Pre-existing Pits Research Articles

Finite element analysis of crack initiation and growth in a rectangular plate with pre-existing pits

Abstract The localized loss of material due to corrosion activities leads to the formation of pits. It produces undesired effect of stress riser which harms the structural integrity. To safeguards the structure with pre-existing pits, finite element analysis (FEA) can be very helpful. The present study investigates the influence of pit shape on crack initiation and propagation in a rectangular plate using the extended finite element method (XFEM). For this purpose, various pit shape such as semi-circular, semi-elliptical, rectangular and bullet-shape were designed. The damage behavior of the assigned material was modelled using cohesive segment approach. Moreover, the rectangular plate was strained by tension and bending loading to study their influence on crack initiation. It was observed that the presence of pit can greatly influence the crack initiation under both tension and bending loading conditions.

Environmental and Health Concerns Associated with the Open Dumping of Municipal Solid Waste: A Lagos, Nigeria Experience

Landfill is the simplest and most commonly used method of disposing municipal solid waste (MSW). In Nigeria, they are better categorized as open dumps, sited for convenience and usually involve the pitching of waste in pre-existing pits. In Lagos, the most populated city in Nigeria, most of these open pits are located near residential quarters and therefore pose a threat to public health and the environment. The absence of containment systems at the sites allows the possible percolation of leachate into groundwater. In addition, the methane gas generated as a result of bacterial degradation of waste is not collected and poses a potential fire hazard. The sites harbour numerous scrap tyres which not only fuel the fires but are also breeding grounds for disease transmitting vermin. Landfill fires especially those involving tyres emit dioxin which can cause reproductive impairment and cancer in humans at extremely low concentrations. Scavenging birds at these sites are potential carriers of diseases. This paper examines the potential environmental and health risks posed by some unsanitary MSW landfills in Lagos, Nigeria and calls for the need to start adopting modern waste management practices to ensure the protection of public health and the environment.

Pitting Corrosion of Co1.5CrFeNi1.5Ti0.5Mo0.1in Chloride-Containing Nitrate Solutions

Abstract The purpose of this study was to investigate the effect of nitrate ions (NO3−) on the critical pitting temperature (CPT) of Co1.5CrFeNi1.5Ti0.5Mo0.1 in chloride solutions using the potentiodynamic polarization method. Results indicate that the values of CPT for the alloy in 1 (mol/L) M sodium chloride (NaCl), 1 M NaCl with 0.1 M sodium nitrate (NaNO3), and 1 M NaCl with 1 M NaNO3 are 60, 70, and more than 80°C, respectively. Moreover, scanning electron microscopy (SEM) images show that nitrate ions enhance the initiation of pits, but impede the growth of preexisting pits on the surface of the alloy.

Morphological changes of InGaN epilayers during annealing assessed by spectral analysis of atomic force microscopy images

During annealing, the morphologies of thin InGaN epilayers have been observed to change from a terraced structure to a network of interlinking InGaN strips separated by troughs. This change in morphology may contribute to high efficiencies in some GaN-based light emitting diodes (LEDs) if the InGaN is exposed to elevated temperatures without a protective GaN capping layer. Here, we investigate the changes in morphology which occur when InGaN epilayers are annealed at their growth temperature under NH3, N2, and a small H2 flux. We observe that while the layers initially roughen, more extended anneals lead to the surface becoming smooth and terraced once again. Power spectral density analysis of atomic force microscopy data is used to show that the dominant mechanism for roughening is loss of material from pre-existing pits, while the dominant smoothening mechanism is surface diffusion. This mechanistic analysis may be relevant to the growth of InGaN quantum wells in LED structures.

SVET and SECM imaging of cathodic protection of aluminium by a Mg-rich coating

The mechanism of cathodic protection of an aluminium substrate by a Mg-rich coating was investigated using localized techniques. Both scanning vibrating electrode techniques (SVET) and scanning electrochemical microscopy (SECM) were used to investigate the processes occurring at the surface of exposed metal when electrically connected or disconnected in a galvanic couple with the Mg-rich coating. The SVET has shown the evolution of the pit activity with time under conditions of sacrificial protection, whereas the SECM allowed indirect sensing of the cathodic activity above the electrodes. It was shown that the cathodic protection provided by magnesium to aluminium substrates acts by both preventing pit nucleation and inhibiting the growth of the pre-existing pits.

Spatial Interactions among Localized Corrosion Sites : Experiments and Modeling

Interactions among local pit sites were investigated using a array of 25 electrodes consisting of closely spaced, flush-mounted 316 stainless steel wires. Three types of interactions occurred when actively corroding pits existed while the remaining electrodes were initially passive in 0.05 M NaCl solution. Suppression of pitting on nearby electrodes occurred due to ohmic potential drop near pre-existing pits. Enhancement of pitting at initially passive electrodes was observed and attributed to both temporary alternations in the local solution composition and a more persistent effect attributed to some action of the locally altered solution composition on nearby passive surfaces. The exact origin of the damage on the nearby passive surface is unknown. Each of these effects exhibited different periods of persistency after pit deactivation. A phenomenological model was developed and used to simulate some of the features of the experimental findings. The model included stochastic pit initiation and interactions promoted by surface damage and concentration effects or inhibited by ohmic drop. The model successfully reproduced some of experimental observations and illustrated the formation of nonrandom corrosion damage patterns even on a simulated homogeneous surface. © 2002 The Electrochemical Society. All rights reserved.

