Chromate and dichromate ions are widely used to inhibit the corrosion of Al and Al alloys. They are effective when added to a corrosive liquid, when incorporated in a conversion coating, or added as a pigment in paint. While conversion coatings or paint can reduce corrosion by forming a protective barrier, it has also been considered that chromate in these layers acts by dissolving into local environments and inhibiting the dissolution reaction. It is therefore of interest to know how chromates affect the dissolution kinetics in localized corrosion reactions. In this work the kinetics of pit growth in thin film aluminum samples are studied. Thin films have been found to be a useful vehicle for the study of pitting kinetics. Essentially, the pitting process is simplified by the removal of one dimension so that pit growth is restricted to two dimensions. 2-D pits in thin films grow in a steady-state fashion with a constant pit depth and pit current density that is constant with time but is a function of applied potential. Analysis of images of growing pits provides a useful approach for determining the pit current density as a function of potential.
Pit growth in 2090 A thick sputter-deposited Al thin films is being studied. Chloride solutions with and without the addition of inhibitor ions is used. In order to maintain a similar conductivity, sulfate is added to inhibitor-free solutions in equal normality. A striking dependence of pit growth in Al thin films on the concentration sodium dichromate has been found. When the dichromate:chloride normality ratio was 1:10, dichromate had no effect on the pitting behavior. Pits formed and repassivated at similar potentials with and without dichromate present. The pit current-density/ potential relationship was also unaffected. For a dichromate:chloride normality ratio of 10:1, it was not possible to create a pit at all, even though a variety of approaches to do so was attempted. Pits that were growing at high potential in a dichromate-free chloride solution immediately repassivated when dichromate at a 10:1 normality ratio was injected. At a ratio of 1:1, pits were found to form in the Al thin films. However, the presence of dichromate altered the pit growth kinetics. The i-E curves in solutions containing 0.1 or 0.01 N each of chloride and dichromate were shifted in the noble direction by 100-150 mV compared to solutions of identical normality containing sulfate instead of dichromate. This shift in the i-E curve is similar to what has been observed for alloying of sputter-deposited Al with small amounts of a number of elements. The critical pit current density for repassivation was higher in the presence of dichromate ions. This indicates that a higher cation concentration is needed in the pit environment to achieve the critical conditions required to prevent repassivation. The influence of the dichromate: chloride ratio will be studied in detail as will the effects of other inhibitor ions such as nitrates, molybdates, and cerium ions.
The detection of localized corrosion on a large and complex structure such as an airplane is very difficult. Many different sensors and techniques are being developed to detect corrosion. However, they are only effective if they are at the place where the corrosion is happening. The basic idea in this study is to modify paint in order to make it a sensor for corrosion since paint has access to the whole surface of a plane. The approach is to develop an indicative coating system that will change in an easily-recognizable and quantifiable fashion when corrosion occurs in the underlying or nearby metal. As a first step, clear polymers mixed with pH indicators have been studied. The hope is to be able to detect pH increases at the polymer/metal interface near sites where the cathodic reaction takes place. The cathodic reaction typically occurs closer to boldly-exposed (and thus visually-accessible) locations than the anodic reaction, which may happen, for instance, deep within the lap joints of the skin of an airplane. Issues to be addressed include optimal schemes for data collection and analysis, calibration of color change in the polymer with pH change, correlation of pH and color change with severity of corrosion, and corrosion protectiveness of the modified polymer coating. It is of coarse important that the coating be extremely protective.
Preliminary studies have used mixtures of acrylic paint with different pH indicators (having a range of pH for color change). Al alloy samples have been coated with these mixtures, and then overcoated with pure acrylic paint. Observation of the surface following exposure to chloride solution indicates that the coatings do change color when corrosion occurs underneath, and that the color change depends on the corrosive conditions and the indicator used. For instance, the higher the concentration of NaCl solution the shorter the time required for observable color change. The different indicative coatings have characteristic intensities and area of color change. It is concluded that corrosion can redistribute pH values on the surface of specimen beneath a coating and induce color change of an indicative coating.
IBM is developing a polymer/metal composite for use as an interconnect material. This composite is electrically conductive owing to the percolation of Ag particles in the polymer matrix. Corrosion is a concern because of the tendancy for Ag to react, and also to exhibit electrolytic migration under an applied bias. This study is investigating the properties of the composite paste and model structures comprising blanket metal layers convered with blanket thin films of polymer.
Friction stir welding (FSW) is a relatively new process that potentially offers considerable improvements in the properties of welded Al alloy structures. With FSW, a butt joint is created between two plates using heat generated by friction. This heat comes from motion of a tool similar to a milling machine along the seam between the two plates, instead of relative motion of the parts as in the case of standard friction welding. The resultant FSW joint is achieved by hot flowing accompanied by dynamic recrystallization without melting of the metal. No filler is used so the weld nugget has a composition identical to that of the base metal. Furthermore, since this is a solid state process, elemental segregation associated with solidification is eliminated. These two factors should help reduce the corrosion susceptibility of the welds. In this work, the localized corrosion and stress corrosion cracking properties are determined for friction stir weldments of various Al alloys. Comparisons are made both to the base alloys and to arc-welded samples. Two Al alloy systems are examined: alloy 5454 and an experimental alloy of composition Al-1.8Li-2.7Cu. Pitting corrosion is examined in chloride solutions, and stress corrosion cracking is assessed via slow strain rate and u-bend tests.
The Air Force has decided that the effort to develop an effective and environment-friendly replacement for chromate will be greatly facilitated by the development of a better understanding of the mechanism of chromate inhibition itself. A team of researchers from academia, a government lab, and an industrial research center will address this problem from many different aspects. The team includes Susan Smialowska, Professor Emeritus of Materials Science and Engineering, and Richard McCreery, Professor of Chemistry, both at Ohio State. Also involved are Dr. Hugh Isaacs from Brookhaven National Lab, Prof. Clive Clayton from SUNY Stony Brook, Dr. Richard Granata from Lehigh University, Prof. Martin Stratmann from Univ. of Erlangen, and Dr. Martin Kendig from Rockwell Science Center. A summary of the work to be done is given below:
kinetics and mechanism of chromate reduction reaction and anodic behavior of Al in chromate solutions
role of intermetallics, effect of chromates on intermetallics
scratching and breaking experiments
topographical studies of corrosion and coating formation processes at intermetallic particles, AFM "scratching", local surface charge measurement
XPS SAM, profiling Scanning Photoemission Microscopy and imaging, Small Angle X-Ray Scattering, White Beam X-Ray Topography, Forward Recoil Elastic Scattering, Nuclear Reaction Analysis, RBS
characterization of coatings, in situ monitoring of conversion coating formation and corrosion process
piezo-electrocapillary experiments
Use sputtered oxide analogs to simulate chromate conversion coatings
Photoelectrochemistry and EIS to study electronic and semiconducting properties, and chemical behavior
X-ray Absorption Spectroscopy to study composition of coatings, behavior of sputtered analogs, leaching of chromates from coatings, and chemistry of chromates in artificial pit environments
Positron Annihilation Lifetime Spectroscopy, surface tension, cathodic disbondment, IR spectroscopy
detect the local potential of the intact substrate/polymer interface
integration of results into predictive comprehensive model (All)