July 15-20, 2018
In work sponsored by the Office of Naval Research a corrosion fatigue test method to better replicate real world corrosion conditions using salt film, relative humidity, ozone and UV-light, is being developed . This method is being used to evaluate the effect of atmospheric corrosion conditions in AA7075-T651 with inhibitive coatings including epoxy chromate and three chromate replacement coatings (water and epoxy based rare earth primers and an aluminum rich primer). The corrosion fatigue results are being paired with leaching studies under traditional immersion and atmospheric conditions on the primers to determine how the leaching rates relate to the ability of a primer to inhibit fatigue damage.
Research shows that chromate in concentrations related to leaching rates can slow fatigue crack growth in aluminum alloys in stress ranges relevant to airframe maintainers [2,3]. The fatigue tests showing inhibition with low levels of chromate (0.05 mM) were completed with chromate added to a full immersion sodium chloride solution. It remains unclear if epoxy chromate based and other polymeric inhibitor coatings can affect corrosion fatigue under atmospheric corrosion conditions. The protection provided by corrosion inhibitors undergoing fatigue can be affected by loading conditions (∆K, frequency) and also likely by the environment due to changes in coating leaching. An improved understanding of how environmental and loading parameters influence a coating’s ability to offer protection against corrosion fatigue damage would greatly help the coating community to design more robust coating protection systems.
Another focus area of the ONR sponsored research is in quantifying the corrosion damage to fatigue crack transition. A standardized specimen and testing protocol to evaluate the relative influence of material, environment, inhibitors, loading spectrum and other inputs on the pit-to-crack transition was developed [2,3]. The methodology uses a narrow plate specimen with a centrally located hole with a preferential pit (diameter approximately 150 µm) placed at the corner of the hole; current work is being completed on legacy aluminum alloy AA7075-T651. The plate thickness and hole diameter are consistent with commercial and military airframe applications. The method uses direct current potential drop (DCPD) to measure the crack length. In the current research the test methodology is being transitioned to evaluate galvanic interactions when a fastener of either stainless steel or titanium in placed in the hole. The fastener contact geometry is limited to two boundary conditions either the hole bore and starting pit are bare or only the starting pit is left unprotected. The fastener shank is left unprotected as well. The galvanic potential between the aluminum and the fastener is measured and then used to control the corrosion potential during the corrosion fatigue testing. This method allows for bench top electrochemical tests to be used to control the mechanical testing and also removes the time effect for galvanic corrosion to begin.
The overarching objective of the research is to improve and transition the results on the effect of environmentally assisted fatigue in high performance metallic alloys (crack growth rate data) to the DoD research and depot maintenance activities by integrating all data into the AFGROW fatigue crack prediction software allowing for the inclusion of corrosion damage and environment effects on fatigue crack life predictions. Likewise better methods for coating and material evaluation are being produced.
1 D. Laing, et al., “Effects of Sodium Chloride Particles, Ozone, UV and Relative Humidity on Atmospheric Corrosion of Silver,” Journal of the Electrochemical Society, 2012, 157 (4) pp. C146‑C156.
2 S.E. Galyon Dorman, J.W. Rausch and S.R. Arunachalam, “Examination and Prediction of Corrosion Fatigue Damage and Inhibition,” Corrosion Reviews, ISSN (Online) 2191-0316, ISSN (Print) 0334-6005, DOI: https://doi.org/10.1515/corrrev-2017-0057.
S.E. Galyon Dorman, et al, (2016) Managing Environmental Impacts of Time-Cycle Dependent Structural Integrity of High Performance DoD Alloys, SAFE Inc., SAFE-RTP-16-045.
Sarah E. Galyon Dorman, Scott A Fawaz, and Justin W Rausch, "Examination of Factors Influencing FCG in legacy aluminum alloys" in "Stress-Assisted Corrosion Damage V", A.K. Vasudevan, ONR (retired) Ronald M. Latanision, Exponent, Inc. N. J. Henry Holroyd, Luxfer Eds, ECI Symposium Series, (2018). http://dc.engconfintl.org/sacd_v/2