Pitting Corrosion

AISI 316 austenitic stainless steels have been plasma nitrided using a dc glow discharge unit in order to investigate the influence of gas composition on microstructure and corrosion behavior of treated samples. Corrosion properties of untreated and plasma nitrided 316 steels ...

The corrosion and nanomechanical characteristics of tool steel coated with vanadium carbide thin films deposited by DC reactive magnetron sputtering were investigated. Cyclic polarization and electrochemical impedance spec-troscopy measurements in a 3.5% sodium chloride solution indicated that corrosion decreases as the C content and the substrate temperature during deposition increase. The maximum hardness is reached for VC coatings with C/V ratios around unity, decreasing for either higher or lower C/V ratios. Complementary physicochemical analyses, made here or elsewhere, are used to clarify the reasons for such behavior. The present results are discussed in terms of the optimization of the deposition parameters aiming at concomitantly good corrosion resistance and high hardness.

Among the light metals, Mg and its alloys occupy a prominent position due to its low density, excellent machinability, and high specific strength. However, a relatively poor resistance to corrosion and wear are serious impediments against wider application of Mg alloys. In the present study, an attempt was made to enhance pitting corrosion resistance and microhardness of a commercial Mg alloy, MEZ (Zn 0.5%, Mn 0.1%, Zr 0.1%, rare earth elements 2%, Mg remaining percentage) by laser surface melting. The study included a detailed characterization of laser surface melted zone in terms of microstructure, phase analysis and its correlation with process parameters to optimize the laser processing routine. Mechanical properties like microhardness, wear, and electrochemical properties like pitting corrosion resistance of the surface melted layer were studied in detail. Microhardness of the laser surface melted layer was improved to 85–100 VHN as compared to 35 VHN of the as-received MEZ. Pitting corrosion resistance of the laser surface melted MEZ, significantly improved in a 3.56 wt.% NaCl solution because of grain refinement and redistribution of the intermetallic phases following rapid quenching associated with the process. The wear resistance of laser surface melted layer was also improved as compared to as-received MEZ.

The present study concerns laser surface alloying (LSA) of AISI 304 stainless steel (304-SS) with predeposited Mo (by plasma spraying) to enhance pitting and erosion–corrosion resistance of the substrate. A parametric correlation between the alloyed zone (AZ) depth (w) vis-à-vis incident laser power density (Q) and linear scanning speed (v) indicates that w is directly related to Q but varies inversely with v. Both microstructure and chemistry of the AZ are strong functions of the LSA parameters. LSA seems to improve the microhardness level in the AZ by 2–3 times that of the substrate. The optimum LSA conditions in terms of predeposit thickness, Q and v have been identified to achieve the desired microstructure, composition and mechanical properties. LSA under such optimum conditions significantly improves the pitting corrosion resistance in terms of critical potential for pit formation (EPP1) and pit growth (EPP2). In addition, the erosion–corrosion resistance in 20 wt.% sand in 3.56 wt.% NaCl solution registers a marked improvement. Thus, it is concluded that LSA of 304-SS with Mo is an appropriate technique to enhance the resistance to pitting and erosion–corrosion in stainless steel.

The Biomaterials Inspection  Laboratory has a long list of material properties that it can test. The area of biomedical engineering has been expanded . The need to educate the multidisciplinary students who will make up the work force in research and industry in biomedical engineering has also correspondingly increased. These experiments will continue to be prepared in order to cover basic tests for biomaterial inspections in the following fields:: Biological Testing of Biomaterials, In Vivo Assessment of Tissue Compatibility, Basic Toxicology and Inspection of Blood-Material Interactions.

There is quite frequent requirement to provide load-bearing structures in order to safely access components installed at height. This implies, even for the initial and preliminary analysis activities, the construction of scaffolding or the use of special equipment such as lifting platforms. Therefore, considerations regarding technical times and costs to be incurred are objective and necessary. Nowadays, the technological development of the RPAS (Remotely Piloted Aircraft Systems) allows the effective bypass of the above-mentioned difficulties, allowing the carrying out of various checks useful to formulate a possible following campaign of punctual and localized checks. The small size and the sophisticated handling systems gives the possibility of locating with precision in the space at different heights, even in areas characterized by access difficulties. The on-site activities must be conducted by devices and personnel in possession of appropriate authorizations and certifications.
The objective of this article is to propose basic guidelines for carrying out surveys with the support of RPAS. In particular, the technical considerations to be considered for the choice and characteristics of the device and its accessories as well as the necessary actions such as the preparation of the flight plan, risk assessment and communication to the competent groups.

