I was visiting my hobie 16 this weekend after long winter brake, and I shocked to find long and ugly corrosion damages along the mast.

What should I do to stop it?

It must have been in contact with some other dissimilar metal or something. Was the mast lead up an plain steel supports or something ?

Wouter

no, it is standing up in natural position with and no contact with any metal.

Maybe it was hit by lightning?

thats a good thought

alloy has great corrosion resistance as long as the tough oxide outer remains unbroken. if anything is wearing this layer off the corrosion can get a good hold. on my nacra 5.2 the jib halyard has a small block on the bottom end for luff tensioning. if the boat is left on the beach with the jib halyard ready to raise the jib that small block is up against the mast just below the mast hound and by moving around against the mast will wear off the oxide layer and promote corrosion. also on the boom i found pitting corrosion where stainless had been riveted direct to the alloy. after drilling the money rivets the pitting corrosion drilled out with little diamond bits and a dremel, then bright alloy was painted with a thick layer of anti-rust primer, then silver paint, then alloy tape and the stainless riveted back on with stainless rivets.

probably not the perfect method but all the materials were easily on hand, the job was simple to do and anything is better than doing nothing! pics attached

this is on the mast and not as bad as the boom but this is the stainless/alloy galvanic pitting corrosion that just gets deeper and deeper until you clean it out and stop it

How old is the mast? (older masts were of different alloys that were not so tolerant to corrosion, particularly in a marine environment, however it was so long ago that most of these mast have corroded themselves out of a job by now anyway.)

Is it painted? (sometimes painting an aluminium extrusion can cause severe pitting corrosion because if water gets below the paint and in contact with un-oxidised aluminium the oxide layer may never form because it can't get any oxygen to form the oxide. It can be thought of as aluminium needing to breathe so that after a bit of a knock (which destroys the oxide layer) the oxide layer can reform and protect the aluminium again (the oxide layer is very hard compared with the base aluminium alloy). If the oxide layer never formed on aluminium, it would be a much less useful material. Thank you aluminium oxide layer!<img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />)

Was a halyard (wire or rope) bearing against the mast?

Got photos?

If the damaged area is not in contact with any other metal, it is probably the result of contamination (salt held in contact by something such as oil, bird poo, construction dust, etc). I would wash the mast with fresh water&soap and then rinse. I would probably watch the area a while before I tried to repair the damage.