Modelling immersion corrosion of structural steels in natural fresh and brackish waters

The multi-phase mean-value model previously proposed for modelling the marine immersion corrosion of low carbon and low alloy structural steels is examined for application to fresh and brackish waters. Use is made of field data for brackish and fresh waters available in the literature, supplemented with data or estimates for water temperature, pH, hardness and nutrient levels. It is shown that the data exhibits consistency with the model and that it is a function of average water temperature. Corrosion in brackish and fresh waters corrosion depends on water hardness, pH and nutrient levels, with higher pH levels and lower water hardness associated with higher aerobic levels of corrosion but these are not significant for anaerobic corrosion. In the anaerobic phases 3 and 4 of the model, the available data and associated trends are interpreted as showing that elevated levels of nutrients produce higher rates of corrosion. Conversely, these phases showed very low rates of corrosion for fresh waters with very low nutrient levels. Consistent with basic corrosion theory and with laboratory observations, salinity by itself is not a clear distinguishing characteristic. The model provides a new approach to interpreting the available data for corrosion in fresh and brackish waters. It permits plausible explanations for previous apparently inconsistent observations for corrosion in brackish waters. Finally, it reinforces the need for full and detailed reporting of corrosion testing programs, including details of precise timing, location, orientation and environmental conditions including means and variations in water temperatures, DO, salinity, pH, water hardness, carbonates and various nutrients.     

Electrochemical corrosion performance of Cr and Al alloy steels using a J55 carbon steel as base alloy

Cr- and Al-modified alloy steels using J55 carbon steel as base alloy were produced by remelting in a vacuum. Their corrosion resistance was estimated by open circuit potential, electrochemical polarisation measurements and immersion tests in a 3.5 wt.% NaCl solution. The modified alloy steels exhibit higher corrosion resistance with a more positive open circuit potential, lower corrosion current density and higher impedance than J55 steel. The immersion tests showed that the new alloy steels have lower corrosion rates and smaller pitting depth than J55 steel and a low-Cr steel.     

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.     

Role of climatic conditions on corrosion characteristics of structural steels

Corrosion behaviour of low alloy (LASS) and plain carbon (PCSS) structural steels exposed in different types of climatic conditions and nature of rust formed on their surfaces have been studied after 2 years of exposures. The test sites were chosen to represent four types of environments who strongly influence the corrosion of metals and alloys. They include (a) humid-saline, (b) humid-saline-urban (c) humid-industrial and (d) plain dry-urban environments. Mass loss, Raman spectroscopy, scanning electron microscopy and electrochemical impedance spectroscopic studies have been performed to study the corrosion behaviour and characterise the nature of rusts formed on these steels. Mass loss measurement technique has been used to determine the loss of thickness of steels during their atmospheric and salt spray exposures. Results indicate that the corrosion rate of steels is strongly influenced by the climatic conditions prevailing at the exposure sites. The presence of SO₂ and salinity in the environments change the structure and protective properties of rust formed on the steels’ surface. Electrochemical impedance and cyclic polarisation studies of the steels in simulated environments have been performed to understand the mechanism of corrosion in different climatic conditions.     

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.     

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.     

The influence of pre-straining on the corrosion fatigue performance of two hot-dip galvanised steels

The corrosion fatigue behaviour of a hot-dip galvanised ferritic pearlitic steel and of a hot-dip galvanised Ti-alloyed high-strength interstitial free steel was investigated. The testing mode was constant displacement, fully reversed plane bending. Both materials were tested as received and 10% uniaxially pre-strained. The effect of pre-straining on the zinc coating was investigated using polarisation resistance measurements and the scanning reference electrode technique (SRET). It was found that pre-straining is detrimental to the corrosion fatigue behaviour of both steels, due to damage to the zinc coating, leading to increased localised corrosion and in general higher corrosion rates.     

Effect of alternating voltage on corrosion of X80 and X100 steels in a chloride containing solution – Investigated by AC voltammetry technique

AC corrosion of the X80 and X100 steels in 0.1 M NaCl solution were studied by the AC voltammetry technique. Corrosion electrochemical kinetics and solid/solution interface structure changes under the influence of AC voltage were characterized. Results illustrate that corrosion potential of the two steels shift negatively with the increase of AC amplitude and decrease of AC frequency. The anodic processes are under charge-transfer control and the anodic Tafel slopes increase with the increase of AC magnitude. The cathodic processes are under diffusion control at low AC amplitudes, while they become increasingly under charge-transfer control with higher AC amplitudes.     

