Corrosion behavior of as-cast Mg68Zn28Y4 alloy with/-phase

Mg₆₈Zn₂₈Y₄ alloys with stable icosahedral quasicrystals (Zn₆₀Mg₃₀Y₁₀) were prepared by cast method. By simulating the environment of ocean, the alloy was eroded in 3.5% (mass fraction) NaCl for 2, 4 and 30 h. The microstructures of the samples and eroded alloys were analyzed by OM and SEM. The compositions and the quasiperiodic structures were identified respectively by EDS and TEM. And the corrosion potential and corrosion current density before and after immersion were measured by potentiodynamic polarization measurements in 3.5% NaCl. The results show that I-phases grow in the mode of conglomeration, piling and transfixion. The Mg₇Zn₃ matrix and α(Mg) solid solution are eroded badly, while W-phase is eroded partially. At the same time, the I-phases exhibit excellent corrosion resistance property. The resistance to corrosion of Mg₆₈Zn₂₈Y₄ alloy is improved by increasing exposed I-phases. With adding element Y to Mg₆₈Zn₃₂ alloy, the corrosion current is decreased by one order of magnitude. And after the immersion of as-cast Mg₆₈Zn₂₈Y₄ alloy for 30 h, the corrosion current density is reduced by two orders of magnitude compared with that of uneroded Mg₆₈Zn₃₂ alloy.     

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 of a Single Crystal Ni-Base Superalloy by Na-Salts at 900°C

The hot corrosion behaviors of a single crystal (SC) Ni-base superalloy coated with Na₂SO₄ and 75 wt.% Na₂SO₄-25 wt.% NaCl mixture were studied in air at 900°C. The results showed that the corrosion productions were laminar structure, porous and easily spalled. And sulfides formed quickly in the deep SC superalloy under the corrosion production. The addition of NaCl into Na₂SO₄ considerably accelerated the corrosion of the SC superalloy, and the corrosion scale became more porous. The hot corrosion process was explained based on sulfide formation and its subsequent oxidation.     

Corrosion property of copperized layer on Zr formed by double glow plasma surface alloying technique

Zr and its alloys have excellent mechanical properties as new structural material,but in specific application environment,its corrosion resistance still needs to be further explored.In this work,double glow plasma surface alloying technique was used for copperizing on pure Zr surface.Besides,X-ray diffraction(XRD),scanning electron microscope(SEM) and energy dispersion spectrum(EDS) were employed to characterize the samples.Furthermore,research was also conducted on the polarization curve of the samples in different solutions.Copperizing on surface can improve corrosion resistance of pure Zr in 3.5 % Na Cl and 0.5 moláL-1Na OH solutions.Especially in 0.5 moláL-1Na OH solution,the corrosion resistance can achieve significant improvement.However,copperizing has no influence on the improvement of corrosion resistance of pure Zr in 0.5 moláL-1H2SO4 solution.The results may provide new insight into way for improving the corrosion property of zirconium alloys.     

Tensile properties of hot rolled Mg_(97)Zn_1Y_2 alloy sheets at elevated temperatures

The elevated temperature tensile properties of Mg97Zn1Y2 magnesium alloy sheets, hot rolled at 390, 420 and 450 ℃ respectively, were tested in a temperature range from room temperature to 250 ℃ with a strain rate of 1.0×10-3 s-1. The results show that the variations in yield strength for Mg97Zn1Y2 magnesium alloy sheets hot rolled at 390 ℃ and 420 ℃ with temperature resemble each other due to their similar morphology of the chain-shaped strengthening phase. The yield strength maintains at a high level of 283 MPa before 200 ℃ and decreases significantly at 250 ℃. Despite of the fine lamellar structure of Mg97Zn1Y2 magnesium alloy sheet hot rolled at 450 ℃, its yield strength decreases linearly owing to occurrence of the coarse grain, and drops to 239 MPa at 250 ℃. The elongation for all hot rolled Mg97Zn1Y2 magnesium alloy sheets increases slightly with increasing testing temperature.     

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.     

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.     