Effects of Sulfate and Nitrate Ion Additives on Pit Growth of Pure Aluminum in 0.1 M Sodium Chloride Solution

Abstract Effects of sulfate (SO42−) and nitrate (NO3−) ion additives on pit growth of pure aluminum were investigated in 0.1 M sodium chloride (NaCl) solution as a function of anion concentration using a potentiodynamic polarization experiment, a potentiostatic current transient technique, and optical microscopy. The increase in SO42− and NO3− ion concentrations in NaCl solution raised the pitting potential (Epit) of pure aluminum and increased the steady-state current density at potentials above Epit. This means that SO42− and NO3− ion additives impede pit initiation on pure aluminum, but enhance the growth of preexisting pits. Potentiostatic current transients of artificial pits obtained from the moment just after adding SO42− and NO3− ions to NaCl solution revealed that the artificial pit grows more rapidly in the presence of SO42− and NO3− ions. From observation of the cross-sectional views of the artificial pit, it appeared that the pit grows preferentially in the lateral direction and in the downwar...

Effects of sulphate ion additives on the pitting corrosion of pure aluminium in 0.01 M NaCl solution

Effects of sulphate (SO 4 2−) ion additives on the pitting corrosion of pure aluminium (Al) have been investigated in aqueous 0.01 M NaCl solution as a function of SO 4 2− ion concentration using potentiodynamic polarisation experiment, ac impedance spectroscopy, electrochemical quartz crystal microbalance technique and abrading electrode technique. The addition of SO 4 2− ions to NaCl solution raised the pitting potential ( E pit) of pure Al in value and simultaneously the anodic current density at potentials much higher than the E pit on the polarisation curves. This implies that SO 4 2− ions impede the initiation of pit on pure Al surface below the E pit, but enhance the growth of pre-existing pits, which is validated by optical microscopy. It was found that the values of the Cl − ion-incorporated outer film resistance R out,ox in SO 4 2− ion-containing chloride solutions were much lower than those in SO 4 2− ion-free solution, obtained from the impedance spectra measured at potentials below the E pit. The chloride peak disappeared from the Auger spectra in SO 4 2− ion-containing solutions. The mass decay rate and pit growth rate b were observed to increase in values once the pits were formed in SO 4 2− ion-containing chloride solutions. Based upon the above experimental results, it is suggested that SO 4 2− ions retard the oxide film breakdown by Cl − ion incorporation into the film, while they accelerate the Al metal dissolution through the instantaneous formation of tunnels at the bottom of the pre-existing pits after the exposure of bare surface above the E pit.

Interactions among Localized Corrosion Sites Investigated with Electrode Arrays

Interactions among local pit sites were investigated using an array of 25 electrodes. The 5 x 5 array consisted of closely spaced, flush-mounted 316 stainless steel wires polarized in 0.05 M NaCl solution. Two types of interactions occurred when either singl e or rows of actively corroding pits existed while the remaining electrodes were initially passive. Inhibition of pitting on near by electrodes occurred due to ohmic potential drop near active pre-existing pits. Enhancement of pitting was observed due to alteratio ns in the local solution composition near just deactivated pits. The former effect was stronger than the latter, but the concentrat ion effect could persist after pit deactivation.

Disturbed Flow and Flow-Accelerated Corrosion in Oil and Gas Production

The effect of fluid flow on corrosion of steel in oil and gas environments involves a complex interaction of physical and chemical parameters. The basic requirement for any corrosion to occur is the existence of liquid water contacting the pipe wall, which is primarily controlled by the flow regime. The effect of flow on corrosion, or flow-accelerated corrosion, is defined by the mass transfer and wall shear stress parameters existing in the water phase that contacts the pipe wall. While existing fluid flow equations for mass transfer and wall shear stress relate to equilibrium conditions, disturbed flow introduces nonequilibrium, steady-state conditions not addressed by these equations, and corrosion testing in equilibrium conditions cannot be effectively related to corrosion in disturbed flow. The problem in relating flow effects to corrosion is that steel corrosion failures in oil and gas environments are normally associated with disturbed flow conditions as a result of weld beads, pre-existing pits, bends, flanges, valves, tubing connections, etc. Steady-state mass transfer and wall shear stress relationships to steel corrosion and corrosion testing are required for their application to corrosion of steel under disturbed flow conditions. A procedure is described to relate the results of a corrosion test directly to corrosion in an operation system where disturbed flow conditions are expected, or must be considered.