The evolution of the corrosion of iron in hydraulic binders analysed from 46- and 260-year-old buildings

Corrosion patterns on samples taken on binder and rebars from two buildings, respectively, aged of 46 and 260 years old have been characterised by coupling different analytical methods at microscopic scale. Different corrosion patterns have been observed. The first one is constituted by the initial mill scale made of wustite, magnetite and hematite. The second one contains remains of this mill scale embedded in phases formed after aqueous corrosion: oxyhydroxides as goethite or lepidocrocite containing marblings of ferrihydrite, maghemite and magnetite. On the thicker and older layers, marblings were only constituted of magnetite and maghemite. It is proposed that the structural evolution of the pattern and their marbling is linked to wet/dry cycles and/or pH condition evolution during the corrosion processes.     

Influence of pH on the nitrite corrosion inhibition of reinforcing steel in simulated concrete pore solution

Measurements of corrosion rate of reinforcing steels have been carried out in solutions simulating electrolytic chloride environments in corrosion pits in the propagation period with sodium nitrite. A significant corrosion inhibition has been observed at relatively low chloride contents of 0.045 M due to the presence of nitrite in these systems, but its efficiency decreases when the pH is reduced. The corrosion intensity seems to be related to the [Cl⁻]/[OH⁻] ratio. Three different pH regions from acid to alkaline have been observed in terms of corrosion activity. An explanation on the behaviour of nitrite at different pH is given.     

Interfacial morphology and corrosion resistance of Fe–B cast steel containing chromium and nickel in liquid zinc

The interfacial morphology and corrosion resistance of low carbon Fe–B cast steels in zinc bath at 520 °C were investigated. The results show Fe–B cast steel containing high Cr and Ni exhibits the best corrosion resistance to liquid zinc. The corrosion layers are composed of Γ-Fe₃Zn₁₀, δ-FeZn₁₀, ξ-FeZn₁₃ and η-Zn. The corrosion behaviour of Fe–B cast steels includes the following processes: the preferential leach and dissolution of Cr and Ni, the formation of Fe–Zn compounds controlled by zinc atom diffusion, and the spalling of borides without the supporting role of α-(Fe, Cr) matrix corroded by liquid zinc.     

Comparative studies on the corrosion properties of a Fe-Mn-Al-Si steel and an interstitial-free steel

Recent developments in a group of super ductile Fe-Mn-Al-Si steels with high-manganese content demands for more research in the corrosion behavior of such steels. The corrosion properties of the Fe-Mn-Al-Si steel was studied in acidic (0.1 M H₂SO₄), alkaline (0.1 M NaOH) and chloride-containing (3.5% NaCl) environments, using immersion and polarization experiments and compared with that of interstitial-free (IF) steel. In acidic solution, the Fe-Mn-Al-Si steel exhibited significantly lower corrosion resistance than that of IF steel. Though the Fe-Mn-Al-Si steel showed lower corrosion resistance as compared to IF steel in chloride solution, the difference was not as substantial as observed in acidic medium. However, in alkaline solution, the Fe-Mn-Al-Si steel showed no significant difference in the corrosion resistance in comparison with that of IF steel, and moreover exhibited substantially high corrosion resistance than in acidic and chloride solution. The post-corrosion characterization studies showed higher corrosion attack of the Fe-Mn-Al-Si steel exposed to acidic solution as compared to that in alkaline and chloride solutions, which is consistent with the corrosion measurement data.     

Long-term immersion corrosion of steels in seawaters with elevated nutrient concentration

Data from a variety of field exposure programs is used to quantify the effect of concentration of dissolved inorganic nitrogen (DIN) on long-term seawater immersion corrosion loss of structural steels. A linear correlation model that asymptotes the long-term part of the previously proposed bi-modal corrosion loss model is used. It allows for average seawater temperature. Model parameters and their variability are determined and reported. The model permits prediction of long-term corrosion loss in nutrient polluted waters of known average temperature. An example shows that anthropological pollution of seawater potentially is a major hazard for corrosion of steel infrastructure.     

