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 AIP NiCoCrAlYSiB coating in salt spray tests

NiCoCrAlYSiB coatings were deposited by arc ion plating (AIP) and annealed/pre-oxidised under various conditions. The corrosion behaviour of as-deposited and annealed/pre-oxidised coatings was studied by salt spray testing in a neutral mist of 5 wt% NaCl at 35 °C for 200 h. The results showed that the as-deposited NiCoCrAlYSiB coating behaved poorly while the annealed and pre-oxidised ones performed much better in salt spray tests. The dense microstructure in annealed coatings and formation of α-Al₂O₃ scales on the surface during pre-oxidation improved the corrosion resistance in salt spray test. The corrosion process was investigated from the aspects of corrosion products, and its electrochemical mechanism was proposed as well.     

Oxidation and hot corrosion behavior of gradient NiCoCrAlYSiB coatings deposited by a combination of arc ion plating and magnetron sputtering techniques

NiCoCrAlYSiB coating was prepared by arc ion plating and Al and Cr gradient NiCoCrAlYSiB coating by a combination of arc ion plating and magnetron sputtering. The results of EPMA show that the Al and Cr are uniformly distributed in the NiCoCrAlYSiB coating but have a graded distribution in the surface layer of the gradient coating. Compared to the bare DSM 11, both the NiCoCrAlYSiB coating and the gradient coating improved the oxidation resistance greatly. After 100 h hot corrosion, the gradient coating showed the best corrosion resistance. The oxidation and hot corrosion mechanisms were discussed.     

Corrosion characterization of tin-lead and lead free solders in 3.5 wt.% NaCl solution

The corrosion resistance of Sn-Pb and several candidate lead free solders were investigated in 3.5 wt.% NaCl solution through potentiodynamic polarisation. Results showed that in NaCl solution lead free solders had better corrosion resistance than Sn-Pb solder and the corrosion resistance of lead free solders was similar, but the corrosion resistance of Sn-Ag solder was better than that of Sn-Ag-Cu and Sn-Cu solders. The corrosion products for Sn-Pb solder had a two-layered structure with Sn-rich phases at the outer layer and looser Pb-rich phases at the inner layer. The loose Pb-rich layer was detrimental to the corrosion property. The corrosion product on the surface of all these solders was tin oxide chloride hydroxide.     

Oxidation resistance and corrosion behavior of hot-dip aluminized coatings on commercial-purity titanium

Aluminizing is an effective method to protect alloys from oxidation and corrosion. In this article, the microstructure, morphology, phase composition of the aluminized layers and the oxide films were investigated by SEM, EDS and X-ray diffraction. The high temperature oxidation resistance and electrochemical behavior of hot dip aluminizing coatings on commercial-purity titanium had been studied by cyclic oxidation test and potentiodynamic polarization technique. The results show that the reaction between the titanium and the molten aluminum leads to form an aluminum coating which almost has the composition of the aluminum bath. After diffusion annealing at 950 °C for 6 h, the aluminum coating transformed into a composite layer, which was composed of an inner layer and an outer layer. The inner layer was identified as Ti₃Al or Ti₂Al phase, and the outer layer was TiAl₃ and Al₂O₃ phase. The cyclic oxidation treatment at 1000 °C for 51 h shows that the oxidation resistance of the diffused titanium is 13 times more than the bare titanium. And the formation of TiAl₃, θ-Al₂O₃ and compact α-Al₂O₃ at the outer layer was thought to account for the improvement of the oxidation resistance at high temperature. However, the corrosion resistance of the aluminized titanium and the diffused titanium were reduced in 3.5 wt.% NaCl solution. The corrosion resistance of the aluminized titanium was only one third of bare titanium. Moreover, the corrosion resistance of the diffused titanium was far less than bare titanium.     

