Cavitation erosion behaviour of 20SiMn low alloy steel in Na2SO4 and NaHCO3 solutions

Most of the former studies on cavitation erosion (CE) in corrosive media were carried out in NaCl solutions. In contrast, the effects of and ions, which are dominant anions in most rivers of China, on cavitation erosion have not been investigated systematically. In this paper, the cavitation erosion behaviour of 20SiMn low alloy steel in Na₂SO₄ and NaHCO₃ solutions was investigated by using a magnetostrictive-induced CE facility. The micrographs of damaged surfaces were observed by scanning electron microscope (SEM). It was found that the CE rate of 20SiMn low alloy steel in 0.003% Na₂SO₄ solution was higher than that in 0.003% NaCl solution, but in 0.03%, 0.3%, and 3% solutions their CE rate was almost the same. The CE rate of 20SiMn low alloy steel in 0.003-3% NaHCO₃ solutions was higher than that in the equivalent 0.003-3% Na₂SO₄ and 0.003-3% NaCl solutions, which may be related to stress corrosion cracking or corrosion fatigue mechanism in NaHCO₃ solutions. The role of corrosion was analyzed by using polarization curves, electrochemical impedance spectroscopy (EIS), linear polarization resistance and the corrosion potential with or without cavitation. Independent of the test media, cavitation shifted the corrosion potential in the positive direction and strongly enhanced the cathodic current density in the polarization curves. In the interaction of cavitation erosion and corrosion, the corrosion-induced erosion component was predominant. Cavitation also greatly reduced the magnitude of impedance though the initial impedance under cavitation conditions in 3% NaHCO₃ solution was almost one order larger than that in 3% Na₂SO₄ solution.     

Corrosion behaviour of aluminium in copper containing environment

Corrosion behaviour of pure aluminium galvanically connected to metallic copper or in the presence of Cu²⁺ ions was investigated by electrochemical measurements in Na₂SO₄ and Na₂SO₄ + NaCl test solutions. It has been found that in aerated Cl⁻ ion containing solutions pitting corrosion of aluminium emerged immediately, while in the absence of oxygen this process was less violent. Effect of passivating pre-treatment of aluminium surface on corrosion behaviour Cu-Al bimetallic system is also demonstrated.     

A novel phosphate conversion film on Mg-8.8Li alloy

The super light Mg-Li alloys exhibit excellent formability due to the addition of lithium, but the corrosion resistance is deteriorated. A novel conversion film is developed to improve the corrosion resistance. The surface morphology of conversion film was observed using scanning electron microscopy (SEM). The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The corrosion behaviors of Mg-8.8Li alloy and conversion film were investigated with electrochemical and immersion tests. The experimental results indicated that the Mg-8.8Li alloys with and without the protection of conversion film can both be used in NaOH solution safely. But the Mg-8.8Li substrate was susceptible to corrode in NaCl and Na₂SO₄ solutions, and the conversion film can prevent them from corroding. Compared with the NaCl solution, the Na₂SO₄ solution was a strong corrosive medium to the conversion film.     

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 and chemical behavior of Mg97Zn1Y2-1wt.%SiC under different corrosion solutions

To explore the corrosion properties of magnesium alloys, the chemical behavior of a high strength Mg₉₇Zn₁Y₂-1 wt.%Si C alloy in different corrosion environments was studied. Three solutions of 0.2 mol·L^-1 NaCl, Na₂SO₄ and NaNO₃ were selected as corrosion solutions. The microstructures, corrosion rate, corrosion potential, and mechanism were investigated qualitatively and quantitatively by optical microscopy(OM), scanning electron microscopy(SEM), immersion testing experiment, and electrochemical test. Microstructure observation shows that the Mg₉₇ Zn₁Y₂-1 wt.%Si C alloy is composed of α-Mg matrix, LPSO(Mg₁₂ ZnY) phase and Si C phase. The hydrogen evolution and electrochemical test results reflect that the Mg₉₇Zn₁Y₂-1 wt.%SiC in 0.2 mol·L^-1 Na Cl solution has the fastest corrosion rate, followed by Na₂SO₄ and NaNO₃ solutions, and that the charge-transfer resistance presents the contrary trend and decreases in turn.     

