Distribution Of Corrosion Pits Research Articles

The Underoil Droplet Corrosion Behavior of X100 Pipeline Steel

Canadian natural bitumen production will reach a peak of 112 million barrels per day in 2078 [1]. Pipelines are primarily used to transport this bitumen. Bitumen must be diluted in order to be pumped through a pipeline effectively because it is otherwise too viscous to flow [2]. Underoil droplet corrosion of diluted bitumen (dilbit) pipelines may lead to the loss of pipe wall thickness, which deteriorates the structural reliability and increases the risk of leaking of pipelines. The current models for atmospheric droplet corrosion, where no limitation to oxygen access is imposed, cannot be applied to underoil droplet corrosion in dilbit pipeline conditions. In this study, a NaCl droplet on X100 pipeline steel covered by paraffin oil is used to simulate the corrosive environment encountered in diluted bitumen pipelines. A new distinctive corrosion mechanism is proposed to explain the corrosion of X100 pipeline steel beneath an underoil salty droplet. This mechanism accounts for the depleted oxygen levels in the droplet environment. In the initial stage (1 h), the distribution of corrosion pits was heterogeneous with one area under the droplet presenting a higher pit density. As the corrosion proceeded (24 h), the localized corrosion was governed by pH development due to initial pit density. Regions of initially higher pit density switched to general corrosion, while regions with lower pit densities continued pitting. The effects of droplet volume, chloride ion concentration, and temperature on the underoil droplet corrosion behavior of X100 pipeline steel were also studied by scanning electron microscopy, Raman spectroscopy, and profilometry. At 23 °C, lepidocrocite and hematite were observed in the region where pitting was initially prevalent. However, as this region switched to uniform corrosion, goethite, lepidocrocite, and hematite were formed. The same distribution of crystal structures was identified for the droplets with varying volume and chloride ion concentration. When the temperature was increased to 60 °C, hematite was detected in both regions. Decreased droplet volumes, increased chloride concentration, and raised temperatures increase corrosion penetration when uniform corrosion is prevalent. This study provides important findings to better understand the mechanisms behind droplet corrosion underoil and informs future predictions of the corrosion rate of pipeline steels.

A New Corrosion Mechanism for X100 Pipeline Steel Under Oil-Covered Chloride Droplets

A 1.7 mM NaCl droplet on X100 pipeline steel covered by paraffin oil is used to simulate the corrosive environment encountered in heavy oil or bitumen pipelines. The development of corrosion under the droplet was monitored and explored in two stages. In the initial stage (1 h), the distribution of corrosion pits was heterogeneous with one area under the droplet presenting a higher pit density. As the corrosion proceeded (24 h), the localized corrosion in the area under the droplet with the higher pit density switched to general corrosion, while the other region of the droplet continued to pit. The mechanisms driving this new distinctive corrosion form developed beneath an underoil droplet are explained.

Effect of simulated pitting corrosion on the tensile properties of steel

The buildings including steel-structured bridges, plants and ocean platforms which take steel as the main material are prone to fall into pitting corrosion. Mechanical properties are greatly affected by pitting corrosion and hence the security of buildings can hardly be guaranteed. The mechanical drilling and milling method is employed to form conical blind holes and then to simulate corrosion pits in present work, after which the effects of the shape, depth and distribution of corrosion pits on the tensile properties of steel have been systematically investigated. The results indicate that the fracture location of steels with corrosion pits occurred at the cross section with larger pitting corrosion factor. The load-displacement curves of the steels were changed due to the existence of corrosion pits, the yield platform got shorter and even disappeared, accompanied with poorer ductility and brittle fracture. Pit shape, depth and distribution impacted little on the ultimate load of tensile samples, the ultimate strength almost correlated only with the maximum cross-sectional area. However, the yield load of steels was affected by both pit shape and distribution. The elongation was mainly affected by pit denseness and depth, denser and deeper pits per unit area would give rise to smaller elongation. Finally, multivariate regression analysis was performed to acquire the prediction model of elongation.