(NiCoCrAlYSiB+AlSiY)复合涂层热腐蚀行为的研究

采用电弧离子镀（AIP）技术在镍基单晶高温合金基体上制备了NiCoCrAlYSiB涂层（普通涂层）和（Ni-CoCrAlYSiB+AlSiY）复合涂层,研究了高温合金基体与2种涂层分别在900和700℃下的涂盐（Na₂SO₄+K₂SO₄和Na₂SO₄+NaCl）热腐蚀行为.结果表明:高温（900℃）热腐蚀条件下,基体合金表面主要生成NiO;普通涂层表面主要生成Cr₂O₃,而且涂层内部出现内氧化和内硫化现象;复合涂层表面主要生成Al₂O₃,外层出现程度较轻的内氧化,涂层表层Al含量仍然较高,维持表面Al₂O₃膜的形成和修复.低温（700℃）热腐蚀条件下,基体合金表面主要生成NiO;普通涂层表面主要生成Cr₂O₃,涂层内部出现严重的内氧化;复合涂层表面也出现了内氧化,高Cr的内层未受腐蚀,有助于提高涂层的抗腐蚀性能.     

Corrosion protection of steel by bi-layered polypyrrole doped with molybdophosphate and naphthalenedisulfonate anions

Bi-layered polypyrrole (PPy) coating was investigated for corrosion prevention of steels. The PPy coating consisted of the inner layer doped with PMo₁₂ (PMo₁₂) and ions and the outer layer doped with naphthalenedisulfonate (NDS) ions. PMo₁₂ ions doped in the inner layer make the passive oxide film on steel stabilized and the outer layer doped with large organic ions of NDS restricts decomposition and release of PMo₁₂ ions in the inner layer. Corrosion tests were made in 3.5% NaCl aqueous solution for the steels covered by the single PPy-PMo₁₂, single PPy-NDS, and bi-layered PPy-PMo₁₂/PPy-NDS coatings. The single PPy-PMo₁₂ and PPy-NDS coatings maintained the steel in passive state for 34 h and 24 h, respectively. The bi-layered coating kept the steel in passive state and prevented corrosion of the steels for the longer time period.     

Corrosion behavior of the composite ceramic coating containing zirconium oxides on AM30 magnesium alloy by plasma electrolytic oxidation

The deterioration process of a plasma electrolytic oxidation (PEO) coating containing zirconium oxides on AM30 magnesium alloy in 3.5 wt.% NaCl solution have been investigated. The coating consists of an outer porous layer and an inner dense layer. The content of MgF₂ is high in the pores and an MgO-rich layer is evident in the inner layer. The corrosion resistance of the outer layer gradually decreases in the initial immersion stage (96 h) due to the decomposition of MgO, and the deterioration of the inner layer is delayed by the blocking effect of the outer layer.     

Al2O3-ZrO2 mixed oxide composite incorporated aluminium rich zinc coatings for high wear resistance

Corrosion resistance and wear resistance are the two important parameters for high performance of zinc galvanic coating. In the present work, the improvement of these two characteristics was achieved by the incorporation of Al₂O₃-ZrO₂ mixed oxide composite in the coating. Al₂O₃-ZrO₂ mixed oxide composite was synthesized from ZrOCl₂·8H₂O. Aluminium rich zinc coatings with high sliding wear resistance was developed from a galvanic bath containing the mixed oxide. Based on the performance of the coating during physicochemical and electrochemical characterization, the concentration of mixed oxide composite in the bath was optimized as 0.50 wt% Al₂O₃-0.50 wt% ZrO₂. While rich in Al-metal content in the coating caused high corrosion resistance, the incorporation of the mixed oxide improved structural characteristics of the coating resulting in high wear resistance also. The coating was nonporous in nature and even the interior layers had high stability. The coatings have potential scope for high industrial utility.     

The effect of counter anions on corrosion resistance of steel covered by bi-layered polypyrrole film

The bi-layered polypyrrole (PPy) coatings were investigated for corrosion prevention of a carbon steel. The inner layer was doped with the Keggin structure anions of (PMo₁₂) and anions for stabilization of the passive oxide film at the metal-polymer interface, and the outer layer was doped with four organic anions of dihydroxynaphthalenedisulfonate (DHNDS), naphthalenedisulfonate (NDS), anthraquinonedisulfonate (AnqDS) or dodecylsulfate (DoS) for inhibition of the decomposition and release of PMo₁₂. The corrosion tests were performed in 3.5 wt.% NaCl aqueous solution. The corrosion resistance of the steels covered by the bi-layered PPy films was found in the following order: PPy-PMo₁₂/PPy-DHNDS < PPy-PMo₁₂/PPy-NDS < PPy-PMo₁₂/AnqDS < PPy-PMo₁₂/PPy-DoS. The performance of corrosion protection related to the oxidized state of the polymer was discussed.     