Anion effects on the stress corrosion cracking behaviour of aluminium alloys

The stress corrosion cracking behaviour of plate material of the aluminium alloys 2024‐T351, 8090‐T8171, 7475‐T651, and 7075‐T7351 was investigated performing constant load tests. Short transverse tensile specimens were permanently immersed in aerated aqueous 0.6 M Na₂Cl solutions with additions of Na₂SO₄, NaNO₃, NaHCO₃, NH₄HCO₃, Na₂HPO₄, Na₂SO₃ or Na₂CO₃. The concentration of the added salts was 0.06 M. The applied stress was 100 MPa, except with 7075‐T7351 specimens, which were loaded at 300 MPa. Environment induced failure was not observed in neutral 0.6 M NaCl solution. The various salts added promoted intergranular stress corrosion cracking with the alloys 2024‐T351, 8090‐T8171, and 7475‐T651. Threshold stresses were generally below 100 MPa. For 8090‐T8171 exposed to chloride containing electrolytes with additions of sulfate, hydrogen phosphate, or sulfite, threshold stresses were approximately 100 MPa or higher. Similar results were obtained for 7475‐T651 plate when immersed in chloride‐hydrogen phosphate and chloride‐carbonate solutions. Alloy 7075‐T7351 was resistant against intergranular stress corrosion cracking. Specimens suffered pitting corrosion during immersion in the corrosive environments. Failure observed with 7075‐T7351, in particular when exposed to the chloride‐nitrate solution, was associated with reduction of cross‐sectional area due to pitting and transgranular stress corrosion cracking.     

Einfluß der Hydrodynamik auf die Korrosion von Eisen und Stahl in neutralen belüfteten NaCl- und Na2SO4-Lösungen

Influence of hydrodynamics on the corrosion of iron and steel in neutral aerated NaCl and Na₂SO₄ solutions The corrosion behaviour of iron and steel in aerated neutral NaCl and Na₂SO₄ solutions is mainly determined by the transport-controlled cathodic oxygen reduction. Its reaction zone depends on the physical and chemical properties of the 3-D layer formed on the corroding surface. The influence of hydrodynamics is restricted to the transport of the dissolved oxygen to the reaction zone. The obtained results are independent of laminar or turbulent flow conditions.     

Improvement of corrosion resistance of AZ31 Mg alloy by anodizing with co-precipitation of cerium oxide

Anodizing of AZ31 Mg alloy in NaOH solution by co-precipitation of cerium oxide was investigated. The chemical composition and phase structure of the coating film were determined via optical microscopy, SEM and XRD. The corrosion properties of the anodic film were characterized by using potentiodynamic polarization curves in 17 mmol/L NaCl and 0.1 mol/L Na₂SO₄ solution at 298 K. The corrosion resistance of AZ31 magnesium alloy is significantly improved by adding cerium oxide to alkaline solution. In addition, the surface properties are enhanced and the film contains no crack.     

Leaching of heavy metal elements in solder alloys

Leaching behavior of heavy metal elements from Sn–3.5Ag–0.5Cu, Sn–9Zn, and Sn–37Pb solder alloys and their joints was investigated in typical H₂SO₄, NaCl and NaOH solutions. The leaching amount of Sn from solder joints was more than that from solder alloys and the leaching amount of Sn from Sn–3.5Ag–0.5Cu solder joint in the NaCl solution was the most. The surface corrosion products on the solder and their joints were composed of oxide, oxide hydroxide or oxychloride of the component element. Much more surface oxides for the samples treated in the NaCl solution produced than that in the NaOH and H₂SO₄ solutions.     

Electrochemical behaviour of high nitrogen stainless steel in acidic solutions

The electrochemical behaviour of high nitrogen stainless steel in acidic solutions was studied by potentiodynamic polarization, EIS, Mott-Schottky and XPS. The passive film formed in neutral NaCl solution was very stable, but the stability of the film decreased with the addition of H₂SO₄ into the solution. The passive film formed in acidic Na₂SO₄ has a superior protective ability than that in acidic NaCl solution. The stability of the film formed in tested solution decreased with increase of applied potentials. The film formed on steel surface was of n-type semiconductor. Chloride penetration mechanism was proposed for the observed passive film breakdown.     