Influence of P,As and Sb on the susceptibility to SCC of α-β′ brasses

The influence of P, As and Sb on the susceptibility to SCC of α-β′ brasses in different non ammoniated solutions was studied. Two series of brasses were tested: the first one with only 5 – 10% of β′ phase and the second one with 54 – 56% of β′ phase. The VB group elements were present at two different concentrations. Susceptibility to SCC was evaluated by SSRT, at a strain rate of 1 × 10⁻⁶ s⁻¹, in concentrated (1M) or diluted (50 and 200 ppm) solutions of Na₂SO₄, NaNO₂, NaCl, at room temperature. The brasses of second series were found to be more susceptible to SCC than those of the first series in all the environments studied. Sb drastically increased SCC susceptibility of the brasses of the first series, while As showed a beneficial effect. The three VB group elements produced a limited inhibiting effect on susceptibility to SCC in the second series of brasses. Susceptibility to SCC of all the brasses was compared with the tendency to dezincification, measured by means of the 6509 ISO Standard Test. The influence of the VB group elements on the two types of corrosion was not comparable.     

Corrosion behavior of T24, T92, VM12, and AISI 304 steels exposed to KCl–NaCl–K2SO4–Na2SO4 salt mixtures

The corrosion mechanisms of T24, T92, VM12, and AISI 304 steels are studied under the influence of NaCl–KCl, NaCl–Na₂SO₄, and KCl–K₂SO₄ salt mixtures in a dry air atmosphere at 650°C for 15 days. NaCl–KCl was the most aggressive deposit and AISI 304 stainless steel exhibited the highest corrosion resistance. There was no relation between the Cr content of the ferritic steels and their corrosion resistance in NaCl–KCl. In contrast, the resistance of high-Cr steels was better when exposed to NaCl–Na₂SO₄ and KCl–K₂SO₄. The high-Cr and the low-Cr steels were more susceptible to NaCl–Na₂SO₄ and to KCl–K₂SO₄, respectively.     

The Hot Corrosion of Fe-Mn-Al–C Alloy with NaCl/Na2SO4 Coating Mixtures at 750°C

The high-temperature corrosion behavior of Fe-30.1Mn-9.7Al-0.77C alloy initially coated with 2 mg/cm² NaCl/Na₂SO₄ (100/0, 75/25, 50/50, 25/75 and 0/100 wt.%) deposits has been studied at 750°C in air. The result shows that weight-gain kinetics in simple oxidation reveals a steady-state parabolic rate law after 3 hr, while the kinetics with salt deposits all display multi-stage growth rates. The corrosion morphology of the alloy with 100% Na₂SO₄ coating is similar to that of simple oxidation. NaCl acts as the predominant corrosion species for Fe-Mn-Al-C alloy, inhibiting the formation of a protective oxide scale. For the alloy coated with over 50% NaCl in salts, NaCl induces selective oxidation of manganese and results in the formation of secondary ferrite in the alloy substrate as well as void-layers with different densities of voids layer by layer in the secondary-ferrite zone.     

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。     

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.     

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.     

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 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.     

Passivation behaviors of 6082 aluminium alloy under stress corrosion process

We systematically studied the passivation process of 6082 aluminium alloy under the bending stress situation by combining electrochemical measurement techniques with three-point bending stress fixture designed by our lab, and then examined the microstructures of corroded specimens to analyze electrochemical corrosion mechanism in 1.5% NaCl solution. The results show that secondary Mg₂Si phase acts as the anodic electrode, leading to the self-corrosion of Mg₂Si phase. As a result, the spots of self-corroded Mg₂Si phase within grains act as initial pitting corrosion site, combined with tiny, massive precipitated Mg₂Si particles at the grain boundaries and bending stress, leading to the failure of surface of the 6082 aluminium alloy. The corrosion current density increases from 3.422 × 10⁻⁷ to 13.77 × 10⁻⁷ A/cm² when bending stress level was increased from 0% up to 100% of yield stress. Passive film formation process occurred between polarization potential area of −1.05 and −0.65 V. Sectional microstructural investigations show that the corrosion starts penetrating vertically into the material before it develops corrosion paths extending parallel to the surface, leading to massive stress-induced corrosion cracks. The maximum corrosion depth increases from ∼24 μm on specimen without any stress applied to 85 μm when bending stress of 100% yield strength is applied.     

