Pitting Corrosion of Metals: A Review of the Critical Factors

Pitting corrosion is localized accelerated dissolution of metal that occurs as a result of a breakdown of the otherwise protective passive film on the metal surface. This paper provides an overview of the critical factors influencing the pitting corrosion of metals. The phenomenology of pitting corrosion is discussed, including the effects of alloy composition, environment, potential, and temperature. A summary is then given of studies that have focused on various stages of the pitting process, including breakdown of the passive film, metastable pitting, and pit growth.Portions of this work and the preparation of this manuscript were supported by the United States Air Force Office of Scientific Research contracts F49620-96-1-0479 and F49620-96-0042 administered by Major H. De Long

Corrosion Study of AA2024-T3 by Scanning Kelvin Probe Force Microscopy and In Situ Atomic Force Microscopy Scratching

The localized corrosion of AA2024-T3, and the behavior of intermetallic particles in particular, were studied using different capabilities of the atomic force microscope (AFM). The role of intermetallic particles in determining the locations and rates of localized corrosion was determined using scanning Kelvin probe force microscopy in air after exposure to chloride solutions. Al-Cu-Mg particles, which have a noble Volta potential in air because of an altered surface film, are actively dissolved in chloride solution after a certain induction time. Al-Cu-(Fe, Mn) particles are heterogeneous in nature and exhibit nonuniform dissolution in chloride solution as well as trenching of the matrix around the particles. Light scratching of the surface by rastering with the AFM tip in contact mode in chloride solution results in accelerated dissolution of both pure Al and alloy 2024-T3. The abrasion associated with contact AFM in situ resulted in the immediate dissolution of the Al-Cu-Mg particles because of a destabilization of the surface film.This work was supported by the Air Force Office of Scientific Research under contact no. F49620-96-1-0479, Major H. De Long, Administrator. P. S. was partially supported by the Swiss National Foundation of Research

Electrochemical Quartz Crystal Microbalance Study of Corrosion of Phases in AA2024

The electrochemical quartz crystal microbalance (EQCM) was used to directly measure the dissolution rate at cathodic potentials, and thus the cathodic corrosion rate, of thin-film analogs of phases in AA2024. Thin films of pure Al, Al-4% Cu, and Al2Cu were studied in 0.1 M NaCl containing 0, 10^-4, or 10^-2 M Cr2O7 . A range of cathodic potentials was studied for each material. The true cathodic current density was calculated from the difference of the net current density and the dissolution rate, which was determined by the EQCM. For pure Al and Al-4Cu, the cathodic corrosion rate was large relative to the net current density, so the true cathodic current density was considerably larger than the measured net current density. The cathodic current density was almost identical to the net current density for Al2Cu because the dissolution rate was very small compared to the cathodic reaction rate. Various potentials in the limiting oxygen reduction reaction region were examined, but the effect of the applied potential was small. The presence of dichromate in solution decreased both the cathodic corrosion rate and the cathodic current density on these thin-film analogs. In particular, it decreased more effectively the cathodic reaction rate on Al2Cu, which can support faster cathodic reaction rates.This work was supported by the United States Air Force Office of Scientific Research Grant no. F49620-96-1-0479 under the guidance of Dr. Paul Trulove

Electrochemical Behavior of AISI 304SS with Particulate Silica Coating in 0.1 M NaCl

This paper presents electrochemical behavior of AISI 304 stainless steel with a silica layer in a stagnant bulk solution of 0.1 M NaCl. Layers composed of densely packed 350 nm diam silica particles were deposited cathodically on stainless steel at a constant voltage by electrophoretic deposition (EPD). Quite smooth and crackfree silica layers less than about 80 μm in thickness were obtained and the thickness of the layer depended linearly on the deposition time. It is proposed that silica layers deposited by EPD can be used as simulated particulate layers to investigate localized corrosion of corrosion-resistant alloys under atmospheric environments. Electrochemical properties of silica-coated stainless steel samples in 0.1 M NaCl were investigated. The cathodic polarization behavior depended on the thickness of the silica layer; the limiting current density for oxygen reduction reaction decreased with increasing silica layer thickness. The effect of the silica layer on anodic polarization behavior was not remarkable.The work was performed under the Corrosion and Materials Performance Cooperative, DOE Cooperative Agreement Number: DE-FC28-04RW12252

Corrosion Inhibition of AA2024-T3 by Vanadates

The speciation of vanadate solutions and the resultinginhibition of oxygen reduction and corrosion of AA2024-T3 wereinvestigated. 51V NMR is very useful for assessing vanadatespeciation. Clear metavanadate solutions contain nodecavanadate, which forms whenever the pH was decreased by theaddition of acid. Orange decavanadate solutions contain nomonovanadate, even when the pH is adjusted to high values.Monovanadate is a potent inhibitor in contrast to decavanadate. Inhibition by monovanadate seems to result from an adsorptionmechanism rather than reduction. Monovanadate effectivelyprotects S phase particles. Aging of high-pH decavanadatesolutions does not improve the inhibition performance or resultin complete depolymerization of the decavanadate