On the corrosion behaviour of novel high carbon rail steels in simulated cyclic wet-dry salt fog conditions

Eutectoid rail steels are prone to excessive corrosion in the coastal locations in India. In order to minimise this problem, four new rail steels with microalloying elements, Cu, Cr, Ni and Si were designed. The corrosion behaviour of these four rail steels were compared with the behaviour of three rail steels already in commercial application. Quantitative evaluation done by weight loss measurements after simulated wet-dry salt fog exposure test showed similar weight loss values for all rail steels. The FTIR spectra of rust samples revealed the presence of Fe_3−xO₄ as the major phase in both inner and outer layers of rusts on all the rail steels. Relative amounts of the different rust phases have been compared. SEM micrographs of the rusted samples revealed that the rust on Cr-Cu-Ni and Cr-Cu-Ni-Si rail steel was more compact than other rail steels. Impedance spectroscopy showed that the rust formed on Cr-Cu-Ni and Cr-Cu-Ni-Si rail steels resulted in the higher impedance in the high frequency region, compared to other rail steels.     

Clarification of chemical state for alloying elements in iron rust using a binary-phase potential-pH diagram and physical analyses

A binary-phase potential-pH diagram has been investigated to evaluate the chemical stability of various kinds of double oxide rusts (Fe-X) to get a principle for alloy design enhancing the corrosion resistance of steels. It was found that there are the following types of alloying elements enhancing the corrosion resistance of steels in the rust: (1) iron substitution type (Ni), (2) oxide formation type (Al), (3) metallic type (Ru), and (4) oxygen-acid salt type (WO₄). X-ray photoelectron spectroscopy and transmission electron microscopy analyses have been conducted on the rust formed on the low alloy steel in a saline environment. The analytical results were discussed using potential-pH diagrams. The iron substitution type and the oxide formation type elements make spinel double oxides with iron. In the corrosion tests, steels added with Ni or Al had high corrosion resistance. Thus it is possible to obtain high corrosion resistance by the creation of spinel double oxide such as Fe₂NiO₄ and FeAl₂O₄ in an inner layer.On the other hand it was found that the metallic type and the oxygen-acid salt type elements were not contained into the iron rust. In particular the oxygen-acid salt elements were excluded from the iron rust and concentrated into the defects of the rust. It is suggested that insoluble salts like FeWO₄ are formed on the base metal in the defects to act as an anodic inhibitor. Thus, the addition of a small quantity of W gives high corrosion resistance.The penetration of Cl ions can be prevented by the spinel double oxide in an inner layer and the oxygen-acid salt in the defects. In this way, the high corrosion resistance by the addition of these elements can be understood from the potential-pH diagram and the physical analyses.     

Electrochemical behaviour and structure of rust formed on Si- and Al-bearing steel after atmospheric exposure

The corrosion resistance of Si- and Al-bearing steel was estimated by atmospheric exposure test, and the structure of the rust was examined by EPMA (electroprobe X-ray microanalysis) and TEM (transmission electron microscopy) analysis. Moreover, the electrochemical behaviour of rust was investigated by EIS (electrochemical impedance spectroscopy).The Si- and Al-bearing steel exhibited excellent corrosion resistance in the exposure test as compared with carbon steel (SM). EPMA and TEM analysis showed that Si and Al mainly existed in nanoscale iron complex oxides in the inner rust formed on this steel. The Al K spectrum of the rust exhibited a peak that was the same as that of Al₂O₃ in the EPMA and TEM-EELS (electron energy loss spectroscopy) analysis. This result suggests that Al was present in the complex oxides as Al³⁺. In the same way, Si was identified as being in an intermediate state in the complex oxides of the inner rust.EIS measurement of the exposure test samples revealed much higher rust resistance (R_rust) and corrosion reaction resistance (R_t) of Si- and Al-bearing steel compared to that of SM. Finally, it was found that nanoscale complex iron oxides formed in the inner rust of Si- and Al-bearing steel, resulting in increased R_rust and R_t, and corrosion suppression.     

Effect of nickel on ion-selective property of rust formed on low-alloying weathering steel

The effect of nickel on the ion-selective property of the rust layers formed on the low alloy weathering steels under marine atmosphere was investigated by an alternate wet-dry accelerated corrosion test.Four types of low-alloying steels with different Ni contents were prepared for the corrosion test.The rust formed at various stages of corrosion was characterized by electrochemical impedance spectra(EIS),electron microprobe analysis(EMPA) and scanning electron microscopy(SEM).Afterwards,the corrosion resistance property of the rust was studied in the light of the experimental results.It is shown that increasing Ni content has significantly improved the corrosion resistance of the low-alloying steels under marine atmosphere.Furthermore,it is noted that an evident cation-selective property emerges in the rust layers with the Ni content of the rust layers up to 1.82%.However,when the Ni content of steel the matrix is below 1.5%,it has little effect on the cation-selective property.The ratio of Ni content in between the rust layer and the steel substrate decreases with the increase of Ni content in the steel substrate and varies little as the corrosion proceeds.     

