Papers by Keyword: Pitting

Abstract: Gears in service conditions experience inevitable tooth damage. These damages can cause changes in the time-varying mesh stiffness depending on the mode of damage. The time-varying mesh stiffness of gears is an essential input in calculating gear dynamic responses. Several researchers have evaluated the mesh stiffness of gear teeth with single-mode damage like pitting, spalling, root cracks and wear using either or any combination of analytical, numeric and experimental models. However, limited research has been done on investigating the mesh stiffness of gear teeth undergoing multi-mode damage. In this work, an analytical model is proposed to evaluate the mesh stiffness of a tooth on the pinion with single-mode damage, including pitting, spalling, and surface crack, separately. In addition, a gear tooth with a combination of pits, spalling and surface cracks is also evaluated. The volume of damage on the tooth is kept constant to provide a basis for comparison. The comparison highlights the possible effects of the combined damage modes, which is a more realistic occurrence in gears in service.

Abstract: This study investigated the impact from nitrogen content in backing gases on the microstructure and corrosion resistance of food grade stainless steel weld metal. Three types of backing gases were employed: 100%Ar, 85%Ar+15%N₂, and 100%N₂. Statistical analysis using ANOVA revealed a significant effect from nitrogen content on the ferrite phase fraction within the weld metal microstructures (p-value = 3.5E-05), indicating a reduction in the ferrite phase with increasing nitrogen content. Moreover, increasing nitrogen content positively shifted the pitting corrosion potential, indicating enhanced corrosion resistance. Optical microscopy confirmed lower pit density in samples with nitrogen backing gas as compared with samples with argon backing gas. These findings underscore the crucial role of nitrogen content in backing gases at influencing microstructure and corrosion resistance in stainless steel weld metal, with higher nitrogen levels correlated with improved corrosion resistance.

Abstract: Duplex stainless steels (DSSs) are being increasingly employed in the oil and gas and chemical industries, which, despite their high alloying degree and high resistance to general corrosion, are subject to pitting and crevice corrosion. According to their resistance to pitting and crevice corrosion, steels are ranked according to the PREN. However, nowadays there are many grades of DSSs, in which the content of Cr, Mo and N varies in different quantities, therefore the selection of the grade must be carried out with great care, considering not only PREN, but also the production technology, operating conditions, and the geometry of products. The crevice corrosion behaviors of three grades of duplex stainless steels quenched from 1050, 1100 and 1200 °C were studied in FeCl₃ solution at 50°C. It is shown that PREN allows to rank only different grades in terms of corrosion resistance. With a constant PREN value, but with different contents of ferrite and austenite in steel, completely different values of the corrosion rate can be obtained, therefore, PREN must be used with great care. It was found that in the studied steels, the optimal ferrite content, at which the lowest crevice corrosion rate is achieved, is at 65 vol.%.

Abstract: Stainless steel reinforcing bars show excellent corrosion resistance in concrete exposed to harsh environments. In this combined electrochemical and surface analytical work, an explanation for this behavior is proposed. XPS surface analytical results (thickness, composition of the passive film and of the interface beneath the film) obtained on black steel, FeCr alloys, and a series of stainless steels after exposure to alkaline solutions simulating concrete are reported. Pitting potentials were determined in the same solutions with electrochemical experiments. It is shown that the pitting potentials of the steels can be related to the Cr (III) oxy-hydroxide and Mo (VI) content in the passive film. It is proposed to calculate a Cr and Mo oxide equivalent similar to the well-known pitting resistance equivalent number (PREN). A correlation between the critical chloride content in concrete (reported in literature for CEM II A/LL and CEM I) and the pitting potential for carbon steel, Fe12%Cr alloy, DIN 1.4301 and DIN 1.4571 stainless steels is proposed to link results of solution analysis and performance in concrete.

Abstract: The pitting corrosion resilience and passivation behaviour of 434 ferritic stainless steel was studied in 1 M H₂SO₄ solution at 0% to 2% NaCl concentration by potentiostatic technique and optical illustration characterization. Data showed 434 steel underwent metastable pitting activity at the lowest metastable pitting potential and current by reason of the effect of SO₄^2- anions within the electrolyte. Addition and increase in Cl^- anion concentration decreased the potential while simultaneously increasing the current value at which metastable pitting occurs till 1% NaCl concentration. The presence of chlorides decreased the passivation range of the steel with regards to Cl^- anion concentration compared to the electrolyte without chlorides till 0.5% NaCl concentration. Beyond this concentration no further decrease in the steel’s passivation range was observed despite increase in chloride concentration. Beyond 1% NaCl concentration passivation of the steel completely collapsed signifying threshold chloride concentration for optimal steel utilization on astringent conditions. Potentiostatic data showed variation in chloride concentration from 0.25% to 1% had strong effect on the pitting resistance of 434 steel. Optical images of 434 steel morphology at 0%, 0.25% and 2% NaCl concentration in 1 M H₂SO₄ solution showed important details. At 0% and 0.25% NaCl concentration, corrosion pits were clearly visible though the dimension and depth of the pits from the electrolyte with 0.25% NaCl concentration were more defined and deeper. At 2% NaCl concentration, corrosion pits were larger and the surface morphology showed severe morphological deterioration by reason of the synergistic action of SO₄^2- and excess Cl^- anions.

