Anisotropic 3D growth of corrosion pits initiated at MnS inclusions for A537 steel during corrosion fatigue

The 3D growth behaviour of pits initiated at MnS inclusions for A537 steel under cyclic load condition has been investigated by confocal scanning laser microscope and finite element analysis. The results indicated the growth of pit for cyclic-stressed samples showed anisotropic behaviour. Significant higher growth rate was found in the direction perpendicular to load axis. Finite element analysis indicated that localized plastic strain played an important role on pit growth. The growth kinetics of pit in width direction is dominated by plastic deformation. Coalescences of pits further increase the growth rate of pit as predicted by finite element analysis.     

An alternate explanation for the abrupt current increase at the pitting potential

A critical transition potential is shown to be associated with the abrupt increase in the number of metastable pits and the onset of spatial clustering of pits amongst nearest neighbor defects in both commercial and model stainless steels. This critical transition potential is correlated with the parameter, E_pit commonly observed in near-neutral NaCl solutions when stainless steels form pits. The explosive growth of pit sites, the onset of clustering of pits, and E_pit all depend on the diffusion length associated with aggressive corrosion products formed at pits and thus on the stirring rate. E_pit is observed to increase significantly and pit sites remain random to high potentials in both experiments and modeling when the diffusion length is decreased below the nearest neighbor distance (NND) associated with surface defects such as non-metallic inclusions that form a random array of susceptible sites across the otherwise passive surface. This phenomenon is confirmed by experiments and modeling using model stainless steel alloys with decreased inclusions densities and, thus, increased defect NNDs. If the NND is greater than the lateral diffusion length, then E_pit will increase, clustering of pit sites will decrease and explosive growth in the number of pits will be suppressed at a given applied point.     

New insight into the pit-to-crack transition from finite element analysis of the stress and strain distribution around a corrosion pit

A finite element (FE) analysis has been undertaken to evaluate the stress and strain distribution associated with a single corrosion pit in a cylindrical steel specimen stressed remotely in tension. A key observation was the localisation of plastic strain to the pit walls (just below the surface of the specimen). Simulation of a growing pit in a static stress field indicated corresponding plastic strain rates that were commensurate with values associated with stress corrosion cracking. This observation introduces a wholly new concept in understanding of the evolution of stress corrosion cracks from pits and correlates with recent X-ray tomography measurements.     

In situ pH responsive fluorescent probing of localized iron corrosion

A fast and versatile method has been demonstrated to detect the early stage of iron corrosion in situ, with minimum external perturbation, by using a pH sensitive fluorescence dye. The decreasing fluorescence intensity inside corrosion pits induced by hydrogen ions produced during iron corrosion is used as a signal indicator. Identification of corrosion pits (with resolution of <1 μm) and quantification of corrosion rate based on local hydrogen concentration were achieved. The developed technique has been applied to show that surface roughness, under a constant current condition, influences the corrosion of iron. Corrosion occurs preferentially adjacent to local topographic features.     

Electrochemical investigation of chloride induced pitting of stainless steel under the influence of a magnetic field

The effect of a magnetic field on chloride induced pitting of stainless steel was studied by potentiodynamic measurements in aqueous NaCl solution in a cylindrical cell with the field perpendicular to the surface under test. Compared to identical tests without field, a significant shift of the repassivation potential E_r in the cathodic direction was observed, together with the formation of small pits at high density in the periphery of the electrode. These pits develop under the influence of a vortical flow induced by the magnetohydrodynamic effect. The shift of E_r is explained as the consequence of the occluded morphology of these pits.     

Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions

Prestressing steels occasionally fail by a process named “stress corrosion cracking”. This process has not been fully elucidated and several theories exists in order to explain the cases in which real structures have collapsed. This paper briefly mentions the different theories and identifies the progress in understanding whether it is necessary to use a testing method, which is able to separate the different steps and mechanisms contributing to the failures.This paper presents the methodology used for inducing controlled localized attack to study the susceptibility of the high strength steels resistance to stress corrosion cracking (SCC). The method is designed to study the growth of cracks initiated from a mechanical notch; the crack is not produced by fatigue.It consists of several stages: coating of the bar with epoxy resin, generation of a small notch, constant load and controlled potential test in the media, mechanical test in air and fractographic study. It allows us to calculate the crack propagation rate and the fracture toughness in the same test.Finally, it has been possible to apply the surface mobility mechanism (SMM) in order to identify the SCC mechanism that operates.     

Effect of sulfuric acid on pit propagation behaviour of aluminium under AC etch process

Pit propagation on high purity aluminium electrode in 2 M HCl solutions with and without H₂SO₄ under an alternating current (AC) has been examined. Pit development and potential transients were dependent on the H₂SO₄ concentration. In the sulfate-free etchant, most pits developed from the pretreated surface, with little tendency to form clusters of pits. With increasing H₂SO₄ concentration the size of the pit clusters increased. There is an optimal H₂SO₄ concentration, which is 0.01 M H₂SO₄ in this study, to form a deep etched layer of uniform thickness with high surface area. At H₂SO₄ concentrations higher than 0.01 M, the pit propagation proceeded on limited foil surface sites and deep etched regions were formed locally, since sulfate ions assisted passivation and reduced the number of pit nucleation sites on foil surface. Analysis of potential transients during the anodic half-cycle supports the hypothesis that sulfate ions retarded the pit nucleation.     

