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.     

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.     

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.     

Environmental factors affecting the corrosion behaviour of reinforcing steel. V. Role of chloride and sulphate ions in the corrosion of reinforcing steel in saturated Ca(OH)2 solutions

The corrosion behaviour of reinforcing steel in saturated naturally aerated Ca(OH)₂ solutions in absence and presence of different concentrations of NaCl, NH₄Cl, Na₂SO₄ and (NH₄)₂SO₄ is followed by measuring of the open circuit potential complemented with SEM and EDS investigation. These salts cause breakdown of passivity and initiation of pitting corrosion. The rates of oxide film thickening by OH⁻ ions and oxide film destruction by the aggressive ions follow a direct logarithm law and depend on the concentration and type of aggressive salts anions and cations. The values of the activation energies for oxide film thickening are calculated and discussed.     

Surface oxidation and/or corrosion behavior of glass-like carbon in sulfuric and nitric acids, and in aqueous hydrogen peroxide

Surface oxidation and/or corrosion behavior of glass-like carbons (GLCs) in concentrated H₂SO₄, HNO₃ and H₂O₂aq. were investigated with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and by weight changes. With GLCs treated in H₂SO₄, formation of sulfur-containing chemical species was observed, with some weight gain. However, no change was observed in morphology. With the samples treated in HNO₃, the weight initially increased and, then, decreased. No nitrogen-containing chemical species were observed. Crack formation and scaling off of the upper layers were observed. With the samples treated in H₂O₂aq., rapid weight loss and void formation were observed.     

Continuous and cyclic oxidation of T91 ferritic steel under steam

The oxidation behaviour of T91 ferritic steel in steam has been studied under isothermal and non-isothermal conditions within a temperature range between 575 and 700 °C. Isothermal treatments resulted in parabolic oxidation kinetics. Three clearly defined oxide layers constituted the oxide scales. The innermost layer was a (Fe,Cr)₃O₄. The intermediate layer was porous magnetite (Fe₃O₄) followed by a compact thinner layer of hematite (Fe₂O₃). Under non-isothermal conditions the oxide scales were irregular and evidently cracked. An increase of the oxidation temperature produces an acceleration of the oxidation process, causing an increase of the oxide scale thickness that depends on the temperature increase and the exposure time.     

Effects of blending on surface characteristics of copper corrosion products in drinking water distribution systems

Copper scales formed over 6-months during exposure to ground, surface and saline waters were characterized by EDS, XRD and XPS. Scale color and hardness were light red-brown-black/hard for high alkalinity and blue-green/soft for high SO₄ or Cl waters. Cl was present in surface or saline copper scales. The Cu/Cu₂O ratio decreased with time indicating an e transfer copper corrosion mechanism. Cu₂O, CuO, and Cu(OH)₂ dominated the top 0.5-1 A° scale indicating continuous corrosion. Cu₂O oxidation to CuO increased with alkalinity, and depended on time and pH. Total copper release was predicted using a Cu(OH)₂ model.     

Oxidation behavior of molten magnesium in atmospheres containing SO2 and air in a sealed furnace

The microchemistry and morphology of the films formed on molten magnesium in atmospheres containing SO₂ and air in a sealed furnace were examined. Based on the results and thermodynamic calculations, a protection mechanism of molten magnesium is presented. The protection effects are closely related to the effective pressure of SO₃ in the atmosphere. When the effective pressure of SO₃ was higher than the equilibrium SO₃ pressure of MgSO₄, MgSO₄ could form on the melt surface and promote the formation of protective surface film. The film with network structure mainly contained MgO and MgS and had different layers.     

