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Corrosion, Wear, and Degradation of Materials
Published in Mahmoud M. Farag, Materials and Process Selection for Engineering Design, 2020
Pitting is a form of localized attack that produces pits or holes in a metal. Pitting corrosion occurs when one area of the surface becomes anodic with respect to the rest of the surface due to segregation of alloying elements or inclusions in the microstructure. Surface deposits that set up local concentration cells, dissolved halides that produce local anodes by rupture of the protective oxide film, or mechanical ruptures in protective organic coatings are also common sources of pitting corrosion. Differences in ion and oxygen concentrations create concentration cells, which can also initiate pits.
Open-Circuit Metal Dissolution Processes
Published in Madhav Datta, Electrodissolution Processes, 2020
Pitting corrosion is the localized corrosive attack of a passive metal that leads to the development of small cavities known as pits. Their growth eventually leads to perforation of a metal plate or pipe wall. Pitting corrosion damage, therefore, can be quite important, even if the absolute amount of corroded material is small. Pitting corrosion requires the presence of aggressive anions, most often chloride ions, and of an oxidizing agent such as oxygen or ferric ions. A corrosion cell forms between the growing pit which is the anode and the passive surface surrounding the pit which serves as the cathode. Because the anode/cathode surface ratio is small, dissolution inside the pit can be very fast. Corrosion products often cover the pits. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system. Pitting corrosion is a dangerous form of corrosion damage since it is difficult to detect and predict. Figure 1.1a shows an example of pitting corrosion in cast iron water mains.
Corrosion and Protection
Published in Zainul Huda, Metallurgy for Physicists and Engineers, 2020
Pitting Corrosion. Pitting corrosion is a localized form of corrosion by which pits or “holes” are produced in the material (see Figure 11.7). Pitting corrosion is most likely to occur in the presence of chloride ions, combined with such depolarizers as oxygen or oxidizing salts.
A review on weldability and corrosion behaviour of L-PBF printed AlSi10Mg alloy
Published in Canadian Metallurgical Quarterly, 2023
Navdeep Minhas, Varun Sharma, Shailendra Singh Bhadauria
Pitting corrosion is localised in nature and forms pits-like structures over the metal surface with diameter and depth. Aluminium shows the sign of pitting corrosion generally in a medium close to the neutral value of pH, such as, the naturally available fresh and sea water. The rupture of passive film leads to corrosion pit and further propagation in Aluminium. The pitting corrosion behaviour for AlSi10Mg alloy, concerning the chlorine concentration exposure (NaCl solutions, with a concentration ranging from 0.01 to 0.6 mol/L) presents the most detrimental results. The pits formation is subjected to some critical value of chloride concentration where chlorine ions do not allow the re-passivation phenomenon. The surface morphology shows significant variation in the corrosion behaviour for the rough and polished surface for a value lower than 0.05 M. With an increase in the chloride ions concentration, the localised corrosion decreases. The corrosion is initiated after the film breakdown, the selective corrosion begins and progresses along the melt pool boundaries by the dissolution of the α-Al. The Si-rich and alpha phase shows susceptibility as a preferred site at melt pool boundaries for the corrosion pits initiation due to higher potential difference, Figure 15(a) [133].
Effect of friction stir processing and heat treatment on the corrosion properties of AZ31 alloy
Published in Australian Journal of Mechanical Engineering, 2022
B. Mohan Bharathi, R. Vaira Vignesh, R. Padmanaban, M. Govindaraju
Figure 9 shows the surface morphology of the corroded base material after 48 hours of immersion. Figure 9(a) shows the corrosion pits on the surface of the base material after 48 hours of immersion. Pitting corrosion drastically reduces the strength of the material leading to sudden failure of the component, which is not the recommended corrosion mechanism for engineering materials. Figure 9(b) shows the magnified image of the pit formed. In Figure 9(c) corrosion products formed were observed as a layer that acted as a barrier for further corrosion process. Figure 9(d) shows the cluster of corrosion products formed on the surface of the base material after 48 hours of immersion. The surface morphology of the specimen HT2 after 48 hours of immersion is shown in Figure 10. Figure 10(a) shows the crack developed over the surface of the specimen after 48 hours of immersion. The formation of magnesium hydroxide cracked the surface of the specimen. As observed in Figure 10(b), an intense cluster of corrosion products covered the specimen in the due course of the corrosion test period. Figure 10(c) shows the corrosion layer and Figure 10(d) shows the magnified corrosion products in the specimen HT2 after 48 hours of immersion. The formation of a layer of corrosion products decreased the corrosion rate in the HT2 specimen.
A review of the effect of biodiesel on the corrosion behavior of metals/alloys in diesel engines
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
Anh Tuan Hoang, Meisam Tabatabaei, Mortaza Aghbashlo
Concerns over the unfavorable biodiesel properties in particular corrosiveness to (especially metal) diesel engine and fuel system compartments have limited commercial utilization of neat biodiesel fuel. Therefore, understanding and prevention of corrosion of mechanical parts of diesel engines when exposed to biodiesel is a major challenge to scientists, engineers, and manufactures. The higher corrosion rates of metals in biodiesel in comparison with fossil diesel were comprehensively discussed herein while the underlying mechanisms were also presented. The factors affecting the corrosion level were also elaborated. Based on the results of various investigations, metals and alloys generally used in the fabrication of diesel engine and fuel systems such as copper, aluminum, and steel could be easily corroded by biodiesel. Pitting corrosion is the most common type of corrosion for nonferrous metals and alloys and carbon steel, while stainless steel seems to be immune to this type of corrosion. Among different metals, copper is the most prone to corrosion by biodiesel followed by aluminum and carbon steel.