According to para. 4.5.8 API 571-2011 copper-based alloys are susceptible to stress corrosion cracking in contact with sulfate solutions. Is pump impeller made of 1.4517 duplex SS (2.8-3.5% Cu) suitable for periodic pumping of a sulfate solution.
Cu based means that Cu is the predominant component of the alloy.
A few % Cu in a SS will be fully in solution and will not behave like a Cu alloy.
Cu bearing SS are commonly used in sulfuric acid service.