Corrosion of Mg alloys EV31A,WE43B,and ZE41A in chloride- and sulfate-containing solutions saturated with magnesium hydroxide

This study studied corrosion in 0.1 M Na₂SO₄, 0.1 M NaCl, and 0.6 M NaCl, all saturated with Mg(OH)₂, using weight loss, hydrogen evolution, and electrochemical measurements. Corrosion was similar in all cases. Nevertheless, the corrosion rates were alloy-dependent, were somewhat lower in 0.1 M Na₂SO₄ than in 0.1 M NaCl, and increased with NaCl concentration. The corrosion damage morphology was similar for all solutions; the extent correlated with the corrosion rate. The corrosion rates evaluated by the electrochemical methods were lower than those evaluated from hydrogen evolution, consistent with the Mg corrosion mechanism involving the unipositive Mg⁺ ion.     

Hot corrosion behaviour of a cobalt-base super-alloy K40S with and without NiCrAlYSi coating

A NiCrAlYSi coating was deposited by arc ion plating on a cobalt-base super-alloy K40S to improve its hot corrosion resistance at 1173 K in air. The K40S suffered from accelerated corrosion and formed non-protective scale with poor adherence when its surface was beneath Na₂SO₄ and Na₂SO₄ containing 25 wt.% NaCl salt deposits. After the K40S was coated with NiCrAlYSi coating, a protective α-Al₂O₃ scale was formed on the coating. Although the NiCrAlYSi coating changed into NiCoCrAlYSi during corrosion processes, it still possessed good corrosion resistance. In addition, the corrosion mechanisms were discussed on a basis of basic fluxing model.     

Corrosion and self‐healing behaviour of AZ91D magnesium alloy in ethylene glycol/water solutions

Corrosion behaviour of magnesium alloy‐based engine parts in cooling system is an urgent fundamental issue in automotive field where magnesium alloys are increasingly used. In the present work, the corrosion behaviour of AZ91D magnesium alloys in various ethylene glycol/water solutions was studied by electrochemical measurements and immersion tests at room temperature. The surfaces of the samples after immersion tests were examined using scanning electron microscope (SEM) and X‐ray diffraction (XRD). The results showed that the corrosion rates of AZ91D magnesium alloys decreased with the increase of ethylene glycol concentration in ethylene glycol/water solutions and the corrosion process was dominated by pitting corrosion. A continuous protective film transferred from corrosion products was formed on the corroded surface after sufficient immersion duration in ethylene glycol/water solutions, which is able to heal the corrosion pits. The self‐healing behaviour inhibited the further corrosion of AZ91D magnesium alloy.     

Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

Effect of alloying elements on the electrochemical polarization behavior and passive film of Fe-Mn base alloys in various aqueous solutions

The corrosion properties of austenitic Fe-Mn, Fe-Mn-Al, Fe-Mn-Cr and Fe-Mn-Al-Cr alloys with compositions of 23-30 wt% Mn, 2.8-8.2 wt% Al and 4.9-6.9 wt% Cr in various aqueous solutions of pH −0.8 to 15.3 and the passivating mechanism induced by the presence of Al, Cr, or Al and Cr have been studied using electrochemical measurements and Auger electron spectroscopic/X-ray photoelectron spectroscopic analysis. Binary Fe-Mn alloys can be passivated only in 10-50% NaOH solutions, and alloying of binary Fe-Mn alloy with Al or Cr or combination of Al and Cr seems not so obviously beneficial to corrosion resistance in HNO₃ or Na₂SO₄ solutions. All of the experimental Fe-Mn based alloys and steels for comparison cannot passivate in either 10% HCl or 3.5% NaCl solution. The Fe-Mn based alloys containing Al or Cr or Al and Cr can passivate in 10-50% HNO₃ or 1 mol l⁻¹ Na₂SO₄ solutions and rainwater. In general, Fe-Mn based alloys can passivate in oxidizing acid, neutral and basic solution, but cannot passivate in reducing acid or solution containing active Cl⁻ ions. In the passive film formed on the surface of Fe-Mn base alloys in various aqueous solutions, bound water and hydroxides are present at the surface of the film, while mixed oxides of Al, Cr, Mn and Fe are located in the inner part. The resistance to corrosion is imparted by a barrier film of bound water, hydroxides and oxides of Al, Cr or Fe, while the Mn oxides in passive film reduce the corrosion resistance.     

