Molecular modeling of corrosion processes: development and engineering applications

Abstract

The effect of two polyacrylate based polyelectrolytes, one cationic, the other anionic, on the corrosion of mild steel in aqueous solutions has been examined. It is shown that under conditions where they accelerate corrosion the effect arises from different properties of the compounds in each case. The cationic polymer is a mild oxidising agent and, in alkaline solution, is aggressive to passive films producing localised attack. The anionic compound acts as an anodic stimulator in neutral and alkaline solutions possibly by virtue of its ability to form a complex ion with the emerging cation. Both compounds therefore accelerate corrosion in neutral and alkaline solutions. By contrast, the anionic polyelectrolyte is a mild mixed inhibitor in acid solutions. It is suggested that not all soluble polymers will adversely affect the corrosion of mild steel and that by redesign of the molecule it might be possible for polyectrolytes generally to act as both water treatment chemicals and corrosion inhibitors.     

Hydrothermal corrosion of TiAlN and CrN PVD films on stainless steel

Hydrothermal corrosion of thin TiAlN and CrN PVD films (of 3μm thickness) in 100 MPa water over a temperature range of 20-950 °C is compared to the behavior of TiN films over the same T-P conditions. Corrosion resistance increases in the sequence TiN → TiAlN → CrN. A FeTiO₃ (ilmenite) layer on the surface of the TiAlN film is almost chromium-free and provides protective properties up to 700 °C, whilst ülvospinel formation leads to spallation of oxide scale due to high level growth stresses. Formation of a very stable spinel scale on the surface of the CrN films provides long-term corrosion protection in 100 MPa water up to 800 °C. Nitride films on low-alloyed steel can substitute for expensive super alloy in wet air oxidation systems, with working temperatures up to 700 °C in the case of TiAlN, or 800 °C in the case of CrN coatings.     

The wet corrosion of Mo and Mo‐Ti alloy thin film – Part IV: The effect of nitriding

A considerable number of detailed investigations have been carried out on the deposition and characterisation of molybdenum and molybdenum‐titanium‐nitride films by employing a variety of techniques. However, very little is currently known about the effect of composition (N₂/Ar flow rate) on the corrosion properties of MoN and MoTiN thin films for aggressive ambient conditions. In this work, the electrochemical and corrosion behaviour of MoN and MoTiN thin films, produced by Physical Vapour Deposition (PVD) with different N₂/Ar flow rates, has been investigated by Electrochemical Impedance Spectroscopy (EIS) in aerated alkaline chloride solution and compared with the behaviour of pure molybdenum in the same environment. Results obtained indicate that increasing nitrogen content in the film leads to a beneficial effect on the corrosion resistance, but results in decreased electrical conductivity of the film that may limit their application as back contact in photovoltaic modules.     

Untersuchungen über einen Lochfraßindikatortest an Chrom- und Chrom-Nickel-Stählen in chlorid- und bromidhaltigen Lösungen

Investigations into a pitting corrosion indicator test on chromium and chromenickel steels in solutions containing chloride or bromide An investigation has been carried out into the validity of the results of an indicator test for the examination of the pit corrosion proneness of stainless ferritic chromium steels and austenitic Cr-Ni and Cr-Ni-Mo steels in neutral solutions containing chloride, at room temperature. Suitable indicators are K₃(Fe(CN)₆) and equimolar mixtures of K₃(Fe(CN)₆) and K₄(Fe(CN)₆). The latter are preferable because their redox potential has a better constancy over time, because of the pH independence of this potential in neutral and alkaline solutions, and because of the position of the redox potential in the vicinity of the potential assumed by stainlèss steels in neutral aerated solutions. But the position of the redox potential depends on the concentration of the complex salts. The added indicatior inhibits pit corrosion at low chloride contents by shifting the pit corrosion potential. In diluted solutions, the indicator test is therefore only suited for comparative investigations on different steels, and not for the determination of the absolute value of the pit corrosion potential. Investigations carried out at higher temperatures show that, above 30° C, the age-resistance ranges of the steels examined are largely independent of steel composition and temperature. At room temperature, the well-known favourable influence of increasing molybdenum contents is apparent. In solutions containing bromide, pit corrosion was found to be generally less pronounced with solutions containing chloride.     

