XPS and AES studies of the high temperature corrosion mechanism of Fe-30Cr alloy

Fe-30Cr alloy specimens were pre-oxidized at two different oxygen partial pressures (10^?16 and 10^?19 atm) at 900°C and subsequently exposed to environments containing both oxygen and sulfur. The sulfur and oxygen partial pressures were maintained such that Cr₂O₃ was the stable phase. The Cr₂O₃ scales formed during pre-oxidation became rapidly unstable when exposed to an environment whose composition approached the chromium sulfide-chromium oxide phase boundary; but when exposed to a higher oxygen partial pressure with the same sulfur partial pressure, the preformed scales remained intact. For elucidating the sulfidation mechanisms, the reaction products on the surface were analyzed at different stages of sulfidation by X-ray photo-electron spectroscopy and Auger electron spectroscopy. Correlation of the reaction mechanisms with thermodynamic and transport parameters are discussed.     

用XPS和AES研究锌表面彩色防腐蚀膜

用阴极极化法从钼酸盐和磷酸盐的混合溶液中获得了锌表面彩色转化膜。此膜具有良好的防腐蚀和装饰效果。盐雾试验结果表明,金黄色膜耐蚀性最佳。XPS和AES分析结果表明,该膜层厚度约为75nm;膜表面钼以Mo(VI)状态存在,而在膜内则以Mo(VI)和Mo(IV)共存。从AES深度分布曲线的组成恒定区求得各组成元素的相对原子百分浓度(A.C.%)为:O 56.9%;Mo 26.8％;P 11.0％;Zn 5.3％。     

The stability of localized corrosion

Chloride pitting of iron group metals at noble potentials proceeds in the polishing state dissolution, provided that metal chloride in the pit solution is maintained above a critical concentration. It ceases to progress by pit repassivation if the pit is smaller than a critical size, or transforms into the active state pitting if the pit size is greater. The boundary potential between the polishing state and the active state pitting may be represented by the passivation-depassivation potential in the pit solution of the critical chloride concentration. Crevice corrosion is characterized by the crevice protection potential, at which the hydrogen ion concentration in the crevice solution is equivalent to pH_pd—the passivation-depassivation pH of the crevice metal. It continues to corrode at more noble potentials than the protection potential, where the crevice solution is more acidic than pH_pd, but is inhibited in the less acidic crevice solution at less noble potentials.     

Special aspects of localized High-temperature corrosion

As in aqueous corrosion a localized corrosive attack can be important in high-temperature corrosion of metallic materials and become, unvoluntarily, a lifetime determining factor for combustion systems. Deposit-induced corrosion is the most important form of localized corrosion on alloy 800 and other materials in advanced coal combustion systems. Coal-ash deposits may cause a sulfidation/oxidation reaction which propagates preferentially along grain boundaries into the underlying metallic material. Preferred grain boundary corrosion may also occur in the walls of the fuel nozzle tubes of gas combustion systems. In this case, an oxidation reaction takes place. The lifetime of the tubes can be increased by use of a special alloy 601 H grade. Pitting may be observed in coal combustion systems. It has also been observed on alloy 601 after service in ceramic firing kilns where it is promoted by the presence of chlorine. Additionally, pitting occurs under carburizing conditions (metal dusting) as does cracking being similar in appearance to stress corrosion cracking in aqueous corrosion. Compared to deposit-induced and grain boundary corrosion, crevice corrosion is of minor importance in high-temperature corrosion. Only one example has been identified so far on alloy 600 but which may also be interpreted as a kind of deposit-induced corrosion.     

Indoor atmospheric corrosion in Cuba. A report about indoor localized corrosion

Indoor weight loss of steel, chloride, sulphur compounds and dust deposition rate were determined in six storehouses having different characteristics. Relative humidity and temperature were determined in three storehouses. A model for indoor corrosion of steel depending on time of exposure and deposition of dust, sulphur compounds and chlorides is proposed. Dust deposition plays an important role indoors. The position of the sample has also a significant influence on corrosion. Indoor corrosion aggressivity in Cuban storehouses ranges in classification IC3 and IC4 according to the new ISO proposal of indoor aggressivity.A report about the presence of localized corrosion indoors (filiform like) using a special designed sample is made.     

Protection of metals against corrosion

Metal Science and Heat Treatment -     

Bacterial corrosion of metals. II. Gravimetric investigations of the corrosion of nonferrous metals

The specific kinetic features of the bacterial corrosion of AK-4, AK-6, Д-16л, Д-16п, AMц-8 aluminum alloys; MA-8 magnesium alloy; and БpAЩMц 10-3-1.5 bronze were investigated with the use of tropical strains of bacteria. The protective effectiveness of anodic-oxide coatings applied to aluminum alloys, and that of TsIATIM-221 and AMG-10 compositions were also tested under these conditions.     