Understanding corrosion via corrosion product characterization: II. Role of alloying elements in improving the corrosion resistance of Zn–Al–Mg coatings on steel

Corrosion products are identified on Zn, ZnMg and ZnAlMg coatings in cyclic corrosion tests with NaCl or Na₂SO₄ containing atmospheres. For Mg-containing alloys the improved corrosion resistance is achieved by stabilization of protective simonkolleite and zinc hydroxysulfate. At later stages, the formation of layered double hydroxides (LDH) is observed for ZnAlMg. According to thermodynamic modeling, Mg²⁺ ions bind the excess of carbonate or sulfate anions preventing the formation of soluble or less-protective products. A preferential dissolution of Zn and Mg at initial stages of corrosion is confirmed by in situ dissolution measurement. The physicochemical properties of different corrosion products are compared.     

Effects of Cr addition on the glass-forming ability and the corrosion behaviors of FeCBSiP amorphous alloys

The corrosion behaviors of Fe_83-XC₁B₁₁Si₂P₃Cr_X (X = 0, 1, 2, and 3 at%) amorphous alloys in 0.1 M NaCl solution have been investigated by immersion and electrochemical tests. With the addition of Cr content from 0 to 3 at%, the corrosion rate of amorphous alloys gradually decreases from 2.57 × 10⁻¹ to 1.04 × 10⁻¹ mm·year⁻¹. The minor addition of Cr improves the corrosion resistance through the increase in the E_corr value, which makes it easy to reach a passive state and suppress pitting corrosion. The corroded morphology and products of amorphous alloys have been tested by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The SEM/EDS analysis shows that the high corrosion resistance is due to the formation of dense and stable chromium oxide during immersion in NaCl solutions.     

Effects of NaCl and SO2 on the initial atmospheric corrosion of zinc

The influence of NaCl deposition on the corrosion of zinc in atmospheres with and without SO₂ was studied via quartz crystal microbalance. Regularity of the initial corrosion of zinc under these conditions was analyzed. The results show that NaCl can accelerate the corrosion of zinc. Mass gain of zinc increases with the exposure time, which can be correlated by using exponential decay function. The relationship between mass gain and amount of NaCl deposition is well linear at any time in air containing 1 ppm SO₂, but follows quadratic function in air without SO₂. More amount of NaCl deposition will slow down the corrosion to some extent after exposure for certain time in the presence of SO₂. The combined effect of NaCl and SO₂ on the corrosion of zinc is greater than that caused by each single component. Fourier transform infrared spectroscopy and X-ray diffraction were used to characterize the corrosion products of zinc. In the absence of SO₂, simonkolleite, Zn₅(OH)₈Cl₂·H₂O and zincite, ZnO are the dominant corrosion products, while zinc hydroxysulfate (Zn₄SO₄(OH)₆·3H₂O), zinc chloride sulfate hydroxide hydrate (Zn₁₂(SO₄)₃Cl₃·(OH)₁₅·5H₂O) and simonkolleite dominate in the presence of SO₂. Brief discussion on the mechanisms of atmospheric corrosion under these conditions was introduced.     

Effects of NH4 +, Na+, and Mg2+ ions on the corrosion behavior of galvanized steel in wet–dry cyclic conditions

The effect of cations on the corrosion of galvanized steel (GS) is scarcely reported. In this study, a wet–dry cyclic test was conducted to study NH₄ ⁺, Na⁺, and Mg²⁺ cation effect on the corrosion behavior of GS available in Nepal. Fourteen wet–dry cycles (18 h wet and 6 h dry) were performed by exposing samples at 298 K with a relative humidity of 90% in a wet cycle and 50% in a dry cycle for 14 days. The cations strongly affect the corrosion rate of the GS sample estimated by weight loss and potentiodynamic polarization. The potentiodynamic polarization curves showed the inhibition of cathodic and anodic reactions by Mg²⁺ ion, while the NH₄ ⁺ ion only changed the cathodic reaction. Mg²⁺ ion was found to shift the corrosion potential to noble values compared with NH₄ ⁺ and Na⁺ ions. A compact and thin corrosion products layer was developed in Mg²⁺ salt solution in contrast to a thick and porous corrosion products layer in NH₄ ⁺ and Na⁺ salt solutions. Red rust due to corrosion of underlying steel appeared in the presence of NH₄ ⁺ and Na⁺ salt solutions. Finally, the weight loss data revealed that the corrosivity of cations for GS decreased in the order Na⁺ > NH₄ ⁺ > Mg²⁺.