Studies on newer chemical passivation treatment for stainless steel

Abstract

An attempt has been made to develop a chemical passivation treatment for AISI 304 stainless steel based on nitric acid in the presence of various oxidising agents such as sodium molybdate, potassium dichromate and potassium permanganate. The stability of the passive film formed through different chemical passivation treatments has been studied by potential–time measurement in 0·5 M sulphuric acid and the operating parameters of chemical passivation have also been optimised by this technique. Results of the study show that chemical passivation of stainless steel in nitric acid shifts the open circuit potential in the positive direction. Increase of concentration of nitric acid, treatment time and temperature further ennobles the potential. Addition of either dichromate or permanganate ions to nitric acid helps in forming a more stable passive film whereas molybdate, even at higher concentrations does not have any beneficial effect on passive film formation.     

Three-dimensional modelling of selective dissolution and passivation of iron-chromium alloys

A three-dimensional model has been developed for modelling the selective dissolution and passivation of alloys. The model has been used to simulate the passivation of iron-chromium alloys. The real structure of the alloy is taken into account (bcc in the present case), as well as the structure of the initial surface. The passivation is modelled in considering the formation of “oxide” nuclei, resulting from the presence of local chromium-rich clusters. During the dynamic evolution of the model, based on the Monte Carlo method, surface diffusion and dissolution of atoms occur according to probabilities dependent on the nature of the atom (Cr or Fe) and on its chemical environment. The conditions of simulation can be changed through a set of parameters defining the rules for surface diffusion, selective dissolution and number of Cr atoms in the Cr clusters required to initiate locally the passivation. The effects of these parameters on the simulation have been tested for an alloy containing 22 at.% Cr and compared with experimental data. The results show that the diffusion of Fe has little influence on the course of passivation while the diffusion of Cr has a marked effect. When the number of surface chromium atoms required to form a nucleus of passive film increases, the passivation becomes less rapid, with a marked effect on the composition of the passivated layer. The extent of the chromium enrichment in the passivated surface obtained in the model for the initial stages of passivation is not as high as the one measured experimentally in the stationary state of passivity.Other simulations have then been performed with various chromium contents in the alloy. The results show the existence of a transition, which is not sharp but progressive, between alloys that cannot be passivated to alloys that are passivated.     

铈基钝化法在金属防护中的应用研究进展

铈盐由于其自身无污染、成本低、性能好、用途广等优点,已成为代替铬酸盐钝化的重要方法之一.简要总结了国内外研究人员的发现,得出铈盐可以自行成膜,且铈盐有着良好的自愈能力.同时,也发现纯铈膜的防腐效果不是很理想,易出现裂纹、附着力差等问题.为解决此问题,可以将铈盐与其他金属盐结合,得到混合钝化液,所得到的复合膜效果较好,能减少裂纹的形成;也可以在铈盐钝化液中加入有机化合物,形成的有机和无机复合钝化膜效果更佳,防腐效果甚至高于铬酸盐技术.铈盐以及铈盐复合钝化膜能广泛地应用在各类金属及金属合金表面,极大程度地扩展了铈盐钝化液的应用范围,提高了在工业上的使用率.对铈盐钝化液的机理、钝化方法、应用领域等方面的最新进展进行简单地总结与探讨.为解决铈盐钝化液在钝化机理以及钝化方法方面存在的不足,对最新的工艺方法或配方改善方法进行了提炼,并对未来铈盐钝化液能有效解决原铬基钝化液的污染问题和提高涂层的防腐能力进行了展望.     

Fe78Si9B13非晶粉末的钝化工艺对其磁粉芯性能的影响

对Fe78Si9B13非晶合金带材破碎粉末制备磁粉芯中的粉末钝化工艺进行了研究。通过添加3类不同的钝化剂对铁基非晶粉末进行表面钝化处理后,可显著改善非晶磁粉芯的成形性及磁性能。改变钝化剂的配方和浓度、控制钝化程度是控制非晶磁粉芯性能（有效磁导率、品质因数Q、损耗、频率特性等）的有效手段之一。     

