diacetyl Fc. Adsorption of both Diacetyl Fc and Diformyl Fc obey Langmuir adsorption isotherm with very low value of free energy of adsorption ΔG^° for the Diformyl Fc (physisorption) while adsorption of BIM Fc follows that of Frumkin with high negative value of ΔG^° (chemisorption). Both Diformyl Fc and BIM Fc act as mixed-type inhibitors with predominant effect on the anodic dissolution of iron. Analysis of the polarization curves and impedance spectra indicates that charge transfer process mainly controls mild steel corrosion in H₂SO₄ solution without and with ferrocene compounds. The mechanism of corrosion inhibition or acceleration by ferrocene derivatives was discussed in the light of the molecular structure of the additives.     

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.     

Copper corrosion inhibition in O2-saturated H2SO4 solutions

Corrosion inhibition of copper in O₂-saturated 0.50 M H₂SO₄ solutions by four selected amino acids, namely glycine (Gly), alanine (Ala), valine (Val), or tyrosine (Tyr), was studied using Tafel polarization, linear polarization, impedance, and electrochemical frequency modulation (EFM) at 30 °C. Protection efficiencies of almost 98% and 91% were obtained with 50 mM Tyr and Gly, respectively. On the other hand, Ala and Val reached only about 75%. Corrosion rates determined by the Tafel extrapolation method were in good agreement with those obtained by EFM and an independent chemical (i.e., non-electrochemical) method. The chemical method of confirmation of the corrosion rates involved determination of the dissolved Cu²⁺, using ICP-AES (inductively coupled plasma atomic emission spectrometry) method of chemical analysis. Nyquist plots exhibited a high frequency depressed semicircle followed by a straight line portion (Warburg diffusion tail) in the low-frequency region. The impedance data were interpreted according to two suitable equivalent circuits. The kinetics of dissolved O₂ reduction and hydrogen evolution reactions on copper surface were also studied in O₂-saturated 0.50 M H₂SO₄ solutions using polarization measurements combined with the rotating disc electrode (RDE). The Koutecky-Levich plot indicated that the dissolved O₂ reduction at the copper electrode was an apparent 4-electron process.     

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.     

2,2′-Dithiobis(3-cyano-4,6-dimethylpyridine): A new class of acid corrosion inhibitors for mild steel

The title compound (PyS)₂ has been synthesized and its inhibiting action on the corrosion of mild steel in 1-5 M H₂SO₄ solutions at 35-50 °C has been investigated by polarization resistance (R_p), polarization curves and electrochemical impedance spectroscopy (EIS). (PyS)₂ showed excellent performance and its efficiency did not affect either by increasing the acid concentration or rise of temperature. Polarization curves indicated that (PyS)₂ behaves mainly as anodic inhibitor in 1 M H₂SO₄ solutions and as a mixed-type inhibitor in 3 and 5 M H₂SO₄ solutions at different temperatures. Adsorption of (PyS)₂ on the steel surface followed Temkin’s adsorption isotherm with a very high negative value of the free energy of adsorption . The activation parameters of the corrosion process were calculated. EIS showed that the charge transfer controls the corrosion process in the uninhibited and inhibited solutions.     

Atmospheric corrosion of field-exposed AZ91D Mg alloys in a polluted environment

The atmospheric corrosion behaviour of cast and high pressure die-cast AZ91D alloy in a polluted environment were investigated by SEM, XRD and FTIR. The atmospheric corrosion rate of the ingot was higher than that of the die-cast specimen. SO₂ gas played an important role in atmospheric corrosion. The effects of dew condensation on the ingot were greater than that on the die-cast specimen. The corrosion was initiated in the less noble α phase of the samples. The β phase, on the other hand, remained and acted as a barrier to corrosion.     

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.     

Corrosion product formation on Zn55Al coated steel upon exposure in a marine atmosphere

The formation of corrosion products on Zn55Al coated steel has been investigated upon field exposures in a marine environment. The corrosion products consisted mainly of zinc aluminium hydroxy carbonate, Zn_0.71Al_0.29(OH)₂(CO₃)_0.145·xH₂O, zinc chloro sulfate (NaZn₄(SO₄)Cl(OH)₆·6H₂O), zinc hydroxy chloride, Zn₅(OH)₈Cl₂·H₂O and zinc hydroxy carbonate, Zn₅(OH)₆(CO₃)₂ were the first three phases were formed initially while zinc hydroxy carbonate Zn₅(OH)₆(CO₃)₂ was formed after prolonged exposure in more corrosive conditions. The initial corrosion product formation was due to selective corrosion of the zinc rich interdendritic areas of the coating resulting in a mixture of zinc and zinc aluminium corrosion products.