Adding a Drain-Off to the Heat Exchanger

[Lou-In-NJ]

My heat exchanger is made out of 316 stainless steel and the boat spends most of its time in brackish water. The question is, for the purposes of slowing corrosion, should I add a small drain tube that will passively allow water to drain out of the heat exchanger when the engine is not running, or is it better to have the H.E. filled with raw-water at all times to limit oxygen?

BTW, this isn't a question about mechanics, it's a question about corrosion.

[SailingHarmonie]

The BEST thing to do would be to give it a fresh water flush if it is not being used

Stainless is not the best choice for a raw water heat exchanger, unless you are using a VERY expensive and hard to get alloy. Stainless is susceptible to cracking at stress points, and to issues with weld quality that can impact its corrosion resistance. And on engines with Al blocks or heads it can cause galvanic corrosion. Standard Cu-Ni heat exchangers are easier to fabricate, cheaper, and have an excellent service life for this application. But you got what you got... and for brackish water, it's likely to not be an issue.

Draining it is actually NOT a good idea, but not for the reasons you think. Any remaining salt water will slowly evaporate, and increase the concentration of chloride ions in the remaining wet spots a LOT. That would be bad.

Stainless actually NEEDS to be exposed to oxygen to develop its corrosion protective layer. However, the amount needed here is quite small and the amount dissolved in the water would be sufficient for a long time, unless there are iron parts in the system, which is excellent at scavanging oxygen out of solution.