The effects of water pollution on the immersion corrosion of mild and low alloy steels

The short-term immersion corrosion of mild and low alloy steels in seawaters is known to be proportional to the concentration of dissolved oxygen (DO) in the bulk water. Longer-term corrosion is a function of the activity of sulphate-reducing bacteria and is influenced by the concentration of nutrients in the bulk water. These influences are examined in more detail for the corrosion of steels in the brackish waters of the River Thames and for several immersion corrosion sites on the Eastern Australian seaboard and in the North Sea. The published data sources were supplemented with plausible assumptions about environmental conditions. New interpretations of the data are provided based on the previously published model for immersion corrosion. For waters with negligible salinity and sulphate levels early corrosion loss was shown to depend on the dissolved oxygen content of the waters, and later corrosion loss was a direct function of nitrogenous nutrient (pollution) levels. This also applies to longer-term corrosion.     

Monitoring corrosion rates and localised corrosion in low conductivity water

Monitoring of low corrosion rates and localised corrosion in a media with low conductivity is a challenge. In municipal district heating, quality control may be improved by implementing on-line corrosion monitoring if a suitable technique can be identified to measure both uniform and localised corrosion. Electrochemical techniques (LPR, EIS, crevice corrosion current) as well as direct measurement techniques (high-sensitive electrical resistance, weight loss) have been applied in operating plants. Changes in the corrosion processes are best monitored in non-aggressive, low conductivity media with sensitive electrical resistance technique and crevice corrosion current measurements.     

Damage analysis and cracking model of reinforced concrete structures with rebar corrosion

The damage of concrete cover in reinforced concrete structures induced by reinforcing steel corrosion is investigated in this study. The damage process of the concrete cover can be divided into two distinct stages: the non-cracking stage and the partial cracking stage. An analytical model based on damage mechanics and elastic mechanics is developed to predict the concrete cracking due to steel corrosion. Based on this model, the expansive pressure and the radial loss of steel bar are discussed. Parametric studies are carried out to examine the effects of the correlative factors on the expansive pressure and the steel loss.     

Application of harmonic analysis in measuring the corrosion rate of rebar in concrete

The corrosion rate (CR) of rebar embedded in cement mortar, concrete and cement extract is determined using harmonic analysis technique (HA). Simultaneously using other electrochemical techniques such as impedance spectroscopy (EIS) and Tafel extrapolation (TET), the CR was determined and compared with the weight loss method. CR obtained from HA is comparable to that of EIS provided that the Stern–Geary constant (B value) obtained from HA is used in the calculation. In concrete, comparable corrosion rates are obtained between TET and HA only under active condition of the rebar whereas under passive state, the corrosion current (i_corr) by TET is 10 times lower than that of HA. A good agreement is obtained between the HA and weight loss method. The outcome of the result suggests that HA is capable of providing a higher degree of accuracy than that of EIS and TET in the determination of i_corr in the medium like rebar in concrete having very low rate of corrosion.     

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.     

Surface chemistry and corrosion behaviour of 304 stainless steel in simulated seawater containing inorganic sulphide and sulphate-reducing bacteria

Although many studies have been carried out regarding the role of sulphide anions in promoting microbial corrosion of various metal substrates, very little is known about the differences between inorganic sulphide and biogenically-derived sulphide by sulphate-reducing bacteria (SRB) and what the reasons for differing corrosion behaviour between the two types of sulphide may be towards common metals. In this study, various electrochemical and surface analytical techniques were employed to study the effect of the inorganic and biogenic sulphide (active SRB present) on the surface chemistry and corrosion behaviour of 304 stainless steels in a simulated seawater-based modified Baar’s (SSMB) medium. Clear differences in the surface chemistry of the sulphurised passive film by inorganic and biogenic sulphide (active SRB present) were quantified by X-ray photoelectron spectroscopy (XPS). The transformation of metal sulphides in abiotic and biotic sulphide solutions with the exposure time was correlated with different corrosion behaviour of 304 stainless steels.     

Pitting corrosion in low carbon steel influenced by remanent magnetization

A new type of experiment is performed using a closed magnetic circuit configuration in order to study the influence of the magnetic field on pitting corrosion in low carbon pipeline steel. The statistical assessment of the pitting corrosion data shows that the magnetic field in the material under test reduces the extreme pit depths and also the average depth of the pit population. It is proposed that reduction in pit depth under the influence of remanent magnetization can be explained based on the behavior of the paramagnetic corrosion products under the influence of the magnetic field gradient produced inside pits.