Localized Corrosion -- Measurement, Mechanisms and Mitigation: Light Metals
Sponsored by: MS&T Organization
Program Organizers: Gerald Frankel, Ohio State University
Monday 2:00 PM
October 17, 2011
Room: D244/245
Location: Greater Columbus Convention Center
Session Chair: Gerald Frankel, The Ohio State University
2:00 PM
Microstructural Aspects That Accompany Localised Corrosion of Sensitised AA5083: Nick Birbilis1; 1Monash University
The sensitisation of aluminium alloy 5083 leads to a dramatic deterioration in corrosion resistance, compromising the integrity of the alloy (often when in-service). The microstructural changes in the alloy that accompany senistitsation are studied using a combination of quantitative TEM and high resolution SEM of carefully prepared specimens. It is revealed that localised corrosion, namely intergranular corrosion, is closely linked with critical aspects of the stereology of precipitated particles. Quantification of the microstructure-corrosion relationship allows for one to typify the impact of sensitisation on a microstructural basis, as opposed to a time-temperature basis that has been used in prior studies.
2:20 PM
Atmospheric Pitting Corrosion of Al Alloys: Sean Morton1; Jinfeng Li2; Gerald Frankel1; 1Ohio State University; 2Central South University
Atmospheric corrosion involves open circuit exposure of metals to thin and sometimes concentrated electrolyte layers. Localized corrosion occurs during atmospheric conditions as the result of the drying of a salt solution droplet on the surface of a metal as temperature increases or RH decreases, and the associated increase in chloride concentration. A Scanning Kelvin Probe (SKP) was used to investigate atmospheric corrosion of Al alloys. Small droplets of MgCl2 solution were placed on an Al-0.63Mg-0.28Si alloy and allowed to dry in both fixed and cycled RH environments while monitoring the Volta potential (which correlates to the corrosion potential) with SKP. Observations of corrosion phenomena vary with initial concentration of MgCl2 and relative humidity, and include metastable pitting, stable pitting, and a unique filiform-like corrosion at the edge of the droplets. Additional observation of the development of these corrosion phenomena have been investigated via direct optical interrogation.
2:40 PM
Understanding the Role of Intermetallic Particles in Localized Corrosion: A Multiscale Modeling Approach: Jie Xiao1; Hyunwook Kwak1; Santanu Chaudhuri1; 1Washington State University
Intermetallic particle (IMP)-induced localized corrosion is a complex phenomenon that involves fully coupled (electro)chemical and physical processes over a wide range of length and time scales. Important processes that have been identified in recent experimental work include galvanic coupling and selective dissolution of IMPs, and etching of oxide film and matrix. The interrelationship between them, however, is still poorly understood. This work targets new insights into particle-induced corrosion through multiscale modeling. The bulk and interfacial electrochemical environment is obtained from a FEM-based model with IMPs and the matrix galvanically coupled by the mixed potential theory. Localized environment then serves as inputs into kMC simulations of the metal-electrolyte interface, which evolves in both surface topology and the composition. We will present how corrosion develops on 2000 series aluminum alloys containing both θ-phase and S-phase when exposed to chloride solutions, focusing on demonstrating the interplay between IMP coupling, dealloying and pit formation.
3:00 PM
The Effect of Weak Acids on Localized Corrosion Morphologies in High Strength Aluminum Alloys: Alexandra Neeley1; Rudy Buchheit1; 1OSU
Previous studies have suggested that pits associated with noble intermetallic particles in high strength Al alloys are deeper when a weak acid is present in solution. This has been attributed to local alkalinity from oxygen reduction on noble intermetallic particles by buffering action of weak acids, and consequent adjacent matrix dissolution. Optical profilometry was used to further characterize effects of weak acids on localized corrosion of 2024-T3 and 7075-T6 aluminum alloys exposed to dilute NaCl solutions containing 0.01M H3BO3 adjusted to pH 7. For 2024, pit-number-density decreased in the presence of weak acid, but pit depths increased at a much faster rate. In 7075-T6, weak acid also decreased pit-number-density, but did not accelerate pit deepening. Pitting results, characterized by optical profilometry and SEM analysis, will be presented, as well as differences in pit damage accumulation rates between the two alloys in terms of intermetallic particle compositions and oxygen reduction kinetics.