高温防护涂层的熔盐热腐蚀研究进展

高温热腐蚀是热元件主要失效形式之一,Na₂SO₄和NaCl熔盐会加速高温下的热腐蚀,甚至导致灾难性事故发生。本文就Na₂SO₄和/或NaCl熔盐引起的热腐蚀进行了讨论,其中Na₂SO₄是主要的腐蚀反应物,详细介绍了2种典型的热腐蚀行为和性能特点。重点介绍了几种热腐蚀模型和机理,以及Na₂SO₄、NaCl、Na₂SO₄+NaCl熔盐的反应公式和腐蚀机理。根据目前的研究状况来看,制备防护涂层是缓解热腐蚀的最佳途径,总结了近年来MCrAlY涂层、NiAl涂层、热障涂层和新型涂层的发展情况,并探讨了进一步提高涂层耐腐蚀性能的方法。最后,展望了防护涂层的未来发展方向。     

Study of pitting corrosion on mild steel during wet–dry cycles by electrochemical noise analysis based on chaos theory

Electrochemical noise (EN) was used to investigate the corrosion behaviour of mild steel (Q235) in 0.1 M Na₂SO₄ and 0.1 M NaCl aqueous solutions during wet–dry cycles. The positive fraction and value of the Largest Lyapunov Exponent (LLE) of electrochemical current noise (ECN) were found out to represent the number and isolation degree of the pits formed in two electrolyte conditions. The calculated results indicate that metastable pits are more plentiful and uniformly distributed in wet cycles and in Na₂SO₄ solution than those in dry cycles and in NaCl solution respectively.     

Preparation and hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy

A gradient NiCoCrAlYSiB coating was prepared on a Ni-base superalloy using arc ion plating (AIP) and subsequent gaseous phase aluminisation techniques. Hot corrosion of normal NiCoCrAlYSiB and the gradient coating in pure Na₂SO₄ and Na₂SO₄/NaCl (75:25, wt./wt.) salts was performed at 900 °C in static air. The corrosion results indicated an enhanced corrosion resistance to both salts for the gradient NiCoCrAlYSiB coating, which the improved performance of it should be attributed to the β aluminide ‘‘pool” at the surface layer. By partially sacrificing Al₂O₃ (i.e. Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts. The grain growth during the gaseous phase aluminisation and the corrosion mechanism, including the role NaCl played in the mixture salt corrosion, are discussed.     

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.     

Localized corrosion behavior of a zirconium-based bulk metallic glass relative to its crystalline state

To date, few detailed corrosion studies of the new bulk metallic glasses (BMGs) have been presented. In the present work, the aqueous electrochemical corrosion properties of BMG-11, 52.5Zr–17.9Cu–14.6Ni–5.0Ti–10.0Al (atomic percent), were investigated. Cyclic-anodic-polarization tests were conducted on amorphous and crystalline specimens in a 0.6 M NaCl solution (simulated seawater) and on amorphous specimens in a 0.05 M Na₂SO₄ solution (simulated moisture condensation, as related to ongoing fatigue experiments in humid air), all at room temperature. In the NaCl solution, both amorphous and crystalline materials were found to exhibit passive behavior with low corrosion rates (15 μm/year or less). However, susceptibilities to pitting corrosion were observed. The amorphous material was found to be more resistant to the onset of pitting corrosion under natural corrosion conditions. In the 0.05 M Na₂SO₄ solution, the amorphous BMG-11 was found to exhibit passive behavior with a very low corrosion rate (0.4 μm/year), and to be immune to pitting corrosion. Furthermore, when the protective passive film was removed by scratching with a diamond stylus, it was found to quickly reform. This result suggested that a corrosion influence on the fatigue properties of BMG-11 in humid air would be minimal.