Passivity of polycrystalline NiMnGa alloys for magnetic shape memory applications

The corrosion and passivation behaviour of bulk polycrystalline martensite Ni₅₀Mn₃₀Ga₂₀ and austenite Ni₄₈Mn₃₀Ga₂₂ alloys was compared in electrolytes with different pH values. Linear anodic and cyclic potentiodynamic polarisation methods and anodic current transient measurements have been conducted for the alloys and their constituents to analyze free corrosion, anodic dissolution and passive layer formation processes. Electrochemically treated alloy surfaces were characterized with scanning electron microscopy (SEM) and angle-resolved x-ray photoelectron spectroscopy (XPS). The electrochemical response of both alloys is in principal similar and is dominated by the Ni oxidation. In acidic solutions (pH 0.5 and 5) a slightly higher reactivity is detectable for the martensitic alloy which is mainly attributed to enhanced dissolution processes at the multiple twin boundaries. In weakly acidic to strongly alkaline solutions (pH 5-11) both alloys exhibit a low corrosion rate and a stable anodic passivity. While air-formed films comprise NiOOH, Ga₂O₃ and MnO₂, passive films formed in near neutral media (pH 5-8.4) are composed of Ni(OH)₂, NiOOH and Ga₂O₃ in the outer region and of NiO, MnO₂ and MnO in the metal-near region.     

Corrosion properties of glassy Mg70Al15Ga15 in 0.1 M NaCl solution

The corrosion behavior of glassy Mg₇₀Al₁₅Ga₁₅ was investigated in 0.1 M NaCl solutions of various pH values. Potentiodynamic polarization measurements confirm a pronounced passivity for 4 ≤ pH ≤ 12. Corrosion currents in neutral solutions are found to be as low as 10^?6 A/cm². Optical microscopy and Auger electron spectroscopy were performed on samples immersed under open-circuit conditions in neutral, acidic and alkaline 0.1 M NaCl solutions to observe their time-dependent passive film formation. A significant incorporation of aluminum in the surface layers was found. The alloys show an unusual stability against dissolution in chloride-containing solutions, raising expectations of a highly corrosion-resistant Mg-based alloy. The time-dependent limitations of this stability are discussed in context with the surface films observed.     

Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy

Effects of pH solution and chloride (Cl⁻) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy.The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.003 wt% and 5.5 wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting E_pit and corrosion E_cor potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits.Polarisation resistance measurements in acidic (pH = 2) and alkaline chloride solutions (pH = 12) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH = 6).     

Corrosion Behaviors of Nitride Coatings on Titanium Alloy in NaCl-Induced Hot Corrosion

TiN and TiAlN coatings were deposited by arc ion plating on titanium alloys to study their hot corrosion resistance when they were exposed to NaCl at 600 ℃.The microstructure and corrosion behaviors of nitride coatings were studied using scanning electron microscope,X-ray diffraction,electro-probe microanalyzer and X-ray photoelectron spectroscopy.The results showed that nitride coatings with the different compositions and the ones with the same composition but different thicknesses presented different hot corrosion resistance.TiN and thin TiAlN coatings showed poor corrosion resistance.Serious internal oxidation attacked the alloy substrate.Their corrosion products were mainly consisted of non-protective TiO2 and sodium salt.By contrast,the thick TiAlN coating presented outstanding corrosion resistance.Besides sodium salt,the corrosion products were composed of protective Al2O3.The increasing thickness of TiAlN significantly enhanced the hot corrosion resistance.The corrosion mechanisms of alloy,TiN and TiAlN coatings were discussed in detail.     

