A chemical that reacts with dissolved oxygen (O₂) to reduce corrosion, such as sulfite (SO₃^-2) and bisulfite (HSO₃^-) ions that combine with oxygen to form sulfate (SO₄^-2). This is a redox reaction and requires a nickel or cobalt catalyst. Removal of air from a mud by defoaming and mechanical degassing is an essential first step before a scavenger can lower the dissolved oxygen content.