(NiCoCrAlYSiB+AlSiY)复合涂层热腐蚀行为的研究

采用电弧离子镀（AIP）技术在镍基单晶高温合金基体上制备了NiCoCrAlYSiB涂层（普通涂层）和（Ni-COCrAlYSiB+AlSiY）复合涂层,研究了高温合金基体与2种涂层分别在900和700℃下的涂盐（Na₂SO₄+K₂SO₄和Na₂SO₄+NaCl）热腐蚀行为.结果表明:高温（900℃）热腐蚀条件下,基体合金表面主要生成NiO;普通涂层表面上要生成Cr₂O₃,而且涂层内部出现内氧化和内硫化现象;复合涂层表面主要生成Al₂O₃,外层出现程度较轻的内氧化,涂层表层Al含量仍然较高,维持表面Al₂O₃膜的形成和修复.低温（700℃）热腐蚀条件下,基体合金表面主要生成NiO;普通涂层表面主要生成Cr₂O₃,涂层内部出现严重的内氧化;复合涂层表面也出现了内氧化,高Cr的内层未受腐蚀,有助于提高涂层的抗腐蚀性能.     

Influence of pre-oxidation on the hot corrosion of Ti3SiC2 in the mixture of Na2SO4-NaCl melts

Polycrystalline Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 °C in air for 2 h. A duplex oxide scale with an outer layer of TiO₂ and an inner layer of a mixture of TiO₂ and SiO₂ was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C for 50 h. However, during the hot corrosion the outer layer of TiO₂ would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti₃SiC₂ corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.     

Microstructure and oxidation behaviour of an AlSiY/NiCrAlYSi composite coating at 1150 °C

A composite coating consisting of an outer AlSiY layer and an inner NiCrAlYSi layer has been prepared by a two-step arc ion plating method. The isothermal and cyclic oxidation behaviour of the composite coating at 1150 °C, including the growth of oxide scale and the microstructure transformation of the coatings, have been investigated comparing with the reference coating, NiCrAlYSi. The results show enhanced oxidation performance of the composite coating, which is concerned with its abundant possession of β-NiAl phase reservoirs, providing it the long term healing capacity of re-growing the α-Al₂O₃ scale.     

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.     

Chemistry of corrosion products on Zn-Al-Mg alloy coated steel

Zn-Al-Mg alloy (ZM) coating provides a decisively enhanced corrosion resistance in a salt spray test according to DIN EN ISO 9227 (NSS) compared to conventional hot-dip galvanised zinc (Z) coating because of its ability to form a very stable, well adherent protecting layer of zinc aluminium carbonate hydroxide, Zn₆Al₂(CO₃)(OH)₁₆·4H₂O on the steel substrate. This protecting layer is the main reason for the enhanced corrosion resistance of the ZM coating. Surface corrosion products on ZM coated steel consist mainly of Zn₅(OH)₆(CO₃)₂, ZnCO₃ and Zn(OH)₂ with additions of Zn₅(OH)₈Cl₂ · H₂O and a carbonate-containing magnesium species.     

Structure and oxidation behavior of Si-Y co-deposition coatings on an Nb silicide based ultrahigh temperature alloy prepared by pack cementation technique