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.     

Stainless Steel Bipolar Plates Deposited with Multilayer Films for PEMFC Applications

A chromium nitride (CrN, Cr₂N)/chromium (Cr)/indium-tin-oxide (ITO) system and a gold (Au)/titanium (Ti) system were separately deposited using a sputtering method and an E-beam method, respectively, onto stainless steel 316 and 304 plates. The XRD patterns of the deposited stainless steel plates showed the crystalline phase of typical indium-tin oxide and of metallic phases, such as chromium, gold, and the metal substrate, as well as those of external chromium nitride films. The nitride films were composed of two metal nitride phases that consisted of CrN and Cr₂N compounds. The surface morphologies of the modified stainless steel bipolar plates were observed using atomic force microscopy and FE-SEM. The chromium nitride (CrN, Cr₂N)/chromium (Cr)/indium-tin-oxide (ITO) multilayer that was formed on the stainless steel plates had a surface microstructural morphology that consisted of fine columnar grains 10 nm in diameter and 60 nm in length. The external gold films that were formed on the stainless steel plates had a grain microstructure approximately 100 nm in diameter. The grain size of the external surface of the stainless steel plates with the gold (Au)/titanium (Ti) system increased with increasing gold film thickness. The electrical resistances and water contact angles of the stainless steel bipolar plates that were covered with the multilayer films were examined as a function of the thickness of the ITO film or of the external gold film. In the corrosion test, ICP-MS results indicated that the gold (Au)/titanium (Ti) films showed relatively excellent chemical stability after exposure to H₂SO₄ solution with pH 3 at 80 °C.     

The mechanism of high temperature aqueous corrosion of steel

A new model is presented which describes the growth of the duplex layers of Fe₃O₄ on mild steel in high temperature, deoxygenated, neutral or alkaline aqueous solutions. It is shown that the layers grow by the ingress of water along oxide micropores to the metal-oxide interface and by the rate-limiting outward diffusion of Fe ions along oxide grain boundaries. The new model accounts for the observed temperature-dependence and pH-dependence of the corrosion, the morphology of inner and outer layer crystallites, the segregation of alloying elements, and the location of hydrogen evolution. The model can also be generalized to other steels and alloys.     

射频磁控溅射沉积TiAlN薄膜的电阻率温度系数(英文)

TiAlN薄膜是一种有可能作为喷墨打印头中传统的TaN或TaAl发热电阻的替代品。采用TiN和AlN作靶材,在400°C下用射频磁控溅射共沉积方法在Si(100)基底上制备TiAlN薄膜,研究了磁控溅射沉积时等离子体功率密度对TiAlN薄膜电阻率温度系数和抗氧化性能的影响。结果表明,TiAlN薄膜的结晶度、晶粒尺寸和表面粗糙度随着等离子体功率密度的增加而增大,从而导致大晶粒和小晶界。X射线光电子能谱分析得到的Ti、Al和N的键合能表明,TiAlN中氮素化学计量学亏缺使TiAlN薄膜的电阻更大。在最高等离子体功率密度下制备的TiAlN薄膜具有最高的抗氧化性能和最低的电阻率温度系数(765.43×106K1)。     

Impedance of Cr18Ni10 stainless steel in sulphate solutions after a low‐temperature plasma nitriding

The 00Cr18Ni10 stainless steel was examined before and after plasma nitriding in a 80%N₂/20%H₂ gas mixture at 425°C for 30 h, resulting in the formation of a supersaturated solid solution of nitrogen in austenite (S phase) with 12.9 wt.% N at the surface. Electrochemical measurements were made on the as‐nitrided surface and after abrasion to various depths. Polarisation curves and impedance were measured in 0.1 M Na₂SO₄ acidified to pH 3.0; some measurements were also made at pH 6 and 9. It was found that nitrogen decreased the corrosion resistance of the steel in the acidified solution (pH 3.0), but increased it in the neutral and near‐neutral solutions (pH 6.0 and 9.0). The deleterious effect of nitrogen at pH 3.0 was at potentials of the active state much stronger and more dependent on the nitrogen concentration than in the passive state. It was suggested that the increased anodic reactivity of the nitrided steel in the active state is inherent for the lower thermodynamic stability of the supersaturated solid solution of nitrogen, whereas the increased corrosion resistance in the neutral and near‐neutral sulphate solutions is associated with the effect of nitrogen on anodic films.     