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 an alumina forming austenitic steel exposed to supercritical carbon dioxide

Microstructure characterization of corrosion behavior of an alumina forming austenitic (AFA) steel exposed to supercritical carbon dioxide was conducted at 450–650 °C and 20 MPa. At low temperature and short exposure times, the oxidation kinetics were parabolic and the oxide scales were mainly composed of protective and continuous Al₂O₃ and (Cr, Mn)-rich oxide layers. As the temperature and exposure time increased, the AFA steel gradually suffered breakaway oxidation and its oxide scales showed a multilayer structure mainly composed of Fe₃O₄, (Cr, Fe)₃O₄, NiFe/FeCr₂O₄/Cr₂O₃/Al₂O₃, FeCr₂O₄/Al₂O₃, and NiFe/Cr₂O₃/Al₂O₃, in sequence. The corrosion mechanism based on the microstructure evolution is discussed in detail.     

The semiconducting properties of passive films formed on AISI 316 L and AISI 321 stainless steels: A test of the point defect model (PDM)

The semiconductor properties of passive films formed on AISI 316L in 1 M H₂SO₄ in three temperatures and AISI 321 in 0.5 M H₂SO₄ were studied by employing Mott–Schottky analysis in conjunction with the point defect model (PDM). Based on the Mott–Schottky analysis in conjunction with PDM, it was shown that the calculated donor density decreases exponentially with increasing passive film formation potential. Also, the results indicated that donor densities increased with temperature. By assuming that the donors are oxygen ion vacancies and/or cation interstitials, the diffusion coefficients of the donors for two stainless steels are calculated.     

Differences between corrosion products formed on copper exposed in Tokyo in summer and winter

We analyzed the copper corrosion products that formed during a month in summer and a month in winter at three sites in Tokyo using several analytical techniques. The X-ray diffraction patterns revealed that cuprite Cu₂O and posnjakite Cu₄SO₄(OH)₆·H₂O formed on copper exposed in summer. By contrast, only cuprite was found in winter exposed copper. The X-ray fluorescence results indicated that the amounts of sulfur and chlorine on the copper plates exposed in summer were much greater than those in winter. This could be explained by the change in particulate sulfate and sea salt concentrations. Depth profiling analysis by Auger electron spectroscopy revealed that the oxide layer formed in summer was thicker than that in winter. This difference in oxide layer thickness could have been due to the differences in temperature, relative humidity, and the amount of sulfur and chlorine on the copper plate.     

Oxidation of 316 stainless steel in supercritical water

The oxide scales of 316 stainless steel (316 SS) have been examined after exposure to supercritical water (SCW) with 2.0% H₂O₂ for up to 250 h. The exposed samples were analyzed using weight measurement, scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD). It was found that mass gain of all samples increased with increasing temperature and exposure time. Higher temperature SCW resulted in rougher surfaces and thicker oxide scales. Duplex layer oxide structures with Ni-enrichment at the oxide/metal interface developed on all samples exposed to SCW, which were identified as Fe₂O₃/Fe₃O₄ + spinel/Cr₂O₃/Ni-enrichment/316 SS from the outer to inner layer. The possible oxidation mechanisms are also discussed.     

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.     

Corrosion behaviour of Ni-Zr-Ti-Si-Sn amorphous plasma spray coating

Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ amorphous material was deposited by a vacuum plasma spraying technique onto steel and copper substrates in order to investigate their behaviour in a corrosive environment. For comparison, the same alloy was prepared as amorphous ribbons by melt spinning. The amorphous nature of the coatings and ribbons was characterized by XRD, DSC and TEM, while XPS and AES analyses were performed to understand the origin of passivation and mode of corrosion. The corrosion behaviour of the coating was studied in H₂SO₄ and HCl solutions open to air at room temperature. Potentiodynamic polarisation and galvanic coupling tests were carried out on the substrate and the coating. It was found that the formation of Zr-, Ti- and Si-rich passive oxide layers provide a high corrosion resistance in H₂SO₄ solution while the breakdown of the passive layer by chloride ion adsorption was responsible for pitting corrosion of the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ amorphous ribbons in HCl solution. Galvanic corrosion was the dominant corrosion mechanism for the coating/copper hybrid structure, in contrast to the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ amorphous coating, which efficiently protected the steel substrate in the corrosive environment.     