3:20 PM Break
3:40 PM Student
Fabrication and Characterization of Porous Aluminum Alloys by Chemical Dealloying: Effect of Concentration and Cooling Rate: Elvin Estremera1; O. Marcelo Suarez1; Arturo Hernandez-Maldonado1; 1University of Puerto Rico
Macroporous aluminum alloys were fabricated through chemical dealloying of Al-Zn alloys with 7-40 at% Zn under free corrosion conditions in a 25 wt% NaOH solution. The alloys were quenched from liquid to solid at various rates. The microstructure of the resulting porous sponge was characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and surface area (BET). The results show that porosity depends highly on alloy concentration and cooling rate. Concentrations ranging from 11 to 15 at% Zn in Al show better overall porosity, porous interconnection and surface area. Faster cooling rates promotes better Zn atoms distribution and consequently better macroporosity.
4:00 PM
The Effect of Stress on Localized Corrosion of AA7050: Junfeng Chen1; Gerald Frankel2; Liang Zhen1; Wenzhu Shao1; 1Harbin Institute of Technology; 2Ohio State University
Creep age-forming is a stress ageing that is widely used in the aerospace industry. This paper compares the location corrosion behavior of AA7050 after age-forming and tradition ageing. Age-forming induced the precipitation of MgZn2 and the formation of a precipitate free zone growth. These precipitates are anodic to the matrix. Cyclic polarization curves in deaerated and aerated 3.5% NaCl solution show that the corrosion potential and corrosion current density of most age-formed samples are lower than traditional aged samples. AFM analysis of the surfaces also suggests that the age-formed samples are more corroded. The size and distribution of precipitates, which were altered by the age-forming process, have a large influence on the localized corrosion behavior. Furthermore, stress has an effect on the grains during age-forming process, which proved by EBSD results. It is well known that the grain shape and grain boundary composition has significant influence on intergranular corrosion.
4:20 PM
First-Principles-Based Analysis on the Role of RE Doping on Mg Alloy Corrosion: Hyunwook Kwak1; Santanu Chaudhuri1; 1ISP/Applied Sciences Laboratory, Washington State University
Rare-earth (RE) doping of Mg alloys has been of great interest for corrosion protection of the material for various applications in automotive and defense technologies. Owing to the complex nature of its corrosion mechanism however, atomistic-scale understanding of the role of RE doping and the larger length scale implication of such results have been limited to date. In this work, recent progress in our first-principles-based corrosion analysis on RE-doped Mg alloy systems coupled to kinetic Monte Carlo (kMC) simulation of Mg-RE alloys will be presented. The results show dramatic reduction in localized corrosion rates after the film failure. In addition, for comparable microstructures of the oxide layer, the role of RE in altering the selective dissolution process will be demonstrated. The work provides key insight to the role of RE doping for corrosion protection of Mg alloys and possible model for selective enhancement of grain boundary stability under humid environment.
4:40 PM Student
Surface Corrosion Behavior of Electron-Excited Pure Mg: Keisuke Funatsu1; Rei Takei1; Junko Umeda2; Katsuyoshi Kondoh2; 1Graduate school of osaka university; 2Osaka university
Magnesium (Mg) is highly corrosive due to its low standard electrode potential (SEP). The conventional methods to improve its corrosion resistance are mainly anodic oxide film formation and chemical conversion treatment. However, they are expensive, and cause the poor recyclability of Mg components with these surface treatments. In the present study, new corrosion protection method of Mg using surface potential change by electron excitation was investigated. Surface potential variation of pure Mg was measured by using Scanning Kelvin Probe Force Microscopy (SKPFM), used to determine anodic/cathodic area at the interface of different phases in the local galvanic cell. The electronically excited area by electron beam irradiation using SEM showed lower surface potential than that of the as-polished surface. SEM image after salt water immersion test and topographic change obtained by AFM indicated corrosion resistance improvement in electronically excited area containing less corrosion products compared to as-polished area.
5:00 PM
Aluminum Weld Metal Corrosion: Philip Vormelker1; Adrian Mendez-Torres1; 1Savannah River National Laboratory
Experience at the Savannah River Site has shown that aluminum welded coupons reveal heavy pitting after long term storage in a basin with deionized and oxygen saturated water at ambient temperatures with a pH of 6. This is the same basin where spent nuclear fuel is stored. Specifically, the weld metal (Al 4043) and heat affected zone of the base metal (Al 6061-T6 and Al 6063-T5) are attacked. Based on the literature, corrosion potentials of these aluminum alloys were previously reported as being negative 0.82-0.83 volts (0.1 N calomel scale) in a salt solution. This alloy combination is then recommended for welding. Since the water in the storage basin is more neutral, potential measurements were taken on alloy coupons in both water solutions and compared. The results are reported in this paper.