Definition - What does Sulfide Stain mean?
A sulfide stain refers to the change in color of a coated surface that results from a chemical reaction between hydrogen sulfide and pigmented metals such as copper or mercury.
Corrosionpedia explains Sulfide Stain
Sulfide stains are usually brown or red in color and are caused by the reaction of highly poisonous elements such as lead or mercury in an acidic environment. Proper care and safety precautions must be taken when remediating a stain. Sulfide stains are most likely to occur during long periods of confined storage of a given metal surface, which enables the reactant components to be trapped at high concentrations.
Understanding Corrosion in Pumps and How to Deal With It