An ellipsometric approach to localized corrosion processes

Qualitative uses of ellipsometry, that can provide valuable insights into localized corrosion processes are reviewed. Without attempting to determine optical constants or film thickness with any accuracy, interesting data can be obtained, particularly correlations of passive film growth rates, as a function of localized corrosion factors, with the velocity of stress corrosion cracking or the rates of pit nucleation. Several applications of qualitative ellipsometry are described, relating to pitting and crevice corrosion, and stress corrosion cracking of carbon steels, stainless steels and titanium alloys.     

The localized corrosion of Cr-Ni stainless steel

The kinetics of localized corrosion of Cr-Ni stainless steel is characterized by the quadratic time dependence of the total current density, by the linear increase in the number of pits and by the decrease of the current density in the pits. The pits have a shape of a rotary ellipsoid. The Tafel slope of the metal dissolution in the pits is 0.50 V. The dissolution rate is the highest on the pit bottom and the lowest at the mouth of the pit. The different rate of metal dissolution is caused by the different concentration of chloride ions over the pit surface.     

The localized corrosion of resistant alloys in chloride solutions

A potentiostatic scratching technique has been used to study the breakdown potentials of stainless steel 304, Incoloy, Inconel 600, Monel 400 and Hastelloy C in chloride solution at pH 8 as a function of chloride activity, temperature and metal cold work. For the first four alloys breakdown occurs through pitting. For Hastelloy C there is no pitting process, and breakdown occurs by a transpassive reaction, probably involving oxidation of Mo and Cr in the film to soluble anions.     

The localized corrosion of resistant alloys in chloride solutions

A potentiostatic scratching technique has been used to study the breakdown potentials of stainless steel 304, Incoloy, Inconel 600, Monel 400 and Hastelloy C in chloride solution at pH 8 as a function of chloride activity, temperature and metal cold work. For the first four alloys break-down occurs through pitting. For Hastelloy C there is no pitting process, and breakdown occurs by a transpassive reaction, probably involving oxidation of Mo and Cr in the film to soluble anions.     

Outdoor-indoor corrosion of metals in tropical coastal atmospheres

Results of indoor and outdoor atmospheric corrosion tests conducted during a long period of time at Cuba and Campeche (Mexico) indicated very high corrosion rates at both sites which have humid-tropical marine climate. We found that the outdoor corrosivity ranges from C3 to >C5 according to ISO 9223 nevertheless metals exposed to sheltered conditions presented higher corrosion rates compared to outdoors, whereas in closed (indoor) environments the corrosion rate significantly decreased. It is recommended to define an additional level of corrosivity for tropical coastal atmospheres in outdoor and sheltered conditions as corrosion depends on the geographical position and exposure conditions.     

Galvanic reactions during localized corrosion on stainless steel

During localized (crevice and pitting) corrosion, a local cell is established between an anode within a crevice or pit and a cathode on the surrounding passive surface. Data are presented to show that concentrated acidic chloride solutions, simulating corrosion product hydrolysis within a crevice or pit, produce potentials which are active (negative) to the normal surface passive potential. This behaviour explains the previously observed active drift of corrosion potential after initiation of crevice or pitting attack in dilute chloride solutions. The active state in concentrated chloride solutions was quite noble (positive) compared to the active state in more dilute solutions. Thus, there is no need to invoke ohmic resistance effects to account for the active state within a crevice or pit.Experiments were devised in which the local anode within a crevice was physically separated from the nearby passive-surface cathode. When the two were coupled together electrically, the cathode surfaces were polarized nearly to the unpolarized local anode potential, with only a few millivolts anodic polarization at the anode within the crevice. The rate of localized corrosion appears from the data to be limited by the rate of dissolved-oxygen reduction on the cathode surfaces. Thus, localized corrosion in dilute chloride solutions will be increased by (a) raising the temperature, (b) adding an oxidizer such as Fe³⁺ ions, or (c) substituting external anodic polarization for dissolved oxidizers.The overall potential, E_corr acquired by a specimen undergoing pitting or crevice corrosion is demonstrated to be near the protection potential, E_p below which pitting corrosion cannot propagate. Any potential active to E_corr and E_p results in cathodic polarization and suppression of the anode reaction in a crevice or pit. Since both E_corr and E_p vary with the extent of previous localized attack, E_p is not a unique property of the alloy as has been sometimes suggested and is of limited value in classifying alloy resistance to localized corrosion.     

