M.W. Kendig

Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings

Abstract This review covers the aqueous chemistry of chromium and the origins of chromium toxicity. Evidence from older and more recent studies is presented showing that inhibition of Al corrosion is derived from both inhibition of oxygen reduction and inhibition of metal dissolution reactions mainly due to a delay in the onset of pitting. Inhibition of corrosion by chromates appears to be closely linked to their ability to irreversibly adsorb on to metal and oxide surfaces. With respect to chromate conversion coatings (CCCs), the evolution of modern coating formulations is presented with a focus on key advances that led to process simplification and improved coating performance. The current understanding of CCC formation, protection, and aging is presented. In the last section of the paper, processing and properties of selected chromate-free conversion coating chemistries is discussed. This discussion focuses on coatings that offer particularly attractive attributes such as self-healing and excellent adh...

The mechanism of corrosion inhibition by chromate conversion coatings from X-ray absorption near edge spectroscopy (xanes)

Abstract X-Ray absorption spectra of chromate conversion coatings on Al 2024-T3 and 3003-H14 have been obtained as a function of processing time and environmental exposure. The spectra reveal the presence of a significant ratio of unreacted hexavalent chromium in the chromate conversion coatings which decays with the time of exposure in air-equilibrated 0.5 M NaCl. These results suggest that the unique corrosion-protective property of chromated Al lies in the presence of a source of the hexavalent chromium species as an active inhibiting species.

XANES investigation of the role of cerium compounds as corrosion inhibitors for aluminum

Abstract Cerium-containing films on aluminum and Alloy 5052 have been investigated using XANES (X-ray Absorption Near Edge Structure). The Ce L III and Ce L I absorption edges allow a clear distinction to be made between 3- and 4-valent cerium. Measurements were made of the fluorescent X-ray signal with X-rays incident at glancing angles to give surface sensitivity. Cerium in the 3-valent state was deposited by galvanostatic reduction from solutions of sufficiently high pH for oxygen to oxidize the cerium to a 4-valent state. From XANES measurements, it was determined that these films contained cerium mainly in the 3-valent state. Exposure of these films to a NaCl solution converted the cerium to the 4-valent state. Open circuit exposure of aluminum to low concentrations of cerium ions gave films with either 3- or 4-valent cerium depending on the exposure time.

The Mechanism of Corrosion of Poly butadiene-Coated Steel in Aerated Sodium Chloride

Abstract Polybutadiene-coated steel exhibits highly localized corrosion sites that spread with time. Delamination of the coating occurs in the cathodic areas immediately adjacent to the anodic corrosion sites. Data obtained from electrical measurements, corrosion potentials, and literature sources were used to arrive at an overall mechanism of corrosion.

An electrochemical impedance spectroscopy study of reactions at the metal/coating interface

Abstract A segmented two electrode system has been used to obtain impedance spectra under a polymer coating. These spectra have been compared with those obtained across the coating between the short circuited steel electrodes and a counter electrode. A transmission line model has been used to obtain estimates for the delaminated volume and the polarization resistance of the steel surface as a function of exposure time to a NaCl solution.

A new atmospheric corrosion rate monitor―development and evaluation

Abstract A research program has been carried out in which a new instantaneous atmospheric corrosion rate monitor (ACRM) has been developed and evaluated and equipment has been constructed which will allow the use of many sensors in an economical way in outdoor exposures. The ACRM was tested in flow chambers in which relative humidity and gaseous and particulate pollutant levels can be controlled. A relatively inexpensive electronics system for control of the ACRM and measurement of atmospheric corrosion rates was designed and built. Calibration of deterioration rates of various metallic and nonmetallic materials with the response of the ACRMs attached to these materials was carried out.

The influence of electrochemical potential on wear

Abstract Electrochemical potentials between −1.4 and +1.2 V (saturated calomel electrode) applied to Ni(200) in the nominally inert electrolyte 1 M NaClO 4 resulted in an observed order of magnitude variation in the wear rate of the nickel under fretting and sliding conditions. The pin-on-disc experiments were carried out with a fused Al 2 O 3 pin loaded at 9.8 and 19.6 MPa sliding on discs at 10 m min −1 , and fretting wear was simulated by a high frequency (10 5 –10 7 cm −2 s −1 ) low amplitude impact of a vibrating Al 2 O 3 pin on the disc. These results suggest that it is possible to control wear in ionically conducting fluids through the application of appropriate electrochemical potentials.

Evaluation of cutting fluid effectiveness in machining using electrochemical techniques

Abstract A correlation has been found between the extent of electrosorption of surfactants from cutting fluids at metal surfaces and cutting forces in machining. The extent of adsorption was determined from electrochemical impedance and polarization data. This suggests that the effectiveness of cutting fluids in machining as characterized by cutting forces may be assessed using rapid electrochemical techniques, thereby reducing the need for time consuming and costly machinability tests.

Effects of Trace Elements on Corrosion in Total Immersion and under Wet/Dry Conditions

Abstract The effects of 13 trace elements in a base line solution that is considered to represent flue gas desulfurization environments have been studied by electrochemical measurements for five alloys that were totally immersed or immersed for 1 h/day. Polarization resistance and corrosion potential measurements were performed continuously for 10 electrodes per test solution over a 6-day period, resulting in approximately 600 measurements per test. A computerized system was used to store test data and analyze results. Very large differences in corrosion behavior were observed for the five alloys in a solution containing a given combination of trace elements and for a given alloy in solutions containing different combinations of trace elements. A statistical analysis has been performed to determine the trace elements that have significant effects on corrosion behavior.