Nonlinear stress–strain behaviour of corrosion-damaged reinforcing bars including inelastic buckling

In the seismic design and assessment of reinforced concrete structures in earthquake zones buckling of longitudinal reinforcement in plastic hinge regions is an important limit state that needs to be considered. If the structure is located in an environmentally aggressive area, it is also subject to material deterioration over its service life. Corrosion of reinforcement is the most common type of deterioration of reinforced concrete (RC) structures and bridges. In this paper the nonlinear stress–strain behaviour of corroded reinforcing bars has been investigated by extensive experimental testing. The effect of different corrosion levels on the tension and compression behaviour of bars with different slenderness ratios is presented. The results of this study show that a corrosion level above 15% mass loss significantly affects the ductility and plastic deformation of reinforcement in tension and that corrosion changes the buckling collapse mechanism of the bars in compression. The results of buckling tests show that 10% mass loss produces about a 20% reduction in the buckling capacity of corroded bars. The results also show that the distribution of corrosion pits along the length of corroded bars is the most important parameter affecting the stress–strain response in both tension and compression. Furthermore, a constitutive material model to predict the post-yield buckling behaviour of high-strength steel without a yield plateau is also developed. The proposed analytical model is based on Dhakal–Maekawa buckling model. The analytical model has been validated against experimental tests on uncorroded and corroded bars. The results of this corrosion extended buckling model show a good agreement with the physical testing.

Research on Corrosion Fractal Character of LY12CZ Aluminum Alloy Based on Image Processing Technique

The number of corrosion pits with different corrosion times were got through extracting the corrosion character of LY12CZ aluminum alloy based on binary image disposal technique, through statistical fractal theory we can see that the distribution of corrosion pits have fractal character; the fractal dimension and weight loss rate increased with the corrosion time and the change rule of fractal dimension with different corrosion time consistent with weight loss rate, both are accordant with power function, so the fractal dimension can be regarded as an important parameter to evaluate the corrosion degree.

Distribution of corrosion pits on a metal surface

The distribution of corrosion pits on the surface of stainless steel samples treated in a 3% NaCl solution is studied. It is established that the distributions are thermodynamically optimized and can be described by the Gibbs expression for a canonical distribution of the thermodynamic probability of energy fluctuations. A wide occurrence of the pitting corrosion and the broad scatter of pits with respect to size are explained.

Ｓ２５Ｃ炭素鋼同種摩擦圧接継手の人工海水中における腐食挙動

Friction welded butt joints are widely used as machine members and structural elements, but valid evaluation method of the joint performance, especially under service load conditions, has not been established yet. Evaluation of corrosion fatigue behaviors of the joint is one of the important problem, for some joints are used in pollutive environments. In our previous paper, corrosion fatigue behaviors in artificial seawater were investigated on S25C/S25C friction welded butt joints. The results indicated that the S-N curve for corroision fatigue strength of the joint specimen showed steeper slope than that of the base material and the strength of the joint took almost the same value as that of the base material specimen in long life region, and the scatter of fatigue lives became smaller in accordance with the increase in fatigue life contrary to the case of the base material specimen. Detailed observations of corrosion pits and non-propagation cracks of run-out specimens suggested that the above mentioned corrosion fatigue characteristics of the joint was governed by corrosion pits generated preferably on exposure surface of the transitional plane of fiber flow directions developed at the final stage of friction welding process.The present study was carried out to clarify the corrosion behaviors of the joint in more detail. Joint specimens were immersed in artificial seawater at stress free condition during 5 and 10 days. Distribution of corrosion pits near the weld interface and geometrical factors of pits were evaluated precisely, and the difference of corrosion behaviors between the joint and the base material were also discussed. The results of this study gives valid support for the estimation on the corrosion fatigue characteristics of the joint indicated in our previous paper.