The hydration of barrier oxide films on aluminium and its inhibition by chromate and phosphate ions

Compact oxide films were formed on aluminium in a neutral borate solution (pH 7.4, 20°C) by applying a constant potential of 50 V (SCE), and then immersed in distilled water, a 0.001 mol dm^?3 chromate solution (pH 7.0), or a 0.001 mol dm^?3 phosphate solution (pH 7.0). Changes in the composition profile of the oxide caused by the immersion were examined by X-ray photo-electron spectroscopy (XPS) combined with a film sectioning technique. After a 72 h immersion in distilled water, the OH^? content in the outer part of the film increased to about three times as that before immersion, and the film sustained only 30 V instead of the initial 50 V. As the profile of BO₂^? ions remained unchanged, the deterioration of the film is exclusively due to hydration of the oxide caused by the penetration of water molecules or OH^? ions into the oxide lattice. It was found that the hydration is strongly inhibited by chromate and phosphate. XPS and chemical analysis showed that CrO₄^2? and PO₄^3? ions adsorb on the oxide surface to form a mono- or bi-layer, hindering the penetration of water molecules. The inhibiting behaviour, stabilizing oxide films, is important in explaining the function of these anions as corrosion inhibitors for metals in near neutral solutions.     

Modeling steel corrosion under supercritical CO2 conditions

Corrosion in the presence of CO₂ was studied under supercritical conditions (high pressure and moderate temperature) using different carbon steels and various corrosion resistant alloys (CRA's). An objective of this work was, among others, to put the results in perspective relative to various CO₂ corrosion models that have been developed and published over the years. In particular, the NORSOK, FREECORP, and OLI models were used for comparison, while others were discussed as well. The systems investigated were (a) supercritical CO₂ (SC CO₂) saturated with water (no separate water phase), (b) a water mist phase in equilibrium with SC CO₂, (c) a water/brine phase in equilibrium with SC CO₂. It was found that the OLI model best simulates the experimental corrosion rates observed in system (c) where coupons were corroded in the aqueous phase at high velocities in the rotating cage. The varying susceptibility of different carbon steels to corrosion under these conditions is highlighted. CRA steels are comparatively more resistant.     

Fabrication of Co−Ni−P film with excellent wear and corrosion resistance by electroplating with supercritical CO2 emulsion

To avoid the defects caused by the hydrogen evolution and improve the corrosion and wear properties of the electroplated films in the traditional aqueous bath electrodeposition, a supercritical carbon dioxide (Sc-CO₂) emulsion was proposed to electrodeposite ternary nanocrystalline Co?Ni?P alloy films. Microstructure, corrosive and tribological properties of the Co?Ni?P films were investigated and compared with the ones electroplated by conventional method. The results show that the Co?Ni?P films produced with Sc-CO₂ assisted electrodeposition exhibit a more compact microstructure. The preferred orientation plane of hcp (110) for the Co?Ni?P films produced in conventional aqueous bath is changed to be hcp (100) for the one prepared in emulsified Sc-CO₂ bath. The microhardness, corrosion resistance and tribological properties of the Co?Ni?P films are substantially improved with the assistance of Sc-CO₂ in the electrodeposition bath.     