A pitting corrosion study of welded joints of austenitic stainless steels (AISI 304 and 316L) has been addressed. A specific small-scale electrochemical cell (minicell) has been used. Four different weldment zones have been studied. The electrochemical methods applied were the potentiodynamic anodic polarization test and cyclic potentiodynamic polarization measurements. These techniques showed that the HAZ was the most critical zone for pitting corrosion for both materials. The results were correlated to the microstructural features of the materials. Finally, a comparative study between minicell and lacquer coating techniques has been done. The results show that it is advisable to use the minicell device if reliable results are wanted.

This paper emphasis on the study of typical premature failure of water wall tubes of two thermal power plant boiler of same capacity (250 MW) and same operational parameter but with different boiler water chemistry. The investigation concludes on the waterside corrosion in both of the case. One boiler is running with coordinated phosphate treatment (CPT) and another with all volatile treatment (AVT). The causes of corrosion were discovered and proposed measures for their elimination were given. Visual examination, chemical analysis of deposits, oxide scale thickness measurement and micro structural examination were carried to ascertain the probable cause/causes of failure. From the investigation, it was finally concluded that the combination of localized high tube metal temperature and wall thinning due to under deposit corrosion led to the premature tube failure in boiler running with coordinated phosphate treatment and localized pitting corrosion in boiler running with AVT. Based on the results and discussions, a possible way to combat the corrosion was proposed.

Corrosion Engineering–Principles and Solved Problems is based on the
author’s experience teaching undergraduate and graduate corrosion courses entitled Corrosion Engineering, Advanced Corrosion Engineering, and Electrochemical and Corrosion Techniques at the University of South Carolina. The book provides an extensive and in-depth theoretical analysis of thermodynamics kinetics, mass transfer, potential theory, and passivation, creating a foundation for understanding the electrochemical nature of the corrosion process and corrosion protection strategies discussed in the book’s second part. Around the world, the students who currently attend corrosion-engineering courses are enrolled in different engineering programs. This fact requires additional topics to be included in the book, and to this end, the book reviews the corrosion processes ,protection strategies, and testing for civil-engineering structures; corrosion in chemical process engineering; mechanical and nuclear corrosion engineering; and metallurgy. The fundamental principles of corrosion and related protection strategies are explained
through solved problems, exercises, and case studies, and the book helps upper-level undergraduate and graduate students learn the subject through an extensive theoretical description of corrosion theory, passivity, corrosion prevention strategies, and corrosion protection system design. The author has attempted to organize the book so the instructor can use it as the basis for a course in corrosion engineering for undergraduate students and also graduate students.

The effects of corrosion on the metallurgical characteristics and structural rigidity of metallic alloys has been a subject of interest for decades. This paper delineates the various researches on pitting corrosion. It reviews the techniques used to study pitting corrosion resistance, susceptibility and inhibition of stainless steel alloys. An extensive discourse can be found on pitting corrosion addressing the initiation and propagation of pits from the work of different authors due to localized corrosion in the presence of chlorides and other halogens and its effects on the electrochemical and metallurgical properties of the steel. Review of various research shows pitting corrosion is excercebated with increase in majorly chloride ion concentration leading to rapid deterioration of the steel alloys. Research also shows pitting corrosion can be controlled through conventional electrochemical techniques and changes substrate metal. The effect of changes in environment on the mechanism and rate of pitting corrosion is discussed.

Localized corrosion damage in Type 7075-T73 aluminum alloy was investigated for various anodizing pretreatment solutions. The postexposure surface corrosion was characterized by use of scanning electron microscopy (SEM) examination. In addition, SEM and energy dispersive spectroscopy (EDS) were used for second-phase (constituent) particle identification for those found to induce pitting corrosion during solution exposure. The pitting mechanisms were identified as circumferential where the particles are noble with respect to the matrix phase and by selective dissolution where they are anodic. The designated category-1 degreasing and category-2 inhibited alkaline solutions did not initiate localized corrosion after 1200 s exposures. However, the category-3 high-pH NaOH and category-4 low-pH HNO3 based solutions were found to initiate pitting attack, with the NaOH being significantly more aggressive. It was hypothesized that if the pits initiating during the pretreatment exposures were beyond a threshold size, on the order of 10-20 μm, a higher current density existed at these locations during subsequent electrochemical processes, thus resulting in larger and deeper pit structures. These surface defects are of primary concern with respect to accelerated fatigue crack nucleation. For smaller pits, on the order of 1-5 μm, the anodic process had a smoothing affect where the film growth tended to passivate the pits.