Korrosion von Chemieemail durch fluoridhaltige Säuren

Corrosion of chemical service glass enamel by acids containing fluoride In cases a material must be found suitable for reactions that involve strong acids, the preferred protection against corrosion taken will be chemistry grade glass enamel. The high chemical resistance of these glass enamels depends on their silica basis which represents 65–75% in the oxide formula. The excellent anti-corrosion properties of these enamels may be endangered under service conditions if the glasses get into contact with traces of fluoride which may enter a process as contaminations of a raw material. In order to determine quantitatively the influence of the various parameters in this corrosion reaction of hydrofluoric acid with the glass network forming silica corrosion tests were carried out with glass-lined samples and fluoride containing acids. The corrosion rate was determined from the weight loss of the sample. Besides the liquid phase attack further corrosion testing was evaluated of glass enamel which was exposed to acid condensate containing fluoride. The attack of hydrofluoric acid on glass enamel was found to be more severe than expected from theoretical considerations. The relation of this attack has been determined in respect to time of exposure, volume to surface ratio, concentration of fluoride, pH value, temperature, and glass quality. The possible passivation of an acid which contains fluoride with silica additions has been checked in the experiments. The user of glass-lined equipment gets with the results presented a practical tool for a better understanding of the individual corrosion situation which helps to more precise decisions on counter measures.     

Untersuchung vielkomponentiger,Nitrilotrimethylenphosphonsäure enthaltender Systeme als Inhibitoren für den Metallschutz in neutralen wäßrigen Medien

Study of multicomponent systems containing nitrilo-trimethylene phosphonic (NTMP) acid as inhibitors for the protection of metals in neutral aqueous media The inhibitor mixtures studied contain nitrilo-trimethylene phosphonic acid, oxalic acid or phthalic acid anhydride and zinc sulfate. On the basis of the measurement of the differential electric double-layer capacity using the bridge method the thermodynamic characteristics of the adsorption of the compounds studied and their mixtures on the surface of armco iron have been determined in aqueous potassium fluoride solutions. The best inhibitor mixture was then used as the starting material for optimizing the composition ratios. Using gravimetric techniques the influence of this mixture on the rate of corrosion of steel ST3S in simulated industrial water of variable chemical composition and in water being chlorinated. In addition the influence of this mixture on the rate of corrosion of copper (Cu 99,9 E), brass (CuZn 37), zinc (97,5) and aluminum alloy (AlCu4Mg1) has been studied as well and considerable decrease of the corrosion rate by the inhibitor mixtures has been established. Nitrilo-trimethylene phosphonic acid, in combination with phthalic acid anhydride and zinc ions can therefore be considered as a perspective inhibitor for the corrosion protection of industrial cooling and heating water installations.     

Elektrochemisches Verhalten und Deckschichtbildung hochlegierter Manganstähle

Electrochemical behaviour and scaling of high alloy manganese steels Passivating surface layers are considered to be one of the indispensable requirements for stress corrosion cracking of metallic materials. It is shown by potentiostatic and potentiokinetic current density-potential curves that the steel X 40 MnCrN 19 in neutral aqueous chloride solutions has a passive potential region. The passivation behaviour Of precipitation hardened samples is in agreement with the chromium depletion theory. The effect of alloying on the passivation behaviour of low carbon Mn steels is studied in 3 % NaCl solution at 20 and 100 °C Increasing proportions ε-martensite reduce the passivation of susceptibility. Increasing the Mn content has the same effect. The vital factor concerning passivation behaviour, however, is chromium content. Increasing the temperature of the corrodent results in an increased tendency to form scales of steels containing less than 8 % Cr. Long-term corrosion tests have shown, that increasing the Cr content produces a continuous transition from general localized and even pitting Corrosion. Tests made without applied current in aerated solutions have shown, that the variation in time of corrosion potentials depends from the tendency to be passivated of the materials and from the oxygen content of the solutions. In oxygen containing solutions passivable steels exhibit a pronounced corrosion in the pitting region, because with such alloys anodic dissolution current densities equal to those of the limiting diffusion current of oxygen reduction are obtained only at potentials above the pitting potential.     