Electrodeposition of polypyrrole on 316L stainless steel for corrosion prevention

The electrosynthesis of polypyrrole films onto 316L stainless steel from near neutral and alkaline solutions containing molybdate and nitrate is reported. The corrosion behavior of the coated electrodes was investigated in NaCl solutions by electrochemical techniques and scanning electron microscopy. The polymer formed potentiostatically in a solution of pH 12 is the most efficient in terms of adhesion and corrosion protection. The coating significantly reduces the pitting corrosion of the substrate. The results are interpreted in terms of the nature of dopants, the good electroactivity of the polymer formed in alkaline solution and the passivating properties of the oxide layer.     

Electrochemical characteristics of steel coated with TiN and TiAlN coatings

The corrosion protection characteristics of titanium nitride (TiN) and titanium–aluminum nitride (TiAlN) coatings produced on cemented carbon steel targets were investigated in aqueous sodium chloride solution. All coatings were produced by cathodic arc plasma deposition. The results indicated that it was possible to follow the corrosion behavior of the coated systems over a period of 300–900 h of immersion. It was found that the TiN and TiAlN coatings had a lower corrosion rate (current density), about three orders of magnitude lower than the untreated steel substrates. The metal substrate was actually passive in these experimental conditions, and exhibited an electrochemical impedance response that could be described by means of the same equivalent circuit than for the coating. Nevertheless, the analysis of the impedance parameters allowed for direct information concerning the enhancement of the corrosion resistance of the coated system as compared to the passive uncoated metal substrate to be extracted. The major corrosion mechanism for the coated samples arises from electrolyte penetration in the pores of the deposits, which may eventually lead to the development of localized forms of corrosion.     

Preparation of TiAlN/ZrN and TiCrN/ZrN multilayers by RF magnetron sputtering

Titanium-based nitride coatings on cutting tools, press molds and dies can be used to prolong their life cycle because of their superior corrosion and oxidation resistance. TiAlN/ZrN and TiCrN/ZrN multilayer coatings were prepared by RF magnetron sputtering, and their microstructural evolution and corrosion resistance during heat treatment were investigated. The TiAlN/ZrN and TiCrN/ZrN multilayer coatings are degraded by heating up to 600 °C with the formation of oxides particles on the surface. During the heat treatment, the TiCrN/ZrN and TiAlN/ZrN multilayer coatings show the lowest corrosion current density and the highest polarization resistance at temperature range of 400–500 °C. Consequently, the TiAlN/ZrN and TiCrN/ZrN multilayer coatings show good corrosion resistance at temperature range of 400–500 °C during heating.     

空心阴极离子镀TiAlN复合薄膜结构及抗氧化性能的研究

利用IPB30/30T型空心阴极离子镀膜机并改变蒸发源料中Ti、Al的比例,在不锈钢表面沉积了不同Al含量的TiAlN薄膜;电子探针分析结果表明涂层为内层富铝,外层富TiN的梯度涂层;X-ray衍射分析表明,薄膜相结构主要为δ-TiN的B1NaCl结构,薄膜的择优取向随着镀料中Al含量的增加由(111)向(220)转变.与TiN相似,TiAlN薄膜为柱状晶结构,但Al的引入使薄膜中使针孔数量和直径减小,致密性改善.600℃～800℃下静态空气中恒温氧化实验表明,TiN和TiAlN薄膜氧化时都在表面形成金红石结构的TiO2,但添加Al的薄膜具有比TiN薄膜更好的抗氧化性.扫描电镜观察表明,添加Al的薄膜表面的氧化膜平整致密,无孔洞;而TiN薄膜在氧化过程中形成数量众多的孔洞.600℃时在NaCl和水蒸气的综合作用下,不锈钢基体材料腐蚀严重,TiN涂层对不锈钢基体有一定的保护作用,但其表面腐蚀产物部分脱落,出现大量的腐蚀空洞,而TiAlN涂层表面腐蚀产物均匀致密,无明显空洞.加入Al引起薄膜结构的改善可能是薄膜抗氧化性和抗腐蚀性提高的原因.