Corrosion Science and Technology
Objectives: Research is conducted at CEEL to determine the electrochemical kinetics and mechanisms of corrosion phenomena for a wide range of materials and environments. Based on a detailed knowledge of corrosion mechanisms improved approaches to corrosion monitoring and control are investigated. Emphasis is placed on the evaluation of various methods of corrosion protection such as the use of environmentally acceptable inhibitors and coatings. The major areas of current interest are:
1. The Interaction of Bacteria and Metal Surfaces. Traditionally it has been assumed that the interaction of bacteria with metal surfaces always causes increased corrosion rates ("microbiologically influenced corrosion" (MIC)). However, more recently it has been observed that many bacteria can reduce corrosion rates of different metals and alloys in many corrosive enviroments. It has been found that certain strains of Shewanella can prevent pitting of AI 2024 in artificial seawater, tarnishing of brass and rusting of mild steel. It has been observed that corrosion started again when the biofilm was killed by adding antibiotics. The mechanism of corrosion protection seems to be different for different bacteria since it has been found that the corrosion potential Ecorr became more negative in the presence of Shewanella ana and algae, but more positive in the presence of Bacillus subtilis. These findings have been used in an initial study of the bacterial battery in which Shewanella oneidensis MR-1 was to a cell sontaining AI 2024 and Cu in a grouth medium. It was found that the power output of this cell continuously increased with time. In the microbial fuel cell (MFC) bacteria oxidize the fuel and transfer electrons directly to the anode. In initial studies EIS has been used to charaterize the anode, cathode and membrane properties for different operating conditions of a MFC that contained Shewanella oneidensis MR-1. Cell voltage (V) - currnet density i curves were obtained using potentiodynamic sweeps. The current output of a MFC has been monitored for different experimental conditions.
2. Green Technology in Corrosion Protection. Rare earth metal salts (REMS) have been used as replacements of toxic chromates in corrosion protection of commercial aluminum alloys. For Cu-bearing alloys a Cu removal step has been added to the process. Exceptional corrosion resistance has been obtained with this approach. REMSs have also been used to replace chromates as sealants in Boeing's boric acid-sulfuric acid anodizing process and other commercial anodizing procedures. Improved paint adhesion of the sealed anodized surfaces has also been achieved. The application of REMSs as inhibitors or components of conversion coating processes is also being evaluated for other materials such as steels.
3. Corrosion Protection of Composite Materials. Composite materials such as Al/SiC, Al/graphite or Mg/graphite have very desirable mechanical properties, but very poor corrosion resistance. For metal matrix composites corrosion protection by chemical conversion coatings, anodizing and/or application of polymer coatings has been evaluated using sophisticated electrochemical techniques such as electrochemical impedance spectroscopy (EIS) which allows detection of changes of the coating properties and of the metal surfaces properties due to exposure to corrosive environments. The use of REMSs for corrosion protection has been evaluated.
4. Corrosion Protection by Inhibitors and Coatings. Surface modification by inhibitors, passive films and coatings is an important approach for corrosion protection. Theoretical and experimental work is being carried out to develop a better understanding of the corrosion behavior of "real" surfaces and the changes of these properties due to surface modification. The present effort focuses on corrosion inhibition in neutral, aerated media and the description of the electrochemical properties of inhomogeneous surfaces which has important applications in localized corrosion processes. The use of EIS and electrochemical noise analysis (ENA) as tools for monitoring the corrosion behavior of polymer coated metals and determining the remaining lifetime of a coating is being evaluated.
5. Electrochemical Noise Analysis (ENA). Naturally occurring fluctuations of potential and current in corroding materials can be used to determine their corrosion characteristics. Normally the electrochemical noise (ECN) data are analyzed in the time domain. It has been shown that more detailed information becomes available if these data are analyzed in the frequency domain. The concept of "spectral noise plots" has been developed and it has been shown that spectral noise plots and impedance plots should be identical. The noise resistance Rn which is determined from ECN data in the time domain depends on the bandwidth Df of the ECN measurement and equals the polarization resistance Rp only if the ECN and impedance data are independent of frequency f within Df. Satisfactory agreement between corrosion rates determined by weight loss and ECN data obtained for the same electrodes has been observed in field tests suggesting that ENA can be a valuable tool for corrosion monitoring.
6. Microbiologically Influenced Corrosion (MIC). The electrochemical and microbiological reactions occurring on stainless steels, Cu-based materials and Ti have been evaluated during exposure to natural seawater using electrochemical and surface analytical techniques. The impact of microorganisms on corrosion protection of steel by polymer coatings has been studied during exposure to natural seawater using EIS and ENA. A novel experimental approach for collection of electrochemical data from remote test sites has been developed. These investigations have been supported by laboratory investigations. The spatial relationships between marine bacteria and localized corrosion have been determined for polymer coated steel.
7. Scanning Kelvin Probe Analysis (SKPA). SKPA has been performed on welded stainless steel to determine the presence of local anodes and cathodes formed in the welding process and their elimination as a result of proper passivation treatments. The potential distribution in the Evans drop experiment has been established. Initiation of localized corrosion under small drops of electrolyte containing chlorides and its inhibition by chromates has been demonstrated. The events taking place during the initial stages of intergranular stress corrosion cracking of stainless steel are been studied by SKPA. Correlations between locations of maximum stress and potential variations have been established experimentally.
8. Materials Damage Due to Acid Rain and Fog. Laboratory studies have been conducted to determine the effects of acid deposition on structural materials of economic significance. The role of pollutants such as S02, N02 and 03 and the chemistry of acid fog which may contain sulfuric and/or nitric acid have been studied under carefully controlled conditions in atmospheric test chambers for materials such as zinc, nickel, aluminum, house paints and polymers. Atmospheric corrosion rate monitors which provide continuous records of the corrosivity of the atmosphere have been exposed at various test sites in Southern California and in the test chambers. These data have been collected with a computerized data logging system and have been used in statistical analyses of the correlations between corrosion damage and atmospheric chemistry.
9. Corrosion Behavior in Low-Conductivity Media. Electrochemical techniques have been evaluated which will allow the determination of the corrosion kinetics in media of low-conductivity such as very dilute aqueous solutions and organic solutions. The corrosion behavior of stainless steels, Al and Ti alloys has been studied in hydrazine and nitrogen tetroxide.
Facilities: Research in corrosion and applied electrochemistry is carried out in the Corrosion and Environmental Effects Laboratory (CEEL) which is equipped with modern electrochemical equipment such as potentiostats, transfer function analyzers and noise analyzers. Most of this equipment is computerized for collection and analysis of the experimental data. Software has been developed for the collection of electrochemical data from remote test sites and analysis of these data based on appropriate models. Test chambers for corrosion studies under controlled atmospheric conditions are available.
The following classes related to corrosion and applied electrochemistry are being taught by Prof. Mansfeld:
MASC 521 - Corrosion Science
MASC 522 - Corrosion Technology
MASC 523 - Principles of Electrochemical Engineering
MASC 524 - Techniques and Mechanisms in Electrochemistry
MASC 440 - Materials and the Environment