Abstract: The general and localized corrosion resilience of 2101 LDX duplex stainless steel (2101ST) was assessed in specific concentrations of H₂SO₄ electrolyte by Potentiodynamic polarization, potentiostatic evaluation, open circuit potential analysis and optical illustration characterization. Data output shows 2010ST exhibited sufficient resistance to general corrosion (0.28 mm/y and 1.01 mm/y) at low concentrations of H₂SO₄ solution (1M and 2 M H₂SO₄) whose values correlates to polarization resistance of 42.47 Ω and 19.74 Ω. Further increment in H₂SO₄ concentration results in substantial increase in degradation rate which culminated at 58.32 mm/y at 6M H₂SO₄ and polarization resistance of 0.10 Ω. Due to the destructive action of SO₄^2- anions. Corrosion potential shifts indicates dominant anodic dissolution reactions with respect to H₂SO₄ concentration. Metastable pitting activity was observed on the plot following anodic polarization but prior to passivation of the steel. The metastable pitting current and passivation potential of the steel grew with increase in H₂SO₄ concentration. This weakened the localized corrosion resistance of the steel at higher H₂SO₄ solution. This is evident in the decrease in passivation range values plot configuration. Open circuit potential plots depict significant passivation of the steel at 1M H₂SO₄ solution compared to the plot at 6M H₂SO₄ which indicates strong tendency to corrode despite its thermodynamic stability. Optical images of 2101ST exterior after corrosion at 1M H₂SO₄ solution shows mild surface deterioration coupled with miniature corrosion pits. This contrasts the morphology of the steel at 6M H₂SO₄ which showed the presence of deep and enlarged, corrosion pits, and a badly etched surface showing the grain boundary coupled with a severely degraded exterior.

Abstract: The vulnerability of Al matrix composites to general and preferential corrosion is due to the intrinsic proneness of binary materials to undergo advanced deterioration. Control of the prevalent sites for evolution and proliferation of confined corrosion strongly influence the corrosion resistance of the composites. The problem for enhanced utilization of composites, has exacerbated with attention on the productive life and resilience to environmental degeneration during operational service. This can be achieved through proper comprehension of the electrochemical mechanism, the intriguing nature of SiC grains and their importance on the secondary phases, metallurgical configuration, and manufacturing process routes. This review confirms the relevance of secondary phases, microstructures and manufacturing processes in relation to SiC particles on the corrosion invulnerability of Al matrix composites to further add corrosion mitigation in design and and technological advancement.

Abstract: In the process of layer inspection and hole mending, it was found that the corrosion of tubing in a well was serious, and perforation and fracture occurred. Part of the tubing was found to be cracked from the failed pipe samples, and relatively serious pitting corrosion pits were found on the surface of the outer wall. The fracture morphology and corrosion products were analyzed by means of macroscopic analysis and metallographic microscope, SEM and EDS. The result show that the mechanical damage of the outer wall of the tubing was the primary condition for accelerating corrosion, and the severe corrosion thinning of the inner and outer walls of the tubing was the main reason for the failure of the tubing string. The corrosion perforation of tubing was mainly caused by internal corrosion.

Abstract: This current work investigates the failure of high pressure (HP) scrubber lining in urea fertilizer production plant after 20 years in service. AISI 316L urea grade (UG) stainless steel scrubber lining was showed severe damage. In order to find out the root cause of failure, cracks were observed by visual inspection together with microstructure examination Chemical analysis of the base metal and corrosion products were analyzed Chemical analysis of the lining base alloy is conformed to the AISI 316L UG. Results revealed existence of chloride assisted stress corrosion cracking (SCC) initiated from pitting formed in the inner surface of the scrubber lining. Corrosion products analysis revealed existence of chloride (Cl^-). Localized pitting corrosion is attributed to Cl attack. The expected sources of chloride are water during hydrostatic testing, water vapor during passivation, chloride containing atmosphere during transportation or storage the base alloy in coastal areas and water vapor in carbamate gasses (process fluid). Pits initiated at the lining inner surface act as stress concentration sites assisted SCC initiation. Chloride SCC is caused by simultaneous action of both tensile stresses and a corrosive environment containing chloride. To overcome the scrubber failure, chloride must be eliminated and residual stresses must be controlled. Upgrading the scrubber base alloy using high-alloy duplex stainless steel (Safurex) is highly recommended.

Abstract: The corrosion behavior of maraging hardened stainless steel (MHSS) in different Cl^- medium was investigated by thermodynamics simulation and electrochemical experiments. The simulation results show that the thermodynamic stability zone decreases with the increase of the concentration of Cl^-. Some of chromium transformed into Cr(OH)²⁺ and adsorbed on the surface of stainless steel, and others generated Cr₂O₃ protecting the matrix. Mo reacted with O₂ to form MoO₄^2- adsorbed on the surface of the material, which inhibited the destruction of Cl^-. The electrochemical experiments indicate that the concentration of Cl^- is in the range of 2%-7%. The pitting potential and self-corrosion potential of MHSS decreased linearly with the increase of ion concentration, and the pitting corrosion resistance of MHSS decreased. When the self-corrosion current increases from 1.9888 μA to 2.6524 μA, the corrosion tendency of the material enhances.