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Is Your Reverse Osmosis Watermaker Killing Your Marine Engine?
Topping Off a Marine Engine's
Closed Cooling System With
Reverse Osmosis Water Causes Corrosion
Believe it or not, a watermaker can kill a marine engine.
Surprise: De-Salinated water is very corrosive.
Watermakers with their reverse-osmosis membranes, scrub saltwater squeaky clean until it's fit for drinking. One way they do this is by removing all minerals.
Because all of the minerals have been removed it doesn't exactly taste great.
But when RO water is added to an engine's expansion tank to top off the coolant, the reverse-osmosis-generated water attacks copper and gray iron castings.
Components like temperature senders literally dissolve in the mineral-starved water.
Overheating and premature engine death can result. Don't do it.
One case history documented by Seattle-based Northern Lights/Lugger reveals how reverse-osmosis-generated water dissolved about a dozen temperature senders in a row before the warranty team finally figured out what the problem was. Here's something scary. There have been cases where cylinder liners eroded completely through in as few as 300-engine hours. But don't despair, there's a simple solution.
Desalinated water can corrode an engine
from the inside out
According to, Dick Gee, of Northern Lights, "The only fluid that belongs in an engine's coolant blend, besides antifreeze, is de-ionized distilled water. Do not use reverse osmosis water unless it has been pH neutralized.
Distilled water is purified by a distillation, by applying heat to water, causing it to vaporize into steam. The steam condenses into water free of hard minerals like calcium.
Deionization uses an electrically-charged, ion-exchange resin, or filter. Minerals like magnesium, calcium, iron and others elements are exchanged for positively-charged ions.
Bottled, distilled water from a food store or water supplier is recommended. Tap water often has a high mineral content and should never be used.