Effect of sweep rate on the active-passive transitions of the Ni/H2SO4 systems

The passivation of Ni in 0·1 IN H₂SO₄ has been studied potentiokinetically at sweep rates ranging from 6·5 to 1000 mV/min, and it has been shown that the passivation potential becomes more positive with increase in sweep rate. It is suggested that above a certain critical sweep rate, the rate of diffusion of H₂O dipoles and OH⁻ ions is insufficiently fast for the nucleation of nickel hydroxide, which is considered to be the precursor of true passivation, so that the nickel remains active at a potential at which passivation would normally occur at sweep rates less than the critical value.     

Effect of products of nitric acid reduction on active dissolution and passivation of nickel

For nickel in HNO₃ solutions, the passivation process is impeded and the range of its active dissolution is broadened, as opposed to in solutions of nonoxidative acids, which is stipulated by the interaction processes between metal surface and the products of cathode reduction of the solution. It has been shown that the products of cathode reaction promote the occurrence of a new electronegative reaction of nickel dissolution in the form of a complex with ammonia, the ligand formed on the cathode. Moreover, the intermediate products of NO reduction to NO₂ by interacting with nickel surface displace passivating oxygen and, therefore, retard the process of electrode passivation. The kinetic control of the cathode reaction of HNO₃ reduction within the potential range of active dissolution of nickel is connected with the complicated NO desorption from nickel surface, which virtually eliminates the possibility of the development of homogeneous autocatalysis within a certain potential range.     

Electrochemical impedance spectroscopy as a monitoring tool for passivation and localized Corrosion of aluminum alloys

Pitting and crevice corrosion of Al alloys and Al-based metal matrix composites can be detected by characteristic changes of the impedance spectra in the low frequency region. A pitting model has been developed which is in agreement with the experimental data. A fitting procedure has been used to analyze a large number of data which have been obtained for as-received samples and samples which had been passivated in CeCl₃ solutions. This chemical passivation process produces surfaces which are very resistant to localized corrosion. Al 6061, Al/SiC and Al/graphite which had been passivated in CeCl₃ for one week did not pit in 0.5 N NaCl for at least one month. Electrochemical impedance spectroscopy (EIS) is a convenient tool for monitoring of the passivation and the corrosion processes.     

Untersuchungen zur Passivierung des Eisens in wäßrigen Lösungen durch das System „Sauerstoff/Phthalat”︁

Investigation into the passivation of iron in aqueous solutions of the system “phthalat/oxygen” The passivation properties of dissolved oxygen on iron and mild steel in potassium hydrogenphthalate solutions have been studied by means of potentiodynamic polarisation curves and potential/timemeasurements. It is shown that the critical passivation current in a solution of constant phthalate concentration decreases with increasing dissolved oxygen concentration and the solution pH. The transition from active dissolution to passivation associated with a potential shift toward more noble values depends on critical minimum values of pH and dissolved oxygen concentration in the solution. Under these conditions the oxygen reduction diffusion current is greater than the critical passivation current. This effect is interpreted in terms of interactions of phthalate ions on the interface iron/solution.     

Aminomethylenphosphonsäuren als Korrosionsinhibitoren für Aluminiumpigmente in wäßrigen Medien

Aminomethylenephosphonic acids as corrosion inhibitors for aluminum pigments in aqueous media Flakelike aluminum pigments (aluminum content >99.5%) are corroded in aqueous alkaline mixtures of water and butyl glycol. Aminomethylenephosphonic acids inhibit this corrosion reaction. The temporal progress of the corrosion is determined by volumetric analysis of the evolved hydrogen. Direct comparison of aminomethylenephosphonic acids with analogous aminomethylenecarboxylic acids shows that the phosphonic acid group inhibits the corrosion significantly better than the carboxylic group. The actual corrosion inhibitors are in both cases (soluble) aluminum(III)-chelates, which are formed by a chemical reaction of the aminomethylenphosphonic resp.-carboxylic acids with the aluminum surface. This is proved by measuring the concentration of dissolved aluminum(III) in the corrosion medium with the help of atomic absorption spectroscopy and by using aluminum(III)chelates as corrosion inhibitors.     