Comparison of influences of NaCl and CaCl2 on the corrosion of 11% and 17% Cr ferritic stainless steels during cyclic corrosion test

During the drying stage of the cyclic corrosion test on ferritic stainless steels in the NaCl environment, the current abruptly increased and then decreased to nearly zero, indicating that pits are initiated as the salt concentration is increased, which are then repassivated when the surface is completely dry. During the wet stage, the current remained high, suggesting that pits mainly propagate during the wet stage. In contrast, in the cyclic corrosion test in the CaCl₂ environment, the current was highest during the drying stage, indicating that the electrolyte is not completely dried and corrosion mainly propagates during the drying stage.     

Corrosion and recession behavior of zircon in water vapor environment at high temperature

A corrosion test in static state water vapor environment and a recession test in high velocity steam jet environment for zircon bulk were performed at 1300 °C. The trace of the water vapor corrosion could be recognized on the grain surfaces and at the grain boundaries for the sample after the static state corrosion test. Sand ripple like morphology was generated on the grain surfaces and etch pits with less than 0.1 μm size were formed at the grain boundaries. A porous structure was formed on the bulk surface of the sample after the steam jet test. A glassy phase enriched with silica could be recognized on the surface of the sample after the test. Cracks were induced on the bulk surface during the test. The zircon phase at the bulk surface decomposed into monoclinic zirconia phase and the fraction of silica component at the bulk surface decreased by the steam jet test. The monoclinic zirconia phase was observed to re-generate and grow on the bulk surface.     

Electrochemical and XPS studies and the potential scan rate dependent pitting corrosion behavior of Zircaloy-2 in 5% NaCl solution

Electrochemical studies of Zircaloy-2 in 5% NaCl solution were carried out using polarization and electrochemical impedance spectroscopy (EIS). Scan rate dependent pitting behavior of Zircaloy-2 was observed when polarization experiments were carried out at different scan rates (5 mV/min, 50 mV/min and 500 mV/min). Polarization resistance, double layer capacitance, and the thickness of passive film were calculated from the EIS study. The slope of the Bode plot in the capacitive region showed non-ideal behaviour, suggesting formation of pits. A distribution of chloride concentration was seen at the surface of the electrode when the test solution was sampled from different parts of the sample surface and analyzed for chloride. Inclusion of chloride ions in the passive film was observed from the XPS study and the chloride concentration increases with the higher anodic polarization. The outermost layer of the passive film was rich in hydroxide and oxy-hydroxide. A bi-layer passive film structure on the alloy surface is proposed.     

Environmental factors affecting pitting corrosion of type 304 stainless steel investigated by electrochemical noise measurements under potentiostatic control

Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated.     

Pit growth behaviour of aluminium under galvanostatic control

The pit growth process on (1 0 0) aluminium under anodic pulse current in a mixed solution of 1 M HCl and 0.1 M H₂SO₄ at 30 °C has been evaluated using potential transient measurements and pit size distributions obtained by scanning electron microscopy. Sustained pit growth is observed for all pits during the initial anodic potential rise before reaching a steady-state etch potential, whereas a substantial fraction of the pits passivate at the steady-state etch potential. The pit growth rate during the initial potential rise is 3.4 μm s⁻¹, which is similar to that at the steady-state etch potential. The growth rates of active pits are potential-independent.     

Classification of corrosion defects in NiAl bronze through image analysis

Corrosion surface damage in the form of pitting and micro-cracks is observed in many metals. Cracks usually initiate from the pits and grow under cyclic stresses and eventually lead to material failure. An image analysis based on wavelet transforms and fractals was used to study the corrosion morphology of nickel aluminum bronze metal under varying corrosion conditions and applied stresses. Image feature parameters were extracted and analyzed to classify the pits/cracks in the metal samples. The results obtained indicate that classification of pits/cracks is possible with image analysis and may be used for correlating service/failure conditions based on corrosion morphology.     

Evaluation of metastable pitting on titanium by charge integration of current transients

The metastable pitting of titanium has been studied under potentiostatic control in solutions containing chloride ions. An approach based on the charge integration of current transients was proposed for a quantitative determination of metastable pitting. A pit density (d_mpit) was defined as the number of metastable pits per unit area per unit time (cm⁻² h⁻¹) with a typical size, instead of a size distribution. The calculated d_mpit of titanium at 0.5 V_SCE in 0.6 M NaCl was about 1.0 × 10³ cm⁻² h⁻¹ with a typical radius of 0.12 μm. An exponential potential dependence of d_mpit was obtained through the integration approach.     