The measurement of localized corrosion using electrochemical noise

Measurements of self-generated electrochemical potential fluctuations on electrodes undergoing either pitting or crevice corrosion show that these forms of localized attack have quite distinct noise “signatures” and that these two types of attack can be detected within seconds of their initiation.     

The corrosion behavior of Fe-Mn-Al weld metals

The corrosion resistance of a newly developed iron-base, Fe-Mn-Al austenitic, and duplex weld metal has been examined in the NACE solution consisting of 5 wt.% NaCl, 0.5 wt.% acetic acid, and the balance distilled water. The electrochemical techniques such as potentiodynamic polarization, Tafel plots, linear polarization, cyclic polarization, and open-circuit potential versus time were employed. The Fe-Mn-Al weld metals did not passivate and exhibited high corrosion rates. Fe-Cr-Ni (310 and 316) weld and base metals were also examined in the NACE solution at room temperature. The 310 and 316 base metals were more resistant to corrosion than the as-welded 310 and 316 weld metals. Postweld heat treatment (PWHT) improved the corrosion performance of the Fe-Mn-Al weld metals. The corrosion resistance of Fe-Mn-Al weld metals after PWHT was still inferior to that of the 310 and 316 weld and base metals.     

Electrochemical noise analysis of localized corrosion by wavelet transform

Electrochemical noise (EN) was measured during pitting and stress corrosion tests in NaCl solutions for type 403 stainless steel tempered at 390 °C for 2 h. The EN data were analyzed mainly by wavelet transform in terms of the energy contribution of smooth crystal and detail crystals in three frequency regions. In the early stage of the pitting tests, general corrosion occurs, and contribution of smooth crystal to the total energy was greater than that of low frequency detail crystal, but less with the development of meta-stable pitting. A similar trend was observed in the early stage of stress corrosion cracking (SCC) tests. The contribution of low frequency detail crystals to the total energy decreased as stable pits were formed, and it was less than the energy contribution of smooth crystal when crack propagation occurred in the later stage of SCC. In addition, the contribution of energy of high frequency detail crystal is shown to be indicative of the occurrence of plastic deformation. These results demonstrate that the onset of detailed processes in pitting and SCC can be sensitively detected by wavelet analysis in terms of the energy contribution of smooth crystal and low and high frequency detail crystals, but not with that based on the change in position of peak in energy contribution of detail crystals.     

The electrochemistry of intermetallic particles and localized corrosion in Al alloys

Intermetallic particle electrochemistry plays significant role in localized corrosion and stress-corrosion cracking of aluminum alloys. This article presents specific examples from recent and on-going studies of stress-corrosion cracking in Al-Mg alloys and localized corrosion in Al-Cu alloys. The common approach in these examples is an explicit accounting of the intermetallic particle-dissolution characteristics, which is developed by measuring the electrochemical behavior of bulk analog compounds. Findings from these types of measurements combined with results from new and advanced small-length scale measurements confirm some of the established ideas about localized corrosion in aluminum alloys, but also lead to new insights that are important for a thorough understanding of corrosion damage accumulation. For more information, contact R.G. Buchheit, The Ohio State University, Department of Materials Science and Engineering, 477 Watts Hall, 2041 College Road, Columbus, Ohio 43210; (614) 292-6085; fax (614) 292-9857; e-mail buchheit.8@osu.edu.     

Probing the atmospheric corrosion of metals. Zinc

Scanning vibrating capacitor (Kelvin probe) and electrochemical impedance spectroscopy are used in electrochemical monitoring of the atmospheric corrosion of zinc. The effect of the corrosion product layer on the surface potential and capacitance is studied. Upon the local application of NaCl, the spatial separation of the cathodic and anodic reactions was noticed in the profiles of the electrode potential and chemical composition of the corrosion products (infrared microscopy). The effect of corrosion inhibitors and activators on the electrochemical atmospheric corrosion of zinc is studied. Original Russian Text ? A.P. Nazarov, D. Thierry, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 5, pp. 478–492.     

Contact corrosion of metals in aqueous media. Part I

The corrosion behavior of aluminum, steel, and cast iron in aqueous solutions is studied both for individual metal specimens and short-circuited systems (aluminum-steel, aluminum-cast iron, steel-cast iron, and aluminum-steel-cast iron). It is shown that aluminum behaves as a cathode with respect to steel and cast iron. The high corrosion rates of ferrous metals are caused by the specificity of iron surface state in aqueous solutions of electrolytes. Original Russian Text ? V.N. Esenin, L.I. Denisovich, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 4, pp. 390–395.