Ni对耐候钢在模拟海洋大气环境下耐蚀性的影响

通过周期浸润加速腐蚀实验,研究了不同Ni含量的耐候钢在模拟海洋大气环境下的腐蚀规律.采用失重法评价耐候钢的耐蚀性,并利用扫描电镜 (SEM),X射线衍射(XRD) 和电化学方法对耐候钢表面生成的锈层进行了分析.结果表明,随着Ni含量的增加实验钢的耐蚀性逐渐增加,当Ni含量超过3%(质量分数) 其耐蚀性较对比钢提高了两倍;Ni的存在能够提高实验钢的自腐蚀电位,并促进保护性腐蚀产物α-FeOOH的形成;电化学阻抗测试结果表明,实验钢中Ni含量越多,实验钢的电阻越大,锈层的保护性越好.     

Raman mapping of corrosion products formed onto spring steels during salt spray experiments. A correlation between the scale composition and the corrosion resistance

There is a growing trend in the automotive industry to reduce vehicles weight so as to increase fuel efficiency and therefore reduce CO₂ emissions. For many automotive components such as springs, weight reduction is sought through an increase in the mechanical properties (allowing smaller components size).For ultra high strength springs, a good corrosion resistance becomes essential to avoid surface damage that will be detrimental to the corrosion-fatigue resistance. Corrosion-fatigue failures indeed often initiate on surface defects caused by corrosion in service (corrosion pits). Therefore, while of moderate importance in conventional spring steels, the corrosion resistance of ultra high strength spring steels is of primary importance.Fine changes in steel chemical composition can have an important effect on corrosion resistance. To understand the individual action of each element on the corrosion resistance of spring steels, corrosion products formed on samples exposed to NaCl environments were characterized using Raman spectroscopy, in a purposely designed experimental tool that allows mapping of corrosion products on the steel surface (by nature and mass fraction).Different steel grades were thus characterized after accelerated corrosion tests, and a clear correlation was established between weight loss and the nature of the corrosion products.     

Studies on the initial stages of zinc atmospheric corrosion in the presence of chloride

A four-year exposure program was carried out in Taiwan in which 23 test sites with different climatic and pollution conditions were chosen and evaluated according to ISO standards 9223-9226. Examination of the results indicated that most of the tests sites were very corrosive to zinc specimens and there was a severe white rust problem for freshly galvanized items stored in high humidity outdoors environments. In addition, the initial stages of zinc atmospheric corrosion in the presence of chloride were studied quantitatively in a non-aqueous electrolyte (methanol) using ex situ electrochemical impedance spectroscopy (EIS) to determine polarization resistance (R_p). The samples were exposed to the synthetic atmospheres with careful controlled relative humidity, temperature, and contaminating salts. It was observed that a change of R_p was accompanied by a change in the corrosion product on the zinc surface, and that the R_p increased with relative humidity (RH) during pre-exposure. Furthermore, the corrosion products of zinc were analyzed qualitatively by grazing-angle X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Zinc hydroxycarbonate (Zn₅(CO₃)₂(OH)₆) and zinc oxide (ZnO) in this surface layer were found to provide protection against chloride contaminants.     

Susceptibility to corrosion damage of pipeline steels under coating disbondments

This study provides an experimental investigation on the corrosion behaviour of three carbon steels used for pipeline application. The susceptibility of these materials to corrosion damage was analysed in order to simulate its service conditions particularly under disbonded coating. Monitoring of open‐circuit potential (E_free), polarization resistance (R_p) and measuring of the weight loss during immersion time were used to evaluate the corrosion behaviour of the studied materials. All the corrosion experiments were performed in two aqueous solutions: natural seawater and synthetic one (3 wt% NaCl solution). The morphology of the corrosion products was examined by optical microscopy. The results obtained from electrochemical tests have shown different behaviour for the studied steels into the retained corrosive environments: more stable potentials (E_free), higher R_p‐values with large fluctuations evolution were found in natural seawater. The gravimetric measurements have also shown a continuous variation of the weight loss throughout the exposure period in the sodium chloride solution. However, it seemed that a passive behaviour was observed in natural seawater. A little difference was observed between all the studied steels in terms of corrosion kinetics. The steel, having the little ferritic grain size, seems to be more resistant to corrosion damage. Qualitatively, a porous and non‐adherent oxide film was observed on the corroded surface in the synthetic solution; while, the rust layer, which is formed in the natural seawater, has acted as a barrier of corrosion process. Finally, all the results obtained from both electrochemical tests and weight loss measurements were in reasonably good accordance. The important common point that can be concluded was that all the tested materials seem to be more suitable for natural seawater than 3 wt% NaCl solution. Also, they are not recommendable to be used in an environment where chloride attack is possible and important.     