AFM study of microbial colonization and its deleterious effect on 304 stainless steel by Pseudomonas NCIMB 2021 and Desulfovibrio desulfuricans in simulated seawater

The biofilm colonization dynamics of Pseudomonas NCIMB 2021 and Desulfovibrio desulfuricans (ATCC 27774) on 304 stainless steels (304 SS) was evaluated using atomic force microscopy (AFM) in simulated seawater-based media under aerobic and anaerobic conditions. Results showed that the biofilm formed on the coupon surface by the two strains of bacteria increased in the coverage, heterogeneity and thickness with exposure time, thus resulting in the deterioration of the steel substratum underneath the biofilm in the form of pitting corrosion. The depth of pits induced by D. desulfuricans was larger than that by Pseudomonas NCIMB 2021, which was mainly attributed to the enhanced corrosion of 304 SS coupons by the biogenic sulfide ions, as revealed by the results of X-ray photoelectron spectroscopy (XPS) and Tafel polarization curves. AFM was also used to determine cell attachment/detachment processes of the Pseudomonas and D. desulfuricans bacteria on the coupon surface by quantifying the tip-cell interaction forces. The interactive forces between the tip and the bacterial cell surface were considerably smaller than those between the tip and the cell-cell interface due to the accumulation of extra-cellular polymeric substances (EPS) for both strains. Furthermore, the adhesion forces over the Pseudomonas cells were verified to be more attractive than those of D. desulfuricans due to the former being a slime-producer.     

The nucleation and growth of metastable pitting on pure iron

The nucleation and growth of metastable pitting on pure iron surface in NaNO₂ + NaCl solution were investigated by potentiodynamic tests. The current fluctuations appear as cluster and overlapped. The active sites for nucleation depend mainly on the surface geometry. The growth behaviour of pure iron is different from stainless steels and carbon steels. On pure iron surface, many pits pile up together to form huge damage area. Pits are shallow, do not grow deeply and pits array one by one along the direction of abrasion grooves. The growth of stable pitting is different from metastable pitting.     

Metal dusting behaviour of Cr-Ni steels and Ni-base alloys in a simulated syngas mixture

Long-term laboratory exposure tests for various Cr and Ni content steels and Ni-base alloys were conducted at 650 °C in a 60vol.%CO-26%H₂-11.5%CO₂-2.5%H₂O gas mixture simulating syngas environments. Upon isothermal heating, alloys with 15% and 20% Cr had many pits on the surface after a brief exposure, while no pit was found on alloys containing of 60% Ni and more than 23% Cr exposed for up to 5000 h. The thermal cycling accelerated pit initiation drastically, resulting that all test specimens except 30%Cr-60%Ni alloy suffered from metal dusting. From a measurement of pit depths, Ni proved to be an effective alloying element to retard the pit growth: growth rate for 75% Ni alloy has achieved double-digit decrease compared to that for 20% Ni. Microscopic observations has revealed that platelet graphite aligned perpendicular at the boundary of gas/metal of pits. The length of the platelet graphite for high Ni alloys was appreciably longer than that for low Ni steels. This can be interpreted from the difference of super saturation of carbon.     

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.     

Stable pit formation on AA2024-T3 in a NaCl environment

Scanning vibrating electrode (SVE), particle induced X-ray emission spectroscopy (PIXE) and standard electrochemical measurements were used to study the establishment of stable pits on AA2024-T3 in neutral sodium chloride solution (0.1 M NaCl). Pits were allowed to develop until hydrogen evolution was observed. Typical current at the mouth of the pits were in the vicinity of 1 mA. PIXE maps revealed the intermetallic (IM) particle distributions in the surface as well as significant chloride buildup around the pits. A significant fraction of the small selection of pits examined here appeared to have an S-phase particle (or remnant) within 20 μm of a AlCuFeMn type IM particle suggesting a coupling between the two. The electrochemistry of the coupling between different IM particle types was further investigated using potentiodynamic scans in 0.1 M aqueous NaCl solution of macroscopic electrodes made according to the IM particle compositions. Current densities at the open circuit potential of AA2024-T3 were largest, typically (0.2 up to 1 mA/cm²) for phases that were anodic with respect to AA2024-T3. Coupling of the IM particles was examined by comparing the degree of clustering around chloride attack sites to the average particle density for each map. There were significantly higher number of IM particles surrounding pit sites than the average IM particle densities indicating that local clustering played an important role in pit initiation.     

The effect of impurity inclusions on the pitting corrosion behaviour of beryllium

An electrochemical polarisation study has been conducted to determine the effect of impurity inclusions on the pitting corrosion behaviour of beryllium. Three grades of commercial beryllium differing in impurity levels were used. This work showed a strong relationship between the pitting potential of the beryllium and the level of impurity inclusions, the grade with the least number of overall inclusions being the most resistant to pitting corrosion. The work also revealed the preferred sites for pit initiation, with most corrosion pits initiating at intermetallic Fe/Al/Be inclusions, and to a smaller extent at inclusion sites containing either elemental silicon or carbides.