A failure analysis was performed on bolts (Monel K-500) that serve to support and join critical components of an electrical submergible pump (ESP) located in a saline solution environment. Based on the experimental results, it can be concluded that pitting corrosion was the primary cause of failure. All fracture surfaces displayed an initial zone of intergranular fracture promoted by pitting corrosion and a propagation zone with transgranular fracture due to mechanical load. High concentrations of sulfur (S) and chlorine (Cl) were found on two corrosion product layers covering the bolt surface, revealing that an aggressive electrochemical reaction occurred in the stagnant environment. Under these
layers, small pits of 10–30 lmin depth were observed that initiated the cracking paths that eventually became complete fractures.

The rapid galvanostatic pulse technique was applied on site on a large number of measuring points with reinforcement varying from severely corroding to passive state. The measurements provide reliable results on corrosion potential, ohmic resistance and polarization resistance in very short time. The overall scatter of the data is not bigger than with any other instrumentation used for corrosion rate determination in the field. For actively corroding zones the current from the counter electrode is self-confined, no guard-ring is needed and the corrosion rate can be calculated on the basis of the reinforcement area under the counter-electrode. For passive zones the calculated “corrosion rate” is overestimated (on a very low level). Corrosion rate calculated from polarization resistance data are always instantaneous values. For engineering application (residual service life) the daily and seasonal changes in corrosion rate have to be considered. In the frequent case of chloride induced localized corrosion the local penetration rates calculated from Rp data can vary up to a factor of 5–10 and local penetration rates of 1 mm/year may occur. This uncertainty on a very high level of corrosion rate is much more important than variations induced by using devices with or without guard ring.

The transition potential and the repassivation potential of AISI type 316 stainless steel was investigated in the absence and the presence of 0.01 M thiosulfate in chloride containing media. The pencil electrode method was employed to explore the key factors affecting the pit transition potential and the repassivation potential in thiosulfate containing solution. Using this method the pit chemistry at various temperatures was also evaluated. A good correlation was found between the pitting potential and the pit transition potential at various temperatures. Moreover, there was a significant decrease in the repassivation potential by addition of 0.01 M thiosulfate. The results were in accordance with the theory suggesting that the chemistry of the pit governs the pit repassivation potential.

Thin films on aluminum–tungsten alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source, on glass and sapphire substrates. Completely amorphous films were obtained in the Al80W20–Al67W33 composition range. Passivity and corrosion behavior of amorphous Al–W alloys were investigated in 1 M deaerated hydrochloric acid solution using polarization and impedance spectroscopy measurements and have been correlated with the properties of pure alloy components. Tungsten and sputter-deposited Al–W thin films are inherently passive materials while aluminum undergoes pitting corrosion in hydrochloric acid solution. The passive film formed at the OCP on each alloy possesses excellent electric and dielectric properties comparable to those of the isolating film on tungsten. The absolute impedance increases with increasing tungsten content in the alloy. According to electrochemical polarization measurements, alloying Al with W in solid solution significantly enhances the material's resistance to pitting corrosion by shifting the breakdown potential above 2000 mV (Al67W33) and lowering the corrosion rate at the OCP by more than two orders of magnitude. The most likely mechanism explaining the passivity of amorphous Al–W alloys, the Solute Vacancy Interaction Model (SVIM), involves the formation of complexes between highly oxidized solute atoms (W+6) and mobile cation vacancies, which restrict the transport of Cl− through the oxide film and inhibit its breakdown in hydrochloric acid solution. The role that film stress relaxation effects and microscopic defects in amorphous Al–W films, of the some composition, and deposited on various substrates play in their corrosion resistance is discussed.

The pitting corrosion of two duplex stainless steels was studied in chloride solutions. Phase compositions were determined by micro-analytical techniques and correlated with pitting behavior. The study showed that the pitting potential and the preferential pitting of the ...

... Development System (NDS) software package. Designed by Nestor, Inc. to run on personal ... 9 (Sept 1996): 61-66. Brody, Herb. “The Neural Computer.” Technology Review 93, no. 6 (August-Sept 1990): 42-49. Cheng, Wei, Bruce W. McClain and Christopher Kelly. ...