Throughhulls & seacocks

It is every boatowners fear to imagine a leak in a seacock, or a downright breakage. Since Iris is built in 1998 and launched in 2000, we decided to change all the underwater throughhulls and seacocks before we set out on our circumnavigation. It turned out to be the best decision ever – read why


By Kaare   |   2022-03-29

A tale of thru-hulls, seacocks and corrosion

I want to share my experience with you regarding seacocks and thru-hulls. It is not a scientific discussion/paper, but nevertheless it is based upon facts and observable evidence.

Iris is first launched in 2001.

This year I decided to change the all the seacocks and thru-hulls because I was a little worried about corrosion, more specifically the dezincification of brass when in contact with seawater. I have no knowledge of the specific material used for neither the seacocks, nor the thru-hulls or the other fittings, but I expect them all to be brass.

For those who are unfamiliar with the phenomena it is the dissolvent of zinc from brass. When the zinc is removed from the brass the remaining metal turns pink, and the metal becomes very brittle. If this happens to a thru-hull or a valve they can break without warning, and the hull is suddenly breached. Imagine if that happened to you out on the ocean!!!

Iris has a total of 12 thru-hulls, and the first 11 showed different signs of dezincification. Not alarming, but enough to indicate that replacement was becoming due.

The first picture beside is taken from the outside of the hull where I drilled out the thru-hull, and deliberately broke the collar of the thru-hull in half. Notice the pink color which is the brittle material, and the inner yellow color which is the untouched brass. It is clear that dezincification moves inwards and have nearly affected the entire collar of the thru-hull. Another indicator was some spot-corrosion marks on some of the seacocks themselves.

In the picture below you see the spot-corrosion mark on the outside of the valve, and the next picture is where I cut through the spot-corrosion and polished the cut section to see the effect on the material. There is some discoloration of the brass in the polished cut, it is not (yet) pink, so it is not brittle, but it reaches all the way to the surface of the valve. Another valve showed the same evidence and behavior. To me the spot-corrosion is an indicator that something is wrong and needs to be addressed. Must they be changed immediately? – No there is still strength, but the remaining lifetime of the seacock is limited.

The thru-hull simply broke in half!!!

That was the first 11 Thru-hulls

Thru-hull no. 12 looked harmless. It was actually beautiful, and there was no sign of corrosion at all, neither inside nor outside. I had actually decided to leave it alone (honest truth), but when I finished all the others, I figured I might as well change the last one also.

I drilled loose the thru-hull collar and went inside to disconnect the rubber hoses and to check if I could move the sea-cock assembly. I pushed at the “Y” on just underneath the handle and the thru-hull simply broke in half!!!!

Please note that this is a 1¼ inch pipe – it is as strong as they come.

It completely sheared off and broke cleanly in two. It was a clear brittle failure and there was no warning whatsoever! It broke just above the thru-hull nut, and just where the thru-hull enters into the elbow-fitting.

Looking at the failure it is evident why there is a failure, because the thru-hull material is completely dezincified and pink all over, and the break shows all the signs if a brittle material-failure. Picture no. 1 below. 

In order to examine the thru-hull a little further I cut the thru-hull approx. 10mm on the outside of the Thru-hull nut leaving me with a 20mm wide ring of the thru-hull, complete with the thru-hull nut attached. Picture no. 2 below.

I polished the end cut, and in comparison, the two ends of the thru-hull are shown in the pictures below. Picture no. 2 and 3 below.

The inside end is a little broken at the top (picture no. 3) and both have some saw-marks, but that is from me handling the pipe to get it loose from the hull. The outside cut (picture no. 2) shows clear signs of dezincification, but not alarmingly, but the frightening thing is that only 20mm away from the polished cut the pipe is completely dezincified and almost without any strength at all!!!!


  1. If your seacocks display the spot-corrosions shown in the second photo you need to change them because the remaining lifetime of the seacocks will then be limited.
  2. The pink dezincification is clear to see. It will come quickly on the surface of the material, but if it starts moves more into the material you might have a problem.
  3. And most importantly even though you brass looks shiny and yellow, you might have dezincification in another place just a short distance away.

My thru-hulls and seacocks were 20 years old, and I am happy I changed them now.

To see the pictures in original size click on the section below.