Why nimonic coating is provided on valve stem of exhaust valves of marine engines??

The purpose of Nimonic coating in exhaust valve of modern marine engines is to prevent hot corrosion of the valve stems, increase valve life and to minimize the effect of valve burning.

Modern marine engines, operaing on HFO with a bore greater than 300mm, the one propelling my current vessel is MAN B&W 6G50ME, operates with a relatively high exhaust tempertaures of 350-400 deg celsius. Under this temperature the uncoated valve stems may give up ther strength, leading to overall reduction in valve life. Also under high temperatures the valves stems are subjected to hot corrosion. Nimonic have a very high temperature stability and resists hot corrosion.

Full nemonic coated valves are also available from engine manufaturers , which will increase overall lifetime of the valve, but increase the cost marginally.

Valve seats are usually treated with a harder alloy compound, stellite.

MAN B&W are now introducing DURA SPINDLE valves with W seats to still increase the overall life, with a promised run time of 20000 hours.