**1. Introduction**

Due to its good mechanical properties, corrosion fatigue resistance, corrosion resistance and cavitation resistance, Ni-Al bronze (NAB) has been widely used for marine components, such as ship propellers, pumps and valves [1–3]. NAB parts used in seawater are inevitably subject to corrosion by the seawater. To date, there are many studies on NAB corrosion behavior in seawater [1,3–8]. The results showed that an α phase corroded while a κ phase did not corrode in near-neutral seawater and 3.5% NaCl solution [1,8,9]. Neodo et al. [7] studied the corrosion behavior of NAB in a 3.5% NaCl solution with di fferent pH values. It was found that the κ phase is preferentially corroded when the pH<4 but the α phase dissolves first when the pH > 4. The results of Weill-Couly et al. [10] showed that the corrosion resistance of NAB in seawater was related to its microstructures. The phase transformation of a small amount of β'→ α + κ could e ffectively improve the corrosion resistance of the alloy in seawater. The excellent corrosion resistance of NAB is derived from a corrosion product film produced on the surface [4,11–14]. Schüssler et al. [4] proposed that the corrosion product film produced on the surface of NAB contained an inner layer of Al2O3 and an outer layer of Cu2O; furthermore, a thickness of 800–1000 nm could reduce the corrosion rate by 20–30 times. If time is su fficient, corrosion products such as Cu2(OH)3Cl will also form on the outer layer of the film [12,13].

The as-cast NAB pieces used for ship propellers are very large, weighing as much as 200 t. Therefore, cast defects are unavoidable. Large and concentrated defects can be repaired in various ways to reduce the adverse e ffects of defects. For example, Sabbaghzadeh et al. [15] repaired NAB by fusion welding, and the galvanic corrosion current between the welded and the as-cast microstructures was only a few nA. Hanke et al. [16] applied a friction surface treatment for as-cast NAB, resulting in refinement and homogenization of the surface microstructure, which repaired cast defects. Ni et al. [17] used friction stir processing for as-cast Ni-Al bronze and compared the weight loss of the treated NAB and as-cast NAB in 3.5% NaCl solution. It was found that the weight loss of the treated NAB was significantly smaller than that of as-cast NAB, improving corrosion performance. That is, welding can improve the corrosion resistance of copper alloys with large cast defects in seawater.

However, for small and dispersed defects, they are not only unavoidable but also di fficult to repair. Therefore, NAB propellers that are put into use generally have small cast defects, and their effect on the corrosion behavior of NAB in practical applications should be considered. Unfortunately, there is little literature available on this issue. Therefore, the e ffect of small cast defects on the corrosion behavior of the UNS C95810 alloy will be investigated in this paper by comparing the corrosion rate, morphology, product composition and electrochemical corrosion parameter of the specimen with and without defects.
