Abstract
The corrosion behaviour of an HVOF Ni–5Al/WC–17Co coating on Al-7075 is investigated in 0.5 M H 2SO 4. In the temperature range of 25–45 °C, the coating exhibits pseudopassivity that effectively protects from localized corrosion. At 25 °C, pseudopassivity proceeds via three stages: during the first stage, oxidation of W in the binder phase occurs. The second stage is characterized by oxidation of W in both the binder and the carbide particles. The third stage is characterized by intensive hydration of WO 3 and formation of Co 3O 4. During the second and third pseudopassive stages, the formation of a bi-layer surface film is postulated. The inner layer, consisting of anhydrous oxides, has a barrier character. The outer layer, composed of WO 3 · xH 2O, is unstable. In case of surface film disruption, the bond coat successfully hinders corrosion propagation into the Al-alloy. Higher electrolyte temperatures lead to faster corrosion kinetics and higher tendency for pitting.
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