In order to improve the oxidation resistance of silicide coatings on Nb silicide based alloys, Y-modified silicide coatings were prepared by co-depositing Si and Y at 1050, 1150 and 1250 °C for 5-20 h, respectively. It has been found that the coatings prepared by co-depositing Si and Y at 1050 and 1150 °C for 5-20 h as well as at 1250 °C for 5 h were composed of a thick (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X)₅Si₃ inner layer, while the coatings prepared by co-depositing Si and Y at 1250 °C for 10-20 h possessed a thin outer layer composed of (Ti,Nb)₅Si₃ and Ti-based solid solution, a thick (Nb,X)Si₂ intermediate layer and a thin (Nb,X)₅Si₃ inner layer. EDS analyses revealed that the content of Y in the (Nb,X)Si₂ layers of all the coatings was about 0.34-0.58 at.% while that in the outer layers of the coatings prepared by co-depositing Si and Y at 1250 °C for 10-20 h was about 1.39-1.88 at.%. The specimens treated by co-depositing Si and Y at 1250 °C for 10 h were selected for oxidation test. The oxidation behavior of the coating specimens at 1250 °C indicated that the Si-Y co-deposition coating had better oxidation resistance than the simple siliconized coating because the oxidation rate constant of the Si-Y co-deposition coating was lower than that of the simple siliconized coating by about 31%. The scale developing on the Si-Y co-deposition coating consisted of a thicker outer layer composed of SiO₂ and TiO₂ and a thinner SiO₂ inner layer.     

Hydroxyapatite coating of AZ31 magnesium alloy by a solution treatment and its corrosion behavior in NaCl solution

Hydroxyapatite (HAp) coatings were formed directly on AZ31 magnesium alloy and pure Mg in a 250 mmol/L C₁₀H₁₂N₂O₈Na₂Ca aqueous solution of pH 8.9. Treatment time was varied from 2 h to 6 h. Crystal phase, morphology and composition of the coatings were investigated. Immersion and polarization tests in a 3.5 wt.% NaCl solution were performed to examine the corrosion behavior of the HAp-coated specimens. The HAp coating of AZ31 with short treatment time had defects which decreased with an increase in treatment time. The HAp coatings of AZ31 consisted of an inner dense layer and an outer coarse layer in the similar manner for pure Mg. The inner layer on AZ31 was composed of dome-shape precipitates densely packed. The outer layer was composed of rod-like crystals growing from each dome in the radial direction. The (002) plane of HAp of inner layer and rod-like crystals roughly oriented to the substrate. Magnesium ion-release and corrosion current density were remarkably reduced with HAp coatings. Each of these values was on the same order of magnitude between HAp-coated AZ31 and pure Mg. The ion release from AZ31 slightly decreased with an increase in treatment time. The original inner dense layer of AZ31 remained after the immersion. It is suggested that the protectiveness of HAp coating relays on the inner layer and does not significantly depend on the kind of Mg substrate.     

A study on mechanism of corrosion protection of polyaniline coating and its failure

Polyaniline (PANI) coatings were electrochemically deposited on substrates of stainless steel and platinum in solutions of 0.2 M H₂SO₄ and 0.1 M aniline by cyclic voltammetry. The corrosion protection of the PANI coatings and their failure were investigated in 0.2 M H₂SO₄ solution. It was observed that the corrosion protection ability of the coating to steel substrate was increased with the increase of the coating thickness. The corrosion protection ability was mainly attributed to the passivating effect of PANI due to its oxidizing ability in its emeraldine state. During its operation, the PANI coating in emeraldine state tended to gradually lose its corrosion protection ability. This gradual failure of the PANI coating, but faster than expected, was confirmed to be related to a gradual reduction of the emeraldine PANI and a gradually increased resistance between the PANI coating and the stainless steel substrate. These findings lead to a new mechanism for the corrosion protection of PANI coating and its failure.     

Electrochemical corrosion behaviour of nanotubular Ti-13Nb-13Zr alloy in Ringer’s solution

Nanotubular oxide layer formation was achieved on biomedical grade Ti-13Nb-13Zr alloy using anodization technique in 1 M H₃PO₄ + 0.5 wt.% NaF. The as-formed and heat treated nanotubes were characterized using SEM, XRD and TEM. Corrosion behaviour of the nanotubular alloy was investigated employing potentiodynamic and potentiostatic polarization. The alloy after nanotubular oxide layer formation exhibited significantly higher corrosion current density than the bare alloy. The lower corrosion resistance of the nanotubular alloy was suggested to be associated with the distinctly separated barrier oxide/concave shaped tube bottom interface. A heat treatment at 150 °C appreciably enhanced the corrosion resistance property.     

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.