Effect of long-term neutral salt spray exposure on durability of adhesive-bonded Zr–Ti coated aluminum joint

In this study, the effect of long-term neutral salt spray (NSS) exposure (i.e., 50 g/L concentration of salt solution at 25 °C) on the retained strengths of Zr–Ti coated and bare lap-shear aluminum joints bonded with hem flange adhesive Henkel Terokal 8021 NB was investigated. A one-part toughened epoxide adhesive and Zr–Ti coated aluminum substrates (i.e., AA6014-T4 and AA6016-T4) were selected. Adhesive-bonded coated aluminum joints (ACJ) and bare aluminum joints (ABJ) were fabricated and exposed in NSS environment for various times. Quasi-static tests were conducted immediately following removal of the joints from the salt chamber at the ambient condition. It was found that while NSS exposure for 720 h degraded little the strength of ACJ, it decreased the strength of ABJ. The Zr–Ti coating protected aluminum substrates from electrochemical reaction and consequently minimized the strength degradation. As the exposure time was prolonged to 1400 h, the strength of ACJ was reduced drastically while the strength of ABJ was only degraded slightly. Fractography, differential scanning calorimetry, electrochemical potentiostatic polarization measurements, and X-ray photoelectron spectroscopy analyses revealed the strength degradation of ACJ was caused primarily by the corrosion of Zr–Ti coating. The passive film on bare aluminum provided a long time protection against NSS exposure, and consequently minimized the strength degradation for ABJ.