Aluminum Surface Finishing Corrosion Causes and Troubleshooting
by W. John Fullen, Boeing Research and Technology & Jennifer Deheck, Boeing, Seattle, Washington, USA
Editor’s Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2014 in Cleveland, Ohio on June 10, 2014. A printable PDF version is available by clicking HERE.
Aluminum corrosion is commonly encountered when performing chemical process operations involving surface finishing, predominantly in preparation for paint application. The protective oxide film of aluminum is only stable in a pH range of 4.5 -8.5. However, many process solutions intentionally exceed this pH range for the purpose of cleaning, metal removal and subsequent smut removal. These process solutions are formulated so as not to cause deleterious pitting or preferential etching. However, the susceptibility of aluminum to pitting depends on many extraneous factors, such as chloride ion concentration, pH control and initial surface condition. Electrochemical measurements via potentiodynamic scans have been shown to be an effective tool in analyzing the propensity of certain process solutions to contribute to observed pitting conditions. In this paper, a review of several process solutions, examining coolants, solvent cleaning, alkaline clean/etch and deoxidizing/desmutting, listing intended and unintended chemical reactions along with possible mechanisms that would favor corrosion formation. Further explanation is provided for the role of incoming water that is used for process solution make-up and the myriad of rinse tanks. Recommendations are provided for electrolytic processes that might be prone to stray currents affecting auxiliary equipment and thereby introducing deleterious contaminants into process solutions as a result of the corrosion products of compromised piping, fittings and fasteners from heating and cooling units. Strict adherence to process specification controls, regular monitoring of suspect contaminants, sound housekeeping and part handling best practices can alleviate many aluminum part processing corrosion occurrences.
Keywords: aluminum, aluminum surface finishing, corrosion causes, corrosion troubleshooting
A protective oxide film of aluminum is only stable in a pH range of 4.5 to 8.5.1 Chemical operations for the metal surface of aluminum include many process solutions that intentionally exceed this pH range for cleaning, metal removal and subsequent smut removal. These process solutions are formulated to avoid deleterious pitting or preferential etching. However, the susceptibility of aluminum to pitting depends on many factors, such as chloride ion concentration, pH, dissolved oxygen in the corrosion environment and surface condition.2 Furthermore, aluminum alloys themselves can contribute to pitting problems due to preferential etching. For instance, aluminum 7075, which contains magnesium and zinc, is more prone to pitting than aluminum 2024 even though the primary alloying element is copper.3 The purpose of this paper is to highlight the major areas of aluminum corrosion that can be encountered during metal finishing operations with the benefit of effectively troubleshooting these occurrences when they inevitably happen.
Pitting potential is a term used to describe the likelihood of a metal to pit when electrochemically analyzed using a potentiodynamic scan of the metal and process solution system. A Boeing R&D effort was initiated to develop a usable test method that quantifies the pitting potential of a process solution relative to a selected alloy. The scope of the research has been limited to metalworking fluids and a degreasing solution, but the test method could be applied to other chemical process solutions.
For this electrochemical potentiodynamic method, potential (volts) versus current density (amps/cm2) is evaluated. The voltage starts cathodic (negative) and is slowly ramped up while measuring current. Two key voltage levels are marked on the scan. The first is corrosion potential (ECORR), which is the potential at electronic neutrality, also known as the open circuit potential. The other key voltage level is breakdown potential (Eb), which is the potential at which the anodic polarization curve shows a marked increase in current density, leading to breakdown of the passive film and pit initiation. Consequently, the closer Eb is to ECORR, the greater the probability that pitting will occur (Fig. 1).