Cu,P,Cr 和 Ni 对低碳钢耐蚀性的影响简

选择Cu-P-Cr-Ni钢、Cu-P-Cr钢和Q235碳钢,在0.01 mol/L的NaHSO3溶液中进行周期浸润、阻抗谱和极化曲线实验,研究了Cu-P-Cr-Ni系合金钢相比Q235碳钢在模拟工业大气(SO2)环境下的耐腐蚀性能;利用SEM,EPMA面扫描和XRD分析腐蚀锈层的形貌、组成及Cu,Cr和Ni的元素分布情况。结果表明:Cu-P-Cr-Ni系钢的腐蚀诱发敏感性最低,其次为Cu-P-Cr钢,腐蚀速率分别为Q235碳钢的59.5%和52.8%;锈层分为内、外两层,致密的内锈层明显发生Cu的颗粒状、Cr的团聚状富集,外锈层主要有Cr的富集,Ni富集不明显。Cu和Cr等的富集可形成致密的内锈层,提高低碳钢的耐蚀性。     

Effect of carbon on stress corrosion cracking and anodic oxidation of iron in NaOH solutions

Anodic behaviour of decarburised iron and of quenched Fe-C materials with up to 0.875 wt% C was examined in 8.5 M NaOH at 100 °C to explain the role of carbon in caustic stress corrosion cracking (SCC) of plain steels. Removal of carbon from Armco iron strongly reduced its intergranular SCC. Slip steps on grains did not initiate cracks. It has been shown that carbon at low contents deteriorates the passivation of iron, whereas at high contents it promotes the formation of magnetite. High resistance to SCC of high carbon steels can be explained by an intense formation of magnetite on these steels.     

Influence of carbon content and microstructure on corrosion behaviour of low alloy steels in a Cl containing environment

Corrosion behaviour of low alloy steels (A and B) with different carbon content was studied by a salt fog test and an outdoor test. A commercial weathering steel 09CuPCrNi was used for comparison. The corrosion resistance of steels A and B with homogeneous microstructures was better than that of the commercial weathering steel 09CuPCrNi in the salt fog test. Steel A with an ultra-low-carbon content had far less weathering resistance than the other steels in the outdoor test. Selective corrosion of large pearlite produces stress in initial corrosion product films. Uniform corrosion product films with few cracks tend to form on homogeneous microstructures such as ferrite and bainite, and this is advantageous for the formation of a compact rust layer in the initial stage of atmospheric corrosion. However, uniform microstructures will result in over even interfaces between rust layers and bases, which will lead to frequent peeling of rust layers from bases because stress is induced by large temperature fluctuations and wet-dry alternations. Protection of the rust layer on a low alloy steel is dependent on the rust density and the bonding performance of the rust-base rather than the proportion of the rust phase in the initial stage of atmospheric corrosion. These results indicate that homogenous microstructures, proper amounts of carbon content and fine carbon-rich phases that are produced by appropriate processes are beneficial for the corrosion resistance of steels.     

In-depth distribution of rusts on a plain carbon steel and weathering steels exposed to coastal-industrial atmosphere for 17 years

In-depth distribution of rusts on two weathering steels and a plain carbon steel exposed to atmosphere for 17 years under a bridge at a coastal + industrial region in Japan were studied. In the rust layer on all specimens, α-FeOOH, β-FeOOH, γ-FeOOH, Fe₃O₄ and so-called amorphous rust were found. Within rust layers, there were thick parts and thin parts, which were finely and complicatedly distributed on steels. Among these rust species, α-FeOOH was dominant on all specimens. α-FeOOH appeared almost homogeneously through the rust layer. Its concentration was higher on weathering steels than on plain carbon steel. β-FeOOH was found mainly at thick parts and was scarce at thin parts of rust layers. Concentration of α-FeOOH was higher and that of γ-FeOOH was lower on weathering steels than on plain carbon steel. Amorphous rust was located at the bottom of the rust layer irrespective of steel types. Concentration of magnetite was negatively correlated with concentration of β-FeOOH.     