Influence of silicate ions on the formation of goethite from green rust in aqueous solution

We investigated the influence of silicate ions on the formation of goethite converted from hydroxysulphate green rust, which was synthesized by neutralizing mixted solution of Fe₂(SO₄)₃ and FeSO₄ with NaOH solution, by O₂ in an aqueous solution. The pH and oxidation-reduction potential of the suspension and the Fe and Si concentrations in supernatant solutions were analyzed. X-ray diffraction results for the solid particles formed during the conversion were consistent with the results of the solution analyses. The results indicated that silicate ions suppressed the conversion from green rust to α-FeOOH and distorted the linkages of FeO₆ octahedral units in the α-FeOOH structure.     

Electrolytic pickling of the oxide layer on hot-rolled 304 stainless steel in sodium sulphate

Electrolytic pickling of hot-rolled 304 stainless steel in Na₂SO₄ electrolyte was investigated with electrochemical, weight loss and SEM-EDX measurements. Pickling took place upon both active and transpassive polarisations. Mechanism and kinetics of pickling during active and transpassive polarisations were unravelled. Metallic phase in oxide layer was dissolved during active polarisation while the oxide layer was significantly undercut. Chromium oxide in the oxide layer was oxidised to soluble anions during transpassive polarisation, while iron oxide and metallic phase could either be remained or removed. Pickling due to undercutting with active polarisation was highly pronounced.     

Electrodissolution of 304 stainless steel in neutral electrolytes for surface decontamination applications

Brightening of 304 stainless steel in concentrated electrolytes of Na₂SO₄, Li₂SO₄, NaNO₃, LiNO₃, NH₄NO₃, and Ca(NO₃)₂ was studied. It is observed that the current efficiency and quality of the surface depend on the electrolyte concentration, ions present in the electrolyte and the operating current density. While sulfate electrolytes etch the electrodes, oxidizing electrolytes generally yield brightened surfaces. Surface brightening is achieved at the lowest current density in LiNO₃/Na₂Cr₂O₇ composite electrolyte at a pH of 5.2. The results are explained on the basis of salt film formation near the specimen surface.     

Corrosion resistance of the Ti–50Zr at.% alloy after anodization in different acidic electrolytes

The stability of oxide films potentiodynamically (50 mV s⁻¹) grown up to 8.0 V(SCE) on Ti–50Zr at.% in H₂SO₄, HNO₃, CH₃SO₃H, and H₃PO₄ (pH ≈ 1) was assessed in the growth electrolyte itself or in a Ringer solution. For all anodizing electrolytes, oxide films become stabler as their thickness increases. In the Ringer solution, the oxide film stability is affected by the anodizing electrolyte; for oxides grown up to 8.0 V(SCE), films obtained in H₃PO₄ are slightly less stable than the ones obtained in the other acids, whereas films obtained in H₂SO₄ are clearly the stablest ones.     

Characterization of microstructure and corrosion properties of cold worked Alloy 800

X-ray diffraction studies indicated that cold worked (∼50%) Alloy 800 was austenitic and transmission electron microscopy revealed the presence of a small volume fraction of hexagonal ε-martensite along with deformation bands, high dislocation density and primary TiN particle with a few dislocations within it. The passivity of cold worked alloy was very stable in H₂SO₄ solution but unstable in HCl solution at room temperature. The exposure of cold worked alloy in 673 K steam (initial pH of water was 10.1) for a period of 264 h showed almost nil corrosion rate. Scanning electron microscopy revealed a number of small oxide particles on the surface exposed in steam indicating initiation of oxide formation. Energy dispersive X-ray analyses of the surface containing small oxide particles indicated that the surface composition was similar to bulk composition of the alloy. X-ray photoelectron spectroscopy revealed that the alloy surface exposed in steam contained mixed oxides of iron and chromium as well as elemental form of iron, nickel and chromium.