Scientific Basis for Storage Criteria for Interim Dry Storage of Aluminum-clad Fuels

AbstractAn engineered system for dry storage of aluminum-clad foreign and domestic research reactor spent fuel owned by the United States Department of Energy is being considered to store the fuel up to a nominal period of 40 years prior to ultimate disposition. Scientifically-based criteria for environmental limits to drying and storing the fuels for this system are being developed to avoid excessive degradation in sealed and non-sealed (open to air) dry storage systems. These limits are based on consideration of degradation modes that can cause loss of net section of the cladding, embrittlement of the cladding, distortion of the fuel, or release of fuel and fission products from the fuel/clad system. Potential degradation mechanisms include corrosion mechanisms from exposure to air and/or sources of humidity, hydrogen blistering of the aluminum cladding, distortion of the fuel due to creep, and interdiffusion of the fuel and fission products with the cladding.The aluminum-clad research reactor fuels are predominantly highly-enriched aluminumuranium alloy fuel which is clad with aluminum alloys similar to 1100, 5052, and 6061 aluminum. In the absence of corrodant species, degradation due to creep and diffusion mechanisms limit the maximum fuel storage temperature to 200°C. The results of laboratoryscale corrosion tests indicate that this fuel could be stored under air up to 200°C at low relative humidity levels (< 20%) to limit corrosion of the cladding and fuel (exposed to the storage environment through assumed pre-existing pits in the cladding). Excessive degradation of fuels with uranium metal up to 200°C can be avoided if the fuel is sufficiently dried and contained in a sealed system; open storage can be achieved if the temperature is controlled to avoid excessive corrosion even in dry air.

A Critical Appraisal of Some Popular Laboratory Electrochemical Tests for Predicting the Localized Corrosion Resistance of Stainless Alloys in Sea Water

Abstract Evidence is presented to demonstrate that although the critical pitting potential is qualitatively related to the resistance of a material to passivity breakdown by pit initiation, it is of questionable value in predicting the corrosion performance off a structure containing crevices. Experiments conducted on a 30Cr-3Mo-Fe alloy indicated total immunity to passivity breakdown by pit initiation during anodic polarization in 1M NaCl and under freely corroding conditions in acidified 10% FeCl3. However, on specimens containing an artificial crevice, passivity breakdown by crevice corrosion initiation was observed in the same two media. The long term implications of these data were confirmed by 16 month immersion tests in sea water, where severe crevice attack was noted equal in extent to that observed on A9SS Types 304 and 316 stainless steels whose pit initiation properties were considerably inferior to those of the 30Cr-3Mo-Fe alloy. The results of cyclic potentiodynamic anodic polarization experiments conducted on A8S1 Type 430 stainless steel are presented to demonstrate that the so called protection potential, Ep, is not a unique material parameter. It is shown that Ep varies in a semi logarithmic manner with the amount of localized attack induced by polarization; that is, with the chemistry changes in the growing pit caused by hydrolysis of corrosion products and chloride ion buildup in the pit cavities. On this basis, it is concluded that Ep data (however determined) cannot be used alone as a criteria for protection against the propagation of pre-existing pits or crevices in an engineering structure. Based on cyclic polarization procedures, a method of assessing the susceptibility and resistance of an alloy to crevice corrosion is suggested. It is shown that, for alloys which initiate pits during anodic polarization, the magnitude of the “difference potential” (Ec-Ep) is related to the crevice corrosion weight loss. For alloys that have no critical pitting potential, the presence of hysteresis during cyclic polarization with an artificial crevice indicates susceptibility to crevice corrosion.

Significance of protection potential in pitting, intergranular corrosion and stress-corrosion cracking

Generally speaking, the solutions existing within living pits and within active stress-corrosion cracks of many passive metals and alloys are locally acid, as a result of hydrolysis phenomena, and, if chloride exists in the corroding solution or in the metal, this acid is hydrochloric acid, which is particularly aggressive. This also occurs in other cases where corrosion proceeds under conditions of restricted diffusion, such as intergranular corrosion and crevice corrosion. In all these cases, a full knowledge of the electrochemical data (of a thermodynamic and kinetic nature) concerning both the behaviour of the active cavities existing in pits or cracks (anodes), which are generally acid, and the behaviour of passive areas outside these cavities (cathodes), which are generally alkaline, helps to aquire a full knowledge of the mechanism of the corrosion; it thus helps to overcome the corrosion. As a general rule, pitting corrosion, stress-corrosion cracking, intergranular corrosion and crevice corrosion may be annihilated by keeping the electrode potential below a given critical value, called the “protection potential”. This protection potential (below which pre-existing pits, cracks and crevices may not grow) should not be mistaken for the “rupture” or “pitting potential” (above which pits may be formed). This protection potential is approximately equal to the potential existing inside acid pits or cracks. Attention is given to the case of copper, iron, stainless steel, high strength steels, titanium and aluminium alloys. Emphasis is given to the usefulness of a long range research programme for the systematic electrochemical study of the corrosion phenomena occurring under conditions of restricted diffusion: pitting corrosion, intergranular corrosion, stress-corrosion cracking and crevice corrosion.