Atmospheric corrosion of field-exposed magnesium alloy AZ91D

The magnesium alloy AZ91D was exposed in three different types of atmospheric environment, viz. urban, rural and marine exposure sites. Corrosion rates, corrosion products formed, and the influence of the microstructure on the corrosion behaviour of the alloy were investigated. The corrosion rate of AZ91D exposed in the marine environment was 4.2 μm/year, and in the rural and urban environments 2.2 and 1.8 μm/year, respectively. The main corrosion product found was magnesium carbonate hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), which was formed at all three exposure sites. The corrosion attack started in the -phase in larger grains at the boundary between the -phase and the eutectic -/β-phase. Microgalvanic elements were formed with the eutectic -/β-Mg phase as cathodic site and the -Mg grains as anodes. The Al–Mn particles played a minor roll in the initiation process, even though these particles are the most noble in the microstructure and thus the driving force for a corrosion attack around these particles could be expected to be high. A close resemblance was observed between the corrosion mechanisms operating under the field-exposure conditions described here and the mechanisms operating under the previously reported laboratory conditions.     

A new interpretation of the corrosion loss processes for weathering steels in marine atmospheres

Most available data sets for the long-term corrosion loss of various grades of weathering steel exposed to marine atmospheric environments are demonstrated to be consistent with the multi-phase corrosion model previously proposed for steels exposed to marine environments. This means that the early corrosion of weathering steels by oxidation is gradually inhibited by the build-up of corrosion products. These produce anoxic and sub-oxic conditions that may permit microbiological activity to govern the longer-term corrosion loss process. This new interpretation for the long-term corrosion of weathering steels may have implications for the design of such steels.     

Assessment of protective function of steel rust layers by N2 adsorption

The specific surface area (SA) of the rusts formed by exposing various kinds of steels in different situations was determined by N₂ adsorption. The SA values of the rusts increased with the increase of corrosion rate, implying that the rust layers with large SA exhibit a high resistance to corrosion. The suppression of rusting by compact rust layers was interpreted by the blockage of pores in rust layers by the adsorption and capillary condensation of water. The SA values clearly reflect the corrosion levels estimated by the external observation. It was convinced that the SA measurement is a universal quantitative technique to appraisal the protective function of rust layers.     

Steam oxidation resistance of new 12%Cr steels: Comparison with some other ferritic steels

The oxidation resistance in pure steam at the 600-650 °C temperature range of a newly developed 12%Cr steel has been investigated for long-term exposures (224 days = 5,376 h). The laboratory and industrial heats were tested in comparison with other ferritic 9-13% chromium steels. Corrosion rates were determined by direct measurements of mass losses obtained after a reducing descaling process. Weight loss and metallographic results confirm the good corrosion resistance in steam of the new steel and allow classing the tested steels in 2 families: one classical with average oxidation behaviour, “T91-type” and another one with low mass losses, varying very slightly with the temperature and the exposure time increasing. To have a better understanding of the observed phenomena, the possible influences of the main alloying elements (Cr, Si, Mn, Mo, W) of steels mentioned by different authors were reviewed and compared to the results obtained for the ten 9-13%Cr studied steels. It appears that the alloying elements cannot be considered separately: as a matter of fact they have not only a specific influence but also a joint influence on the steam corrosion behaviour of the 9-13%Cr ferritic steels.     

A study on the initial corrosion behavior of carbon steel exposed to outdoor wet-dry cyclic condition

The initial corrosion behavior of carbon steel subjected to outdoor wet-dry cyclic exposure and exposure under natural environments have been investigated. The weight loss results indicate a transition from corrosion acceleration to deceleration during the early stage of corrosion of carbon steel under both conditions. The corrosion kinetics under both conditions follow empirical equation D = Atⁿ. Outdoor wet-dry cyclic exposure significantly promoted the initiation but the rate of corrosion was about three times as fast. The morphology of corrosion surfaces and cross-section of rust layer have been examined using SEM and the compositions have been analyzed using XRD and EPMA.     

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H₂SO₄ at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.     

The influence of chromium content on the electrochemical behavior of weathering steels

To investigate the influence of chromium content on corrosion characteristics of weathering steels, the electrochemical measurements were performed on the steels containing 0–9% Cr (wt.%) in NaHSO₃ aqueous solution. The results indicated that the open circuit potential of these steels shifted to the positive direction remarkably, because the additions of Cr improved the passivation capability of the steels. The corrosion current density of the steels containing more than 7% Cr (wt.%) decreased significantly after pre-rusted treatment, implying the corrosion resistance could be enhanced by the formation of protective goethite rust layer.