Why some of electric valve is black

Why are some common electric valve is black? Believe everyone is curious, but there is a reason, below with every instrument valves and see it! After the phosphating process of electric valve will be black, the main role is rust. Ping into the maintenance of electric valve note items: 1, the need to keep before installing the valve pipe welding slag, iron and other foreign bodies, so as not to close the valve through the valve sealing surface. 2, bolt, screw need to add lubricating oil regularly, maintenance, in order to avoid rusting parts. 3, in order to avoid leakage valve packing with long aging, need to periodically check the replacement valve packing. This is the reason why electric valves are black.