Eis and XPS study of surface modification of 316LVM stainless steel after passivation

The effects of surface finish, nitric acid passivation and ageing in air on corrosion resistance of 316LVM stainless steel in 0.5% H₂SO₄ have been investigated by EIS, potentiodynamic polarization measurements and XPS. The results indicate that a smoother surface exhibits to a higher corrosion resistance. The effectiveness of the passivation treatment strongly depends on nitric acid concentration, passivation time and temperature. The passivation treatment significantly increases the corrosion resistance due to a high Cr content in the passive film and increased film thickness. Ageing after passivation increases the corrosion resistance whereas ageing before passivation has little effect.     

膦酸缓蚀剂的合成及其缓蚀性能研究

合成了7种膦酸缓蚀剂。用失重法考察了膦酸浓度及溶液pH值对缓蚀性能的影响,分析了膦酸分子结构与缓蚀性能的关系。通过钢铁表面形态及膜成分分析,证明膦酸是一种吸附型缓蚀剂,在金属表面形成一层致密的憎水保护膜,使腐蚀反应得到抑制。     

铜防变色表面处理的研究

通过对AMT、BTA、TTA等铜缓蚀剂以及AMT复配物作为钝化液对铜进行表面处理的研究，开发了铜的化学转化膜表面处理新技术，替代铜的铬酸盐处理技术。经过处理提高了铜的抗氧化变色能力，采用该法可以避免铬酸盐处理造成的环境污染。     

Soft chemical stripping of aluminide coatings and oxide products on Ni superalloys

Turbine components are submitted to very aggressive environments and particularly to high temperature corrosion and/or oxidation. Because of their high cost, they have to be repaired instead of being replaced. Prior to refurbishment and recoating, the components have to be fully stripped to remove the oxide products and defective coatings.In this work, various soft chemical stripping methods are studied based on pitting acids and agents as well as oxidising acids in organic and aqueous media. The effect of the immersion time is also investigated. The feasibility of the dissolution reactions has been evaluated through thermodynamic calculations. It will be shown that the most effective baths are those containing relatively high concentrations of pitting reagents in organic media. Aqueous solutions lead to pitting of the superalloy. The stripping mechanisms have been discussed in terms of the dissolution of coating grain boundaries once the solution has passed through the oxide scales.     

Enhancement of the photoluminescence of porous silicon by sputter deposition of semitransparent metal films

Optoelectronic devices based on porous silicon (PS) undergo substantial degradation in luminescence with aging due to atmospheric oxidation. The passivation of PS has been reported with a transparent conducting material or a semi-transparent metal. In this paper, we report enhancement of the photoluminescence (PL) of PS by the passivation of PS with ultrathin metal films such as silver (Ag), aluminum (Al), and gold (Au). It has been found that Ag and Au are respectively most and least effective in enhancing the PL of PS among those three different metals. The highest PL enhancing effect of Ag is mostly attributed to the high electrical conductivity of Ag, whereas the lowest PL enhancing effect of Au is due to the lowest optical transmittance of Au. The details of the PL enhancing effect of metal passivation are discussed with the aid of FTIR analysis results.     

Corrosion under Heat Transfer: Service Failures and Laboratory Testing*

Abstract

Corrosion resistance of metals under conditions of heat transfer can be different from that under conditions where there is no heat transfer. In order to provide additional data to aid material selection for chemical plant, etc., a laboratory test has been developed in which specimens under applied heat flux are exposed to process 1iquids. The equipment used is described and details are given of results obtained in a wide range of applications including one in which anodic passivation has been used to eliminate crevice corrosion of titanium. In conclusion methods are suggested for reducing the detrimental effect of heat transfer.     

An X-ray photo-emission spectroscopy study of iron-oxide,iron-benzoic acid inhibitor films

Surface oxide and benzoic acid films on pure iron have been examined by X-ray photo-emission spectroscopy (XPS). The passivation and inhibitor properties of these films is correlated with chemical features arising in the spectra. Surface hydroxyl groups are observed on surfaces having good passivation properties. Benzoic acid forms an oriented salt film.     

铝合金三价铬钝化液的研制和应用

分析了铝合金三价铬钝化液的钝化机制,研究了复合盐钝化剂用量、钝化处理时间和温度对钝化膜质量的影响。结果表明,经铝合金三价铬钝化液钝化后的试样,其抗腐蚀性能优于铬酸钝化液,可在常温下进行处理,且处理时间较短,有利于减轻铬酸钝化液中六价铬离子对环境的影响。